Transpower says the power emergency it declared in the North Island was due to a fault on one of the two Cook Strait cables which brings electricity up from the South Island.
It issued a grid emergency notice at 7.15am warning there was a risk of insufficient generation to meet demand in the North Island due to the a fault on the HVDC cable.
Some power was still flowing across the cable but a second fault could suddenly stop the flow of electricity, the operator said.
It was working with lines companies to manage controllable load such as hot water systems, it said.
It asked consumers to be mindful of their electricity use on Friday morning and did not rule out some customers being disconnected.
Transpower chief executive Alison Andrew told Morning Report the problem had been fixed and the system had been restored.
“We dropped about 180 megawatts of controllable load – that’s things like hot water heating – managed with the distribution companies,
“We’re now restoring all that power back so the system is back in full strength.”
She said the fault was on a piece of equipment on the receiving end of the cable, and its cause was not yet known.
“We obviously have to do do a full investigation.”
The power system has been “tight” this winter, she said, and there had been situation where it had less extra capacity than Transpower would like, she said. However she did not believe the fault was due to the extra load being brought up from the South Island in the unusually cold spring weather.
Andrews said the system had worked as it should and consumers would not have noticed any impact.
Earlier on Friday, Transpower issued a warning notice at 5.37am that there was a risk of insufficient generation and reserve offers to meet demand in the North Island. It had asked for more offers of power and moves to decrease demand if necessary.
The cold snap was lingering early on Friday with below-zero temperatures in Rotorua, which was on -3C, and Hamilton (-2C) while in the south Blenheim and Queenstown were on -1C at about 6.30am.
On Tuesday the operator said the Antarctic blast this week could set an October record for electricity demand, and though it expected the lights to stay on it would issue a notice requesting extra generation if needed.
Power systems consultant Brian Leyland told Morning Report if there had been a continued problem with the cable, the North Island could not have produced enough extra electricity for its needs in time.
He believed the system did not allow for enough spare capacity. “Under the way the market works, having generating capacity spare and idle and available for this sort of thing doesn’t make money.”
Transpower has said it had made changes since it was held responsible by the electricity watchdog for widespread outages on one of the coldest nights of last year.
Transpower liked to carry a buffer of about 200 megawatts of generation over and above what the predicted load would be, Andrews told Nine to Noon.
“At times that residual, excess generation over expected demand, has been under what we like.
“That’s when we have been talking to industry, issuing customer advisories, having industry conferences.”
In addition the market carries reserve capacity, aimed at stopping a major fault on the system turning into a ‘cascade failure’ or widespread loss of electricity load.