The earliest power distribution systems transmitted DC. Soon enough simple circuit analysis showed that transmitting power at higher voltages would reduce line losses (higher voltage, lower current, lower I2R copper losses). The transformer made AC power step up and step down possible, and coupled with other reasons the world quickly switched to AC transmission and distribution. Power levels transmitted were perhaps in the kilowatts which gradually swelled to Megawatt and Gigawatt levels. Transmission of such huge amounts of power, and interconnection of grids across continents for balancing demands and production, demanded reducing power losses further. This demanded raising the voltage levels even higher. 765 kilovolt is now a standard transmission voltage for long distances and large powers.
High voltage transmission reduced the copper losses due to line resistance, but long lines have appreciable series inductances and shunt capacitances. Series inductance means the voltage at the receiving end will be less than that at the sending end, and the difference will keep changing with the load current. Also the source and the load both see a reactive line creating problems in maximum power transfer. Line voltage ‘compensation’ techniques, based mostly on distributed shunt capacitance have their own problems. At the same time, shunt capacitance of the line, and the shunt compensation capacitors cause a shunt current leakage which increases with increasing transmission voltage. The leakage current also causes a power loss in the wire resistance. The power loss of this type is proportional to square of the current, and hence, square of the transmission voltage. At high voltages and high power levels, this loss cannot be ignored.
AC Power (green curve) Direct Current (red curve)
Engineers thought again about DC transmission. High power semiconductor devices are now available to efficiently convert AC to DC for transmission, and then back from DC to AC for step down and distribution. Therefore the switch from long distance AC transmission to DC transmission is on its way.
HVDC transmission for Renewables
Solar PV, solar thermal PV, and wind power all have a defect (and a half) for grid operation. None of them is available 24 hours a day. Further, none of the three are available every day. The random hourly variations, are particularly problematic for grid supply. Grid supply must reliably meet the demand which itself has its own peculiar variations. Luckily daily demand variations at two distant locations are displaced in time also, and having interconnect helps smooth the demand deficits. Wide-spread grids , like trans-Europe, trans-America, or trans-continental, have the potential to similarly average out the random fluctuations in PV, STPV, and wind power etc. Thus grid power based purely on renewable generating stations will also become viable if the generation is spatially dispersed over a large region, and HVDC transmission is used.