常见的电力专业英语14-电力系统中的高电压技术.ppt

Lighting Conductors Lighting Conductors Lightning Shielding of Transmission Lines When lightning makes a direct hit on a transmission line, it deposits a large electric charge, producing an enormous overvoltage between the line and the ground. The dielectric strength of air is immediately exceeded and a flashover occurs. Lightning Shielding of Transmission Lines The arc between the line and the ground produces a highly ionized path which behaves like a a conducting short-circuit. Consequently, the normal AC line voltage immediately delivers a large AC current that follows the ionized path. This follow-through current may sustain the arc until the circuit breakers open at the end of line. Lightning Shielding of Transmission Lines Direct hits on a transmission line are rare. More often, lightning will strike the overhead ground wire that shields the line. In the latter case, a local charge shill accumulates on the line, producing a very high local overvoltage. Overhead Ground Wires Overhead Ground Wires Lightning Arresters Travelling Waves The concentrated charge immediately divides into two waves that swiftly move in opposite directions at close to the speed of light. The height of the impulse wave represents the magnitude of the surge voltage that exists from point to point between the line and the ground. The peak voltage may attain one or two million volts. * 电力系统中的高电压技术 电气工程学院 High-Voltage Technology In Power Systems Topics Insulation Grounding Overvoltage Lightning Shielding Insulation 1 Insulation is required to keep electrical conductors separated from each other and from other nearby objects. The choice of material is dictated by the requirements of the particular application and by cost. Insulation 2 The coils of generators and motors may be insulated with various tapes. The insulation on power transformer windings is commonly paper tape and pressboard operated under oil. Insulation 3 Overhead lines are supported on porcelain insulators. Between the