Decision on 11 July 2025 under-frequency event

The Electricity Authority Te Mana Hiko has determined that Transpower New Zealand Limited, in its role as System Operator, caused an under-frequency event on 11 July 2025.

Ahead of a planned maintenance outage at Haywards substation, the System Operator decided not to instruct the grid owner to apply required high-voltage direct current (HVDC) control settings, departing from established operating procedures. As a result, a significant drop in HVDC power transfer into the North Island occurred, causing system frequency to fall below 49.25Hz and triggering the under-frequency event.

The Authority has determined that the System Operator’s decision not to instruct the grid owner to apply the required HVDC control settings was the root cause of the event.

Although the event was resolved without widespread impact, it created a risk of a more significant power system event. This highlights the importance of following established operational procedures and ensuring robust decision-making processes when departing from those procedures.

The Authority consulted on a draft determination between December 2025 and January 2026 before reaching its final decision. The System Operator was the only party to make a submission and disagreed with our draft determination, stating the grid owner should be the causer. Read the System Operator’s submission.

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 $74,800.

Any participant who is substantially affected by this decision may dispute it by referring the matter to the Rulings Panel by 5pm, Wednesday 20 May.