Substations and Switchgear Installations: 16 Equipment | Electricity

The following points highlights the sixteen equipment required for substations and switchgear installations. The equipment are: 1. Main Bus-Bars 2. Current Limiting Reactors 3. Station Insulators 4. Switchgear 5. Switches 6. Fuses 7. Circuit Breakers 8. Protective Relays 9. Power Transformers 10. Instrument Transformers 11. Lightning Arresters 12. Indicating and Metering Instruments 13. Batteries and others.

1. Main 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 a rectangular-section bus-bar 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.

2. Current Limiting Reactors:

A current limiting reactor, also sometimes called a series reactor, is an inductive coil having a large inductive reactance in comparison to its resistance and is used for limiting short circuit currents during fault conditions. These are installed in feeders and ties, in generator leads, and between bus sections to reduce the magnitude of short circuit currents and the effect of the resulting voltage disturbances.

Their cost is often more than offset by the saving in circuit breaker cost as a result of the lower short circuit ratings that can be used; their use is, therefore, restricted to the interconnection of large power systems. The reactors allow free interchange of power under normal conditions but under short circuit conditions the disturbance is confined to the faulty section. As the resistance of reactors in comparison to their reactance is very small, the efficiency of the system is not affected appreciably.

Types of Reactors:

There are two types of reactors in use viz., bare type and shielded type:

In the bare or unshielded type reactors, circular coils or bars of stranded copper are embedded in a number of specially designed concrete slabs. Such an arrangement affords a very rigid mechanical support against the mechanical forces developed due to the flow of short circuit currents through the reactor windings.

The mechanical forces act in the form of a couple that tries to compress the winding axially and gives rise to radial expansion and for this reason only circular coils are used as in the case of transformers. The individual turns of the winding are inclined with respect to the horizontal plane. The necessary insulation to earth is provided by a concrete base and porcelain post insulators which also serve the purpose of supports. Such reactors are also known as cast concrete type or dry type reactors.

Dry type reactors are usually cooled by natural ventilation and are sometimes designed with forced air and heat exchanger auxiliaries.

Dry type reactors are very simple from the constructional point of view and are robust but have the following disadvantages:

(i) Large space is required because the magnetic field on account of load current is practically unrestricted.

(ii) Difficulty is experienced in cooling of large coils by fans.

(iii) These are unsuitable for outdoor services.

(iv) Their use is limited to the voltages of 33 kV.

The shielded or oil-immersed type reactors employ insulation and cooling arrangement similar to those of the ordinary transformers. In an air-cored construction, there must be laminated-iron shields or copper shields around the outside of the conductors in order to prevent the magnetic flux entering the tank walls and causing excessive losses and heating.

In iron- cored construction air gaps are introduced in the core to prevent saturation and to give a magnetising current of the desired value. Such current limiting reactors can be used up to any voltage level, for outdoor or indoor constructions.

Other advantages of oil-immersed current-limiting reactors are:

1. They have size smaller than the dry type air-cored reactors because of ease of cooling due to oil immersion.

2. They have higher factor of safety against flash-over.

3. They have higher thermal capacity.

4. There is no magnetic field outside the tank to cause heating or magnetic forces in adjacent reactors or metal structures during short circuit.

The air-cored type, having no iron, has a constant reactance at all currents, but the reactance of the iron-core or iron-shielded types may drop by about 10 per cent due to saturation during short-circuits.

3. Station Insulators:

Station insulators are used in generating stations and substations to fix and insulate the bus­-bar systems. They may be subdivided into post and bushing (through) type.

A post insulator consists of porcelain body, cast iron cap and flanged cast iron base. The hole in the cap is threaded so that the bus-bars are either directly bolted to the cap or fixed by means of bus-bar clamp. Post insulators are available with round, oval, and square flanged bases for fixing respectively, with aid of one, two or four bolts. Each base, in addition, also has an earthing bolt.

A bushing or through insulator consists of porcelain-shell body, upper and lower locating washers used for fixing the position of bus-bar or rod in shell, and mounting flange with holes drilled for fixing bolts and supplied with an earthing bolt.

For current rating above 2,000 A, the bushings are designed to allow the main bus-bars to be passed directly through them.

Each phase of the bus-bars is coated with paint according to a fixed colour code—red, yellow and blue so that phase of the main bus-bars can be identified.

4. Switchgear:

During the operation of the power system, the generating plants, transmission lines, distributors and other electrical equipments are required to be switched on or off under both normal and abnormal operating conditions. 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.

In home or office, a simple tumbler switch with a fuse somewhere in the back ground serves to control and protect lights, domestic apparatus or other equipment and is in its own way a type of switchgear. With appliances of higher rating ordinary fuse may not give the desired result as such high rupturing capacity (HRC) fuses are used along with switches. But with the advancement of electrical power systems, the lines and other equipment operate at very high voltage and carry large current.

Whenever a short circuit occurs, a heavy current flow through the equipment causing considerable damage to the equipment and interruption of service, so in order to protect the lines, generators, transformers and other electrical equipment from damage automatic protective device or switchgear is required. An automatic protective switchgear mainly consists of the relays and circuit breakers.

When fault occurs on any section of the system, protective relay of that section comes in operation and close the trip circuit of the breaker, which disconnects the faulty section. The healthy section continues supplying loads as usual and thus there is no damage to equipment and no complete interruption of supply.

Broadly speaking switchgear is of two types viz.:

(i) Outdoor type, and

(ii) Indoor type.

For voltage above 66 kV outdoor type is almost universally used because for such voltages building work will unnecessarily increase the installation cost owing to large spacing between conductors and large size of insulators. Below 66 kV there is no difficulty in providing building work for the switchgear at a reasonable cost. Moreover this type of switchgear is of metal clad type and is compact. Owing to compactness, safety clearances for operation are also reduced thus reducing the area required.

5. Switches:

Switches, on the other hand, are used either for breaking a circuit where current is within the normal capacity or for breaking a circuit carrying no current. Where used for the latter purpose, switches are called isolators. Two forms of switchgear are in general use nowadays- air-break switches and oil-break switches.

As their name imply, air-switches are those whose contacts are opened in air, while oil switches are those whose contacts are opened under oil. Oil switches are usually employed in very high voltage heavy current circuits.

Sequence of Operation during Opening/Closing of a Circuit:

While Opening:

Open circuit breaker, open isolator and then close earthing switch, if any.

While Closing:

Open earthing switch, close isolator and then close circuit breaker.

6. Fuses:

Fuse is perhaps the simplest and cheapest device used for interrupting an electrical circuit under short circuit, or excessive overload, current magnitudes. As such, it is used for overload and/or short-circuits protection in high voltage (up to 66 kV) and low voltage (up to 400 V) installations/circuits. In high voltage circuits their use is confined to those applications where their performance characteristics are particularly suitable for current interruption.

The action of a fuse is based upon the heating effect of the electric current. In normal operating conditions, when the current flowing through the circuit is within safe limits, the heat developed in the fuse element carrying this current is readily dissipated into the surrounding air, and therefore, fuse element remains at a temperature below its melting point.

However, when some fault, such as short circuit occurs or when load connected in a circuit exceeds its capacity, the current exceeds the limiting value, the heat generated due to this excessive current cannot be dissipated fast enough and the fusible element gets heated, melts and breaks the circuit. It thus protects a machine or apparatus or an installation from damage due to excessive current.

The time for blowing out of fuse depends upon the magnitude of the excessive current. Larger the current, the more rapidly the fuse will blow i.e., the fuse has inverse time-current characteristic. Such a characteristic is desirable for protective gear.

Essentially, a fuse consists of a fusible element in the form of a metal conductor of specially selected small cross-sectional area, a case or cartridge to hold the fusible element, and in some cases, provided with a means to aid arc extinction. The part which actually melts and opens the circuit is known as the fuse element. It forms a series part of the circuit to be protected against short circuit or excessive overloads.

Fuses have following advantages and disadvantages:

Advantages:

(i) It is the cheapest form of protection available.

(ii) It needs no maintenance.

(iii) Its operation is inherently completely automatic unlike a circuit breaker which requires elaborate equipment for automatic action.

(iv) It interrupts enormous short-circuit currents without noise, flame, gas or smoke.

(v) The minimum time of operation can be made much smaller than that with the circuit breakers.

(vi) The smaller sizes of fuse element impose a current limiting effect under short-circuit conditions.

(vii) Its inverse time-current characteristic enables its use for overload protection.

Disadvantages:

(i) Considerable time is lost in rewiring or replacing a fuse after operation.

(ii) On heavy short circuits, discrimination between fuses in series cannot be obtained unless there is a considerable difference in the relative sizes of the fuses concerned.

(iii) The current-time characteristic of a fuse cannot always be correlated with that of the protected device.

The function of fuse wire is:

(i) To carry the normal working current safely without heating, and

(ii) To break the circuit when the current exceeds the limiting current.

Distribution circuits are protected from ground and short- circuit currents by fuses or circuit breakers so arranged as to disconnect the faulted equipment promptly from its source of supply. Fuses are used almost exclusively for the protection of cables in low-voltage light and power circuits and for transformers of rating not exceeding 200 kVA, in primary distribution systems. Circuit breakers are employed for larger amounts of power and in cases where the operation of the overload device is so frequent as to make the use of fuses impractical.

Nowadays several types of fuses are available which find extensive use in low and moderate voltage applications where frequent operations are not expected or where the use of circuit breakers is uneconomical. Most important among these is the motor protective fuse which may be chosen to have a suitable time-delay to permit starting inrush current to flow and also carrying of moderate motor overloads without blowing, and yet be capable of blowing early enough to avoid excessive winding temperature due to overload and at the same time blow very quickly in case of a short circuit.

Modern HRC cartridge fuses provide reliable discrimi­nation and accurate characteristics. In some respects HRC fuses are superior to circuit breakers.

7. Circuit Breakers:

A circuit breaker is a mechanical device designed to close or open contact members, thus closing or opening an electrical circuit under normal or abnormal conditions. It is so designed that it can be operated manually (or by remote control) under normal conditions and automatically under fault conditions. An automatic circuit breaker is equipped with a trip coil connected to a relay or other means, designed to open or break automatically under abnormal conditions, such as overcurrent.

When the circuit breaker is closed, considerable energy is stored in the springs. The contacts are held together by means of toggles. To open the circuit breaker, only a small pressure is required to be applied on a trigger. When the trigger is actuated by the protective relay, it trips and the potential energy of the springs is released and the contacts open in a fraction of second.

A circuit breaker must carry normal load currents without overheating or damage and must quickly open short-circuit currents without serious damage to itself and with a minimum burning of contacts. Circuit breakers are rated in maximum voltage, maximum continuous current-carrying capacity, maximum interrupting capacity and maximum momentary and 4-second current carrying capacity.

The breakers are provided with operating mechanisms which are, in turn, actuated by power applied through suitable relays. In indoor substations, breakers of high rupturing capacities are enclosed in fire-proof compartments. Lift-up or draw-out type breakers which incorporate a disconnect feature on each side are the most common in design and applications.

Air-circuit breakers are often employed instead of oil up to 15 kV in these units and oil re-closer is sometimes employed to cut cost in small rural substations.

Thus the functions of a circuit breaker are:

(i) To carry full- load current continuously,

(ii) To open and close the circuit on no load,

(iii) To make and break the normal operating current, and

(iv) To make and break the short-circuit currents of magnitude up to which it is designed for.

8. Protective Relays:

The protective relay is an electrical device interposed between the main circuit and the circuit breaker in such a manner that any abnormality in the circuit acts on the relay, which in turn, if the abnormality is of a dangerous character, causes the breaker to open and so to isolate the faulty element. The protective relay ensures the safely of the circuit equipment from any damage which might otherwise cause by the fault.

All the relays have three essential fundamental elements, as illustrated by Fig. 16.16 (a).

1. Sensing element, sometimes also called the measuring element, responds to the change in the actuating quantity, the current in a protected system in case of overcurrent relay.

2. Comparing element serves to compare the action of the actuating quantity on the relay with a pre-selected relay setting.

3. Control element on a pick-up of the relay, accomplishes a sudden change in the control quantity such as closing of the operative current circuit.

A typical relay circuit is shown in Fig. 16.16 (b) (only one phase has been shown for clarity).

The connections are divided into 3 main circuits consisting of:

(i) Primary winding of the CT (current transformer) connected in series with the main circuit to be protected,

(ii) Secondary winding of the CT and the relay operating winding, and

(iii) The tripping circuit.

Under normal operating conditions, the voltage induced in the secondary winding of the CT is small and, therefore, current flowing in the relay operating coil is insufficient in magnitude to close the relay contacts. This keeps the trip coil of the circuit breaker de-energised.

Consequently, the circuit breaker contacts remain closed and it carries the normal load current. When a fault occurs, a large current flows through the primary of the CT. This increases the voltage induced in the secondary and hence the current flowing through the relay operating coil. The relay contacts are closed and the trip coil of the breaker gets energized to open the breaker contacts.

9. Power Transformers:

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 voltage;

3. Number of phases (single or three-phase);

4. Rated frequency;

5. Connections (Δ or λ. in case of 3-phase transformer);

6. Tapping’s 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.

10. Instrument Transformers:

AC type protective relays are actuated by current and voltage supplied by current and potential (or voltage) transformers, known as instrument transformers.

The main functions of instrument transformers are:

(i) To provide insulation against the high voltage of the power circuit and to protect the apparatus and the operating personnels from contact with the high voltages of the power circuits.

(ii) To supply protective relays with current and voltage of magnitude proportional to those of the power circuit but sufficiently reduced in magnitude so that the relays can be made relatively small and inexpensive.

(iii) Possibility of different types of secondary connections to obtain the required currents and voltages.

For safety purposes, the secondaries of current and potential transformers (CTs and PTs) are grounded.

For the proper applications of CTs and PTs, required considerations are:

Mechanical construction, type of insulation (dry or liquid), ratio in terms of primary and secondary currents or voltages, continuous thermal rating, short-time thermal and mechanical ratings, insulation class, impulse level, service conditions, accuracy and connections.

11. 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 and 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 air-gaps 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.

12. Indicating and Metering Instruments:

Ammeters, voltmeters, wattmeters, kWh meters, kVARh meters, power factor meters, and reactive-volt-ampere meters are installed in substations to control and maintain a watch over the currents flowing through the circuits and over the power loads.

13. Batteries:

In electric power stations and large-capacity substations, the operating and automatic control circuits, the protective relay systems, as well as emergency lighting circuits, are supplied by station batteries. The latter constitute independent sources of operative dc power and guarantee operation of the above mentioned circuits irrespective of any fault which has occurred in the station or substation, even in the event of complete disappearance of the ac supply in the installation.

Station batteries are assembled of a certain number of accumulator cells depending on the operating voltage of the respective dc circuits. Storage batteries are of two types viz., lead acid and alkaline batteries. Lead acid batteries are most commonly used in power stations and substations because of their higher cell voltage and low cost.

14. Carrier-Current Equipment:

Such equipment is installed in the substations for communication, relaying, telemetering or for supervisory control. This equipment is suitably mounted in a room known as carrier room and connected to the high voltage power circuit.

15. Control Cables:

The control cables and conduit system are required for affecting automatic controls. The control system generally operates at 110 V or 220 V and the cables employed for this purpose are multi-core cables having 10 or 37 or 61 conductors according to requirement. For laying these cables generally ducts are run from control room basement to centrally located junction box from where the conduits are run to the required points.

16. Switchyard:

The switchyard houses transformers, circuit breakers and switches for connecting and discon­necting the transformers and circuit breakers. It also has lightning arrestors for the protection of the power station against lightning strokes.