HVDC Inter-Island
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The HVDC Inter-Island is the high voltage direct current connection between the two main islands of New Zealand, which was put into service in 1965. It was built with mercury arc rectifiers in its static inverters. The project was, until it was upgraded in 1993, a bipolar HVDC line with an operating voltage of 250 kV and a maximum transmission capacity of 600 megawatts.
The transmission line connecting the static inverter stations at Benmore Dam in the South Island and Haywards on the North Island has a total length of 610 kilometres. The line is run overhead for 570 kilometres. A 40 kilometres undersea cable is laid across Cook Strait. In 1993 the system was upgraded with thyristors in its static rectifiers, which uses 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.