NREL Controllable Grid 

Interface (CGI): Overview 

of Progress and Projects

Vahan Gevorgian

Fifth International Workshop on Grid Simulator Testing

Tallahassee, Florida

November 15–16, 2018

NREL/PR-5D00-72886
NREL | 2

Status of Wind and Solar PV Capacity in U.S.

U.S. DOE Wind Vision Study Scenarios

Source: DOE

U.S. Wind Power Cumulative Installed Capacity by State

Source: American Wind 

Energy Association

U.S. Solar Power Cumulative Installed Capacity by State

Top 10 Solar States

Total U.S. Solar 

Capacity: 58.3 GW

Total U.S. Wind 

Capacity: 90 GW

Source: Solar Energy Industries Association Source: Solar Energy Industries Association
NREL | 3

Evolution of the Power System

New challenges in a modern grid: 

• Increasing levels of power electronics-based variable renewable energy (VRE)—wind and 

solar 

• More use of communications, controls, data, and information (e.g., smart grids)

• Other new technologies: electric vehicles, distributed storage, flexible loads 

• Becoming highly distributed—more complex to manage.

Current Power System Future Power Systems

• Large central, 

synchronous 

generation

• Central control

• Carbon intensive

• Highly regulated.

• More VRE

• More info.

• More 

distributed

• Less 

carbon.

Source: B. Kroposki, NREL
• Provide tools and data that enable more informed decisions 

and reduce risks on key issues that influence the future of 

the electric grid/power sector. Institutional Support

• Create grid planning tools that integrate transmission and 

distribution and system dynamics over a variety of time and 

spatial scales.

Design and Planning 

Tools

• Design and implement a new grid architecture that 

coordinates and controls millions of devices and integrates 

with energy management systems.

System Operations, 

Power Flow, and Control

• Advance low-cost sensors, analytics, and visualizations that 

enable 100% observability.

Sensing and 

Measurements

• Develop new devices to increase grid services and 

utilization and validate high levels of variable generation 

integrated systems at multiple scales.

Devices and Integrated 

System Testing

• Develop advanced security (cyber and physical) solutions 

and real-time incident response capabilities for emerging 

technologies and systems. Security and Resilience

Technology Innovation

4

DOE Grid Modernization Initiative
NREL | 5

NWTC Controllable Grid Platform

NWTC Wind Turbines

SunEdison

1 MW PV Array

Controllable Grid Interface (CGI)

for Grid and Fault Simulation

Switchgear 

Building

Xcel

Substation

1 MW / 1 MWh BESS

Controlled grid,

CGI Bus

Regular grid,

Xcel Bus

(7 MVA continuous / 40 MVA s.c.)

115 kV

13.2 kV tie-line

GE 1.5 MW

Siemens 2.3 MW

Alstom 3 MW

Gamesa 2 MW

DC

AC

DC

DC

DC

DC

AC

13.2 kV 

13.2 kV 

First Solar

430 kW PV array

GE 1.25 MW / 1.25 MWh BESS

 

Aerial view of the site AC

AC

AC

Source: NREL
6

Controllable Grid Interface

Power rating

• 7-MVA continuous

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

• 4-wire, 13.2 kV.

Possible test articles 

• Types 1, 2, 3 and 4 wind turbines

• Capable of fault testing largest Type 3 wind turbines

• PV inverters, energy storage systems

• Conventional generators

• Combinations of technologies.

Voltage control (no load THD <3%)

• Balanced and unbalanced voltage fault conditions (ZVRT and 140% 

HVRT ride-through)—independent voltage control for each phase on 

13.2-kV terminals

• Response time: 1 ms (from full voltage to zero, or from zero back to 

full voltage) 

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

magnitude modulations (0–10 Hz)—SSR conditions

• Programmable impedance (strong and weak grids) 

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

Frequency control

• Fast output frequency control (3 Hz/s) within a range of 45–65 Hz

• 50/60-Hz operation

• Can simulate frequency conditions for any type of power system 

• PHIL capable (coupled with RTDS, Opal-RT, etc.).
NREL | 7

Active Projects Involving Energy Storage

• U.S. Department of Energy (DOE) GMLC/WETO: Active power controls 

by wind power combined with energy storage systems

• DOE SETO Hybrid PV/storage plants in collaboration with First Solar

• DOE WPTO: Integrating energy storage with run-of-river hydropower 

plants

• PG&E/EPIC: Development and testing of advanced active power 

controls and grid gault performance for a battery energy storage 

system (BESS)

• DOE OE (U.S.-China CCWG): NREL-China EPRI project 

• AES: Tightly coupled PV/storage plants

• GE: Clusters of hybrid wind/solar PV/storage power plants

• Statoil: Energy storage for offshore wind power plants.
NREL | 8

NREL-PG&E Project on BESS Inertial Controls
NREL | 9

Inertial Response by Large Wind Power Plant

Results of 65 inertial response tests by 1.5 MW wind turbine generator
NREL | 10

Aggregate Inertial Response
NREL | 11

Impacts of Wind and Energy Storage Controls on 

Frequency Response
NREL | 12

Thinking Beyond Traditional Variable Generation Plants 

Traditional Plants

Plants Integrated 

with Storage

Hybrid Renewable 

Energy Plants

Single Renewable Technology Players Hybrid Technology Players

Evolution of Variable Renewable Power Plants

High potential to disrupt the game for 

single-technology players

2020

~

~

=

Synchronous 

Condensers

FACTS Devices

Addtional

Hybridization

with Reliability

Enhancing 

Technologies

Reliability+ • Dispatchability

• Energy shifiting

• Flexibility

• Scalability

• All essential reliability services

• Advanced reliability services

• All forms of APC and RPC

• Short circuit currents

• Grid strength reinforcement

• Real and synthetic inertias

• Synchronizing torque

• Grid forming/black start

• Fast response, ultra-wide 

dynamic ranges

• Resiliency, robustness

• Grid-connected and islanded 

operation 

Flexible, Dispatchable and Reliable Renewable Generation Plants
NREL | 13

Services by Multi-Technology 

(Hybrid) Plants – Dispatchable renewable plant operation

• Long-term and short-term production forecasts

• Capability to bid into day-ahead and real-time energy markets like conventional generation

• Flexibility services. 

– Ramp limiting, variability smoothing, cloud-impact mitigation

– Provision of spinning and nonspinning reserves

– AGC functionality

– Primary frequency response (programmable droop control)

– Fast frequency response

– Inertial response:

• Programmable synthetic inertia for a wide range of H constants emulated by wind generation

• Selective inertial response strategies by wind turbines.

– Reactive power/voltage control 

– Black start, resiliency services 

– Advanced controls: power system oscillations damping, phasor measurement unit measurement-based controls, 

wide-area stability services

– Stacked services

– Plant electric loss reduction, annual energy production increase

– Battery state-of-charge management

– Optimization model-predictive control strategies

– Revenue optimization for transmission- and distribution-level applications.

Hybridization Recipe
NREL | 14

Multi-Technology Plant Controls Validation Platform
NREL | 15

Dispatchable Power Plant

Main control panel

No curtailment

Reduced ramps
NREL | 16

Examples of Dispatchable Operation Demo

Turbine 1 Turbine 2

Total wind power

Turbine 2

stopped

Plant stays on schedule 

after loss of Siemens

Discharhing

Charging

Dispatchable wind power plant
NREL | 17

Reactive Power Capability Testing

Measured P-Q Capability of 1-MW/1-MWh Li-ion BESS

Comparison of Reactive Power Capabilities
NREL | 18

New NWTC Wide-Area Control Test Bed

Source: NREL
NREL | 19

Components of WAC

Test Bed

• Seven GPS-synchronized SEL PMUs installed:

– MV sides of 1.5-MW wind turbine, 1-MW BESS, 430-kW PV 

plant, dynamometer test article

– Coupled with RTDS, so they can be virtually placed at any bus 

in real-time power system model. 

• SEL phasor data concentrator (PDC)

• SEL RTAC

• PMU fiber network

• Integrated with NWTC site controller

• All in parallel with the existing 50-kHz medium-voltage DAS.
NREL | 20

Research Opportunities Enabled by WAC Test Bed

• New groundbreaking capability—does not exist elsewhere in the world!

• Develop continuous feedback control systems for real-time power modulation by inverter-coupled generation and energy 

storage that can be used for enhancing local and wide-area system stability.

• Validate various optimization problems to determine maximum system benefit options for the provision of various power 

modulation schemes for system stability. 

• Develop and test control methods that will use wind, PV, and battery inverters to provide damping for local and inter-area 

modes in the system. Such systems must be developed unique to each resource type to ensure maximum benefit without 

damaging the resource (e.g., wind turbines).

• Develop wind, PV, and battery storage transient and dynamic models with power modulation controls for conventional grid 

integration simulation software tools (PSCAD, PSLF, PSS/E) and validate these models for various grid conditions using the WAC 

test bed.

• Test NREL-developed or client-provided PMU-based WAC systems 

• Use small-signal (linear) and large-signal (nonlinear) impedance-based stability theory for adaptive control development using 

real-time impedance measurements at both the terminals of the device and remote system feedback using PMUs for wide-area 

oscillations.

PMU1

PMU2

PMUN

D

D

D

D

PDC WADC

Power System

Time stamp

Delay

Compensation

Power and Voltage 

Modulation Setpoint

Wide-area Stability 

Control Center

Participating 

PV/storage 

Time stamp plant

Setpoint

Distributed Phasor 

Measurement Units

Plant Controller

Oscillation Damping 

Contribution Example of PV storage system 

participating in WAC services

Source: NREL
NREL | 21

Impedance Characterization of Converter-Coupled 

Generation using CGI

• New automated CGI control was 

developed for measuring small- and 

large-signal impedance response of 

multi-MW-scale inverters

• Injections of voltage perturbations on 

13.2-kV bus using CGI

• Measurement system was developed to 

capture impedance response at different 

perturbation levels.

Example of measured positive-sequence impedance 

response of 1-MW BESS inverters for small-signal (0.5%) 

and large-signal (5%) voltage perturbations using CGI 

Source: NREL
NREL | 22

New Research Opportunities

• New unique capability: Small- and large-signal impedance-based 

characterization of full inverter-coupled resource (wind turbines, 

PV plants, storage systems, etc.), including MV transformers (This 

cannot be done anywhere else in the world.)

• Identification of true voltage and current-control bandwidths of 

inverters 

• New field of research: Develop and validate methods for shaping 

inverter-coupled resource impedances to reduce the resonance 

severities and mitigate oscillatory behavior in power systems. 
NREL | 23

CGI#2 Configurations

Increased capacity Back-to-back configuration

Dual voltage

CGI#2 use cases:

• Larger MW capacity

• Conducting more independent experiments

• Voltage/current source operation

• Full microgrid testing configuration

• PHIL with two independent POIs

• Dual medium-voltage levels.
• Cross-technology grid compliance and ancillary services testing at the multi-MW level under controlled 

grid conditions

• Tool to test for compliance with national and international electrical standards, grid codes, and 

interconnection requirements

• Tool for advanced controls testing and validation

• Real hardware testing combined with simulation (PHIL)

• Helps increase reliability and reduce integration cost of renewable generation and energy storage. 

Value Proposition of Grid Simulators 
www.nrel.gov

Thank you

This work was authored 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 Energy Technologies Office 

and Solar Energy Technologies Office. The views expressed in the article do not necessarily represent the views of 

the DOE or the U.S. Government. The U.S. Government retains and the publisher, by accepting the article for 

publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, irrevocable, worldwide 

license to publish or reproduce the published form of this work, or allow others to do so, for U.S. Government 

purposes.