ECA Director Steven Graham’s briefing on the recent events in South Australia.
What is System Security?
System security refers to maintaining the power system in a secure and safe operating state to manage the risk of major supply disruptions. It deals with
the technical parameters of the power system such as voltage, frequency, the rate at which these might change and the ability of the system to withstand
System security is different from reliability of supply which has a consumer focus and describes the likelihood of supplying all consumer needs with the
existing generation capacity and network capability.
In order to maintain the electricity system in a secure operating state, the frequency of the system must be maintained within a tight operational range.
Rapid changes in frequency or large deviations from normal operating frequency can lead to instability in the system. The ability of the power system
to resist large changes in frequency arising from the loss of a generator, transmission line or large industrial load is determined by the inertia
of the power system. Inertia is naturally provided by large spinning conventional generators that are synchronised to the frequency of the system.
Historically, most generation in the NEM has been synchronous and, as such, the inertia was provided by these generators as an unpaid by-product of
those generators being online.
How are security events managed by the interconnected system?
Generators will automatically disconnect or trip when there is a very rapid change in frequency in order to protect equipment and personnel from harm.
In the case of networks the automatic protection systems will attempt to clear the fault and reconnect either through redundant capacity in the network
or the segment on which the fault originally occurred if the fault can be cleared.
When a power station disconnects, depending on the circumstances of the disconnection and the type of generator, it may take quite some time to restart
and then reconnect to the system. In seeking to reconnect its output it has to be synchronised to the frequency of the system a bit like a car merging
into a high speed freeway traffic flow.
Where a large number of events occur across the system or where a very major event occurs on a key element of the system a cascade effect can be created.
A cascade is usually associated with a major event followed by the mal-operation of the protection system meant to manage the event. This makes the
event even bigger and puts larger stress on the remainder of the system.
What are black system events?
In an extreme event parts of the interconnected system may be disconnected from the rest of the system to prevent a spreading cascade of instability. The
national market operator (AEMO) is required to have contracts in place with market participants to provide energy to restart black systems. This has
to be conducted in a structured and incremental manner so as keep the energised elements of the system stable and operating within their technical
Black system events are extremely rare. There was one was in northern Queensland in 2009 and one New South Wales in 1964.
Under the National Electricity framework after any significant event AEMO is required to conduct an investigation and publish a report.
Who does what in a black system event?
AEMO has overall responsibility for coordinating the restart and restoration process; and determining the fastest way to reconnect the system.
Transmission network companies (in the South Australian case, Electranet) work with the distribution network providers (in South Australia SAPN) to prepare
blocks of load to be reconnected progressively.
The distribution network provider has responsibility for ensuring the local network is ready to have power restored and coordinates reconnection with the
AEMO liaises with nominated parties in a jurisdiction to coordinate the restoration process and if necessary the exercise of emergency services powers.
What happened in South Australia?
Reports indicate that as a result of the storm three transmission lines in South Australia failed due to towers being blown over.
It is also likely that some of the 8,000 lightning strikes would have caused transient faults on networks and at least one lightening strike hit a power
station which may have taken it out of action.
It appears that these events created system security issues which resulted in South Australia being disconnected from the rest of the national network
to protect the rest of the network as described above.
In these situations there is another consequence and that is as parts of the network fail and supply is attempted to be maintained through the rest of
the network an imbalance between the networks capacity to supply and the consumer demand. The loss of one segment of the network can increase current
on other segments. All segments have current limits (congestion) and the response to congestion is to shed load (in your home if you overload a circuit
the fuse blows). Sometimes overloads cause under voltages and even voltage collapse. This also causes load to be shed.
AEMO’s full preliminary report is available here.
Audio: ECA’s Associate Director, Stakeholder Engagement Craig Memery breaks down some of the technical jargon to explain what caused the statewide power
outage in South Australia.