in the North Island as they approach the end of their useful life Reinforcing some North Island transmission towers. Benmore generator transformers The original design of the inter-island link at Benmore was integrated with the design of the 540 MW Benmore hydroelectric power station. The 16 kV generator busbars in the power station were the point of connection between the HVDC link and the South Island grid. The power from the six Benmore generators could flow directly from the 16 kV busbars to the HVDC link via converter transformers, with the interconnecting transformers connecting to the Benmore 220 kV busbar to export or import electricity from the rest of the South Island. The design of the power station was optimised with the HVDC link, and the interconnecting transformers were designed with a significantly lower rating than the maximum output of the Benmore generators, because so much of the generator output power would normally flow to the HVDC link. Following Transpower's decommissioning of the original Pole 1 equipment, there was no longer any direct connection between the generator 16 kV busbars and the HVDC link, and the limited capacity of the Benmore interconnecting transformers would have constrained the maximum output of the station. In co-ordination with the Transpower programme for decommissioning of the Pole 1 equipment, Benmore owner Meridian Energy replaced the interconnecting transformers with new generator transformers. The six generators were reconnected to the 220 kV national grid via six new generator circuit breakers and three 220/16/16 kV three winding transformers. The new transformers each connect two generators, via two 16 kV secondary windings. Future options Fourth Cook Strait cable There are proposals to install a fourth cable underneath Cook Strait (Cable 7), connecting to Pole 2, to allow the HVDC link to increase to 1400 MW. In addition to a fourth cable, new filters would also be installed at Benmore and Haywards, and a new STATCOM at Haywards. As of 2017, there is no definitive timeframe for a fourth cable. North Canterbury tap The Upper South Island north of the Waitaki Valley is generation-poor, yet has many large demand centres, especially Christchurch, Nelson, Ashburton and Timaru-Temuka. Almost all of the electricity has to be imported from the Waitaki Valley, via three major 220 kV lines: the single-circuit Livingstone to Islington line (built 1956), the single-circuit Twizel to Islington via Tekapo B line (built 1962), and the double-circuit Twizel to