Faults on Power Systems
Each year new design of power equipment bring about increased reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults on electric power systems. On the occurrence of a fault , current an voltage conditions become abnormal, the delivery of power from the generating station 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 faulty is the unintentional or intentional connecting together of two or more conductors which ordinarily operate with a difference of potential between them. The connection between the conductors 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 very low value in case the connection is through an arc. Currents of abnormally high magnitude flow through the network to the point of fault. These short-circuit currents will usually be much greater than the designed thermal ability of the condition in the lines or machines feeding the fault . The resultant 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 apparent that the late conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect the faulted equipment from the remanding of the system. Ordinarily it is desirable that no other switches on the system are opened, as such behavior would result in unnecessary modification the system circuits.
A distinction must be made between and an overload. An overload implies only that loads greater than the designed values have been imposed on system. Under 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 current in the overload equipment are high and may exceed the thermal design limits. Nevertheless, 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 occurrences in homes. For example, a housewife might plug five waffle irons into the kitchen circuit during a neighborhood part. Such an overload, 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 fuses or circuit breakers which open quickly when currents above specified values persist. Distribution transformers are sometimes overloads as customers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformers sizes are increased as load grows.
Faults of many types and causes may appear on electric power systems. Many of us in our homes have seen frayed lamp cords which permitted the two 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 conductors. There are sometimes accidentally brought together by action of wind, sleets, trees, cranes, airplanes, or damage to supporting structures. Overvoltages due to lighting or switching nay cause flashover of supporting or from conductor to conductor. Contamination on insulators sometimes results in flashover even during normal voltage conditions.
The conductors of underground cables are separated from each and from ground by solid insulation, which nay be oil-impregnated paper or a plastic such polyethylene. These materials undergo some
deterioration with age, particularly if overloads on the cables have resulted in their operation at elevated temperature. Any small void present in the body of the insulating material will results in ionization of the gas contained therein, the products of which react unfavorably with the insulation. Deterioration of the insulation may result 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 lightening or switching produces transient voltage of abnormally high values between the conductors.
Transformer failures may be the result of insulation deterioration combined with overvoltage due to lightning or switching transients. Short circuit due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltage. Major insulation may fail, permitting arcs to be established between primary and secondary windings or between 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 may also occur between one of the winding 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 section of machine.
Balanced three-phase faults, like balanced three-phase loads, may be handled 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 handing of faults on single-phase lines is of course identical to the method of handing three-phase faults on an equivalent single-phase basis.
Faults may be classified as permanent or temporary. Permanent faults are those in which insulation failure or structure failure produces damage that makes operation of the equipment impossible and requires repairs to be made. Temporary faults are those which may be removed by deenergizing the equipment for a short period of time, short circuits on overhead lines frequently are of this nature. High winds may cause two or more conductions to swing together momentarily. During the short period of contact. An arc is formed which may continue as long as line remains energized. However, if automatic equipment can be brought into operation to service as soon as the are is extinguished. Arcs across insulators due to overvoltages from lighting or switching transients usually can be cleared by automatic circuit-breaker operation before significant structure damage occurs.
Because of this characteristic of faults on lines, many companies operate following a procedure known as high-speed reclosing. On the occurrence of a fault, the line is promptly deenergized by opening the circuit breakers at each end of the line. The breakers remain open long enough for the arc to clear, and then reclose automatically. In many instances service is restored in a fraction of a second. Of course, if structure damage has occurred and the fault persists, it is necessary for the breakers to reopen and lock open.
电力系统故障
每年新设计的电力设备都使系统的可靠性不断提高，然而，设备的使用不当以及一些偶然遇到的外在因素均会导致系统故障的发生。发生故障时，电流、电压变化得不正常，从电厂到用户的送点在相当大得内不令人满意。此时若故障设备不立即从系统中切除的话，则会造成其他运行设备的损坏。
故障是由于有意或无意地使两个或更多的导体接触造成的。导体间本来是有电位存在的，而这种接触可能是金属性接触，也可能是电弧引起的。如果是前者造成的故障，则两部分导体之间电压下降为零；若为后者，则电压变得很低，超常的大电流经过网络流至故障处。此短路电流通常会大
大超出导线以及供电发电机的热承受能力，其结果，温度的升高会导致导体烧毁或绝缘焦化。在允许的期限内，最靠近故障处的电压会变得很低，致使用电设备无法正常运行。显然，系统设计者必须事先考虑到故障可能发生在什么地方，能够推测出故障期间的各种情况，提供调节好的设备，以便驱动为将故障设备切除所必须断开的开关能够跳闸。通常希望此时系统无其他开关打开，否则会导致系统线路不必要的修改。
过负荷与故障是两个概念。过负荷仅指施加于系统的负荷超过了设计值。发生这种情况时，过负荷处的电压可能很低，但并不等于零。这种电压不足的情形可能会超过过负荷处蔓延一定距离，进而影响系统其它部分。过负荷设备的电流变大而超过预定的热极限，但是这种情况比发生故障时的电流要小。此时，供电虽然往往能维持，但电压较低。
过负荷的情况在家里发生，例如请街坊邻居聚会时，女主人可能将五个化夫饼干烘烤器的插头同时插入厨房的插座，诸如次类的过负荷倘若不能迅速处理的话，就会造成电力线发热甚至酿成火灾。为了避免这种情况的发生，须采用保险丝或短路器来保护住宅区电路免受损坏。断路器会在电流超出预定值时迅速切断电路。当用户安装的用电器增加时，也会超过变压器负荷能力，因此有必要不时地监视配电线路以确保在负荷增加时变压器的容量也相应增加。
电力系统会发生各种类型，由各种原因引起的故障。我们在家里看到过破损的照明灯电线，使得其两根导线相触，并会发出弧光。如果此时断路器或保险丝能够正常工作，则电路能被自动切断。
大部分架空明线是用裸导体假设的，又是由于风、雨、雷、或大树、起重机，飞机及支撑物的损坏等因素会使导线偶然碰到一起。由雷电或开关瞬变过程中引起的过电压会在支撑物或导体之间产生电弧，即使在电压正常的情况下，绝缘材料的污染也会引起电弧。
通常采用油浸电缆纸或聚乙烯一类固体塑料绝缘材料将埋地电缆中的导线与导线和导线与地隔开。这些绝缘会随着时间的流逝而老化，尤其是在过负荷引起高温下运行时候更是如此。绝缘材料内的空隙会造成气体的电离，其生成物对绝缘不利。绝缘材料老化会引起绝缘性能下降而导致导线短路。电缆故障的可能性会因雷电或开关瞬间引起的导线的电压骤然变高而增加。
变压器故障可能是由绝缘老化、加上雷电、开关瞬变过程导致的过高压造成的。同一绕组相邻线圈之间由于绝缘问题造成的短路可能是由于突然遇到外加高压电所致。绝缘失败会在一次绕组与二次绕组之间或绕组与接地金属部件如铁芯或变压器外壳之间产生电弧。
发电机故障可能是由于同一槽中相邻线圈之间绝缘被破坏而造成的，其结果会导致发电机匝内短路。绝缘损坏也可能发生在某一绕组与定子铁芯的接地钢架构之间。同一槽内不同绕组之间的绝缘损坏会导致电机大范围短路。
像处理平衡三相负荷一样，处理平衡三相故障也是依照基于由火线到零线的电路或等效单相电路的原则进行。可以通过电压、电流和电阻的规律求解问题。当然，单相线路上故障的处理方法也可以用于在单相等效电路下三相故障的处理中。
故障有永久性故障及暂时性故障之分。永久性故障指绝缘或结构上的损坏，致使设备不能维修则无法运行。暂时性故障指通过给设备临时断电即可排出的故障，架空线路往往就有这个特点。大风可能会使两根导线瞬时间碰在一起，并产生电弧。只要线路通电，此电弧会一直存在。然而如果能借助自动化设备使导线迅速断电的话，就不会造成任何损失了，一旦电弧熄灭，线路即可自行恢复。有雷电及开关切换产生的过电压引起的绝缘材料飞狐则可通过自动化开关设备动作，在严重的结构损坏发生之前便得到排除。
鉴于线路故障的这些特征，许多公司都使用一种叫做高速重合器的装置。故障发生时，线路两端的断路器跳闸，电流即被切断，经过一定的时间间隔，待电弧熄灭后，断路器又自动进行再次合闸，大多数情况下，不到一秒种即可恢复正常供电。当然，如果因结构损坏，故障不能很快排除的话，则断路器必须再次跳闸且保持这种跳闸状态。