影响电力系统运行的因素.doc

Hazards to Power System OperationThe operation of electric power systems may be disturbed as the result of a number of causes. The troubles on the electric circuits are usually in the form of broken conductors or circumstances in which conductors are temporarily (or permanent) connected to each other or to ground. Regardless of the cause of the disturbances, such troubles seriously interfere with the flow of power and require corrective action.Overhead lines are vulnerable to troubles caused by lightning. When lightning strikes a line or a nearby object, a transient voltage is created on the line ,Hihg-voltage circuits have large spacing between conductors and from conductors to grounded objects, such as towers. Direct strokes to such lines are usually diverted to ground through overhead ground wires and may cause no interference with operation. Occasionally strokes terminate directly on the power conductors and introduce onto them transient transient voltages of such magnitude that archer across insulator strings may result. Once an arc is established, power current flows through it until the circuit is deenergized . Lightning voltages may be of high magnitude but are of very short duration; their time is measured in microseconds. As a result, their effects are most noticeable at the point where the stroke occurs. Distribution lines, which operate at perhaps 12kV or less, are mounted on relatively small insulators, which may be flashed over by the voltage induced in a line even though the lightning stroke does not contact the line. Such resulting arcs interfere with operation and usually are eliminated by deenergizing the line. When the line is deenergized . The arc is extinguished and, very commonly, the line may be reenergizing immediately.Overhead lines are subjected to wind and sleet. Although designed to withstand ordinary storm loading, practically they can not be designed for the extreme condition of sleet or wind loading. Hence, occasional line failures occur as a result of these causes. Failures caused by wind or sleet are usually of such nature that repairs must be made before the line may be reenergize .Foreign objects may contact a line and cause damage, which necessitates removal of the line from service. Airplanes may fly into a line, an automobile may knock apole , a construction worker may run a crane into a line ,or an excavator may dig up a cable. Birds, squirrels , snakes and other animals occasionally short-circuit lines and cause outages.Generators, transformers , lines ,or other components may be switched out of service for normal or routine maintenance. When switching operations occur, transients are set up on the power system which sometimes produce serous overvoltages. Such overvoltages in turn may cause flashover of insulators or puncture of solid insulation.Equipment sometimes fails without any particular direct cause. Failure may be due to inadequate design. previous overloads, or natural deterioration of equipment.Human errors may produce power-system outages. Switches may be opened unintentionally. Cutting off loads; temporary grounding electrodes may be put on energized circuits by mistake; equipment handled by hotline devices may be dropped. Such occasions, although rare, must be considered as hazard possibilities in power-system design. 影响电力系统运行的因素影响电力系统正常运行的因素很多。电路故障的形式通常是导体断裂。导体暂时（或永久地）相间接地。不管故障的原因是什么，这些故障都严重干扰电流的输送，需要采取补救措施。雷电容易影响架空线路。当雷电击中输电线路或其附近物体时，线路上产生暂态电压。高压电路的相间以及相对接地体如杆塔）间均有很大的绝缘距离。当接地线遭受雷击时，雷电可经过架空地线流入大地，不会给运行造成什么影响。但是，有时输电线路会遭受雷击，从而在输电线路上产生倡但很高的暂态电压：这种电压会引起线路绝缘子串的电弧放轨一旦绝缘子串上建立电弧，电流会通过它流动而引起线路断电。雷电过电压具有很高的幅值，但持续时间极短，以微秒计算。由此可见，雷电对系统正常运行影响很大。配电线路的运行电压通常为12千伏或更低，它们由相对较少的绝缘子串悬挂，所以即使雷电不足以击倒输电线路。它也会在线路上引起感应电压，从而导致绝缘子串闪络。这种电弧放电会影响系统运行，通常可通过断电来消除故障。跑路一断，电弧熄灭，此时线路可重新合闸架空线易受到大风和冰雹的侵袭。虽然线路设计时需考虑耐受风暴的影响，但是不可能依据最恶劣的气候条件来设计。所以气候条件也会引起输电线路故障，这种故障必须在线路重新供电之前排除。外来物体有可能接触线路造成损害，此时需切断线路。飞机可能会碰到线路，汽车可能会撞倒电线杆，建筑工人的起重机可能会撞到导线，挖掘机也可能会挖出电缆。鸟儿、松鼠、蛇和其它动物偶尔会使线路短路造成停电。在正常或例行检修时，发电机、变压器、输电线路或其它设备需退出运行。当开关操作时，系统中会出现暂态过程，从而引起非常严重的暂态过电压。这种过电压会造成绝缘子串闪络或固体绝缘击穿。有时设备产生故障并没有什么特别的直接原因。故障可能归因于设计不当、过载，或者设备的自然老化。人为过失也会导致电力系统停电事故。开关可能会被无意中拉开而中断负荷；临时接地体可能被错误地与带电回路相连。由带电装置控制的设备可能被接地。这种情况虽然很少，但在电力系统的设计当中必须考虑它们可能带来的危害。New Words1. magnitude 幅值 2. arcover 电弧放电，跳火，飞弧，击（打）穿 3. insulator绝缘体，绝缘子 4. arc弧 5. reenergize断路（开） 6. kV千伏 7. induce感应 8. reenergize 使又通上电流 9. necessitate 使成为必要 10. excavator挖掘机 11. short-circuit使短咱 12. outage停电 13. transformer变压器 14. overvoltage 过电压 15. unintentionally 无意地 16. grounding接地（电） 17. electrode