IEEE Power & Energy Magazine - September/October 2021 - 48

QLD
4 × Syncon (by Mid 2021)
SA VIC
North
Area
NSW
Contribution to SA
System Strength:
Strong
Metro
Area
VIC
South
East
TAS
SA System Strength Contribution
(Relative Detailed View)
figure 1. The contribution of different NEM regions and subregions to SA system strength. QLD: Queensland; NSW: New
South Wales; VIC: Victoria; TAS: Tasmania; SA: South Australia.
and the portions that do get limited system strength support
from other regions, as Figure 1 highlights. The system's very
large size, with several hundred kilometers between the IBR
concentration areas and synchronous generators, creates a
further challenge in the SA region.
To determine minimum unit commitment from a system
security perspective, AEMO had to develop a wide-area electromagnetic
transient (EMT) model of the SA region and surrounding
areas and later apply the same approach for all other
regions. Determining adequate system strength as a function
of generation dispatch and operating conditions requires a
thorough understanding of complex interactions between
IBRs themselves and with the wider network, and these cannot
be generally simulated with phasor-domain modeling.
Stability of System
Operating Point
Unstable
The combinations of synchronous generators for maintaining
sufficient system strength correspond to a minimum
baseline level of strength required for the system as
a whole, regardless of dispatch pattern. The level of outof-merit
dispatch-referred to as " directions " -applied to
meet this baseline level is not generally sufficient to ensure
unrestricted operation of IBRs. Further directions to allow
unrestricted IBR operation would lead to a significant residual
cost, and in some instances, there may not be enough
capable synchronous generators available. The total output
of most IBRs collectively is, therefore, curtailed to a systemwide
limit to ensure power system security is maintained for
a given synchronous generating unit combination. This level
is based on EMT studies and could differ depending on the
exact combination of online synchronous generators.
Figure 2 summarizes key points from AEMO's several thousand
simulation case studies to determine SA system strength
requirements in operational timeframes. The color intensity
illustrates the extent of system stability. Darker colors indicate a
highly stable system, and paler colors show a system on the verge
of instability. As Figure 2 indicates, the higher the penetration of
IBRs, the lower the system strength will become unless it is compensated
by additional synchronous generators. This implies that
the higher the penetration of IBRs in the system, the greater the
need for sources of system strength like synchronous machines.
The figure also highlights that system strength can only be provided
by synchronous machines that are sufficiently close.
Operating
Point
Strong
Inverse of System Strength
figure 2. Factors influencing system strength.
48
ieee power & energy magazine
Weak
(Proximity to/Number of Synchronous Generators)
Dealing With Synchronous
Unit Scarcity in SA
Maintaining a sufficient number of synchronous generators at
all times has been challenging in practice, especially during
low-demand and low-price periods. Where the normal market
september/october 2021
SA
VIC
Weak/None
Interconnector Type:
ac
dc
Instantaneous Nonsynchronous
Generation (MW)

IEEE Power & Energy Magazine - September/October 2021

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