HVDC Inter-Island

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Haywards substation, site of the North Island side static inverter.

The Inter-Island HVDC link is the high voltage direct current connection between the two main islands of New Zealand, put into service in 1965. The abbreviation stands for High Voltage Direct Current. It was built with mercury arc rectifiers in its static inverters. The project was, until upgrading in 1993, a bipolar HVDC line with an operating voltage of 400 kV and a maximum transmission capacity of 1200 MW.

The transmission line connecting the static inverter stations at Benmore Dam in the South Island and Haywards in the North Island has a total length of 610 km. The line runs overhead for 570 km. A 40-km undersea cable is laid across Cook Strait. In 1993 the system was upgraded with thyristors in its static rectifiers, which use a transfer voltage of 350 kV. During this upgrade the static inverters of the old scheme, which still use the mercury arc valves from 1965, were modified so that they are now switched in parallel.

As of 21 September 2007, part of Pole 1 of the system which still utilized the original mercury arc rectifiers, was shut down 'indefinitely',^[1] though in December 2007, Transpower announced that one half of Pole 1 would be returned to 'warm standby' service prior to the 2008 winter, in order to meet North Island demand if required.^[2] Transpower also noted in November 2007 that by December 2007, it would increase the south to north transfer capacity at Pole 2 from 500MW to 700MW by reconfiguring the three operational undersea cables by switching one of the two undersea Pole 1 cables to Pole 2.^[3].

Transpower announced on 13 March 2008 that the work required to restore half of Pole 1 to service at times of peak North Island demand had been completed, and that this would be available for the winter months. Power transfer would be strictly limited to northwards flow, reducing the stress on the old technology.^[4]