Major components of a power system are- synchronous generators, synchronising equipment, circuit breakers, isolators, earthing switches, bus-bars, transformers, transmission lines, current transformers, potential transformers, relay and protection equipment, lightning arresters, station transformer, motors for driving auxiliaries in power station. Some of the components will be discussed here as shown in Fig. 1.7 .
The synchronous generators used in generating stations are revolving field type owing to its inherent advantages.
The synchronous generators, based on the type of prime movers to which they are mechanically coupled, may be classified as:
(ii) Turbo-generators, and
(iii) Diesel engine driven generators.
Power transformers are used for stepping-up the voltage for transmission at generating stations and for stepping-down voltage for further distribution at main step-down transformer substations. Usually naturally cooled, oil immersed, known as ON type, two winding, three-phase transformers, are used up to the rating of 10 MVA.
The transformers of rating higher than 10 MVA are usually air blast cooled. For very high rating, the forced oil, water cooling and air blast cooling may be used. For regulating the voltage the transformers used are provided with on load tap changer.
They are put in operation during load hours and disconnected during light load hours i.e. they are usually operated at approximately full load. This is possible because they are arranged in banks and can be thrown in parallel with other units or disconnected at will. So power transformers are designed to have maximum efficiency at or near full load (i.e. with iron loss to full-load copper loss ratio of 1: 1).
Power transformers are designed to have considerable leakage reactance than is permissible in distribution transformers because in power transformers inherent voltage regulation is not as much important as current limiting effect of the higher leakage reactance. Power transformers usually make use of flux density of 1.5 to 1.77; have percentage impedance ranging from 6-18% and regulation 6-10%.
The transformer specifications cover the following:
5. Connections (Δ or λ in case of 3-phase transformer);
8. Type (power or distribution);
9. Ambient temperature (generally average 40°C);
10. Type of cooling – (a) cooling medium-air, oil or water (b) circulation type- natural or forced (c) simple or mixed cooling;
11. Temperature rises above ambient in °C depending upon the class of winding insulation;
12. Voltage regulation [(a) Per cent or pu at full load at 75°C unity pf or 0.8 pf lag (b) Impedance-per cent or pu (c) Reactance-per cent or pu];
14. Efficiency-in per cent or pu at full load, 1/2 load, 3/4 load at unity power factor and 0.8 pf.
Power transformers are covered under IS 2026-1962.
The transformers are generally installed upon lengths of rails fixed on concrete slabs having foundation 1 to 1(1/2) metre deep.
Everyone is familiar with low voltage switches and rewirable fuses. A switch is used for opening and closing of an electric circuit while a fuse is used for over-current protection. Every electric circuit needs a switching device and protective device. Switching and protective devices have been developed in different forms. Switchgear is a general term covering a wide range of equipment concerned with switching and protection.
(i) Switching during normal operating conditions for the purpose of operation and maintenance.
(ii) Switching during abnormal conditions such as short-circuits and interrupting the fault currents.
The first of above could be served by relatively simple switches because it is relatively simple as it involves normal currents which are easy to interrupt. The second function is, however, complex. With the advancement of electrical power system the lines and other equipment operate at very high voltage and carry large currents.
Whenever a short-circuit occurs, a heavy current flows through the equipment causing considerable damage to the equipment and interruption of service. In order to avoid such damage every part of the power system is provided with a protective relaying system and an associated switching device.
The function of protective relaying system is to cause the prompt removal from service of any element of a power system when it suffers a short-circuit, or when it starts to operate in any abnormal manner that might cause damage or otherwise interfere with the effective operation of the rest of the system.
About Structure of power system and its components
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