the aging converter system. In May 2009, Transpower placed the remaining capacity of Pole 1 back into service for a short period, at a limited capacity of 200 MW, in response to a temporary loss of capacity on Pole 2. The decommissioning of half of Pole 1 and the operational restrictions placed on the remaining Pole 1 capacity led to the HVDC link operating mostly in monopolar mode, using Pole 2 alone. In 2010, Transpower reported that continuous operation in monopolar mode has caused the HVDC link to act as a galvanic cell with the earth, causing Benmore's Bog Roy earth electrodes to erode as they acted as an anode, and causing the buildup of magnesium and calcium hydroxide deposits on Hayward's Te Hikowhenua shore electrodes as they acted as a cathode. Additional replacement and maintenance work was required. On 1 August 2012, Transpower decommissioned the remaining half of the Pole 1 mercury arc valve converter stations at Benmore and Haywards, after 47 years in service. The Inter Island link at the time was the last HVDC system in the world with mercury arc valve converters in operational service. The Pole 3 Project In May 2008, Transpower submitted an investment proposal to the Electricity Commission for the replacement of the old mercury arc valve Pole 1 converter stations with new thyristor converter stations. In July 2008, the Electricity Commission announced its intention to approve the project. This project involved the construction of new converter stations designated as Pole 3, to operate at +350 kV 700 MW, matching the existing Pole 2 (−350 kV, 700 MW). Site works on the $672 million project were formally commenced on 19 April 2010, when Minister of Energy Gerry Brownlee turned the first sod. The new converter stations were to be commissioned by April 2012, but in May 2011, Transpower announced that commissioning was delayed until December 2012 because of difficulties being experienced by the manufacturer. Work involved in replacing Pole 1 with the new Pole 3 converter stations included: New valve halls adjacent to the Pole 2 valve halls at both Benmore and Haywards, each containing the thyristors converters New transformers connecting the valve halls to the 220 kV buses at both Benmore and Haywards Connecting the Pole 3 thyristors to the existing Pole 1 lines at both Benmore and Haywards Connecting the Pole 3 thyristors to the existing electrode lines at both Benmore