电力系统外文翻译

Minimum Power System

A minimum electric power system is shown in Fig. 1. the system consists of an energy source, a prime mover, a g enerator, and a load.

The energy source may be coal, gas, or oil burned in a furnace to heat water and generate steam in a boiler; it m ay be fissionable material which, in a nuclear reactor, will heat water to produce steam; it may be water in a pond at an elevation above the generating station; or it may be oil or gas burned in an internal combustion engine.

Fig. 1. The minimum electric power system

The prime mover may be a steam-driven turbine, a hydraulic turbine or water wheel, or an internal combustion engi ne. Each one of these prime movers has the ability to convert energy in the form of heat, falling water, or fuel into rotation of a shaft, which in turn will drive the generator.

The electrical load on the generator may be lights, motors, heaters, or other devices, alone or in combination. Prob ably the load will vary from minute to minute as different demands occur.

The control system functions to keep the speed of the machines substantially constant and the voltage within prescr ibed limits, even though the load may change. To meet these load conditions, it is necessary for fuel input to chan ge, for the prime mover input to vary, and for the torque on the shaft from the prime mover to the generator to ch ange in order that the generator may be kept at constant speed. In addition, the field current to the generator must be adjusted to maintain constant output voltage. The control system may include a man stationed in the power pla nt that watches a set of meters on the generator-output terminals and makes the necessary adjustments manually. 3In a modem station, the control system is a servomechanism that senses a generator-output conditions and autom atically makes the necessary changes in energy input and field current to hold the electrical output within certain sp

ecifications.

More Complicated Systems

In most situations the load is not directly connected to the generator terminals. More commonly the load is some di stance from the generator, requiring a power line connecting them. It is desirable to keep the electric power supply at the load within specifications. However, the controls are near the generator, which may be in another building, p erhaps several miles away.

If the distance from the generator to the load is considerable, it may be desirable to install transformers at the gen erator and at the load end, and to transmit the power over a high-voltage line (Fig. 2). For the same power, the hi gher-voltage line carries less current, has lower losses for the same wire size, and provides more stable voltage.

In some cases an overhead line may be unacceptable. Instead it may be advantageous to use an under ground ca ble. With the power systems talked above, the power supply to the load must be interrupted if, for any reason, any component of the system must be removed from service for maintenance or repair.

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Fig 2A generators connected through transformers and a high-voltage line to a distant load

Additional system load may require more power than the generator can supply. Another generator with its associate d transformers and high-voltage line might be added.

It can be shown that there are some advantages in making ties between the generators (1) and at the ends of the high-voltage lines (2and 3), as shown in Fig. 3. This system will operate satisfactorily as long as no trouble develo ps or no equipment needs to be taken out of service.

The above system may be vastly improved by the introduction of circuit breakers, which may be opened and closed as needed. Circuit breakers added to the system, Fig. 4, permit selected piece of equipment to switch out of servi ce without disturbing the remainder of system. With this arrangement any element of the system may be reenergize d for maintenance or repair by operation of circuit breakers. Of course, if any piece of equipment is taken out of s ervice, the total load must then carried by the remaining equipment. Attention must be given to avoid overloads duri ng such circumstances. If possible, outages of equipment are scheduled at times when load requirements are below normal.

Fig. 1-3 A system with parallel operation of the generators, of the transformers and of the transmission lines

Fig. 4A system with necessary circuit breakers

Fig. 5Three generators supplying three loads over high-voltage transmission lines

Fig. 5 shows a system in which three generators and three loads are tied together by three transmission lines. No circuit breakers are shown in this diagram, although many would be required in such a system.

Typical System Layout

The generators, lines, and other equipment which form an electric system are arranged depending on the manner i

n which load grows in the area and may be rearranged from time to time.

Fig. 6 A radial power system supplying several loads

However, there are certain plans in to which a particular system design may be classified. Three types are illustrate d: the radial system, the loop system, and the network system. All of these are shown without the necessary circuit breakers. In each of these systems, a single generator serves four loads.

The radial system is shown in Fig. 6. Here the lines form a “tree” spreading out from the generator. Opening any li ne results in interruption of power to one or more of the loads.

The loop system is illustrated in Fig. 7. With this arrangement all loads may be served even though one line sectio n is removed from service. In some instances during normal operation, the loop may be open at some point, such as A. In case a line section is to be taken out, the loop is first closed at A and then the line section removed. In this manner no service interruptions occur.

Fig. 1-7A loop arrangement of lines for supplying several loads

Fig. 8 shows the same loads being served by a network. With this arrangement each load has two or more circuits over which it is fed.

Distribution circuits are commonly designed so that they may be classified as radial or loop circuits. The high-voltag e transmission lines of most power systems are arranged as networks. The interconnection of major power systems results in networks made up many line sections.

Fig. 8A network of lines for supplying several loads

Auxiliary Equipment

Circuit breakers are necessary to deenergize equipment either for normal operation or on the occurrence of short ci rcuits. Circuit breakers must be designed to carry normal-load currents continuously, to withstand the extremely high currents that occur during faults, and to separate contacts and clear a circuit in the presence of fault. Circuit break ers are rated in terms of these duties.

When a circuit breaker opens to deenergize a piece of equipment, one side of the circuit breaker usually remains e nergized, as it is connected to operating equipment. Since it is sometimes necessary to work on the circuit breaker itself, it is also necessary to have means by which the circuit breaker may be completely disconnected from other energized equipment. For this purpose disconnect switches are placed in series with the circuit breakers. By openin g these disconnests, the circuit breaker may be completely deenergized, permitting work to be carried on in safety.

Various instruments are necessary to monitor the operation of the electric power system. Usually each generator, ea ch transformer bank, and each line has its own set of instruments, frequently consisting of voltmeters, ammeters, w attmeters, and varmeters.

When a fault occurs on a system, conditions on the system undergo a sudden change. Voltages usually drop and currents increase. These changes are most noticeable in the immediate vicinity of fault. On-line analog computers, c ommonly called relays monitor these changes of conditions, make a determination of which breaker should be open ed to clear the fault, and energize the trip circuits of those appropriate breakers. 'With modern equipment, the relay action and breaker opening causes removal of fault within three or four cycles after its initiation.

The instruments that show circuit conditions and the relays that protect the circuits are not mounted directly on the power lines but are placed on switchboards in a control house. Instrument transformers are installed on the high-vol tage equipment, by means of which it is possible to pass on to the meters and relays representative samples of th e conditions on the operating equipment. The primary of a potential transformer is connected directly to the high-vol tage equipment. The secondary provides for the instruments and relays a voltage which is a constant fraction of vol tage on the operating equipment and is in phase with it. Similarly, a current transformer is connected with its primar y in the high-voltage circuit. The secondary winding provides a current which is a known fraction of the power-equip ment current and is in phase with it.

Bushing potential devices and capacitor potential devices serve the same purpose as potential transformers but usu

ally with less accuracy in regard to ratio and phase angle.

Faults on Power Systems

Faults and its Damage

Each year new designs of power equipment bring about increased reliability of operation. Nevertheless, equipment f ailures and interference by outside sources occasionally result in faults on electric power systems. On the occurrenc e of a fault, current and voltage conditions become abnormal, the delivery of power from the generating 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 conductors which ordinarily operate wit h a difference of potential between them. The connection between the conductors may be by physical metallic cont act 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 hig h 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 conductors in the lines or machines feeding the fault. The resultant rise in t emperature may cause damage by the annealing of conductors and by the charring of insulation. In the period duri ng which the fault is permitted to exist, the voltage on the system in the near vicinity of the fault will be so low th at utilization equipment will be inoperative. It is apparent that the power system designer must anticipate points at which faults may occur, be able to calculate conditions that exist during a fault, and provide equipment properly adj usted to open the switches necessary to disconnect the faulted equipment from the remainder of the system1. Ordi narily it is desirable that no other switches on the system are opened, as such behavior would result in unnecessar y modification of the system circuits.

Overload

A distinction must be made between a fault 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 under voltage 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 l imits. Nevertheless, such currents are substantially lower than in the case of a fault. Service frequently may be mai ntained, 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 party. 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 prote cted by fuses or circuit breakers which open quickly when currents above specified values persist. Distribution transf ormers are sometimes overloaded as customers install more and more appliances. The continuous monitoring of dist ribution circuits is necessary to be certain that transformer sizes are increased as load grows.

Various Faults

Faults of many types and causes may appear on electric power systems. Many of us in our homes have seen fray ed lamp cords which permitted the two conductors of the cord to come in contact with each other. When this occur s, 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. These are sometimes accidentally brought to gether by action of wind, sleet, trees, cranes, airplanes, or damage to supporting structures. Over voltages due to li ghtning or switching may 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 other and from ground by solid insulation, which m ay be oil-impregnated paper or a plastic such as polyethylene. These materials undergo some deterioration with ag e, particularly if overloads on the cables have resulted in their operation at elevated temperature. Any small void pr esent in the body of the insulating material will result in ionization of the gas contained therein, the products of whi ch react unfavorably with the insulation, deterioration of the insulation may result in failure of the material to retain i ts 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 may be the result of insulation deterioration combined with over-voltages due to lightning or swi tching transients. Short circuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied over voltages. Major insulation may fail, permitting arcs to be established between primary and se condary windings or between a winding and grounded metal part 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 shor t circuit in a single turn of the generator. Insulation breakdown may also occur between one of the windings and th e grounded steel structure in which the coils are embedded. Breakdown between different windings lying in the sam e slot results in short-circuiting extensive sections of machine.

Balanced three- phase faults, like balanced three-phase loads, may be handled on a line to-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 s ingle-phase basis.

Permanent Faults and Temporary Faults

Faults may be classified as permanent or temporary. Permanent faults are those in which insulation failure or struct ure failure produces damage that makes operation of the equipment impossible and requires repairs to be made. T emporary faults are those which may be removed by deenergizing the equipment for a short period of time, short ci rcuits on overhead lines frequently are of this nature. High winds may cause two or more conductors to swing toget her momentarily. During the short period of contact, an arc is formed which may continue as long as the line remai ns energized. However if automatic equipment can be brought into operation to deenergize the line quickly, little ph ysical damage may result and the line may be restored to service as soon as the are is extinguished. Arcs across insulators due to over voltages from lightning or switching transients usually can be cleared by automatic circuit-brea ker operation before significant structure damage occurs.

Because of this characteristic of faults on lines, many companies operate following a procedure known as high-spee d 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 man y 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

最低限度的电力系统

最低电力系统显示图.1 .该系统包括能源，主要动力，一台发电机和负荷。

能源可能是煤，天然气或石油燃烧炉加热水，并产生蒸汽的锅炉;这可能是可裂变材料，在一个核反应堆，用热的水来生产蒸汽，它可能是水池塘海拔高于发电站;或可能是石油或天然气燃烧的内燃机。

图. 1 .最低的电力系统

电动机可能是一个蒸汽驱动涡轮机，水力涡轮机或水车，或内燃机。每一个这些主要推动者有能力转换能源形式的热量，减少水，或燃料旋转轴，这反过来将驱动发电机。电力负荷的发电机可共照明，电机，电暖炉，或其他设备，单独或联合。负荷将可能发生有所不同的需求。

即使负荷可能会改变，控制系统的功能保持高速的机器常数和电压在规定限度。为了满足这些负载条件下，有必要对燃料投入的变化，为原动机的投入不相同，以及扭矩上的骨干从原动机发电机改变，以使该发电机可保持在恒定速度。此外，该领域目前的发电机，必须加以调整，以保持恒定输出电压。控制系统可能包括一个人驻扎在该电厂监控发电机输出终端，并进行必要的手动调整。在调制解调器管理站，控制系统根据发电机输出的条件，自动作出必要的变化，目前能源的投入和电气输出应控制在一定的规格。

更为复杂的系统

大多数情况下，负载不是直接连接到发电机终端，更常见的是负载要求的电源线连接与发电机有一段距离，为了保持电力供应在负载范围内的规格，这是可取的。然而，为了接近控制的发电机，这可能是在另一个工程，也许在几英里之外。

如果距离产生相当大的负荷，安装变压器在发电机和负载结束，并转交权力高压线路（图2 ），可能是可取的。出于同样的功率，高电压线路进行低电流，低损失的大小相同的线路，并提供更稳定的电压。

在某些情况下，架空线可能是不可使用的。相反，它可能更有利于使用地下电缆。电力系统出现谈到的上述情况，供电负荷必须被打断，如果因任何理由，任何组成部分的系统，必须从服务，维修或修理。

图2发电机连接，以一个长负荷连接变压器和高压线路

额外系统负载，可能需要更多的发电机来供应。另一个发电机及其相关变压器及高压线路可能会增加。

有事实表明，有一些优势，使关系发电机（ 1 ），在两端的高电压线路（ 2 ， 3 ）所示，图. 3 .只要不烦琐开发或设备没有需要采取的服务这一系统的运作将令人满意。

根据上述系统引进断路器可大大提高打开和关闭的需要。断路器加入该系统，图. 4在不干扰系统的其余部分允许选定设备切换的服务。这一安排可能是重供能的保养或修理的操作断路器的任何系统。当然，如果任何一件设备被带离服务，总负荷必须由其余的设备供应。在这种情况下必须注意避免超载。如果可能的话，停电设备预定时候负载要求低于正常值。

图.3.系统并联运行的发电机组，变压器和输电线路

图.4.系统提供的必要的断路器

图.5.发电机负载提供三个以上高压输电线路

图.5显示系统，其中3台发电机和三个负载绑在一起的三个输电线路。尽管许多将需要在这样的制度，但没有断路器是在此图显示。

典型的系统布局

在负载增加在该地区，并可能重新定时，发电机，线路，设备和其他设备构成电力系统安排取决于以何种方式。

图. 6径向提供几个电力系统负荷

但是，有某些计划在某一特定系统的设计可进行划分。三种类型的说明：径向系统，闭环系统，以及网络系统。所有这些都表明没有断路器的必要。在每一个系统，一个单一的发电机提供四个负载。

径向系统中显示图. 6. 从发电机这里的线形成一个“树”延伸。中断任何线，会关闭一个或多个负载。

在闭环系统的说明图.7 .中有了这个安排即使同一从服务所有负载都可送达。在正常操作期间在某些情况下，循环可能在某个时候开放，如答：首先是环收盘时的线，然后一节删除。如果在一条线科将要采取的那样，以这种方式没有任何服务中断发生。

图. 7.A条回路安排提供几个负荷

图.8显示了同样载荷提供服务的网络。有了这个安排每个负荷拥有两个或两个以上电路。

配电线路一般设计，使它们可归类为径向或循环电路。高压输电线路的电力系统最大的安排网络。互连的主要电力系统网络的结果作出了许多线路区段。

图.8A条网络线路提供几个负荷

辅助设备

无论正常运作还是发生短路断路器是必要的断开设备。断路器的设计必须履行正常的负载电流不断，承受极高电流期间发生故障，并单独接触和明确的电路存在故障由断路器负责。当断路器打开或断开一台设备，一方断路器通常仍然连接，因为它是连接在操作系统的设备。因为它有时需要断路器本身工作，还必须有办法使断路器可完全断开其他带电设备，为此断路器与一系列断开开关放置在一起。通过开放这些开关，断路器可完全被控制，它是允许工作继续下去的安全设备.监测运作电力系统，不同的线路开断是必要的。通常每个发生器，每个变压器，每一行都有自己的一套工具，经常组成的电压表，电流表，瓦特表和无功功率表. 一个系统发生故障，系统的条件突然发生变化，通常会电压下降和电流上升，这些变化是最为显着邻近故障。在线模拟计算机，通常称为继电器监测，在这些变化的条件下，作出决定，其中断路器应明确是否出错开断，并连接这些线路适当的电路断路器。有现代设备，中继断路器开断行动和清除故障原因在三个或四个周期后发起.仪器显示电路条件和继电器保护电路不直接安装在供电线路，但都放在交换机的控制房间。仪器变压器上安装了高电压设备，通过这一手段，可以传授给继电器有代表性的样本条件的经营设备。首要的一个潜在的变压器直接连接到高电压设备。规定的设计和继电器的电压是恒定电压部分进行作业设备。电流互感器是它的主要的高电压电路。二次绕组的电流提供了一个电力设备，是在当前阶段设备和装置电容器服务于同一目的，但潜在的变压器通常较少，准确性方面比较差。

电力系统故障及损伤

每年新设计的电力设备带来更高的可靠性运行。不过，设备故障和干扰，外部来源偶尔导致电力系统的故障。发生故障，电流和电压条件变得异常，提供功率从发电站的负载可能是相当重要地区，如果开断设备不及时断开余下的系统，破坏可能会导致其他经营设备.A件故障是有意或无意连接在一起的两个或两个以上的导体这通常与不同的潜在关系。之间的连接导线可能是因身体的金属接触，也可以是通过一个弧形。在故障，电压两部分之间是减少到零的情况下在金属与金属的接触，或以一个非常低价值的情况下，连接是通过一个弧形。电流的异常高强度流经网络故障点。这些短路电流通常会远远超过设计能力的热导体中的线路或设备故障喂养。由此产生的温度上升可能造成损害的退火导线和烧焦绝缘。在期间，故障被允许存在，电压对系统在不久的断层附近的将是如此之低，利用设备将无法运作。显然，电源系统设计必须预测点，这可能会发生故障，能够计算条件中存在的故障，并提供设备的适当调整，打开开关必须断开断设备的剩余系统。通常，可取的做法是没有任何其他系统上的开关打开，因为这种行为会造成不必要的修改系统电路。

超负荷

必须区分故障和超负荷，超负荷意味着只有加载大于设计值已经实施的系统。在这种情况下的电压在超负荷可能低一点，但不是零。这电压条件下可以延长一段距离超出了超载点到其余的系统。电流中的重载设备的高，可能会超过热设计极限。然而，这种电流大大低于如故障。服务经常可维持，但低于标准电压. 在家中超载是常见的现象。例如，一

个家庭主妇可能在街道插件5胡扯铁杆到厨房电路，这种超负荷，如果允许继续下去，将导致取暖的电线难以承受，并可能最终断路。为了防止这种麻烦，住宅电路保护熔断器或断路器开放时迅速电流值坚持上述规定。配电变压器超负荷有时为顾客安装越来越多的家用电器。连续监测电路的分布是必要的肯定，变压器的大小随着负荷的增长。

各种故障

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常驻故障和临时故障

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