Decision on 21 December 2024 under-frequency event

In June, the Authority consulted on a draft determination of causer for an under-frequency event (UFE) that occurred at 2.45pm on 21 December 2024.

The UFE occurred when two 220kV circuits supplying Hawke’s Bay tripped and frequency dropped to 49.2Hz. We received one submission from Transpower as grid owner on our draft determination.

Our 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 UFE.

The reasons for the Authority’s determination are:

a) 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

b) 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

c) the exception provided for in subparagraph (c) of clause 1.1 definition of “causer” in Part 1 of the Code does not apply

d) no other asset was identified as potentially causing the UFE

e) 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.

Under-frequency event determinations