200,000 affected by power cut at Huntly

A fault in Huntly set off a series of automatic triggers that rippled across the country, cutting electricity to 200,000. Photo / APN

Technicians at Huntly worked overnight to reroute circuits around a fault lurking somewhere between the plant's generators and their connection to the national grid just outside the property boundary.

The fault yesterday set off a series of automatic triggers that rippled across the country, cutting electricity to 200,000 customers before drawing on reserves at other stations.

The outage joins other incidents in recent years when national infrastructure faltered to highly localised issues - in one case a single metal shackle that blacked out half of Auckland, in another a wayward forklift that bumped into a power circuit.

Electricity was cut to homes and businesses throughout the North Island, including Auckland and Wellington, after a fault at the Huntly power station about 12.38pm.

Reserve power is being used until the problem is fixed.

A spokesman for Genesis Energy, which operates the gas- and coal-powered station, said technicians had brought a small generator back on to the grid last night by rerouting circuits to work around the problem.

They hoped to bring up to two others online for today, the spokesman said.

Three of six generators at Huntly had been operating when they lost their connection to the switch yard outside the plant, which transfers the electricity to the national grid.

The generators immediately shut down and about 500MW of power was gone from the North Island electricity supply, leading to a widespread, controlled blackout by a process known as automatic under-frequency load shedding, or Aufls.

About 10,000 houses and businesses lost power for up to an hour in Auckland, mostly in the North Shore and Hillsborough.

Elsewhere, 95,000 Powerco customers in the Bay of Plenty, Manawatu, Wanganui and south Taranaki experienced outages, as well as 20 per cent of Unison customers in Taupo, Rotorua and the Hawkes Bay.

In the Wellington region, electricity was lost to pockets of the lower and central Hutt Valley, Petone, and suburbs including Hataitai and Karori.

However, reserve generation from other plants around the country was brought online to allow retailers to restore electricity to customers within hours.

Transpower said that by last night power had been restored to 90 per cent of customers,.

"Technicians are working at Huntly to locate the cause of the outage," the Genesis Energy spokesman said.

"Huntly remains offline at this stage but there is sufficient reserve generation to meet demand."

Both Transpower and Genesis Energy, which owns the Huntly station, were yet to determine the cause of the outage.

In June 2006, it was discovered that a problem with a simple metal shackle - called a D-shackle - caused a failure at a substation in Otahuhu, causing outages that affected more than 700,000 people.

In 2009, 280,000 homes and businesses were affected when a forklift carrying a shipping container accidentally hit a power circuit.

Two months ago, gas supplies were cut across the region following a leak in the pipeline from the Maui fields.

Auckland suffered a five-week long blackout in 1998 when a line failure caused a chain reaction leading to the failure of three other lines.