The Transmission of electrical power by means of high voltage direct current, frequently abbreviated to HVDC, has received a tremendous amount of attention in the last decade. Theory and experience indicate that transmission of large amounts of electrical energy for distances of several hundred miles and over is more economical by HVDC than by the usual high voltage alternating current. The development of hydroelectric power generation has made long transmission lines essential to move the generated power to industrial centers. Air and thermal pollution problems have discouraged increased construction of fossil fuel and nuclear steam power plants close to industrial load centers. For long distance power transmission high voltage is essential. Consideration of electrical fundamentals shows that doubling the transmission voltage reduces the current by 50 percent for the same power transmitted. This means smaller conductors and lighter towers can be used. A.C. power is usually transmitted in the three-phase configuration, requiring three conductors. If the A.C. can be converted to high voltage D.C., two wires can be used, lower line losses will be experienced and, because of lower peak voltage, smaller insulators are required. If direct ground return can be used, then only one conductor is needed, resulting in much greater transmission economy, but with hazards which will be discussed later.
