11 July 2025 under-frequency event

The Electricity Authority sought feedback from participants who may be substantially affected by the under-frequency event (UFE) of 11 July 2025.

On 11 July 2025, part of the Haywards substation was to be released from service for planned maintenance. Before the outage, the System Operator decided to modify the established release procedure by omitting changes to two specific HVDC control settings. These settings are associated with the Harmonic Performance Controller, which normally manages DC power/current levels when harmonic filters are switched in or out by the Reactive Power Controller.

The System Operator informed the grid owner of its decision about an hour before the planned outage. The decision was a deviation from the System Operator’s standing temporary written instructions which specify that the System Operator issues instructions to apply Harmonic Performance Control settings. The equipment was taken offline at about 8.00am on 11 July 2025, and at about 1.04pm there was a sudden drop in HVDC power transfer into the North Island, from 178MW to 68MW, which caused the system frequency to drop to 49.24Hz. The failure to apply the Harmonic Performance Control settings was the root cause of the UFE.

The Electricity Authority’s draft determination is that Transpower New Zealand Limited, as the System Operator, is the causer of the UFE. The definition of causer was broadened in May 2025 to include all participants. Previously it was limited to generators or grid owners.

Although the UFE was resolved without widespread impact, it posed a risk of a much more significant event. This underlines the importance of following procedures and ensuring robust independent decision-making processes where there is deviation from those procedures.

We sought feedback on the draft determination of causer and the System Operator’s calculation of megawatts of power lost during the event. We will consider all submissions before a determination on the UFE is finalised.

Note: on 20 January 2026 the submission deadline was extended from 23 to 30 January 2026.