ransmission of electrical power by high voltage dc syst

ransmission of electrical power by high voltage dc system is superior to that by high voltage ac system in many respect such as saving of copper (d c power can be transmitted through only two wires) no inductance capacitance phase displacement and surge problem; no skin effect no charging current low corona loss reduced interference with communication circuits better voltage regulation for the same load and sending and voltage no need to stabilizer and reduced potential stress ( the potential stress on the insulation in case of dc system is 1/v2 times of that in ac system for the same working voltage). The only difficulty is of obtaining the necessary high voltage required for transmission as neither electrical power can be generated at high voltage because of commutating difficulties nor voltage can be stepped up.In case of ac systems the main advantage is that the voltage can be stepped up at generating end by means of step up transformers to the required value for transmission and then stepped down at distributing end by means of step down transformers for distribution with ease and efficiency. The other advantages of a c systems are that electric power can be generated at high voltage easily and the maintenance of a c sub-stations is easier and cheaper. The main draw-backs of ac systems are more spacing between the overhead conductors more copper requirement continuous power loss on account of charging current in case of long transmission lines synchronization of alternators and control of alternator speed within very low limits.Distribution by ac system is undoubtedly superior to that by dc system as in ac system voltage control is easy by means of transformers. Transmission of electrical power by dc system has now become possible because of introduction of mercury arc rectifiers and thyratrons which can convert ac into dc and vice-versa efficiently and at reasonable cost. Such devices can handle 30 MW at 400 kV.
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