Minimum Pow er System
A minimum electric pow er system is show n in Fig. 1. the system cons ists 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 w ater and generate steam in a boiler; it m ay be fissionable mater ial w hich, in a nuclear reactor, w ill heat w ater to produce steam; it may be w ater in a pond at an elevation above the generating station; or it may be oil or gas burned in an internal combust ion engine.
Fig. 1. The minimum electric pow er system
The prime mover may be a steam-driven turbine, a hydraulic turbine or w ater wheel, or an internal combustion engi ne. E ach one of these prime movers has the ability to convert energy in the form of heat, falling w ater, or fuel into rotation of a shaft, w hich in turn w ill drive the generator.
The electrical load on the generator may be lights, motors, heaters, or other devices, alone or in combination. P rob ably the load w ill 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 w ithin 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 pr ime 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 pow er pla nt that w atches a set of meters on the generator-output ter minals and makes the necessary adjustments manually. 3In a modem station, the control system is a servomechanis m that senses a generator-output conditions and autom atically makes the necessary changes in energy input and field current to hold the electrical output w it hin certain sp
ecifications.
More Complicated Systems
In most situations the load is not directly connected to the generator ter minals. More commonly the load is some di stance from the generator, requir ing a pow er line connecting them. It is desirable to keep the electric pow er supply at the load w ithin specifications. How ever, the controls are near the generator, w hich may be in another building, p erhaps several miles aw ay.
If the distance from the generator to the load is considerable, it may be desir able to install transformers at the gen erator and at the load end, and to trans mit the pow er over a high-voltage line (Fig. 2). For the same pow er, the hi gher-voltage line carries less current, has low er losses for the same w ire size, and provides more stable v oltage.
In some cases an overhead line may be unacceptable. Instead it may be advantageous to use an under ground ca ble. With the pow er systems talked above, the pow er 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 transfor mers and a high-voltage line to a distant load
Additional system load may requir e more pow er than the generator can supply. Another generator w ith its associate d transformers and high-voltage line might be added.
It can be show n that there are some advantages in making ties betw een the generators (1) and at the ends of the high-voltage lines (2and 3), as show n in Fig. 3. This system w ill 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 br eakers, w hich may be opened and closed as needed. Circuit breakers added to the system, Fig. 4, per mit selected piece of equipment to sw itch out of servi ce w ithout disturbing the remainder of system. With this arrangement any element of the system may be r eenergize d for maintenance or repair by oper ation 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 over loads dur i ng such circumstances. If possible, outages of equipment are scheduled at times w hen load requirements are below nor mal.
Fig. 1-3 A system w ith parallel oper ation of the generators, of the transformers and of the trans mission lines
Fig. 4A system w ith necessary circuit breakers
Fig. 5Three generators supplying three loads over high-voltage trans mission lines
Fig. 5 show s a system in w hich three generators and three loads are tied together by three trans mission lines. No circuit breakers are show n in this diagram, although many w ould be required in such a system.
Typical System Layout
The gener ators, lines, and other equipment w hich form an electric system are arranged depending on the manner i
n w hich load grow s in the area and may be rearranged from time to time.
Fig. 6 A radial pow er system supply ing several loads
How ever, there are certain plans in to w hich a particular system des ign may be classified. Three types are illustrate d: the radial system, the loop system, and the netw ork system. All of these are show n w ithout the necessary circuit breakers. In each of these systems, a single generator serves four loads.
The radial system is show n in Fig. 6. Here the lines form a “tree” spreading out from the generator. Opening any li ne results in interruption of pow er 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 dur ing nor mal operation, the loop may b
e 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 show s the same loads being served by a netw ork. With this arrangement each load has tw o or more circuits over w hich it is fed.
Distribution circuits are commonly des igned so that they may be classified as radial or loop circuits. The high-voltag e trans mission lines of most pow er systems are arranged as netw orks. The interconnection of major pow er systems results in netw orks made up many line sections.
Fig. 8A netw ork of lines for supplying several loads
Auxiliary E quipment
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 nor mal-load currents continuously, to w ithstand 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 ter ms of these duties.
When a circuit breaker opens to deenergize a piece of equipment, one side of the circuit breaker usually rem ains e nergized, as it is connected to operating equipment. Since it is sometimes necessary to w ork on the circuit breaker itself, it is also necessary to have means by w hich the circuit breaker may be completely disconnected from other energized equipment. For this purpose disconnect sw itches are placed in series w ith the circuit breakers. By openin g these disconnests, the circuit breaker may be completely deenergized, per mitting w ork to be carried on in safety.
Various instruments are necessary to monitor the operation of the electr ic pow er system. Usually each generator, ea ch transformer bank, and each line has its ow n set of instruments, frequently consisting of voltmeters, ammeters, w attmeters, and var meters.
When a fault occurs on a system, conditions on the system undergo a sudden change. Voltages usuallreactor 翻译
y 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 deter minat ion of w hich 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 w ithin 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 pow er lines but are placed on sw itchboards 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 w hich is a constant fraction of vol tage on the operating equipment and is in phase w ith it. Similarly, a current transformer is connected w ith its primar y in the high-voltage circuit. The secondary w inding provides a current w hich is a know n fraction of the pow er-equip ment current and is in phase w ith it.
Bushing potential devices and capac itor potential devices serve the same purpose as potential transformers but usu

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