The Lightning of Transmission Line
Overvoltages on power systems are traceable to three basic caus, lightning, switching, and contact with circuits of higher voltage rating. The power system designer eks to minimize the number of the occurrences ,to limit the magnitude of the voltages produced,and to control their effects on operating equipment.
Lightning results from the prence o{ clouds which have become charged by the action of falling rain and vertical air currents, a condition commonly found in cumulus cloudsVoltages may be t up on overhead lines due to direct strokes and due to indirect strokes . In a direct stroke, the lightning current path is directly from the cloud to the subject equipment--an overhead line. From the llne, the current path may be over the insulators and down the pole to ground. The voltages tup on the line may be that necessary to flash over this path to ground. In the direct stroke, the lightning current path is to some nearby object, such as the tree shown In Fig. 10 lb. The voltage appearing on the line is explained as follows As the cloud comes over the line, the positive charges it carries draw negative ch
arges from distant points and hold them bound on the line under the cloud in position as shown. The voltage on the ]ine is zero assuming that the line is not energized, IF the cloud is assumed to discharge on the occurrence of the stroke in zero time, the positive charges suddenly disappear, leaving the negative charges unbound. Their prence on the llne implies a negative voltage with respect to ground. On the occurrence of a stroke, lightning clouds do not discharge in zero time. Instead,the stroke current ris from zero value to maximum value (perhaps 50, 000 amperes) in a few microconds and is completed in a few hundred microconds.
Direct lightning strokes to lines as shown in Fig. lO-la are of concern on lines of all voltage class ,as the voltage that may be t up is in most instances limited by the flashover of the path to ground, Increasing the length of insulator strings merely permits a higher voltage before flashover occurs. The most generally accepted method of protection against direct strokes is by u of the overhead ground wire For simplification only one ground wire and one power conductor are shown.
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gta5最高画质The ground wire is placed above the power conductor at such a position theractically all lightning-stroke paths will be to it instead of to the power conductor. Stroke current then flows to the ground most of it passing through the tower footing ground resistance R~whde a smaller part goes down the line and to ground through the adjacent tower footings. The tower ris in voltage due to the current I1 through the resistance R1 to a value which is Approximately this voltage appears between the tower and the power conductor (which was not struck). If this voltage is less than that required to cau insulator flashover, no trouble results. Protection by this method is improved by using two carefully placed ground wires and by making tower footing ground resistance of low value.
The lightning record of lines supported on towers 80 to 90 feet tall substantiates the simple theory of line protection just prented. The poorer record of lines on towers over 100 ft in height indicates that other factors, perhaps the inductance of the path down the tower, should be considered. low-voltage lines supported on small insulators. They are of little importance on high-volt-age lines who insulators can withstand hundreds of kilovol
ts without flashover.91校长
Insulation is required to keep electrical conductors parated from each other and from other nearby objects. Ideally, insulation should be totally nonconducting, for then currents are totally restricted to the intended conductors. However, insulation does conduct some current and so must be regarded as a material of very high resistivity. In many applieatlons, the current flow due to conduction through the insulation is so small that it may be entirely neglected. In some instances the conduction currents, measured by very nsitive instruments, rve as a test to determine the suitability of the insulation for u in rvice.
Although insulating materials are very stable under ordinary circumstances, they may change radically in characteristics under extreme conditions of voltage stress or temperature or under the action of certain chemicals. Such changes may, in local regions, result in the insulating material becoming highly conductive. Unwanted current flow brings about inten heating and the rapid destruction of the insulating material. The
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insulation failures account for a high percentage of the equipment troubles on electric power systems. The lection of proper materials, the choice of proper shapes and dimensions and the control of destructive agencies are some of the problems of the insulation-system designer.
Many different materials are ud as inaulation on eIectrle-power systems. The choice of material is dictated by the requirements of the particular application and by cost. In residences, the conductors ud m branch ctrcults and m the cords to appIlances may be insulated with rubber or plastics of veral different kinds. Such materials can withstand necessary bending, are relatively low electrical stress.
High-voltage cables are subjected to extreme voltage stress;in some cas veral hundred kilovolts are impresd across a few centimeters of insulation. They must be manufactured in long ctions, and must be sufficiently flexible as to permit pulling into duets of small cross etion. Tbe insulation may be oil-impregnated paper, varnished cambric, or synthetic materials such as polyethylene.
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The coils of generators and motors may he insulated with tapes of various kinds. Some of the are made of thin sheets of mica held together by a binder, and others are of fiber glass impregnated with insulating varnish. This insulation must be capable of withstanding quite high operating temperatures, extreme mechanical forces, and vibration.
The insulation on power-transformer windings is commonly paper tape and pressboard operated under oil, The oil saturates the paper, greatly increasing its insulation strength, and, by circulating through ducts, rves as an agent for carrying a way the heat generated due to IZR loss and core loss in the transformer. IThe transformer insulation is subjected to high electric stress and lo large mechanical forces, The shape and arrangemer~t of conducting metal parts is of particular concern in transformer design.曹操的外貌描写
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