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.

Second International Workshop 

on Grid Simulator Testing of 

Wind Turbine Drivetrains— 

Clemson University, North Charleston, 

South Carolina

Vahan Gevorgian

September 17, 2014

NREL’s Controllable Grid 

Interface for Testing 

Renewable Energy 

Technologies

NREL/PR-5D00-62998
2

First Workshop—June 2003

Workshop report: http://www.nrel.gov/docs/fy14osti/60246.pdf 

Workshop website: http://www.nrel.gov/electricity/transmission/grid_simulator_workshop.html

Image from http://upload.wikimedia.org/wikipedia/commons/

9/9e/Mollweide_projection_SW.jpg
3

U.S. Multi-MW Facilities for Grid Integration Testing of 

Renewable Energy Technologies

• U.S. Department of Energy Wind 

Program investment in world-class 

testing facilities

• Component, wind turbine, plant levels

• Key enabler for wind technology 

validation and commercialization

• Specific focus on testing ancillary 

service controls

Energy Systems Integration Facility (ESIF)

NREL

Clemson University’s SCE&G Energy Innovation 

Center and Duke Energy’s eGRID Facility

1+ MW

7 MVA

15 MW

Photos from NREL and Clemson

National Wind Technology Center (NWTC)
4

NWTC Test Site

Siemens

2.3 MW

Alstom

3 MW

GE

1.5 MW

Gamesa

2 MW

Two Research 

Turbines

650 kW 

Photovoltaic 

Array 1.1 MW

2.5-MW dynamometer

• Total of 11 MW of variable renewable generation currently at the NWTC test site

• Many small wind turbines (less than 100 kW) installed as well

• 2.5-MW and 5-MW dynamometers 

• 7-MVA controllable grid interface (CGI) for grid-compliance testing 

• Multi-megawatt energy storage testing capability under development 

5-MW dynamometer

7-MVA CGI

Photo from Dave Simms, NREL
5

CGI Main Technical Characteristics

Power rating

• 7-MVA continuous

• 39-MVA short-circuit capacity (for 2 s)

Possible test articles 

• Types 1, 2, 3, and 4 wind turbines

• Capable of fault testing world’s largest, 6.15-MW 

Type 3 wind turbine

• Photovoltaic (PV) inverters, energy storage systems

• Conventional generators

• Combinations of technologies

Voltage control (no load THD <5%)

• Balanced and unbalanced voltage fault conditions (ZVRT and 

130% HVRT)—independent voltage control in each phase 

• Long-term symmetrical voltage variations (+/- 10%) and 

voltage magnitude modulations (0 Hz to 10 Hz)—SSR

• Programmable impedance (strong and weak grids) 

• Programmable distortions (lower harmonics 3, 5, 7)

Frequency control

• Fast output frequency control (+/- 3 Hz)

• 50-Hz/60-Hz operation

• Simulate frequency response of various power systems 

• RTDS/HIL capable
6

• Installed at NWTC test site—November 2012

• Commissioning and characterization testing—end of 2013

• Row 4/turbine bus connection—FY14

• Energy storage site connection—end of 2014

NWTC’s 7-MVA CGI 

Photo from Mark McDade, NREL
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No-load L-to-L voltage No-load voltage harmonics (THD=3.4%)

0 5 10 15 20 − 30

− 20

− 10

0

10

20

30

Voltage A-B

Voltage B-C

Voltage C-A

TIME (ms)

VOLTAGE (kV)

L-to-L voltage under 2.75-MW load Voltage harmonics under 2.75-MW 

load (THD=2.5%)

CGI Voltage Waveform
8

FERC LVRT requirements (Order 661-A)

IEC 61400-21 LVRT Testing Matrix IEC 

recommended 

fault emulator

Testing to All Interconnection Requirements 

and Grid Codes
9

CGI

GE 2.75-MW Installed in NREL Dynanometer

Photo from NREL

New 5-MW dynamometer facility
10

~3MW Type 4 drivetrain

Voltage amplitude, frequency, phase 

angle, and harmonic content are 

controlled on CGI terminals

Type 4 ~3-MW Turbine Operation with CGI
11

Example Test Result: Single-Phase Fault Emulated on 

MV Terminals of 2.75-MW Wind Turbine
12

Two-Phase Fault
13

Three-Phase Fault 
14

Single-Phase Fault—Slow Recovery
15

Three-Phase Fault—Slow Recovery
16

Single Phase—130% Overvoltage
17

Two Phase—130% Overvoltage
18

Three Phase—130% Overvoltage
19

Three Phase Overvoltage—Slow Recovery 
20

Frequency Events in the Western Interconnection
21

Frequency Events in the Eastern Interconnection
22

Frequency Events in the Electric Reliability Council 

of Texas (ERCOT)
23

Frequency Response of an Island Power System
24

GE 1.5-MW Field Turbine/CGI Interconnection
25

• FY14—Testing multi-turbine controls 

coordination when operating with 

PE terminal (HVDC replica), 

NREL/DONG Energy project

FY14/15 Multi-Turbine Testing Project
26

CGI

Site 3.1

10-MW rated three-phase 

13.2-KV cable in 4" 

conduit

15'x20' concrete pad for equipment 

(e.g., customer-supplied transformer)

5-MW 

Dyno. 

New: Interconnect CGI to 

switchgear building (7-MW 

rated three-phase 

13.2-KV cable)

Switchgear building:

- Turbine bus

- Building bus

- CGI bus

New vista 

switch(es) to 

connect MW 

turbines to CGI 

or turbine bus

Existing

To DOE 1.5

To CART’s

To Alstom, 

Gamesa, 

Siemens 

(Future)

Site 4.0

251 

13.2-KV GES test pad interconnection

New array of up to six transfer (Vista) switches 

(1 ea. per GES test pad) in an outdoor-rated 

expandable cabinet. Enables each GES test pad 

to be interconnected to:

- CGI bus, or

- Turbine bus, or

- Building bus

Photo from Dave Simms, NREL, 6/6/13

Proposed Electrical and Facility Infrastructure for Grid Energy Storage (GES) Test Pads and 

Row 4 Turbine Interconnection to CGI

Notes: 

- Graphical infrastructure depiction only, not to scale—locations shown are approximate; final siting should be based on cost/ practical considerations

- GES test pads sized to house customer-supplied GES test articles (pictured) plus customer-supplied transformer and other equipment

- Translucent items depicted are optional depending on budget; plan and install as much as possible/practical anticipating future expansion

- The 5-MW Dyno. Control Room or the Site 3.1 Data Shed (partial N area) could serve as a client facility for GES test control/DAS/customer use
27

• CGI-connected tests for storage inverter LVRT testing and frequency response testing 

• Utility-connected tests in parallel with real MW-scale wind and PV resource variability

• Ideal conditions to test energy storage for frequency regulation, variability smoothing, 

and ramp-limiting applications

Focused on these 

applications

Energy Storage Testing for Ancillary Services
28

Possible tests

• MMPT algorithms

• 50-Hz/60-Hz operation

• Voltage fault tests

• Advanced testing

• Operation with

reserves

• Frequency response

• Voltage control

• Testing with energy

storage

PV Inverter Testing Concept Using NWTC CGI
29

• Highly flexible and configurable system-level multi-MW testing/demonstration platform 

• Most components in place and operational

• Energy storage testing facility to become on-line by the end of 2014

• Possible to test grid-connected generation

• Micro grids/EMS

• Combination of technologies/advanced controls

NWTC Dual-Bus Test Setup
30

• Evaluate impacts of 

wind and PV on 

distributed 

generation grid 

reliability

• Estimate the size of 

energy storage

• Control strategies

A system like this can be 

replicated at the NWTC in 

full scale for CGI testing. 

(See next slide.)

Diego Garcia Wind/PV Integration Study—30% Case 
31

NWTC CGI for Microgrid Testing
32

NWTC/ESIF Real-Time Interconnection
33

• Cross-technology grid-compliance and ancillary services testing at multi-MW level 

under controlled MV grid conditions

• Tool for renewable energy industry to test for compliance with national and 

international electrical standards, grid codes, and interconnection requirements

• Tool for advanced controls testing and validation

• Helps increase reliability and reduce integration cost of renewables generation 

CGI Value Proposition
Thank you.

Questions?