26 July 2024 under-frequency event

We're seeking 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.

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 is 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 does not accept that it was the causer.

Contact Energy does not accept that it was the causer.

We welcome submissions on the draft determination of causer to compliance@ea.govt.nz by 5pm, 13 May 2025 to inform our final determination of causer.

We also invite feedback on the system operator’s calculation of the megawatts of power lost during the event.