Top Energy workers at Kaikohe substation after Thursday afternoon's "catastrophic failure". Photo: RNZ / Peter de Graaf
Far North lines company Top Energy says two massive power outages are not connected, despite affecting the same areas and occurring just six days apart.
The company is also appealing to Far North residents to conserve power this weekend while repairs continue and the crucial Kaikohe substation continues to limp along on its smallest transformer.
More than 23,000 homes and businesses across the southern part of the Far North, including the Bay of Islands, Kaikohe and South Hokianga, lost power about 5.20pm on Thursday.
A Kaikohe resident who was the first to call 111 said she heard a loud explosion and saw the doors of a substation building had blown open.
She could also see smoke or water vapour wafting out the doors as firefighters entered the building in full protective gear.
Power was restored to most areas by 11pm, an outage of more than five hours.
Just six days earlier exactly the same area lost power, from 3.50pm to about 7pm.
Top Energy chief executive Russell Shaw said the two outages, despite occurring less than a week part, were not related.
Thursday's power cut was caused by a "catastrophic failure" of a protection breaker within the Kaikohe substation.
A protection breaker was similar to a circuit breaker inside a home, but on a much larger scale.
The failure damaged part of a 33,000-volt switchboard and triggered the substation's fire suppression systems.
Shaw said the failure caused significant arcing and heat but no fire, and no widespread physical damage.
However, half the substation's 33kV switchboard had been damaged and was no longer functioning.
That meant the substation was currently running on its smallest transformer, limiting the amount of power than could be supplied.
Unfortunately, the outage occurred as the nearby Ngāwhā geothermal power station was offline for annual maintenance, so the usual back-up supply was not available.
The company was instead using diesel generators at Ōmanaia, in South Hokianga, to support the southern part of the network.
Shaw said staff were assessing the damaged switchboard, which was about 10 years old, and planning repairs and replacements.
They were also working to understand the cause of the failure and reduce the chance of it happening again.
The nearby Ngāwhā geothermal power station has been shut down for maintenance so it can't provide back-up power. Photo: Peter de Graaf
It was not yet known how long repairs would take, he said.
Meanwhile, last week's outage was caused by a tree falling onto a 33kV line to Mount Pokaka, south of Kerikeri.
The line's protection systems did not work as intended, "tripping" the substation and causing a much wider outage across the southern half of the district.
Shaw said Far Northerners could help prevent any further problems in coming days by using power carefully, especially during morning and evening peaks; not using multiple large appliances at once; and turning appliances off at the wall when not in use.
It could also take time before street lights around the southern half of the district were functioning normally again.
During both power cuts, the northern part of the Far North, including Kaitāia, Doubtless Bay, North Hokianga and the Aupōuri Peninsula, was unaffected.
The Whangārei and Kaipara districts that make up the rest of Northland were likewise unaffected.
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