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 AEMC’s Reliability Panel defines power system security and reliability standards necessary to provide a reliable and secure electricity market - against which Australian Energy Market Operator (AEMO) and network businesses operate the system. The ongoing challenge is to determine the best ways to keep the power system stable as the generation mix changes and transforms in response to carbon emissions policy.
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.
A contingency event is an event that affects the power system in a way which would likely to 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.
- 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 s hort-circuiting can occur 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
They can also occur on transformers where internal insulation failure can lead to pressure build up due to:
- insufficient maintenance (oil)
- 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 bush fires or storms.
Operating in a satisfactory state
The power system is said to be operating in a satisfactory state after a single credible contingency 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.
A satisfactory operating state is defined by the
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.
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:
- ambient temperature
- amount of cooling installed
- remedial action such water sprinklers.
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 resulty 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.
Black system events
Security events are caused by sudden equipment failure. The sudden, unexpected loss of a major source of supply can cause very rapid changes in system frequency which undermines the security of the electrical system.
Networks and generators will automatically disconnect or “trip” when there is a very rapid change in frequency in order to protect equipment and personnel from harm.
AEMO maintains power system security and has overall responsibility for coordinating the restart and restoration process; and determining the fastest way to reconnect the system:
- Transmission network companies work with the distribution network providers 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 transmission business.
- AEMO liaises with nominated parties in a jurisdiction to coordinate the restoration process and if necessary the exercise of emergency services powers.
All generators and network service providers (distribution and transmission) prepare and maintain the local black system procedures which must be approved by AEMO. These procedures outline the actions that would be taken in the event of a major supply disruption. Information within the procedures allows AEMO to effectively coordinate the safe implementation of the system restoration plan. This plan is prepared by AEMO for the purpose of managing and coordinating system restoration activities during any major supply disruption.