What is system strength?
System strength can broadly be described as the ability of the power system to maintain and control the voltage waveform at any given location in the power system, both during steady state operation and following a disturbance. A power system with inadequate system strength raises the risk of system instability and supply interruptions to energy consumers. In a system with low system strength:
- generators may be unable to remain connected during disturbances on the power system;
- control of the system voltage becomes more difficult; and/or
- protection systems that ensure safe operation of the network may not operate correctly.
System strength in Australia’s electricity system has traditionally been provided by synchronous generators, as an intrinsic by-product of producing energy and reserves. As thermal generators retire or change their operating patterns, and as new inverter-based resources connect to the network, new sources of system strength will be required to ensure the power system remains secure.
Transgrid’s role in providing system strength
Under the National Electricity Rules (NER), Transgrid is responsible for meeting specified levels of power system security services in NSW, including system strength, inertia and voltage control. As the System Strength Service Provider (SSSP) for NSW, Transgrid must ensure sufficient system strength services are available at all times to maintain the stability of the power system.
Under the AEMC’s 2021 final determination for the Efficient management of system strength on the power system (System Strength Rule Change), from 2 December 2025 Transgrid is responsible for delivering system strength on a forward-looking basis to the standards determined by AEMO, as set out in NER S5.1.14, including providing sufficient system strength services to meet the:
- Minimum fault level requirements, for the safe and secure operation of the power system; and
- Efficient level of system strength, to facilitate the stable voltage waveform of new inverter-based resources (asynchronous generating units and inverter-based loads) surrounding specified system strength nodes.
In December 2022 Transgrid commenced a Regulatory Investment Test for Transmission to meet system strength requirements in NSW; For more details, visit the system strength requirements in NSW page.
What is a stable voltage waveform?
For the efficient level of system strength, AEMO has specified several criteria that must be met to ensure a stable voltage waveform can be maintained (rather than specifying a required level of fault current). A stable voltage waveform is defined by four criteria:
- Voltage magnitude: the positive-sequence RMS voltage magnitude at a connection point does not violate the limits in the operational guides for the relevant network.
- Change in voltage phase angle: changes in the steady-state RMS voltage phase angle at a connection point should not be excessive following the injection or absorption of active power at a connection point.
- Voltage waveform distortion: the three-phase instantaneous voltage waveform distortion at a connection point should not exceed acceptable planning levels of voltage waveform distortion for pre- and post-contingent conditions.
- Voltage oscillations: any undamped steady-state RMS voltage oscillations anywhere in the power system should not exceed an acceptable planning threshold as agreed with AEMO.
Transgrid will provide system strength services to ensure the stable voltage waveform for an efficient quantity of new connecting generators, as per AEMO’s forecasts.