IEEE Power & Energy Magazine - November/December 2017 - 24

Scheduled Generation

50.2
50.1

Scheduled Generation

33.5

50.0

33.4

49.9

33.3

49.8
49.7

33.2
Semischeduled Generation

50.2
50.1

0.7

50.0

0.6
Semischeduled Generation

0.5

49.9
49.8

0.4

49.7

0.3
Nonscheduled Generation

Active Power (GW)

0.5
0.4

50.2
50.1

Nonscheduled Generation

0.3

50.0

0.2

49.9

0.1

49.8

0.0
16:20

Frequency (Hz)

Active Power (GW)

0.8

16:21

16:22

16:23

NEM Frequency

16:24
16:25
16:26
Time at 2017-02-10
Frequency Standards

16:27

16:28

16:29

Frequency (Hz)

Active Power (GW)

33.7
33.6

South Australia is one region that is pushing the limits of
what was previously thought possible. This small system-
2,895-MW peak load-is interconnected to Australia's much
larger National Electricity Market (NEM) through a series of
small interties. The system has experienced instantaneous penetration levels of 100%, and it reached an annual penetration
level of 35% in 2015-2016. In South Australia, the challenges
of accommodating very high penetration levels of wind are
spurring new emphasis on maintaining sufficient system inertia.
The precision with which thermal generation follows dispatch
signals in Australia has changed considerably in recent years,
with numerous cases of generators blocking their governors
to prevent an automatic generation control (AGC) signal from
being followed. This has resulted in a documented decrease in
frequency control. Figure 1 shows the impact of synchronous
generation (scheduled) on system frequency compared to asynchronous generation (semischeduled and nonscheduled).
The documented change in thermal power plant performance in NEM is largely driven by market design features
that value following 5-min dispatch more than responding
to frequency deviations. The declining frequency response
of NEM is driven by the frequency control market and

Frequency (Hz)

by surprise. However, recent research in India suggests that
even systems relying on coal-fired generation can extract significant flexibility from the existing fleet and transmission system to enable the integration of gigawatts of wind and solar
generation. Variable renewables such as wind and solar are no
longer being built only because of mandates; they are being
built because they make economic sense and because operators agree that modest penetration levels of wind and solar,
such as 20-30%, can be reliable, profitable, and affordable.
However, much progress to date has been a result of the
inherent flexibility that was built into the grid decades ago.
Leaders in renewables integration are efficiently making use
of the inherent flexibility in their systems and exploring new
technologies and tools to reach ever increasing levels. At
the point where renewables' penetration levels fully utilize
the grid's inherent flexibility, subsequent variable renewable
generation will face rapidly rising integration challenges due
to excessive curtailment. Using the existing bulk transmission network and building new capacity to enable sharing
among regions can provide one solution to rising curtailment.
Another approach is using energy storage that enables generation to be shifted to hours when it is needed most.

49.7
16:30

Stabilization Frequency

figure 1. System frequency and generator schedules in NEM.
24

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