the developed world relies on inter-regional and international transmission lines to deliver the power that energy-hungry cit and industries consume in such vast quantities. As the world urbanizes and becomes more industrialized, and as more dispersed generators appear on the network, the demand for, and on such power distribution infrastructure will only increase. It has long been recognized that power is more efficiently transmitted using DC than AC and early is visionaries worked hard at perfecting he valves necessary for converting the high-voltage AC produced by generators and vice versa This conversion is the key challenge for HVDC transmision and the HVDC story can be into two con version technology dominated by mercury arc valves and a later one in which they were supplanted by thyristor-based valves