Transpower is asking generators to avoid planning further maintenance work in May and June so that it can maintain a sufficient capacity buffer during peak winter demand.

The national grid operator has highlighted two dates in mid-May when a combination of planned power station outages and transmission work could leave that margin lower than it would like, should one of the big North Island thermal plants drop out of the system at peak time.

In the unlikely event that two major generation or transmission assets were out of action, there could be potential shortfalls in early April, late May and much of June.

In order to reduce that risk, state-owned Transpower has asked generators and other participants to avoid scheduling any more maintenance during those months that could shut or constrain generation.

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Market participants should also "where possible, consider moving existing outages in this period" from existing maintenance plans, Transpower's System Operator arm said in a note to customers this week.

Transpower is charged with maintaining sufficient spare capacity in the system to ensure the lights stay on even if the largest asset on the network – such as a pole of the high-voltage Cook Strait cable, or either of the 400 MW gas-fired power stations operated by Contact Energy and Genesis Energy – were to unexpectedly trip off.

To do that, it helps coordinate the hundreds of maintenance projects needed on lines, substations, switch yards and power stations around the country annually. It provides a monthly generation balance report to highlight any potential tight spots during the following six months.

As recently as November, it had to cut back maintenance work it had started on part of the HVDC link when cold, still weather left the North Island short of generation.

Transpower had already signalled the looming tight spot in mid-May, due to generation outages and work on key transmission lines on both islands, and had cancelled one of two projects it had planned on the HVDC link that month.

In its latest generation balance report this week, that shortfall risk now extends through the rest of May, due to work Transpower plans on circuits between Clyde and Cromwell.

Chief executive Alison Andrew noted that the work Transpower has scheduled for May includes some of the HVDC maintenance not completed during the shortened November outage and an important upgrade for its substation at Clyde.

"Transpower has already been in discussions with affected parties about the Cromwell-Clyde project and have reprogrammed the work to reduce the outage from 30 to 16 days," she said.

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"The work to replace circuit breakers at the Clyde substation is essential to ensure the integrity of the assets and is reliant on bringing in experts from Europe to complete this specialised work."

Transpower's System Operator arm was now carrying out a more detailed assessment based on the updated forecasts, she said. That includes reviewing load forecasts and holding discussions with participants, including Transpower in its role as owner of the grid.

Andrew said the next generation balance report "could show a different position" as a result of that work.