26 July 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 New Zealand Limited, as grid owner, was the causer of the under-frequency event (UFE) at 7.21am on 26 July 2024.

The Authority’s reasons for the determination are:

• the interruption of energy injected into the grid occurred at Clyde power station units 2 and 3, which belong to Contact Energy Limited (Contact)
• Contact and Transpower as grid owner share use of the same direct current (d.c.) secondary power supply at the Clyde generation station and grid connection point
• the Contact generation assets tripped due to a 125V d.c. earth fault on a grid owner’s disconnector located at the Clyde grid connection point. This introduced an incorrect protection signal that triggered Contact’s 220kV primary protection system and tripped Clyde generating units 2 and 3
• in the system operator’s view, Transpower as grid owner is the causer of the under-frequency event.
• the grid owner accepts, based on the strict definition of ‘causer’ in the Code, that it is the causer of this under frequency event
• the Authority did not receive any feedback that disputed its draft determination.

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 $213,750.

Any dispute of this final determination can be lodged with the Rulings Panel by 5pm 22 September 2025.

This consultation sought feedback from generators, the grid owner and other participants who may have been substantially affected by an under-frequency event on 26 July 2024 and our draft determination.

It sought feedback on the draft determination of causer and the system operator’s calculation of the megawatts of power lost during the event.

At 7:21am on 26 July 2024, an under-frequency event occurred when 231MW of generation injection was interrupted or lost at the Clyde grid connection point. This resulted in a drop in frequency to 49.14Hz in the South Island.

Our draft determination is that Transpower as grid owner was the causer of this under-frequency event because:

• the interruption of energy injected into the grid occurred at Clyde power station units 2 and 3, which belong to Contact Energy Limited (Contact)
• Contact and Transpower as grid owner share use of the same direct current (d.c.) secondary power supply at the Clyde generation station and grid connection point
• the Contact generation assets tripped due to a 125V d.c. earth fault on a grid owner’s disconnector located at the Clyde grid connection point. This introduced an incorrect protection signal that triggered Contact’s 220kV primary protection system and tripped Clyde generating units 2 and 3
• in the system operator’s view, Transpower as grid owner is the causer of the under-frequency event.

Transpower did not accept that it was the causer.

Contact Energy did not accept that it was the causer.