IEEE Power & Energy Magazine - September/October 2021 - 47

World-leading levels of distributed energy resources have
increased the complexity of operating this power system.
Operating the NEM power system has caused several
unique operational challenges that require developing several
first-of-their-kind solutions. Each NEM region has specific
operational challenges, and a solution applied in one
region may not necessarily work for another. This article
discusses selected operational challenges experienced across
the NEM and solutions implemented to manage power system
security with increased penetration of IBRs.
In addition, this article discusses challenges related to the
impact of the reduced commitment of synchronous generators
and increased uptake of IBRs. These challenges could apply at
any given time, including when the entire network is intact and
all sources of generation are visible and controllable, or when part
of the network is an island and a large share of generation from
distributed IBRs is not visible and only partially controllable.
Increasing the commitment of synchronous generators above
a certain level provides only a marginal benefit to the grid with a
high share of IBRs, particularly in remote parts of the network,
and there is a large electrical distance between IBRs and synchronous
generators. Under these circumstances, localized solutions
provide more benefits, and such solutions are discussed.
This article also outlines other factors that increase the complexity
of power system operation. These include increasing
uptake of distributed energy resources and, in particular, distributed
photovoltaic (PV) systems and the NEM's recent experience
of several contingency events involving the loss of multiple power
system elements due to extreme weather-related events. This,
combined with aging network and generation assets, has resulted
in increased occurrences beyond the next credible contingency
for which power systems are typically planned and operated.
Such events include multiple concurrent planned outages
of network and generation assets, rapid forced withdrawal of
several larger thermal units, and severe thunderstorms and
tornados damaging several transmission towers. This article
presents examples of complexities associated with multiple
planned outages and sustained islanded operation of a normally
interconnected system with a high share of IBRs.
Insufficient Synchronous
Unit Commitment
Background
Historically, synchronous generators have been the primary
source of electricity supply for meeting demand. They have
also provided several inherent or controlled characteristics,
such as system strength (as characterized by the ability to
maintain stiff voltage magnitude and phase angle across the
system), inertia, and frequency control.
Some characteristics have been defined as ancillary services
under the umbrella of the ancillary services market, providing
further revenue streams for the generator owner. However,
such characteristics can only be provided when the generator is
dispatched to supply the demand. The revenue from ancillary
september/october 2021
services is often not enough to bring a generator online that
would otherwise opt out of bidding into the energy market due
to the availability of cheaper sources of energy generation.
In the NEM, the dispatch of synchronous generators has
been decreasing due to several factors, such as the growing
share of transmission-connected IBRs (a cheaper source of
energy), increased uptake of distributed PV systems reducing
supply demand from transmission-connected generators,
the retirement of synchronous generators, and reduced reliability
of aging synchronous generator assets. This trend has
been experienced in all five NEM regions but most notably
in South Australia (SA). This is primarily due to the region's
abundance of existing utility-scale IBRs and distributed
energy resources, and its reliance in recent years on one type
of fuel source (natural gas) for all synchronous generation.
In late 2016, SA was operated with one synchronous generator
online because of natural market operation, which prompted
actions and technical analysis to determine the absolute minimum
requirements to be online. The studies determined the
minimum number of synchronous generators, typically at least
four, required to be online at any given time. Determining the
number of synchronous generators required to maintain system
security is a nontrivial task as it depends on many factors,
including unit size, vicinity to other synchronous generators,
and electrical distances between those generators and areas of
IBR concentration. These synchronous generator combinations
need to be predetermined based on detailed power system studies
before they can be used operationally.
The remainder of this section outlines how AEMO first
determined the minimum unit commitment for SA to maintain
sufficient system strength and then applied the same methodology
to other NEM regions, considering subtle differences
between different regions and impact on unit commitment.
Determining Unit Commitment for SA
The SA power system has the highest penetration of IBRs, not just
in Australia but varguably worldwide among systems of its size.
The region covers an area 40% larger than Texas in the United
States but with only 7% of that state's population. The power
system's demand is highly variable, ranging between 300 and
3,300 MW, with a median demand of 1,400 MW. SA has one of
the highest percentages of existing IBRs worldwide. It has 2,700 MW
of grid-connected IBRs primarily located in a concentrated area
referred to as North Area, as shown in Figure 1. Operationally,
instantaneous IBR penetration has, at times, exceeded 150% of
demand. SA also has another 1,400 MW of distributed PV systems,
largely in the Adelaide metropolitan area where most of SA's
demand and synchronous generators are located. As mentioned
earlier, all of the region's existing large synchronous generators
are gas fired with a relatively high fuel cost compared to IBRs.
Despite being interconnected with other NEM regions
via a double-circuit ac line and a single-circuit dc link, SA's
sheer size (approximately 1 million km2 with a large part
uninhabitable) makes for a relatively weak power system.
Thus, a sizable portion of SA has no electrical infrastructure,
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IEEE Power & Energy Magazine - September/October 2021

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