Third big fault in national grid this year

Transpower should know today how long a critical part of the interisland link will be out of action after the third major fault in the national grid this year.

Gear at Benmore failed on Monday night, causing a transformer to trip out and leaving the country reliant on brittle 44-year-old backup equipment to shift power between the two islands for up to a week.

North Island generators are running normally and the island is well supplied but Transpower's chief executive Patrick Strange said if there was a serious generation equipment failure the position would get more critical.

"If we lost major plant then we'd be tighter but probably still OK. It's removed one line of defence. Of course we get nervous."

Hydro generators in the South Island have excess water after torrential rain and are spilling water from dams.

In two weeks during February, a transformer failure cut power to more than 70,000 Auckland consumers and high voltage lines fell on to houses in the city, trapping residents.

Energy Minister Gerry Brownlee yesterday used the latest breakdown to highlight the fragility of the grid and take a swipe at underinvestment by the previous government.

"What this incident highlights is the continuing concern I have about the state of our transmission network and how fragile security of supply is," he said.

"If this incident happened at the height of winter there would have been potentially significant consequences for industrial and domestic users."

Transpower is spending $672 million on replacing the 1960s vintage Pole 1 - a collection of switchgear and valves at Benmore and Haywards Hill in the Hutt Valley at either end of high-voltage overhead lines in the South Island and the Cook Strait cable.

Power historically flows from south to north although during winter shortages during the past few years this has been reversed.

Dr Strange said Pole 2 which was knocked out on Monday was commissioned in 1991.

"This is nothing to do with age or maintenance it's just the nature of heavy equipment and things occasionally go wrong - that's why we have backup," he said.

Repairs were expected to take between two and seven days, depending on the exact cause of the fault.