IEEE Power & Energy Magazine - May/June 2021 - 42

Simulation Challenges for IBR
Short Circuit Analysis
The protection system design must be informed by a reliable
short circuit study of the microgrid. Conventionally, protection software packages perform short circuit studies in the
phasor domain assuming linear Thevenin equivalent short
circuit models. Unfortunately, those linearized models tend
not to closely represent the fault current behaviors of IBRs.
Recent efforts by the IEEE Power & Energy Society's Power
System Relaying and Control Committee, under Working
Group C24, have helped create phasor-domain short circuit models of IBRs, where IBRs are included in the fault
analysis as nonlinear elements defined simply by their -output
-characteristics. Though industrial programs have shown
good results with this approach on transmission systems with
limited IBR penetration, there are still many challenges with
convergence under very high penetration of IBRs and with
simulating grid-forming inverters and isolated microgrids.
IBR protection simulation packages can be categorized
into iterative and time-domain solutions. In general, to correctly represent IBRs in protection studies, the IBR model in
the protection software should be able to
✔✔ model the IBR current limitations with their nonlinearities
✔✔ include different IBR control modes (i.e., grid forming
and grid following)
✔✔ account for different IBR reactive power or voltage support control modes (e.g., constant active/reactive power

or constant power factor control modes) in terms of their
impact on current-voltage phase angles
✔✔ model the IBR's phase lock loop and droop control
characteristics
✔✔ produce proper levels of zero- and negative-sequence
fault currents
✔✔ account for the IBR's fault ride-through capability.

Microgrid Protection Solutions for IBRs
Overview of Inverter Operation Under Faults
Figure 5 shows a simulation of a grid-forming inverter in
an islanded grid with a line-to-ground fault starting at 8.0  s
and removed after 0.15 s. The voltage and current plots at the
inverter terminal illustrate that the system recovers to prefault
conditions in a reasonable timeframe. Inverter 1 is a grid-forming, 1,500-kVA, 480-V inverter with a current limit of 1.5 pu.
The current limit is shown by the light blue line in Figure 5
at a current peak of 3,827 A. Inverter 2 is a grid-following,
1,000-kVA, 480-V inverter with a current limit of 1.1 pu, or a
peak current shown in the graph of 1,871 A.
Figure 5 shows that the inverters respond according to
design, with the grid-forming inverter 1 providing unbalanced currents both before the fault for the unbalanced
load and during the single-line-to-ground fault. On the
other hand, the grid-following inverter 2 always provides
a balanced current with no negative-sequence current. The
inverters are connected with a delta/Yg 480-V/4.16-kV

4
vA
vB
vC
vpos
vneg

0

-0.5
7.95

8

8.05

8.1
8.15
Time (s)
(a)

8.2

Current (kA)

Voltage (kV)

0.5

-2
8

8.1

8.2
Time (s)
(b)

8.3

8.4

8

8.1

8.2
Time (s)
(d)

8.3

8.4

2
iA
iB
iC
Ipos
Ineg
Ilimit

0

Current (kA)

Voltage (kV)

0

-4

8.25

0.5

-0.5
7.95

2

1
0
-1
-2

8

8.05

8.1
8.15
Time (s)
(c)

8.2

8.25

figure 5. Inverter behavior during a phase A to ground fault in islanded mode. (a) Grid-forming inverter terminal
voltage, (b) grid-forming inverter output current, (c) grid-following inverter terminal voltage, and (d) grid-following
inverter output current.
42	

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