National Renewable Energy Laboratory 

Innovation for Our Energy Future 

A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

Advanced Wind Turbine 

Program Next Generation 

Turbine Development Project 

June 17, 1997 — April 30, 2005 

GE Wind Energy, LLC 

Tehachapi, California 

Subcontract Report 

NREL/SR-500-38752 

May 2006 
Advanced Wind Turbine 

Program Next Generation 

Turbine Development Project 

June 17, 1997 — April 30, 2005 

GE Wind Energy, LLC 

Tehachapi, California 

NREL Technical Monitor: S. Schreck

Prepared under Subcontract No. ZAM-7-13320-26 

Subcontract Report 

NREL/SR-500-38752 

May 2006 

National Renewable Energy Laboratory 

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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Notice Regarding Change of Subcontractor Name 

At the time of the submission to the U.S. Department of Energy of the proposal which resulted in 

award of NREL Subcontract Number ZAM-7-13320-26, the company now known as GE Wind 

Energy LLC was known as Zond Energy Systems, Inc. Zond was subsequently acquired by another company and eventually was sold to GE Power Systems Corp., a subsidiary of General 

Electric Corporation, and renamed GE Wind Energy. The present document uses only the names 

GE Wind Energy, or simply GE and refers to the company’s products as the GE 750 or the GE 

1.5, regardless of the time at which work being referenced was conducted. The titles of some 

subcontractor reports submitted during earlier stages of this contract and referenced herein contain the older name Zond or the interim names Enron Wind or Enron Wind Energy Systems Corporation (EWESC), and these are the only exceptions to the exclusive use of the GE name. 

Acknowledgments 

This report summarizes the results of nearly seven years of wind turbine research and development by GE Wind Energy and its predecessors. Such a comprehensive undertaking has involved 

the contributions of scores of GE employees and dozens of consultants, subcontractors, vendors, 

and suppliers. Unfortunately, their number is too great to list them all here, but GE is sincerely 

appreciative of all of their individual and collective efforts to accomplish the work presented in 

this report. 

We have also received substantial technical and project management assistance from dozens of 

personnel of the U.S. Department of Energy, the National Renewable Energy Laboratory, and 

Sandia National Laboratories. Again, the contributions of all of these people are greatly appreciated. We would specifically like to express our gratitude to Alan Laxson, Brian Smith, Scott 

Schreck, Ed Cannon, and Paul Migliore, each of whom served as the NREL Project Manager 

during some portion of this project, and to Neil Wikstrom for his seven years as the Subcontract 

Administrator for this project. 

i 
Abstract 

This document reports the technical results of the Next Generation Turbine Development Project 

conducted by GE Wind Energy LLC. This project is jointly funded by GE and the U.S. Department of Energy’s National Renewable Energy Laboratory through Subcontract Number ZAM-7-

13320-26. 

The goal of the NGT Program is for DOE to assist the U.S. wind industry in exploring new concepts and applications of cutting-edge technology in pursuit of the specific objective of developing a wind turbine that can generate electricity at a levelized cost of energy of $0.025/kWh at 

sites with an average wind speed of 15 mph (at 10 m height). 

GE's NGT Project has consisted of three broad activities: 

• Concept Studies 

• Design, Fabrication, and Testing of the Proof of Concepts (POC) turbine 

• Design, Fabrication, and Testing of the Engineering and Manufacturing Development 

(EMD) turbine. 

GE Wind personnel, working with consultants, have completed investigations of a number of 

wind turbine system and component concepts. The purpose of these studies has been to determine the trade-off between cost and improvement in energy capture resulting from each of the 

concepts. These studies have focused on three broadly defined categories of concepts: 

• Electromechanical systems 

• Rotor and structural design 

• Controls. 

The electromechanical systems studies have focused upon a large number of configurations created by changing five key parameters: 

1. generator synchronous speed and the corresponding required gearing 

2. type of generator 

3. number of generators 

4. speed regulation, that is, variable speed or fixed speed 

5. power conversion options. 

The result of all of the electromechanical concept studies is that no concept produces a significant improvement in the COE delivered by wind power relative to the existing GE Wind turbine 

configuration. Only two concepts, 1) a medium-speed wound rotor induction generator operating in variable-speed, constant-frequency mode, and 2) a medium-speed wound rotor synchronous generator operating in fixed-speed mode show any improvement at all in cost of energy 

(COE), and even then, the estimated improvement is less than $0.01/kWh. 

The control strategies which have been investigated in the present study are: 

• Coupling of Blade Pitch and Generator Torque Control 

• Tracking of Peak Power Coefficient 

• Tower Vibration Feedback Control 

ii 
• Independent Blade Pitch for Asymmetric Load Control 

• Load-Limiting Control 

• Alternative Yaw Control Strategies 

• Damage Monitoring and Feedback Control 

• Adaptive Drive Train Damping. 

All of these concepts except for the alternative yaw control strategies show potential for improving the cost of energy, either through reductions in loads or improvement of energy capture. 

The rotor and structural concepts investigated include: 

• Rotor and other turbine structural flexibility 

• Concurrent aerodynamic and structural design optimization 

• Carbon composite rotor blades 

• Aeroelastic tailoring of rotor blades 

• Variable diameter rotors. 

Again, all of these concepts show potential for improving COE through either reduced component costs or improved energy capture. 

The implications of the concept studies that show innovations most likely to produce near-term 

COE benefits at risk levels acceptable to the wind energy financial community are: 

• Optimized low-solidity rotor blades • Larger rotor enabled by sophisticated load-alleviating controls systems 

• Advanced controls systems, including: 

o Independent blade pitch to effect asymmetric load control 

o Tower top accelerometer feedback for tower damping 

o Coupling of pitch and generator torque control 

• Taller, more flexible towers. 

Achieving the originally stated NGT goal of $0.025/kWh at IEC Class II sites does not appear to 

be achievable in the near term for technologies with market-compatible risk levels. GE has identified through its NGT concept studies, high-risk concepts that can provide additional reductions 

in COE between 10%–25%, thereby making the $0.025/kWh goal achievable. The most important of these factors are the variable diameter rotor and aeroelastic tailoring of rotor blades. Additionally, damage identification and feedback control may offer benefits to COE in the 5%–10% 

range. However, these three ideas entail risk perceived by the market as excessive, and they have 

not been pursued to the hardware stage in the NGT Program. 

As the first of the concept studies were being completed, GE began designing the POC turbine. 

Installation of the turbine was completed at the GE wind farm in Tehachapi, California, in April, 

2000. This turbine is rated at 1.5 MW and features a three-bladed, upwind rotor of 70.5-m diameter driving a six-pole wound rotor asynchronous generator through a three-stage planetary 

gearbox. The POC turbine employs several innovations identified in the concept studies: 

• Flexible, low-solidity rotor blades employing high-lift, thicker airfoils 

• Coupling of pitch and torque control 

• Taller, soft hybrid steel/concrete towers. 

iii 
In addition to innovations resulting from the concept studies, the POC turbine also employs a 

water-cooled generator. 

Certification testing of the POC turbine for noise, loads, power performance, and power quality 

has been completed under the supervision of personnel of the National Renewable Energy Laboratory. Power performance exceeds the predicted results and the noise results indicate that the 

POC1.5/70.5 configuration is the quietest turbine on the market among units of similar size. 

The EMD turbine was installed in Tehachapi, California, in April 2002. The EMD turbine is an 

evolution from the POC turbine incorporating the following improvements in its design: 

• A larger 77-m diameter rotor 

• 37-m blades with tip curvature 

• Tower-top accelerometer feedback for tower damping 

• Independent blade pitch control for asymmetric load control. 

Certification testing of the EMD turbine for noise, loads, and power performance has been completed under the supervision of personnel at the National Renewable Energy Laboratory. Additional load testing to verify the performance of the asymmetric load control system has also been 

performed by GE personnel. 

Through the application of the above-mentioned near-term concepts in the EMD turbine, GE has 

been able to shave the cost of energy from wind power by 13% relative to the baseline configuration, without even adjusting for inflation. 

GE Wind Energy has commercialized the POC turbine as the GE1.5s. As of December 31, 2004, 

GE had manufactured and deployed 1160 turbines of this configuration in the United States 

alone. GE Wind has commercialized the EMD turbine as the GE1.5sle. As of December 31, 

2004, over 1000 1.5sle-turbines were on order for delivery during 2005. 

iv 
Contents 

List of Tables ............................................................................................................................... vii 

1 Introduction and Project Overview ........................................................................................... 1

1.1 Project Background ........................................................................................................... 1 

1.2 Scope of Work.................................................................................................................. 1 

2 Concept Studies ......................................................................................................................... 3 

2.1 Electromechanical Systems Concepts ............................................................................... 4 

2.1.1 Conventional-Speed Generators and Multi-stage Gearboxes ................................ 8 

2.1.1.1 Alternative Converter Topologies. ........................................................... 8 

2.1.1.2 High-Speed, Variable-Speed Operation of Alternative Generators ......... 9 

2.1.1.3 Fixed-Speed Operation of Conventional Generators ............................. 10 

2.1.1.4 Multiple Conventional-Speed Generators .............................................. 11 

2.1.2 Medium-Speed Generators and Single-Stage Gearboxes .................................... 12 

2.1.2.1 Medium-Speed Wound Rotor Induction Generator ............................... 12 

2.1.2.2 Medium-Speed Squirrel Cage and Synchronous Generators ................. 13 

2.1.2.3 Multiple Medium-Speed Generators ...................................................... 14 

2.1.3 Direct-Drive Generators ....................................................................................... 14 

2.1.3.1 Fixed-Speed Direct-Drive Machines...................................................... 15 

2.1.3.2 Variable Speed Direct-Drive Machines ................................................. 15 

2.1.3.3 Tubular Nacelle (External Monocoque)................................................. 16 

2.1.4 Summary of Drive Train Concepts ...................................................................... 18 

2.2 Controls Concepts ........................................................................................................... 19 

2.3 Rotor and Structural Design Concepts ............................................................................ 20 

2.3.1 Rotor and Structural Flexibility............................................................................ 21 

2.3.1.1 Upwind Rotor Blade and Tower Flexibility........................................... 22 

2.3.1.2 Machine Carrier Yaw and Nodding Compliance ................................... 25 

2.3.1.3 Flexible Downwind Rotor Concepts ...................................................... 29 

2.3.2 Concurrent Aerodynamic and Structural Design Optimization ........................... 31 

2.3.3 Aeroelastic Tailoring of Rotor Blades ................................................................. 33 

2.3.4 Cast Blade Roots .................................................................................................. 35 

2.3.5 Boundary Layer Control....................................................................................... 38 

2.4 Variable Diameter Rotor ................................................................................................. 38 

2.5 Summary of Concept studies........................................................................................... 40 

3 Proof of Concepts Turbine ...................................................................................................... 41 

3.1 Design of the POC1.5/70.5.............................................................................................. 41 

3.1.1 Tower and Foundation Design ............................................................................. 45 

3.1.2 Drive Train Design and Testing ........................................................................... 45 

3.1.3 Controls System Design ....................................................................................... 46 

3.1.4 POC 34-m Rotor Blade Design and Testing ........................................................ 49 

3.1.5 POC Hub Analysis ............................................................................................... 56 

3.2 Installation and Testing of the Proof of Concepts Turbine ............................................. 56 

3.2.1 Power Performance Testing of the POC Turbine................................................. 58 

3.2.2 Power Quality Testing of the POC Turbine ......................................................... 58 

v 
3.2.3 Mechanical Loads Testing of the POC Turbine................................................... 58 

3.2.4 Acoustic Noise Testing of the POC Turbine........................................................ 59 

3.3 POC Certification ............................................................................................................ 59 

4 Engineering and Manufacturing Development Turbine, EMD1.5/77..................................... 61 

4.1 IEC Class S High Energy Capture Rotor Configuration and Loads ............................... 61 

4.2 EMD1.5/77 Component Designs .................................................................................... 65 

4.2.1 EMD Gearbox Modification ................................................................................ 65 

4.2.2 EMD 37-m Rotor Blade Design and Testing ....................................................... 65 

4.2.3 EMD Controls Systems ........................................................................................ 67 

4.2.3.1 Tower-top Accelerometer Feedback ...................................................... 67 

4.2.3.2 Rotor Feedback Control ......................................................................... 69 

4.3 Installation and Testing of the EMD Turbine ................................................................. 70 

4.3.1 Power Performance Testing of the EMD Turbine ............................................... 71 

4.3.2 Mechanical Loads Testing of the EMD Turbine.................................................. 72 

4.3.3 Acoustic Noise Testing of the EMD Turbine....................................................... 72 

4.3.4 Testing of Asymmetric Load Control System...................................................... 73 

5 Commercialization of GE NGT Technology .......................................................................... 74 

6 Conclusions ............................................................................................................................. 75 

List of Figures 

Figure 1. Multiple Generator Configuration................................................................................. 11 

Figure 2. Concept layout of Single Stage Gearbox, Medium Speed Generator System .............. 13 

Figure 3. Direct-Drive Generator Wind Turbine System ............................................................. 15 

Figure 4. Direct-Drive Support System ........................................................................................ 17

Figure 5. Fatigue Damage Equivalent von Mises Stress Distribution .......................................... 18 

Figure 6. Effect on Fatigue from Reducing Rotor Blade and Tower Stiffness............................. 22 

Figure 7. Effect on Fatigue from Reducing Rotor Blade and Tower Stiffness............................. 24 

Figure 8. Impact of Yaw and Nodding Compliance on Fatigue & Motion .................................. 25 

Figure 9. Impact of Nodding Nacelle on Fatigue ......................................................................... 26 

Figure 10. Impact of Rotor, Tower, and Tower Top Flexibility on Fatigueas Determined 

through Simulations ...................................................................................................... 28 

Figure 11. Summary of Downwind Configurations ..................................................................... 30 

Figure 12. Impact on Energy Production from Blade Root Inserts ............................................. 37 

Figure 13. Illustration of S-VADER Blade Concept .................................................................... 39 

Figure 14. Illustration of S-VADER Operation............................................................................ 39 

Figure 15. Predicted Power Performance for the POC1.5/70.5 Turbine ..................................... 41 

Figure 16. Schematic of the Proof of Concepts Turbine ............................................................. 44 

Figure 17. POC Tower in Tehachapi, California .......................................................................... 45 

Figure 18. Drivetrain Layout of NGT Proof of Concept 1.5 MW Turbine .................................. 46 

Figure 19. POC Water-Cooled Loher 1.5-MW Generator ........................................................... 47 

Figure 20. POC Drive Train Components .................................................................................... 47 

Figure 21. POC Drive Train on the NWTC Dynamometer Test Stand ........................................ 48 

vi 
Figure 22. POC Main Control Cabinet ........................................................................................ 49 

Figure 23. POC 34-m Blade Planform......................................................................................... 51 

Figure 24. NGT POC Airfoil Family........................................................................................... 52

Figure 25. 18% Airfoil Model in the WSU Wind Tunnel Test .................................................... 52 

Figure 26. Prototype Set of the GE34a Blades at Tecsis ............................................................. 53 

Figure 27. Static Blade Tests ........................................................................................................ 54 

Figure 28. Static Blade Test Setup................................................................................................ 54 

Figure 29. NWTC Fatigue Test Setup ......................................................................................... 55 

Figure 30. POC Hub ..................................................................................................................... 56 

Figure 31. POC Turbine Rotor Being Installed ........................................................................... 57 

Figure 32. POC Turbine Installed in Tehachapi, California ......................................................... 57 

Figure 33. Comparison of Theoretical and Measured POC Power Curves .................................. 58 

Figure 34. Comparison of the POC Noise Levels to Historical Levels ........................................ 59 

Figure 35. Comparison of Noise Levels and Rotor Tip Speeds ................................................... 60 

Figure 36. Benefits to COE from Increasing Rotor Size and Turbine Rating .............................. 64 

Figure 37. Predicted Power Performance for the EMD1.5/77 Turbine ....................................... 65 

Figure 38. EMD 37-m (GE37a) Blade.......................................................................................... 66 

Figure 39. EMD Tower-Top Accelerometer Package ................................................................. 68 

Figure 40. Flowchart of the EMD Tower Top Accelerometer Feedback .................................... 68 

Figure 41. EMD Turbine Installed in Tehachapi, California........................................................ 71 

Figure 42. Comparison of Theoretical and Measured EMD Power Curves ................................. 71 

Figure 43. Comparison of the EMD Noise Levels to Historical Levels ....................................... 73 

Figure 44. Leverage of USDOE R&D Funding by GE Wind Energy.......................................... 74 

Figure 45. Return on Investment of NREL Funds in GE Wind................................................... 74 

List of Tables 

Table 1. Summary of the Key Drive Train Design Parameters and Configurations Options......... 6 

Table 2. Summary of Drivetrain Configurations Investigated........................................................ 7 

Table 3. Proof of Concepts Turbine Configuration Description................................................... 42 

Table 4. Summary of Required Airfoil Characteristics ................................................................ 51 

Table 5. Test Equivalent Damage ................................................................................................. 56 

Table 6. Engineering & Manufacturing Development Turbine Configuration ............................ 62 

vii 
List of Symbols and Abbreviations 

ALC Asymmetric Load Controller 

CSI Current Source Inverter 

DOE Department of Energy 

EMD Engineering & Manufacturing Development 

GE General Electric Corp. 

GL Germanischer Lloyd 

GW Gigawatts (power) 

GWh Gigawatt·hours (energy) 

Hz Hertz (cycles per second) 

IGBT Integrated Gate Bipolar Transistor 

kg Kilograms (mass) 

kV Kilovolts 

kVA Kilovolt-amps 

kW Kilowatts (power) 

kWh Kilowatt·hours (energy) 

MW Megawatts (power) 

MWh Megawatt·hours (energy) 

NGT Next Generation Turbine 

NREL National Renewable Energy Laboratory 

NWTC National Wind Technology Center 

POC Proof of Concepts 

PWM Pulse-width modulated 

SCR Silicon Controlled Rectifier 

VAC Volts, Alternating Current 

VAR Volt-amps, reactive (reactive power) 

VSCF Variable Speed Constant Frequency 

VSI Voltage source inverter 

viii 
1 Introduction and Project Overview 

This document summarizes work completed by GE Wind Energy LLC on the Next Generation 

Turbine Development Project. This project is jointly funded by GE and the U.S. Department of 

Energy’s National Renewable Energy Laboratory (NREL) through Subcontract Number ZAM-7-

13320-26. 

1.1 Project Background 

The U.S. Department of Energy began sponsoring the Turbine Research Program (then called the 

Advanced Wind Turbine Program) in 1990. The first phase of this program – Conceptual Design 

Studies – was completed in 1992 and identified incremental improvements and advanced configurations that could improve the competitiveness of wind energy. 

Near Term Product Development, the second phase of the program, provided funding to several 

U.S. wind energy companies to design, fabricate, and test prototype turbines designed to produce 

electricity for $0.05/kWh or less at 5.8 m/s (13 mph) sites. Among these were two projects directed by GE. The first resulted in the development of the Z-40, a 550-kW turbine, of which GE 

Wind (then operating as Zond) installed 93 in China, Greece, Ireland, Korea, Mexico, and the 

U.S. The second project, the Near Term Research and Testing program, resulted in significant 

value engineering improvements to GE’s 750-kW series of turbines. This project resulted in a 

substantial reduction in COE delivered by wind power. 

The third phase of the Turbine Research Program is the Next Generation Turbine Development 

Project (NGT). The goal of the NGT Program is for DOE to assist the U.S. wind industry in exploring new concepts and applications in cutting-edge technology to develop a wind turbine that 

generates electricity at a levelized COE of $0.025/kWh at sites with an average wind speed of 15 

mph (at 10-m height). 

Through the NGT program, GE submitted a proposal to NREL in May 1995, for a project to develop an advanced-technology turbine capable of achieving the $0.025/kWh objective. GE received notice of award in May 1996. The NGT Project has proceeded in two stages. The first 

stage, initiated when GE and NREL signed a letter subcontract in April 1997, involved concept 

definition studies, which were intended to develop reliable performance and cost estimates for 

GE's proposed systems, along with a preliminary work plan, budget, and schedule for the second 

stage of the project, Prototype Development. The latter began in June 1998, when GE and NREL 

signed Subcontract Number ZAM-7-13320-26, authorizing full funding of GE’s NGT Project. 

This Subcontract has been jointly funded by GE and NREL through a cost-sharing arrangement. 

Technical work under this contract was completed in April 2004, at which time personnel from 

GE, NREL, and the Sandia National Laboratories conducted the Project Final Design Review. 

1.2 Scope of Work 

The Statement of Work (SOW) governing this Project requires GE to design, fabricate, install, 

and test two prototype turbines: 

• Proof of Concepts (POC) turbine 

• Engineering and Manufacturing Development (EMD) turbine 

1 
The SOW also stipulates that GE seek certification of the POC turbine. The scope of work for 

this project requires GE to select a wind turbine system for development that will achieve a combination of improved performance, increased reliability, and decreased cost, such that the COE 

objective of the project is met. 

The SOW stipulates that GE investigate through a series of comprehensive studies a number of 

concepts, the best of which would be incorporated into the POC turbine. The baseline turbine 

(GE's 750-kW turbine) would serve as a foundation for these studies as well as the starting point 

for development of the POC. The EMD turbine would represent a technological evolution from 

the POC, incorporating new innovations resulting from continuing concept studies. 

Following signing of the subcontract, GE sponsored a kickoff meeting in July 1998, at its facility 

in Tehachapi, California. Present at this meeting were key GE engineering personnel and technical consultants. The purpose of the meeting was to identify a number of technologies and concepts that might be considered good candidates for further study in an effort to significantly reduce the cost of electricity generated by wind power. Concepts included both near-term technologies that might constitute incremental improvements to existing GE products, as well as advanced concepts requiring several years to develop. 

On the basis of this meeting, GE identified a number of potentially beneficial concepts and 

signed several subcontracts with consultants to assist the company in completing studies of these 

ideas. The initial concept studies portion of the NGT project continued through December 1999. 

Some of the concepts were incorporated into the POC turbine, while other innovations required 

further study and have been incorporated into the EMD turbine. The concept studies are discussed in detail in Section 2 of this report. 

As the first of the concept studies were being completed, GE began the design of the POC turbine, installation of which was completed at the GE wind farm in Tehachapi, California, in April 

2000. This turbine is rated at 1.5 MW and features a three-bladed, upwind rotor of 70.5-m diameter driving a six-pole wound rotor asynchronous generator through a three-stage planetary 

gearbox. The turbine is installed on a free-standing 63-m tubular steel tower. Details of the 

POC turbine design, fabrication, installation, and testing are reported in Section 3. 

As the initial phase of concept studies was ending and the installation of the POC turbine was 

underway, the design of the EMD turbine began in early 2000. As noted above, this effort began 

with continuing concept studies and then progressed to preliminary, detail, and final design efforts. A critical design review was conducted at NREL in March 2001. The EMD was installed 

in two phases. In the first phase, GE installed the 77-m rotor on the EMD turbine in Tehachapi 

in April 2002. In the second phase, advanced controls systems were installed on the turbine in 

2003. Details of the EMD design are reported in Section 4. 

2
2 Concept Studies 

GE approached the design of its next generation turbine from the premise that any turbine concept or technology that might yield improvements in COE should be considered for inclusion in 

the design. As a result, at the onset of the project, GE chose not to focus all engineering efforts 

on designing and developing a preconceived concept for an advanced technology wind turbine. 

The company instead chose to identify a number of innovative concepts and technologies that 

might be expected to produce improvements in COE. At project commencement, GE expected 

to spend approximately 6 to 8 months conducting studies of these concepts and then, based upon 

the results of these studies, to select the most promising, feasible, and mutually compatible technologies to incorporate into the preliminary design of a POC turbine. As research and development progressed during the NGT project, new concepts were continually proposed and concept 

studies continued in parallel with the development of the POC and EMD turbines. As some of 

the best concepts were refined, they were introduced into the POC and EMD turbine designs, as 

explained in Sections 3 and 4 below. 

GE Wind personnel and consultants have completed investigations of several wind turbine system and component concepts. The purpose of these studies has been to determine the trade-off 

between cost and improvement in energy capture resulting from each of the concepts. These 

studies have focused on three broadly defined categories of concepts: 

• electromechanical systems 

• controls systems 

• Rotor and structural design. 

Each of these categories of concept studies is discussed in Sections 2.1 through 2.3. 

The concept studies were initially conducted at a rated power of 750 kW so that the baseline 

Z48/750 turbine could be used as a basis for comparison. As is noted in Section 3, the POC turbine was developed at 1.5 MW. As a result, some of the concept studies conducted subsequent 

to the initiation of the POC design were also conducted at 1.5 MW so that their results could be 

compared to the POC turbine. 

Because of market advantages and GE experience, it was decided from the outset that the NGT 

design should favor the three-bladed, upwind, variable speed, and pitch-regulated configuration, 

although some alternatives to this were briefly examined. Additional guiding principles 

throughout the concept studies have been: 

• Use of readily available (commercial off-the-shelf or COTS) components 

• Modularity (i.e., distributed design) 

• Simplicity and ease of operations and maintenance 

• Manufacturability 

• Transportability 

• Scalability. 

3 
2.1 Electromechanical Systems Concepts 

This category encompasses concepts relating to the drive train and all of the mechanical, structural, and electrical systems and components that support the drive train. This includes everything located at the tower top (except for the rotor), primary mechanical components (such as the 

gearbox), and electrical systems (such as the generator, power inverter, bedplate, and nacelle). 

These components are grouped together because they are so highly integrated. It is impossible to 

discuss fundamental changes to the gearing without considering the impact on the generator, and 

it is further impossible to discuss major changes to the generator without further considering how 

this impacts the power inverter. 

The studies have identified wind turbine drive train concepts that differ fundamentally from the 

baseline configuration in terms of either: 

a) The electromechanical arrangement they employ to convert the low-speed rotary motion 

of the wind turbine rotor into high-quality, three-phase, 60 Hz electrical power, or 

b) The infrastructure they use to support the drive train. 

Concept studies have been conducted using the GE 750-kW and 1.5-MW turbines as bases for 

comparison. The drive trains of the two turbines differ primarily in that the 750-kW uses an integrated design in which the gearbox casing serves as the main support frame and yaw deck to 

which the generator and the nacelle attach directly, while the 1.5-MW features a distributed design in which a main frame bedplate serves as the yaw deck to which all of the component–main 

bearing, gearbox, generator, and nacelle– are attached. 

The salient features common to the drive trains of both the baseline turbine and the 1.5-MW turbine and from which all proposed changes have been measured are: 

• Two- or three-stage gearbox, providing output-to-input speed ratios of approximately 

40:1 to 100:1 

• Doubly-fed (wound rotor) induction generator with 4 or 6 poles on stator and rotor, designed to generate 60 Hz at either 1200 rpm or 1800 rpm or 50 Hz at 1000 rpm or 1500 

rpm 

• Variable speed operation, with the wound rotor generator operating over a speed range of 

approximately 25%–30% sub-synchronous slip to 25%–30% super-synchronous slip 

• Partial power conversion of only the generator rotor-supplied (or fed) current. 

This arrangement is employed on all of the wind turbine models manufactured by GE. 

The use of the wound rotor generator allows the stator to be electrically connected directly to the 

main electrical grid operating at either 50 Hz or 60 Hz, depending upon the location in the world. 

As the entire system operates at variable speeds, the generator rotor produces variable frequency 

current that is converted to 60 Hz power by the power converter. When the generator is operated 

at speeds above the synchronous speed (e.g., 1200 rpm for a 6-pole generator in the U.S.) the rotor feeds real power to the converter. When operated at speeds less than synchronous speed, the 

converter must feed real power to the rotor. 

4
The advantage of this system is that from 70%–80% of the power is produced by the stator, 

which is connected directly to the electrical grid without need of power conversion. Only the 

20%–30% of the power derived from the rotor requires power conversion. This arrangement allows for variable speed operation of the wind turbine rotor, thereby maximizing energy capture, 

while allowing the use of a power converter sized for only 20%–30% of the wind turbine rated 

power. Partial power conversion improves the system efficiency (i.e., energy production) while 

significantly reducing the cost of the power electronics. Furthermore, 4- and 6-pole induction 

generators and motors are widely manufactured and relatively inexpensive. 

In moving away from the baseline drive train configuration, the designer can work with five key 

parameters: 

• Generator synchronous speed and the corresponding required gearing 

• Type of generator 

• Number of generators 

• Speed regulation (variable speed or fixed speed) 

• Power conversion options. 

Table 1 summarizes the configuration options associated with each parameter and the anticipated 

cost and performance trade-offs that have motivated the studies of these options. A total of 216 

permutations of the configurations shown in Table 1 exist. However, many of the permutations 

obviously can be eliminated. For example: 

• The direct-drive option only works with one generator. The use of multiple generators 

requires multiple output shafts from a gearbox. 

• Only the wound rotor induction generator can utilize partial power conversion. 

• Fixed speed operation requires no power conversion. 

Table 2 summarizes the 36-drive train configurations examined in the present study. Every configuration in Table 2 does not need to be examined separately, because several of the design parameters can be changed independently. For example: 

• Changing from variable speed to fixed speed operation will impact the aerodynamic energy capture similarly regardless of the drive train configuration. 

• Changing from variable speed to fixed speed operation will eliminate the power converter 

regardless of whether the generator is direct-drive, medium speed, or conventional speed. 

• Use of multiple generators should not significantly impact the cost of the power converter, as the cost of the latter is mostly a function of the total power rating. 

• Use of multiple generators should not significantly impact the energy capture. 

As a result of these assumptions, the field of candidate configurations for study have been narrowed to 11 categories, as defined in Table 2. These 11 categories of drive train configurations 

are each discussed in the following subsections 2.1.1 through 2.1.3. 

A summary of the COE impacts on proposed design changes is summarized in Section 2.1.4. 

5 
Table 1. Summary of the Key Drive Train Design Parameters and Configurations Options 

Parameter Configuration Options Trade-offs 

Performance Cost 

Generator Synchronous Speed 

• Low-Speed (Direct-Drive 

Generator) 

• gearboxes represent a 

significant source of 

• multi-stage gearboxes represent a significant fraction 

• Medium-Speed (Single 

Stage Gearbox) 

mechanical loss 

• gearboxes represent a 

of the cost of a wind turbine 

• Conventional (1200 or 

1800 for 60 Hz, 1000 or 

significant source of 

maintenance require­

• unconventional generators 

can be expected to be more 

1500 for 50 Hz) ments expensive. 

Type of Generator • Wound rotor induction • Squirrel cage machines • Squirrel cage and wound 

generator and synchronous ma- rotor synchronous ma­

• Squirrel cage induction 

generator 

• Wound rotor synchronous 

generator 

• Permanent magnet synchronous gen. 

chines either require full 

power conversion or 

must be operated fixed 

speed. Both options 

will reduce energy capture. 

chines are widely manufactured and easily manufactured, making them very 

low-cost options. 

• Permanent magnet machines can be very compact 

and hence relatively low 

cost 

Speed Regulation • Variable Speed • Fixed speed operation • Fixed speed operation 

• Fixed Speed generally results in a 

loss of several percent 

eliminates the need for expensive power converters 

of energy capture 

Number of Genera­ • Single Generator • Multiple generators • Use of more and smaller 

tors • Multiple Generators might allow a better 

coupling of the aerodygenerators might provide a 

small manufacturing cost 

namic (rotor) power – savings due to economies 

vs– wind speed curve of scale 

and the generator power 

–vs– shaft speed curve, 

improving efficiency 

Power Conversion • Full Power Conversion 

• Partial Power Conversion 

• No Power Conversion 

• Rectifying and inverting 

more power will result 

in greater power losses 

• The more power for which 

the converter is sized, the 

more expensive it will be. 

6
Table 2. Summary of Drivetrain Configurations Investigated 

Configuration 

Generator 

Synchronous 

Speed 

Generator Type Number of 

Generators 

Speed 

Regulation 

Power 

Conversion 

1.1. Baseline Conventional Wound-Rotor Induction 1 Variable Partial 

1.2 

Conventional Squirrel Cage Induction 1 Variable Full 

Conventional Wound-Rotor Synchronous 1 Variable Full 

Conventional PM Synchronous 1 Variable Full 

1.3 

Conventional Wound-Rotor Induction 1 Fixed None 

Conventional Squirrel Cage Induction 1 Fixed None 

Conventional Wound-Rotor Synchronous 1 Fixed None 

Conventional PM Synchronous 1 Fixed None 

1.4 

Conventional Wound-Rotor Induction Multi Variable Partial 

Conventional Squirrel Cage Induction Multi Variable Full 

Conventional Wound-Rotor Synchronous Multi Variable Full 

Conventional PM Synchronous Multi Variable Full 

Conventional Squirrel Cage Induction Multi Fixed None 

Conventional Wound-Rotor Synchronous Multi Fixed None 

Conventional PM Synchronous Multi Fixed None 

2.1 Medium Speed Wound-Rotor Induction 1 Variable Partial 

2.2 

Medium Speed Squirrel Cage Induction 1 Variable Full 

Medium Speed Squirrel Cage Induction 1 Fixed None 

Medium Speed Wound-Rotor Synchronous 1 Variable Full 

Medium Speed Wound-Rotor Synchronous 1 Fixed None 

Medium Speed PM Synchronous 1 Variable Full 

Medium Speed PM Synchronous 1 Fixed None 

2.3 

Medium Speed Wound-Rotor Induction Multi Variable Partial 

Medium Speed Squirrel Cage Induction Multi Variable Full 

Medium Speed Wound-Rotor Synchronous Multi Variable Full 

Medium Speed PM Synchronous Multi Variable Full 

Medium Speed Squirrel Cage Induction Multi Fixed None 

Medium Speed Wound-Rotor Synchronous Multi Fixed None 

Medium Speed PM Synchronous Multi Fixed None 

3.1 

Direct Drive Squirrel Cage Induction 1 Fixed None 

Direct Drive Wound-Rotor Synchronous 1 Fixed None 

Direct Drive PM Synchronous 1 Fixed None 

3.2 Direct Drive Wound-Rotor Induction 1 Variable Partial 

Direct Drive Squirrel Cage Induction 1 Variable Full 

3.3 Direct Drive Wound-Rotor Synchronous 1 Variable Full 

3.4 Direct Drive PM Synchronous 1 Variable Full 

7
2.1.1 Conventional-Speed Generators and Multi-stage Gearboxes 

These options are most closely related to the baseline configuration. Possible changes are: 

• Use alternative converter topologies. 

• Use generators other than wound rotor induction machines. 

• Operate at fixed shaft speed. 

• Use more than one generator. 

Each of these proposed changes is discussed further below. 

2.1.1.1 Alternative Converter Topologies. 

 The baseline Z750 turbine uses an IGBT-based current source inverter with an active bridge rectifier (AR-CSI). The 6-pole-generator on the baseline turbine has a synchronous speed of 1,200 

rpm at 60 Hz. In the Z50 configuration, it is operated from approximately 850–1300 rpm, which 

means it is operating both sub-synchronously and super-synchronously. As a result, the power 

converter must be designed to be bi-directional, that is, it must both feed and take power from 

the rotor. 

Unidirectional Current Source Inverter 

GE investigated using a unidirectional CSI with a passive (uncontrolled) diode rectifier (DRCSI) by operating the wound rotor induction generator only sub-synchronously. This concept 

probably represents the smallest change to the baseline configuration of any concept studied in 

the project. The proposal was to operate such a generator from approximately 720–1100 rpm, 

remaining entirely sub-synchronous. Although operating at 720 rpm would represent higher slip 

and more rotor current than operating at 850 rpm, it would probably be more cost effective to 

only construct the converter to handle unidirectional power. 

However, the study showed that the higher rotor currents would not only require larger rotor 

windings, but they would likely produce greater problems with generator cooling and overall 

losses. Although they did not conduct a detailed design, engineers at one of GE's generator suppliers briefly examined the concept and concluded that they would have to increase the frame by 

one, if not by two frame sizes, and that it could produce intractable cooling problems. Another 

GE-commissioned study concluded that the increase in rotor power would increase the generator 

volume, weight, and cost. Furthermore, this work indicated that the rotor power increase was 

also a more significant cost driver for the converter, and estimated that the cost of a unidirectional power converter for subsynchronous generator operation, assuming the same aerodynamic 

rotor speed range, would cost more than the bidirectional arrangement. 

Results indicated this concept was not worth pursuing further. 

Voltage Source Inverter Topologies 

GE also examined voltage source inverter (VSI) topologies, including: 

8 
• VSI with passive diode bridge rectifier (DR-VSI) 

• VSI with DC-to-DC Boost Converter (DR-B-VSI) 

• VSI with Active Rectifier (AR-VSI) 

The basic DR-VSI configuration, although an economical converter solution, is not the best 

choice from a system design standpoint. This is due to the difficulty of maintaining a constant 

AC line voltage over the generator speed range. In fact, the generator terminal voltage is a function of the generator speed; in the absence of DC bus voltage regulation, the relatively wide 

speed range would lead to a very large DC bus voltage fluctuation. This fluctuation of the DC 

bus voltage would force design changes of the system for full voltage at minimum speed. Allowing the DC bus voltage to grow as the speed increases would result in tremendous converter 

oversizing and poor machine utilization. 

The DR-B-VSI solution is sometimes adopted in adjustable speed drives when AC voltage regulation is desired without pulse width modulation (PWM) harmonics and ripple. In general, the 

DC-to-DC converter is a buck converter or step down converter with a different arrangement of 

the devices. In the case of the wind power generator, the constant voltage side would be the output of the regulator, and the variable voltage side would be the input. This configuration is very 

flexible and can achieve good DC bus voltage regulation even if the machine terminal voltage is 

very low. It has two extra switches, and one extra inductor. Both the inductor and the diode 

carry the entire DC bus current, and contribute significantly to the total converter losses. All the 

active switches can be either IGBTs (preferred) or SCRs with commutation circuits. In addition 

to the PWM inverter, the passive three-phase bridge, and the DC link inductance loss, this topology has the diode and switch loss of the boost converter; these extra losses are expected to add 

an additional 0.5% loss at full load. 

The AR-VSI technology should be readily available, even if the diode bridge front end version is 

more common. IGBTs are the power devices of choice in this case. This AC-to-AC converter 

can provide input and output power factor control and obtain the best machine utilization. The 

converter losses are somewhat higher than the other configurations but the generator efficiency 

can be improved because of a more flexible and accurate machine control. 

The conclusion of the study was that the baseline configuration AR-CSI is probably the least expensive of the topologies appropriate for use with the wind turbines. As noted above, the 

DR-CSI unit is inappropriate for bi-directional operation, and the DR-VSI unit is inappropriate 

for use with wind turbines. This leaves only the AR-VSI or DR-B-VSI units, both of which 

demonstrate higher losses and cost more than the baseline converter topology. As a result, these 

studies did not present any superior alternatives to the baseline system. 

2.1.1.2 High-Speed, Variable-Speed Operation of Alternative Generators 

The option of using generators other than wound rotor induction machines (squirrel cage induction generators, wound rotor, or permanent magnet synchronous generators) in a variable speed 

mode was briefly considered at the outset of the concept studies because of the assumed higher 

cost associated with manufacturing the wound rotor machines. The use of any of the machines 

9
other than the wound rotor induction generator, however, involves the need for full power conversion. 

In terms of cost, the choice of generator represents a simple trade-off between the cost of the 

generator and the cost of a power converter sized for full power conversion versus one sized for 

partial power conversion. At the time these studies were conducted in 1998 and 1999, it was estimated that the power converter for operating a single squirrel cage induction generator over the 

same speed range as the GE 750-kW operated generator would cost nearly as much as the cost of 

the wound rotor machine and partial power converter together. Furthermore, investigations 

showed that the best expected price which could be expected for a commercial off-the-shelf 750­

kW squirrel cage generator was approximately 75% that of the wound rotor machine, and it was 

not clear that these machines would have the service factor sufficient to allow their use in wind 

turbine applications. The squirrel cage generator and full-power converter package would cost 

approximately 35%–-40% more than the baseline configuration hardware. 

Similar prices were identified for 750-kW wound rotor synchronous generators. No permanent 

magnet synchronous generators rated at 750 kW were available commercially, but GE and its 

consultants agreed at the time that it could be expected that such a machine custom-built could 

cost as much or more than a conventional wound rotor synchronous generator. Furthermore, 

studies estimated the cost for a full power converter for a PM synchronous generator at approximately 120%–140% of the cost of the baseline generator/converter combination. Again, the 

combined cost of these two components would be well more than that of the baseline configuration. 

In terms of energy capture, all four types of machines can be fabricated with high electrical and 

mechanical efficiencies, so the impact of the generator choice on energy capture is probably negligible. The typical efficiency associated with the power converter is approximately 98%–99%. 

Therefore, if all of the current is converted instead of rather than at most 20% of the current, this 

change represents an additional loss of approximately 0.5%–1.5% of energy capture. Such a relatively small loss might be justifiable if significant cost savings result, but given the probable increase in cost resulting from these configuration changes, this energy loss only exacerbates the 

problem. 

Furthermore, Dynamic VAR capability is frequently demanded by customers of GE wind turbines and is an important marketing consideration. Cost of energy being equal, options that provide dynamic VAR control are preferable. A squirrel cage induction generator cannot provide 

dynamic VAR control unless it utilizes a converter that can supply VARs to the stator. This results in a double cost penalty for these machines: the rating of the converter is higher because of 

the reactive power to be supplied, and the cost per kVA is substantially higher because of the active rectifier. 

2.1.1.3 Fixed-Speed Operation of Conventional Generators 

Some commercially available wind turbines employ wound rotor induction generators operating 

at fixed speeds. The motivation for this is unclear, as GE identified no significant advantage to 

using a wound rotor machine versus a squirrel cage machine for fixed speed operation, other than 

10
possibly an improvement in generator efficiency. As a result, GE did not give significant consideration to this option. 

GE, instead, examined the option of connecting squirrel cage or synchronous machines directly 

to the grid and eliminating the need for power converters. Squirrel cage asynchronous generators 

and wound rotor synchronous generators can be purchased for approximately 60%–70% of the 

cost of a wound rotor asynchronous machine. The converter cost is eliminated. The cost of a 4­

pole or 6-pole permanent magnet synchronous machine was not thoroughly investigated by GE, 

as it was decided that the machine would be more expensive than a wound rotor synchronous 

generator. On the performance side, however, some energy capture is lost from fixed speed operation. This energy capture was estimated by GE personnel at approximately 3%. The impact 

on COE is negligible, as the reduction in cost is offset by a reduction in energy capture. As a 

result, GE decided not to pursue these fixed speed options further. These fixed-speed options 

also lack the ability to provide dynamic VAR control offered by the baseline system. 

2.1.1.4 Multiple Conventional-Speed Generators 

GE investigated the use of a two-stage hybrid planetary/helical gearbox driving multiple generators. Figure 1 illustrates the proposed drive train layout. Although originally proposed with 

wound rotor asynchronous generators in mind, the concept would apply equally well to squirrel 

cage machines or synchronous generators. The motivation to consider this option is the possibility that reduced generator frame size and increased part count could result in reduced generator 

cost. 

Use of multiple generators might also provide for improved energy capture, as the wind speed, 

shaft speed, aerodynamic rotor power, and generator power can be more optimally matched with 

multiple generators than with only one generator. Two generators have been used in the past, 

most notably in the U.S. by Kenetech. 

The perceived cost advantages of this 

concept did not materialize upon 

closer inspection. For the volumes 

being purchased by an OEM customer such as GE, the cost per kilowatt rated for wound rotor induction 

generators is nearly constant as generator rating changes. The size at 

which wind turbines are now being 

manufactured means that few economies of scale were found in cutting 

the generator rating in half or by a 

third. Examination of the volume 

costs of off-the-shelf squirrel cage 

generators showed similar trends. 

Furthermore, the benefits to the energy capture were believed to be generally small. As a result, the use of 

multiple generators does not seem to Figure 1. Multiple Generator Configuration 

11 
offer compelling advantages to COE. The conclusion, therefore, is that short of some other significant justification for using multiple generators (e.g., a significant change to the main frame 

which lends itself to multiple generators), GE could not justify moving in this direction on the 

basis of COE considerations. 

2.1.2 Medium-Speed Generators and Single-Stage Gearboxes 

A more significant departure from the baseline is represented by the use of a medium-speed generator coupled to a single-stage gearbox. This concept was originally proposed as an alternative 

to the direct-drive generator concepts discussed in Section 2.1.3. The direct-drive generators 

tend to be extremely large to the point of seeming almost impractical. On the other hand, the 

conventional arrangement of multi-stage gearbox and high-speed generator leads to a configuration in which the gearbox is substantially larger than the generator. Therefore,, it was speculated 

that a compromise between the two approaches employing a single gear stage and a mediumspeed generator might provide for a more optimal matching of those two components. In fact, it 

was proposed that they could be integrated into a geared generator contained in one housing. 

The concept was originally proposed in conjunction with an innovative new tubular nacelle concept in which the single gear stage and the generator could be integrated into the tubular nacelle, 

eliminating the gearbox and generator casings. This concept is discussed further in Section 

2.1.3.3, but for the purposes of the present analysis, a more appropriate evaluation of this concept 

would result from considering stand-alone gearbox and generator components or a stand-alone 

geared generator. 

2.1.2.1 Medium-Speed Wound Rotor Induction Generator 

GE developed a design for an integrated, geared, medium-speed, wound rotor induction generator (Figure 2). Previous work on the direct-drive generator as reported in Section 2.1.3.2 showed 

it was not possible to design an induction generator operating at 20–35 rpm, the stator of which 

could be directly connected to the electric grid at 60 Hz, allowing for only partial power conversion. As a result, no great advantage exists for the induction design for direct-drive applications. 

For medium-speed applications, however, interest remains in demonstrating a wound-rotor induction machine that could be directly connected. As with the conventional-speed generators, 

such a machine would offer the advantages of variable speed operation with only partial power 

conversion, but with reduced gearing. 

Several alternative generator/power electronics configurations were identified, including 32–, 

40-, and 50–pole machines operating both sub- and supersynchronously as well as subsynchronously only. The study showed that the bidirectional systems (40zB and 50zB) could be constructed to cost no more than the conventional speed system. In both cases, the converters cost 

slightly more than the conventional converter, while the generators cost slightly less than the 

conventional system. The latter result was surprising. However, one consequence of the increase 

in pole count was a loss of approximately 2%–2.5% of system efficiency. This represents a significant loss in energy capture detracting strongly from any cost savings that might result from a 

reduction in gearing. 

12
Figure 2. Concept layout of Single Stage Gearbox, Medium Speed Generator System 

The initial concept planned for the unit to be integrated into a tubular nacelle, as described in 

Section 2.1.3.3. Therefore, some of the projected support structure cost savings resulted from 

that integration. Nevertheless, data shows that the stand-alone medium-speed generator should 

weigh and cost no more than a conventional generator, while it can be expected that simply integrating the single-stage gearbox with the medium-speed generator would result in a significant 

weight and cost savings for the gearbox, translating into a cost savings of nearly 2.5% of the installed turbine cost. 

No estimate was made of the improvement in gearbox efficiency resulting from elimination of 

the second and third stages of the conventional gearbox, although it can be expected that anywhere from 1%–2% loss is associated with each stage. Therefore, it would not be unreasonable 

to assume that the loss of efficiency associated with the medium-speed generator is offset by a 

gain in efficiency on the gearing. The impact on the energy capture could be expected to be 

minimal. 

Therefore, this concept could potentially result in some small reduction in COE. However, a 

relatively high amount of risk is believed to be associated with the concept, given that no medium-speed wound rotor induction generators are known to exist, and it would represent a substantially new technology. 

2.1.2.2 Medium-Speed Squirrel Cage and Synchronous Generators 

GE did not expend substantial effort in studying medium-speed squirrel cage induction, wound 

rotor synchronous, or permanent magnet synchronous generators. Based upon the results reported in Section 2.1.2.1, it is possible to design a medium-speed wound rotor induction generator similar in weight and cost to a conventional-speed machine. The NGT design team con13
cluded that the same would likely be true for squirrel cage and synchronous machines. It can be 

expected that a small savings in generator cost would result. 

Furthermore, the costs estimated for the power electronics for conventional-speed versions of 

these machines were very similar to the costs estimated for the power electronics for direct-drive 

versions of these machines. As a result, the NGT team further concluded that the cost for the 

power electronics for variable-speed operation of medium-speed squirrel cage and synchronous 

generators would also be similar to the costs for the direct-drive and conventional-speed versions. As noted in Section 2.1.1.2, these converter costs are substantially greater than power 

electronics costs for the wound rotor induction generator and more than offset any savings in 

generator cost that result from using squirrel cage or synchronous designs. As a result, it is safe 

to conclude that these configurations have no advantage over the wound rotor induction machine 

for variable speed systems. 

As for fixed speed operation, the same conclusions reached in Section 2.1.1.3 apply to mediumspeed machines as well. The cost savings from replacing the more expensive wound rotor induction generator with a synchronous machine and eliminating the power electronics is more than 

offset by the loss in energy capture. Furthermore, fixed speed machines are incapable of providing critical VAR support. 

2.1.2.3 Multiple Medium-Speed Generators 

The concept of using multiple stand-alone geared, medium-speed generators was not extensively 

investigated. However, drawing analogies from the results of Sections 2.1.1.4, 2.1.2.1, and 

2.1.2.2, it is safe to conclude that this concept would hold no particular cost or performance advantage over using one large geared, medium-speed generator. 

2.1.3 Direct-Drive Generators 

Interest in direct-drive generators derives from the ability to completely eliminate the gearbox. 

The latter component is among the most expensive components in a wind turbine, and wind turbine gearboxes, in general, have been prone to require extensive maintenance industry wide. 

14
2.1.3.1 Fixed-Speed Direct-Drive Machines 

It would seem desirable to directly connect an 

electrical generator to the existing 60-Hz power 

grid. Although, it would have to operate as a

fixed speed machine, sacrificing a portion of the 

input wind energy, power and frequency conversion with accompanying losses and costs could 

be avoided. But studies showed that direct connection forces unwieldy structure sizes for the 

shaft speed ranges that are typical for large horizontal axis wind turbines. A 30-rpm machine at 

60-Hz drive must have a pole count of 240. For 

large pole count AC machines, the required 

physical length of a pole pitch at the stator air 

gap surface–lengths required by slot area and 

shape considerations–forces very large values of 

air gap diameters, in excess of 7 m. Machines of 

this size would clearly be unwieldy and very expensive to support and stiffen. 

The best machine GE designed was a wound Figure 3. Direct-Drive Generator Wind 

field synchronous generator. Two aspects of this Turbine System 

machine design are unacceptable: the excessively 

large air gap diameter necessary to obtain a reasonable pole pitch length and the low rated electrical efficiency of 84.5%. By replacing the field windings in the rotor with permanent magnets, 

one could increase the efficiency to very high levels, but the machine size would remain unchanged because of the number of poles and the 60 Hz nominal operating frequency. Most of the 

machine loss is in rotor resistance heating due to operation at the stator unity power factor, forcing high rotor currents necessary to push magnetic flux across the machine air gap at each of the 

240 poles. 

It is has been discovered that it is simply impossible to design a 30-rpm-direct-drive asynchronous machine that could be directly connected to the grid at 60 Hz. Iterations on air gap diameter and pole pitch length did not converge to a generator with reasonable power factor or efficiency. 

As a result, fixed-speed operation of direct-drive machines without use of power conversion was 

impractical. The only means of solving this problem is to reduce the number of poles, which 

means that at the rated power shaft speeds typical of large wind turbines (18-35 rpm), the generator will produce power at frequencies much less than 60 Hz. This will require full power conversion. Once the converter is added, there is no longer an advantage to operating as a fixed speed 

machine. For the same reason, it is not possible to design a wound rotor asynchronous machine 

with a stator that can be directly connected to the grid at 60 Hz, thereby allowing variable speed 

operation with only partial power conversion. This method of operation, which is a key advantage of the baseline configuration, is simply not available to direct-drive generators. 

2.1.3.2 Variable Speed Direct-Drive Machines 

Both wound rotor and squirrel cage asynchronous machines were examined. One of the requirements imposed was that the variable-speed machines be able to provide dynamic VAR control. The advantages of this capability are discussed in Section 2.1.1.1. This requirement has an 

extremely negative effect on the squirrel cage machines. The squirrel cage induction generator 

15
cannot be designed for unity power factor operation. As a result, it must utilize a converter that 

can supply VARs to the stator. This results in a double cost penalty for these machines: the rating of the converter is higher because of the reactive power to be supplied, and the cost per kVA 

is higher because of the active rectifier. 

The study showed that a direct-drive wound rotor generator can be expected to cost less than the 

combined cost of the gearbox and generator for the baseline turbine. However, the need for full 

power conversion increases the cost of the power electronics substantially, such that the estimated cost for the direct-drive generator and power electronics is significantly greater than the 

cost for the gearbox, generator, and converter for the baseline turbine. 

Results indicated the wound rotor synchronous machine actually made the best candidate for a 

direct-drive generator. The rotor converter in the wound field synchronous machine concept is 

required to supply DC current to the rotor field windings. The wound field synchronous machine 

is a machine with a stator construction similar to an induction machine but with a rotor containing DC field windings that must be fed from an external supply. As a result, the wound rotor 

synchronous and asynchronous generators can be very comparable in construction and cost, 

while the power electronics for the synchronous machine will be less expensive. The total cost 

for a direct-drive wound rotor synchronous generator and power electronics is estimated to be 

slightly higher than the equivalent components in the baseline turbine. 

Two different topologies for permanent magnet (PM) synchronous machines were examined: • Surface mount PM synchronous generator: This is an AC machine (sometimes, paradoxically, referred to as a brushless DC machine) with square-like current and voltage wave 

shapes rather than the sinusoidal wave shapes of the squirrel cage induction, doubly-fed 

induction, and wound field synchronous machines. The machine excitation is due to rotor magnets. Since there is no resistance heating loss for the machine excitation, the PM 

machine has the potential for very high operating efficiency. There is, however, a very

real cost for the rare earth PM material. 

• Doubly-salient PM synchronous generator is a PM synchronous machine type with the 

excitation permanent magnets mounted on the stator structure rather than in or on the rotor. The rotor of the DSPM machine is a very simple and rugged construction of stacked 

electrical steel laminations with protruding (salient) "poles". The stator windings of the 

DSPM machine are wound on salient poles of stator-laminated steel. As the rotor poles

move across the faces of the stator poles, the stator PM excitation flux is switched from 

stator phase to stator phase producing pulsating flux variations within the stator phase 

windings and thus an AC excitation voltage. 

Extensive trade-off studies of stack length, active material weight, and rated efficiency as functions of air gap and air-gap diameter were conducted for the PM machines. As with the squirrel 

cage induction generator, the stator converter must supply reactive power or VARs to the DSPM 

machine. This requires that the machine-side portion of the stator converter must be an active 

rectifier, usually an IGBT bridge. The typical DSPM machine is not controlled through a standard three- phase power converter but requires a dedicated converter topology. As a result, the 

direct-drive PM synchronous configuration showed no advantages in COE for variable-speed 

operation. 

2.1.3.3 Tubular Nacelle (External Monocoque) 

GE studied the tubular nacelle structure shown in Figure 4, wherein a direct-drive generator 

would be supported directly by the nacelle structure itself. The nacelle could be constructed of 

sheets of steel welded together. Using this approach, substantial savings in the overall main 

frame cost would result for a direct-drive configuration. Although this concept was initially pro16
posed in conjunction with the direct-drive generator concept, GE’s concept evolved to consider it 

as a possible replacement for the bedplate main frame regardless of the drive train arrangement. 

` 

(a) Large Bearing Support (b) Tubular Nacelle 

Figure 4. Direct-Drive Support System 

The motivation for using welded steel sheet is that the latter could be cut, rolled, and welded 

quite inexpensively, either in a factory setting or even in a field assembly facility. Minimal finish 

machining would be required, and the entire structure should save substantial money. Significant 

main frame weight reductions were predicted. 

Extensive structural analysis of the tubular nacelle concept was conducted. This analysis was

based around replacing the bedplate for the 1.5 MW POC turbine with the welded steel tubular 

nacelle. Internal structure was added in order to add rigidity to the cylindrical skin structure and 

to distribute the loads to the skin and the rest of the support structure in a reasonable fashion. An 

optimization of the tubular nacelle design was conducted in an attempt to minimize the amount 

of material used while maintaining structural integrity. Welded steel does not have particularly 

high fatigue strength, only 4.5 ksi, or 31 MPa. Figure 5 below illustrates the stress distribution 

for the final “optimized” design. Even this design, exhibiting areas of excessive fatigue (gray 

areas in Figure 5), weighs over twice what the existing bedplate weighs. As a result, this configuration is expected to cost more than the GE 1.5 MW bedplate and fiberglass nacelle. Further optimization of the design might be expected to shave more cost,, but not enough to provide a significant benefit. As shown in Figure 5, the distribution of material does not match the distribution of stress. A great amount of material above the centerline and behind the tower is very 

lightly stressed. This design was an inefficient structure for this application. 

17
Figure 5. Fatigue Damage Equivalent von Mises Stress Distribution 

for “Optimized” Cylindrical Steel Nacelle 

2.1.4 Summary of Drive Train Concepts 

The result of all of the electromechanical concept studies is that no concept produces a significant improvement in the COE delivered by wind power. Only two concepts–the medium-speed 

wound rotor induction generator operating in VSCF mode and the medium-speed wound rotor 

synchronous generator operating in fixed-speed mode–showed any improvement at all in COE, 

and even then the estimated improvement was less than $0.01/kWh, probably within the uncertainty range of these analyses. The remainder of the concepts produced no improvement or increase in COE. 

As a result, GE decided that none of the concepts investigated showed potential for improving 

COE. The only one that has piqued some enduring interest is the medium-speed wound-rotor 

induction generator. This concept clearly allows for the elimination of all but the first stage of 

the gearbox, thereby guaranteeing some cost savings. However, the costs saved by eliminating 

the second and third stages of the gearbox are not overwhelming and may not justify the risk associated with development of a 40-pole generator. 

Significant improvements in COE must be identified elsewhere, as noted in the subsequent sections. 

18
2.2 Controls Concepts 

Modern wind turbines such as those manufactured by GE feature electronic control systems that 

are relatively sophisticated compared to systems employed by wind turbine manufacturers only a 

decade ago. The GE baseline 750i turbine features both variable speed operation of the generator and active pitching of the blade for power regulation. Both systems are controlled by sophisticated power electronics. 

The control system monitors the generator speed and torque. At speeds below rated, the pitch is

generally maintained at fine pitch, and the generator torque is adjusted by controlling the current 

in the rotor winding in order to maintain turbine operation on the optimal speed-torque curve for 

maximum energy capture. At speeds above rated, the blade pitch is regulated to try to maintain 

constant shaft speed, while the torque is maintained at rated torque. This maintains constant 

power operation. 

The baseline 750-kW turbine featured a collective pitch system through which the pitch settings 

of all three blades were substantially equal and controlled simultaneously. The collective pitch 

was effected mechanically through revolute joints on each blade connected to a hydraulically 

actuated cam. As part of GE’s Near Term Research and Testing (NTRT) program jointly funded 

with NREL, this hydraulic/mechanical system was replaced on the Value Engineering (VE) turbine by a system of separate electromechanical drives on each blade. The pitch bearings on the

VE turbine are geared on the inside. The pitch angle of each blade is controlled by a geared AC 

motor driving a pinion meshed with the bearing gear. At present, this system still features collective pitch, in that all three blades are maintained at substantially the same pitch setting at all 

times. 

Despite the relative sophistication of this system, a number of innovative controls and monitoring concepts have been proposed during the NGT program, offering the potential for: 

• Improving energy capture 

• Reducing extreme or cyclic loadseducing fatigue damage accumulation, reactive maintenance, component failures, or other factors that result in reduced turbine life or operations 

and maintenance expenses. 

The control strategies investigated in the present study are: 

• Coupling blade pitch and generator torque control 

• Tracking peak power coefficient 

• Tracking tower vibration feedback control 

• Tracking independent blade pitch control for asymmetric load control 

• Tracking load-limiting control 

• Tracking alternative yaw control strategies 

• Tracking damage monitoring and feedback control 

• Tracking drive train damping control. 

Among the more intriguing benefits of advanced control concepts is that the cost of implementing them is frequently minimal compared to the overall turbine cost. For some of the control 

schemes, the cost consists only of reprogramming software. For some of the other schemes, implementation might require a slight upgrade of computational hardware or the addition of a few 

sensors. Therefore, the trade-off becomes less of a cost-benefit analysis than a comparison of 

benefits in some areas (e.g., load reduction) versus penalties in other areas (e.g., reduction in energy capture). 

19
2.3 Rotor and Structural Design Concepts 

The rotor arguably represents the most important component of a wind turbine. It is the component responsible for extracting energy from the wind, but it is also the component responsible for 

extracting loads from the wind and transmitting them to the rest of the turbine. Furthermore, the 

energy in the wind is unsteady and stochastic, and the rotor also becomes the principal conduit of 

dynamic loads experienced by the turbine. These loads induce fatigue and drive the design of 

most wind turbine components. 

Therefore, a significant portion of GE’s concept studies have focused on rotor design concepts

that can significantly improve energy capture with a less than proportionate increase in dynamic 

loads and turbine cost. The challenge inherent in achieving this goal is that trying to extract more 

energy from the wind generally entails extracting more of the dynamic loads. 

The most obvious and simplest design change that can affect energy capture is increasing the rotor size for a given drive train. There is no wind industry standard for the diameter of a rotor to 

be coupled with a generator of a given size. In recent years, however, all of the manufacturers of 

utility-scale turbines of roughly 500–1,650 kW rating have offered them with rotors corresponding to a loading of between 380 and 450 W/m2

. For reference purposes, this corresponds to rotors with a diameter between 38 and 41 m for a 500-kW turbine, between 46 and 50 m for a 750 

kW turbine, and between 65 and 71 m for a 1,500-kW turbine. 

Early in the concept studies, GE personnel began examining the possibility of reducing the rotor 

loading (i.e., increasing the diameter) to as low as 320 W/m2, corresponding to a 55-m rotor on 

the baseline 750-kW turbine or a 77-m rotor on a 1.5-MW turbine. Such a design change is not

trivial, however, for several reasons: 

• For a pitch-regulated, variable speed wind turbine, increasing the rotor diameter will only 

increase the energy capture at wind speeds below rated. Therefore, a 20% increase in rotor swept area may result in a 20% increase in power production at wind speeds below 

rated; but it will result in less than a 20% increase in annual energy production, probably 

nearer a 8%–12% increase, depending upon the wind characteristics at the site. 

• Acoustic emissions considerations may require that the rated shaft speed be reduced proportionately to the rotor diameter increase, in order to avoid increasing the rotor tip 

speed, which strongly affects aerodynamic noise. This will cause a proportionate increase in the low-speed shaft torque with concomitant effects on several major components, including the main shaft, gearbox, and bedplate. 

• Increasing the rotor diameter increases the loads more than the energy capture. The 

power production is obviously proportional to the swept area, which increases as the 

square of the rotor diameter. If the shaft speed-vs-wind speed schedule remains unchanged up to rated power, however, then the tip speed increases proportionately with the 

rotor diameter, and the aerodynamic pressure at the tip increases roughly as the square of 

the tip speed. Assuming a roughly constant-chord section is added to the tip, the area 

over which the pressure acts is also proportional to the diameter, so the overall loading 

increases as the cube of the rotor diameter. If the entire blade is scaled, including the 

chord length, the loading scales as the fourth power of the diameter. The thrust loading 

and in-plane loading induce strong moments on the tower base and drive the design of the 

tower. Furthermore, the moment arm from the aerodynamic loading to the blade root is 

also proportional to the increase in diameter, so the increase in blade root bending moment scales as the fourth to fifth power of the rotor diameter. These loads drive the design of the blades, hub, and drive train mechanical and structural components. If the rated 

20
power of the turbine does not change, the mean loads on the system are not increased. 

However, every wind gust that strikes the turbine when it is operating at rated power is 

converted to a dynamic load on the turbine without producing any additional power. This 

can result in a substantial increase in fatigue on mechanical and structural components. 

Therefore, unless the rotor diameter increase is affected in an intelligent manner, it is 

possible that increasing the rotor diameter will impact the cost of fatigue-driven components in such a way that the turbine cost is increased more than the energy capture. 

• Average wind speed and turbulence intensity strongly affect the importance of increased 

rotor diameter. For sites with strong winds or high turbulence, the dynamic loads experienced at and above rated wind speed can become a bigger design driver than additional 

energy capture below rated. The use of larger rotors is largely a consideration for wind 

farm sites with relatively low mean speeds and relatively low turbulence, where the turbine operates less often at and above rated wind speeds and is less likely to experience 

the strong wind gusts which induce fatigue damage. 

The subsequent parts of this section address several alternative concepts investigated by GE for 

significantly increasing energy capture without proportionately increasing the loads or turbine 

cost. GE also investigated several alternative concepts that attempt to reduce rotor cost without 

impacting energy capture. These concepts include: 

• Rotor and other turbine structural flexibility • Concurrent aerodynamic and structural design optimization • Carbon composite rotor blades • Aeroelastic tailoring of rotor blades • Rotor blade aerodynamic boundary layer control • Variable diameter rotor. 

2.3.1 Rotor and Structural Flexibility 

The main aim in exploring the implications of rotor structural flexibility is that, while some 

steady state loads cannot be altered, dynamic loads on the wind turbine system are often design 

drivers and may be reduced with the energy content of the input wind loading in various frequency ranges partly dissipated by aerodynamic damping of blade motions. Hence, reduced loading is passed further into the wind turbine system. The other aspect of structural flexibility that 

is specific to a rotor system, as opposed a static cantilevered beam, is the effect of centrifugal 

force in providing relief of blade out-of-plane bending moments, both steady and dynamic. Reducing the stiffness of a tubular tower may also allow for a reduction in tower material, and introducing damped compliance into the machine carrier yaw and pitch (i.e., nodding) degrees of 

freedom may similarly reduce the absorption by the bedplate, yaw deck, or tower top of dynamic 

loading imparted by the rotor. It is likely that tower and blade flexibility may be cheaper and easier to engineer than yaw deck compliance and may even be a source of mass and cost reduction 

of blades and tower rather than an added cost to be paid for by other benefits. All concepts are

investigated here. 

The reduction in loads that might result from increased structural flexibility could result in benefits to COE either as benefits to component designs or through increased energy capture by enabling the use of a larger rotor with loads no higher than would be produced by a smaller, stiffer 

rotor. 

21
2.3.1.1 Upwind Rotor Blade and Tower Flexibility 

Two different studies were commissioned by GE to investigate the impact of reducing the wind 

turbine rotor blade stiffness. Both studies were conducted using the baseline 750i turbine as the 

study control. The studies were conducted in slightly different ways, as explained below. 

In the first study, a model of the baseline 750i turbine using the baseline 48-m rotor was developed. A new “softer” blade was modeled with stiffness equal to 50% of the baseline value and 

mass reduced to roughly 70% of the baseline. Though softer, the blade is still stiff (above 2p). In 

addition to the baseline tower, a so-called “soft-soft” tubular tower with a first natural frequency 

less than the turbine main shaft maximum rotational rate was also modeled. Two towers and two 

rotor configurations yield four configurations for study. 

Figure 6 shows the impact on several key loads resulting from the softening of the tower and rotor blades. All load increases and decreases are measured relative to the baseline configuration. 

The softening of the tower alone, Configuration 3, reduces all loads except tower top thrust when 

m=10 and blade edgewise bending moments for both m=4 and 10. Softening of the rotor alone, 

Configuration 2, reduces all loads except for a slight increase in blade flapwise bending moment 

and a fairly substantial increase in tower top thrust. 

Percent Change in Load Percent Change in Load 

Loads Decrease Loads Increase Loads Decrease Loads Increase

-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 

Blade Flapwis e Moment 

Blade Edgewise Moment 

Shaft Bending Moment 

Tower Top Yaw Moment 

Tower Top Tilt Moment 

Tower Top Thrust 

Blade Flapwise Moment 

Blade Edgewise Moment 

Shaft Bending Moment 

Tower Top Yaw Moment 

Tower Top Tilt Moment 

Tower Top Thrust 

li

Baseline Tower, S oft Rotro 

S oft-S oft Tower, Base ne Rotor 

S oft-soft Tower, S oft Rotor 

 (a) Vwind=11.3 m/s, m=4 (b) Vwind=11.3 m/s, m=10 

Percent Change in Load Percent Change in Load 

Loads Decrease Loads Increase Loads Decrease Loads Increase

-30 -20 -10 0 10 20 30 -30 -20 -10 0 10 20 30 

Blade Flapwis e Moment 

Blade Edgewise Moment 

Shaft Bending Moment 

Tower Top Yaw Moment 

Tower Top Tilt Moment 

Tower Top Thrust 

Blade Flapwise Moment 

Blade Edgewise Moment 

Shaft Bending Moment 

Tower Top Yaw Moment 

Tower Top Tilt Moment 

Tower Top Thrust 

(c) Vwind=17 m/s, m=4 (d) Vwind=17 m/s, m=10 

Figure 6. Effect on Fatigue from Reducing Rotor Blade and Tower Stiffness 

Simulations of the Baseline 750i Turbine 

Finally, softening both the tower and the rotor blades, Configuration 4, results in a fairly substantial reduction of all loads except for tower top thrust. It is predicted to have an 18% reduction in 

22
damage equivalent flap moment, 25% reduction in edge moment, 20% reduction in yaw moment, 

23% reduction in tilt moment, and 20% reduction in hub moment (My) in 17 m/s winds. The reduction in edge moment corresponds approximately to the reduction in blade weight. There does 

not appear to be any reduction or amplification due to the reduced natural frequency. 

The only predicted increase is in tower-top thrust (7%). This is attributed to tower motion interaction with the pitch control system and predicted that this increase could be reduced or eliminated using nacelle acceleration feedback to the controller. It is interesting to note that the thrust 

(m=4) and tower clearance are worse for configuration 2 than for configuration 4. The soft-soft 

tower provides some mitigation of the negative aspects of the soft blade. Use of the soft blade 

with the baseline tower (configuration 2) is not recommended. This configuration is predicted to

have less benefit and a greater reduction in tower clearance than configuration 4. 

The blade tip/tower clearance is reduced by up to 25% for Configuration 4. This is the largest 

challenge of the softer configurations. This problem can be reduced with little apparent negative 

consequence by further increasing the precone angle. The energy yield is predicted to decrease 

by 4%. Configuration 2 (with the same precone angle as configuration 4) has less energy loss at 

11.3 m/s than configuration 4. This means that not all of the energy loss is due to the larger precone angle of the softer blade. Based upon projected area alone, the expected energy loss due to 

a 2° increase in preconing would be about 0.3%. Therefore, most of the loss is attributed to the 

poorer speed regulation with the softer systems. Further “tuning” of the control system can 

eliminate much of this loss. 

This study demonstrated clear benefits in terms of reducing blade mass and loads in general, but 

the study did not quantify in terms of impact on COE any of these benefits or costs such as energy loss. The second study was similar, but in addition to examining the impact of rotor flexibility on loads, this study also addressed several practical considerations in more detail: • The relationship between mass and stiffness • Tower clearance with more flexible rotor blades • Impact on COE • Engineering implementation issues, including scaling concerns. 

In this study, the blade stiffness was halved, and the blade mass was reduced approximately 

30%. A lifetime fatigue analysis and an extreme load calculation were performed. The extreme 

loads analyses yields approximately equal maximum blade deflection. The extreme blade deflection for the baseline design is 1.5 m. For the softened blade it is about 3.0 m. The principal

challenge of reducing blade stiffness with an upwind rotor configuration is to provide a safe 

tower clearance to accommodate extreme blade deflections. The main options for increasing 

tower clearance are: • Increased rotor overhang • Increased rotor tilt • Increased blade coning • Introduction of blade curvature (out-of-plane). 

The introduction of a moderate amount of tilt (up to 10°) will add cyclic loads that are small in 

comparison to the stochastic loads induced by turbulence. Simulations have been carried out with 

8° of shaft uptilt and 3° of forward precone. This configuration provided for a maximum tip deflection substantially equal to that of the baseline configuration. The results are summarized in 

Figure 7, which can be compared directly to the results shown in Figure 6, which were obtained 

for similar cases with slightly different values of uptilt and precone. Most of the fatigue reduction is quite comparable, but the increases seen in the first study in a few components of fatigue 

have been eliminated by careful tuning of the controller to avoid tower/blade resonance. Overall, 

the results mentioned above are confirmed, in as much as substantial reductions of critical components of fatigue can be realized through relaxing the rotor blade stiffness. 

23
10 

Percent Change in Load Percent Change in Load 

-40 -30 -20 -10 0 10 -40 -30 -20 -10 0 

Blade Flapwis e Moment Blade Flapwise Moment 

Blade Edgewise Moment Blade Edgewise Moment 

Tower Top Yaw Moment Tower Top Yaw Moment 

Tower Top Tilt Moment Tower Top Tilt Moment 

Baseline Tower, S oft Rotor 

S oft-S oft Tower, Baseline Rotor 

S oft-S oft Tower, S oft Rotor 

(a) m=4 (b) m=10 

Figure 7. Effect on Fatigue from Reducing Rotor Blade and Tower Stiffness 

Simulations of the Baseline 750i Turbine 

The cumulative effects of softening both the tower and the rotor do not differ substantially from 

softening simply the rotor with the exception of possible benefits to blade flapwise bending. This 

is a somewhat different result than what was obtained in the first study, which showed substantial improvements in softening the tower in conjunction with a softer rotor, and is presumably 

attributable to the differences between the controllers used in the two simulations. Softening the

tower in conjunction with softening the rotor, however, does not detract the benefits of rotor 

softness. Furthermore, softening the tower allows for a substantial reduction of tower mass, a 

critical conclusion which served as one of the design drivers for the towers for the POC and 

EMD turbines. 

The COE impacts of implementing a more flexible rotor were also examined. The reduction in 

system fatigue loads that results from increased blade flexibility may be taken as a benefit to 

component design. Alternatively, a longer blade may be considered in order to increase energy 

capture while using structural flexibility to keep within the design loads of the baseline machine. 

The greatest reduction in COE using the baseline rotor diameter is about 3%. An extended, tilted 

rotor (50.77 m) is the most economic option, producing a 5% reduction in COE. 

Four main conclusions can be drawn: 

• Extending the blade to recover energy loss from tilting and/or coning options is always 

preferable for COE. 

• Providing increased blade tip clearance solely by increasing overhang is not economical. 

• Providing increased blade tip clearance by means of increased rotor tilt and compensating 

for energy loss by extending the blade is the most economical solution. 

• Exploiting fatigue load reduction by increasing rotor size is more economical than taking 

benefit to components at standard rotor size. 

The latter is of critical importance in evaluating candidate designs for the POC and EMD turbines, as discussed in Sections 3 and 4. The conclusion that increased rotor flexibility should be 

exploited by increasing rotor size was instrumental in driving the preliminary design of the POC 

turbine towards a larger, more flexible rotor. 

24
Series1

Series2

Series3

Series4

Series5

Series6

Series7

2.3.1.2 Machine Carrier Yaw and Nodding Compliance 

GE commissioned two studies to investigate the impact on loads from introducing compliance 

about the drive train tilt (or nodding) and yaw axes. Both studies were conducted using the baseline 750i turbine as the study control. The studies were conducted in slightly different ways. 

Figure 8 illustrates the results of introducing yaw and nodding compliance as determined by the 

first study for different values of natural frequency and damping for each degree of freedom. 

Flap bending moment, tower top yawing moment, and hub (shaft) bending moment are all most 

strongly affected by the addition of yaw compliance, with the nodding, fixed yaw cases providing little or no load relief. Tower top yaw moments are further reduced by increased softening of 

the yaw degree of freedom (i.e., reduced natural frequency), but blade flap bending moments and 

hub bending moments are not otherwise appreciably affected by the magnitude of yaw compliance. The magnitude of tilt compliance has little effect on these three components of fatigue. 

The tower top tilt moment is obviously most strongly affected by the softness of the tilt degree of 

freedom, with increased softening of the yaw degree of freedom actually offsetting some of the 

loads improvement. 

20 between tilt and yaw motion. After

examining all of these simulation results, it was concluded that the stiffest 15

case (i.e., ωtilt=1.3P,ωyaw=1.2P showed 

the best combination of load reduction 

with minimal yaw and tilt motion. 

This case is seen as the yellow bars in

Figure 8. Fatigue is reduced by 34% 

10 

5 

Configurations with yaw frequency of 

at least 0.7P are able to keep the yaw 60% angle range within 10°, which was a

stated goal. The Configurations with 

70% 

50% 

tilt natural frequency of 1.3P were

able to keep the tilt angle within the 

Tilt and Ya w Angle Ranges [°] Reduction of Fatigue 

40% 

30% 

desired range of ±2.5° and minimized 

the increase in maximum tip deflec­ 10% tion. The stiffness is considerably 

higher than the value originally sought 0%

for the best load isolation. A larger -10% 

yaw damper also helps to limit tilt motion. For a given tilt natural frequency, the tilt motion increases as the 

yaw natural frequency drops. The 

simulations do show an interaction 25 

20% 

ω ζ

ω ζ

ω ζ ω ζyaw

ω ζ ω ζyaw

ω ζ ω ζyaw

ω ζ ω ζyaw

ω ,ζ ,ωyaw ,ζyaw

tilt=1.3P, tilt=0.3,fixed yaw 

tilt=0.7P, tilt=0.3,fixed yaw 

tilt=1.3P, tilt=0.3, yaw=1.2P, =0.3 

tilt=1.3P, tilt=0.3, yaw=0.7P, =0.3 

tilt=1.3P, tilt=0.3, yaw=0.4P, =0.3 

tilt=0.7P, tilt=0.5, yaw=0.7P, =0.5 

tilt=0.7P tilt=0.3 =0.4P =0.3 

blade flap bending tower top yaw moment tower top tilt moment hub bending moment 

moment 

(a) Reduction of Fatigue 

Increase in Maximum Tip Deflection [m] 

0 

Tilt Angle Range Yaw Angle Range Increase in Tip Deflection 

for yaw moment, 35% for tilt moment, 

6% for blade flap moment, and 14% 

for low-speed shaft bending. The (b) Tilt &Yaw Angles & Tip Deflection tower clearance is reduced by 0.54m. Figure 8. Impact of Yaw and Nodding Compliance on 

As a progression from this study, a Fatigue & Motion 

more detailed analysis of tower top 

flexibility was completed. This study introduced a drive train nodding degree of freedom to the 

flexible blade configurations studied above. 

From Figure 9, it is evident that introducing nodding freedom into the tower top has only a small 

and mixed effect on most of the components for fatigue on the baseline configuration as well as 

25

0.0 

0.5 

1.0 

1.5 

2.0 

2.5 
the other three combinations of rotor and tower flexibility. The only component of fatigue that 

strongly benefits from the introduction of nodding compliance is the tower top tilt moment. Energy capture was negligibly affected by the introduction of nacelle nodding. While the maximum blade tip deflection was increased significantly by introduction of nacelle nodding, the 

blade tip-to-tower clearance was not strongly affected. 

Percent Change in Fatigue Relative to Baseline Configuration 

Loads Decreas e Loads Increas e 

-40 -30 -20 -1 0 0 1 0 20 30 40 

Blade Flapwis e Moment 

Blade Edgewise Moment 

S haft Bending Moment 

Tower Top Yaw Moment 

Tower Top Tilt Moment 

Tower Top Thrust 

li i lle 

li i le 

igi

i lle 

Baseline Tower,Baseline Rotor,Nodding Nacelle 

Baseline Tower,Softer Rotor,Rigid Nacelle 

Baseline Tower,Softer Rotor,Nodding Nacelle 

S oft-s oft Tower,Bas e ne Rotor,Rig d Nace

S oft-s oft Tower,Bas e ne Rotor,Nodd ng Nacel

S oft-s oft Tower,S ofter Rotor,R d Nacelle 

S oft-s oft Tower,Softer Rotor,Nodd ng Nace

Figure 9. Impact of Nodding Nacelle on Fatigue 

As Determined through Simulations, m=4, Vavg=11.3 m/s 

The second study also built upon the earlier second study of rotor and tower flexibility to add 

nodding and yaw compliance. This study introduces a drive train nodding degree of freedom and 

a yawing compliance degree of freedom. Two different configurations of nodding and compliance were introduced: 

• Case a: The nodding hinge is introduced at the center of gravity (c.g.) of the baseline 

nacelle plus rotor. The hinge stiffness is set to provide a natural frequency in nodding 

of ωtilt=1.3P and a damping ratio of ζtilt=30%. The yaw compliance is set to provide a 

natural frequency of ωyaw=1.2P and a damping ratio of ζyaw=30%. • Case b: The nacelle center of gravity is adjusted to lie on the tower centerline. A 

natural frequency of ωtilt= ωyaw=0.7P and a damping ratio of ζtilt= ζyaw=30% is applied

to both nodding and yaw rotations. 

The results illustrated in Figure 10 are quite similar to those shown in Figure 8 and Figure 9; 

namely, the impact on fatigue from adding tower top compliance to any of the other four configurations is actually quite minor except for the tower top tilt and yaw components. The most 

significant reductions in blade flapwise bending fatigue come from addition of rotor and tower 

softness, with the addition of tower top compliance that add little additional benefits. The most 

significant reductions in blade edgewise bending fatigue derive from the softening of the rotor, 

with the addition of tower top compliance actually increasing fatigue for the baseline rotor and 

offsetting some benefits of the soft rotor. 

While it is evident that the softest tower top system provides the greatest fatigue load alleviation 

there, the results show that the differential benefit in fatigue load reduction of Case b compared 

to the stiffer hinge/yaw system of Case a is not great. The angular range of nodding motion of 

26
Case b is about twice that of Case a. It may not be worth engineering a system to cope with the 

large motions of Case b when most of the benefit can be provided with the Case a system. This 

view is reinforced when the effect of nodding rotations on tower clearance is considered. The 

results show that the Case a configurations are marginal for tower clearance (probably inadequate for lifetime extreme tip deflections), and that the Case b hinge is unrealistic for an upwind 

machine configuration. 

Overall, there is some cumulative benefit in having a soft tower, soft blade, and soft tower top 

connection but it is much less than simply additive, and it seems necessary to have a soft blade to 

prevent increase of blade loading because of tower head flexibility. The benefits from reductions 

in tower top tilt and yaw fatigue are questionable, as the only wind system component driven by 

these components of fatigue is the bedplate-tower interface, which will increase in cost and complexity anyway, in order to introduce the compliance. A brief engineering analysis of the mechanical requirements for implementing the tower top compliance was conducted. The conclusions show the Case a system added to the baseline 750i turbine featuring a soft-soft tower and a 

softer rotor and would cost approximately the same as the yaw drive system, which is the only 

clear cost savings implemented from yaw or tilt compliance. There is no benefit to blades, and 

unless a substantial benefit to tower costs, nacelle structure, or drive train can be established, it 

will be hard to justify the costs. 

27
Percent Reduction in Component of Fatigue Relative to Baseline Configuration

-20 -10 0 10 -40 -30 -20 -10 0 10 20 -60 -50 -40 -30 -20 -10 0 10 -50 -40 -30 -20 -10 0 10

Bl i

Rig

ade Flapw s e Moment Blade Edgewise Moment Towe r Top Yaw Mome nt Towe r Top Tilt Mome nt 

Baseline Tower, Baseline Rotor, 

Nodding & Yawing Nacelle A 

Baseline Tower, Baseline Rotor, 

Nodding & Yawing Nacelle B 

Baseline Tower, Softer Rotor, 

id Nacelle 

Baseline Tower, Softer Rotor, 

Nodding & Yawing Nacelle A 

Baseline Tower, Softer Rotor, 

Nodding & Yawing Nacelle B 

Soft-Soft Tower, Baseline Rotor, 

Rigid Nacelle 

Soft-Soft Tower, Baseline Rotor, 

Nodding & Yawing Nacelle A 

Soft-Soft Tower, Baseline Rotor, 

Nodding & Yawing Nacelle B 

Soft-Soft Tower, Softer Rotor, 

Rigid Nacelle 

Soft-Soft Tower, Softer Rotor, 

Nodding & Yawing Nacelle A 

Soft-Soft Tower, Softer Rotor, 

Nodding & Yawing Nacelle B 

Figure 10. Impact of Rotor, Tower, and Tower Top Flexibility on Fatigueas Determined through Simu

28
2.3.1.3 Flexible Downwind Rotor Concepts 

Wind turbines featuring downwind rotors with flexible blades have been designed, built, and 

tested by a number of researchers and turbine manufacturers in the past, with varying degrees of 

success. Some were technically unsuccessful, and none have proved commercially successful. 

The reasons are varied. Some were probably overly ambitious to the degree of flexibility attempted. Some may have been affected by the number of technical innovations introduced simultaneously in the turbine design, which challenged the ability of engineering modeling and analysis at the time. Inadequate modeling led to unforeseen dynamic problems during testing and 

proved insurmountable. 

From this experience, the utility-scale wind industry has, during the course of nearly a quartercentury of progress, converged on the three-bladed, upwind rotor configuration. Every utilityscale wind turbine currently manufactured either in the U.S. or in Europe, features this design. 

As noted at the beginning of Section 2, GE believes there are market advantages to the threebladed upwind rotor configuration and decided, from the outset of the NGT project, that the 

NGT design should favor this concept. 

Nevertheless, a downwind rotor featuring flexible rotor blades represents one means of overcoming the most significant challenge to implementing more flexible rotor blades in an upwind rotor 

configuration–accommodating blade tip tower clearance requirements. In order to halve the 

stiffness of the blade in an upwind rotor arrangement, approximately 2.5% of energy capture is 

sacrificed by incorporating forward coning sufficient to ensure adequate tip tower clearance, 

while approximately 1.75% is sacrificed through use of uptilt. These represent fairly significant 

losses that offset a substantial portion of the COE savings resulting from the use of more flexible 

rotor blades. 

GE decided to commission a study of the downwind rotor concept to determine whether introducing flexible rotor blades downwind could produce savings in COE sufficient to justify further 

pursuit of this concept. The study was quite broad, examining a number of variations on the 

downwind flexible rotor concept, including: 

• Degree of rotor blade stiffness 

• Number of blades 

• Hub type (i.e., rigid, teetered, or hinged/flapping) 

• Tower height and type (free standing vs. guyed) 

• Rotor solidity and tip speed. 

Various rotor configurations were examined. The 3-bladed, upwind, rigid hub configuration was 

used as a baseline and the other configurations were compared to that case. The “hinged” hub

configuration modeled blades independently hinged flapwise at their roots. The “soft” blade had 

a flapwise stiffness of 0.5 and a mass of 0.67 of the corresponding regular blade properties. The 

same blade was used on both the two- and the three-bladed rotors. In order to make the rotors 

equivalent and to have the same performance curve, the rotor speed of the three-bladed version 

was reduced from 42 rpm to 35.5 rpm. 

To obtain information on both the peak loads and the fatigue loads, the study concentrated on 

two operating conditions: normal operation in a “standard” turbulence with a mean wind speed 

of 18 m/s, and the extreme coherent gust with direction change (ECD) at rated power. The simulations of 10-minute records of normal operation were carried out four times, each with a different random seed. 

29
Design rules were used to estimate the effect of different blade designs on weight and cost. The 

performance and annual energy production of a number of different blade and rotor designs were 

also investigated. The most promising way in which the rotor might be redesigned to reduce the 

COE, was to sweep an area as large as possible with as little blade as possible, while minimizing 

the impact on drive train torque. This implies that the blade must move faster, which leads to a 

higher rotor shaft speed that also reduces the shaft torque and the gearbox cost. All of the modified rotors had larger diameters than the baseline and also had lower solidities. The maximum 

tip speed was close to 90 m/s, considerably higher than for most current rotors. The relative effects of changes in loads and rotor configuration on blade weight and cost were examined. 

The impact on COE is summarized in Figure 11. The minimum COE for each configuration 

shown in Figure 11 does not necessarily occur at the same rotor size or turbine rating for each 

configuration, and so the COE values shown reflect variously sized turbines. The order in which 

the results are presented in Figure 11 were selected to reflect the level of perceived technical risk 

associated with the respective configurations. Some of the configurations use technologies that 

are speculative, such as the hinged blades. Those architectures have been placed at the end of 

the list. 

Figure 11. Summary of Downwind Configurations 

In the end, GE concluded that a slightly more conventional version of Option 4, using an upwind 

configuration and conventional free-standing tower, could achieve a COE nearly as good as that 

reported for Option 4 with less risk. Only Options 7 through 9 offer noticeable improvements in 

COE, and those improvements (on the order of 10% relative to Option 4) do not justify the risk 

entailed. Similar savings can be found elsewhere with less risk. 

30
2.3.2 Concurrent Aerodynamic and Structural Design Optimization 

The design of a wind turbine rotor blade is generally accepted to be a compromise between aerodynamic performance and structural integrity. Aerodynamic considerations relate to maximizing 

energy capture and drive the designer towards the use of relatively thin sections with high lift-todrag ratios. Satisfying structural requirements relating to static strength, fatigue resistance, buckling, and tip deflection (i.e., avoiding tower strikes) while minimizing material usage suggest the 

use of thick, structurally efficient sections. It is frequently possible to introduce thicker sections 

inboard for structural considerations without seriously impacting energy capture. It may be similarly possible to relax structural considerations outboard in order to maximize energy capture. 

This is especially true if more blade flexibility is permitted, or even desired, as a result of the 

concept studies discussed in Section 2.3.1. Furthermore, entirely new approaches to blade optimization may be possible, and were accomplished with alternative materials such as carbon fiber 

reinforced plastic. 

Developing an optimal compromise between these criteria is a serious design undertaking and 

one which GE approached rigorously in the development of the POC rotor blades, as discussed 

further in Section 3. To provide guidance in the POC design, GE commissioned two different 

studies of methods for optimizing rotor blade design considering aerodynamic and structural requirements, concurrently. 

The first study focused on developing not only an aerodynamically optimum blade but also 

"planform envelopes" of greater and lesser solidity than the optimum planform, which show the 

variations in planform allowable within a certain specified energy sacrifice. The results showed 

that a structural designer can contemplate a much wider range of potential designs than had been 

previously thought possible and may therefore be able to achieve a greater degree of structural 

optimization and blade cost saving. 

The second study focused on the influence of airfoil section thickness on the performance and 

mass of rotor blades. While this study did not touch on many of the issues addressed by the first 

study, it did provide a more realistic assessment of the ability to simultaneously optimize aerodynamic and structural considerations dependent on airfoil section thickness. The study involved development of a roughly designed but customized family of airfoils with thickness-tochord ratios of 16, 18, 21, 24, 27, and 30 percent. The design objectives for these airfoils were: 

• Design lift coefficient between 1.1 and 1.2 at the Reynolds numbers typical of the baseline turbine rotor blades 

• Limited upper surface laminar flow in order to minimize sensitivity to leading edge surface roughness. This criterion is in sharp contrast to the NREL families of airfoils, which 

are specifically designed to produce extensive laminar flow. 

Aerodynamically optimum rotor blades employing these airfoils in various distributions from 

hub to tip were then designed. The objective for the blade designs was to maximize gross annual 

energy production (GAEP), which was based on a Rayleigh wind speed distribution with an average wind speed of 8.5 m/s (IEC Class II wind regime). The first step in the blade design process was to identify the optimum lift distribution to be prescribed. To quantify the effect of the lift 

distribution on gross annual energy production and rotor thrust loads, a sensitivity study was per31
formed using the baseline rotor design. In this sensitivity study, constant distributions of Cl were 

prescribed. The results showed a variation of at most 0.1% in GAEP, meaning that there is a relatively wide range of Cl yielding similar performance in terms of energy output and thrust loads. 

Several key conclusions of this study are: 

• Significant improvements over the baseline design are possible. 

• Use of higher thickness-to-chord ratio section does not automatically result in excessive 

energy loss. 

• Excessive thickening of the tip should be avoided. 

• If one expects surface-roughened conditions, then the blades should be optimized using 

the rough-surface (i.e., fixed transition) aerodynamic characteristics. 

These results were encouraging that it might be possible to concurrently optimize the rotor aerodynamic and structural performance by thickening inboard stations while leaving outboard stations thinner. With this in mind, GE studied the benefit or detriment to blade mass using the 

same thickness and chord distributions. They assumed the baseline turbine rotor blade tip deflection limitation and also a relaxed deflection limitation (i.e., twice the baseline deflection). 

The results of the structural analysis were compiled along with the aerodynamic analysis to reflect the combined impact on COE resulting from both the aerodynamic and structural changes. 

The final conclusion is that all of the changes have a relatively minor impact on overall COE.. 

Changes are in the ±1% range for the baseline tip deflection and under 2% reduction for the relaxed tip deflection. The best improvement to COE can be realized by using softer blades derived 

from increasing the rotor diameter and energy capture rather than from trying to realize significant savings in blade cost. Section 3 describes the strong influence the conclusions of this section had on the design of the POC turbine. 

32
2.3.3 Aeroelastic Tailoring of Rotor Blades 

GE was motivated to determine whether composite rotor blades can be designed and costeffectively manufactured in such a way that they provide an aeroelastic response tailored to ameliorate fatigue-inducing transient loads and concomitant extreme blade deflections. Traditionally, 

aeroelastic response–that is, the tendency of a structure to exhibit coupling between aerodynamic 

loading and structural behavior was something the designer, whether aerospace, civil, or mechanical, had to accept with design considerations. In the past three decades, the advent of composite materials have allowed for tailoring of aeroelastic properties through deliberate design of 

desired flap-twist coupling. Off-axis (e.g., 20°) orientation of reinforcement fibers (e.g., glass, 

carbon, Kevlar, etc.,) along a supporting spar will cause the spar to twist under sufficient bending 

strain. This can be used to tailor the bend-twist coupling of an aircraft wing or a wind turbine 

blade. 

GE’s pitch-regulated wind turbines maintain constant power above rated wind speed by constantly pitching the blades in response to wind speed gustiness. When the baseline turbine is operating at wind speeds just less than or at rated wind speed. it is most vulnerable to rapidly rising 

wind gusts in terms of their ability to induce damaging transient loads and blade deflections. 

The blade pitch is at a fine setting to optimize the power factor, and so any gust that hits the turbine is seen as a sudden and significant increase in angle of attack along the entire blade span. 

Since for optimal power factor, the blade design and pitch are set so each station is operating 

near the angle of attack for maximum lift-to-drag ratio, any increase in angle of attack results in 

increased lift. The change in lift combined with the change in inflow angle results in a sudden 

increase in both in-plane and out-of-plane loading on the blade. Such transient loads induced by 

wind gusts combined with the response of the turbine are the most significant contributions to 

structural fatigue on wind turbines. 

Presently, the pitch control system responds to increased rotor torque by pitching the blades to 

smaller angles of attack, thereby reducing the blade loading and torque. As the wind subsides, 

the pitch is returned to normal. Nevertheless, the pitch system can respond only so quickly, particularly when it must transition from Region 2 to Region 3 of the power curve. As a result, the 

wind turbine will still realize significant transient increases in loads before the pitch system can 

counteract them. 

An aeroelastically tailored blade could reduce such transient loads. As the gust would induce 

flapwise deflections, the tailored bend-twist coupling of the blade would passively induce a 

twisting of the blade towards reduced angles of attack, thereby relieving the loads and reducing 

pitch activity. This will prevent the loads and hence the deflection from rising to the levels they 

would reach if the blade were not twist-bend coupled. Although it might be desirable to have an 

entirely passively controlled blade, GE has never believed this was achievable, nor that it was the 

goal of research for the concept. The reason is because of a simple but unavoidable quandary– 

the blade can only reduce loads by twisting into the wind, and it will only twist more if it flaps 

more, and it only flaps more if it is subjected to higher loading. It is simply not possible to shed 

all transient loads passively. But trimming the amplitude of transient loads and reducing pitch 

activity are both extremely desirable goals for twist-bend coupling. 

Introduction of twist-bend coupling is not without cost, however. First, twist-bend coupling can 

cost some energy capture if not implemented carefully. If a turbine is operating at a mean wind 

speed near rated, the blades will be subject to some relatively high mean loading that will pro33
duce an average flapwise deflection of the blades and will in turn induce some coupled twist distribution along the blade span. If a blade originally designed without twist-bend coupling,(with a 

chord length and twist distribution optimized for energy capture) is altered to introduce twistbend coupling (without changing the unloaded twist distribution), then at rated wind speed, the 

blade’s twist distribution will no longer be optimal due to flap-induced twisting. This will cost 

energy capture. Energy minimization can be partially demonstrated by designing the blade to 

achieve its optimal twist distribution at a wind speed at or near rated, such that the blade operates 

off the optimal twist distribution at predominately lower wind speeds. 

Second, introducing diagonal plies into the blade construction might be challenging and more 

expensive. The blades of the baseline turbine feature unidirectional and ±45° bias plies. The 

latter are substantially more expensive than the former, but introducing material such as ±20° 

plies for twist-bend coupling could be even more expensive. 

Third, introducing off-axis plies could soften the blade. While this could be desirable, as discussed in Section 2.3.1.1, if one cannot accommodate additional blade deflection, then some 

means of increasing the blade stiffness will have to be introduced, most likely through additional 

material and cost. 

As the concept studies progressed, it also became apparent that in-plane sweep of the blade could 

be employed to achieve aeroelastic coupling. While this presents some challenges to manufacturing and shipping, it avoids some of the problems associated with the use of oriented fibers. 

Studies of aeroelastic coupling continued over a period of nearly six years on the NGT Project. 

Many of the early studies served as foundation for more sophisticated studies to follow. The results of all of these investigations of aeroelastic tailoring were assimilated in one final comparison of comparably coupled blades that achieve coupling through sweep or oriented fibers, or a 

combination of both. This final investigation attempted to develop blades with practical levels of 

sweep or coupling. 

The study focused initially on the design and analysis of the structure of four new carbon/glass 

hybrid blades with varying levels of twist-flap coupling: 

• Carbon Spar Baseline: The GE37a all-glass blade and the glass spar cap replaced with 

one constructed of unidirectional carbon fibers of appropriate thickness to ensure adequate tower clearance and acceptable axial strain values in the carbon fibers 

• Coupled Blade 1: The Carbon Spar baseline with the very modest coupling added 

through oriented fibers in the skin 

• Swept/Coupled Blade: Coupled Blade 1 with in-plane sweep 

• Coupled blade 2: Coupled Blade 1 with additional oriented fibers in the spar cap. 

The operation of the EMD turbine employing a set of each of these blades was dynamically 

simulated. Different pitch control systems were modeled: Version B for the baseline collective 

pitch controller, and the Enhanced Controller A2, which includes independent blade pitch control. The operation of the EMD turbine employing all the various blades and controller combinations was simulated for a variety of IEC DLCs, as well as fatigue. 

The conclusions to be drawn from the results are: 

• Coupling (via oriented fibers and/or sweep) combines with the enhanced controller to 

produce reductions in nearly all key design-driver loads. 

34
• Oriented fibers and sweep are complementary. 

• Oriented fibers and sweep produce very similar impacts on loads and fatigue for comparable levels of twist-flap coupling. 

• Twist-flap coupling and the enhanced controller algorithms are complementary. 

An analysis of the impact on COE to be expected from aeroelastic coupling was conducted. 

When presented with a concept that offers loads reductions in a wind turbine system, two different means exist for translating those reductions into reductions in COE: 

1. Increase the rotor diameter until loads are returned to baseline levels, thereby realizing an 

improvement in the energy capture 

2. Redesign the components affected by the reduced loads to realize cost savings. 

The basic principles of the first approach are: 

• Increase the rotor diameter until key driver loads/fatigue are returned to baseline levels, 

taking account of: 

o Influence of rotor diameter on loads 

o Influence of blade mass on loads (see below) 

• Account for impact on energy capture, including: 

o increased energy capture due to larger rotor diameter 

o reduction in energy capture due to coupling 

• Account for increased blade mass/cost required to maintain adequate blade tip-tower 

clearance, taking account of any initial changes in the minimum tip clearance due to 

changes in blade stiffness, coupling, sweep, etc. 

• Account for any increases in component costs to accommodate any loads/fatigue that are 

allowed to increase above baseline levels 

• Determine net impact on COE. 

The results of the COE analysis show that reductions of approximately 6% can be expected from 

the moderate aeroelastic tailoring. The enhanced controller operating alone shows a potential for 

approximately 2% reduction in COE, while combining it with the uncoupled carbon blade shows 

a potential for approximately 3% reduction in COE. These conclusions are based upon substantial scaling of the rotor diameters. The fundamental conclusion of this study is that aeroelastic 

tailoring allows for a significant increase in the size of the rotor placed on a given drive train. 

The reductions in fatigue and ultimate loads may also allow a wind turbine to be certified to operate in a higher wind class than that for which it is presently certified. Reductions in COE in the 

range of 9%–11% appear achievable by increasing the rotor size while increasing the wind class. 

Incorporation of sweep requires development of a brand new blade mold, the cost of which was 

beyond the scope of the NGT project. Similarly, incorporation of off-axis carbon fibers requires 

a substantial materials testing program that was beyond the scope of the NGT program.. Aeroelastic tailoring was therefore not incorporated into the EMD turbine. 

2.3.4 Cast Blade Roots 

A substantial portion of fiberglass blade weight and cost is contained in the root and transition 

sections, which contribute negligibly to energy capture. Less expensive steel or cast iron pieces 

could be inserted between the hub and the blades, which in turn could be shortened. In other 

words, the fiberglass blade root would be replaced by a steel or cast iron blade root. The goal 

35
would be to reduce the COE by reducing the turbine cost by a greater percentage than the energy 

capture. The use of steel blade roots could also possibly reduce the cost of transporting large 

blades or even enable the use of larger rotor diameters without resulting in significant transport 

problems. 

As part of a structural and cost analysis of steel or cast iron blade roots, the following studies 

were conducted: 

• Impact studies of the impact on the aerodynamic performance and energy capture of the 

34-m blades for the GE 1.5 MW wind turbine resulting from adopting cylindrical blade 

root (extension lengths 6 m, 8 m, and 9 m) and assuming that: 

o Blade T-bolts similarly employed now would be used with a relatively long radial 

transition from the round root to the aerodynamic section would be required to avoid 

blade buckling 

o A new means of clamping the blade to the bearing could be created, such that the 

blade would feature no structural transition to a round section, and the blade could be 

fitted with a radially short cosmetic piece to provide rapid transition from the round 

blade root (extender) to the aerodynamic sections 

• Structural analyses of four different blade root concepts in an effort to determine the materials requirements: 

o Cast iron blade roots 

o Rolled and welded sheet steel blade roots 

o Cast iron Y-shaped hub 

o Rolled and welded sheet steel Y-shaped hub 

• Cost-benefit analysis of these concepts. 

Five different blade geometries were produced, two each (rapid cosmetic transition and long 

structural transition) for the 3- and 6-m blade roots, and one only (rapid transition) for the 9-m 

blade roots (or their equivalent Y-shaped hubs). Figure 12 summarizes the expected annual energy capture to be derived from each of the six blades for the defined IEC Class II wind. The 

expected energy losses are small with the 6-m roots, but rather significant with the 8- and 9-m 

roots. On the basis of the structural analysis, the only configuration that makes any sense is one 

with cast iron cylinders 2 m in diameter, with inboard flanges connected to the hub, and outboard 

flanges connected to the blade. The design is driven by fatigue considerations, and the fatigue 

strength of weldments is so much less than that of ductile cast iron that a second configuration 

employing a cylinder constructed of welded steel sheet could not be optimized to a realistic 

weight. The same concept applied even greater to a Y-shaped hub constructed of welded steel 

sheet. 

36
9 m Root, Short Transition 

8 m Root, Long Transition 

8 m Root, Short Transition 

6 Root, Long Transition 

6 m Root, Short Transition 

Baseline 

GE1.5/70.5 Turbine 

IEC Class IIA 

6.042 

5.995 

5.909 

5.826 

5.698 

5.707 

5.5 5.6 5.7 5.8 5.9 6.0 

Annual Energy Production Per Turbine [GWh] 

Figure 12. Impact on Energy Production from Blade Root Inserts 

The conclusions drawn from this study are: 

• Extensions of the cylindrical blade root beyond 6 m from the current blade root (i.e., beyond 7.25 m radial location) lead to unacceptable loss of energy production and outweighs any cost savings. 

• Any blade root lengthening concept is dependent upon the use of radially short transitions 

from the round sections to the aerodynamic sections. The use of radially long transitions 

in the blade structure itself is not acceptable. 

• Cast iron blade roots 6 m in length might offer a slight improvement in COE, but the present study shows savings of no more than 1%, or less than $0.05/kWh. 

• The use of cast iron blade roots of 6 m in length could increase the total rotor weight by 

as much as 10,000 lb, or nearly 20% of the current rotor weight. 

• A large cast iron Y-shaped hub does not appear to offer any advantages over separate cast 

iron blade roots and creates severe logistical (transportation) problems. 

• Welded structures appear to be entirely inappropriate for hub and blade applications 

given the relatively low fatigue endurance limits of welded steel and the relatively high 

fatigue to which the blades and hubs are subjected. 

The concept might have advantages, however, not from a cost perspective but from the perspective of transportation. As the blades on larger, future turbines grow in length, it may become 

necessary to adopt two-piece blades for transportation reasons. The cast iron blade root presents 

an alternative with minimal or no negative impact on performance. 

37

6.1 
2.3.5 Boundary Layer Control 

GE commissioned a review of the current state of the art aerodynamic boundary layer control via 

blowing and suction and an evaluation of the potential for applying these concepts to a wind turbine blade. The study identified four motivations for employing boundary layer control in fluid 

dynamic systems: 

1. Boundary Layer Control (BLC) via blowing or suction to eliminate leading edge laminar 

separation

2. Laminar Flow Control (LFC) using suction to delay or avoid transition of the laminar 

boundary layer to turbulent

3. BLC to control the development of the turbulent boundary layer 

4. BLC to delay or avoid turbulent boundary layer separation using suction or blowing. 

Only Items 2 and 4 have potential for wind turbine applications. GE's study includes an exhaustive review of research on laminar flow control (LFC and boundary layer control (BLC, including the drag reductions and lift improvements possible, as well as the requirements for magnitudes of blowing and suction to achieve desired results. The study also examined practical 

methods for implementing suction and blowing, derived equations for calculating the power required for flow control, and estimated the cost of implementing a flow control system into a 

wind turbine blade. 

These results were used to conduct power performance and cost modeling of four possible configurations for introducing suction into a wind turbine rotor blade. All of the system modeling 

uses the GE37a blade as a baseline. The key conclusions drawn from this study are that the 

maximum benefit expected from applying suction for flow control on the GE37a blade is near a 

2% reduction in the COE. 

2.4 Variable Diameter Rotor 

The essence of the variable diameter concept is to have a rotor that may augment its diameter 

and increase energy capture in the frequently occurring moderate wind speeds below rated wind 

speed where most energy is available. At the same time, the diameter can reduce in higher wind 

speed and avoid loads which would otherwise penalize a rotor of increased diameter. The cost 

reducing principle applicable to the present concept is to introduce adaptive characteristics responding to the value (i.e., energy) and avoiding the penalty (i.e., loads). 

Two differing implementations of the variable diameter concept were identified. The first concept, subsequently referred to as S-VADER (where “S” refers to sliding), employs an outer section of blade that can retract “telescopically” into the main blade section. The second concept,

referred to as C-VADER, changes the swept area of the rotor by coning the blades. GE subsequently decided that only the S-VADER concept merited further consideration. 

The S-VADER concept employs a telescoping rotor blade configuration, in which an outer, extensible blade section fits inside of a main blade (see Figure 13). This rotor concept could feasibly be incorporated into the existing upwind, pitch-regulated configuration. The main blade 

would bolt via a pitch bearing to the hub, and the pitch of the blade would be controlled in the 

same manner as presently. The outer blade would be extended and retracted by a second mechanical system in response to gross changes in wind speed (see Figure 14), while pitch control 

would still be employed to regulate the power. 

38
mai l

extension piece 

n b ade 

Figure 13. Illustration of S-VADER Blade Concept 

Figure 14. Illustration of S-VADER Operation 

The key is to engineer a system without having an aerodynamically inefficient rotor. The requirement that the outer blade fit within the inner blade, combined with the need for bearings and 

mechanical drive systems will necessitate the use of an outer blade with a relatively short chord 

length. This requires airfoils of much higher design lift coefficient on the outer blade section. 

A rotor optimized for a specified speed schedule has a unique lift distribution. This is a determinate independent of airfoil selection. The lift is proportional to the product of airfoil design lift 

coefficient and blade chord. Therefore, any airfoils with adequate general performance characteristics will realize optimal blade performance with a chord distribution that suits the design lift 

coefficients of the airfoils. For a variable diameter rotor, it was initially unclear what type of airfoils and what chord widths might be preferable. Two basic criteria were established: 

1. The main blade airfoils must be wide enough to accommodate the extension piece and the 

bearing system. 

2. The variable diameter system must be operated within boundaries set by a conventional 

reference wind turbine of the same rated electrical output. Specifically, this defines limits on 

power, torque, thrust, and tip speed. 

39
Perhaps the most important outcome of the COE investigations are an appropriate choice of design parameters. The concept shows a COE benefit of just a little over 12%, using steady state 

power curve comparisons and prototype rotor costs. Using turbulent wind power curves, increasing tower height to optimum for the VADER and assuming that in production, VADER added 

rotor cost can be further reduced, leads to a predicted COE reduction of approximately 18%. 

The study further suggested that improved control could realize about 3.5% more. 

The original goal of the NGT Program was a reduction of COE to $0.025/kWh at the NREL Reference Site 1. GE estimates that this COE would require a reduction in COE of 30% from the 

baseline values. Therefore, it is concluded that with a little optimization, the VADER concept 

could move GE two-thirds or more towards the goal. Combined with other improvements identified elsewhere in this report, it is believed that the S-VADER concept could achieve the stated 

NGT goal. 

2.5 Summary of Concept studies 

The implications of the concept studies show the best chance for providing near-term reductions 

in windpower COE at risk levels that are acceptable to the wind energy financial community are: • Optimized low-solidity rotor blades • Larger rotor enabled by sophisticated load-alleviating controls systems • Advanced controls systems, including: • Independent blade pitch to effect asymmetric load control • Tower top accelerometer feedback for tower damping • Coupling of pitch and generator torque control • Taller, more flexible towers. 

All of these features were incorporated into either or both of the POC and EMD turbines, as 

noted in the following sections of the report. Although at the outset of the project GE had defined the 750-kW machine as the baseline, as a result of the concept studies, it was decided to 

develop the POC at the 1.5-MW scale. New 34-m and 37-m low-solidity blades were designed 

for these machines using custom-designed high-lift, thick airfoils. The controller of the POC 

machine was modified to include coupling of pitch and torque control. The tower of the POC 

turbine at 70 m is taller than the 50-m-tower of the baseline machine, and the 70-m tower is 

much softer. Installation constraints at the Tehachapi, California, test site prevented employing 

an even taller 80-m-tower, which would have been desired. The EMD machine incorporates 

asymmetric load control and tower top damping control. 

The electromechanical concept studies showed two concepts–the medium-speed wound rotor 

induction generator operating in VSCF mode and the medium-speed wound rotor synchronous 

generator operating in fixed-speed mode–that might produce small improvements in COE, but 

less than $0.01/kWh. These were not felt to justify moving away from proven drive train technology with significant commercial advantages for GE Wind. 

Of all the concepts examined, only 2 seem to offer potential for substantial (10%-25%) reductions in the COE: aeroelastic tailoring (via either oriented fibers or in-plane sweep) and the variable diameter rotor (S-VADER). Damage identification feedback shows potential for 5%-8% 

reductions in COE. Consideration of the financial and marketing risks led GE not to prototype 

hardware incorporating these concepts as part of the NGT Project. 

40
3 Proof of Concepts Turbine 

The POC turbine was erected at GE’s facility in Tehachapi, California, in April 2000. It is rated 

at 1.5 MW and features a three-bladed, upwind rotor of 70.5-m diameter driving a six-pole 

wound-rotor asynchronous generator through a three-stage planetary gearbox. The turbine is installed on a free-standing 63-m tubular steel tower on a 5-m concrete pedestal foundation. Power Production [kW 

3.1 Design of the POC1.5/70.5 

Relative to the GE1.5/65 or the baseline 750-kW turbine, the innovations incorporated into the 

POC turbine are: 

• flexible, low-solidity, longer (34-m) rotor blades employing high-lift, thicker airfoils 

• coupling of blade pitch and generator torque control 

• taller (70-m), soft hybrid steel/concrete tower 

• a water-cooled generator. 

Table 3 summarizes the POC turbine configuration as it was finally designed, built, and tested. 

Figure 16 shows a schematic of the turbine as installed. The POC turbine’s innovative features 

were subsequently adopted for the GE1.5/70.5 production machine. 

Loads analysis of the POC turbine in the form of its production version, the GE1.5/70.5, have 

been performed. A comparison of the results of those loads analyses with the design loads for 

the original GE1.5/65 turbine led GE to conclude that the GE 34-m blades could be installed on 

the existing GE1.5/65 platform to form the POC machine without any changes to the remaining 

GE1.5 hardware. GE has also generated the predicted performance curves summarized in Figure

15. The annual energy production data presented in Table 3 is based upon this predicted performance. The following subsections detail the design and testing of several of the components 

or component systems of the turbine, including the tower and foundation, the drive train, controls 

systems, and the rotor blades. 

1,500 

1,000 

500 

0 

Steady Winds 

9% Turbulence 

0 5 10 15 20 25 30 

Wind Speed [m/s] 

Figure 15. Predicted Power Performance for the POC1.5/70.5 Turbine 

41
Table 3. Proof of Concepts Turbine Configuration Description 

General Configuration 

Type Grid Connected 

Rotation Axis Horizontal 

Orientation Upwind 

Number of Blades 3 

Rotor Hub Type Rigid 

Rotor Diameter 70.5 m 

Hub Height 71.0 m 

Performance (Sea level 10 m) 

Rated Electrical Power 1,500 kW 

Rated Wind Speed 11.4 m/s 

Cut-in Wind Speed 3.0 m/s 

Cut-out Wind Speed 25.0 m/s 

Extreme Wind Speed 59.5 m/s 

Gross Annual Energy Production 

at NREL Reference Site 1 4.5 GWh/yr 

at NREL Reference Site 2 5.6 GWh/yr 

Rotor 

Swept Area 

Rated Rotational Speed 

Cut-in Rotational Speed 

Coning Angle (forward precone) 

Shaft Tilt Angle 

Blade Pitch Principle 

Blade Pitch Actuation 

Blade Pitch Angle 

Hub Material 

Direction of Rotation 

Power Regulation 

Over-speed Control 

3,902 m

2 

20.0 rpm 

11.8 rpm

1.5° 

4.0° 

Independent Blade Pitch 

Independent Electric Drive 

Fully Variable 

Ductile Cast Iron 

Clockwise 

Variable Speed/Pitch 

Variable Pitch 

Blades 

Make GE34a 

Material Glass Fiber-Reinforced Epoxy 

Length 34.0 m 

Airfoil Types Proprietary GE 

42
Table 3. Proof of Concepts Turbine Configuration Description (Continued) 

Drive train 

Gearbox Type 

Gearbox Rated Power 

Gearbox Ratio 

Shaft Brake Type 

Shaft Brake Location 

Generator 

Generator Type 

Number of Poles 

Synchronous Speed 

Rated Generator Speed 

Minimum Power Generator Speed 

Speed Range 

Turbine Line Voltage 

Rotor Rated Current 

Turbine Generating Frequency 

Generator Insulation Class 

Cooling Systems 

Yaw System 

Wind Direction Sensor 

Yaw Control Method 

Yaw Bearing Type 

Control System 

Make, Controller 

Power Conversion Strategy 

Power Logic/Device Type 

Controller Logic Systems 

SCADA 

Tower 

Type 

Material

Height 

Foundation 

Type 

Material

Height 

3-Stage, Planetary/Helical 

1,660 kW 

1:72 

Single Disk Active 

High-Speed Shaft 

Doubly Fed Asynchronous 

6 

1,200 rpm 

1,440 rpm 

850 rpm 

800-1,600 rpm 

575 VAC 

585 A 

60 Hz 

F 

Water-to-Air Cooled 

Wind Vane 

Planetary Drive 

Friction 

GE Winteligence 

Doubly Fed 

PWM/IGBT 

Microprocessor-based 

GE VisuPro 

Tapered Tube 

 Certified Steel 

63.1 m 

Pad/Pedestal 

Concrete 

5.0 m 

43 
Figure 16. Schematic of the Proof of Concepts Turbine 

44
3.1.1 Tower and Foundation Design 

A hub height of 70 m was achieved with a new 68-m tower designed using a new hybrid approach utilizing a 5-m concrete pedestal and a 63-m steel tower to achieve an 80-m hub height. 

Several advantages of the hybrid approach are significant. First, using concrete at the base 

minimizes the diameter of the steel tower that needs to be shipped. Shortening the height of the 

steel section also helps resolve buckling considerations. Finally, access doors can be installed in the concrete much more 

easily than in the welded steel construction. Studies showed, 

however, that for the POC design, using more than 5 m of 

concrete was not cost effective. These studies indicated, however, that for a larger turbine, a taller concrete pedestal might 

be cost effective. Figure 17 shows the tower being installed at 

the test site in Tehachapi, California. 

Figure 17. POC Tower in Tehachapi, California 

3.1.2 Drive Train Design and Testing 

Figure 18 illustrates the drive train layout. The drive train is substantially similar to that of the 

GE1.5/65 turbine with the addition of the water-cooled generator (Figure 19). Figure 20 illustrates additional components of the drive train, including the main shaft, gearbox, the high-speed 

shaft, and the yaw drive. The POC 1.5-MW drive train–from the main shaft to the generator– 

was delivered to the National Wind Technology Center (NWTC) at NREL in Golden, Colorado, 

for comprehensive testing at their large-scale dynamometer facility. Figure 21 illustrates the test 

setup at NWTC. The objectives of this testing were: 

• Verify proper load transfer at the gear mesh using contact patterns test. 

• Measure the efficiency of the gearbox and generator systems at different power levels using thermal loss measurements. 

45
• Measure thermal characteristics of the drive train after stabilization at rated power. 

• Run a gearbox endurance test equivalent to 20 years turbine operation. 

• Verify sufficient lubrication in idling situations without grid (oil supply pump not running) with different oil temperatures. 

The unit was tested from November 2002 until February 2003 for a total of 1,600 hours of test 

time. The target test torque load was approximately 130% of the design load to accelerate test 

time to equal gear tooth fatigue of 20 years. A teardown inspection of the gearbox was performed following the testing. The conclusions of the inspection were that after a test of fatigue 

damage equal to the design life of 20 years, the transmission was in serviceable condition. There 

were no components with breakage or extraordinary wear. 

Figure 18. Drivetrain Layout of NGT Proof of Concept 1.5 MW Turbine 

3.1.3 Controls System Design 

The coupling of pitch and torque control was identified as a promising concept in the concept 

studies phase of the project and is incorporated in the POC turbine. The POC main control cabinet is illustrated in Figure 22. 

46
Figure 19. POC Water-Cooled Loher 1.5-MW Generator 

(a) Main (Low-Speed) Shaft (b) Gearbox 

(c) High-Speed Shaft (d) Yaw Drive 

Figure 20. POC Drive Train Components 

47
Figure 21. POC Drive Train on the NWTC Dynamometer Test Stand 

48
Gen. Torque (Nm) 

12000 

10000 

8000 

6000 

4000 

2000 

0 

-2000 

800 900 1000 1100 1200 1300 1400 1500 1600 

PI(D) control of speed 

by blade pitch, 

max. torque saturated 

min. torque saturated 

max. torque saturated PI control of speed 

blades saturated at fine pitch 

by generator torque, 

blades saturated at 

blades saturated at fine pitch 

Optimal tip speed fine pitch 

PI control of speed ratio tracking 

by generator torque, 

blades saturated at Swith over max. min. 

fine pitch torque saruration 

Gen. Speed (rpm) 

Figure 22. POC Main Control Cabinet 

3.1.4 POC 34-m Rotor Blade Design and Testing 

The POC1.5/70.5 configuration uses 34-m blades (GE34a), as shown in Figure 23. The GE34a 

blade employs a family of airfoils custom designed as part of the NGT project for the POC turbine. GE personnel and consultants designed a family of airfoils with the characteristics summarized in Table 4. In addition to the quantitative drivers shown in the table, GE also required that: 

49
• The lift and drag characteristics of the airfoils with leading edge surface roughness 

should be optimized, as opposed to “clean” performance. 

• Given all other design constraints, the designs should seek to maximize the lift-to-drag 

ratio of each airfoil section. • The airfoils should be designed as a family such that they can be physically blended into 

a smooth rotor blade surface. • The maximum lift coefficients should be insensitive to leading edge surface roughness. 

The six airfoils produced by this effort (14%, 18%, 24%, 27%, 30%, and 45% thick sections) are 

shown in Figure 24. These airfoils have been patented in the U.S. (USPTO Number 6,503,058), 

Europe (EPO Number 1152148A1), and internationally (WIPO Number WO 01/83983A1). 

Models of three of the NGT Airfoils (the 18%, 24%, and 30% thick sections) were actually built 

and tested. The models were designed and fabricated by subcontractors in response to requirements established by GE. The testing was performed in the Wichita State University (WSU) 

7 x 10-foot Low-Speed Wind Tunnel (LSWT). Each model was nominally 84 inches long with a 

chord of 36 inches, and was mounted vertically in a floor-to-ceiling arrangement in the LSWT 

test section. 

50
Figure 23. POC 34-m Blade Planform

 Table 4. Summary of Required Airfoil Characteristics 

Airfoil t/c Design Cl 

(Cl at L/Dmax) 

Rec at Rated Rec at Cutin Blade 

Location 

r/R 

1 14% 1.25 8 million 4 million 90% 

2 18% 1.25 8 million 4 million 75% 

3 24% 1.25 8 million 4 million 55% 

4 27% 1.35 6 million 3 million 40% 

5 30% 1.50 4 million 2 million 25% 

6 45% Maximized 3 million 2 million 15% 

51
Figure 24. NGT POC Airfoil Family 

For each model, the two-dimensional airfoil properties (lift, drag, and pitching moment coefficients) were measured at a pre-determined schedule of tunnel speeds and angles of attack. Aerodynamic forces were measured indirectly using a combination of surface static-pressure taps and 

wake dynamic pressure surveys. In addition, the LSWT floor and ceiling balances were used to 

measure and record the direct aerodynamic forces on the model. 

The majority of the test technical objectives were met. One requirement for the testing was to 

reach a chord Reynolds number of 4 million, and values of 3.8, 3.7, and 3.5 million were attained 

for the 18%, 24%, and 30% airfoils, respectively. This 

was deemed acceptable. Two-dimensional force and 

moment measurements were made to a high level of 

confidence for all three airfoils in both smooth and 

rough surface conditions. 

The test data lift curve slopes agree quite well with 

thin airfoil theory. A review of the predicted lift data 

showed that the predicted lift curve slopes were approximately 10% higher than the value from thin airfoil theory. It has, therefore, been concluded that the 

wind tunnel lift data are of high confidence, and that 

the calculations over-predicted the lift curve slope. 

Comparisons of measurements with predictions 

showed: 

• Minimum drag values and width of drag bucket 

showed good agreement with predictions. 

• Measured CL,max was between 5% and15% 

lower than predicted by XFOIL (expected result). 

• Stall for all airfoils was sharper than predicted, 

with the 24% and 30% thick airfoils showing Figure 25. 18% Airfoil Model in the WSU particularly sharp stall. Wind Tunnel Test 

52
A finite element model of the blade structure was built in ANSYS and used to analyze the blade's 

structural integrity with respect to: 

• Tip deflection 

• Buckling stability 

• Extreme strain 

• Laminate Fatigue 

• Root attachment failure due to fatigue, extreme static loads, and flange liftoff 

• Natural frequency requirements. 

Structural analyses showed that tip deflection constraints were the overriding design driver. The 

target tip deflection was 65% of tower clearance. Forward coning was added to the blade at the 

5.5-m station in order to provide an additional 0.7-m of tip offset and reduce spar cap material 

usage. Coning from 5.5 m keeps most mass on the pitch axis. Nevertheless, tip deflection requirements still required an increase in blade mass. Full-scale tests indicated that 59.7% of tower 

clearance was being consumed, versus the 70% maximum allowed by Germanischer-Lloyd. The 

maximum strain on the blade is well below the strain allowable. Fatigue damage remains everywhere under unity. Buckling analyses indicated that no core material was needed in the spar cap. 

The POC blade design departs from the approach used by the baseline 750-kW turbine's 24-m 

blades, which employ balsa in the spar cap for buckling resistance. By narrowing and thickening 

the spar cap of the POC blade, the use of balsa was avoided for equal cost. The GE34a uses a Tbolt root attachment developed in the 750-kW NTRT program. The blade uses 54 M30 T-bolts. 

The prototype set of blades is shown in Figure 26. 

Figure 26. Prototype Set of the GE34a Blades at Tecsis 

53 
One of the prototype blades was initially subjected 

to a flapwise load test while still at the fabrication 

facility, as shown in Figure 27. The tests conducted were a two-point static load test, a singlepoint static load test, and a natural frequency measurement, all of which were conducted under the supervision of GE Wind Energy and Germanischer 

Lloyd personnel. Figure 28 illustrates the arrangements of the two-point and single-point tests. 

The magnitudes of the loads which were applied 

Figure 27. Static Blade Tests were provided by GE Wind Energy personnel to 

Tecsis test engineers. These tests confirmed the 

structural integrity of the blade before it was first fielded. The blade did not fail under 100% 

loading. Maximum values of strain that were measured were approximately 0.4%. 

(a) Two-Point Load Test Setup 

(b) Single-Point Load Test Setup 

Figure 28. Static Blade Test Setup 

54
A blade certification testing program was conducted at the National Wind Technology Center 

(NWTC) in Golden, Colorado, under the supervision of personnel of the National Renewable 

Energy Laboratory. Fatigue testing of a GE34a blade began in August 2001, and was completed 

January 10, 2002. The same blade was used as had been subjected to the static testing described 

above. The objective of the fatigue test was originally to conduct a limited life fatigue test in order to give basic assurance that no major catastrophic fatigue failures are likely. The target number of test cycles was 300,000 for a ten percent life. This was based on observations that past 

fatigue testing has revealed problems early in the test duration. Later, this objective was expanded to include a full 20-year life of damage using a one-million-cycle load duration. The test 

operated for 1.9 million cycles. 

(a) Blade Setup 

(b) Two-Axis Hydraulic Actuator 

Figure 29. NWTC Fatigue Test Setup 

Loads were provided to NWTC by GE Wind to represent a 3-million-cycle design life. The testing began with the 3 million cycle loads. Beginning at approximately 928,000 cycles, however, 

the test loads were increased to accelerate the test to achieve 100% damage in 1 million cycles. 

Table 5 summarizes the operationally equivalent fatigue damage resulting from the fatigue test at 

various spanwise locations on the blade. These results were determined via finite element analysis of the blade. These results indicate that the critical test regions of the blade were tested to at 

55
least 20 years of equivalent loading, the design life of the blade, without significant loss of global 

properties. In summary, NWTC concluded from the fatigue tests that the blade sustained the full 

load spectrum without significant loss of properties, including stiffness and ability to carry load. 

Table 5. Test Equivalent Damage 

Spanwise Test Life (Years) Notes 

Station 

z [m] 

Flap Edge 

6.4 16.4 2.0 

9.2 25.9 5.6 

12.8 37.3 21.7 Critical Flap Station 

16.9 20.2 37.6 Critical Edge Station 

20.9 0.1 5.0 

3.1.5 POC Hub Analysis 

Figure 30 shows the POC hub on the floor of GE's manufacturing plant. During the NGT program, concern arose that much better fatigue properties for the GGG40.3 ductile cast iron used in 

the hub were required. A subcontract was issued to conduct an exhaustive materials characterization effort for this material. Analyses of the hub static stresses and fatigue concluded that, with a 

redesign of one small part, the hub does not appear to have any fatigue-driven problems. Therefore, it was concluded that the hub is acceptable in fatigue for greater than 20 years. 

3.2 Installation and Testing of the Proof of Concepts Turbine 

The POC turbine was erected at the GE facility in Tehachapi, California. Installation was completed in April 2000. The turbine was installed on a free-standing 63-m tubular steel tower on a 

5-m concrete pedestal foundation. The rotor installation is illustrated in Figure 31, and the completed POC turbine is shown in Figure 32. 

Certification testing of the POC turbine for noise, loads, power performance, and power quality 

has been completed under the supervision of personnel of the National Renewable Energy Laboratory. 

Figure 30. POC Hub 

56 
Figure 31. POC Turbine Rotor Being Installed 

Figure 32. POC Turbine Installed in Tehachapi, California 

57
nron 6-21 01

reliminar NREL

3.2.1 Power Performance Testing of the POC Turbine 

Power performance testing was conducted on the POC turbine between February 9, 2001 and 

March 21, 2001. A total of 265 hours of data with wind out of the acceptable wind directions 

(295° to 335°) and with the turbine available was collected during that time. Figure 33 compares 

the measured and theoretical power curves. Power (kW) 

1800 

1600 

1400 

1200 

1000 

800 

600 

400 

200 

0 

0 5 10 15 20 

i l 

E 2/ / 

P y NREL Measured 

Theoret ca

GEWE Measured 

Wind Speed 

Figure 33. Comparison of Theoretical and Measured POC Power Curves 

3.2.2 Power Quality Testing of the POC Turbine 

Power Quality testing of the POC turbine was conducted between May 31, 2001 and June 25, 

2001. A total of 973 hours of data were collected while the turbine was available, but only 315 

hours of data was recorded during 10-minute intervals during which the turbulence intensity was 

between 8% and 16%, as required by the IEC 61400-21 Power Quality Standard. The testing

consisted of a long-term phase and a short-term phase, conducted between 31 May and 9 June. 

The tests conducted as part of the power quality testing were: 

• Maximum measured power (60-second and 0- and 2-second), reactive power demand 

• Turbine start and stop tests 

• Voltage fluctuations (flicker) 

• Current imbalance measurements 

• Voltage and current harmonics. 

The results of the testing indicated no power quality problems for the POC turbine. 

3.2.3 Mechanical Loads Testing of the POC Turbine 

Mechanical loads testing was conducted on a production GE1.5/70 turbine installed in a wind 

farm at Desert Sky, Texas. Loads were determined to be acceptable. 

58
3.2.4 Acoustic Noise Testing of the POC Turbine 

Turbine and background acoustic noise data were collected between July 6, 2000 and July 8, 

2000. The A-weighted sound power level as 8 m/s wind speed was calculated using two methods for determining wind speed. The first method uses wind speed as measured on a temporary 

10-m meteorological tower upwind of the turbine. The second method derives the wind speed 

from measurements of turbine power and a power curve obtained from power performance testing. Additional acoustic testing included wind speed sensitivity, directivity, one-third octave 

spectra, and tonal analysis. Figure 34 shows a comparison of the POC noise level of approximately 98 dBA to levels historically associated with wind turbines of various sizes. It is significant to note that the POC turbine exhibits noise emissions substantially below those of turbines 

of similar size. Figure 35 shows that this is true in spite of the POC's relatively higher rotor tip 

speed. 

+ POC1.5/70 

Figure 34. Comparison of the POC Noise Levels to Historical Levels 

3.3 POC Certification 

In December 2000, Germanischer Lloyd (GL) granted GE wind certification of design compliance to IEC 61400-1 Class IIA for the production derivative of the POC turbine, the GE1.5/70.5. 

59
104

103

102

101

100

Maxi mum Ti p Speed (m/s) Sound Power Level [dB(A)] 99 

98 

97 

Enercon E- POC GE1.5s Jacobs NEG Nordex N- SudWind Vestas 

66 Turbine (APX 70) MD70 Micon 60 S70 V66 

1500 

90

80

70

60

50

40

30

20

10

0 

Enercon POC GE1.5s Jacobs NEG Nordex N- SudWind Vestas 

E-66 Turbine (APX 70) MD70 Mic on 60 S70 V66 

1500 

Figure 35. Comparison of Noise Levels and Rotor Tip Speeds 

60
4 Engineering and Manufacturing Development Turbine, 

EMD1.5/77 

The Engineering and Manufacturing Development (EMD) turbine is an evolution from the POC 

turbine incorporating the following improvements in its design: 

• A larger 77-m-diameter rotor 

• 37-m blades with tip curvature 

• Tower-top accelerometer feedback for tower damping 

• Independent blade pitch control for asymmetric load control. 

These improvements were implemented in two stages. The POC configuration was converted to 

the EMD configuration officially in April 2002, with the installation of the 37-m blades. Following the power performance, loads, and noise testing of that configuration, the EMD was further 

upgraded with the addition of the asymmetric load controller (ALC) and associated turbine sensor system. The system in its final configuration was then, in June 2003, subjected to additional 

loads testing. 

Table 6 provides a summary of the EMD turbine configuration. 

4.1 IEC Class S High Energy Capture Rotor Configuration and Loads 

The development of the high energy capture 77-m rotor was motivated by two considerations. 

First, earlier design efforts conducted as part of the development of the POC turbine showed that 

the rotor diameter for the GE1.5 turbine could be increased to at least 77 m and possibly higher, 

without a significant increase in loads if the design took advantage of the fatigue-reducing benefits of increased rotor flexibility. 

The second motivation for the 77-m rotor involved consideration of the operating conditions. 

Analysis of wind speed histories from several commercial wind farms in the U.S. led to the conclusion that one could expect 50-year gusts in level great plains areas at the 65–80 m hub heights 

to be less than 52.5 m/s. The analysis also shows that both IEC Class 2A and 2B overpredict the 

level of turbulence intensity at all wind speeds compared to every wind farm analyzed. The value 

of k=2 for the Weibull coefficient that is typically used to define frequency distributions of wind 

speed generally overpredicts the frequency of very high winds relative to what is occurring at 

typical wind farms. 

Since the distribution of very high winds and the turbulence intensity are factors that most define 

extreme wind and fatigue, study results indicate using the IEC standard wind classes will result 

in the design of a turbine that is overly conservative for the conditions typical at the sites where 

wind farms are being installed in the U.S. Use of a larger Weibull factor than 2.0 and turbulence 

intensities lower than IEC Class 2B would allow the use of a much larger rotor and result in the 

proper projection of significantly improved energy capture and reduced COE. 

Therefore, GE decided to define a special S-Class environment for purposes of designing the 77­

m rotor that was felt to be typical of real operating environments: • A 52.5 m/s 50-year 3-second-average extreme gust • 9 m/s annual average wind speed • A Weibull distribution of wind speeds with coefficient k=2.3 • IEC Class B turbulence intensity • 1.2 kg/m3

 annual average air density. 

61 
Table 6. Engineering & Manufacturing Development Turbine Configuration 

General Configuration 

Type Grid Connected 

Rotation Axis Horizontal 

Orientation Upwind 

Number of Blades 3 

Rotor Hub Type Rigid 

Rotor Diameter 77.0 m

Hub Height 71.0 m

Performance (Sea level 10 m)

Rated Electrical Power 1,500 kW

Rated Wind Speed 13.0 m/s 

Cut-in Wind Speed 3.0 m/s 

Cut-out Wind Speed 25.0 m/s 

Extreme Wind Speed m/s 

Gross Annual Energy Production 

at NREL Reference Site 1 4.8 GWh/yr 

at NREL Reference Site 2 5.9 GWh/yr 

Rotor 

Swept Area 

Rated Rotational Speed 

Cut-in Rotational Speed 

Coning Angle (forward precone) 

Shaft Tilt Angle 

Blade Pitch Principle 

Blade Pitch Actuation

Blade Pitch Angle 

Hub Material

Direction of Rotation 

Power Regulation

Over-speed Control 

4,657 m

2 

18.3 rpm 

10.9 rpm

1.3° 

4.0° 

Independent Blade Pitch 

Independent Electric Drive 

Fully Variable 

Ductile Cast Iron 

Clockwise 

Variable Speed/Pitch 

Variable Pitch 

Blades 

Make GE37a 

Material Glass Fiber-Reinforced Epoxy 

Length 37.25 m 

Airfoil Types Proprietary GE Wind 

62
Table 6. Engineering & Manufacturing Development Turbine Configuration (Continued) 

Drive train 

Gearbox Type 

Gearbox Rated Power 

Gearbox Ratio 

Shaft Brake Type 

Shaft Brake Location 

Generator 

Generator Type 

Number of Poles 

Synchronous Speed 

Rated Generator Speed 

Minimum Power Generator Speed 

Maximum Generator Speed 

Turbine Line Voltage 

Turbine Generating Frequency 

Generator Insulation Class 

Cooling Systems 

Yaw System 

Wind Direction Sensor 

Yaw Control Method 

Yaw Bearing Type 

Control System 

Power Conversion Strategy 

Power Logic/Device Type 

Controller Logic Systems 

SCADA 

Tower 

Type 

Material

Height 

Foundation 

Type 

Material

Height 

3-Stage, Planetary/Helical 

1,660 kW 

1:78.246 

Single Disk Active 

High-Speed Shaft 

Doubly Fed Asynchronous 

6 

1,200 rpm 

1,432 rpm 

850 rpm 

1,600 rpm 

575 VAC 

60 Hz 

F 

Water-to-Air Cooled 

Wind Vane 

Planetary Drive 

Friction 

Doubly Fed 

PWM/IGBT 

Microprocessor-based 

GE VisuPro 

Tapered Tube 

 Certified Steel 

63.1 m 

Pad/Pedestal 

Concrete 

5.0 m 

63 
GE briefly considered the costs and benefits of not only increasing the rotor diameter, but also 

increasing the turbine rating to 1.7 MW. However, the analysis showed that the costs associated 

with increasing the turbine rating more than offset the benefits in terms of increased energy capture, resulting in an increase in the COE. This is true for both Rayleigh and non-Rayleigh wind 

speed distributions, as shown in Figure 36. Percentile 

100% 

80% 

60% 

40% 

20% 

1.5/70.5 

1.5/77 

1.7/77 

0% 

9 m/s 

6.5 m/s 

Cost of Energy 

Figure 36. Benefits to COE from Increasing Rotor Size and Turbine Rating 

Four different sets of loads have been generated for the EMD 1.5/77 Turbine: 

1. Loads generated using DIBT WZ2 conditions (similar to the IEC S-Class) with the baseline controller and heavy blades reflective of the prototype blade weight 

2. Repeat number 1 for lighter blades reflective of product improvements 

3. Loads using IEC Class III-A conditions, an intermediate blade weight, and the baseline 

controller 

4. Loads using IEC Class III-A conditions, heavier blades, and the ALC. 

The key conclusion to be drawn from the loads data is that the EMD1.5/77 turbine can operate in 

either the IEC Class S defined above or in IEC Class 3A with loads less or on par with the IEC 

Class 2A loads for which the POC1.5/70.5 machine was certified. Finally, the ALC promises to 

produce some substantial reductions in several key components of both ultimate loads and fatigue. 

As a consequence of these loads predictions, none of the components of the POC turbine would 

need to be redesigned in order to accommodate loads changes effected by the conversion to the 

EMD configuration. In the following sections, only detailed design information about the 

changed components–the gearbox, rotor blades, and the control system–will be presented. 

Figure 37 summarizes the predicted power for the EMD1.5/77 turbine, as determined via simulations. 

64
1,500 

1,000 

Power Production [kW 

500 

0 

0 5 10 15 20 25 30 

Steady Winds 

9% Turbulence 

Wind Speed [m/s] 

Figure 37. Predicted Power Performance for the EMD1.5/77 Turbine 

4.2 EMD1.5/77 Component Designs 

4.2.1 EMD Gearbox Modification 

To maintain the rotor tip speed at the baseline value of 73.8 m/s when operating at rated power,

the gear ratio was increased to 78.25:1 rpm. The gearbox supplier analyzed the gearbox and determined that the change could be effected by modifying the third stage only. The first stage

gear can accommodate the increased ultimate loads. 

4.2.2 EMD 37-m Rotor Blade Design and Testing 

Several approaches to designing the 37-m blades were considered, including adding material to 

either or both of the root and the tip of the GE34a blade. GE finally decided to choose the approach motivated by energy capture and add all of the 3.25 m of new blade to the tip. The contour is exactly the same as that of the GE34a blade out to the 30.95-m spanwise location. Details 

of the blade geometry are shown in Figure 38. The GE37a blade employs another innovative 

feature: blade curvature. During the concept studies (see Section 2.3) it was noted that one alternative to blade coning is the use of blade curvature, that is, gradually curving the blade out of the 

rotor plane. For the GE37a blade, such curvature is only employed at the tip of the blade in order to provide a small amount of additional tower clearance. This is needed because of the extra 

length of the blade. The blade still employs 1.3° of forward precone in the root. 

65
Figure 38. EMD 37-m (GE37a) Blade 

66
The laminate schedule of the spar cap of the GE37a blade consists of substantially more layers 

than that of the GE34a in order to ensure proper tower clearance. Structural analyses show that

the peak strains are well within the allowables. Moreover, the GE37a blades have sufficient 

margin with respect to both buckling and fatigue as a consequence of meeting deflection requirements. Finally, the T-bolts are acceptable based upon on a comparison of the rotated blade 

root fatigue damage equivalent loads (m=4), which are only 4% greater in the 9 m/s average 

wind S-Class with the 37a blades than with the GE34a blades in IEC Class 2A. Margins on the

POC T-bolts were greater than 4%. As a result, reanalysis of the T-bolts was not required. 

In the end, GE has been able to achieve a 19% increase in rotor swept area with only a 3% increase in blade mass. 

The prototype GE37a blade underwent ultimate load tests in December 2001 and survived to 

greater than 100% of the S-Class extreme flap and edge loads, including the 1.35 safety factor 

required by GL. The National Wind Technology Center has conducted comprehensive fatigue 

testing of a GE37a blade in connection with the NGT Program. 

4.2.3 EMD Controls Systems 

The EMD turbine initially operated with the baseline POC controller. This portion of the EMD 

development was referred to as the EMD Phase 1. The subsequent phases consisted of modifications to the EMD control system to implement an ALC with independent blade pitch actuation 

and tower top damping. The changes were implemented in several phases as follows: • EMD Phase II: develop and refine the software and hardware necessary for the ALC • EMD Phase III: Asymmetric loads control testing • EMD Phase IV: ALC refinement. 

At the conclusion of the NGT Program, Phases II and III were complete and Phase IV was ongoing. This section documents the efforts completed in Phase II, while Section 4.3.4 summarizes 

the results of Phase III. Phase IV will be completed outside of the NGT program and is not 

documented in this report. 

The control objective using the ALC is to maximize energy capture while minimizing load subject to the following constraints: • Rotational speed constraints • Maximum power output • Maximum loads • Limitations on actuation force and rate 

The control schemes implemented in the EMD controller include: • Independent blade pitch for reduction of asymmetric loads due to wind shear, yaw misalignment, and turbulence • Tower-top accelerometer feedback driving collective blade pitch for damping fore-aft 

tower oscillations due to pitch changes and turbulence 

The two control approaches are coded as separate tasks in the controller, which communicates to 

the main task through the SVI Interface. 

4.2.3.1 Tower-top Accelerometer Feedback 

The tower-top accelerometer feedback was identified during the concept studies as a promising 

means of reducing transient loads that were actually being induced by the pitch system itself. 

67
Development of the concept required more time, however, than would permit incorporation into 

the POC machine. Therefore, it has been incorporated at the EMD stage. 

As Phase 2 of the development, a wide variety of tower top accelerometers were subjected to exhaustive testing. Accelerometers from several different vendors were evaluated with respect to 

the performance, reliability, and quality of the manufacturer. GE selected a modified version of 

the unit that was already being employed for diagnostic monitoring in the GE1.5 turbines. 

Figure 39 illustrates this unit and its location on the EMD turbine. 

Figure 39. EMD Tower-Top Accelerometer Package 

Fore-aft acceleration is measured from the unit via an output that is wired directly to the controller analog input. The control algorithm is PI from the filtered signal to collective pitch increment, as illustrated in Figure 40. Error traps are programmed for mean value drift, mean value 

significantly different from zero, non-changing signal, and large change in signal in one time 

step. The system was tested in emulation with serial communication to the controller before being implemented on the operating turbine. 

PCH PI 

∆

Filters 

Error Traps 

Pitch 

Figure 40. Flowchart of the EMD Tower Top Accelerometer Feedback 

68 
4.2.3.2 Rotor Feedback Control 

Results of the concept studies showed that independent blade pitch for asymmetric load control 

could produce significant reductions in key loads with little or no impact on the energy capture. 

As noted above in Section 4.1, EMD loads analysis supported significant loads reductions from 

using the ALC. 

The POC turbine featured independent blade pitch actuation, insomuch as the pitch of each rotor 

blade was controlled by an independently functioning electric drive. This concept was inherited 

from both the value-engineered version of the baseline 750-kW turbine and the original version 

of the GE1.5 turbine. However, like those turbines, the control system of the POC turbine was 

designed to effect synchronized actuation of the blades. Absent a control fault or a failure of one 

of the pitch drives, all three blades should ideally be at the same pitch setting at all times. 

The EMD control system implements the functionality to control the pitch setting of each blade 

independently. As first noted during the concept studies, independent blade pitch control can be 

used to ameliorate asymmetric rotor loads that cannot be otherwise mitigated using collective or 

synchronized pitching of the blades. Implementation of asymmetric load control requires two 

sets of measurements: 

1. Blade or rotor loads 

2. Blade azimuthal position. 

The primary tasks in Phase II were: • Evaluate, install, and document candidate input sensors for the ALC. • Select the best sensors from the suite of candidate test sensors for ALC. • Design and test communications (hardware/software) interface between the candidate 

control sensors and the PLC. • Test and verify sensor signal data acquisition in the PLC by passing acquired signals 

through PLC to the ADAS DAQ system. 

• Implement and test PLC control code (software) changes for ALC system. 

• Perform system dynamics study to determine controller parameters: 

o ALC algorithm coding on PLC 

o ALC simulation and emulation. 

• Develop and verify ALC system safety features including: 

o Signal plausibility and error-trapping routines to prevent spurious input signals from 

damaging the turbine through unstable ALC feedback 

o Implementation of adjustable ALC pitch modulation limiter 

o System safety verification prior to field testing via ALC simulation and emulation 

o algorithm functionality and sensitivity of gain settings. 

The first step in the development of the system consisted of an exhaustive survey of measurement approaches and appropriate sensors. An engineering evaluation of the sensors was conducted to assess sensor performance and reliability. This evaluation included numerous tests. 

First an industry review of vendors was conducted to screen sensors meeting Critical To Quality 

requirements. A series of screening tests were performed on the selected sensor and target material in order to determine significant performance factors. Three factors were investigated: power 

supply rejection, target temperature and sensor temperature. Power supply rejection and target 

temperature were found to have very little influence on the sensor output and were not characterized. Sensor temperature was found to be a significant factor and was incorporated into the 

characterizations tests. 

69
Bench tests verified PLC control module operation, and confirmed hardware/software interface 

and wiring schematics. Bench tests also confirmed program routines for flagging software safety 

traps, such as when a sensor signal is out of range. Software safety traps included signal failure, 

signal range errors, and plausibility errors. Laboratory bench tests also included testing pitch 

control algorithms under various turbulent wind conditions using software to simulate and emulate turbine operation. The objective of simulations and PLC emulation was to test the algorithms 

to ensure they were working properly and to ensure safety when testing ALC operating on the 

prototype turbine. Thus, bench scale laboratory tests were used to confirm proper wiring and interfacing between the sensors and the PLC software/hardware and to predict the behavior of the 

ALC system before “real-world” testing. These tasks included: 

• PCH raw signal output tests 

• PCH system self-test being initiated by PLC 

• PCH system self-test failure relay detection by PLC 

• Sensor operation and interface to PLC 

• Software subroutine timing measurements 

• Test software error traps for sensor input signals (upon failure, write flag code to text file 

and shut down ALC control functions). 

4.3 Installation and Testing of the EMD Turbine 

The first stage of the conversion of the prototype 1.5-MW machine in Tehachapi, California, 

from the POC configuration to the EMD configuration was completed in April 2002, with the 

installation of the 37-m rotor blades. The turbine remained installed on the free-standing 63-m 

tubular steel tower on a 5-m concrete pedestal foundation. The completed EMD turbine is 

shown in Figure 41. 

Certification testing of the EMD turbine for noise, loads, and power performance have all been 

completed under the supervision of personnel of the National Renewable Energy Laboratory. 

70
Electrical Power (kW) 

Figure 41. EMD Turbine Installed in Tehachapi, California 

4.3.1 Power Performance Testing of the EMD Turbine 

Power performance testing was conducted on the EMD turbine between May 4, 2002 and June 

17, 2002. A total of 495 hours of data with wind out of the acceptable wind directions (295° to 

335°) and with the turbine available was collected during that time. Figure 42 compares the 

measured and theoretical power curves. 

EMD Turbine, 77-m Rotor, Sea-Level 

1600

1400

1200

1000

800 

600 

400

200

0

0 5 10 15 20 25 

Wind Speed (m/s) 

NREL Measurements Predct ons i i

Figure 42. Comparison of Theoretical and Measured EMD Power Curves 

71 
4.3.2 Mechanical Loads Testing of the EMD Turbine 

Mechanical loads testing was conducted on the EMD turbine between June 11, 2002 and September 17, 2002. A total of 77 hours of valid data were collected during normal operation of the 

turbine. The objective of the loads testing was to determine the relationships between wind conditions and loads on the POC turbine under all normal and emergency operating conditions. 

Loads were found to be acceptable. 

GE conducted an internal study of the comparison between the measured mechanical loads and 

those predicted by the dynamic simulations. The results show that the dynamic simulations do 

an acceptable job of predicting the blade flapwise fatigue and the shaft torsion and bending fatigue. They also generally do a good job of predicting the mean shaft torque. 

The figures also show that the simulations tend to slightly underpredict the blade edgewise fatigue and tower bending fatigue. This discrepancy is due, in part, to discrepancies in the calculation of the rotating axes in the dynamic simulations versus the measured data. In general, however, the agreement between predicted and measured data seems quite reasonable and inspires 

confidence in the use of the simulations. 

4.3.3 Acoustic Noise Testing of the EMD Turbine 

Turbine and background acoustic noise data were collected between May 2, 2002 and May 4, 

2002. The A-weighted sound power level at 8 m/s wind speed was calculated using two methods 

for determining wind speed. The first method uses wind speed as measured on a temporary 10­

m meteorological tower upwind of the turbine. The second method derives the wind speed from 

measurements of turbine power and a power curve obtained from power performance testing. In 

the case of the present testing, the two methods yielded very similar results of, respectively, 

103.0 dBA and 103.3 dBA sound power levels. Additional acoustic testing included wind speed 

sensitivity, directivity, one-third octave spectra, and tonal analysis. Figure 43 shows a comparison of the EMD noise levels to levels historically associated with wind turbines of various sizes. 

It is significant to note that the EMD turbine exhibits noise emissions below the average for turbines of similar size. 

72
+ POC1.5/70 

+ EMD1.5/77 

Figure 43. Comparison of the EMD Noise Levels to Historical Levels 

4.3.4 Testing of Asymmetric Load Control System 

Following implementation of the ALC system into the EMD, tests were conducted to document 

the differences in blade, main shaft, and tower loads ascribed to using ALC versus normal control operation on the GE EMD1.5 wind turbine at various wind speeds. The five main objectives 

of Phase III testing are:

1. Validate computer simulations of fatigue loads during ALC operation of the 1.5 MW 

EMD turbine. 

2. Compile 10-minute datasets in sufficient quantity and with sufficient consistency to be 

used in scatter plots comparing ALC operation to ordinary operation. 

3. Provide fatigue loads reduction information that can ultimately be used to estimate the effect on overall COE. 

4. Gain knowledge of the ability of the turbine’s pitch system to handle the increased activity that arises from ALC operation. 

5. Verify the proper operation of the sensors used as inputs to the ALC algorithm. 

This test was conducted on the EMD 1.5 wind turbine during the month of June 2003. The turbine generally functioned appropriately with the ALC control active. A total of 221 ten-minute 

sets of valid data were collected. Of those, 73 sets were collected during ALC operation of the 

turbine, and 148 sets were collected during normal or ORD operation. The data sets were obtained by manually switching between normal operation and ALC operation. The procedure was

generally to switch operation between collection of 10-minute data sets in an attempt to obtain 

similar wind statistics for comparison. 

The results show that blade flap bending fatigue is reduced by approximately 12% over the range 

of wind speeds where the ALC is active. If we assume an IEC class II wind distribution, this results in approximately a 12% reduction in total fatigue. The maximum, minimum, and maximum range values for flap bending seen in the data are also reduced for ALC operation. Blade 

edge bending DELs were not affected by the control method. There is also a reduction in the 

fixed frame mainshaft bending loads as well as the main shaft torsion fatigue. The reduction of 

the blade fluctuating loads may contribute to the reduction in torsion fatigue. The results confirm 

the motivation for the ALC concept. 

73
5 Commercialization of GE NGT Technology 

GE Wind Energy commercialized the NGT Proof of Concepts (POC) turbine as the GE1.5s. GE 

Wind commercialized the NGT Engineering and Manufacturing Development (EMD) turbine as 

the GE1.5sle. As of December 31, 2003, GE had manufactured and deployed 902 GE1.5s turbines in the U.S. alone. As of April 2004, when technical work on the NGT project was completed, an additional several hundred turbines of both configurations were on order for delivery 

during 2004 and 2005. 

GE Wind has been the beneficiary of several R&D contracts from the U.S. Department of Energy during the past decade. Figure 44 shows how USDOE funds were leveraged by GE internal 

R&D funding. Over the course of the last decade, GE matched with internal resources nearly $6 

for every $1 of USDOE funding. Figure 45 shows the overall return on that investment to the 

U.S.: $98 million worth of R&D, of which DOE contributed 15%, has translated into $1.9 billion 

worth of wind turbine hardware manufactured and deployed in the U.S., with nearly four-fifths 

domestic content. 

NREL/DOE 

GE Wind Budget 

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 (1994-2003) 

GE Wind Energy's Product Development Budget 

Figure 44. Leverage of USDOE R&D Funding by GE Wind Energy 

MW 900 

800 

550kW 

750kW 

1.5MW

550kW

750kW

$1.9 Billion Total 

700

600

79% 

21% 

Foreign Content 500 Domestic 

Content 400

300

Cumulative Total/U.S. Production 200 (1994-2003) 

100 

0

'94 '95 '96 '97 '98 '99 '00 '01 '02 '03

Figure 45. Return on Investment of NREL Funds in GE Wind 

40.0 

35.0 

30.0 

25.0 

20.0 

15.0 

10.0 

5.0 

0.0 

$98 million 

NREL/DOE Share GE Wind 

15% 

85% 

NREL/DOE Share GE Wind

15%

85%

Total GE Wind Product 

Development Budget 

74
6 Conclusions 

Conclusions of the seven-year effort spent on this project include: 

• GE NGT Concept Studies identified the following concepts as those with the best chance 

for providing near-term reductions in the COE from windpower at risk levels acceptable 

to the wind energy financial community: 

o Optimized low-solidity rotor blades 

o Larger rotor enabled by sophisticated load-alleviating controls systems 

o Advanced controls systems, including: 

 Independent blade pitch to effect asymmetric load control 

 Tower top accelerometer feedback for tower damping 

 Coupling of pitch and generator torque control 

 Taller, more flexible tower. 

• Through the application of these concepts in the EMD turbine, GE has been able to shave 

the COE from windpower by 13% relative to the baseline configuration, without adjusting for inflation 

• Achieving the originally stated NGT goal of $0.025/kWh at IEC Class II sites does not 

appear to be achievable with technologies with near-term market-compatible risk levels 

• GE has identified through its NGT Concept Studies, high-risk concepts that can provide 

additional reductions in COE between 10% and 25%, making the $0.025/kWh goal 

achievable. The most important of these are: 

o Variable diameter rotor 

o Aeroelastic tailoring of rotor blades. 

75 
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1. REPORT DATE (DD-MM-YYYY)

May 2006 

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6/17/97 - 4/30/05 

5a. CONTRACT NUMBER

DE-AC36-99-GO10337 

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4. TITLE AND SUBTITLE

Advanced Wind Turbine Program Next Generation Turbine 

Development Project 

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

NREL/SR-500-38752 

5e. TASK NUMBER

WER5.3203 

6. AUTHOR(S)

GE Wind Energy, LLC 

5f. WORK UNIT NUMBER

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GE Wind Energy, LLC 

13681 Chantico Road 

Tehachapi, CA 93561 

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ZAM-7-13320-26 

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NREL 

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National Renewable Energy Laboratory 

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NREL/SR-500-38752 

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14. ABSTRACT (Maximum 200 Words)

This document reports the technical results of the Next Generation Turbine Development Project conducted by GE 

Wind Energy LLC. This project is jointly funded by GE and the U.S. Department of Energy’s National Renewable 

Energy Laboratory.The goal of this project is for DOE to assist the U.S. wind industry in exploring new concepts and 

applications of cutting-edge technology in pursuit of the specific objective of developing a wind turbine that can 

generate electricity at a levelized cost of energy of $0.025/kWh at sites with an average wind speed of 15 mph (at 10 

m height). 

15. SUBJECT TERMS

wind energy; wind turbine; next generation turbine; cost of energy benefits 

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