Development of HVDC Transmission Technology in India
High Voltage Direct Current (HVDC) power transmission made a modest beginning in 1954 when a 100 kV, 20 MW DC link was established between Swedish mainland and the island of Gotland. The converter stations utilized mercury arc valves which were problematic due to the problem of arc backs. Yet, 6000 MW of HVDC links (including the Pacific Intertie in USA and Nelson River Bipole 1) were built using mercury arc valves till 1972. The thyristor valves were first installed in a Back to Back (BTB) HVDC link at Eel River in Canada which interconnected Hydro Quebec system and the New Brunswick electric power system. Thyristor valves did not have the problem of arc backs in mercury arc valves which prevented fast control of power in the DC link. Also, the thyristor valves are made up of series connection of thyristor devices and permitted the choice of the voltage and current ratings of the HVDC link for a specified power flow. Further developments in the technology involved microcomputer based converter control, light triggered thyristor (LTT), capacitor commutated converter (CCC) and UHV DC transmission (at ±800 kV).