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 Scope

The system operator has scoped a programme of work to deliver by the end of 2024 which will review selected credible event categories, processes, and methodologies. In 2017 we established a process to identify priorities for the credible event review, a similar approach has been applied to develop the 2024 scope. As each review is completed a report will be published to this page and if there is a change to an event categorisation, or methodology used to categorise an event, we will run a consultation process to gather feedback from the industry prior to finalising the outcome.

2024 Published Reports

As we publish reports in the 2024 review, the reports, industry feedback, and the system operator responses will be added below.

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.