at each end of HVDC link used large mercury-arc rectifiers and inverters – 1960s technology – to convert between AC and DC. The South Island converter station was established at the Benmore hydroelectric power station in the Waitaki Valley. The North Island converter station was built at Haywards in the Hutt Valley near Wellington. The HVDC transmission line that connects Benmore and Haywards converter stations has an overall length of 610 kilometres. The overhead transmission line is supported by 1649 transmission towers and has a total route length of 570 km. The submarine cables underneath Cook Strait are 40 km long. Until it was upgraded in 1993, the HVDC Inter-Island link had normal operating voltages of ±250 kV, and a maximum power transmission capacity of about 600 MW. The HVDC link was originally designed to transfer power northwards from Benmore to Haywards. In 1976, the control system of the original scheme was modified to allow power to be sent in the reverse direction, from Haywards to Benmore. Engineering heritage status The original HVDC link was recognised as a significant part of New Zealand's engineering heritage by the Institution of Professional Engineers New Zealand, (now Engineering New Zealand), during the "Engineering to 1990" project, which helped to celebrate the country's sesquicentenary in 1990. The Hybrid Upgrade Project In 1987, the Electricity Corporation of New Zealand began investigations to find the best means of upgrading the inter-island link. A hybrid upgrade was chosen over total replacement, for economic reasons. The term "hybrid" was adopted because the increase in capacity was to be obtained through a combination of voltage and current upgrades. The upgrade project involved continued use of the existing mercury arc valve converter equipment alongside new solid-state thyristor converter stations. The scope of work included: Providing three new HVDC submarine cables underneath Cook Strait, to supplement and ultimately replace the original cables. Each new cable was rated at 350 kV, 1430 A, giving a maximum power capacity of 500 MW per cable. The three new power cables were installed in 1991 by the cable laying vessel Skagerrak. New cable terminal stations at Fighting Bay and Oteranga Bay The existing mercury arc valve converters at each end of the link were reconfigured to operate in parallel at each station (they had previously operated with opposite electrical polarity). They were redesignated as Pole 1. The operating voltage of the mercury arc valve converters