HVDC converter stations. The unique planning considerations for the overall proposal included: The hydroelectric generators at Benmore would need to be capable of absorbing the harmonic currents that would be created by the operation of the mercury arc converters. The Benmore generators were proposed to have an operating voltage of 16 kV, which was a new high for New Zealand hydroelectric generators at the time. The 16 kV circuit breakers required at Benmore would be state of the art. The mercury arc valves would be larger than any previously constructed, and would require water-cooled cathodes. The overhead HVDC transmission line would be one of the longest and most difficult built in New Zealand up to that time. The Cook Strait submarine cables would have to be specially designed for the seabed and tidal conditions, and require special armouring at the Oteranga Bay end, of a kind that had not been used before. In 1958, BICC laid two 0.8 km trial lengths of cable off Oteranga Bay in Cook Strait to demonstrate their ability to resist the abrasion, bending and vibration caused by conditions on the seabed. These trial lengths were recovered and inspected in 1960, and by October that year, BICC reported that the trial had been successful and that the prototype cable would provide good service underneath Cook Strait. In the period 1958 to 1960, some differing views were offered to Government about the most appropriate power developments for the country as a whole, and there were reservations about the risks involved in the planned Cook Strait cable crossing. However, in March 1961, against a background of increasing urgency in meeting the forecast demand, the Government approved the project. A NZ£6.5 million contract was placed with ASEA for the design, manufacture, installation and commissioning of the converter plant at Benmore and Haywards, and a NZ£2.75 million contract was placed with BICC for the manufacture, delivery, laying and testing of the Cook Strait submarine cables. Construction The HVDC inter-island link was designed and built between 1961 and 1965 for the New Zealand Electricity Department. The major equipment suppliers were ASEA and British Insulated Callender's Cables. The original Cook Strait cables were installed in 1964, from the cable laying ship Photinia. When it was completed, the New Zealand HVDC link was the world's longest HVDC transmission scheme, with the highest power rating, and the largest undersea power cables. The terminal stations