NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

NREL and Clemson University 

Drivetrain Test Facility 

Collaboration

Cooperative Research and 

Development Final Report

CRADA Number: CRD-13-509

NREL Technical Contact: Gevorgian Vahan

CRADA Report

NREL/TP-5D00-72093

July 2018
NOTICE

This work was authored in part by the National Renewable Energy Laboratory, operated by Alliance for 

Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. 

Funding provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy Wind and 

Water Technologies Office. The views expressed in the article do not necessarily represent the views of the DOE 

or the U.S. Government.

This report is available at no cost from the National Renewable 

Energy Laboratory (NREL) at www.nrel.gov/publications. 

U.S. Department of Energy (DOE) reports produced after 1991 

and a growing number of pre-1991 documents are available 

free via www.OSTI.gov. 

Cover Photos by Dennis Schroeder: (left to right) NREL 26173, NREL 18302, NREL 19758, NREL 29642, NREL 19795.

NREL prints on paper that contains recycled content. 
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Cooperative Research and Development Final Report

In accordance with Requirements set forth in the terms of the CRADA agreement, this document 

is the final CRADA report, including a list of Subject Inventions, to be forwarded to the DOE 

Office of Science and Technical Information as part of the commitment to the public to 

demonstrate results of federally funded research.

Parties to the Agreement: Clemson University Restoration Institute

CRADA number: CRD-13-509

CRADA Title: NREL and Clemson University Drivetrain Test Facility Collaboration

Joint Work Statement Funding Table showing DOE commitment:

Estimated Costs NREL Shared Resources

a/k/a Government In-Kind

Year 1

Year 2

Year 3

$ 90,000.00

$ 85,000.00

$ 85,000.00

TOTALS $ 260,000.00

Abstract of CRADA Work:

The National Renewable Energy Laboratory (NREL) and Clemson University have a mutual 

interest in collaborating in the development of wind turbine drivetrain testing facilities and grid 

simulators. NREL and Clemson University will work together to share resources and experiences 

in the development of future wind energy test facilities and capabilities. The CRADA includes 

sharing of facility topologies and capabilities, modeling efforts, commissioning plans, test 

protocols, infrastructure cost data, test plans, pro forma contracting instruments, and safe 

operating strategies. Further, NREL and Clemson University will exchange staff for training and 

development purposes, including collaborative participation in commissioning and customer 

testing.

DOE has provided NREL with over 10 years of support in developing custom facilities and 

capabilities to enable testing of full-scale integrated wind turbine drivetrain systems in 

accordance with the needs of the US wind industry. NREL currently operates a 2.5MW 

dynamometer and is in the processes of building a 5MW dynamometer and a grid simulator 

(referred to as a “Controllable Grid Interface” or CGI). Clemson University is currently building 

a drivetrain testing facility with two dynamometers, 7.5 MW and 15 MW, and 15 MW Hardware 

in the loop grid simulator.
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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Contents

1 NREL and Clemson University Drivetrain Test Facility Collaboration............................................ 3

2 Major Accomplishments ...................................................................................................................... 3

3 Importance of PHIL Testing of Utility-Scale Wind Turbine Generators .......................................... 4

4 Value Proposition ................................................................................................................................. 4

4.1 Electrical Hardware in the Loop ................................................................................................... 4

4.2 Mechanical Hardware in the Loop ................................................................................................ 5

5 The Electrical Strategy......................................................................................................................... 5

5.1 NREL Controllable Grid Interface Configuration......................................................................... 6

5.2 Clemson Electric Grid Research, Innovation and Development Center Configuration................ 7

6 The Mechanical Strategy ..................................................................................................................... 9

7 Conclusion .......................................................................................................................................... 11
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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

1 NREL and Clemson University Drivetrain Test

Facility Collaboration

The National Renewable Energy Laboratory (NREL) and Clemson University had a mutual 

interest in collaborating in the development of drivetrain testing facilities that resulted in signing 

a multi-year CRADA agreement between to organizations in 2012. NREL and Clemson 

University worked together to share resources and experiences in the development of future wind 

energy test facilities. The CRADA included sharing of test protocols, infrastructure cost data, test 

plans, pro forma contracting instruments, and safe operating strategies. NREL and Clemson 

University also exchanged staff for training and development purposes.

Another major purpose of this CRADA was to facilitate the development of NREL and Clemson 

University drivetrain test facilities by leveraging NREL’s and Clemson University’s resources 

and experiences in this area. The joint modeling capabilities development task included:

1. Joint development of standard, flexible modeling approach for wind turbine testing 

Modeling platforms to be considered include RTDSTM (Real Time Digital Simulator) 

that solve electromagnetic transient simulations in real time and HIL (Hardware In the 

Loop) in which a mathematical expression of related dynamic systems are included in 

wind turbine testing.

2. Development of RTDS/HIL testing scenarios for various types of power grids (large 

interconnected system or island, different generation mixes, various renewable 

penetration levels, etc.)

3. Development of “larger” HIL capability by connecting both facilities via real-time link 

for multi- system testing. This may also include ESIF for multi-infrastructure HIL 

testing including distribution. Such inter-lab real-time link will create unique testing 

environment for power system level HIL testing

4. Development and validation of complete dynamometer / grid simulator models for 

conducting preliminary “virtual” tests for performance verifications, controls tune up 

and safety margin identification for each test article prior to conducting real physical 

tests.

2 Major Accomplishments

The following major task have been accomplished under this CRADA:

• Organization of four international Grid Simulator Workshops at NREL (2013), Clemson 

(2014), FSU (2015) and NREL (2017). These workshops became a major international 

venue on the topic of grid simulator testing of renewable energy technologies and energy 

storage: https://www.nrel.gov/grid/workshop-grid-simulator.html

• Number of visits and staff exchanges for training and development

• Joint testing activities and model validation efforts

• Joint conference papers and journal publications

• Joint simulation-based study to quantify the influence of abstraction on the dynamic 

response of a wind turbine nacelle
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• Final Task: Development of and demonstration PHIL technique for grid simulator testing 

of wind turbine drivetrains on dynamometers (next sections describe the 

accomplishments under this final task)

3 Importance of PHIL Testing of Utility-Scale Wind 

Turbine Generators

Historically, wind turbine prototypes were tested in the field, which was–and continues to be–a 

slow and expensive process. As a result, wind turbine dynamometer facilities were developed to 

provide a more cost-effective alternative to field testing. New turbine designs were tested and the 

design models were validated using dynamometers to drive the turbines in a controlled 

environment. Over the years, both wind turbine dynamometer testing and computer technology 

have matured and improved, and the two are now being joined to provide hardware-in-the-loop 

(HIL) testing. This type of testing uses a computer to simulate the items that are missing from a 

dynamometer test, such as grid stiffness, voltage, frequency, rotor, and hub. Furthermore, wind 

input and changing electric grid conditions can now be simulated in real time. This recent 

advance has greatly increased the utility of dynamometer testing for the development of wind 

turbine systems.

Large-scale grid integration of variable wind (and solar) generation will have a profound impact 

on grid stability and reliability. As a result, wind turbine manufacturers, plant operators, and 

utilities need to continuously adapt to evolving ancillary service markets and reliability rules and 

regulations, changing grid codes and interconnection requirements, improved operational 

practices, and so on. Under such conditions, traditional testing and certification of individual 

system components is no longer sufficient to ensure lower costs, reliability, and quality of the 

power supply. Therefore, more complex HIL testing schemes combining real power hardware 

and controllers with real-time dynamic simulation of other real-world components (e.g., wind 

rotors, power grids, market rules, and price signals) are needed to verify that wind turbine 

generators (WTGs) are capable of meeting their design goals in a safe and reliable manner.

4 Value Proposition

4.1 Electrical Hardware in the Loop

HIL grid simulator testing capability provides a great deal of value to the wind power industry 

and other renewable energy industries, including:

1. Wind power grid compliance and ancillary services testing at the multimegawattlevel under controlled medium-voltage (hereafter referred to as MV per the 

International Electrical Commission) grid conventions

2. The ability to test for compliance with national and international electrical standards, 

grid codes, and interconnection requirements

3. Tools for advanced controls testing and validation

Additionally, grid simulator testing helps increase reliability and reduce integration costs of wind 

power generation.
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4.2 Mechanical Hardware in the Loop

Mechanical HIL testing benefits the wind turbine testing community by:

• Allowing wind turbine test benches to be used for performance and structural testing, which 

is accomplished by incorporating the nacelle control system into the physical test. For 

example, modern wind turbines often use sensors to measure key structural responses, such 

as blade strain or main-shaft deflection, for the purpose of estimating mechanical loads in 

real time. In this paradigm, instrumented wind turbine subsystems are operating as 

mechanical load transducers. Measured mechanical loads are fed back to the pitch and/or 

generator control systems to reduce fatigue loading. HIL provides a highly realistic, 

controlled environment to test real-time, closed-loop, load mitigation systems.

• Providing realistic mechanical boundary conditions for the test article. The HIL system 

allows transient events (mechanical or electrical) that evolve over the course of seconds to 

take place as though the nacelle were in the field.

• Offering a more fully integrated system test beyond testing just the mechanical and electrical 

components. Incorporating the nacelle controller allows test engineers to observe nearly all 

the components of the nacelle interacting with one another in a controlled and repeatable 

environment.

5 The Electrical Strategy

For years, the importance of hardware in the loop (HIL) for dynamometer testing has been 

recognized by the testing community as introducing the impacts of “missing” turbine components 

(e.g., the blade pitch system and wind rotor dynamics) into the testing process of wind turbine 

drivetrains. The same level of understanding is being developed for the grid simulator side of 

testing as well, in which the power hardware-in-the-loop (PHIL) capability will introduce the 

impacts of “missing” electrical components from the grid side. Creating a real-time PHIL test 

system that encompasses both the mechanical and electrical systems will also allow for greater 

scalability when the dynamics of an entire wind power plant or power system can be emulated.

Historically, testing the grid compliance aspects of wind generation involved a single wind 

turbine connected to a strong power grid (International Electrotechnical Commission 61400-21 

power quality testing standard for wind turbines). But, as levels of wind penetration increase, 

new requirements for such testing have arisen. For example, a single wind turbine can no longer 

be tested in isolation from the other elements of the power system. Fortunately, PHIL-capable 

grid simulators can now be used to test the grid integration aspects of wind power.

Ongoing research work at Clemson University and the National Renewable Energy Laboratory 

(NREL) is targeted at helping the wind power industry by developing advanced PHIL testing 

methods that can address these new challenges. In particular, these methods combine a real-time, 

multimegawatt grid simulator and variable frequency drive and nontorque (NTL) systems with 

real-time dynamic simulation hardware (Real Time Digital Simulator [RTDS], Opal RT, NI PXI, 

NI cRIO) in both facilities to create a unique environment suitable for testing, under repeatable 

conditions, the full wind turbine generator drivetrain with controls and auxiliary equipment. Such 

simulations can model turbine behavior when exposed to the most severe perturbations coming 

from both grid and variable wind resource sides. In addition, suites of advanced services to the 
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grid, such as various forms of active and reactive power controls and fault performance, can be 

tested under most realistic conditions.

5.1 NREL Controllable Grid Interface Configuration

An example of NREL’s PHIL testing setup includes a dynamometer with a test article, a 

controllable grid interface, and the possibility of connecting additional power hardware (e.g., 

field turbines and energy storage) is shown in Figure 1. Additional real-time links with other 

facilities such as NREL’s Energy Systems Integration Facility and the Idaho National Laboratory 

are also available, allowing NREL to use its real-time simulation resources for more advanced 

experiments involving additional power hardware and controls.

Figure 1. NREL’s PHIL testing setup

One example of a PHIL application for wind turbine testing is being developed by NREL to 

investigate the impacts of wind active power controls and low-voltage ride-through (LVRT) 

controls on the structural loading of drivetrain components. For active power controls, NREL is 

testing new controller designs that are capable of simultaneously actively de-rating, following an 

automatic generation control (AGC) command, and providing primary frequency control (PFC). 

For LVRT controls, the focus is to evaluate the impacts of various grid code requirements on 

turbine loads. For this purpose, the NREL team is working on developing the experimental scheme 

shown in Figure 2. A real-time model of a power system consisting of conventional generators, 

wind power, a transmission network, and loads is simulated in RTDS, which provides MV voltage 

waveform set points to the controllable grid interface. At the same time, the measured MV voltage 

and current waveforms are supplied to the RTDS model for closedloop simulation. This and 

similar experimental configurations create unique PHIL platforms for testing the interactions 

between many wind turbine generator control loops (e.g., aerodynamic, electromechanical, and 

grid side) under most realistic but repeatable and fully controlled scenarios and conditions.
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This report is available at no cost from the National Renewable Energy Laboratory (NREL) at www.nrel.gov/publications.

Figure 2. Example of NREL’s PHIL experiment

5.2 Clemson Electric Grid Research, Innovation and Development 

Center Configuration

The initial motivation for HIL testing at the Duke Energy Electric Grid Research Innovation and 

Development (eGRID) Center was to provide full electrical testing for wind turbines as well as 

mechanical testing at the Wind Turbine Drive Train facility at the SCE&G Energy Innovation 

Center at the Clemson University Restoration Institute. For this purpose, the Clemson team also 

developed a one-of-a-kind solution to perform fault ride-through testing by combining a reactive 

divider network and a large power converter. This unique hybrid method of performing fault 

ride-through analysis enables the research team at eGRID to investigate the complex differences 

between the alternative methods of performing fault ride-through evaluations, ultimately 

advancing the science behind this testing.

Figure 3 shows a simplified, single-line electrical diagram of the Energy Innovation Center. 

Switchgear is built into the system for easy coupling of the wind turbine dynamometers to 

eGRID. The 15-megawatt (MW) HIL Grid Simulator can also be independently partitioned off 

and utilized for the testing of other devices in the electrical test bay area. Power amplifier units 

(PAUs) are the key components of the 15-MW Grid Simulator. The modular design of PAUs 

allows series and parallel configurations of amplifier sections to obtain desired voltage and 

power levels. The installed configuration of the power amplifier consists of eight parallel 

converters, two per unit, with an output overvoltage capability of 133% at rated current. Each 

PAU consists of two sets of four series-connected H-bridge converters per phase arranged in 

slices. Through the use of phase-shifted carrier pulse width modulation, a large degree of 

harmonic cancellation is achieved. The addition of a reactive divider network enhances the 

system’s capability to conduct fault testing and reduces the short-circuit duty of power amplifier 

units (PAUs). This electrically isolated circuit can tolerate up to 100-MVA fault duties. It also 

enables a hybrid method of performing fault ride-through testing. The reactive divider network is 

easily reconfigurable to create any symmetrical and asymmetrical fault conditions by using 

adjustable air-core reactors and resistor banks. Figure 4 shows a typical HIL setup at eGRID.
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Figure 3. The electrical single-line diagram of the Energy Innovation Center. 

Schematic by Curtiss Fox, Clemson University.

Figure 4. A typical HIL setup at eGRID.
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The PAUs use a distributed control system based on National Instrument’s RIO components with 

three major levels. The control system architecture matches the flexibility of the series-connected 

H-bridge power stage topology in the sense that each slice carries its own controller board. The 

slice controller connects to a mains controller that serves as a communication hub for the PAU 

and contactor control. The master controller is the highest and final layer of the control system 

and controls up to four PAUs in this configuration. The HIL application requires the 

coordination of additional components aside from the amplifier, so an additional control layer 

was added in the form of a National Instrument’s PXI chassis, referred to as the interface 

controller. This controller communicates with the master controller digitally using a plastic 

optical fiber physical layer on a 12-kilohertz clock. Furthermore, it provides an instantaneous 

value interface and sends a serial packet containing instantaneous voltage set points for all three 

phases to the power amplifier. These voltage values can come from either the RTDS (HIL 

application) or can be internally generated according to the loaded test protocol.

6 The Mechanical Strategy

Both the Clemson and NREL dynamometers are configured similarly from a mechanical 

perspective. All of them use an electric prime mover, a speed-reduction gearbox, and a load 

application unit (LAU) or nontorque loading (NTL) unit to apply nontorque loads at a point 

along the main shaft of the device under test (referred to as the hub point). These loads can be 

specified ahead of time or calculated in real time as part of an HIL testing scenario.

HIL testing is useful when the device under test has an influence on the loads it experiences. In 

the real world, a device may interact with its environment, thereby influencing the input coming 

from the environment. In a test environment, the effect of the device output must be calculated 

by a real-time simulation and the test equipment must be made to replicate these effects at full 

scale (see Figure 5).

Figure 5. Interaction structure of a device in the real-world environment and in an 

HIL test environment

In the case of a wind turbine, the loads experienced at the hub point depend on both the input to 

the turbine (i.e., the wind) and the control action of the turbine (i.e., the pitch, yaw, and generator 

torque). As a result, an HIL testing strategy will allow the test bench to provide loading 

conditions that are consistent with both the system input (i.e., the wind) as well as the control 

actions coming from the nacelle.
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At both the Clemson and NREL test benches the nacelle being tested is mounted to either a short 

tower or directly to the floor; the hub of the nacelle (including the blades and pitch actuation 

systems) is removed, and the nacelle is coupled to the test bench at the end of the main shaft. The 

mechanical properties of the nacelle are altered significantly by removing the hub and attaching 

the nacelle to a test bench. Not only must the effects of the now-missing hub be emulated, but the 

different mass, stiffness, and damping properties of the test bench must be compensated for as 

well.

Figure 6 shows a proposed wind turbine nacelle HIL configuration designed to account for the 

effects of the missing hub and being mounted to a test bench, and to compensate for the 

structural properties of the test bench. With this configuration, the control actuation signals for 

the blade pitch mechanisms and the yaw mechanism are intercepted and routed to a real-time 

model of a wind turbine. This model takes those control signals, along with a simulated wind 

field, and computes the loading at the interface of the nacelle. The test equipment then replicates 

9 these loads at full scale. This control loop allows the device under test to influence the loads it 

is experiencing the same way it would in the field.

Figure 6. An interaction diagram between the device under test, the computer simulation, 

and the test hardware.
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7 Conclusion

The arrival of faster computing capabilities coupled with modern wind turbine dynamometer 

testing provides a new and unprecedented opportunity for wind turbine developers. Now, testing 

time of prototype wind turbines can be significantly reduced. In parallel, design models can be 

validated and upgraded using dynamometer hardware-in-the-loop data, which are quicker and 

less expensive to collect than field-based experimental data.
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Subject Inventions Listing:

None

ROI #:

None

Report Date:

June 22, 2018

Responsible Technical Contact at Alliance/NREL:

Vahan Gevorgian

Name and Email Address of POC at Company:

Nikolaos Rigas, nrigas@clemson.edu

NREL Team:

R. Wallen, S. Lambert, J. Keller, Y. Guo, V. Gevorgian

Clemson Team:

C. Fox, R. Schkoda, R. Hadidi, A. Bibo

DOE Program Office:

Office of Energy Efficiency and Renewable Energy Wind and Water Technologies Office

This document contains NO confidential, protectable, or proprietary information.