Brief Introduction to The Electric Power System
Part 1 Minimum electric power system
A 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 loadservice fault 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 Systems
In 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.
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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 p
ower 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.
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