The reason so many people lost power yesterday is because the substation at Penrose is very old and supplies a disproportionate amount of power to a large area of the city, says an energy expert.
Bryan Leyland, a power system engineer and former Vector board member, said Penrose was one of four or five major substations in Auckland that receive bulk 220,000-volt and 110,000-volt power from Transpower. The power is broken down to 33,000 and 22,000 volts for Vector to feed about a dozen smaller substations.
Speaking last night from London, Mr Leyland said: "I remember someone saying a few years ago it [Penrose] supplies a disproportionate amount of power."
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He said the cables went out in all directions to Vector substations but many of these cables would be in a common trench.
There was nothing wrong with putting several cables in a single trench, but you had to provide fire protection, Mr Leyland said. He did not want to speculate on the cause of the fire.
John Clarke, Transpower general manager of system operations, said the ground was basalt which meant the cables were bunched more closely above ground, rather than being buried as would be more common. The uncommon set-up meant one cable catching fire was more likely to spread and affect others, he said, though it was not yet clear exactly what happened.