Power system security relates to:

• the technical parameters of the power system such as voltage and frequency
• the rate at which these parameters might change
• the ability of the system to withstand faults.

The power system is secure when technical parameters such as voltage and frequency are maintained within defined limits. To maintain frequency the power system has to instantaneously balance electricity supply against demand.

The system security and reliability standards needed for a reliable and secure electricity market are defined in the National Electricity Rules and also by the AEMC’s Reliability Panel. Australian Energy Market Operator (AEMO) and network businesses operate the system in line with these standards.

The ongoing challenge is determining the best ways to keep the power system stable as the generation mix changes, with a large number of wind and solar farms, and storage including pumped hydro, set to connect in coming years while older synchronous generators are retiring.

Secure operating environment

When the system is operating within the range of acceptable limits it is considered to be secure. For frequency, the optimal operation of the system is 50 cycles per second, or 50 Hertz.

A secure power system is designed to withstand a single credible contingency event.

Contingency events

A contingency event is an event that affects the power system in a way which would likely involve the failure or sudden and unexpected removal from operational service of a generating unit or transmission element.

There are two categories of contingency events.

Credible contingency events

Credible contingency events are events that AEMO considers to:

• be reasonably possible to occur
• have the potential for a significant impact on the power system.

These include:

• the loss of single element or generator
• a single phase or phase to phase line fault.

Credible contingency events can occur on transmission and distribution lines where there is short-circuiting due to:

• ionised particles
• wind causing conductors to clash
• pollution of insulators due to salt or dirt build-up
• mechanical failure due to cracking, tower damage
• lightning.

They can also occur on transformers where internal insulation failure can lead to pressure build up due to:

• insufficient maintenance (oil)
• age
• manufacturing problems
• the power system not being satisfactory (high voltages and overloads).

Generators can also be the cause of credible contingency events due to:

• mechanical problems due to interruption in the fuel supply
• electrical insulation failure or overloading/overheating.

Non-credible contingency events

Non-credible contingency events are contingency events other than credible contingency events. These are generally considered to be events that are rare in occurrence, such as the combination of a number of credible contingency events occurring at the same time.

AEMO can re-classify non-credible events as credible when the risk of rare events more becomes likely, including during extreme weather such as bushfires or storms.

Protected events

Through its Power System Frequency Risk Review, AEMO is required to regularly and transparently assess risks to power system operation caused by events that are unlikely but would have high impacts if they were to happen. 

If AEMO believes that there are more transparent and cost-effective ways of managing any of the risks it identifies it can request that the Reliability Panel declare a risk as a ‘protected event.’ 

The Reliability Panel will then consider the net economic benefits of managing the event as a protected event. If the Panel declares a protected event, AEMO can take additional steps to proactively manage the risk.

Operating in a satisfactory state

The power system is said to be operating in a satisfactory state when:

• voltage and frequency are within standards
• current flow is within ratings
• power quality is within standards
• the system is stable
• all equipment is operating within ratings
• fault levels within capability of switch gear
• there are no safety issues related to operations.

Managing power quality

Generally power quality problems are localised to a small part of the power system. Networks businesses are responsible for managing power quality against standards, and they can impose conditions based on connection agreements.

To mitigate problems with power quality they can:

• make modifications or add additional equipment at consumer’s site
• make modification to connections or augmentations to the network
• limit transfers of energy within the power system to maintain the system in a secure state.

Thermal limits

Variation of temperature, heating and overheating of physical infrastructure within the power system can have significant impacts.

Thermal variation on components caused by

• increased power flow or loading
• ambient temperature
• wind speed and direction
• rain, snow, humidity
• cloud cover and sun intensity
• locations of individual towers (sheltered valleys etc)

can have significant impacts, therefore:

• there is a maximum allowable temperature
• allowable flow in a conductor is capped.

Thermal variation on power lines can lead to:

• sagging, which reduces the clearance between the power line and the ground and can lead to:
  • an increased risk of a fault (short circuit) as the clearance reduces
  • a safety issue for vehicles etc. under the lines
• permanent deformation (sag) resulting in reduced mechanical strength.

Thermal variation on transformers can lead to tripping, failure or an explosion. Conductor temperature depends on:

• loading
• ambient temperature
• amount of cooling installed
• remedial action such water sprinklers.

Unsatisfactory states

An unsatisfactory state within the power system can occur due to:

• a non-credible contingency
• multiple credible contingencies
• non-conformance with technical standards
• system security maintenance issues.

An unsatisfactory state within the power system can result in:

• mal-operation of computers and electronic controls
• flickering of lights
• equipment heating up
• overheating of motors and generators
• tripping of generators and other equipment
• cascading failure
• load shedding.

System Restart Standard

The System Restart Standard is set by the AEMC’s Reliability Panel.

The standard specifies the time frame and generation supply capability to be restored following a major supply disruption. The System Restart Standard is a procurement standard under which AEMO contracts System Restart Ancillary Services (SRAS) from generators.

After a power outage most generators need to get energy from the grid to start generating electricity again. If supply from the system is lost, most generators are not capable of independently restarting in the event of tripping off.

Some generators have specialised equipment that allows them to restart without external support. These generators are a backup providing dependable restart capability. In the event of a major supply disruption, contracted SRAS and any other available resources may be called on by AEMO to supply energy to restart power stations; and begin the process of restoring the power system.

System black events

A large-scale blackout of the power system is called a system black event. These events are extremely rare – the last ones were in South Australia in 2016 and before that in northern Queensland in 2009.

System black events can occur when a sudden, unexpected loss of a major source of supply causes very rapid changes in system frequency which undermines the security of the electrical system.

Generators and networks automatically disconnect or ‘trip’ when there is a very rapid change in frequency in order to protect equipment and personnel from harm. The disconnection of multiple generators can lead to cascading failures and ultimately a system black if not addressed in time by emergency measures such as load shedding.

If grid supply is lost, most generators are not capable of independently restarting. They need power from the grid to start running again.

How is the system restarted?

Following a large-scale blackout AEMO coordinates the restart and restoration process using emergency procedures set out in its system restart plan.

Some generators have specialised equipment that allows them to restart after a system black, without needing a ‘kick start‘ from the grid. These generators provide what are called system restart ancillary services (SRAS). AEMO contracts SRAS from generators throughout the power system.

The restart and restoration process involves these steps:

• AEMO calls on contracted SRAS to supply enough energy to restart power stations and begin the process of restoring the power system. AEMO may also be able to draw on power via interconnectors in neighbouring states to speed up the restart process.
• Transmission network companies work with AEMO to establish network paths to generators.
• Transmission network companies work with the distribution network providers to prepare blocks of load to be reconnected progressively.
• Distribution network providers prepare local networks to have power restored and coordinate reconnection with the transmission business.
• AEMO, the transmission and distribution network companies must coordinate the restoration process with each state’s system security coordinator. These are known as jurisdictional system security coordinators.

What is the System Restart Standard?

• The system restart standard is set by the AEMC’s Reliability Panel. It is a procurement standard under which AEMO contracts system restart ancillary services (SRAS) from generators.
• It sets out several key parameters for system restart including the maximum time within which the services are required to restore supply to other generators in electrical sub-networks to specified levels.
• The standard does not set out the level of, or time in which, supply needs to be restored to consumers. This is because restoring supply to consumers depends on many things that are beyond the scope of the standard, such as the extent of network damage that may have happened during a system black event. It would be difficult and unhelpful for AEMO to be required to estimate the time to do this and provide for it when buying SRAS.

AEMC’s role in system security and reliability

The AEMC’s system security and reliability work program is focused on developing market frameworks which allow continued take-up of new generating technologies while keeping the lights on at the least cost to consumers. Learn more about our work to help keep the energy system secure and reliable.