of over 3,900 guyed towers and 96 self-supporting towers across varied terrain. Permafrost in some areas led to foundation settling of up to 3 feet (1 m). The loan by the Government of Canada was discharged when Manitoba Hydro bought the line and outstanding debt in 1992. In 1997 a tornado damaged 19 towers of the DC lines. During repairs, some major customers were advised to curtail load, but imports over the 500 kV lines from adjacent utilities in the United States prevented serious interruption of power. A third such line, called Bipole 3 was proposed, to run along the west side of Manitoba. On October 26, 2009, the Canadian Taxpayers Federation, along with engineering and environmental experts, released an analysis which they claimed refuted each of the government's claims for why the line must be built down the west side of the province. The line was constructed on the western route and completed in 2018. System components The transmission system is currently composed of three bipole transmission lines with their converter stations and ground return electrodes to enable monopole operation. Bipole 1 Bipole 1 runs from Radisson to Dorsey. It was originally rated to run at a maximum potential difference of ±450 kilovolts and a maximum power of 1620 megawatts. This results in an electric current of 1800 Amperes. Bipole 1 consists of six, 6-pulse converter groups at each end (three in series per pole), each originally rated at 150 kV DC, 1800 A. Each converter group can be bridged at the DC side with a vacuum switch. Subsequent upgrades have increased the current rating to 2000 A and the voltage rating of most equipment to 166 kV per bridge (i.e., 500 kV total), although as of January 2013 Manitoba Hydro still report that the line is operated at +463 kV/−450 kV. When it was built between March 1971 and October 1977, mercury-arc valves were used to rectify the alternating current. These valves, supplied by English Electric, each had six anode columns in parallel and were the most powerful mercury arc valves ever built. Each of them had a weight of , a length of , a width of and a height of . Between 1992 and 1993 the mercury arc valves of Pole 1 were replaced with thyristor valves by GEC Alsthom, increasing the maximum power and voltage of the line to its current levels. The mercury arc