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
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Credible Event Review Scope.pdf (PDF 805.43 KB)Summary of Event Classifications.pdf (PDF 786.67 KB)
- Core grid busbar classifications due to thermal and voltage
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Busbar Classification July 17 - Summary of Submissions.pdf (PDF 455.1 KB)Busbar Classification - Results GZ01 Northland.pdf (PDF 1.06 MB)Busbar Classification - Results GZ02 Auckland.pdf (PDF 1.16 MB)Busbar Classification - Results GZ03 Hamilton.pdf (PDF 1.08 MB)Busbar Classification - Results GZ04 Edgecumbe.pdf (PDF 1.41 MB)Busbar Classification - Results GZ05 Hawkes Bay.pdf (PDF 781.46 KB)Busbar Classification - Results GZ06 Taranaki.pdf (PDF 1.07 MB)Busbar Classification - Results GZ07 Bunnythorpe.pdf (PDF 1.22 MB)Busbar Classification - Results GZ08 Wellington.pdf (PDF 786.1 KB)Busbar Classification - Results GZ09 Nelson.pdf (PDF 935.31 KB)Busbar Classification - Results GZ10 Christchurch.pdf (PDF 901.38 KB)Busbar Classification - Results GZ11 Canterbury.pdf (PDF 689.08 KB)Busbar Classification - Results GZ12 West Coast.pdf (PDF 875.91 KB)Busbar Classification - Results GZ13 Otago.pdf (PDF 985.06 KB)Busbar Classification - Results GZ14 Southland.pdf (PDF 1.1 MB)
- 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.
Busbar Classification for Frequency.pdf (PDF 1.67 MB)Busbar Classification for Frequency - Summary of Submissions.pdf (PDF 539.64 KB)- 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 Classification Review.pdf (PDF 1021.15 KB)Appendix 1 - North Island ICTs.pdf (PDF 3.27 MB)Appendix 2 - South Island ICTs.pdf (PDF 2.05 MB)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.
Appendix 1 - North Island ICTs - minor update 2021.pdf (PDF 1.66 MB)Appendix 2 - South Island ICTs - minor update 2021.pdf (PDF 1.07 MB)