Construction, Installation and Maintenance Of 132 Kv Grid Sub-Station

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Construction, Installation and Maintenance Of 132 Kv Grid Sub-Station


1.1 Sub-station

A sub-station is an assembly of equipment desired to receive power supply from a higher voltage system, convert it to a form suitable for local distribution, via different feeders through switching equipment desired to protect service from affects of faults.

1.2 Purpose of sub-station

The purpose of a sub-station is to give reliable service at minimum cost and minimum power supply interruption. Sub-Station can generally be safely operated with less reserve than generating stations owing to

a) The fewer and shorter periods of scheduled outages

b) The shorter period of outage due to repairs or replacement

c) The greater possibility of over loading during emergency

d) The smaller amount of load affected in case dropping of load



The following equipment are generally used in a 132 grid sub-stations

2.0 High voltage power circuit breaker

Power circuit breakers arc available today in two main types known as oil and oil less designs. 111gb voltage power circuit breakers arc considered to be those applied in circuits from above 132 KV to 34$ KV. While the oil circuit breaker is the type in general use for out door service and oil less circuit breakers of the air to of magnetic air types for indoor service. About 90’/o of indoor installations are now being made with oil less types. Some applications of oil less type power circuit breakers have been made outdoors and it is anticipated that that this trend will gain momentum with time, The reasons tk,r these changes are:

a) Oil tired hazard

b) Case of inspection and maintenance in oIl less breakers

c) Longer outages necessary to maintain oil circuit breakers

d) experience shows satisfactory performance with in oil less circuit breakers.

2.1 Oil Circuit breakers

Oil circuit breakers are made in two general types. The dead-tank construction and the live, tank construction. The former are

Fig. 2.3
Fig. 2.2

available in all classifications of voltage and interrupting rating for indoor and outdoor applications, while the later It ye generally been restricted to voltage of 14, 4.kv and below, although application up w 34. $00 volts have been made.

2.4 A dead-tank breaker

A dead tank breaker consist of a steel tank partially field with oil, through the cover of cover of which are carried porcelain on composition bushing. Contacts at this bottom of the bushing are bridged by a conducting crosshead carried by a wood or composition which in common design drops by cavity, thus the breaker. Accclcradng springs arc frequently used to Increase the rule of opening. In many designs two bushings and one crosshead give two breaks per pole, while in some the contacts aid crosspieces are designed 1:0 give four six or more breaks per pole with a redact ion in the length of tic : give adequate arcing distance. An insulating, fibrous fibrous tank liner aids in preventing the arc from striking the tank walls.

2.5 Single tank breakers

Single tank breakers with all poles of a pole breaker in one tank arc available for ratings up to 14,400 volts and 500.000 KVA Interrupting. Insulating fibrous barriers are inserted between the phases in the tank in sonic designs.

2.6 Multi tank breakers

Multi lank break with each pole in a seperate tank arc used for higher voltages and intimating ratings and generally far large out door breakers. They arc available in many of the smaller ratings.

however and are commonly wed for main generating station breakers of 1,00,000 KVA rupturing rating and larger. They necessary for the isolated phase type of construction.

2.7 Oil-tight features

Oil-tight features are standard in modern oil breakers. A vent with oil separating feature permits the escape of the gases generated by the arc but prevents the escape of the entrained oil.

2.8 Plain-break breakers :

Plain-break breakers have been largely supplanted in recent years by designs having special arc controlling devices which materially shorten the arc an arcing time, thereby reducing the arc energy. Arc controlling devices have been added modernize many of the huger plain-break breakers in survice to improve their performance at the original rating in some cases to obtain higher interrupting ragings.

2.9 Air-Control devices

Many types of air-control devices have been developed during the pat some years for oil circuit breakers. Four of wich are used at present on high voltage out door circuit breakers, All make use of oil pressure. generated by a pump or by gas created by the are, or both to force fresh oil through the arc path in such quantity to provide, the Succession insulation at current zero to prevent a restricted of the arc and thereby interrupt the circuit, The interrupters contains the high pressure within their chambers and greatly reduce the stress to which the main oil tank would otherwise be subjected. The gas as which arc generated, after being effectively cooled, pass out through the tank vent pipe to the open air.

2.10. Oil-blast features:

The particular designs vary depending upon the breaker type and rating, In large tank type, high voltage breakers the interrupter is a special cross-oil blast-design. The construction for 138 KV, 10,000 MVA to 330 KV, 25000 MVA. The gas pressure developed in the upper chamber forces a blast of oil horizontally across the are at the intenupting contacts. The oil pump also assists this. It is particularly effective at low current. The blast of oil prevents the re-establishment of the are after an early current zero. In smaller breakers the tank is divided into two portions by insulating barriers with the contact in one portion. A port is located in the barrier close one gap of each pole. The gas formed by the are forces oil across the gap nearest the port and into the other chmber, quickly extinguishing the are.

2.11 Expulsion interrupter

Expulsion interrupters as used on high voltage breakers have tulip-type contact arranges. So the are is drawn on different surfaces from those that carry load current oil circulation is from pressure-producing sources located within or adjacent to the interrupter. Baffles and ducts direet the follow so fresh oil collapses into the gaps as current zero is approached.

2.12 Deion grids :

Deion grids are standard on many westing house oil breakers of 15000 volts and above and simplified deion interrupter are common on lower voltage breakers. The deion grids surround the arcing contacts and consist of a series of insulating plates having interspersed plates of magnetic material all go disposed and vented that the are is moved laterally into oil pockets where it vaporizus the oil. The resulting gases of are stream to deionize them and extinguish. The are is extinguished before the contacts are with drawn from the grid.

2.13 Live-tank breakers

Live-tank breakers of general electric type-H designs make use of two small-diameter cylindrical tanks or pain pcr pole. Each pale is mounted on an insulator and fonts part of the circuit. A conducting crc. stead above the. pots carries two rods, one of which extends through a porcelain in the top of each pot into the tank, and when the breaker is closed, makes contacts with flexible contact in the bottom of the pot. The breaker is opened by fitting the rods. Insulating shuffles in the designs and oil blast. features in the later ddesigns aid in extinguishing the arc and increase the nurturing capacity. The breakers arc ordinarily operated by a combination of spring and motor mechanisms.

2.14 Low oft content, high speed. 230 KV oil circuit breaker

This breaker has an interrupting raring of 10,000 ,000 KVA in less than 3 cycles (I/Ø see). ‘The oil content is out one-tenth. of that of a conventional breaker of the same rating. and only about one tenth of ibis oil is to arcing. The breaker is of the impulse type, in which a positive and definite.

2. 15 Indoor oil circuit breaker

Indoor oil circuit breakers are generally mounted in steel or masonry compartments. Large dead tank breaker are sometimes frame mounted. Live tank breaker should always be mounted in cells.

2.16 Outdoor station type oil circuit breaker

Outdoor station type oil circuit breakers are generally frame mounted up to 69000 volt rating and floor mounted above 69,000.

2.17 Oil less power breakers

Oil less power circuit breakers have received wide acceptance in all fields during the past ten to fifteen fly practically displaced

the oil circuit breaker from indoor application and one gaining rapidly in outdoor installation. The oil circuit breaker remains favored for indoor uses only, where adverse conditions such as dirty atmospheres, steam, high humidity and the like are present. Oil less breakers are manufactured in two main classes, i.e. Magnetic air and Compressed air types.

2.18 Magnetic air circuit breakers

Magnetic air circuit breaker are usually Solenoid operated and interrupt their main circuit in the normal atmosphere under the influence of a strong magnetic field which acts to force the arc deep into a specially designed arc clute. The clute cools and lengthens the arc to appoint where the circuit can not be maintained by the voltage of the system and interruption is accomplished. The zone between the main contacts is clear of ionized air by the time interruption is obtained in the arc clute and so restricting at this point is not a problem. Since the magnetic effect is not great at low currents. Such as load, Transformer magnetizing cable charging current, etc. Most designs use an air pump “Puffer” actuated by the operating mechanism, which blows a blast of air across the arc and thereby assures its entering the arc chutes and giving repaid interruption at the low current values also when the circuit breaker is opened the arc transfers from the main arcing contact to fixed arcing .horns which are within the arc chute the magnetic field is produced by coils in the main current circuit wound around a magnetic core which magnetizes soft iron plates in the sides of each arc chute.

2.21 Compressed air circuit breaker

Compressed air circuit breaker are used to full fill the heavy duty requirements of air circuit breakers in high voltage circuits. They are used mainly to provide the indoor ratings at 14.4 KV and 34.5 V through air blast designs have been used at some lower voltage ratings. They are also used in out door applications up to 10,000 MVA at 138 KV.

The first air blast circuit breaker was for 13.8 KV generating station service, it was rated 15 KV. The modern design of this circuit breaker and its housing is shown. Compressed air is used to operate the breaker as well as to provide the medium for arc interruption. A blast of air at 150 (westing house) or 250 (General Electric cr.) PSIg is directed across the path of the arc, forcing it into a special arc claute where the arc products are cooled and deionizer to a degree to effect efficient interruption. Contact erosion is small and maintained is small and easy to accomplish in these interrupters which are available at 1000-1500 and 2500 MVA duties.

The 138 KV air blast circuit breaker is a new development in the united states, though it has been used in Europe for many years. The first development in American factories is the largest interruption rating available at this voltage viz. 10,000 MVA, 3 phase, at 130 to 145 Ky. This circuit breaker when enclosed in a metal housing and used in connection with metal enclosed buses is readily adaptable to installation in metropolitan areas where the open type of construction could not be used because of space limitation and cost.

Air blast circuit breaker 138 KV, 10,000 MVA interrupting rating 3- phase unit. The supporting porcelain columns provide housing for the necessary current. Transformers for relaying and metering and provide tapes for capacitance type potential measuring devices control and air supply lines are also located inside the vertical support columns. Interrupting elements moving contacts, operating lineage, and air blast valves are allocated in the steel pressure vessels near the top of each porcelain columns. High pressure air is maintained in the steel pressure vessel, where it is immediately available to interrupt the high voltage arc drawn when the moving contacts are opened. High pressure air blasts radially into the arc stream when the down stream blast valve is opened, causing interruption at an early current zero. Sufficient high pressure air is contained in the interrupter chamber to provide for several complete opening and closing operations. Replenishing air is fed from a common air compressor and storage facility which is normally connected to all of the high voltage breakers located in a single station.

2.25 Vacuum circuit breaker

When two current carrying contacts are separated in a vacuum module, an arc is drawn between them. An intensely hot spots are created at the instant of contact separation from which metal vapour shoot off, constituting plasma. The amount of vapour in the plasma is proportional to the rate of vapour emission from the electrodes; hence to the arc current with alternating current arc, the current decreases during a portion of wave, and tends to zero. There by the rate of vapour emission tends to zero and the amount of plasma tends to zero. Soon after natural current zero, the remaining metal vapour condenses and the dielectric strength builds up rapidly, and restricting of arc is prevented.

This principle is used in vacuum circuit breakers. The vacuum circuit breaker comprises one or more sealed vacuum-interrupter units pole. The moving contact in the interrupter is connected to insulating operation rod linked with the operating mechanism. The contact travel is of the order of a few millimeters only. The movement of the contacts within the sealed interrupter unit is permitted by metal bellows.

The range of vacuum switching devices includes:

  • Vacuum interrupters rated 3.6/7.2/12/36 KV for indoor metal clad switch gear
  • Vacuum contactor’s rated 1.2/3.6/7.2 KV for indoor metal enclosed control gear.

Vacuum circuit breaker

Fig. Vacuum circuit breaker

  • Vacuum interrupters rated 3.6/7.2/36 KV for outdoor porcelain housed single interrupter per pole, circuit breaker
  • Multi interrupter out door porcelain housed circuit breakers for 145 KV and above.

The structural configurations of the switchgear mentioned above is quite different, through the basic interrupter unit is based on same principle of operation.

The multi-unit vacuum circuit breakers rated 72.5 KV and above have been developed and installed in England and USA. However, they are not vary popular and are not preferred any more.

For voltage up to 36 Ky, vacuum circuit breakers employing a single interrupter unit have become extremely popular for metal enclosed switchgear, arc-furnace installations, auxiliary switch gear in generating stations and other industrial applications.

Single phase 25 Ky, 25 KA vacuum circuit breaker having two interrupters per pole are used for railway track side 25 KV single phase substations. Vacuum switching devices have several merits such as high speed of dielectric recovery after repaid and silent operation, suitability for repeated operation. Simple operating mechanisms, freedom from explosion, flexibility design, long of life etc.

The unique merits of vacuum interrupter’s are small contact travel and less weight of moving parts. The vacuum interrupters have a very long life of the order of several thousand operations on rated normal current. However for outdoor installations the external insulation requirements must be fulfilled and the advantages of high dielectric strength of vacuum cannot be fully utilized. Some recently developed 36 KV — GIS utilized SF6 gas as insulation and vacuum interrupters for arc interruption. Such circuit breakers are commercially manufactured in USA and Japan.


Having located the breaker truck in the respective busber location, the breaker is dully locked in position by operating the shoot bolt castle lock. By operating the close push button manually or electrically the breaker is closed so that the load on the cable side is connected to the bus side. The breaker can be switched OFF by electrical by means of TNC switch (mounted on instrument panel front door) or manually by a push button on the right hand side of the mechanism.


  1. VCB is self contained and does not need filling of gas or oil. They do not need auxiliary air system, oil handling system etc. No need of periodic refilling.
  2. No emission of gases, pollution free
  3. Modest maintenance of the breaker, no maintenance of interrupters. Hence economical over long period.
  4. Breakers unit which can be installed at any required orientation. Breaker unit is compact and self contained.
  5. Non explosive
  6. Silent operation
  7. Large number of operation on load, or short circuit. Suitable for repeated operating duty long life.
  8. Suitable for capacitor switching, cable of switching industrial load Constant dielectric. There are no gas decomposition products in vacuum and the hermetically seated vacuum interrupter keeps out all environmental effects.
  9. Constant contact resistance. In vacuum, the contact cannot oxidize, which ensures that their very small resistance is maintained through out their life.
  10. High total current switched. Since contact piece erosion is small rated normal current can be interrupted upto 30,000 times and rated short circuit breaking current on average a hundred times.

2.27 Sulphur Hexafluoride (SF6) Circuit breaker


Sulphur hexafluoride (SF6) is an inert, heavy gas having good dielectric and arc extinguishing properties. The dielectric strength of the gas increases with pressure and is more than that of dielectric of oil a pressure of 3 kg/cm2 SF6 is now being very widely used in electrical equipment like high voltage metal enclosed cables, high voltage metal dade switch gear, capacitors, circuit breakers, current transformers, bushings etc. This gas liquifies at certain low temperatures, the liquefaction temperature increases with pressure. This gas is commercially manufactured in many countries and is now being extensively used by electrical industry in Europe, USA and Japan.

Several types of SF6 circuit breakers have been developed by various manufactures in the world during last fifteen years, for rated voltage from 36 to 760 Ky.

SF6 as insulated metal clad switchgear comprises factory assembled metal clad, sub-station equipment like circuit breakers, isolators, earthing switches, bus bars, etc. These are filled with SF7 gas. Such sub-stations are compact and ire being favored in densely populated urban areas.

Sulphar hexa fluoride gas is prepared by burning coarsely crushed roll sulphar in the fluorine gas, in a steel box, provided with staggered horizontal shelves, each bearing about 4 kg of suiphar. The steel box is made gastight. The gas thus obtained contains other fluorides such as S2F10,SF4 and must be purified further SF6 gas is generally supplied by chemical firms. The cost of gas is low if manufactured on a large scale.

The gas transported in lequid form in cylinders. Before filling the gas the circuit breaker is evacuated to the pressure of about 4mm of mercury so as to remove the moisture and air. The gas is then filled in the circuit breaker. The gas can be reclaimed by the gas handling unit.

2.29 Operations

The arc extinction process in SF6 circuit breakers is deferent from that air blast circuit breakers. During the arcing period SF6 gas is blown axially along the arc. The gas removes the heat from the arc by axial convection and redial dissipation. As a result the arc diameter reduces during the decreasing mode of the current wave. The diameter becomes small during current zero and the are is extinguished due to its electronegative, and low arc time constant, the SF6 gas regains its dielectric strength rapidly after the final current zero, the rate of rise of dielectric strength is very high and the time constant is very small.

In SF6 circuit breakers, the gas is made to flow from a high pressure zone to a low pressure zone through a convergent divergent nozzle. The mass flow is a function of nozzle throat diameter, the pressure ratio and the time of flow. The nozzle is located such that the flow of gas covers the arc. The gas flow attains almost super sonic speed in the divergent portion of the nozzle, there by the gas takes away the heat from the periphery of the arc, causing reduction in the diameter of the arc. Finally the arc diameter becomes almost zero and the arc is extinguished. The arc space is filled with fresh SF6 gas and the dielectric strengths of the contact space is rapidly recovered due to the electro negativity of the gas.

2.30 Merits of SF6 circuit breaker

  1. Out door EHV SF6 has less number of interrupters per pole than ABCB and MOCB outdoor SF6 CB is simple, less costly, maintenance free and compact.
  2. The gas is non-inflammable and chemically stable. The decomposition products are not explosive. Hence there is no danger of fire after explosion.
  3. Same gas is recalculated in the circuit. Hence requirement of SF6 gas is small.
  4. Ample overload margin. For the same size of conductors the current carrying ability of SF6 circuit breakers is about 1.5 times that of airblast circuit breakers because of superior heat transferability of SF6 gas.
  5. The breaker is silent and does not make sound like air blast circuit breaker during operation.
  6. The sealed construction avoids the contamination by moisture, dust, sand etc. No, costly compressed air system like ABCB.
  7. The maintenance required is minimum. The breaker may need maintenance once in four to ten yeard.
  8. Ability to interrupt low and high fault currents, magnetic currents, capacitive current, without excessive over-voltages. The SE6 gas circuit breaker can perform the various duties like clearing short-line faults, opening unloaded transmission lines, capacitor switching, transformer, reactor switching etc. much smoothly.
  9. Excellent insulating, arc extinguishing, physical and chemical properties of SE6 gas is the greatest advantages of SF6 breakers.
  10. No frequent contact replacement contact corrosion is very small due to inertness of SF6 gas. Hence contacts do not suffer oxidation.
  11. No over voltage problems. Due to particular properties of SF6 gas, the arc is extinguished at natural current zero with out current chopping and associated over voltage originating in circuit breaker.

2.31 Demerits of SF6 circuit breaker

  1. Sealing problems arise due to the time of construction used. Imperfect joints lead to leakage of gas.
  2. Arced SF6 gas is poisonous and should not be in inhaled or let out.
  3. Influx of moisture in the system is very dangerous to SF6 gas circuit breakers. Several failures are reported due to this cause.
  4. The double pressure SE6 circuit breakers are relatively costly due to the type of construction and the complex gas system.
  5. The internal parts should be cleaned thoroughly during periodic maintenance under clean dry environment.
  6. Special facilities are needed for transporting the gas transferring the gas and maintaining the quality of gas. The deterioration of quality of the gas affects the reliability of the SF6 circuit breakers.

2.32 Minimum oil circuit breaker (MOCB)

Descriptions : In minimum oil circuit breakers, dielectric oil is used as an arc quenching medium and dielectric medium.

For voltages up to 36 KV. minimum oil circuit breakers are generally enclosed indrawout type metal clad switchgear. For 36KV, 72.5 Ky, and 145 KV ratings MOCB’s are outdoor type, with one interrupter per pole and single operating mechanism for three poles.

For 245 KV and above, modular construction is necessary. In such a construction, the twin interrupter units are connected in series in T or Y formation. The number of units per pole depends upon the rated voltage and voltage per interrupter.

Bulk oil circuit breakers (Tank type circuit breakers) are becoming obsolete and have been described here in brief.

Minimum oil circuit breakers have the following demerits:

Short contact life

  1. Frequent maintenance
  2. Possibility of explosion
  3. Larger arching time for small currents
  4. Prone to restrickes

2.33 Principle of Arc-extinction oil breakers

As the current carrying contacts are separated under oil, the heat of the arc causes decomposition of the oil. The gases formed due to the decomposition expand causing increase in pressure. The pressure build-up and the flow of gases is influenced by the design of arc-control device, speed of contact travel, the energy liberated by the arc etc. The gas flowing near the contact zone causing cooling and splitting of the arc. The contact space is filled with fresh dielectric oil after the final arc interruption at a current zero.

Arc control devices are fitted to the fixed contact of minimum oil circuit breaker. Arc control devices modify the behavior of circuit breakers. These are, enclosures of dielectric materials fitted to contacts of the circuit breakers such that the actual contacts are separated inside the cavity of the device. The arc control devices are based on axial flow and / or cross flow principle. Axial flow is employed in circuit breaker upto 25KA. Cross flow principle is preferred for higher breaching currents.

The heat of arc causes decomposition of the dielectric oil, the products of decomposition being hydrogen gas and other gases like acetylene. The gases result in increased pressure inside the arc control device. As the moving contact travels, the arc length increases. The amount of gas produced in the jet pot is a function of heat of arc, arching time related with speed of contact travel. The pressure generated within the arc control device is related with the amount of gas produced and the area and the location of the side vents. At lower values of currents, the amount of gas produced is relatively less. Hence the size of vents to cause enough pressure difference should also be relatively small. For larger currents the size of vents should be larger to avoid excessive pressure difference. The high pressure gas’ in the arc control device tries to escape through the side vents towards the top of the interrupter while doing so, it tries to cool the arc and length in arc into the vents. At current zero of the waves, the arc diameter is very small and the gas flow is able to interrupt the arc. The interruption of the stops the generation of gas and flow of oil.

The contact space contains hot gases during the brief period after the interruption of arc and high rate of the rise of TRV can cause arestrike. To avoid this, the contact travel is extended well beyond the arc control devices so the fresh dielectric oil filled the contact space after the arc extinction. The other techniques adopted to increase the rate of gain of dielectric strength after final current zero are:

  • Flushing the contact space by fresh dielectric oil forced into the contact space by means of position action.

A position attached to the moving contact compresses the dielectric oil in a cylinder. The oil at a high pressure in the cylinder flows into the contact space.

  • Maintaining the pressure on the oil in the interrupter if the oil in the interrupter is maintained at higher pressure by means of an inert gas, the oil flow into the contact space and the hot gases travel upwards. pressure reduces the size of gas bubbles.

These techniques are used in minimum oil circuit breakers to avoic restrikes during capacitor switching.sss

2.34 Transformer

“Transformer is a static magnetic machine (equipment) which transforms alternating current from one A/C voltage to another AC voltage at the same frequency (say 50 Hz) and at the same apparent power (KVA)”. A transformer transfers power from one winding (circuit) to another winding (circuit) via a common magnetic core.

2.35 Basic principle of power transformer

From faraday’s law of electro magnetic induction, we know that, “emf. is induced in a closed conducting circuit when the magnetic flux linking with that circuit changes in time. The emf. induced is proportional to the rate of change of flux linkage”.

A Transformer has a closed magnetic circuit called the core. Two or more windings are placed on the core one of the winding (called the primary winding) is supplied with alternating voltage of power frequency (50Hz). The primary winding takes no load current (10) from the supply and set-up alternative magnetic flux of 50 Hz in the core. As the secondary winding is also placed on the same core, the magnetic flux linking with the secondary winding also changes continuously at a rate of 50Hz. Thereby emf is induced in the secondary winding induction principle. The emf induced in the secondary winding has the same frequency as that of the magnetic flux and primary exciting current. However, the emf has a direction opposite to applied voltage.

A single phase transformer has two electrically isolated windings on a common single laminated core. The core is of laminated soft iron sheets, insulated from each other. Laminated cold rolled grain oriented (CRGO) silicon iron sheets to minimize hysterics loss and eddy current loss. When primary with number of turns Ni is connected sinusoidal AC voltage VI, the AC current in primary set up continuously alternating magnetic field (?) in the core.

The flux linkage with the secondary winding changes sinu soidaly and sinu soidal voltage V2 is induced in the secondary winding with turns N2.

2.37 Commissioning of Transformers

All the basic safety rules should be followed. Some are mentioned below. Before starting the maintenance or clase inspections.

To disconnect the transformer from supply and ground the line terminals, ground the tank.

  1. To disconnect both the primary and secondary. The purpose of disconnecting secondary is to eliminate and possibility of a system feedback.
  2. To lock the disconnecting switches in open position.
  3. To remove the fuses or fuse cut outs to a special inaccessible area so they cannot be accidentally reused.
  4. To check the permanently installed transformer tank ground.
  5. To remove the tank top or manhole cover of liquid immersed transformer to relieve internal tank pressure.
  6. For askarel as an insulating liquid while working with Transformer. To avoid direct contact with askarel as it has an irritating effect on the skin, especially the eyes, nose and lips.
  7. Do not open the transformer containing ask are when it is hot as the fumes are toxic, if it is necessary, it should be done in a well ventilated area and direct exposure to the fumes should be avoided.
  8. It should be mandatory procedure that when a man enters a transformer tank a second man be on duty out side the tank man should not enter nitrogen filled tank.
  9. No tools or other equipments are dropped or left inside the tank.
  10. Do not take a chance. Play safe.
  11. Do not assume that a transformer is dead.
  12. To make sure all is clear before energizing, after work is completed. Primary windings : Enameled electrolytic copper wire is used for primary windings of intermediate voltage transformer.

Secondary windings: Enameled electrolytic copper wire is used for secondary windings of intermediate voltage transformer.

Maintenance : Maintenance free, simple recommendations:

  • A complete and through inspection should be carried out in every year.
  • Clean the porcelain bushing at regular intervals depending on the degree of pollution.

2.38 Description

Primary and secondary windings are housed in a porcelain bushing. Including and above 245 KV EMEK VT’S are two piece designs. Commonly known as cascade. These are basically two independent VT’S connected in series. They are named the top unit and bottom unit. Transformers are hermetically sealed.

Frequency: 50 Hz, 60 Hz.

2.39 Rated secondary voltage

110?3, 100?3 V etc. inconformity with customer requirement.

Rated tertiary voltage: 110 100 I, 110/3, 100/3, 110 V etc. inconformity with customer requirement.

Highest system voltage: Highest system voltage from 72.5 to 420 KV

Number of secondary windings: In conformity of customer requirements, maximum 4 secondary windings.

Voltage factor: 1.2 x Un continuous/ 1.9 x Un for 30 sec.

Oil insulation : The insulating oil is meneral oil in conformity with IEC 296. Before impregnation, moisture and gases are removed from the oil by special process.

Drain plug for taking samples and oil refilling facilities are provided on request.

Floating type oil level indicator is standard supply

2.40 Transport, installation and maintenance

Packaged for transport in horizontal or vertical position. There is no need of any special tool for the mounting. Hermetically sealed, free maintenance spare parts not necessary. They have a valve for oil sampling, emptying and refilling, as well as an oil indicator on the head.

2.41 Classification of Transformers

Transformers and their characteristics are difined in the following paragraphs together with various considerations that are closely associted with their designed, application and operation.

2.41 Constant potential transformer

Transformer having nearly constant ratio at all loads, are most generally used for transmission and distribution of power. They are then designated power transformers. When used for the purpose of raising the voltage they are referred to as step-up transformer, and for lowering the voltage as step-down transformers.

2.43 Potential transformer

Potential transformer is a transformer intended fir measurement or control purposcs4 which is designed to have its primary winding connected in parallel with a circuit the voltage of which is to be measured or controlled.

2.44 Regulating transformer

Transformer having one or more windings excited front the system circuit or 3 separate source and one or m winding connected in series with the system circuit for adjusting the voltage or the phase relation or both in steps without interrupting the load,

2.45 Auto transformer

Transformer in which part of the winding is common to both the primary and secondary circuit.

2.46 Grounding transformer

Grounding transformer is a transformer intended primarily (or the purpose of providing a neutral point far grounding purpose.

2.47 Power transformer

Power transformers generally rated above 500 KVA arc used on primary transmission lines for the transmission and distribution of relatively large quantities of energy.

2.48 Distribution transformer

Distribution transformers generally used 500 KVA and less are used for distributing the energy from transmission lines and net work for local consumption.

2.49 Instrument transformer

Instrument transformer in which the conditions of current or voltage and of phase position in the primary circuit are represented with acceptable accuracy in the secondary circuit. A instrument transformer may be either an instrument current transformer or an instrument potential transformers.

2.50 Current transformer

Current transformer is a transformer intended for measuring or control purposes designed to have its primary winding connected in series with a circuit carrying the current to be measured or controlled.

2.51 Unit Sub-station transformer

A unit sub-station transformer is a transformer which is mechanically and electrically connected to an coordinated in designed with one or more switchgear or motor controlled assembles or combinations thereof.

2.52 Teaser transformer

A teaser transformer is that transformer of two T-connected single phase, unit for 3-phase, to 2-phase, or 2-phase to 3-phase operation which is connected between the mid point of the main transformer and the third wire of the 3-phase system. For the teaser unit of a scott connected pairs of transformers, an 86.6% tap is used for connection to the third wire of the 3- phase system.



Fuses are the simplest device used for interrupting an electrical circuit under short circuit or excessive overload, current magnitudes. Enclosed fuses are on the market and are approved by the under writers laboratory in a wide variety for use up to 250 and 600 volts. They may be used in a-c-or d-c circuits and have a variation in time current characteristics that makes them suitable for many special purpose. More important among these is the motor protective fuse which may be chosen to have a suitable time delay to permit starting in rush current to flow and also they carrying of moderate motor overload 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 short circuit. While a minimum of 10,000 amp interrupting capacity is required in low voltage fuses, some sizes and types today are capable of interrupting upto a 100,000 amp circuit. The standard lines on low voltage fuse are available in several steps of ampere capacity, each of which is a different physical size viz. 1 to 30 amp, 35 to 60 amp 70 to 100 amp, 110 to 400 amp and 450 to 600 amp. Recently a new development in low voltage fuses makes 100,000 Amp interrupting capacity available in sizes above 600 amp- up to several thousand amperes continuous rating. This is sometimes referred to as the silver sand fuse. Law voltage fuse will carry their rated current continuously, if not heated abnormally at the same time say by a loose terminal connection, but will blow in 1 to 5 mini, if the current reaches 115% of rating. They must also meet additional performance and other requirements of under writers laboratories, inc.

3.2 High voltage fuses

High voltage fuses are available in a range of capacities of currents up to 400 amp and voltage up to 138000. They are used for protecting potential transformers, Distribution or small power transformers and occa sionally branch circuit. In some cases they have been installed in series with protective gaps for lightning protection. They are often equipped with contacts so arranged that the fuse and its mounting act as a disconnecting switch. Mechanism has been used in certain size which automatically replace a blown fuse with a new one. When fuses are used to protect transformer branch circuits in series with other fuses or automatic circuit breakers, care must be exercised to insure proper time selectively. Fuse characteristics may be varied some what by the choice and disposition of materials but time and minimum blowing currents arc affected by ambient and currents carried immediately previously. Fuse characteristics arc usually based on test starting cold in a 20 to 30 C ambient. Fuse blowing and clearing characteristics should be coordinated through out the range of short circuit currents to insure a proper margin for selectivity, as fuse characteristics differ with different types and the shape of the curves usually differs appreciably from relay characteristics. Shown in the Fig.

Fig. 3.3
Fig. 3.5

3.6 Several types of fuse

Several types of this fuse are available

  1. Fuse mounted on a porcelain plug set in a weather proof block on enclosed in a fiber base with a porcelain housing. This type is commonly used on distribution circuits and station apparatus for voltages upto 7500 and in some cases up to 15000. They should not be used where the intermitting duty is high.
  2. Fuse enclosed in a metal housing field with Transit oil. This types is used on high current capacities for voltage up to 7500 for protecting small Transformer banks and other station or distribution purposes.
  3. Fuse enclosed in a high strength glass tube filled with special arc- quenching liquid. This type is used on many classes of service. A type of liquid is used which contains carbon tea chloride and does not treez or congeal.
  4. An expulsion “deon” fuse containing boric acid which generates gas under the arc of the blowing of the fuse wire aiding in promptly deonizing the arc and opening the circuit. On the basis of 3-phase 6 cycle circuit the interrupting ratings are below.

Table 3.7 of interrupting of deion fuse

  1. Current limiting fuses are those which open with such speed that the fuse arc resistance is introduced to much less than that which would follow wire the fuse conductor intact. In general these fuses should not be applied to circuits the voltages of which are less than 70% of the fuse voltage ratings.

Typical of these fuses are the General Electric Type EJ.

Interrupting Rating of current limiting fuses

Table 3.8 Interruption Rating of current limiting fuses

3.9 Limiters

Limiters are time delay fusible connectors designed to be installed in low voltage net work mains cables at Street junction points. They are rated in cable sizes and have time current characteristics to

a) allow the cable fault to burn itself clear if it does so promptly, without blowing the limiter

b) blow before the cable insulation away from the fault is roasted and prevent the failure from spreading beyond the junction point, and

c) obtain adequate selectivity so that when installed in a network only those limiters connected to the faulted cable blow.



Disconnection switches are used primarily for the purpose of isolating equipment from buses or live apparatus and for sectionalizing buses or circuits, also for transfer, testing and grounding purposes. They are not intended to break load current except under special conditions and in limited amounts. High voltage outdoor disconnects are sometimes equipped with horn gaps but should not be used to break heavy charging currents until the conditions have been carefully analyzed and known to be satisfactory. If mounted vertically, they should be installed so that they open down. Except for very light duty, disconnects should be equipped with latches or other mechanical mechanisms to prevent their being blown open by the magnetic forces due to short circuit.

4.2 Designs of Disconnecting switches

Designs of disconnecting switches show considerable progress in development of improved types of contact to meet the demand of both indoor and outdoor service. Experience has shown that the ordinary flat blade type of disconnecting switch usually laminated for heavy currents is subject to losses in contact efficiency which eventually seriously impair its carrying capacity. The resulting heating accelerates the trouble, which becomes cumulative, contacts with high pressure features or provided with secondary motions for producing effective pressures have improved the reliability of operation and in creased the serviceability, the trend being toward designs with reduced contact areas but greatly increased contact pressures with adequate cross-sectional dimension and mass of material at contact points. In some of the modem designs the construction provides for only line or multipoint contact. At high pressures the surfaces arc cleaned of dirt, oxides, select etc. modem designs use-silver plated or silver inlaid contact surfaces quite generally as this material is not subject to increased contract resistance and heating due to oxidation.

Switches are available in many different mechanical designs. Some have been built to provide “Snap open” interrupting contacts, which operate after the main current carrying contacts are open. This provides some measures of small interrupting ability, which is useful in some instances. Another design of this general types but providing for the final interruption of the circuit in a specially suited and enclosed interrupting chamber, has made is appearance recently. This unit known, as an “interrupter switch” provides a material step in higher interrupting ability.

4.3 High pressure Contact

A high-pressure contact design for indoor service is illustrated shown s been adapted for switches rated at 132 KV. The switch illustrated is of 600-amp capacity and has 12 contact spots formed in the blades and raised above the surrounding surface.

4.4 High pressure contact design for outdoor service

A high pressure contact design for out door service embraces the feature of the switch blade revolving inside instead of moving parallel with the contact surfaces the blade enters the contact and is then turned when opening the circuit the blade is first turned, the pressure thus being relieved before the blade lifts or moves side wise. The rotation of the blade is accomplished by a yoke connected to a rotating or Oscillating insulator which produces a continuous quick motion with little effort. The total pressure on the contacts varies with the current rating of the switch the design providing V2 lb of pressure per ampere of capacity a 600 amp switch having a total pressure of 300 lb and a 2000 amp switch a total contact pressure of 1000 lb switches of this type have been designed for circuit 132 to 330KV.

4.5 Manual and motor operating mechanisms

Manual and motor operating mechanism are available which permit flexibility in the arrangement of disconnecting switches to suit the requirements of the electrical and structural layout most low voltage indoor and outdoor disconnecting switches of moderate current capacity are of the single pole, hook operated type, while a large portion of the higher voltage switches are of the gang operated manual type. Gang operated switches are often used on moderate voltage indoor station circuit, as the mechanism can



Earthing Switches

Fig. 4.7
Fig. 4.6

4.8 Technical Specification OF 145kv Disconnector/ Disconnecting Switch

Three-pole, outdoor, single central break with two insulators per pole, horizontal mounting, having characteristfr’ and dimensions as shown in our catalogue and in compliance with IEC specifications:

– Disconnector without grounding blade Type

Rated Voltage :145 kV

Rated Current :1250 A

BIL :650kV

Short time current : 31.5 kA x 1 sec.

Peak current : 80 kA

Insulators : according to fEC standard

– Disconnector with grounding blades type

Three-pole isolators as above described, but complete with 3-pole grounding blades, fixed on the same bases of the isolator and designed to withstand the same short circuit current as main blades.

4.9 Operating Mechanism (without grounding blades type)

For the motor operation of the disconnecting switch without grounding blades, complete of the following accessories:

– motor 11OV DC ÷ 10%, -15%;

– thermic and magnetic protection for the motor;

– control and auxiliary circuits for 11OV DC;

– 6 NO + 6 NC auxiliary contacts, cam type, rated current 1OA, breaking and making current 0.5A;

– LOCAL /REMOTE selector switch;

– OPEN/CLOSE control push-buttons;

– anti-condensation resistance 220V AC — 50Hz;

– padlocking facilities in the OPEN/CLOSED positions;

– provision for emergency manual operation complete with crank;

– Electromagnetic interlock to prevent from inserting crank into working position and interlock the manual operation with other devices where required (f.i. with circuit breaker).

– Manual operation is possible only when electromagnetic lock is energized;

– Electrical interlock to prevent the motor operation when the crank is inserted in the working position.

– Terminal blocks capable of accommodating up to 4 mm2 ferrule type terminals;

– Wiring is made of 1.5 mm2 fire resistant, not flammable conductors and only black colored;

– The above equipment with other normally furnished are enclosed in a control cubicle provided with lockable door, and having IP 55 protection degree. This cubicle with suitable dimensions is made of aluminum alloy.

4.10 Contact Systems

Commutating Contacts

Fig 4.11
Fig 4.12

4.13 Motor Operating Mechanisms

Fig 4.14
Fig 4.17
Fig 4.17
Fig 4.21
Fig 4.20


5.1 Metal Clad or metal enclosed switchgear

Metal clad or metal enclosed switch gear is a type of design in which the electrical equipment is located in an enclosing metal structure, generally factory assembled.

5.2 The term metal-clad switchgear

The term “metal-clad switchgear” indicates a type of design in which all these equipment required to control an individual circuit, including bus, circuit breaker, disconnector devices, current and potential transformers, controls, instruments and relays, is assembled in one metal cubicle and the circuit breaker is provided with means for ready removal from the cubicle. Circuit breakers can be of the oil or air type, although the trend is strongly to the use of air circuit breaker.

5.3 Circuit breaker disconnection

It is accomplished by “Vertical lift” and “Horizontal-draw out” designs. The over-all effect is the same in either case.

5.4 Interlocks

Interlocks are provided in metal-clad assemblies to prevent disconnecting or connecting the circuit breaker if it is closed and to protect personnel from coming in contact with the high voltage circuits when the circuit breaker is removed from the cubicle.

5.5 Metal-clad switchgear

It is used for medium and high capacity circuits, for indoor and outdoor installation at (69-132 KV) rating range capacity.

5.6 The term “metal enclosed switchgear”

It indicates a type of design in which the major component parts of a circuit, such as buses, circuit breakers, disconnecting switches, and current and potential transformers, are in separate metal housing and the circuit breakers are of the stationary type in the design.

5.7 Phase segregation

Phase segregation in metal-enclosed switch gear is a type of design in which a 3-phase metal housing is divided into three single-phase compartments by means of single metal barriers as shown in the figure.

Fig 5.8
Fig. 5.9

5.10 Isolated-phase metal enclosed switchgear

It is a type of design in which each phasc is enclosed in a separate metal housing and an air space is provided between the housings. It is conceded to be the most practical and economical way of preventing phase to phase shod circuits through construction methods in large and important plaifls.

5.11 Metal enclosed switchgear

It is used in indusial. commercial and utility Installations, generally on systems that would use swkchgcar rated from 69 KV to 132 KV. having interrupting duties varying from 1000.000 to 2, 50,0000 KV and up to 10,000,000KVA at 132 KV.


Porcelain Surge Arrester 3EP

6.1 Descriptions

3EP porcelain housed surge arresters is most essential, when it comes to reliability in medium and high voltage applications, since 1929 siemens have been manufacturing high voltage surge arresters with porcelain housing for standard specialized applications. The cost effectiveness of this products is under scored by unc