IEEE Power & Energy Magazine - September/October 2021 - 51

Practical Experience
Detailed simulation studies carried out for the zone indicated
sustained postdisturbance oscillations following a loss of
some critical transmission lines. Disconnecting these lines
would further weaken the link between areas of concentration
of synchronous generation in southeast Victoria and IBRs in
the West Murray Area. Considering the high impact on overall
power system security and commercial operation of impacted
IBRs, AEMO decided it was crucial to categorically determine
whether these oscillations are a true reflection of actual
plant behavior or can be attributed to modeling artifacts.
AEMO conducted several staged system tests when actual
system conditions were like the simulation condition that
indicated sustained postdisturbance oscillations. Simulation
studies confirmed that an unfaulted disconnection of some
of the critical transmission lines would be adequate to cause
those oscillations; hence, there was no need to apply an actual
fault to the system during staged tests. Consistent with simulation
studies, the actual staged tests indicated the presence
of sustained low-frequency oscillations with a dominant
frequency of 7 Hz, as shown in Figure 5, which depicts an
example network voltage when switching out a transmission
line. As the figure shows, the magnitude and frequency of
oscillations are largely in alignment between simulation and
field measurements.
Solutions Implemented
This section discusses key outcomes of detailed simulation
studies conducted and different viable options implemented
in practice for addressing the sustained low-frequency oscillations
experienced.
Reduction in the Number of Online Inverters
AEMO found that a reduction in the total MW output of
impacted IBRs would not have a significant impact on the
magnitude of oscillations experienced. For example, constraining
those IBRs to 0 MW while maintaining the same
Voltage-Solar Farm 1
1.08
1.06
1.04
1.02
1
0.98
0.96
0.94
0.92
82 82.5 83 83.5 84 84.5 85
Time (s)
SF1 Real-Time
EMT Simulation
1.015
1.013
1.011
1.009
1.007
1.005
95
95.5
SF1 Real-Time
96
Time (s)
EMT Simulation
figure 5. A comparison of measured system responses against EMT simulation results for staged disconnection of a critical
transmission line.
september/october 2021
ieee power & energy magazine
51
96.5
97
number of online inverters would only result in a marginal
reduction in the level of oscillations.
Constraining the number of online inverters to 50% of the
total installed inverters demonstrated a substantial reduction
of oscillations. The result was that the system could be operated
within its technical envelope in terms of stability and
power quality criteria. Under this scenario, each IBR could
be operated with up to 50% of its nominal power, subject to
resource availability.
Installation of Synchronous Condensers
None of the IBRs with an adverse impact had their own synchronous
condensers to enhance system strength for their stable
operation. Furthermore, no electrically close synchronous
generator was available in the area. However, a few other more
recently connected IBRs were available in the West Murray
Area with dedicated synchronous condensers. Studies confirmed
that the synchronous condensers, in addition to facilitating
the stable operation of associated IBRs, can help suppress
low-frequency oscillations for those IBRs determined as
key contributors to unacceptable voltage oscillations.
Figure 6 shows the impact of one and two synchronous condensers
associated with IBRs without an adverse impact, highlighting
that the addition of two synchronous condensers practically
eliminates unacceptable oscillations on key contributing IBRs.
Inverter Control System Tuning
AEMO's analysis determined that the original tuning of an
inverter control system for the key contributing IBRs was
another key cause of low-frequency oscillations. The original
tuning was developed without recognizing nearby IBRs
and potential adverse interactions.
AEMO recently developed wide-area EMT models of all
NEM regions and an integrated model combining all four
mainland regions (excluding Tasmania). These models are
extensively used for various purposes, including generator
connection studies, long-term planning, operational decision
Voltage-Solar Farm 1
Voltage (p.u.)
Voltage (p.u.)

IEEE Power & Energy Magazine - September/October 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2021