IEEE Power & Energy Magazine - September/October 2021 - 74

The system's resilience is provided in different ways to manage
the risks of most possible noncredible distinct events with only the
most severe events likely to have a black system event risk.
For example, to manage increased risk from a major storm system
crossing a region, AEMO might elect to apply more constraints
on dispatch or procure additional ancillary services.
Figure 4 summarizes the NEM's current arrangements
for distinct events and operating strategies for indistinct
events in the future. As discussed earlier, the power system is
kept in a secure state to manage the risks of discrete credible
contingency events. The system's resilience is provided in
different ways to manage the risks of most possible noncredible
distinct events with only the most severe events likely to
have a black system event risk. For example, this is achieved
through the protected events framework where specific noncredible
distinct events are managed through mixed solutions.
Resilience is also delivered through AEMO's ability
to reclassify distinct noncredible contingencies to credible
during abnormal conditions.
Possible new regulatory arrangements for managing
indistinct event risks are also summarized in Figure 4. At the
time of writing (May 2021), some of these arrangements are
being investigated by AEMC through the " indistinct events
rule change request. "
Defining the size and characteristics of indistinct events
regarded as reasonably possible and credible using probabilistic
methods.
The top left-hand box of Figure 4 describes AEMO's current
approach to the management of credible distinct events.
These contingencies are considered reasonably possible in
the surrounding conditions, and AEMO takes action to manage
them by constraining dispatch and procuring additional
ancillary services.
The bottom left-hand box represents the logical extension
to indistinct events. In this area, the system operator would
take action to operate the power system so that the system is
secure for credible indistinct events. For example, as mentioned
earlier transmission-level contingencies can induce
DER and VRE tripping or drop in outputs due to protection
relay settings and specific control strategies. This may
particularly occur in areas with reduced system strength and
inertia due to the large-scale penetration of inverter-based
technologies and renewables. However, in these cases the
potential size and exact location of output impacts from loss
from VRE and DER are uncertain.
Credible Events
Non-Credible Events
Reclassification
Discrete
Events
Secure for Single
Credible
Contingency
Protected Events
Discrete Standing Risks
Indistinct Standing Risks
Nontraditional
Indistinct
Events
Secure for Low
Impact Indistinct
Contingency
Protected Operation
Abnormal Conditions
Causing Indistinct Risks
Secure Operating State
Resilience Expected to
Avoid Black System
High Impact, Low
Probability Events With
Multiple Distinct Outages
and Indistinct Events
Noncredible Discrete Contingency
Reclassified as Credible During
Abnormal Conditions
High Impact Low
Probability Events With
Multiple Outages
Material Risk of Black
System Event
figure 4. Power system operating strategies to manage credible and noncredible events, including HILP events and different
types of indistinct events (courtesy of AEMC).
74
ieee power & energy magazine
september/october 2021

IEEE Power & Energy Magazine - September/October 2021

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