Event categorisation

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Under the Security Policy section of the Policy Statement, the system operator categorises credible events according to the nature of the control required to manage their effects.  This is determined periodically when we carry out a credible event review, as detailed in the Policy Statement (see  clause 13.1).  Every 5 years a programme of work is scoped and delivered to ensure that risks on the power system are categorised according to their probability, costs, and consequences and any changes are assessed.

2024 Credible Event Review

The system operator completed a programme of work during 2024 which reviewed selected credible event categories, processes, and methodologies. In 2017 we established a process to identify priorities for the credible event review, a similar approach was applied to develop the 2024 scope.

2024 Published Reports

The 2024 review, the reports, industry feedback, and the system operator responses are below.

Interconnecting transformer classifications

We reviewed and updated the methodology to remove Other as an available classification, giving those interconnecting transformers the classification of ECE instead. We also completed an assessment of interconnecting transformers at Bombay, Hamilton and Islington, assessing impacts to equipment loading and related voltage issues. Following this review, participants were invited to provide feedback.

The final report and classifications can be viewed below

HVDC classification

We reviewed our HVDC single pole risk classification and cable discharge modelling method, concluding both should remain unchanged. Following this review, participants were invited to provide feedback, none was received. 

The final reports can be viewed below.

The classification is already in effect.

Fault ride-through non-compliance

We clarified how we will consider risk management options while assessing fault ride-through dispensation requests. For most participants, there will be no impact from this report.

The final report can be viewed below along with a summary of industry feedback.

110 kV busbar classifications

We reviewed our 110 kV busbar fault levels and classifications, concluding an update to the classifications was not warranted. 

The final report can be viewed below.

The classification is already in effect.

Historical Reports

Credible event review for the 2017-2019
Core grid busbar classifications due to thermal and voltage
Core grid busbar classifications due to frequency

We completed an assessment and classification of all busbars connected to the core grid assessing impacts to frequency. The final report and classifications can be viewed below. The categorisations classifications came into effect from 13 January 2019.

Interconnecting transformer classifications

We completed an assessment of all core interconnecting transformers, assessing impacts to equipment loading and related voltage issues. Following this review, participants were invited to provide feedback. 

The final report and classifications can be viewed below along with a summary of industry feedback.

The classifications came into effect on 1 October 2018.

Interconnecting transformer classifications – minor update 2021

Updated versions of the existing 2018 Interconnecting Transformer Classification reports are provided below. Changes to the original 2018 assessments are highlighted in yellow in the reports. 
The following changes have occurred:

  • SFD T9 now replaces NPL T8
  • A 50 Mvar reactor has been installed at KIK, impacting the light load calculation
  • The OTA 110 kV bus is no longer split
  • Correction of error in the costs KIK light load assessment

These classifications came into effect on 1 January 2022.