21 December 2024 under-frequency event

The Electricity Authority's final determination under clause 8.61 of the Electricity Industry Participation Code 2010 (Code) is that Transpower as grid owner was the causer of the 21 December 2024 under-frequency event (UFE).

The reasons for the Authority’s determination are:

• lightning strikes caused two trips on the grid owner’s transmission lines that disconnected the Tauhara B – Wairakei circuit, and caused a double circuit fault which tripped both 220kV circuits supplying Hawke’s Bay
• the double circuit fault resulted in a loss of 391.38MW of electricity into the system and a loss of supply to Hawke’s Bay. The loss of supply to Hawke's Bay (~165MW demand) meant a net loss of generation to the remainder of the North Island of around 224MW and a drop in North Island frequency to 49.2Hz leading to the UFE
• the exception provided for in subparagraph (c) of clause 1.1 definition of “causer” in Part 1 of the Code does not apply
• no other asset was identified as potentially causing the UFE
• the System Operator and the grid owner agree that the grid owner was the causer of the UFE.

In accordance with clause 8.65 of the Code, the clearing manager will rebate the event charges to the relevant participants. The event charge calculated in the System Operator’s report to the Authority is $414,225.

Any participant who is substantially affected by this determination may dispute the determination via the Rulings Panel in accordance with clause 8.62 of the Code by 5.00pm on Wednesday 20 August 2025.

We sought feedback from generators, the grid owner and other market participants who may have been substantially affected by the under-frequency event (UFE) of 21 December 2024.

A UFE occurs when the system frequency falls below 49.25Hz because of a loss of more than 60 megawatts into the grid.

On 21 December, two faults that were likely caused by lightning strikes occurred on the network between Wairakei and Redclyffe, causing both a loss of supply to Hawke’s Bay and disconnecting generation in the region. The reduction of generation and the load demand within Hawke's Bay resulted in North Island frequency dropping to 49.2Hz.

The Electricity Authority’s draft determination is that Transpower New Zealand Ltd as the grid owner is the causer of the UFE. You can read the reasons in full in our draft determination of causer consultation paper below.