Overview

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:

  1. Minimum fault level requirements, for the safe and secure operation of the power system; and
  2. 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.

System strength mitigation requirements for connecting parties

The System Strength Rule Change has established new requirements for connecting parties to meet, mandating a base level capability for each inverter connecting to the network.

Demand side: two new generator access standards

The System Strength Rule Change introduces two new access standards, helping reduce the impact of inverter-based resources on system strength. These new access standards:

  • Require new connecting inverter-based resources (asynchronous generating units and inverter-based loads) and market network service providers (MNSPs) to operate stably and remain connected at a minimum short circuit ratio (SCR) level of 3.0 (NER S5.2.5.15).
  • Prevent generating systems which comprise partly or fully of asynchronous generating units from including a vector shift or similar protection relay that would operate for a voltage phase angle shift less than or equal to 20 degrees, as measured at the connection point (NER S5.2.5.16).

Coordination: system strength mitigation requirement

The System Strength Rule Change introduces a new system strength mitigation requirement, moving away from the current ‘do no harm’ obligations. When a party seeks to connect to Transgrid’s network or when a generator undergoes a 5.3.9 modification, if a general system strength impact due to the connection has been identified, it has two options:

  1. Pay the System Strength Service Provider (e.g. Transgrid in NSW and ACT) the system strength charge for hosting capability, with the charge reflecting the system strength requirements of the connecting party.
  2. Self-remediate its general system strength impact, by proposing an appropriate system strength remediation scheme.

Following a connection enquiry, a System Strength Preliminary Assessment (SSPA) will be undertaken by Transgrid to determine whether the proposed connection will cause a general system strength impact. As part of the SSPA, Transgrid will provide new connections with an estimate of the general system strength impact by reference to the reduction in Available Fault Level and the System Strength Location Factor, to assist the Applicant to determine whether to remediate the general system strength impact or pay the System Strength Charge.

  • If an Applicant elects to pay the System Strength Charge in its application to connect, its submission under clause 5.3.9(b) or under clause 5.3.12(b), a Stability Assessment will be undertaken to verify the stability of the proposed connection.
  • If an Applicant elects to remediate the general system strength impact, an appropriate proposal for a system strength remediation scheme is required to be included in its application to connect, its submission under clause 5.3.9(b) or under clause 5.3.12(b)[3]. A System Strength Full Assessment will be undertaken by Transgrid to verify the general system strength impact of the proposed connection and to assess the effectiveness of the proposed remediation scheme (unless the SSPA indicates that there will be no general system strength impact).

[3] As specified in NER clause 5.3.4B(g)

Further guidance on the pre-requisites for the Full Assessment and the Stability Assessment are provided in AEMO’s System Strength Impact Assessment Guidelines.

When do these requirements commence?

System Strength Mitigation Requirements commence on the 15 March 2023. The applicable transitional arrangements are set out in clause 11.143.8 of the Rules and provide as follows:

  • If a Connection Enquiry has been submitted but a Connection Application has not been made before the commencement date of 15 March 2023 – the new System Strength Mitigation Requirements and the new access standards on the demand side will apply to the project as per the new System Strength Rule Change; or,
  • If a Connection Application has been made prior to 15 March 2023, but an offer to connect has not been received – by default, the project will come under the existing arrangements (i.e. required to comply with ‘do no harm’ and not need to meet the new access standards). However in this case proponents would have the option of electing into the new System Strength Rule Change by requesting this of the Network Service Provider.

System Strength Charge

The System Strength Charge is to be paid by connecting generators that choose not to self-remediate their system strength impact. Each component of the charge is designed to send a pricing signal to connecting parties on how their investment impacts the demand and supply of system strength. The greater the ability of proponent’s inverter-based resources to operate stability in a ‘weak’ grid with lower levels of system strength, the lower their system strength mitigation requirements are. The System Strength Charge is made up of three components:

  1. The System Strength Unit Price (SSUP) is the forward-looking average cost of the System Strength Service Provider supplying system strength at each system strength node. As outlined in Transgrid’s pricing methodology, the System Strength Unit Price is calculated by dividing the total long-run average cost of the solutions used to provide system strength by the total capacity (in MVA fault current) that those solutions provide. This results in a price in $/MVA for each node in NSW that is set by the System Strength Service Provider every 5 years, and reset at the start of the second year of a regulatory control period. View Transgrid’s System Strength Unit Prices for the financial year 2023-24. Transgrid’s System Strength Unit Prices will be reset on 15 March 2024 and will then be fixed for 5 years (subject to an annual inflation escalation) from 1 July 2024 to 30 June 2029. A summary of the methodology for calculating the System Strength Unit Price can be found here.
  2. The System Strength Locational Factor (SSL) is used to determine the relative impact on system strength at different locations within the electrical grid. The greater the electrical distance (or impedance) of a connecting generator from a node, the greater its impacts on system strength. Transgrid will calculate the System Strength Location Factor as per AEMO’s methodology outlined in their System Strength Impact Assessment Guidelines. Transgrid has published Location Factors for the current year in our TAPR map.
  3. The System Strength Quantity (SSQ) is estimated from the size of the connecting party’s plant in megawatts (MW) and its short circuit ratio (SCR) (MVA/MW) requirements. This quantity will be fixed at connection in the generator performance standards of the plant unless the plant is altered to consume less system strength. Transgrid will calculate the System Strength Quantity as per AEMO’s methodology outlined in their System Strength Impact Assessment Guidelines.

The System Strength Charge is payable in equal monthly instalments commencing from Registration Approval.

The three components of the System Strength Charge are illustrated below:

230313 SSUP image for website

Designing your plant to lower its system strength impact

The greater the ability of proponent’s inverter-based resources to operate stability in a ‘weak’ grid with lower levels of system strength, the lower their system strength mitigation requirements are. Examples of ways proponents can reduce the system strength impacts of their plant includes optimising plan design and reducing the balance of plant impedance, as well as inverter tuning and the use of grid forming inverters. If a plant is altered to reduce its system strength impact following the agreement of generator performance standards, then the amended 5.3.9 or new 5.3.12 and 5.3.13 processes must be followed.

Disclaimer

The information on this page has been prepared in good faith for information only. Transgrid gives no warranty or representation as to the accuracy, completeness, adequacy, reliability or sufficiency of the information on this page. Transgrid does not owe any reader of this page a duty of care and the information should not be relied upon. Readers will need to form their own views and should seek their own expert advice, including (without limitation) on the application and interpretation of the National Electricity Rules and other relevant laws.