Power Transmission System

Contents

1. Introduction & History Power map of West Bengal

2. Substation overview Single line diagram of Kasba substation

3. Equipments in the substationA. BusbarB. IsolatorsC. Transmission towers and structuresD. Circuit breakersE. RelaysF. Current transformerG. Potential transformerH. Lightning arrestorsI. InsulatorsJ. Arcing hornsK. Underground cables

4. Conductors used5. Auxiliary power supplies6. Communication in the substation7. Specifications and ratings8. References

1. Introduction & History

West Bengal is the pioneer in power development in the country. The first hydro electric power plant was commissioned more than a century ago in 1987 in a tiny

village SIDRAPONG in Darjeeling and the first thermal power plant at Kolkata in 1899. With urbanization and growth of industries and the advent of green revolution, electricity became an inseparable part of our lives. West Bengal State Electricity Board (WBSEB) was established on 1st May, 1955 and its journey spanning over 4 and half decades has been full of challenges. Sheer determination, hard work and its inherent capability to overcome hurdles has enabled WBSEB to reach its current enviable position.

WBSEB, a major power utility in the state, has completed 54 years of its dedicated service to the people of west Bengal, reaching out to provide power to about 40 lakhs homes, industries, agricultural sectors and hospitals thereby covering about 87000 sq. km out of the total 89000 sq. km area of this state.

In the year 2000, WBSEB was divided into two new boards, namely WBSEB and WBSEPDCL. WBSEPDCL covers the thermal generation part of the previous board. In the year 2007, WBSEB was further divided into two new companies namely WBSETCL AND WBSEDCL. In spite of this partition WBSEB still is committed in building a powerful base for Bengal ensuring harmony and balanced growth of company, with equal emphasis in serving both rural and urban people and thus achieving the twin goals of social development of people and economic regeneration of West Bengal.

2. SUBSTATION OVERVIEW:220 kV system layout:-

The 220 kV consists of two bus-bars, namely the Main bus and the transfer bus. This arrangement is called the double bus bar arrangement. The 220 kV bays at 220 kV substations are:-

1. Jeerat 1 feeder bay.2. Jeerat 2 feeder bay.3. Jeerat 3 feeder bay4. Laxmikantapur feeder bay.5. Transformer TR-1 bay.6. Transformer TR-2 bay.7. Transformer TR-3 bay.

The circuit breakers with isolators on both sides, lightning arrestors are the other equipments. There are CT’s and PT’s used for metering purpose.

Power from 220 kV bus-bar is fed to:-

1. TR-1(150MVA, 220/132 kV)2. TR-2(150MVA, 220/132 kV)3. TR-3(160MVA, 220/132 kV)

132 kV system layout:-There are 3 transformers: TR1, TR2, TR3.The secondary of the transformers are connected to various CTs, isolators etc. The CTs used are 600-300/1-1 i.e. they can step down 300 to 600

ampere current to 1 ampere for protection and metering purposes. The 132kV base is connected to two 132-110 V for metering purpose.

Power flow in 132kV system:1. Sonarpur(132kV) substation through first feeder.2. Behala(132kV) substation through second feeder.3. Saltlake(132kV) substation through third feeder.4. Bantala(132kV) substation through fourth feeder.5. CESC through five different feeders:

1. 3 feeders for CESC transformers.2. 2 feeders to Princep Street through cables only.

33 kV system layout : There are three transformers whose tertiary windings steps down the voltage from 220kV to 33kV. The tertiary winding of TR1 andTR2 are connected to gas-turbine1 and gas-turbine2 respectively. The tertiary winding of TR3 is fed to a 33/.4 kVA transformer.Power flow in 33kV system:After proper stepping down of generated voltage, they are fed to the gas turbines; each can generate 20 MW Power at 11 kV. They are required to provide excessive power demands when required. The secondaries of 33/.4 kVA transformer is used to meet the demand of station service auxiliaries like transformer cooling fans, AC motors etc.

Technically we can divide the substations into three parts:1. Switch Yard.2. Control Room.3. Battery Room.

3. Equipments in the SubstationA. Busbars:

When a number of lines operating at the same voltage have to be directly connected electrically then bus bars are used as the common electrical equipment. Bus bars are copper or aluminum bars and operate at constant voltage. The incoming and outgoing lines in a substation are connected to the bus bars. The most commonly used bus bar arrangements are:

1. Single bus bar system2. Single bus bar system with bus sectionalisation3. Double or duplicate bus bar system4. Double breaker scheme system5. Breaker and a half scheme system6. Main and transfer bus bar system 7. Double bus bar with bypass isolator system8. Mesh scheme system9. Main and common bus bar systemB. Isolators:

In substations it is often desired to disconnect a part of the system for general maintenance and repairs. This is accomplished by an isolating switch or an isolator. An isolator is essentially a knife switch and is designed to open a circuit under no load condition. It should be kept in mind that the isolator should not be opened until the circuit breaker in the same circuit has been opened and should always be closed before the circuit breaker is closed. This is because if the isolator is opened on load there will be arcing in between the contacts and since the isolator is not provided with any arc quenching medium, it will damage the system and may also prove to be fatal to the operator in case the isolator is not a remote controlled one.

From constructional point of view, the isolators may be classified as follows:

1. Three post, centre post rotating type, double break type.2. Two post single break type.3. Triple pole gang operated type.4. Pantograph type.

The operation of the isolator may be manual, remote control, pneumatic or by spring or by counter weight.

An isolator should have the following parts:

Operating handles and keyhole

Base channel Insulators Arcing horns Female contacts Make before-break after contacts Terminal pad Rotating arm with male contacts Stop for rotating arms Earthing blades Female contact for earthing switch Auxiliary switch Interlocking arrangement.

For preventing incorrect operation of isolator, an isolating switch is blocked against respective circuit breaker, earthing switches or other isolating switches. This is called interlocking of an insulator. It may be electrical or mechanical depending on the application field.

C. Transmission towers and structures

Tower is a lattice structure that supports insulators, overhead transmission line and overhead earth wires. Towers and structures are three dimensional fabricated lattice structures made up by bolting, riveting and welding the structural members of galvanized steel.

Due to limitation of transmitting voltage up to 400kV, in India rigid self supporting towers are used for transmission. The towers may be single or double circuit. When a tower has only one circuit it is called vertical tower. Double circuit tower is called horizontal tower.

Depending on several factors towers may be classified as follows:

Straight Line Tangent or Suspension Tower Section Tower or Tension Tower Small Angle Tower ( 2 to 15 degree) Medium Angle Tower ( 15 to 30 degree) Large Angle Tower ( > 30 degree) with dead end Large Angle with Corner End Anchor Tower Long Span Tower Transposition Tower Take-off Tower or Turning Tower Crossing Tower Anti-wind Tower

In Kasba, mainly sectional towers and suspension towers are used. Section towers are classified according to cross arm angles. The different types of towers are:

1. “A” type towers ( angle 2 degree)- normally used in lines2. “B” type towers (angle 3-30 degree)-used in road crossings3. “C” type towers (angle 30-60 degree)- used in turning points

4. “D” type towers (angle 60-90 degree)- used mostly in hilly regions.

D. Circuit breaker: A Circuit Breaker is a piece of equipment which can make or break a circuit either manually or by remote control under normal or faulty conditions.

When the contacts of a circuit breaker are separated under fault conditions, an arc is struck between them. The current is thus able to continue until the discharge ceases. The production of a n arc not only delays the current interruption process but also generates enormous

heat that may cause damage to the system or to the circuit breaker itself. Therefore the main problem in the circuit breaker is to extinguish the arc within the circuit breaker in the shortest possible time so that the heat generated may not reach a dangerous value.

Circuit Breakers are of following type:-

1. Bulk oil circuit breaker.2. Minimum oil circuit breaker.3. Air Blast circuit breaker.4. Vacuum circuit breaker5. SF6 Gas circuit breaker.

A minimum oil circuit breaker has the following parts:-

1. Supporting chamber.2. Circuit breaker Chamber.3. Top Chamber.4. Upper and lower fixed contacts.5. Moving contact.6. Turbulator.7. Breather.8. Oil level indicator.9. Gas vent or vent pipe.10.Arc extinction device and chamber.11.Operating rod.12.Drain valve.13.Cubical box.14.Spring charge arrangement with limit switch

Figure of an SF6 breaker

The MOCB consists of two compartments separated from each other but both filled with oil. The upper chamber is the circuit breaking chamber while the lower one is the supporting chamber. The two chambers are separated by a partition and oil from one chamber is prevented from mixing with the other chamber. This arrangement permits two advantages. Firstly, the circuit breaking chamber requires a small volume of oil which is just enough for arc extinction. Secondly, the amount of oil to be replaced is reduced as the oil in the supporting chamber does not get contaminated by the arc. The circuit breaking chamber contains the upper and lower fixed contacts, moving contacts and the turbulator.

The moving contact is hollow and includes a cylinder which does not move down over a fixed piston. The turbulator is an arc control device and has both axial and

radial vents. The axial venting ensures the interruption of low currents whereas the radial venting helps in the interruption of heavy currents. The top chamber provides expansion space for the oil in the circuit breaking compartment.

When a fault occurs, the moving contact is pulled down by the tripping springs and an arc is struck. The arc energy vaporizes the oil and produces gases under high pressure. This action constrains the oil to pass through a central hole in the moving contact and results in forcing series of oil through the respective passages of the turbulator.

Advantages

1. It requires lesser quantity of oil2. It requires lesser space3. There is reduced risk of fire4. Maintenance problems are reduced

Disadvantages

1. Due to smaller quantity of oil, degree of carbonization is increased.2. There is difficulty of removing the gases from the contact space in time.3. The dielectric strength of oil deteriorates rapidly due to high degree of

carbonization.

In the Sulphur Hexaflouride(SF6) circuit breaker, the arc quenching medium is SF6 gas. It consists of fixed and moving contacts enclosed in a chamber (called arc interruption chamber) containing SF6 gas. This chamber is connected to SF6 gas reservoir. When the contacts are opened, the valve mechanism permits the high pressure SF6 gas from reservoir to flow towards to arc interruption chamber. The fixed contact is a hollow cylinder current carrying contact fitted with an arc horn. The moving contact is also a hollow cylinder with rectangular holes in the sides to permit the SF6 gas to let out through these holes after flowing along and across the arc. The tips of the moving contact, fixed contact and the arcing horn are coated with copper tungsten arc resistant material. Since SF6 gas is costly it is reconditioned and reclaimed by suitable auxiliary system after each operation of the breaker.

In closed position the contacts remain surrounded by SF6 gas, and at a pressure of 2.8 kg/cm2. The breaker operates the moving contact is pulled apart and an arc is struck between the contacts. The movement of the moving contacts is synchronized with opening of a valve which permits SF6 gas at 14kg/cm2 from the reservoir to the arc interruption chamber. SF6 being an electronegative gas absorbs free electrons in the arc path to form immobile negative ions which are ineffective as charge carriers. The result is that medium between the contacts quickly builds up high dielectric strength and causes extinction of arc.

Advantages

1. Due to superior arc quenching property of SF6, such circuit breakers have very short arcing time.

2. Since dielectric strength of SF6 gas is 2 to 3 times as that of air, such breakers can interrupt much larger currents.

3. The SF6 circuit breaker gives noiseless operation due to its closed gas circuit4. The closed gas enclosure keeps the interiors dry so that there is no moisture

problem.5. There is no risk of fire in such breakers as SF6 gas is non-inflammable.6. There is no carbon deposit so that tracking and insulation problems are

eliminated.

Disadvantages

1. Level system case2. Sealed shaft 3. Finger cluster sliding contact4. Moving contact stem of silver plated

copper5. Stationary piston of Al alloy6. Finger cluster type moving contact7. Arcing contact 8. Nozzle9. Stationary contact stem10. Porcelain weather casing11. Gasket12. Protective insulating tube13. springs14. terminals15. Upper cap16. Absorbing substances17. Plug in joints18. Removable cover.

1. These are costly because of high cost of SF6 gas.2. Additional equipments are required for the reconditioning of the SF6 gas.

In Kasba substation motor controlled spring mechanism is used in all the breakers. In this case the closing operation is done by releasing the compressed spring which had been previously charged by electric motor. The main advantage is that the making time is independent of the charging time of the spring and depends only on the elasticity of the spring.

A Circuit Breaker used in the substation.

E. Relay The protective relay is a device that detects a fault and initiates the operation of a circuit breaker to isolate the defective equipment from the rest of the system. The

relays are operated on the principle of electromagnetic attraction and induction. The relays detect abnormal conditions in the electric circuit by constantly measuring the electrical quantities which may change under fault conditions. These electrical quantities are voltage, current, frequency and phase angle. When a short circuit occurs on a line, current flowing through the line increases to an enormous value. This results in heavy current flowing through the relay coil, causing the relay to operate by closing its contacts; this in turn closes the trip circuit of the breaker, making the circuit breaker open and isolating the faulty section from the rest of the system. In order that the protective relay system may perform its operation satisfactorily, it should have the following qualities:

1. Selectivity- It is the ability of the system to select correctly that part of the system that is faulty and disconnect the faulty part without disturbing the rest of the system.

2. Speed- The relay system should disconnect the faulty part as fast as possible.

3. Sensitivity- It is the ability of the relay system to operate with low value of actuating quantity

4. Reliability- It is the ability of the relay system to operate under the pre- determined conditions.

5. Simplicity- The relaying system should be simple so that it can be easily maintained.

6. Economy- The most important factor in the choice of a particular protection scheme is the economic aspect. The relay used for protection should be economic.

The different types of functional relays are described below:

Over current relay: This type of relay works on the induction principle and initiates corrective measures when the current in the circuit exceeds a predetermined value. The actuating source is the current in the circuit supplied to the relay from a current transformer. This relay is used for a.c circuits only and it may be directional or non- directional in nature. A non directional over current relay can operate for fault current flow in any direction. A directional over current relay consists of a directional element and a non directional element. The directional element is essentially a directional power relay where the operation of the relay is dependent on the direction of power flow and the non directional element is a non directional over current relay.

The operation of the relay occurs under the following conditions:

i. Current flows in a direction such as to operate the directional element.ii. Current in the reverse direction exceeds the pre determined value.iii. Excessive current persists for a period corresponding to the time setting

of the over current relay.

Earth fault relay: This relay operates when ever some earth fault occurs in the system and isolates the faulty part from the rest of the system by energizing the trip circuit of the circuit breaker. When there is an earth fault in the system, the vector sum of the current in the three phases is not zero and there is an unbalancing of currents. This vector sum now has a particular value and when this value exceeds the pre determined value of the relay it operates and isolates the faulty part of the circuit from the rest of the system. This relay is directional in nature i.e. it depends upon the direction of the actuating quantity.

Distance or impedance relay: The operation of this type of relay is governed by the ratio of the applied voltage to current in the protected circuit. This relay is called impedance or distance relay because impedance is the electrical measure of distance along a transmission line. In an impedance relay the torque produced by the current element is opposed by the torque produced by a voltage element. The relay will operate when the ratio V/I falls below a pre determined value.

The basic operational principle shows that the voltage element of the relay is excited through a potential transformer (P.T) from the line to be protected. The current element of the relay is excited from a current transformer (C.T) in series with the line. The portion AB of the line is the protected zone. Under normal operating conditions the impedance of the protected zone is ZL. The relay is designed such that it closes its contacts whenever the impedance of the protected zone falls below the pre determined value i.e. ZL in this case. If a fault occurs at a point F1 in the protected zone then the relay operates because the impedance in this case is less than ZL. But if the fault occurs at a point F2 then the relay does not operate as the impedance is more than ZL.

There are two types of distance or impedance relays:

i. Definite-distance relay which operates for fault up to a pre-determined distance from the relay.

ii. Time-distance relay in which the time of operation is proportional to the distance of the fault from the relay point. A fault nearer to the relay will operate it earlier than a fault further away from the relay.

Differential relay: A differential relay is the one that operates when the phasor difference of two or more similar electrical quantities exceeds a pre-determined value. A differential relay compares the currents entering a section with the current leaving the section. Under normal operating conditions the two currents are equal, but when a fault occurs, the difference between the incoming and outgoing current

is arranged to flow through the operating coil of the relay. If this differential current is greater than the pick value, the relay will operate and isolate the faulty part of the circuit. There are two fundamental types of differential protection:

1. Current balance protection2. Voltage balance protection.

The figure shows a current differential relay

In KASBA substation, the relays which are used are:

1. Over Current Relay.2. Earth Fault Relay.3. Distance Relay.4. Buchholz Relay.

When a fault occurs in a circuit and a relay operates to close the trip circuit of the respective circuit breaker, it is called main tripping relay or master tripping relay. Other subsequent relays are called auxiliary tripping relay.

RELAY PANEL

F. TRANSFORMERS:

A transformer is an electrical device having no moving parts. By electromagnetic Induction, it transforms electrical energy from one circuit to another at same frequency, usually with changed values of voltage and current. In its simplest form, it consists of two windings insulated from each other and wound on a common core. In both windings9 i.e. primary and secondary windings), the emf is induced by electromagnetic induction.

In substations, mainly two types of transformers are used, viz.

Power Transformer Distribution Transformer

Power Transformer:

A power transformer is used in a substation to step down the voltage. Except at the power station, all the subsequent substations use step down transformers to

gradually reduce the voltage of electric supply and finally deliver it at utilization voltage. The modern practice is to use 3 phase transformers in substations. These types of transformers have higher flux density as they are not always in continuous operation.

Distribution Transformer:

A distribution transformer is normally used for purposes in day in and out to provide transmission and distribution after the voltage has been stepped down. It has comparatively a lower value of flux density than a power transformer.

Although these two are the basic ones, regarding transmission and distribution, there are other types of transformers which are used mainly in substations providing power supply to the railways. These are the Traction Transformers.

A Traction transformer is a special type of transformer used only for providing power supply to the railways that require an abnormal supply of 23 kV(single phase). It transforms 132 kV L-L voltage to 25 kV L-G voltage. It may be of different capacities like 10, 12.5 or 20 MVA. This type of transformer is generally not used in parallel operation. For this type of transformer, OFF-LOAD-TAP-CHANGER is used. In addition, Buchholz relay, Oil-Temperature Indicator (OTI), and Winding Temperature Indicator(WTI( are used.

Front View of a Power Transformer

Back View of a Power Transformer

TRANSFORMER PROTECTIONAs transformers are widely used over the transmission and distribution area, protection of the same is very much needed because if a transformer ceases to operate the power system has to bear heavy losses caused by shedding of loads. The two main types of protection that are used are as follows:-

BUCHHOLZ RELAY:

It is a type of relay where oil is used as a protective medium. It is a sudden pressure relay & acts as a pressure release valve. Large internal faults (phase to phase, phase to ground) are taken care of by the Buchholz Relay. Buchholz Relay is less sensitive compared to high speed, high set over-current relay. Buchholz relay is used for transformer of rating 500KVA and above.

It is fitted in a pipe between the conservator and the tank. Its operation should be listed before installation y ensuring that the chambers are full of oil. When a serious short circuit fault occurs in the transformer, the increased pressure in the tank makes the oil rush towards the conservator, through the Buchholz relay.

CIRCULATING CURRENT SCHEME FOR TRANSFORMER PROTECTION:

The figure below shows Merz-Price circulating-current scheme for the protection of a 3-phase delta/delta power transformer against phase-to-ground and phase-to-phase faults. The CTs on the two sides of the transformer are connected in star. This compensates for the phase difference between the power transformer primary and secondary. Pilot wires connect the CTs on the two sides and one relay is used for each pair of CTs.

During normal operating conditions, the secondaries of CTs carry identical currents. Therefore, the currents entering and leaving the pilot wires at both the ends are same and no current flows through the relay. If a ground or a phase-to-phase fault occurs, the current in the secondaries of the CTs will no longer be the same and the differential current flowing through the relay circuit will clear the breaker on both sides of the transformer. The protected zone is limited to the region between the CTs on the high-voltage side and the CTs on the low-voltage side of the power transformer.

It is worthwhile to note that this scheme also provides protection for short-circuit between the turns on the same phase winding.

COOLING OF TRANSFORMERS;

The transformer is a static device which converts energy at one voltage level to another voltage level. During this conversion process, losses occur in the windings of the transformer in the form of heat. This heat is dissipated to the surroundings. Hence the types of cooling normally used for dissipation of heat are as follows :-

1. Air Natural(AN)2. Air Blast(AB)3. Oil Natural(ON)4. Oil Natural Air Forced(ONAF)5. Oil Natural Water Forced(ONWF)6. Oil Forced Air Natural(OFAN)7. Oil Forced Air Forced(OFAF)8. Oil Forced Water Forced(OFWF)

Among all the above cooling processes, the two main processes that are most commonly used are:

Oil Natural Air Forced: In this method, the oil circulating under natural head transfers heat to the tank walls. The transformer tank is made hollow and air is blown through the hollow space to cool the transformer. The heat removed from the inner tank

walls can be increased to five or six times that dissipated by natural means and therefore very large transformers can be cooled by this method.

ONAF Cooling Method

Oil Forced Air Forced :- The oil is cooled in external heat exchangers using air blast produced by fans. It is interesting to note that the oil pump and fans may not be used all the time. At higher loads, the pumps and the fans may be switched on by temperature sensing elements. This arrangement results in higher efficiency for the system.

In addition to the above methods, there are some other processes that are also used like:

Oil Forced Air Natural :-In this method, oil is circulated through the transformer with the help of a pump and is cooled in a heat exchanger by natural circulation of air. This method is not commonly used.

Air Blast :-Cooling by natural circulation of air becomes inadequate to dissipate heat from large transformers and hence air blast cooling is employed in order to keep the temperature rise within limits. The blast of air improves the heat dissipation.

G. CURRENT TRANSFORMERS

Dead-Tank type CT.

A Current Transformer is easily a step down transformer which steps down current according to a known ratio.The primary of this transformer consists of one or more turns of thick wire connected in series with the line. The secondary consists of large number of turns of wire which provides for measuring instruments and relays and a current which is a constant fraction of the current in the line.

In KASBA substation, there are wound type multi-ratios and core current transformers which are used in different bays. But in power transformer bushing type CTs are used for differential protection. These CTs are bar type in construction,

For obtaining desirable CT ratio for selected fault current, transformers can be provided with two or more cores. Each core carries its own secondary winding. Primary winding is common to all of them. It is called Multi-Core-Transformer (ex. 2 core CT 400/5/1 Amp.)

H. POTENTIAL TRANSFORMER (PT) :It is essentially a step down transformer that steps down the voltage to a

known value. The primary of this transformer consists of a large number of fine wires connected across the line. The secondary winding consists of a few turns that provides for measuring instruments and relays, a voltage which is a known fraction of the line voltage.

The potential transformers are classified as;

1. Magnetic type (up to 132 kV)

2. Capacitive type (CVT) (above 132 kV)

PTs are important in case of voltmeter, wattmeter, distance, and directional relays. Its one end of the primary winding is grounded so it has only input bushing and it is connected in parallel with the bus. In Kasba substation, the CVT is also used as the coupling capacitor for power line carrier communication, for protecting the low frequency noise.

Potential Transformer

Capacitor Voltage Transformer(CVT)

I. LIGHTNING ARRESTORS: Normally in power stations, the ground wire or the earthing screen used for protection of overhead lines and power stations provide adequate protection against lightning but eventually they are inactive in providing protection the travelling waves. The most common device used for this purpose is the Lightning Arrestor or Surge Diverter.

A surge diverter is a device that is connected between line and earth i.e. in parallel with the equipment to be protected at the substation. A lightning arrestor can be classified according to the components it is comprised of and the function it performs.

Different types of Lightning Arrestors are described below :-

1. Rod-Gap arrestor:This is the simplest form of surge diverter consisting of two 12mm

diameter or square with ends facing each other, one connected to line and the other to earth. These are usually connected across the bushing of various equipments. The rod-gap depends upon the operating voltage of the system.

2. Sphere Gap Arrestor:In such arrestors, the air gap is provided by two similar spheres; one connected to the line and another grounded. The spacing between the spheres is very small compared to their diameter and it be adjusted with the help of gauge.

3. Horn Gap Arrestor:

It consists of two horn shaped pieces of metal separated by a small air gap and connected in shunt between each conductor and earth because of its simplicity. The gap between the horns is less at the bottom and large at the top. These are used in low voltage lines

4. Electrolytic Arrestor:These type of arrestors are the earliest types of arrestors with large discharge capacities. It operates on the fact that a thin film of aluminum hydroxide deposited on the aluminum plates immersed in the electrolyte acts as a high resistance to a low voltage but a low resistance to a voltage above a critical value. Such arrestors are very delicate and need daily supervision. The electrolytic arrestor is used in conjunction with an impulse gap.

5. Expulsion type arrestor:It is an improvement over the rod gap in that it seals the flow of power frequency follow current. Its use is normally restricted to the circuits in which the possible range of power current magnitude following a spark over is not more than about three to one, since an excessive current may cause bursting of tube while a small current may not produce a sufficient pressure to extinguish the arc.

6. Thyrite Lightning Arrestor :This is the most common type of lightning arrestor and is mostly used for protection against dangerously high voltages. It operates on the fact that thyrite, a dense organic compound which is ceramic in nature,

has high resistance decreasing rapidly from high value to low value for currents of low value to those of high value. The current increases 12.6 times on doubling the voltage. It consists of discs of 15 cm diameter and 19 mm thickness.

When lightning takes place, voltage is raised and break-down of gap occurs, the resistance falls to a very low value and wave is discharged to earth. After the surge has passed, the thyrite again comes back to its original position, there being no chemical change in it!

The thyrite arrestor discharges several thousand amperes without the slightest tendency to flashover on the edges. Of all the advantages of this type of arrestor, the most important one is that there is absolutely no time lag in its performance.

J. INSULATORS:

An insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate electrical conductors without passing current through themselves. The term is also used more specifically to refer to insulating supports that attach electric power transmission wires to utility poles or pylons.

Some materials such as glass or Teflon are very good electrical insulators. A much larger class of materials, for ex. rubber-like polymers and most plastic are still “good enough” to insulate electrical wiring and cables even though they may have lower bulk resistivity, these materials can serve as practical and safe insulators for low to moderate voltages ( 100, or even 1000 volts)

Insulators suffer from the phenomenon of electrical BREAKDOWN. When any voltage applied across a length of insulating substance exceeds a threshold breakdown field for that substance, which equals the band gap energy, the insulator suddenly turns into a resistor, sometimes with catastrophic results. During electrical breakdown, any free charge carried being accelerated by the strong E-field will have enough velocity to knock electrons (ionize) ant atom it strikes. These free electrons and ions are in turn accelerated and strike other atoms, creating more charge carriers, in a

chain reaction. Rapidly the insulator becomes filled with mobile carriers and its resistance drops to a lower level. In air, the outbreak of conductivity is called “Corona discharge” or a “Spark”. Similar breakdown can occur when any insulator, even within the bulk solid of a materials. Even a vacuum can suffer a sort of breakdown, but in this case the breakdown or vacuum are involves charges ejected from the surface of metal electrodes rather than produced by the vacuum itself.

Suspended wires of electrical power transmission are bare, except when connecting two houses and are insulated by the surrounding air. Insulators are required at the points at which they are supported by utility poles or pylons. Insulators are also required where the wire enters buildings or electrical devices, such as transformers or circuit breakers. To insulate the wire from the case, these hollow insulators with a conductor inside them are called BUSHINGS.

Insulators are used for High-voltage power transmission are made from glass porcelain or composite polymer materials. Porcelain insulators are made from clay, quartz or alumina and feldspar and are covered with a smooth glaze to shed water. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Porcelain has a dielectric strength of 4-10 kV/mm. Glass has a higher dielectric strength, but it attracts condensation and the thick irregular shapes needed for insulators are difficult to cast without internal strains. Some insulator manufacturers stopped making glass insulators in the late 1960s switching to ceramic materials.

Recently some electrical utilities have been converting to polymer composite materials for some types of insulators. These are typically composed of a central rod made of fiber reinforced plastic and an outer weather shed made of silicon rubber or EPDM.

Composite insulators are less costly, lighter in weight, and have excellent hydrophobic capability. This combination makes them ideal for service in polluted areas. However, these materials do not yet have the long term proven service life of glass and porcelain.

The electrical breakdown of an insulator due to excessive voltage can occur in one of two ways:-

Punctuation voltage: Is the voltage across the insulator (when installed in its normal manner) which causes a breakdown and conduction through the interior of the insulator. The heat resulting from the puncture usually damages the insulator irreparably.

Flashover voltage: is the voltage which causes the air along the surface of the insulator to break down the conduct, causing ‘flashover’ along the outside of the insulator. They are usually designed to withstand this without damage.

High voltage insulators are designed with a lower flashover voltage than puncture voltage, so they will flashover before they puncture to avoid damage. Dirt, pollution, salt and particularly water on the surface of a high voltage insulator can create a conductive path across it, causing leakage currents and flashovers. The flashover voltage can be more than 50% lower when the insulator is wet. High voltage insulators for outdoor use are shaped to maximize the length of the leakage path along the surface from one end to the other, called the creepage length, to minimize these leakage currents. To accomplice this, the surface is molded into a series of corrugations or concentric discs shapes. These usually include one or more shaded, downward facing, cup-shaped surfaces that act as umbrellas to ensure the part of the surface leakage path under the ‘cup’ stays dry in wet weather. Minimum creepage distances are 20-25 mm/kV, but must be increased in high pollution airborne sea-salt areas.

CAP AND PIN TYPE INSULATORS

Higher voltage transmission lines uses modulator cap and pin insulator designs. The wires are suspended from a ‘string’ of identical disc shaped insulators which attach to each other with metal clevis pin or ball and socket links. The advantage of this design is that insulator strings with different breakdown voltages, for use with different line voltages, can be constructed by using different numbers of the basic unit. Also, if one of the insulator units in the string breaks, it can be replaced without replacing the entire string. They are constructed of a ceramic or glass disc with a metal cap and pin cemented to opposite sides. In order to make defective unit obvious, glass units are designed with Class B construction: an overvoltage causes a puncture arc through the glass. The glass is heat-treated so it will shatter, making the damage unit visible. However, the mechanical strength of the unit is unchanged, so the insulator string will stay together. Standard disc insulator units are 10 inches(25 cm) in diameter and 23/4 in (15 cm) long, can support a load of 80-120 kN(18-27 klbf), have a dry flashover voltage of about 72kV, and are rated at an operating voltage of 10-12kV. However, the flashover voltage of a string is less than the sum of its component discs, because the electric field is not disturbed evenly across the string but is strongest at the disc nearest to the conductor, which will flashover first. Metal grading rings are sometime added around the lowest disc, to reduce the electric field across that disc and improve flashover voltage.

SUSPENSION TYPE INSULATORS:

Typical number of insulation discs for standard line voltages

Line Voltage (kV) Discs34.5 346 469 592 7

115 8138 9161 11196 13230 15287 19345 22360 23

K. Arcing horns

Arcing horns (arc horns) are projecting conductors used to protect insulators on high voltage electric power transmission systems from damage during flashover. The horns encourage flash over between themselves rather than across the surface of the insulator they serve to protect. Horns are normally paired on either side of the transformer, one connected to the high voltage part and other to ground. They are frequently seen on insulator strings, or protecting transformer bushings. The horns can take various forms such as simple cylindrical rods, circular guard rings or contoured curves, sometimes known as stirrups.

High voltage equipment particularly that which is installed outside such a overhead power line is commonly subjected to transient overvoltage, which may be caused by phenomena such as lightning strikes, faults on other equipments or switching surges during circuit re-energisation. Over voltage conditions such as these are un-predictable and in general cannot be prevented. Line terminations at which a transmission line connects to a bus bar or a transformer bushing, are at greatest risk to over voltage due to change in characteristic impedance at this point.

An electrical insulator serves to provide physical separation of conducting parts and under normal operating conditions is continuously subject to high electric field which occupies the air surrounding the equipment. Over voltage event may cause the dielectric field to exceed the dielectric strength of air and result in the formation of an arc between the conducting parts and over the surface of the insulator. This is called flashover. Contamination of the surface of the insulator reduces the breakdown strength and increases the tendency to flashover. On an electrical transmission system protective relays are expected, to detect the formation of the arc and automatically open the circuit breakers to discharge the circuit and extinguish the arc. Under a worst case this process may take several seconds. During which time the insulator surface would be in close contact with the highly energetic plasma of the arc. This is very damaging to an insulator and may shatter brittle glass or ceramic discs, resulting in its complete failure.

Arcing horns form a spark gap across the insulators with a lower breakdown voltage than the air path along the insulator surface. So an over voltage will cause the air to breakdown and the arc to be formed between the arcing horns diverting it away

from the surface of the insulators. An arc between the horns is more tolerable for the equipment, providing more time for the fault to be detected and the arc to be safely cleared by the remote circuit breakers. The geometry of some designs encourages the arc to migrate away from the insulator, driven by rising currents as it heats up the surrounding air. As it does so the path length increases, cooling the arc, reducing the electric field and causing the arc to extinguish itself when it can no longer span the gap. Other designs can utilize the magnetic fields produced by the high current to drive the arc away from the insulator. This type of arrangement is known as magnetic blowout.

Design criteria and maintenance regimes may treat arcing horns as sacrificial equipment, cheaper and more easily replaced than the insulator, failure of which can result in complete destruction of the equipment it insulates. Failure of the insulator strings on overhead lines could result in parting of the line, with significant safety and cost implications. Arcing horns thus play an important role in the process of correlating system protection with protective device characteristics, known as insulation coordination. The horns should provide among other characteristics, near infinite impedance during normal operating conditions to minimize conductive current losses, low impedance during the flashover, and physical resilience to the high temperature of the arc.

As operating voltages increase, greater considerations must be given to such design principles. At medium voltages, one of the two horns can be omitted as the horn to horn gap can otherwise be small enough to be bridged by an alighting bird. Alternatively duplex gaps consisting of two sections on opposite sides on the opposite sides of the insulator can be fitted. Low voltage distribution systems, in which risk of arcing is much lower, may not use arcing horns at all. The presence of arcing horn necessarily disturbs the normal electric field distribution across the insulator due to their small but significant capacitance. Most importantly a flashover across an arcing horn produces an earth fault resulting in circuit outage until the fault is cleared by circuit breaker operation. For this reason, non linear resistors known as surge diverters can replace arcing horns at critical locations.

4.Underground cables

An underground cable consists of one or more conductors covered with suitable insulation and surrounded by a protective cover. In practice underground cables are required to deliver three phase power. But mainly cables are used in a substation to connect the high voltage equipments in the yard to the control room for controlling and monitoring purpose. For the purpose single core, 2 cores, 3 core cables are used. But sometimes 4, 8, 12, or 16 core cables are also used.

The various parts of a 3 conductor cable are:

1. Core or conductors2. Insulation3. Metallic sheath4. Bedding5. Armoring6. Serving

Insulation is applied over the conductors. The various types of insulation are classified as:

i. Tapped insulation ( impregnated paper or varnished cambric )ii. Rubber compound insulation and rubber compound sheathsiii. Plastic insulation (polyethylene, PVC, XPLE).

Data for conductor & cable used at distribution system.Sl. No.

Name of conductor

Size in sq.mm

Breaking strength in KN

Resistance at 20C in ohm/Km

Allowable current in amp

Weight in Kg/Km

code name

REMARKS

1 AAC 25 4.52 1.096 124 74 Gnat IS:398(PART-

1)-19962 AAC 50 8.25 0.5525 188 145 Ant IS:398(PART-

1)-19963 AAC 100 15.96 0.2752 287 290 Wasp IS:398(PART-

1)-19964 AACR 20 7.61 1.394 108 85 Squirrel IS:398(PART-

2)-19965 AACR 30 11.12 0.9289 139 128 Weasel IS:398(PART-

2)-19966 AACR 50 18.25 0.5524 196 214 Rabbit IS:398(PART-

2)-19967 AACR 100 32.41 0.2792 290 394 Dog IS:398(PART-

2)-19968 AACR 200 89.67 0.1320 449 974 Panther IS:398(PART-

2)-19969 AACR 250 110.88 0.1102 517 1229 Bear IS:398(PART-

2)-199610 AACR 300 135.10 0.08989 586 1492 Goat IS:398(PART-

2)-199611 AACR 420 130.32 0.06868 685 1621 Zebra IS:398(PART-

2)-199612 AACR 520 159.60 0.05595 777 1998 Moose IS:398(PART-

2)-199613 AAAC 20 6.45 1.5410 103 60.16 IS:398(PART-

4)-199414 AAAC 30 10.11 0.9910 134 94.00 IS:398(PART-

4)-199415 AAAC 50 16.03 0.6210 178 149.20 IS:398(PART-

4)-199416 AAAC 100 29.26 0.3390 258 272.86 IS:398(PART-

4)- 1994

5. AUXILIARY POWER SUPPLY: AC single phase and three phase supplies are necessary for:

i. Illuminationii. Battery chargingiii. Transformer slant adjustment

iv. Oil filtration slantv. Transformer tap changer drivesvi. Power supplies for communication equipmentsvii. Breakers/ disconnect switch motor operation

DC auxiliary supplyDC auxiliary supplies are necessary for closing and tripping of CB, emergency lighting, control board indication etc. During normal operation rectifiers serve the purpose. However in case of failure of rectifiers a storage battery of adequate capacity is provided to meet the DC requirement. Normally storage batteries in substations merely keep floating on the DC subsystem and supply current only in case of failure of rectifiers.The voltages commonly used are 30V, 110V or 220V and 48V for PLCC.

Battery charger specifications:KASBA substation houses 220V/24A float charger and 24A float cum boost charger.

MODEL : BCSERIAL NO. : CON/00278 101-02RATING : 220V, 24V +24VA.C. INPUT : 145+/- 15%, 30A %0 HZ, 3PHASED.C. OUTPUT : 247.5V,24ABATTERY VOLTAGE : 220VAH CAPACITY : 217MAX. AMB. : 50 degree CelsiusMANUFACTURING DATE :P.O. NO. : KD/TR/42/968CUSTOMER : WBSEB

6.COMMUNICATION IN THE SUBSTATION:Communication is a vital part of any substation. It is necessary to connect HQ, CLD and different tie line substations. For load dispatching and better power management, communication is essential.

The KASBA substation employs the following medium for communication are used:

Power line carrier communication using WAVE TRAPPER:

Power line carrier communication means that communication signals are sent through power line along with the power. The supply to communication system is 48V DC. A PLCC system should have the following parts:

1. A telephone exchange system 2. Battery system with battery charger3. A carrier signal generating system4. Coupling capacitor5. Wave trapper6. Earthing switch7. Lightning arrestor8. Drainage coil

The wave trapper is a tuned LC circuit, where the frequency carrier gets blocked and the power frequency electrical signal passes into the electrical circuit. High frequency wave passes through the coupling capacitor but low frequency is blocked here. Drainage coil is used to bypass the over current surges whereas lightning arrestor is kept here additionally to provide safety against over voltages and lightning surges. Generally PLCC is done using R phase. The conductor from the lower end of the coupling capacitor enters the drainage coil. LA and earthing switch enter into a box, known as Line Matching Unit (LMU). From there a coaxial cable enters to carrier set. Phase modulation is employed in the carrier set circuit. The signal is sent to the transmitter and after amplification it is passed to the filter.

In kasba substation the receiving end and sending end frequencies are as follows.

KASBA T.F = 380kHz R.F = 428kHz

Satellite communication:VSAT or Very Small Aperture Terminal communication through satellite is also done in KASBA substation. It is done with other primary grid substation, CLD, HQ authorities. Due to weak, noisy, slow and poor receiving qualities this type of communication is not used now days.

VHF radio communication:This type of communication takes place between 132KV and 33KV substations. The frequency of sending and receiving signals is 167.125MHz. Its main disadvantage is that it is one way communication at a time.

P & T phone and mobile communication:There are two P&T phones in Kasba substation. One is used for normal official works and the other phone line is an emergency line and can be used for CLD or HQ for collecting data from any feeder or bay. Mobile phones are personal and are used only in emergency.

7. SPECIFICATION AND RATING

A. POWER TRANSFORMERS: There are three auto transformers for stepping down the voltage at a level of 220 kV/132 kV/33kV. Two of them are MITSUBISHI Corp., Japan made and the other is CROMPTON GREAVES India made. Their load capacities are 150MVA and 160 MVA respectively. Their tertiary winding voltage is 33kV. The tertiary voltage of the CROMPTON GREAVES transformer has been further stepped down to .4kV by an auxiliary transformer to provide power to the central load of the substation only.

TRANSFORMER 1 and 2 with load tap changer:Maker’s Name MITSUBISHI Corp, JapanMVA Rating 150Volts at NO Load HV- 220 kV

IV- 132 kVLV- 33 Kv

Phases 3Serial No. 7430010302

TRANSFORMER 3 with load tap changer:Maker’s Name CROMPTON GREAVES, IndiaMVA Rating 160Volts at NO Load HV- 220 kV

IV- 132 kVLV- 33 kV

Amperes HV- 429.9 AIV- 699.8ALV- 787.9 A

Phases 3Serial No. 7430010302Year of Manufacturing 1995Temperature Rise in Oil and Winding 45 degree and 60 degreeTypes of Cooling ONAN, ONAF, OFAFConnection Symbol: YNaOd11

CIRCUIT BREAKER SPECIFICATIONS KASBA SF6 C.B.:Maker’s Name CROMPTON GREAVES, JapanRated Voltage 145 kV

Rated Normal Current 1600 A

Rated Short Circuit Current 31.5 kARated SF6 Gas Pressure 6kg/sq cm at 20 degree

Rated Closing Time <130 msRated Opening Time <30 ms

Rated Frequency 50 HzGas Weight 7.5 kgTotal Weight 1450 kg

Type 120 SFM- 32B(3 Pole)Rated Making Capacity 80 KAp

Rated Short Time Current 31.5 kA for 3 secFirst Pole to Clear Factor 1.5Auxiliary Circuit Voltage 1 Ph, 230 V AC, 50 Hz

SPECIFICATIONS OF CTs:CTs are of two types:

1. Dead Tank type- Here CT tanks are situated well below the instrument. They have their major insulation over high current carrying primary.

2. Live Tank type- Here Ct tanks are on the top of the instrument bushings. Primary winding is short and rigid, therefore more reliable and can withstand high short circuit current. They don’t have their major insulation over high current carrying primary, so the heat generated can easily be dissipated, thus having a longer life.

Type IMB 145- Single PhaseHighest System Voltage 145 kVRated Primary Normal Current 650ARated Insulation Level 145/275/650 kVDynamic Current(kA Peak) 80.3Class of Insulation ATotal Creepage 3725Rated Short Time Current 81.5 kA for 3 sec

8. REFERENCES:

Principles of Power System By V.K.Mehta & Rohit Mehta. Electrical Machine Designing by A.K.Sawhney. Wikipedia Electricalandelectronics.org Information obtained from Kasba substation.