IEEE Electrification - March 2021 - 62

NREL. It was commissioned in 2013-2014, and it is used
to conduct a wide variety of grid-integration tests on
wind turbines and PV/storage inverters. Figure 12 presents a schematic of the impedance-measurement system developed at NREL around the CGI. As illustrated,
medium-voltage data-acquisition system nodes are used
to take GPS-synchronized measurements of three-phase
voltages and currents during the injection of voltage perturbations at different frequencies. These measurements

2 Gbit/s Optical Fiber
vabc

RTDS

CG1

iabc
PC
MATLAB

3.3/13.8 kV

MVDAS
Node 1

Filter
CGI Breaker

Measurement Using CHIL Simulations

MVDAS
Server

DUT Breaker
MVDAS
Node 2
DUT

Figure 12. The impedance measurement system at NREL. RTDS:
Real-Time Digital Simulator; DUT: device under test; MVDAS: mediumvoltage data acquisition system.

Magnitude (dBΩ)

-10
-20

Yp(s)

-30
-40
-50

Ycp(s)

-60
10

100
(Hz)

1,000

100
(Hz)

1,000

300

Phase (°)

200

Ycp(s)

100
0
-100
-200
10

Yp(s)

Figure 13. The sequence admittance measurements of a 1.9-MW
wind turbine at NREL.

are postprocessed using fast Fourier transform analysis
to obtain Fourier components at the perturbation, coupling, and fundamental frequencies. These Fourier components are then used to obtain the impedance
response. The impedance-measurement system at NREL
can perform measurements over a wide frequency
range-from a fraction of hertz to several kilohertz. Figure 13 displays the sequence-admittance response of a
1.9-MW wind turbine measured at NREL. The figure
shows the positive-sequence admittance, Yp(s), and the
coupling admittance, Ycp(s), of the turbine. Note that the
coupling admittance is the gain from the injected voltage
perturbation to a current response at the coupling frequency, which is given by fp-2f1, where fp is the frequency
of the injected perturbation, and f1 is the fundamental
frequency. It is evident from Figure 13 that the coupling
admittance cannot be ignored below 120 Hz. Hence, as
noted previously, the frequency-coupling effects must be
considered for the analysis of subsynchronous and nearsynchronous stability problems.

I E E E E l e c t r i f i cati o n M agaz ine / MARCH 2021

In a CHIL simulation platform, the control hardware
consists of controller interface circuits and industrial
controllers, where electrical system control algorithms
are executed. The control system receives operating
commands, processes feedback signals, and sends out
control signals-such as PWM signals and circuit
breaker commands-to the power-stage circuits simulated in a RTDS. Simulated power-stage circuits include
models for the electric grid, wind turbine generators,
power electronics converters, harmonics filters, circuit
breakers, and other passive components. Figure 14
presents a CHIL real-time simulation system at General
Electric Research. It facilitates control algorithm development and time-domain performance evaluation for
different power electronics systems. The control interface boards for different converters (shown in the figure for wind turbines, HVdc converters, and PV
inverters) are modified to interface with RTDS GTIOs.
Note that, compared to a real system, the CHIL platform introduces additional control delays because of
computational delays and I/O conversion delays, whose
effects on the simulation results must be carefully evaluated and addressed.
The impedance responses of IBRs can be calculated
from analytical models based on simplified control and
circuit models. The analytical models are valuable during the control design phase because they reveal key
design factors that dominate the system impedance
responses. However, during the test and validation
phase, when more-accurate impedance responses are
required, it becomes extremely difficult to develop the
analytical models for system-stability analysis due to
complex control algorithms and high system nonlinearity. The impedances can be measured at a grid simulator

IEEE Electrification - March 2021

Table of Contents for the Digital Edition of IEEE Electrification - March 2021