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

Hardware-in-the-Loop Testing of 

Utility-Scale Wind Turbine 

Generators

Ryan Schkoda, Curtiss Fox, and Ramtin Hadidi

Clemson University

Vahan Gevorgian, Robb Wallen, and 

Scott Lambert

National Renewable Energy Laboratory 

Technical Report

NREL/TP-5000-64787

January 2016
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

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Hardware-in-the-Loop Testing 

of Utility-Scale Wind Turbine 

Generators

Ryan Schkoda, Curtiss Fox, and Ramtin Hadidi

Clemson University

Vahan Gevorgian, Robb Wallen, and 

Scott Lambert

National Renewable Energy Laboratory 

Prepared under Task No. WE15.1A07

Technical Report

NREL/TP-5000-64787

January 2016
NOTICE

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Table of Contents

Table of Contents....................................................................................................................................... iii

List of Figures ............................................................................................................................................ iii

1 Introduction........................................................................................................................................... 1

2 Value Proposition ................................................................................................................................. 2

2.1 Electrical Hardware in the Loop ................................................................................................... 2

2.2 Mechanical Hardware in the Loop ................................................................................................ 2

3 The Electrical Strategy......................................................................................................................... 3

3.1 NREL Controllable Grid Interface Configuration......................................................................... 3

3.2 Clemson Electric Grid Research, Innovation and Development Center Configuration................ 5

4 The Mechanical Strategy ..................................................................................................................... 8

5 Conclusion .......................................................................................................................................... 10

List of Figures

Figure 1. NREL’s PHIL testing setup. Illustration by Vahan Gevorgian, NREL .................................... 4

Figure 2. Example of NREL’s PHIL experiment. Illustration by Vahan Gevorgian, NREL ................... 5

Figure 3. The electrical single-line diagram of the Energy Innovation Center. Schematic by Curtiss 

Fox, Clemson University...................................................................................................................... 6

Figure 4. A typical HIL setup at eGRID. Image by Curtiss Fox, Clemson University........................... 6

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

environment .......................................................................................................................................... 8

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

test hardware. Figure by Ryan Schkoda, Clemson University ........................................................ 9
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1 Introduction

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.
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2 Value Proposition

2.1 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:

• Wind power grid compliance and ancillary services testing at the multimegawatt-level 

under controlled medium-voltage (hereafter referred to as MV per the International 

Electrical Commission) grid conventions

• The ability to test for compliance with national and international electrical standards, grid 

codes, and interconnection requirements

• Tools for advanced controls testing and validation.

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

power generation.

2.2 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.
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3 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 loading (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 grid, such as various forms of active and reactive power controls and 

fault performance, can be tested under most realistic conditions.

3.1 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.
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Figure 1. NREL’s PHIL testing setup. Illustration by Vahan Gevorgian, NREL

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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Figure 2. Example of NREL’s PHIL experiment. Illustration by Vahan Gevorgian, NREL

3.2 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 
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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.

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. Image by Curtiss Fox, Clemson University
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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.
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4 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.

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
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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. Figure by Ryan Schkoda, Clemson University

Note: Abbreviations in Figure 6 include TB = test bench, F = force, M = moment, DTA = down 

tower assembly (power electronics), and Tgen = torque, generator.
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5 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.