IEEE Electrification - September 2022 - 40

The frequency measurement, which is measured
using a PLL, is very noisy and inaccurate during the fault
due to the high voltage distortion, but both the hardware
and simulated inverters remain stable. The case with
60 MVA of GFM capacity (not shown) is very similar to the
other three cases. The hardware inverter, when in GFM
mode (blue trace), shows a more aggressive reactive
power response, resulting in a higher residual voltage at
its PCC during the fault and a faster voltage recovery after
the fault. The simulated GFM inverter shows similar
behavior at its bus (not plotted). Otherwise, the three scenarios
are similar. In summary, for this event, there are
some benefits of GFM control, but GFM control is not necessary
for system stability because the synchronous condensers
provide sufficient system strength despite the
fact that inverter-based generation comprises 100% of
online generation.
The faults in scenarios S1 and S2 (not shown) are similar
to those in S6: the system recovers quickly and remains
stable, and the mix of GFM versus GFL inverters makes
only a small difference to the system response. Similarly,
the loss-of-generation event in scenarios S1, S2, and S6 is
stable both with and without GFM inverter controls. Hence,
for these three higher-system-strength scenarios, GFM
inverters are not needed for system stability purposes
(though GFM controls do increase damping somewhat).
This aligns with the results of the PSCAD study.
Finding the Stability Boundary
Figure 5 compares the transition from scenario S3 to S3a
(i.e., the disconnection of the second-to-last synchronous
condenser) for two cases: one with all IBRs in GFL mode
(yellow trace) and one with the hardware inverter in GFM
mode (blue trace). Note that, for the case without GFM
IBRs (yellow), scenario S3 already has sustained lowamplitude
0.7-Hz voltage and frequency oscillations
before the loss of the condenser, indicating this scenario
is already marginally stable; these oscillations are not
present with the hardware inverter in GFM mode. (The
PSCAD study similarly found scenario S3 to be unviable if
all inverters were in GFL mode.) The loss of the synchronous
condenser increases the amplitude of oscillations,
Faster Local Voltage
Recovery and Some Overshoot
1
Higher Residual
Fault Voltage
0.5
61
60
59
45 67
Active Power (pu)
1
-1
45 67
Reactive Power (pu)
2
1
45 67
t (s )
89
Figure 4. The scenario S6 (no synchronous generation) fault event, with and without GFM. GFM was enabled on the hardware inverter in the
blue trace and on the simulated PV-BESS plant in the red trace. (All traces are measured at the hardware inverter's 13.2-kV PCC, as in other
figures.) p.u.: per unit.
40
IEEE Electrification Magazine / SEPTEMBER 2022
89
89
Hardware GFM
45 67
Frequency (Hz)
Simulation GFM
No GFM
89
Voltage Magnitude at 13.2 kV PCC (pu)

IEEE Electrification - September 2022

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