was increased from the original 250 kV to 270 kV New HVDC thyristor converter stations were added at each end of the link. These had an operating voltage of 350 kV, and were designated as Pole 2. The reinsulating of the entire HVDC overhead transmission line to increase its rating to 350 kV. Work on transmission structures and conductors was also carried out to ensure that the line conductors could operate at up to 2000 A on each Pole. The Pole 2 converter stations and new submarine cables were commissioned in March 1991. The upgrade brought the total converter station capacity to 1348 MW (648+700 MW), however the link was restricted to 1240 MW due to the overhead transmission line rating's restricting Pole 1's operating capacity to 540 MW. After the retirement of the last of the original submarine cables, the overall HVDC link transfer capability was restricted further to 1040 MW due to the single Pole 2 cable underneath Cook Strait. In its Asset Management Plan 2018, Transpower indicated that in the regulatory period 2020-2025 it planned significant expenditure to extend the life or replace ageing equipment in the Pole 2 converter stations that is near the end of its original 30-year design life. Decommissioning of Pole 1 On 21 September 2007, the original Pole 1 mercury-arc converter stations were shut down "indefinitely". However, in December 2007, Transpower announced that one-half of the capacity of Pole 1 would be returned to "warm standby" service before the winter of 2008 in order to meet the demand for power in the North Island if needed. The remaining half-pole equipment of Pole 1 was to be decommissioned. Transpower also announced in November 2007 that by December 2007, it would increase the south to north power transmission capacity of Pole 2 from 500 MW to 700 MW. This was done by reconfiguring the three operational submarine cables. One of the two cables previously connected to Pole 1 was transferred to Pole 2. On 13 March 2008, Transpower announced that work had been completed to restore 50% of the capacity of Pole 1 to service at times when the demand for power on the North Island peaked. Several mercury arc rectifiers were cannibalized from the Konti-Skan link between Denmark and Sweden for this restoration. The energy transfer on Pole 1 was strictly limited to the northbound direction, to reduce the stress and strain on