1、中文 3200 字 附录 ： 专业相关文献翻译 Faults on power system Each year new designs of equipment bring about increased reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults on electric power system. On the occurrence of power from the generatin
2、g stations to the loads may be unsatisfactory over a considerable area, and if the faulted equipment is not promptly disconnected from the remainder of the system, damage may result to other pieces of operating equipment. A fault is the unintentional or intentional connecting together of two or more
3、 conductors which ordinarily operate with a difference of potential between them. The connection between the conductions may be by physical metallic contact or it may be through an arc. At the fault, the voltage between the two parts is reduced to zero in the case of metal-to-metal contacts, or to a
4、 very low value in case the connection is through an arc. Currents of abnormally high magnitude flow the network to the point of fault. These short-circuit currents will usually be much greater than the designed thermal ability of the conductors in the lines or machines feeding the fault. The result
5、ant rise in temperature may cause damage by the annealing of conductors and by the charring of insulation. In the period during which the fault is permitted to exist, the voltage on the system in the near vicinity of the fault will be so low that utilization equipment will be inoperative. It is appa
6、rent that the power system designer must anticipate points at which fault may occur, be able to calculate conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect faulted equipment from the remainder of the system. Ordinarily it is de
7、sirable that no other switches on the system are opened, as such behavior would result in unnecessary modification of the system circuits. A distinction must be made between a fault and an overload. An overload implies only that loads greater than the designed value have been imposed on system. Unde
8、r such a circumstance the voltage at the overload point may be low, but not zero. This undervoltage condition may extend for some distance beyond the overload point into the remainder of the system. The currents in the overloaded equipment are high and may exceed the thermal design limits. Neverthel
9、ess, such currents are substantially lower than in the case of a fault. Service frequently may be maintained, but at below-standard voltage. Overloads are rather common occurrence in homes. For example, a housewife might plug five waffle irons into the kitchen circuit during a neighborhood party. Su
10、ch an over-load, if permitted to continue, would cause heating of the wires from the power center and might eventually start a fire. To prevent such trouble, residential circuits are protected by fuse or circuit breakers which open quickly when currents above specified values persist. Distribution t
11、ransformers are sometimes overloaded as customers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformer sizes are increased as load grows. Faults of many types and causes may appear on electric power systems. many of us in ou
12、r homes have seen frayed lamp cords which permitted the tow conductors of the cord to come in contact with each other. When this occurs, there is a resulting flash, and if breaker or fuse equipment functions properly, the circuit is opened. Overhead lines, for the most part, are constructed of bare
13、conductors. These are sometimes accidentally brought together by action of wing, sleet, trees, cranes, airplanes, or damage to supporting structures. Overvoltages due to lightning or switching may cause flashover of supporting or from conductor to conductor. Contamination on insulators sometimes res
14、ults in flashover even during normal voltage conditions. The conductors of underground cables are separated from each other and from ground by solid insulation, which may be oil-impregnated paper or a plastic such as polyethylene. These materials undergo some deterioration with age, particularly if
15、overloads on the cables have resulted in their operation at elevated temperature. Any small void present in the body of the insulating material will result in ionization of the gas contained therein, the products of which react unfavorably with the insulation. Deterioration of the insulation may res
16、ult in failure of the material to retain its insulating properties, and short circuits will develop between the cable conductors. The possibility of cable failure is increased if lightning or switching produces transient voltage of abnormally high values between the conductors. Transformer failures
17、may be the result of insulation deterioration combined with overvoltages due to lightning or switching transients. Short circuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltages. Major insulation may fail, permitting arcs to be estab
18、lished between primary and secondary windings or between a winding and grounded metal parts such as the core or tank. Generators may fail due to breakdown of the insulation between adjacent turns in the same slot, resulting in a short circuit in a single turn of the generator. Insulation breakdown m
19、ay also occur between one of the windings and the grounded steel structure in which the coils are embedded. Breakdown between different windings lying in the same slot results in short-circuiting extensive sections of machine. Balanced three-phase faults, like balanced three-phase loads, may be hand
20、led on a lineto-neutral basis or on an equivalent single-phase basis. Problems may be solved either in terms of volts, amperes, and ohms. The handling of faults on single-phase lines is of course identical to the method of handling three-phase faults on an equivalent single-phase basis. Voltage transformers Voltage transformers are used with voltmeters, watt-meters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and the no-voltage and over-voltage trip coils of
