HVDC Inter-Island

The HVDC Inter-Island link is a high-capacity, bipolar high-voltage direct current (HVDC) transmission system connecting the electricity networks of the North and South Islands of New Zealand together. It is commonly referred to as the Cook Strait cable, which is somewhat of a misnomer, because only a short section of the complete link is in cable, and there are actually three operational HVDC power cables across Cook Strait. However, the term Cook Strait cable is commonly used in the media and press releases. The link is owned and operated by state-owned transmission company Transpower New Zealand.

The 610 km link starts at the Benmore Hydroelectric Power Station, on the Waitaki River in Canterbury, and travels 534 km on an overhead transmission line through inland Canterbury and Marlborough to Fighting Bay in the Marlborough Sounds. From Fighting Bay, the link travels 40 km via submarine cables across Cook Strait to Oteranga Bay, near Wellington, before travelling the final 37 km on overhead lines to Haywards transmission sub-station in Lower Hutt.

The HVDC link was first commissioned in April 1965 to primarily transport electricity from the generation-rich South Island to the more populous North Island. The link originally was a bipolar 600 MW link with mercury arc valves, until the original equipment was paralleled onto a single pole (Pole 1) in 1992, and a new thyristor-based pole (Pole 2) was commissioned alongside it. As of August 2012, the link operates as a 700 MW monopolar link with Pole 2 only. Pole 1 was fully decommissioned effective 1 August 2012, and Transpower is in the process of testing and commissioning a replacement thyristor-based pole, Pole 3, ready for full service in early 2013. This will restore the link to a bipolar 1000 MW configuration.