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Contact Energy had asked the national grid operator as far back as 2003 to investigate a power bottleneck, the Environment Court was told yesterday.
The power company also insisted yesterday it was not its role to buy transmission equipment and it had other priorities.
Contact Energy market and dispatch manager Boyd Brinsdon said the company had asked Transpower in July 2003 to investigate options on the bottleneck at the Roxburgh dam substation.
But it was advised by the national grid operator the existing transformer was in reasonable operating condition and there was no need for a new transformer.
The power company wants a condition imposed on the resource consents for the Mahinerangi wind farm, with capacity increased at transmission lines at the Roxburgh substation and between Roxburgh and Naseby.
Mr Brinsdon said the company was forced to reduce generation at Roxburgh because of line constraints.
He was surprised to hear the Transpower general manager, grid development, Tim George, express confidence in the Environment Court earlier this week of an upgrade to the Roxburgh-Naseby line.
Mr Brinsdon said just because a decision had been made to look into upgrading the 220kV Roxburgh-Naseby line, it would not happen quickly. It would most likely take several years.
The power company did not want to buy the transformer itself as it was slowly upgrading generators at Roxburgh, which was a higher priority. It had been upgrading the eight turbines at the dam since 2000 and should be finished next year.
Mr Brinsdon said the constraint at the Roxburgh transformer happened during high flows, where the Manapouri catchment was also likely to have high in-flows.
So even if a new transformer was purchased, the constraint would still occur at the 220kV line between Roxburgh and Naseby, as Manapouri would be sending through more power.
He said a generator could not acquire priority rights to a transmission system, so Contact Energy would not want to buy a transformer and then have it used, at no cost, by TrustPower.
He said discussions between Transpower and Contact Energy did not get very far.
Mr Brinsdon said talking to TrustPower staff last year he got the indication that company had not considered the constraints when considering the wind farm.
Contact Energy had also highlighted a constraint at the Cook Strait link.
Mr Brinsdon said though Transpower had indicated its support for the grid upgrade the final decision was up to the Electricity Commission.
TrustPower counsel Les Taylor said the reason Contact Energy had appealed the decision was wind was replacing water, which were both renewable resources.
Mr Taylor said, historically, there had been few constraints on the Roxburgh Naseby line.