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Transpower says legal appeals against its ability to increase electricity current along high-voltage overhead lines through Auckland may jeopardise supply security north of the harbour bridge.
The electricity grid operator intends applying to the Environment Court for the right to operate the lines at higher loads until two appeals against a decision of Auckland City planning commissioners can be heard.
National grid general manager David Laurie said yesterday that the security of supply to northern parts of Auckland as well as Northland might otherwise be jeopardised, given an annual increase in electricity demand in those areas of about 30 megawatts, or 3.5 per cent.
Transpower has until now faced a tight restriction on the number of times a year it can exceed a maximum load on each of two circuits running over transmission towers between its Otahuhu and Henderson substations, en route to Northland and North Shore City.
That has effectively kept the amount of electrical current running through each circuit below half the designated maximum, in case a fault on one set of lines should force it to transfer the full load to the other.
But although it has applied for Government approval to spend $521 million on a more direct underground link by 2013 between Pakuranga and Albany via the harbour bridge, it says it needs to be able to pump extra current through the overhead lines until then.
The Auckland City commissioners issued a decision last month - against at least 430 opposing submissions - to allow Transpower to exceed the maximum rating by 31 per cent on any one overhead circuit during a forced "outage" on the other, and to boost supply by up to 45 per cent for an unlimited number of 15-minute periods on such occasions.
Although Transpower insists any such forced occurrences are rare, the Onehunga Enhancement Society says the decision effectively allows the normal operating current on each circuit to be increased by at least 31 per cent on a daily basis.
Society chairman Jim Jackson says his community group would be prepared to accept the extra current as a short-term imposition, but only if Transpower committed itself to a "sunset" clause by laying ducts for future underground supply beneath motorway extension projects through Onehunga and Mt Roskill.
Mr Jackson, who owns an electrical manufacturing business which he says is already affected by interference from the Transpower lines running above it, confirmed that the two appeals to the Environment Court were from him and the society.
He denied holding northern power consumers to ransom, saying he and the society were offering Transpower reasonable steps to become a "good corporate neighbour" by preparing for the eventual dismantling of the overhead lines, given mounting community concerns about possible health effects from electric and magnetic radiation.
Transpower says its plans for a direct link to Northland by 2013 would make it uneconomic to spend money on underground ducts it may never use through the western side of Auckland.
But Mr Laurie said there was a "real risk" of being unable to supply enough electricity to meet demand from northern consumers without the ability to run extra current through the overhead circuits in the meantime.
"It is critical that, should a forced outage occur on the Henderson-Otahuhu line before the appeal hearings are completed, the demand for electricity in the Auckland and Northland areas will still be able to be met."
He said Transpower would therefore ask the Environment Court for permission to operate the circuits at the higher ratings until the appeals could be dealt with.
Despite the growth of electricity demand, Transpower says it would also be too costly to install the new direct link before 2013, even though it has already laid ducts for future high-voltage cables under parts of Central Auckland and the Northern Busway.
