IEEE Power & Energy Magazine - September/October 2021 - 70

In addition to the reasonably possible (credible) contingencies,
the power system can also be exposed to noncredible contingency
events. While some of these can be more severe than
credible events, their probability is considered low. A secure
operating state does not mean the grid would necessarily remain
so after a severe noncredible contingency event. In practice,
this means that load shedding may occur to prevent the system
from collapsing. Therefore, power system security arrangements
require emergency control schemes, such as underfrequency
load shedding (UFLS), as a last line of defense to prevent a cascading
outage. This is represented in the middle of Figure 1.
Abnormal conditions, such as storms or bushfires, can
increase risks to the power system by making otherwise noncredible
contingency events more likely to occur. For example,
the loss of double-circuit transmission lines, normally a rare
contingency event, is classified as noncredible. Making the
system secure for such an event would impose restrictions on
grid operation, increasing the costs of supplying customers by
constraining transmission network flows and dispatching more
ancillary services. However, during approaching storms or
bushfires, the likelihood of a double-circuit line loss is significantly
increased and has historically been " reclassified " from
noncredible to credible. In this way, the resilience of the power
system can be increased for known abnormal conditions.
Resilience of the NEM power system can be further
increased through the declaration of a " protected event. "
This is a noncredible contingency event that is somewhat
possible and likely to lead to a cascading failure, such as the
loss of a major double-circuit transmission line. To prevent
a protected event from leading to a cascading failure, the
system operator can take a portfolio approach that deploys
a mix of ex-ante steps (e.g., installation of special protection
schemes) and ex-post measures (e.g., procuring additional
ancillary services and constraints on network flows). This
is, for example, what AEMO employs to deal with situations
when loss of major interconnections and separation events
of some regions (e.g., South Australia from the rest of the
NEM) may become plausible.
Should a very rare, yet severe, contingency event occurs,
a cascading outage may not be arrested by emergency control
schemes. Parts of the power system could therefore
experience a black system event, like in South Australia in
September 2016. Therefore, the system operator must ensure
that sufficient system restart facilities are available to reenergize
the system following a major supply disruption. This
is represented on the right-hand side of Figure 1.
The decision of whether contingencies are treated as credible
and noncredible crucially defines the technical envelope
for power system operation. This implies a tradeoff between
the ongoing costs to the market for maintaining the grid in a
secure state and the benefits of potentially avoiding the loss
of load following less severe events and the reduced risk of a
major supply interruption.
The Changing Nature of the Risks
to the Operation of the Power System
Current power system security arrangements largely reflect
the risks to the systems prevalent when they were developed.
Transmission Network
Lower Inertia and Higher Impedance
System and
Network
Abnormal Conditions:
Storms/Bushfires
Multiple
Faults
Monitoring
and Control
Disturbances
Volatile
Generation
Weather Induced:
Clouds/Wind Gusts
Contingency
Size
Cyber Security Attack:
Network/Generation
DER
Ride-Through and
Under-Frequency
Load Shedding
Effectiveness
Volatile
Generation
Distribution Network Increased DERs
Market Conditions:
Low/High Prices
figure 2. Indistinct events due to uncertain interactions within the power system.
70
ieee power & energy magazine
september/october 2021
IEEE Power & Energy Magazine - September/October 2021

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