In accordance with our obligations under the Security Policy of the Policy Statement, the system operator assesses the control requirements to categorise events as extended contingent events.
In 2009 the following events were classed as extended contingent events (ECE):
- the loss of an individual 220 kV interconnecting transformer; and
- the loss of an individual 220 kV busbar, 110 kV busbar or 66 kV busbar connected to the core grid.
Following the double circuit outage at Manapouri on the 16th May 2015 the system operator reviewed its extended contingent events policies. The system operator identified the need to review the classification of interconnecting transformers and core grid bus bars as extended contingent events and initiated a project to complete the review and to operationalise a management processes if required.
While the project carries out its investigation an interim policy for the management of core grid bus bars has been adopted to ensure consistent practice by the system operator. The existing policy for management of interconnecting transformers will continue during the review. Both policies are set out below (and which have immediate effect):
- In relation to the loss of an individual 220kV interconnecting transformer:
- For planned outages, we will (in planning time) assess the potential effects of a loss of an individual interconnecting transformer occurring. If such a loss would create potential for causing cascade failure, then we will rely upon any special protection scheme in place to undertake post-event load management.
- If no special protection scheme is in place the system operator will take pre-event action; this can include:
- pre-event load management
- constraint of generation
- cancellation of a planned outage.
- In relation to loss of an individual 220 kV busbar, 110 kV busbar or 66 kV busbar connected to the core grid:
- The system operator will not (subject to the exception below) actively monitor and manage ECE risks presented by core grid busbars. The system operator will, therefore, not manage pre-contingently for the loss of a core grid bus bar during any outage, including single circuit or double circuit outages.
- However with respect to ECE risks posed by a Manapouri busbar relating to frequency, these risks will be managed during both normal (all equipment in service) and outage scenarios (consistent with the existing policy).
- ECE risks posed by a Manapouri busbar relating to voltage will not be monitored and managed during normal (all equipment in service) or outage scenarios. Therefore, the system operator will not manage pre-contingently for the loss of a bus bar during an outage (including single circuit or double circuit outages) into Manapouri for voltage collapse, as it did on 16 May 2015.
- The system operator will not seek to identify or manage potential issues arising from the loss of other core grid busbars until pre-event mitigation actions have been reviewed by the project on both operational and economic grounds.
- The interim policy will remain until the project has identified and the system operator has adopted a permanent policy. This is expected to take anywhere from 12 to 18 months.
For questions regarding the interim policy please contact Scott Avery.
For questions regarding the ECE bus policy review project please contact Justin Blass.