1、The Electric Power System Minimum Power System A minimum electric power system is shown in Fig. 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 f
2、issionable 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
3、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 li
4、ghts, 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 to keep the speed of the machines substantially constant and the voltage within prescribed limits, even though the load may cha
5、nge. To meet these load conditions, it is necessary for fuel input to change, for the prime mover input to vary, and for the torque on the shaft from the prime mover to the generator to change in order that the generator may be kept at constant speed. In addition, the field current to the generator
6、must be adjusted to maintain constant output voltage. The control system may include a man stationed in the power plant 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
7、 a generator-output conditions and automatically makes the necessary changes in energy input and field current to hold the electrical output within certain specifications. More Complicated Systems In most situations the load is not directly connected to the generator terminals. More commonly the loa
8、d 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 gen
9、erator 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. 2). For the same power, the higher-voltage line carries less current, has lower losses for the same wire size, and provides m
10、ore stable voltage. In some cases an overhead line may be unacceptable. Instead it may be advantageous to use an under ground 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 removed from service for
11、maintenance or repair. . 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 associated transformers and high-voltage line might be added. It can be shown th
12、at 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 develops or no equipment needs to be taken out of service. The above system may be vastly impro
13、ved 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 service without disturbing the remainder of system. With this arrangement any element of the system may be reenergi
14、zed for maintenance or repair by operation of circuit breakers. Of course, if any piece of equipment is taken out of service, the total load must then carried by the remaining equipment. Attention must be given to avoid overloads during such circumstances. If possible, outages of equipment are sched
15、uled 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.
16、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
