Meridian Energy is replacing two of its transformers at its Manapouri power station after finding a fault with cooling units.

"The problems with the coolers were detected early enough and action taken before an electrical fault occurred that could've resulted in a transformer fire," a spokesman said.

The failure of the coolers meant that the tubes winding around the two affected transformers were contaminated with a small amount of metal particles, the company said.

"Meridian always takes a conservative view to management of risk at Manapouri and on discovery of the fault with the coolers decided to remove these two transformers from service. The reason for this is that technically there is always a risk of fire with transformer failure," said the spokesman.

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The tube coolers are double skinned and alarmed to ensure that oil does not leak into the cooling water and equally to prevent water from getting into the transformer.

Meridian said it was buying a third transformer so that it had a spare available if required in the future. The total cost to replace each transformer, including design, manufacture, supply, installation and commissioning is about $3 million.

The job of replacing the transformers is scheduled to be finished in March.

The transformers themselves were in good condition, said the spokesman.

They were refurbished and upgraded with new windings between 1998 and 2001 and the company expected to get a further 20 years or more of reliable performance.

Heavy trucks and loaders were used to transport the transformers up the steep Wilmot Pass from Deep Cove. Because of the risk of bio-organisms a delivery ship, the MVFagelgracht, needed a resource consent from Environment Southland to allow it to enter Doubtful Sound.

Hull inspections were done in Brisbane by a diving contractor.

Environment Southland staff were present throughout the time the ship was in the sound to ensure all conditions of the consent were complied with. There were no breaches, the spokesman said.

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