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 .
Component # 1. Synchronous Generators:
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:
1. KVA rating;
2. Rated voltages;
3. Number of phases (single or three phases);
4. Rated frequency;
5. Connections (Δ or λ in case of 3-phase transformer);
6. Tappings if any;
7. Type of core (core or shell);
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];
13. No-load current in amperes or per cent of rated current at rated voltage and rated frequency;
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.
Component # 2. Switchgear:
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.
In a power system switchgear serves two basic purposes:
(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.
The relaying equipment is aided in this task by circuit breakers that are capable of disconnecting the faulty element when they are called upon to do so by the relaying equipment. In addition to circuit breakers and protective relays, the associated equipment for controlling, regulating and measuring can also be considered as switchgear devices.
Thus the apparatus including its associated auxiliaries employed for controlling, regulating or switching on or off the electrical circuits in the electrical power system is known as switchgear. It includes switches, fuses, circuit breakers, isolators, relays, current and potential transformers, indicating instruments, lightning arresters and control panels.
A circuit breaker is a device that interrupts the abnormal or fault currents and in addition performs the function of a switch. Circuit breakers and fuses both possess a time element of operation, whereby they operate practically instantaneously on short-circuits but with definite time lags on overloads. Circuit breakers can be reset in less time and with less trouble that are required to replace blown fuses and spare parts are seldom required.
For low voltage circuits fuses may be used to isolate the faulty circuits; but for higher voltages, say from 3.3 kV upwards, isolation is achieved by circuit breaker. Circuit breakers may also be preferred where continuity of service is an important consideration or where frequent fuse replacement may be expected. However, for low capacities a fuse with circuit breaking arrangement is quite useful and economical.
As the circuit breakers have to be accessible for inspection and maintenance, the Indian Standard regulations require the provision for separation of the breaker from the live parts by means of isolating switches. Isolating switches are employed for separating the disconnected element from the live portion of the system. They should be employed only after the corresponding circuit breakers have been opened. Isolating switches are employed for isolating a piece of equipment from adjacent live elements for purposes of safety.
Circuit breakers are mechanical devices designed to close or open contact members, thus closing or opening of an electrical circuit under normal or abnormal conditions.
Automatic circuit breakers, which are usually employed for the protection of electrical circuits, are equipped with a trip coil connected to a relay or other means, designed to open the breaker automatically under abnormal conditions, such as over-current.
The automatic circuit breakers perform the following duties:
(i) It carries the full-load current continuously without overheating or damage,
(ii) It opens and closes the circuit on no load,
(iii) It makes and breaks the normal operating current and
(iv) It makes and breaks the short-circuit currents of magnitude up to which it designed for.
The circuit breaker performs first three duties satisfactorily but in performing fourth duty i.e., when it is to make or break short-circuit currents, it is subjected to mechanical and thermal stresses. The circuit breakers are rated in terms of maximum voltage, number of poles, frequency, maximum continuous current carrying capacity, maximum interrupting capacity and maximum momentary and 4-s current carrying capacity.
The interrupting or rupturing capacity of a circuit breaker is the maximum value of current which can be interrupted by it without any damage. The circuit breakers are also rated in MVA which is the product of interrupting current, rated voltage and 10-6.
A circuit breaker is a switching and current interrupting device. It consists, essentially, of fixed and moving contacts, which are touching each other and carry the current under normal conditions i.e., when circuit breaker is closed. When the circuit breaker is closed, the current carrying contacts, called the electrodes, engage each other under the pressure of a spring.
During the normal operating condition the circuit breaker can be opened or closed by a station operator for the purpose of switching and maintenance. To open the circuit breaker, only a small pressure is required to be applied on a trigger. Whenever a fault occurs on any part of the power system, the trip coils of the breaker get energized and the moving contacts are pulled apart by some mechanism, thus opening the circuit. The separation of current carrying contacts produces an arc.
The current is thus able to continue until the discharge ceases. The production of arc not only delays the current interruption process but it also generates enormous heat which may cause damage to the system or to the breaker itself. Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest possible time so that heat generated by it may not reach a dangerous value.
The basic construction of a circuit breaker requires the separation of contacts in an insulating fluid which serves two functions:
1. Extinguishes the arc drawn between the contacts when the circuit breaker opens.
2. Provides insulation between the contacts and from each contact to earth.
The insulating fluids commonly used for this purpose are as follows:
1. Air at atmospheric pressure.
2. Compressed air.
3. Oil producing hydrogen for arc extinction.
4. Ultra high vacuum.
5. Sulphur hexa-fluoride (SF6).
The fluids used in circuit breakers should have the properties of high dielectric strength, non-inflammability, high thermal stability, arc extinguishing ability, chemical stability, and commercial availability at moderate cost.
Of the simple gases air is the cheapest and most widely used for circuit breaking. Hydrogen has better arc extinguishing property but it has lower dielectric strength as compared to air. Also if hydrogen is contaminated with air, it forms an explosive mixture. Nitrogen has similar properties as air.
CO2 has almost the same dielectric strength as air but is a better arc extinguishing medium at moderate currents. Oxygen is a good extinguishing medium but is chemically active. SF6 has outstanding arc-quenching properties and good dielectric strength. Of all these gases SF6 and air are used in commercial gas blast-circuit breakers.
Air-circuit breakers are often used instead of oil up to 15 kV in these units and oil reclousures are also sometimes used to cut cost in small rural substations.
Since isolators (or isolating switches) are employed only for isolating circuit when the current has already been interrupted, they are simple pieces of equipment. They ensure that the current is not switched into the circuit until everything is in order.
Isolators or disconnect switches operate under no load condition. They are not equipped with arc-quenching devices. They do not have any specified current breaking capacity or current making capacity. The isolators in some cases are used for breaking charging current of transmission line.
Isolators are employed in addition to circuit breakers, and are provided on each side of every circuit breaker to provide isolation. While opening a circuit, the circuit breaker is opened first, then isolator. If an isolator is opened carelessly, when carrying a heavy current, the resulting arc could easily cause a flash-over to ground.
This may shatter the supporting insulators and may even cause a fatal accident to the operator, particularly in hv circuits. While closing a circuit, the isolator is closed first, then circuit breaker. Isolators are necessary on the supply side of the circuit breakers in order to ensure isolation (disconnection) of the circuit breaker from the live parts for the purpose of maintenance. Automatic switching of isolators is preferred.
Isolators employed in power systems are usually 3-pole isolators, each having three identical poles. Each pole consists of two or three insulator posts mounted on a fabricated support. The fixed and moving conducting parts are of copper or aluminium rods. During the opening operation the conducting rods swing apart and isolation is obtained. The simultaneous operation of three poles is obtained by mechanical interlocking of the three poles.
To prevent the mal-operation, the isolator is provided with the following interlocking’s:
(i) Interlocking between three poles for simultaneous operation.
(ii) Interlocking with circuit breakers – Isolator cannot be opened unless the circuit breaker is opened and circuit breaker cannot be closed unless the isolator is closed.
(iii) Load interrupter switches – In addition to isolators and circuit breakers, there is one more device, called the load-interrupting switch, which combines the functions of the isolator and a switch. They are designed only for breaking and making the load currents.
These switches are designed and used to close and open high voltage circuits under normal working conditions (at normal load). The arc extinguishing device of the load interrupter is made in the form of a split, moulded plastic chute fitted with organic glass inserts. This chute surrounds the moving knife of the arc extinguishing system. The stationary arcing contact is located in the lower part of the chute.
Load interrupter switches are intended only for handling low-energy arcs resulting from the interruption of load current and altogether unsuitable for extinction of high-energy fault current arcs.
When the switch is opened, the working contacts between which the arc is drawn separate. Acted upon by the high temperature of arc, the walls of the organic material insert generated gases (mainly hydrogen), which create a longitudinal blast serving to extinguish the arc. Lever- arm manually operating mechanisms are employed for closing and opening the load interrupter switches.
It is a wide practice to install load-interrupting switches in low-capacity installations like industrial-shop, urban and rural, and like distribution substations where it is possible to provide short-circuit current protection with high-voltage fuses and where the only duty of load- interrupting switch is to make and break the load current.
The installation of a load-interrupting switch, including the high-voltage fuses, is not only cheaper, but also usually requires less space than the installation of a high-voltage circuit breaker.
Earthing switch is connected between the line conductor and earth. Normally it is open and it is closed to discharge the voltage trapped on the isolated or disconnected line. When the line is disconnected from the supply end, there is some voltage on the line to which the capacitance between the line and earth is charged.
This voltage is significant in hv systems. Before commencement of maintenance work it is necessary that these voltages are discharged to earth by closing the earthing switch. Normally, the earthing switches are mounted on the frame of the isolator.
Component # 3. Bus-Bars:
Bus-bar (or bus in short) term is used for a main bar or conductor carrying an electric current to which many connections may be made.
Bus-bars are merely convenient means of connecting switches and other equipment into various arrangements. The usual arrangement of connections in most of the substations permits working on almost any piece of equipment without interruption to incoming or outgoing feeders.
In some arrangements two buses are provided to which the incoming or outgoing feeders and the principal equipment may be connected. One bus is usually called the “main” bus and the other “auxiliary” or “transfer” bus. The main bus may have a more elaborate system of measuring instruments, relays etc. associated with it. The switches used for connecting feeders or equipment to one bus or the other are called “selector” or “transfer” switches.
Bus-bars may be of copper, aluminium or steel. Copper has a comparatively low resistivity and also the advantage of relatively high mechanical strength; this makes it economical to use copper bus-bars in installations of very large capacity where the currents are particularly heavy.
During 1960’s the need for substituting the copper with aluminium became very urgent, particularly in countries like India where copper is imported. Now aluminium is being increasingly used for various switchgear installations due to its numerous advantages over copper such as higher conductivity on weight basis, lower cost for equal current carrying capacity, excellent corrosion resistance and ease of formability.
For proper reliable electrical connections aluminium buses are coated with silver. The aluminium used for bus-bars should have high conductivity, good mechanical properties, high softening temperature etc. Steel bus-bars have a comparatively high specific resistance (about 7 times greater than that of copper).
Furthermore, the losses due to hysteresis and eddy currents when carrying ac are also considerable. The primary advantage is that steel bus-bars cost very little. The influence of the above factors is seen in the wide application of steel bus-bars in low-capacity installations where the load currents do not exceed 200-300 A.
The bus-bars used in substations are usually bare rectangular x-section bars (but they can be of other shapes such as round tubes, round solid bars, or square tubes) as they are more economical in comparison with round solid bus-bars. This is explained by the fact that rectangular- section bus-bars of the same x-sectional area have a higher rate of heat dissipation due to their greater cooling surface.
Furthermore, the ac (or effective) resistance of a round bus-bar is greater than that of rectangular-section bus-bars because of the skin effect. Because of these two facts rectangular-section bus-bars are able to carry larger load currents than round solid bus-bars (for the same cross-sectional area and the same temperature rise).
For the same cross-sectional area and the same temperature rise, copper bus-bars will have the greatest and steel bus-bars the least-permissible current-carrying capacity because of the difference in their resistance. As the size of the bus-bars increases, their heat dissipation capacity falls off and the permissible current density must therefore be reduced.
If the load current to be carried exceeds the permissible current for a single-strip bus-bar of the greatest available size, each phase of the bus-bars shall have to be assembled of several strips arranged in a stack and clamped on post insulators. The air-gaps left between the strips in a bus-bar stack are usually made equal to the thickness of the strips, this is necessary for adequate cooling.
As the number of strips in a bus bar stack is increased, the current with which it is permissible to load the bus-bar cannot be raised in direct proportion to the number of strips; the increase in permissible current must be appreciably less because the conditions for cooling become more unfavourable. Moreover, when dealing with ac it is necessary to take another factor into consideration, i.e., the proximity effect. The result is that the bus-bar metal in a stacked bus-bar is much less efficiently utilised than in a single-strip bus-bar.
It is for the above reason that bus-bars for ac are designed with not more than two, and, rarely, with three strips. For dc, the bus-bars may consist of a large number of strips because there is no proximity effect and current distribution is uniform.
In large-capacity ac installations with extremely heavy working currents it is more effective to install box-shaped types of bus-bars of aluminium or copper. In this case the bus-bar takes the form of a hollow square conductor in which the metal is much more efficiently utilized than it would be in a stacked-strip bus-bar. The box shape also makes for better cooling than is obtained with stacked-strip bus-bars.
When installations operate at 33 kV and high voltages, the bus-bars have to be designed with due consideration for corona effects.
All rigid types of bus-bars mounted on support insulators are coated with enamel paints of the following colours:
Three-phase systems—Red, yellow and blue to indicate different phases.
DC systems—Positive bus-bars-claret-coloured; negative bus-bars—blue.
Coating bus-bars with paint improves their rates of cooling to some extent and therefore permits them to carry a larger load current. By coating steel bus-bars with paint we protect them from corrosion. The use of different colours is important because it helps the operating personnel to distinguish between the different phases of the installation at a glance.
Flexible bus-bars (bare stranded conductors) are not coated with paint. To identify the phases of the bus-bars, disks painted with the respective phase colour are hung from the bus-bars.
The most common sizes of bus-bars are 25 × 6 (150 mm2); 50 × 6 (300 mm2), 75 × 6 (450 mm2); 100 × 6 (600 mm2); 125 × 6 (750 mm2); 50 × 10 (500 mm2); 75 × 10 (750 mm2); 100 × 10 (1,000 mm2); 125 × 10 (1,250 mm2); 150 × 10 (1,500 mm2); 200 × 10 (2,000 mm2); 75 × 12 (900 mm2; 100 × 12 (1,200 mm2); 125 × 12 (1,500 mm2); 150 × 12 (1,800 mm2); 200 × 12 (2,400 mm2); 250 × 12 (3,000 mm2). The bus-bars are of 5 or 6 metres in length.
The early substations were generally with flexible bus design. A flexible bus consists of flexible ACSR or all-aluminium alloy stranded conductors supported by strain insulators from each end. The flexible bus is held at higher level above the different substation equipment. The connections between the flexible bus and the terminals of substation equipment are made by flexible conductors held in vertical or inclined plane.
Rigid bus-bars are easy to maintain. They are at lower height. Connections to substation equipment are easy. Aluminium tubes are preferred for rigid bus-bars.
A substation usually has a combination of rigid bus-bars and flexible bus-bars. ACSR conductors are preferred for flexible bus-bars.
The bus-bars are designed to carry certain normal current continuously. The x-section of conductors is designed on the basis of rated normal current and permissible temperature rise. The value of x-section so obtained is verified for temperature rise under short-time short-circuit current. The bus-bar conductors are supported on post insulators or strain insulators.
Component # 4. Lightning Arresters:
The lightning arrester is a surge diverter and is used for the protection of power system against the high voltage surges. It is connected between the line and earth and so diverts the incoming high voltage wave to the earth.
Lightning arresters act as safety valves designed to discharge electric surges resulting from lightning strokes, switching or other disturbances, which would otherwise flash-over insulators or puncture insulation, resulting in a line outage end possible failure of equipment.
They are designed to absorb enough transient energy to prevent dangerous reflections and to cut off the flow of power-frequency follow (or dynamic) current at the first current zero after the discharge of the transient. They include one or more sets of gaps to establish the breakdown voltage, aid in interrupting the power follow current, and prevent any flow of current under normal conditions (except that gap shunting resistors, when used to assure equal distribution of voltage across the gaps, permits a very small leakage current).
Either resistance (valve) elements to limit the power follow current to values the gaps can interrupt, or an additional arc extinguishing chamber to interrupt the power follow current are connected in series with gaps. Arresters have a short time lag of breakdown compared with the insulation of apparatus, the breakdown voltage being nearly independent of the steepness of the wave front.
Lightning protection by means of lightning arresters and gaps and overhead ground wires is a means of reducing outages and preventing damage to station equipment from lightning disturbances. The amount and kind of protection vary in different applications, depending upon the exposure of the lines, the frequency and the severity of lightning storms, the cost of the protection as an insurance value against damage to equipment and the value of reduced line outages.
Transmission line is protected from direct strokes by running a conductor, known as ground wire, over the towers or poles and earthed at regular intervals preferably at every pole/tower.
Substations, interconnectors and power houses are protected from direct strokes by earthing screen that consists of a network of copper conductors, earthed at least on two points, overall the electrical equipment in the substation.
The ground wire or earthing screen does not provide protection against the high voltage waves reaching the terminal equipment, so some protective devices are necessary to provide protection to power stations, substations and transmission lines against the voltage wave reaching there.
The most common device used for the protection of the power system against the high voltage surge is surge diverter, which is connected between line and earth and so diverts the incoming high voltage wave to earth. Such a diverter is also called the lightning arrester.
Rod gap is a very inferior type of surge diverter and is usually employed as a second line of defence in view of its low cost. Horn gap arrester was one of the earliest type of surge diverters to be developed, and is still used to a certain extent on low voltage lines on account of its great simplicity. Like rod gap arrester it is also employed as an auxiliary protection.
Electrolytic arrester operates on the fact that a thin film of aluminium hydroxide deposited on the aluminium plates immersed in electrolyte acts as a high resistance to low voltage but a low resistance to voltage above a critical value. Such arresters are very delicate, require daily supervision, and the film is required to be reformed whenever destroyed, therefore these arresters have become obsolete nowadays.
Oxide film arrester operates on the fact that certain chemicals (such as lead peroxide) have the property to change rapidly from a good conductor to almost perfect insulator when slightly heated. The great advantage of such an arrester is that it does not require daily charging, and it may be installed at points on transmission systems where daily attendance is difficult or expensive to provide.
Thyrite arrester is the most common and is mostly used for the protection against high dangerous voltages. It operates on the fact that thyrite, a dense organic compound of ceramic nature, has high resistance decreasing rapidly from high value to low value for currents of low value to those of high value.
The expulsion type arrester consists of a tube made of fibre (which is very effective gas evolving material), an isolating spark gap (or external series gap) and an interrupting spark gap inside the fire tube. Expulsion type arresters can be considered as economical means of surge protection for small rural transformers, where valve type arresters may prove expensive and the application of airgaps yield inadequate protection.
These arresters can also be used on special transmission towers of extra height on river crossings where the possibilities of lightning strokes are relatively high. Such arresters can be considered very favourably for use on systems operating at voltages up to 33 kV.
Valve type lightning arrester is very cheap, effective and robust and is, therefore, extensively used nowadays for high voltage systems. This consists of a number of discs of a porous material stacked one above the other and separated by their mica rings. The mica rings provide insulation during normal operation.