IEEE Power & Energy Magazine - September/October 2021 - 40

services. The challenge will be in allocating risk and cost
appropriately while enabling operator confidence and flexibility
within the system to adapt without causing inefficient
overprocurement.
A parallel option includes a unit commitment for security
or synchronous services market mechanism that enables an
operator to schedule synchronous units to minimum levels
for safe operation. The mechanism could then support additional
VRE penetration through competitive provision from
uncontracted resources. This could also be potentially supported
with a " nomogram " (a diagram that facilitates calculation
through geometrical construction), a precursor of
which is the example transfer limit advice table for strength
in South Australia (Figure 6). While not exhaustive, this
example indicates the various combination of synchronous
(gas) units that support different levels of nonsynchronous
(renewable) generation.
The computational complexity of modeling to construct
a table such as this is significant. Additional difficulty
arises from the inclusion of economic considerations to support
efficient decisions in allowing or curtailing renewable
energy. When this economic analysis can be combined with
such a table, it may provide a pathway toward a complete
nomogram to support the greater economic integration of
renewables in the short to medium term. It is not clear how
these various approaches may be married, nor is it evident
how to manage the risks and costs of over- and underprocurement
to customers via network service providers and the
system operator. The emerging capabilities of grid-forming
50 Hz
Settling
Frequency
Nadir
Stabilization Limit
Contingency Limit
inverters (see " Australia's Big Battery " ) will likely play a
part in any future mechanism, requiring review and revision
during the transition.
Interdependencies
Thermal power stations (largely coal fueled) are forecast
to retire at pace in the next two decades from the NEM
and WEM (Figure 7). A key pillar of reform is the consideration
of resource adequacy mechanisms to drive investment
in capacity to ensure that the reliability standard is
met through the transition. However, the power system will
also need investment in resources capable of meeting its
ESSs. If these requirements are not considered when investing
in new generation (or demand-side) capacity, the overall
cost of delivering secure and reliable energy to consumers is
likely to be higher. Investment in system service capability
may take the form of incremental capital expenditures to
new entrant generation, retrofitting existing generators, and
new stand-alone merchant resources with system service
capabilities. A resource adequacy mechanism (e.g., a capacity
market) could be extended to incorporate investment in
system service capabilities by placing an obligation on consumers
(or retailers) to procure additional capabilities.
These interdependencies present a significant challenge to
the overall coordination of reform and market participation
across investment horizons. Historically, grid-scale power
systems have required large investments in equipment to be
economical. In smaller systems and jurisdictions, this has
meant that a single provider may be the most economically
Normal Operating Limit
Inertial
Total System Response
Primary
Secondary
Tertiary/
Rescheduling
6 s 60 s6 min
Time
15 min
figure 5. (a) The system frequency appropriately managed after a contingency event. (b) Frequency management
mechanisms to support the restoration of system frequency following a contingency event. ROCOF: rate of change
of frequency.
40
ieee power & energy magazine
september/october 2021
Aggregate System Response (MW)
System Frequency (Hz)
ROCOF

IEEE Power & Energy Magazine - September/October 2021

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