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Brief Introduction to The Electric Power System Part 1 Minimum electric power systemA minimum electric power system is shown in Fig.1-1, the system consists of an energy source, a prime mover, a generator, 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 may 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. The prime mover may be a steam-driven turbine, a hydraulic turbine or water wheel, or an internal combustion engine. 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. Probably the load will vary from minute to minute as different demands occur.The control system functions （are） to keep the speed of the machines substantially constant and the voltage within prescribed limits, even though the load may change. To meet these load conditions, it is necessary for fuel input to change, for the prime mover input to vary, and for torque on the shaft from the prime mover to change 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 plant who watches a set of meters on the generator output terminals and makes the necessary adjustments manually. In a modern station, the control system is a servomechanism that senses generator-output conditions and automatically makes the necessary changes in energy input and field current to hold the electrical output within certain specifications.Part 2 More Complicated SystemsIn most situations the load is not directly connected to the generator terminals. More commonly the load is some distance 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, perhaps several miles away.If the distance from the generator to the load is considerable, it may be desirable to install transformers at the generator and at the load end, and to transmit the power over a high-voltage line (Fig.1-2). For the same power, the higher-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 underground cable. 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 moved from service for maintenance or repair. Additional system load may require more power than the generator can supply. Another generator with its associated 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 end of the high-voltage lines (2 and 3), as shown in Fig.1-3. This system will operate satisfactorily as long as no trouble develops 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.1-4, permit selected piece of equipment to switch out of service without disturbing the remainder of system. With this arrangement any element of the system may be deenergized for maintenance or repair by operation of circuit breakers. Of course, if any piece of equipment is taken out of service, then the total load must be carried by the remaining equipment. Attention must be given to avoid overloads during such circumstances. If possible, outages of equipment are scheduled at times when load requirements are below normal. Fig.1-5 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. Part 3 Typical System LayoutThe generators, lines, and other equipment which form an electric system are arranged depending on the manner in which load grows in the area and may be rearranged from time to time. However, there are certain plans into which a particular system design may be classified. Three types are illustrated: 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.1-6. Here the lines form a “tree” spreading out from the generator. Opening any line results in interruption of power to one or more of the loads.The loop system is illustrated in Fig.1-7. With this arrangement all loads may be served even though one line section 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.1-8 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-voltage transmission lines of most power systems are arranged as network. The interconnection of major power system results in networks made up by many line sections. Part 4 Auxiliary EquipmentCircuit breakers are necessary to deenergize equipment either for normal operation or on the occurrence of short circuits. 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 breakers 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 energized, 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 opening these disconnectors, 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, each transformer bank, and each line has its own set of instruments, frequently consisting of voltmeters, ammeters, wattmeters, 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, commonly called relays, monitor these changes of conditions, make a determination of which breaker should be opened 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-voltage equipment, by means of which it is possible to pass on to the meters and relays representative samples of the conditions on the operating equipment. The primary of a potential transformer is connected directly to the high-voltage equipment. The secondary provides for the instruments and relays a voltage which is a constant fraction of voltage on the operating equipment and is in phase with it；similarly, a current transformer is connected with its primary in the high-current circuit. The secondary winding provides a current that is a known fraction of the power-equipment current and is in phase with it. Bushing potential devices and capacitor potential devices serve the same purpose as potential transformers but usually with less accuracy in regard to ratio and phase angle.中文翻译：电力系统的简介第一部分：最小电力系统 一个最小电力系统如图1-1所示，系统包含动力源，原动机，发电机和负载。 动力源可能是火炉中的煤，燃气或石油，加热锅炉中的水而产生蒸汽。它也可能是可裂变的材料，如原子核反应堆，加热水而产生蒸汽；也可能是在高海拔发电站上水池的水；或者是石油或燃气在内燃机中的燃烧。 原动机可能是汽轮机，水轮机，水车，或内燃机。这些原动机中的一种都有能力把其他形式的能量转化为热能，势能，或使轴转动的能量，进而驱动发电机工作。发电机上的电负载可能是灯，电动机，加热器或是其他设备的个体或组合。负载可能在每一分钟内都会产生不相同的需要。控制系统的功能是保持机器的速度是个稳定值和保持电压在规定的范围内变化，即使负载可能发生变化。要满足这些负载的条件，需要燃料输入的变化，需要原动机输入的变化，这是为了改变发电机中原动机轴的转矩保持恒定。此外，发电机的励磁电流必须加以调整，以保持恒定电压输出。该控制系统可能包括一个驻扎在电厂里的人，他看管着发电机输出终端的一整套仪表和有时进行必要的手动调整。在一个现代化的电站里，控制系统用来感应发电机输出条件，在能源输入中自动进行必要的改变和磁场电流在规定的范围内保持电力输出的一个伺服机构。第二部分：更复杂的系统在大多数情况下负载不是直接与发电机终端连接在一起的。更常见的是负载与发电机有一段距离，需要电源线把它们连接起来。理想情况下是在规定的范围内对负载保持电能补给，进而控制附近的发电机。发电机可能在另一幢大楼里，也可能有几英里远。如果发电机到负载的距离是合理的，理想的情况是在发电机和负载的端部都安装变压器，通过高压线路传送电能（如图1-2）。 对于相同的电能输送，更高的电压线路会流过更小的电流，对于同样尺寸的电线则降低更多的损耗，提供更稳定的电压。 在某些情况下架空线可能是不合适的。相反，使用地下电缆可能会更有利。上面谈到的电力系统，不管任何原因，负载的电能补给必须被中断，供电中系统的任何元件必须被维护或者被移除。其他系统的负载所需的电能可能比发电机需要的电能更大。另外，与发电机相关的变压器和高压线路可能被增加。由上可以说明连接发电机（1）和高压线路末端（2和3）是有一些优势的，如图1-3所示。这个系统只要没有任何故障或没有需求的设备在供电中被移除时，其运行将令人满意。上面的系统通过引进断路器将有很大的改善，这需要使用断路器的开通和关断功能。增加断路器的系统，如图1-4，允许选择某台设备在供电中被关断，不会影响剩余的系统设备。采用这种布局的系统可能通过断路器的操作对其进行不带电的维护或维修。当然，任何一台设备从供电中被移除，总负载必须在剩余设备中被运行。在上述情况下必须注意的是避免过载。如果可能，在低于正常值的负载需求，设备的停电是要有时间计划的。如图1-5所示，通过三条输电线路，系统的三个发电机和三个负载是捆绑在一起的。在此图中没有断路器，然而在很多时候这样的系统是被需要的。第三部分:典型的系统布局电力系统中发电机，线路和其他设备根据该地区负载的增加和变化不时的被重新排放。然而，真正的计划归类是一个特别的系统设计。三种系统类型说明：放射性系统，循环系统和网络系统。所有这些都说明不需要断路器。这些系统中的每一个都是由一个发电机来供给四个负载。放射性系统如图1-6所示。这些线路形成一棵树的形状，从发电机蔓延出去。开通任何线路可能会导致一个或多个负载电源的中断。如图1-7循环系统的图解。这样的排列能给所有负载供电，即使线路中的一部分在输电过程中被移除。在某些正常操作情况下，循环中的某些点可能被打开。例如A点，部分线路被移除，循环在A点处首先被关闭，然后线路节点被移除。这种方式是不会发生供电中断的。如图1-8所示通过网络对相同负载供电。在这样的排列下，每个负载的供电将通过两条或更多的线路来完成。配电线路有共同的设计方案，以便它们能作为放射型或循环型线路而被分类。大多数高压传输线路作为网络型系统被排放。主要电力系统互相连接，是通过许多线路而组成网络。第四部分：辅助设备在正常运行或发生短路时，断路器是必要的断电设备。断路器的设计必须不断地承受正常负载，承受超高电流，这是为了在发生故障期间，断开连接和清除这条线路的故障。断路器根据其职责来设定额定值。当断路器断开，切断一台设备的电源时，断路器的一侧通常保持通电，因为它是连接在运行中的设备。由于它有时需要对其本身进行操作，也需要通过对断路器与带电设备进行完全隔离的手段。为此，隔离开关被放置与断路器串联。通过断开这些隔离开关，断路器可完全切断电源，使工作安全进行。使用各种仪表去监视电力系统的运行是必要的。一般每一个发电机，每一个变压器组，每一条线路都有它自己的整套仪表，通常包括：电压表，电流表，有功功率表和无功功率表。当系统发生故障时，在这种条件下系统要经受一个突然的改变。电压通常下降，电流上升。这些改变最明显的是紧邻的故障。在线模拟计算机，通常称为继电监视器，去监视这些情况下的改变。断路器的断开，有效得清除故障和带电的跳闸回路。随着现代化设备的发展，故障发生后，继电器的动作和断路器的开启在三个或四个周期内可将故障移除。仪表可显示电路情况和保护电路的继电器有没有直接安置在电源线上，但是仪表被放置在控制房里的开关屏上。仪表互感器被安装在高电压设备上，意味着它可以在设备运行的条件下传递仪表和继电器的典型信号。电压互感器的原边直接连接着高压设备。副边在运行的设备上，为仪表和继电器电压提供一个恒定的电压分数和电压相位。同样地，电流互感器连接着高压回路的原边。副边绕组提供一个已知的电源设备分数和与之同相的电流。潜在的衬套设备和潜在的电容器设备服务于相同的目的，视为潜在的变压器，但通常就变比和相角而论，精确性更低。 10
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