Rule Change: Completed
On 19 September 2017, the AEMC published a final rule to place an obligation on Transmission Network Service Providers (TNSPs) to maintain minimum levels of system strength.
System strength in some parts of the power system has been decreasing as conventional synchronous generators are operating less or being decommissioned. This can mean that the system strength is not sufficiently high to keep the remaining generators stable and connected to the power system following a major disturbance. The relative stability of the power system can also reduce when additional non-synchronous generators connect to the network.
The Commission has concluded that, for the following reasons, the best mechanism to meet the minimum fault level requirements associated with maintaining system security would be through provision of system strength services by TNSPs.
The framework in the final rule clearly allocates responsibility for system strength to the party who is best placed to manage the risks associated with fulfilling that responsibility – that is, the relevant TNSP. The framework enables TNSPs to identify efficient, least cost solutions that support long run efficient operation, use and investment in electricity services.
The framework in the final rule differs to what has was considered in the draft rule and is similar to that being implemented under the Managing the rate of change of power system frequency rule change request.
The final rule provides for a holistic, flexible and technologically neutral solution to issues arising from reduced system strength by requiring TNSPs to maintain system strength at the levels determined by AEMO, under a range of operating conditions specified by AEMO.
TNSPs have a holistic perspective of their network and are able to address system strength in a manner that considers the best options for the entire network, including being able to optimise between sources that can provide system strength services as well as other key services such as inertia. This should result in more efficient outcomes for consumers in the long term by minimising the potential duplication of investment.
The final rule also places an obligation on new connecting generators to 'do no harm' to the level of system strength necessary to maintain the security of the power system.
When a new generator is negotiating its connection with the relevant NSP, a system strength impact assessment will be required to be undertaken by the NSP to assess the impact of the connection of the generating system on the ability of the power system to maintain stability in accordance with the NER, and for other generating systems to maintain stable operation including following any credible contingency event or protected event.
On 8 September 2016, the AEMC initiated the rule making process for the Managing power system fault levels rule change request submitted by the South Australian Minister for Mineral Resources and Energy.
The exit of large thermal synchronous generation, together with an increasing proportion of non-synchronous generation like wind and solar, has contributed to decreases in system strength in some areas of the power system.
A reduction in system strength in certain areas of the network may mean that generators are no longer able to meet their technical performance standards and may be unable to remain connected to the system at certain times. It may also lead to voltage instability and a reduction in the effectiveness of the protection systems used by network businesses, generators and large customers. If not addressed, these effects could lead to system instability and potential major supply interruptions