Bus-bars are merely convenient means of connecting switches and equipment into various arrangements. This usual arrangement of’ connections 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 outgoing feeders and the principal equipment may be connected. On 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 associated with it. The switches used for connecting feeders or equipment to one bus or the other ate called “selector” or “transfer” switches.
The pus-bars used in substations are usually bared rectangular x-section bars bus they can be of other shapes also as round tubes, round solid bars, or square tubes. The bus bars are usually of aluminium (less frequently of copper). The use of aluminum as an electric conductor is made 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 most common. sites of bus-bars are 40 x 4 mm. (60 mm2); 40 x 5 mm (200 mm2); 80 x mm (640 mm2) and 100 x 10 mm. (1,000 mm2) according to the maximum current to be. carried. The bus-bars are of 5 or 6 meters of length. (ii) Insulators The porcelain Insulators employed in substations are of the post and bushing (through) type. They serve as supports and insulation of the bus bars. A post insulator consists of porcelain body, cap and flanged cast iron base, as shown in figure. The hole in the cap is threaded so that the bus-bars so that the bus-bars are either directly bolted to the cap or fixed by means of a 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 the 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 direction 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. (iii) Isolators An isolator or disconnecting switch is used to open some given part of a power circuit after switching off the load by means of circuit breaker. The isolators serve only for preventing the voltage from being applied to some given section of the bus in a switchgear cases isolators may be used a a circuit breaking device but the use for this purpose is strictly limited by define conditions such as the power rating of the given circuit. There are types of isolators.
(1) Single pole isolators and
(2) Three pole isolators. (iv) Circuit Breakers Circuit breakers are installed to perfonn the following duties:
(a) To carry full load current continuously.
(b) To open and close the circuit on no load.
(c) To make and break the normal operating current.
(d) To make and break the short circuit currents of magnitude up to which it is, designed for.
Indoor substations the oil filled and hard gas types of circuit breakers are usually used. (v) Load-interrupter Switches The 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 molded plastic chute fitted with organic glass inserts. This chute surrounds the moving knife of the arc extinguishing system. The stationary acting contact is located in the lower part of the chute.
When the switch is opened, the working contracts between which the are is drawn separate. Acted upon by the high temperature of arc, the walls of the organic material inserts generated gases (mainly hydrogen 0, which create a longitudinal blast serving to extinguish the are. Lever-arm manually operating mechanism are employed for closing and opening the load interrupter switches. (vi) Fuses Fuses have already been described. (vii) Power Transformers Power transformers are used for stepping down the voltage for transmission at generating stations and for stepping down the voltage for further distribution at main step down transformer substations. Usually naturally cooled, oil immersed, known as on type, two windings, three phase transformers, are used up to the rating of 10 MVA. The transformers of raring 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. (viii) (a) Current Transformers (C. T.S) These instrument transformers are connected in a.c. power circuits to feed the current coils of indicating and metering instruments (ammeters, watt-meters, watt hour meters) and protective relays. Thus ‘the CTs broaden the limits of measurements and maintain a watch over the currents flowing in the circuits and over the power loads. In high voltages installations CTs. in addition to above, also isolate the indicating and metering instruments from high voltage. Thus current transformer basically consists of an iron core on which are wound a primary and one or to secondary winding. The primary is directly inserted in the power circuit (the circuit in which current is to be measured and to the secondary winding or windings thee indicating and metering of CT flows through its primary winding a current of 5 amperes will appear in its secondary winding.
The current transformers are rated for rated voltages of the insulations, the rated currents of the primary arid secondary windings and the accuracy class. The accuracy class indicates the limit of the error in percentage of the rated turns ratio of the given current transformer. Current transformer are available in the accuracy classes 0, 1, 3 and 10. (b) Potential Transformers (P. Ts) The potential transformers are employed for voltages above 380 volts to instruments (volt meters, watt meters, watt-hour meters) and relays. These transformers make the primary low voltage instruments suitable for measurement of high voltage and isolate them from high voltage. The primary win4ing of the potential transformers is connected to the main-bus bars of the switch gear installation and to the secondary windings, various indicating and metering instruments and relays are connected When the rated high voltage is applied to the primary of aPT the voltage of 110 volt appears across the secondary winding. The ratio of the rated primary voltage to the rated secondary voltage is known as turns of transformation ratio. The potential transformers are rated for primary and secondary rated voltage, accuracy class member of phases, system of cooling. (ix) Indicating and Metering Instruments Ammeters, voltmeters, watt meters, kwh meters, kvarh meters are installed in the substations to control and maintain a watch over the currents flowing in the circuits and over the power loads. (x) Protective Relays A relay is a low-powered device used to activate a high powered device. In a power system of transmission and distribution, it is the function of protective relays to give the tripping commands to the tight circuit breakers. These are used to protect the feeders and the equipment from damage in the event of fault. (xi) 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. (xii) Control Cables The control cables and conduit system is 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.
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