NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Next Generation Drivetrain 

Development and Test Program

Drivetrain Concepts for Wind Turbines

6th International Conference

Jonathan Keller, NREL

Bill Erdman, Cinch, LLC.

Doug Blodgett, DNV GL

Chris Halse, Romax Technology, Inc.

Dave Grider, Wolfspeed

Bremen, Germany

November 30, 2015

NREL/PR-5000-65497
2

Agenda

• Next-generation drivetrain architecture

• Drivetrain technology development and testing

o Gearbox and inverter software

o Medium voltage inverter modules

• Summary
3

Next-Generation Drivetrain (NGD) Architecture

The NGD is an integrated, medium-speed, medium-voltage drivetrain, 

featuring advances in the gearbox, generator, and power converter 

that increase efficiency, reliability, and annual energy production 

(AEP) while reducing operation and maintenance (O&M) and cost of 

energy (COE).

NREL's research on the NGD has two phases:

• Phase I - Investigated NGD benefits; found 5% AEP increase

and 13% COE decrease at 5 megawatts (MW)

• Phase II - Designed, built, and tested key technologies.

70%

75%

80%

85%

90%

95%

100%

0% 50% 100%

Drivetrain efficiency (%)

Rotor Power (%)

Next Generation 

Drivetrain

Three-stage, 

doubly fed 

drivetrain

+10%

+5.6%

NGD wind turbine concept.

Illustration by Al Hicks, NREL

Comparisons of drivetrain cost and efficiency at 5 MW.

Illustration by Al Hicks, NREL

Similarities to:

• Winergy HybridDrive

• Moventas/The Switch FusionDrive

• ZF Wind Power etc. 
4

NGD Design – Main Bearings

NGD gearbox and generator. Illustration by Josh Bauer, NREL

NGD wind turbine concept.

Illustration by Al Hicks, NREL

• Double tapered roller bearings (from WindPACT Program)

o Support rotor axial loads and moments

o Support planetary carrier

o Oil lubricated
5

NGD Design - Gearbox

• Single-stage planetary gearbox†

o Mechanical simplicity and journal bearings increase reliability

o Multiple planets, flex pins, and premium steel‡ increase capacity

NGD gearbox planetary section. Photos by Chris Halse, NREL 33353 and Jon Keller, NREL 33341

†Two-stages may result in lower drivetrain capital cost

‡Not part of Phase II design, build, and test program
6

NGD wind turbine concept.

Illustration by Al Hicks, NREL

NGD Design – Power Converter

• Medium-voltage, 3-level neutral point clamp design with 

hybrid Silicon/Silicon-Carbide (Si/SiC) modules

o Increase efficiency and energy production

o Lower temperatures increase reliability

– May reduce or eliminate tower cooling

o Decrease pendant cable size and cost

• Inverter utility control algorithms

o Increase drivetrain reliability and support voltage and frequency

Silicon Carbide diodes.

Photo by CREE

Medium-voltage hybrid module. 

Photo by Powerex
7

NGD Design - Generator

• Permanent Magnet (from WindPACT Program)

o Medium-speed and medium-voltage†

o Concentrated windings decrease manufacturing cost

o Segmented stator decreases O&M cost

Edge-wound concentrated winding. 

Illustrations by Global Energy Concepts

Uptower stator extraction.

Illustration by

Global Energy Concepts

Generator rotor.

Photo by Jon Keller, NREL 33343

Uptower rotor extraction.

Illustration by

Global Energy Concepts

†Not part of Phase II design, build and test program
8

Agenda

Next-generation drivetrain architecture

• Drivetrain technology development and testing

o Gearbox and power converter software

o Medium voltage hybrid modules

• Summary
9

Dynamometer Testing

• Gearbox journal bearing and flex pin robustness

o Validate load sharing behavior

o Determine wear in rotor start-stop and dither conditions

• Inverter utility fault control effectiveness

o Use Controllable Grid Interface (CGI) to emulate grid faults

o Validate torque oscillation reduction

Next generation drivetrain.

Photo by Jon Keller, NREL 35206

Controllable grid interface.

Photo by Mark McDade, NREL 29069
10

Flex Pin and Journal Bearing Tests

Flex pin stresses. Illustration by Romax Technology

NGD instrumentation. Illustration by Romax Technology

• Planetary loads

o Generator torque

o Ring gear tooth load (4 locations, 8 tooth gauges)

o Flex pin bending strain (4 pins)

o Gearbox vibration

• Journal bearing operations

o Oil supply pressure and temperatures

o Oil particle count and oil analysis

o Journal temperatures and wear inspection

• Dynamometer and grid conditions

o Speed, torque, etc.

o Active and reactive power

o Grid frequency

o Inverter phase voltages and currents
11

Temperature equilibrium point at higher 

torque where loaded and nonloaded sides 

maintain a temperature differential. 

Temperatures are the steady-state average 

of all four pins.

50

52

54

56

58

60

62

0 100 200 300 400 500

Temperature (°F)

Torque (kNm)

Journal Bearing Temperatures

Load Zone

Non-Load Zone

Manifold

0.0

0.2

0.4

0.6

0.8

1.0

0 100 200 300 400 500

∆Temperature (°F)

Torque (kNm)

∆T Between Load Zone and Nonload Zone

Journal Bearing Temperature

Journal Temperatures.

Illustration by Romax Technology

Manifold

Load Zone

Nonload Zone

Rated

Rated
12

Planetary Load Share 

• As torque increases, flex pins balance loads

Torque = 21 kNm Torque = 250 kNm Rated torque = 417 kNm

Measured planet pin forces 

correlate with design values, 

especially at higher torque. 

Ring Locations Ring Locations Ring Locations

Relative LoadRelative LoadRelative Load
13

Upcoming Gearbox Tests

• Start-stop tests

o 5,000 cycles from

0 to 10 RPM in 10 seconds

• Rotor dither tests

o Unlocked: 14,400 cycles

over ±5° at 1/6 Hz

o Locked: 86,400 cycles

over ±1⁄4° at 1 Hz

• Post-Test Teardown Inspection

o Spindle and journal bearing surfaces

o Sun, planet, and ring gear contact patterns

Rotor dither test setup.

Illustration by Lambert Engineering
14

Utility Faults

• Grid interconnection requirements reviewed

o Eastern and Western U.S. Interconnections, ERCOT, HECO, and PREPA

• Requirements impacting drivetrain selected for mitigation

via power converter control algorithms

o Symmetrical and asymmetrical grid fault responses

o Frequency deviation response

o Main shaft torsional mode active damping

Interconnection areas and fault types.

Illustrations by DNV KEMA
15

Asymmetrical Fault Control

• Traditionally via Positive Sequence Current (PSC) regulator

o Reference currents that are sinusoidal and balanced (BPSC)

o Results in 120 Hz power oscillations under unbalanced voltage conditions

o Power oscillations result in gearbox torque oscillations

• New Positive-Negative Sequence Current (PNSC) regulator

o Unbalanced currents reduce 120 Hz power and torque oscillations

Phase

Voltages

Generator

Torque

Time (s)

Simulation of Phase-to-Phase Fault at High Side of PadMount Transformer

Phase

Currents

BPSC – 120 Hz Oscillations

PNSC

0.25 0.30 0.35 0.40 0.45 0.50
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9 cycles

Example Asymmetric Fault Test

• 2-phase faults, PNSC damping not enabled yet

Drop of 20% voltage = ±20% torque

9 cycles

9 cycles

30 cycles

Gearbox & main shaft

natural frequency

Drop of 50% voltage = +117% torque

Drop of 80% voltage = ?? torque

IEC 61400-21, Table 1, Case VD1-3
17

Fault Control Comments and Caveats

• Fault torques highly dependent on drivetrain technology 

o Full conversion, passive rectifier (NGD topology)

– Probably least severe fault torques

– NGD measurements made for validation and commercialization purposes

o Full conversion, active rectifier

o Partial conversion, doubly-fed induction generator

– Probably most severe due to direct utility connection of stator circuit

• Implementing PNSC with non-unity power factor may be 

complicated with certain (European) standards.

Worthy of 

future study
18

Agenda

Next-generation drivetrain architecture

• Drivetrain technology development and testing

Gearbox and power converter software

o Medium voltage hybrid modules

• Summary
19

Hybrid Si/SiC Inverter Module Testing

• Manufacture 4.5kV SiC barrier diodes

• Install SiC barrier diodes in commercial Si modules 

o Hybrid diode module in clamping location provides largest 

reduction in converter switching losses

o IGBT module reduces losses to a lesser extent

• Test hybrid Si/SiC modules in 5 kV test stand

o Validate switching loss reduction

2.3 MW medium voltage module test stand.

Photo by DNV KEMA

10 mm x 8 mm 4.5 kV/40A SiC barrier diodes.

Photo by CREE

Medium-voltage hybrid module. 

Photo by Powerex
20

Switching Waveforms and Losses

Single Phase of Two-Level Inverter

1.0 PU Switching Losses 0.6 PU Switching Losses*

Si Module Si/SiC Hybrid 

Module

IL

0

1000

2000

3000

4000

5000

6000

7000

8000

Si Hybrid Si/SiC SiC

Power Loss (Watts/Cell)

Switching Losses

Conduction Losses

Q1 Gate 

Drive

1 - 3 kHz

Time

Diode Reverse

Recovery Current

Carried by Diode

Substitute SiC Diode for Si Diode Only 

IGBT Tail

Current

Switching 

Interval

sin ωt

IGBT 

Waveform 

Diode 

Waveform IL

Vge

Diode Reverse

Recovery Current

Carried by IGBT
21

Switching Waveform Results

• IGBT Turn-On, Diode Turn-Off at 1,200 Amps

Diode Reverse Recovery Current Si

Diode Reverse Recovery Current SiC
22

Inductive Load, 3-Level NPC Circuit

Test Conditions:

Vcc = ±2,500 V, Vge = +15/-7 V

Rgon = Rgoff = 2 Ω

Tcase = 85 °C

Switching Energy Results
23

Inverter Loss and Efficiency Estimates

• Switching losses reduced up to 12.6 kW with hybrid modules 

o 2.1 kW per cell for 6 cells sized for 2.3-MW inverter

• Conduction losses essentially the same

1Tomta, G., and Nielsen, R., “Analytical Equations for Three Level NPC Converters”,

9th European Power Electronics Conference Proceedings, EPE 2001.

Medium Voltage, 3-Level,

6th Gen Standard Module

Low Voltage, 2-Level,

4th Gen Standard Module

Medium Voltage, 3-Level, Hybrid Si/SiC Module

X – Inverter Efficiency Data Taken at NREL in 2007 

Inverter Losses Inverter Efficiency 1

Si Switching

SiC Switching

x x

x x

x

x

100% Corresponds to 400 A, 3.3 kV, 5 kVDC, fs = 1 kHz, Unity Power Factor, and 2.3 MW

Conduction Losses Equal
24

125 °C 100 °C

Module Reliability

• Increasing efficiency reduces inverter and module temperatures

• Reducing module temperatures increases their reliability

• Module reliability related to 3 factors:

o Junction average temperature, Tj

o Junction temperature change

over power cycle, ∆Tj

o Case temperature thermal

cycle change, ∆Tc

Module Cross Section

Higher Tj

∆T (°C)

∆Tj

∆Tc

Base Plate Solder
25

Agenda

Next-generation drivetrain architecture

• Drivetrain technology development and testing

Gearbox and power converter software

Medium voltage hybrid modules

• Summary
26

Summary

• Medium-speed, medium-voltage drivetrain developed

o Main bearing, gearbox, generator, and inverter innovations

o Paper study showed reductions in CAPEX, OPEX and COE

• Technology test program is assessing key innovations

o Gearbox flex pin load sharing and journal bearing performance

o Inverter utility fault control algorithm effectiveness

o Hybrid Si/SiC module efficiency

Next generation drivetrain. Photo by Jon Keller, NREL 35206

Jonathan Keller

jonathan.keller@nrel.gov

+1 (303) 384-7011 

NGD Video Link
27

U.S. Department of Energy 

FOA # DE-FOA-0000439

National Wind Technology Center 

(NWTC) at NREL

Funding

Prime contractor, 

testing and

COE Analysis

Technology design, 

manufacturing, 

engineering analysis 

and licensing

Gearbox and Journal 

Bearings

Acknowledgments and NGD Project Team

Hybrid Power Converter 

Modules

Utility Fault Control 

Algorithms

Interested Cost Share Partner

Romax

Technology

Miba

Vattenfall

CREE

Wolfspeed

Powerex

The Cinch

DNV GL