IEEE Power & Energy Magazine - March/April 2021 - 83

example, dynamic Bayesian belief networks are currently
being employed by AEMO to assess the requirements for
short- to medium-term reserves, including the forecasting
of several uncertain parameters, such as conventional generation availability, demand, renewable output, weather, and
market prices. Such a tool could potentially also be extended
to forecasting ramping and various flexibility requirements.
A summary of solutions being implemented or have been
proposed or discussed in Australia to deal with different system security issues is presented in Table 1.

Case Study: Australian System Cascading and
Separation Event, 25 August 2018

may cascade and split into different areas. In particular, the
ROCOF value in Queensland following the interconnector
trip was up to approximately 0.8 Hz/s. This relatively high
value is attributed to local low-inertia conditions in the
separated system due to 23% of the total generation being
nonsynchronous and consisting mostly of photovoltaic units.
Additionally, on one hand, while renewables and DERs
helped support system security by providing some over-frequency response, uncertainty in their response underscored
the importance of both increasing DERs' visibility and making the system less fragile. On the other hand, given that
the interconnector's trip was not considered a credible event,
renewables and DERs contributed (through their frequency
response) to providing system resilience to deal with a noncredible event that might have otherwise ended up in further,
high-impact cascading behavior.
Special consideration should go to the response of the
100-MW/129-MWh Hornsdale Power Reserve plant located in
South Australia, which, at the time, was the largest lithiumion battery system in the world. During the separation event,
the Hornsdale plant provided an extremely fast response for
both low and high frequency when the frequency drop in South
Australia switched to a rise following the trip of the Heywood
interconnector (Figure 5). As it can be appreciated, this is an
impressive performance in terms of the subsecond speed of
activation and ramping capability to mirror the frequency
variation by droop control and, most importantly, in terms of

march/april 2021

13:13:00

13:12:50

13:12:40

13:12:30

13:12:20

13:12:10

13:12:00

13:11:50

13:11:40

13:11:30

Frequency (Hz)

On Saturday, 25 August 2018, lightning strikes on the
transmission towers that support the two circuits of the
330-kV Queensland-New South Wales interconnector led
both circuits to trip, leaving Queensland islanded. As the
Queensland-New South Wales interconnector was transferring 865 MW from Queensland to New South Wales,
Queensland experienced over-frequency conditions while
the remainder of the NEM experienced low frequency. The
generating units in South Australia, including the Hornsdale
battery system, increased their output to restore the system
frequency, which led to a rapid rise in active power flowing
through the Heywood interconnector. The interconnector
eventually tripped due to the activation of dynamic protection mechanisms-about 8 s after the Queensland-New
South Wales interconnector trip.
At the time of separation, Heywood was transferring 430 MW
from South Australia to Victoria.
QNI and Heywood Trips-25 August 2018
This caused a frequency rise in
51
Frequency in Queensland
South Australia and a further frequency drop in New South Wales
50.5 QNI Trip
Frequency in South Australia
13:11:39 h
and Victoria, which led to the activation of multiple under-frequency
50
load-shedding schemes (around
1 GW in total) in these two states.
Frequency in Victoria and
49.5
Droop response and automatic genNew South Wales ( " Mainland " )
Heywood
eration control from synchronous
Frequency in Tasmania
Trip
generation then restored the fre49 13:11:46.901
13:11:47
quency in Victoria and New South
13:11:45
UFLS-Mainland
AUFLS-TAS
Wales, followed by the restoration
(Smelter and Residential)
(Smelter)
48.5
of loads and resynchronization of
the separated states within approximately 2 h. The event frequency
traces for the first minutes of the
Time (hh:mm:ss)
event can be seen in Figure 4.
Legend:
Although the NEM as a whole
QNI - Queensland-New South Wales Interconnector
was not, strictly speaking, operatTAS - Tasmania
ing under low-inertia conditions
UFLS - Under-Frequency Load Shedding
AUFLS - Automated Under-Frequency Load Shedding
at the time, the separation event
illustrates the complexity and,
to some extent, the fragility of a figure 4. The 25 August 2018 separation event frequencies in the different states
large interconnected system that (Source: AEMO).

ieee power & energy magazine

83
IEEE Power & Energy Magazine - March/April 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2021
