IEEE Power & Energy Magazine - September/October 2021 - 73

Partial substitutability implies that it may be possible
to use a single solution, or a combination, to achieve
the same outcome at a lower overall cost.
Complementarity also implies that each solution described
previously might reinforce each other, delivering a material
increase in overall resilience. For example, while procuring significant
additional volumes of regional ancillary services and
constraining interconnector flows will increase resilience (a
mixed bigger/avoid and bigger/survive solution), complementing
this with additional black start and system restoration services
that have been effectively modeled and tested (a mixed
bigger/recover and smarter/recover solution) will result in a
material increase in resilience. The presence of these last-resort
services could mean the system can be recovered in hours
rather than days after a HILP (assuming that the additional
ancillary services/interconnector constraints failed to prevent
a black system), significantly increasing overall resilience and
improving customer outcomes.
In addition to these concepts, a portfolio approach to
resilience should take into account that resilience solutions
can be both targeted and general. Targeted measures might
include specific physical assets (e.g., a special protection
scheme) designed to manage specific risks (e.g., noncredible
loss of a double-circuit transmission line). General measures
might include changes applied across the system (e.g., mandating
more comprehensive technical capabilities from all
generators) designed to provide a general improvement in
the resilience of the system to nonspecific risks.
Several technical studies suggest that adopting a mix
of targeted and generalized solutions may provide an optimal
resilience outcome based on managing both " known "
unknowns and " unknown " unknowns. The existing " protected
events " process in the NEM is a good example of an
existing regulatory framework that reflects these general
principles. Introduced in 2017, this process allows AEMO
to identify specific noncredible contingencies for management
and then propose mixed solutions at the lowest overall
cost. Examples of mixed solutions have included operational
actions (e.g., curtailing power transfer between regions during
abnormal conditions) coupled with investment actions
(e.g., procuring additional fast active power response services
from a battery and upgrading UFLS schemes).
Finally, resilience solutions can also provide system support
for credible events, which should be accounted for in the
relevant cost-benefit analysis. One of the challenges, therefore,
is to adopt a consistent framework for both security
and resilience solutions. In this respect, significant work is
needed to truly understand the role of new technologies in
providing security and resilience while displacing conventional
technologies. For example, frequency response from
september/october 2021
batteries or hydrogen electrolyzers might be a substitute for
primary frequency response from conventional generators in
the case of credible events. However, the fast response of
new technologies may be much more beneficial to the system
in the case of HILP events, especially under low-inertia
conditions. Capturing this kind of behavior from a technical
perspective, and then from a regulatory and market view, is
a work in progress in Australia and worldwide.
New Mechanisms Being Discussed
and Developed to Further Support
Resilience
Power system operators need new mechanisms and tools
to manage resilience that look and feel very different from
those they originally learned how to operate. This is a significant
challenge for policy makers and system operators
mainly due to the increasing uncertainties surrounding
HILP events and the related complexity of assessing associated
costs.
In this regard, the NEM is going through a significant
reform process for transition to a power system based
increasingly around inverter-based, variable, and decentralized
renewable generation. The work of the Energy Security
Board and various market bodies focuses on managing the
rapid transition of the generation fleet, growth in DERs, and
an increasingly active demand-side. This work in progress
will help strengthen the power system and deliver resilience
benefits. AEMC is also progressing several pieces of
work specifically targeted at enhancing system resilience.
This includes working with other market bodies to develop
frameworks to manage " indistinct " events.
As discussed earlier, power system risk profiles are
changing with new classes of " indistinct risk " becoming
more predominant. Unlike a distinct contingency event,
indistinct events require consideration of multiple potential
assets that will be affected and the response of the system.
Therefore, indistinct events require an assessment that recognizes
the uncertainty inherent in the event itself as well
as the uncertainty of the power system's resulting response.
Existing frameworks that only consider the probability of
a distinct event are likely to be inappropriate for managing
indistinct events.
AEMC is considering a new regulatory framework to better
manage these indistinct events. This would allow AEMO to
take actions in addition to those needed to manage a distinct,
credible contingency event when external conditions mean
the power system is likely to face increased indistinct risks.
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IEEE Power & Energy Magazine - September/October 2021

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