ACKNOWLEDGEMENT

 

 

 

It is a technical report of practical training at Rajasthan Rajya Vidhyut Prasaran Nigam LTD. It

was commenced on 15/6/2004 and completed on 15/7/2004. It was of 30 days and taken at 132

kv  G.S.S Jawahar Nagar, jaipur.

 

I feel immense pleasure in conveying my heartiest thanks and deep sense of gratitude to

Mr.Monil Mathur, Head of the Electrical Engineering Department of Kautilya Instiute Of

Technology And Engineering. for his efforts and for technical as well as moral support.

 

I feel indebted to express my heartiest thanks and gratitude to Mr.N.K.Jain.  (AEN),

Mr.M.K.Makheja(JEN) for their valuable time, learned guidance, kind, candid, wise and

illuminating advice during training period.

 

 I am also thankful to our instructors and other technical and non technical staff, for helping in

understanding the various aspects and constructional details of work and site in 132 kV G.S.S.

Jawahar Nagar, Jaipur.       

 

It may not be possible for me to acknowledge the contribution of all my friends, but I am thankful to all

those who came forward to help me. I express my sincere thanks to my colleagues and other trainees for

their valuable ideas and support during practical training.

 

 

                                                                        RAGINI AGARWAL

III B.E. (ELECTRICAL ENGINEERING)

 

 

 

 

DETAILS OF TECHNICAL

 

AND

 

OTHER OBSERVATIONS

 

MADE DURING TRAINING PERIOD

  

 

 

 

 

TRANSMISSION LINES

In this category the EHV lines viz. Extra high voltage lines of 400 kV, 220 kV, 132 kV and 66 kV

are considered/are used voltage from one grid sub-station to other sub-station through six various

types conductors.

THE CONDUCTORS USED FOR

(i)                               For 400 kV line   : Taran Tulla and Marculla conductor.

(ii)                             For 220KV line: Zebra conductor is used composite of Aluminium strands and steel

wires.

(iii)                            For 132 kV line:  Panther conductor is used composite of Aluminium strands and

steel wires.

The material used in these conductors is generally Aluminium conductor steel reinforced

(ACSR).  The conductors run over the tower cross arms of sufficient height with the consideration

to keep safe clearance of sagged conductor from ground level and from the objects (trees,

buildings etc.) either side also.

The work involvement in laying EHT (Extra High-Tension) lines are:

The survey of the route is done keeping in view the shortest route as may be economical and

feasible too per load necessity.  The shortest route between the two sub-stations on which

transmission line route is surveyed keeping in view to that survey of the route is done as may be

economical and feasible too per load necessity is called BEE LINE.  Care is taken for legal

matters as may be due to near by habitants.  When route is finalised and approved by the

authorities on the profiles plotted according to the data obtained at site by use of the theodo lite,

dumpy level, staff level & Measuring Chain/tape.

The steps followed in laying out the transmission lines are as follows:

I.                     Selection of Towers: Foundations is selected as per the requirement of the towers to

be used.

1.                                           The 'A' type for straight conductors called as SUSPENSION TOWERS can be

used also for 2o maximum deviation.

2.                   The 'B' type angle towers of (O o  - 15 o ) for turning/deviation points.

3.                               The 'C' type angle towers (O o - 30 o ) for turning/deviation points.

4.                   The 'D' type angle towers (O o - 60 o) for turning/deviation points and Dead/cut ends. 

These may be for double circuit also with six cross arms.  Other than these some time extension

part of the main height are used for 3m or 6m as may require/to maintain clearance from the

raised ground or building or railway crossing and roads.  The foundation are casted (C.C) to set

the base leg called stubs (ground part block) aligned to the proper slope.  The TEMPLATE is

used which is an arrangement of setting of the stubs resting on jecks before concereting.

II.                   Erection of Towers: As per practice the tower members are jointed in piece meal

method by use of derric pole (wooden or hollow pipe) and tightened by the galvanized nuts and

bolts of various size of 16 mm x 25 mm to 16 mm x 55mm and cross arm and lifted upward to fix.

III.                              Stringing of Conductor: It consists of many procedures:

(a)                                         Paving out of conductors above the towers is done by tractor/winch machine

manually by turning the conductor drum for unwind on turn table and earthwire is also paved out

in same manner.

(b)                                         Suspensions of disc insulators are made to the 'X' arms of towers in suitable

quantity.  The quantity of disc insulators used generally in the suspension of conductors in 132 kV

towers is 9 in numbers and in 220 kV towers is 12 - 14 discs insulators in numbers.

(c)                                         After paving out the conductor one by one is lifted to the height of the cross

arm for hanging in the disc and thereafter these are given tension pulling through the

tractor/winch machine and is finally sagged as per span finally in profile.  The sag between towers

is different in different span.  Thus for double circuit six conductors are drawn and for single

circuit three conductors are used.

(d)                                         The earth wire is also given tension for final sag running on top of the towers

for protections from the line lightening hazards/strokes in rainy season.

(e)                                         The earthing of each tower is done and by connecting the leg to the earthed

pipe by means of steel leads for protection in case of faults.

(f)                                           Finally all trees falling in the danger zone of the line either side is cleared for

10 meters from the center to avoid earth fault and falling on t he conductor.

(g)                                         Main consideration is to maintain the proper clearance for post & telegraphs

lines by seeking permission from the PTCC wing meant only for approval.

(h)                                         Similarly the railway authorities are also involved for giving clearance of P & T

line and Railway Track Clearances

(i)                                           After fulfilling all conditions the testing of the line by use of meggarring is done

to check continuity or obstacles if any.

(j)                                           The vibration dampers are provided at the tension points as well as to

armoring points of suspension clamps at specified distances to suppress the wave due to wind to

save twisting of the conductor so that may not damage due to twist.

 

IV         Total No. Of the Towers are:

(a)                                         Transmission Towers

(b)                                         Dead end towers

(c)                                         Tangent towers

(d)                                         Angle towers

(e)                                         Extension towers

(f)                                           Spiral towers

(g)                                         Section towers

(h)                                         Narrow base towers (used in 33 KV lines)

(i)                                           Board base towers.

TRANSFORMERS

 

Introduction

A transformer is a static (or stationary) piece of apparatus by means of which electric power of

one circuit is transformed into electric power of same frequency in another circuit.

In brief, a transformer is a device that:

1.                   Transfer electric power from one circuit to another

2.                   It does so without change of frequency

3.                   It accomplishes this by electromagnetic induction

4.                   Where the two electric circuits are in mutual inductive influence of each other

A high voltage is desirable for transmitting large powers in order to decrease the IR losses and

reduce the amount of conductor material.  A very much lower voltage, on the other hand, is

required for distribution, for various reasons connected with safety and convenience.  The

transformers make this easily and economically possible.

 

POWER TRANSFORMER

The x- mer is oil immersed with triple rating of 100 MVA auto under ON (natural cooling) (oil

immersed with natural air-cooling) or (oil immersed with forced oil cooling). The tertiary is suitable

for 11MVA continuos synchronous condenser loading. When the tertiary is load the secondary

load should be limited, such that no industrial individual winding is over loaded and are also that

total losses are not exceeds.

However a simultaneous loading of 100MVA at 0.8p.f. (Lag) on the low voltage and 65MVA at

0.8p.f. (Lag) on the territory is fissible the tertiary can also be loaded to 20MVA 0.8p.f.on low

voltage without exceeding temperature rise. The x-mer is suitable for simultaneous parallel

operation.

It is ensured that the tertiary winding will also operate satisfactory with each other. The x-mer is

provided with separate tank of radiators, fans, pumps and associated control equipment. The

control equipment is housed in a tank mounted marshalling commercial. It is provided with on

load tap charge. 

The magnetic circuit is a 3 limbed core type construction each being inter leaved with joints with

top and bottom yokes.  The winding surrounded the three limbs the conditions are made from

high grade caused "cold ratted grain oriented" silicon alloy steel.  The Insulation on laminations is

of varnish.  This has stepped sections with legs and yokes having 100% area cooling ducts are

provided parallel to the plane of laminations.  The yoke laminations are clamped with clamp

plates are legs by means of clamping-cum-sling plates.  They are clamped with bolts.

For lifting the core with winding high lumbers of lifting bolts are provided.  The core clamp plates

are insulated from each other to with stand plates, a test pressure of 2 kV, 50Hz, A.C. for one

minutes.

The innermost coil i.e. nearest to the core transformer is the testing winding.   This is wounded

helical on a Bakelite cylinder with radial coil ducts for cooling purpose. Axial oil ducts are provided

inside and out side the coil; means to tertiary winding is wound the common winding. This coil is

wound as a no. of continuous discs with radial oil ducts in between discs. Thus statics shields

rings are provided one at the top and other at the bottom to control the electrostatic stress

distribution in the winding

Just outside the common winding, the tapping coil is placed.  The tapping coil is wound as an

inter-wound spiral coil on a Bakelite cylinder.  As the tapping arrangement is of the reversing type

the number of sections in the coil is half the number of tapping steps. The required number of

parallel conductors are wound in two parallel conductors respectively one together outside the

coil the outer most coil forms the series winding, the series winding is shielded layer type winding

consists of five layer each wound spiral.  The winding is placed in between two shields.

A X - mer double wound or auto wound has minimum of two voltage, one corresponding to the

supply and second to the load side.  Many time a third winding is introduced in primary and

secondary, winding requires it because another voltage may be required at the place of supply to

load.  In either core the third winding is connected to delta formation and is generally known as

"tertiary" winding in the case of star/star methods of connection three phase shell type X mer is

used  which causes a serious problem of the third harmonic components of the magnetic

currents.  The tertiary delta provides a short circuit for flow of the third harmonic currents

therefore eliminating third harmonic multiple connection are provided on primary and secondary

winding.

The neutral point of such winding is therefore stable and can earthen without any effect to the X-

mer on the system

The territory winding helps:

(1)                 To reduce the unbalancing in the phase of the primary side due to unbalanced three

phase loads;

(2)                 To redistribute the flow of faults currents;

(3)                 To supply an auxiliary load in addition to the main load.  This could be consists of the

power factor improvement synchronous condensers or shunt capacitors.  In such a case the

purchaser of a power X mer should always specify the voltage and power ratings of the tertiary

winding;

(4)                 As compared with star/delta connection of fault current in the event of secondary form

line to neutral tertiary delta consists in the laminations.  This of course further depends upon the

impedance between tertiary and main winding.

(5)                 The X mer having mixed cooling OFB(forced oil air blast cooling) (100 MVA) OB(air

blast) (70 MVA) and ON(natural cooling) (50 MVA) is provided with separately mounted banked

radiators.  There are eight radiators of elements 2920 mm. Long.  The radiators are mounted on

the top and bottom headers, which are supported by facilitated frames.  Each radiator is

connected at the top and bottom with respective header through out let and inlet valves.  The top

and bottom headers are connected to the tank by 200 mm diameter pipes and valves one each

radiators as well as headers.

A Inland propeller type of air pump is connected in the bottom pipes, which circulate air through

the tank and radiators.  A motor is full of the X mer oil, which serves as a cooling medium.  A

window nut is provided on the body of the motor.  The coolers are provided with two 915 mm

(36") dia. (weather proof), each blowing with 440 cubic meters of air minute on the radiators

element directed in such as way that the full length of the element is converted by the blast.

There are two type of control

(1)                 Hand Control and

(2)                 Automatic Control

Hand Control

For hand control selector switch is set to hard position whereby individual motor can be started by

its own starter push buttons already in the event all 'starts' push button are kept in the depressant

position, the cooler bank can be group started.

Automatic Control

The start push button of the starters is kept locked in depresses position and switch 4313 kept in

Auto position.   fans motor conductors are energized by the closing of mercury switch contact

49A-1, 49B-1 or 49C-1.  For winding temperature indicator 49A, 49B, 49C respectively with a

further increase in temperature either of contact 49A=2, 49B = 2  or 49C =2 of winding

temperature indicates closed to start the motor.

The bushings are one per phase porcelain steam type of 12 kV class.  The neutral bushings are

oil filled porcelain steam type of 36 kV class.  The LV and 245 kV class respectively oil filled type

condenser bushings.  The active part of the bushings consists of core made of wound synthetic

resin bounded paper (5 RBP) with condenser layers (Aluminum foil) suitable inter posed to field

control.

The control metal tubes are serves as support for the paper wound core during the winding

process.  The core adhere to the metal tube. The oil being absolutely oil tighten.  The HV, LV

bushings are mounted on the delta detachable adapters, while the neutral bushings are mounted

directly on the cover.

Its temperature operates on the principle of liquid expansion (mercury is in steel).  The

temperature indicator is provided with a max-pointer and two mercury switches.  The mercury

switches are adjustable to make contacts between 50oC and have a fixed difference of 10oC.

This operates on the principle of liquid expansion (mercury in steel) provides in deal local

indication at the marshalling box.  Half of the sheet temperature irrespective of the suitable

conditions, the thermometer bulb is connected by capillary tube to the local indicator.

The marshalling box is a weather proof steel box mounted on the side of X mer tank, the

marshaling box is provided with heater for the prevention of moisture condensation, besides this

the inside of the box is provided with anti condensation point.

 

The following equipment are mounted inside the marshalling box.

S.No.                Name of equipment

1.         Temperature indicator

2.         Auxiliary gear for tap changer control

3.         Cooler Control gear

4.         Heater Switches, illumination lamps

The X mer is in the yard has many tapping on ways every one about. 17 tapping. When the load

on X-mer increase due to regulation of the voltage 90 down to increase the voltage on the

secondary side by the changing their tapping to higher position.  For changing tap simply we have

to close the supply and taken change the tap by mechanically means.   In Heerapura G.S.S.

there is on load taps changer and it is totally remote control.  There are four panels at control

room for X mer and by pushing button, once we increase or decrease the tapping we can see the

number of trip at the panels, all four X mer must have the same number of tapping.

The on load tap changer design is a part of X-mer unit winding (e.g. as line and connection &

witch) .The on load tap changer consists load diverter and a selected switch, the desired winding

is first selects currents legs by a slow switching selector switch then follow the charge over by

means of load diverter connection, the neutral point has even and odd number selector and

contact plates alternatively. This adjustment scheme shows schematically design and mode of

operation of the training centre.

AUTO TRANSFORMERS

Basically auto-transformer comprises of only one winding per phase, part of which is used by

both primary and secondary winding.  This arrangement results into an appreciable saving in cost

as well as higher operating efficiency is achieved, but their extensive use is not being favoured by

power utilities due to certain inherent dis-advantages which are as follows:

1.                   It has got low inherent reactance as such is subjected to severe short circuit conditions.

2.                   Since primary and secondary side uses same windings, there is always possibility of

imposition of higher voltage on secondary in case of fault.

3.                   Both the windings make use of common neutral, as such neutral is required to be

earthed or isolated on both sides.

4.                   Provision of additional insulation on secondary side and increased frame size when

adjustable taps are provided erodes the initial advantage of low cost.

Power transformer are installed with various fittings and devices which are necessary for their

proper functioning.

CONSTRUCTIONAL PART

The chief elements of the construction are :

1.                Magnetic Circuits :  Comprising limbs, yokes and clamping structures.

2.                Electrical Circuits: The primary, secondary and (if any) tertiary windings, formers,

insulation and bracing devices.

3.                Terminals : Tappings, tapping switches, terminal insulators and leads

4.                Tank : oil, cooling devices, conservators, dryers and auxiliary apparatus.

CORE CONSTRUCTION

Special alloy steel of high resistance and low hysteresis loss is used almost exclusively in

transformer's cores.  Induction densities up to 1.35 - 1.55 wb/m2    are possible.  The limit for 50

c/s is being the loss and the magnetizing current.

As the flux in the cores is a pulsating one, the magnetic circuit must be laminated and the

separate laminations insulated in order to retain the advantages of subdivision.  Paper, Japan,

Varnish, China clay or phosphate may be used.

Burring of edges of plates may cause a considerable increase in a core loss by providing paths

for eddy currents should the sharp edges cut through the insulation and establish contacts

between adjacent plates.  Burrs are removed before core assembly.  Silicon alloy steel are hard,

and cause wearing of the punching tools, so that the removal of burrs needs special attention.

Transformer shut sheet are cut as far as possible along the grain which is in the direction in which

the material has a higher permeability.

CONSTRUCTIONAL FRAME WORK

Considerable use is made of channel and angle section rolled steel in the framework of core type

transformers.  a typical construction is to clamp the top and bottom yokes between channel

sections, held firmly by tie-holts.  The bottom pair of channels has cross channels as feet.  The

upper pair carries clamps for the high and low voltage connections.

Windings : Classification of windings maybe done as (a) Circular or rectangular & (b) Concentric

or sandwiched

In core type circular or rectangular type of windings are used and in shell type generally

sandwiched type windings are used.

On account of easier insulation facilities, the low voltage winding is placed nearer to the core.  In

the case of core type and on the outside positions in the case of shell type transformers.  The

insulation spaces between low and high voltage coils also serve to facilitate cooling.

INSULATION

The insulation between the H.V. and I.V. windings, and between I.V. winding and core,

compresses Bakelite paper cylinders or elephantine wrap.

The insulation of the conductors may be of paper, cotton or glass tape being used for air

insulated transformers.  The paper is wrapped round the conductor in a suitable machine,

preferably without overlap of adjacent turns.  In the power transformers, owing to strain on the

insulation between turns t the line end of the high voltage winding, about 5 percent of the turns

are reinforced with the extra insulating material.

LEADS AND MATERIAL

The connections to the windings are copper rods or bars, insulated wholly or in part, and taken to

the bus bars directly in the case of oil cooled transformers.  The shape and size of the conductors

are of importance in very high voltage systems, not on account of the current carrying capacity,

but because of dielectric stresses, corona, etc. at sharp bends corners with such voltages.

BUSHINGS

Up to voltages of 33 kV, ordinary porcelain insulators can be used. Above this voltage the use of

conductor and of oil filled terminal bushings, or, for certain cases, a combination of the two, has to

be considered.  Of course, any conductor can be effectively insulated by air provided that it is at a

sufficient distance from other conducting bodies and sufficiently proportioned to prevent corona

phenomena.  Such conditions are naturally UN-obtainable with transformers where the conductor

has to be taken through the cover of the containing tank, although common enough with over

head transmission lines.

The oil filled bushing consists of a hollow porcelain cylinder of special shape with a conductor

(usually a hollow tube) through its centre.

The space between the conductor and the porcelain is filled with oil, the dielectric strength of

which is greater than that of air.  The dielectric field strength is greatest at the surface of the

conductor, and this breaks down at a much lower voltage in air than in oil. Oil is fed into the

bushing at the top, act as an expansion chamber for the oil when the bushing temperature rises. 

Under the influence of the electric field, foreign substances in the form of dust, moisture or

metallic particles have a tendency to arrange themselves in radial lines giving rise to paths of low

dielectric strength, with constant danger of breakdown.  To prevent such action by unavoidable

impurities in the oil Bakelite tubes are used to surround the conductor concentrically.  The effect

is to break up radial chains of semi-conducting particles.

The capacitor type bushing is constructed of thick layers of Bakelite paper alternating with thin

graded layers of tin foil.  The result is a series of capacitors formed by the conductors and the first

tin foil layer, the first and second tin foil layers. …. an so on.

The capacitance of the capacitors is controlled by their length and the radial separation of their tin

foil plates.

The oil immersed ends of HV bushing maybe reentrant form, reducing the immersed length and

permitting as more uniform distribution of the axial and radial electric stress components.

TANKS

Small tanks are constructed from welded sheet steel, and larger ones from plain boilerplates. 

The lids may be cast iron, or waterproof gasket being used at the joints.  The fittings include

thermometer pockets, drain cock, rollers or wheels for moving the transformer into position, eye

bolts for lifting, conservators and breathers, cooling tubes are welded in, but separate radiators

are individually welded and afterwards bolted on.

CONSERVATOR

Conservator are required to take up the expansion and contraction of the oil to come in contact

with the air, from which it is liable to take up moisture.  The conservator may consist of an airtight

cylindrical metal drum supported on the transformer lid or on a neighboring wall, or of a flexible

flat corrugated disc drum.  The tank is filled when cold and the expansion is taken up in the

conservator.

TRANSFORMER OIL

Oil in transformer construction serves the double purpose of cooling and insulating. In the choice

of oil for transformer use the following characteristics have to be considered.

Viscosity, insulating property, flash point, fire point, purity, slugging, audity.

SYNTHETIC TRANSFORMER OIL

These have been developed to avoid the risk of fire and explosion, present always with normal

mineral oils.  Chlorinated diphenyl, a synthetic oil suitable for transformer is chemically stable,

non-oxidizing, rather volatile, and heavier than water.  Its dielectric strength is higher than that of

mineral oil, and moisture has a smaller tendency to migrate through it.  The permittivity is 4.5,

compared with about 2.5.

TRANSFORMERS: OPERATION

NOISE: Under 'No load condition' the 'hum' developed by energized power transformers

originates in the core, where the laminations tend to vibrate by magnetic forces.  The essential

factors in noise production are consequently:

(a)                 Magnectostriction i.e. the very small extension, with corresponding reduction of cross

section, of sheet steel strips when magnetized.

(b)                 The degree of mechanical vibration developed by the laminations, depending upon the

tightness of clamping, size, gauge, associated structural parts etc.

(c)                 The mechanical vibration of the tank walls.

(d)                 The damping

The total noise immersion may be reduced by:

(a)                 Preventing core plate vibration, which necessities the use of lower flux density and

attention and attention to constructional features such as clamping bolts, proportions and

dimension of the 'steps' in plate width, tightness of clamping and uniformity of plates.

(b)                 Sound insulating the transformers from the tank by cushions, padding, or oil barriers.

(c)                 Preventing vibrations of the tank walls by suitable design of tank and stiffness.

(d)                 Sound insulating the tank from the ground or surrounding air

 

 

TERRITARY WINDING

This core used on the main transformer instead of a separate regulator unit.  The cost of tap

changing unit lies in:

(1)                 The mechanical arrangements for effecting the change

(2)                 The relay devices to prevent incorrect operation or to ensure that transformers in

parallel are all operated together.

(3)                 The electrical operation of the gear.

(4)                 The additional insulation required for tappings, switches, etc.

The booster transformer system is the most expensive, but has the considerable advantage of

employing a standard main transformer, and tap devices dealing only with a small power at low

voltage.

 

TERRITARY WINDING

Transformers may be constructed with territory windings (i.e. windings additional to the normal

primary and secondary) for any of the following reasons:

(a)                 For an additional load which for some reason must be kept insulated from that of the

secondary.

(b)                 To supply phase compensating devices, such as condensers, operated t some voltage

not equal to that of the primary or secondary or with some different connection (e.g. mesh)

(c)                 In star/star-connected transformers, to allow sufficient earth fault current to flow for

operation of protective gear, to suppress harmonic voltages, and to limit voltage unbalance when

the main load is asymmetrical in each case the territory winding is delta connected.

(d)                 As a voltage coil in a testing transformer.

(e)                 To load large split-winding generators

(f)                   To interconnect three supply systems operating at different voltages.

Territory windings are frequently delta-connected consequently, when faults and short circuits

occur on the primary or secondary sides (particularly between lines and earth), considerable

imbalance of phase voltage may be produced, compensated by large territory circulating current. 

The reactance of the winding must be such as to limit the circulating current to that which can be

carried by the copper, otherwise the territory winding may overtreat under fault conditions.

 

PROTECTIVE DEVICES AND THEIR MAINTANENCE

 

 

DE-HYDRATING BREATHING

 

Presence of moisture in oil results into reduction of its di electric strength and may cause failure

of transformer. As such it is necessary to ensure that oil remains clean and dry. Conservator tank

is mounted on the top of the transformer and is connected to main tank by means of a small

diameter pipe. Interior of the conservator tank is connected to a silica gel breather by a pipe.

Silica gel breather absorbs the moisture when the transformer breathes due to expansion and

contraction of the oil on account of changes in temperature thereby ingress of moisture into the

main body of the oil is avoided.

Silica gel breathers are usually dispatched in Position during transit. But in some cases they may

be dispatched separately for transport purposes. In such cases breathers should be fitted as soon

as possible. While fixing the breather make sure that all joints are airtight. Silica gel is blue in

colour when dry with absorption of moisture, its colour changes to violet and then pink. pink

colour indicates that silica gel is saturated and is ineffective. It should then be either replaced or

re activated.

While charging the breather following steps must be considered:-

Remove the wing nuts supporting the body.

Glass container should be squarely fitted on its gasket, then pour reactivated silica gel into the

container up to a level ¼ inch from top.

Fix the assembly to the top plate with inspection window facing out ward from the transformer

and secure it with the wing nuts. Ensure that top gasket is in Position.

Transformer oil should be poured into the oil cup until it overflows through the screw hole and

fixes it to the assembly with the nut.

Applying heat to it in oven until its colour is restored to blue reactivates silica gel.

 

BUCHHOLZ RELAY

 Oil immersed power transformer are fitted with gas actuated relay known as buchholz relay. It

compresses of oil tight cast iron housing and two operating elements. The top operating element

consists of alarm system on gas collection while the bottom-operating element trips the

controlling circuit breaker in case of oil surge. Each element is made up of an aluminum bucket

pivoted on bearings on a fixed stainless steel shaft. Mercury switches are clamped to these

elements control wires is taken to the terminal block. Pet cock and drain plugs are provided at top

and bottom of relay. An inspection window, with graduated scale is provided for viewing the level

of oil.

In case of minor faults, small bubbles of gas while going towards conservator get accumulated in

the relay housing and lower down the level of oil. Top operating element gets tilted and mercury

switch gets closed thereby completing the alarm circuit. In case of major faults due to violent

generation of the gas oil surges towards conservator. Oil surge tilts the bottom element that close

the mercury switch and necessary command is given for tripping the controlling circuit breaker. In

case the level of oil drops below the level of the relay due to heavy leakage, alarm and trip circuit

gets energized and transformer is isolated from the source of power.

Buchholz relay help in detecting various faults as: -

1.                Core lamination short circuit.

2.                Overheating of the windings.

3.                Arcing due to bad conductors.

4.                Earth faults.

5.                Short circuit of winding.

6.                Puncture of bushing insulators inside tank.

 

 

 

PRESSURE RELIEF VALVE/EXPLOSION VENT

In case of small transformers when they are not provided with conservator tanks, relief valves are

incorporated at the top for the release of the gas, which may be generated on account of

overheating, arcing or short circuit under the oil.  Transformers of medium or large capacities are

fitted with explosion vent.  Explosion vent is fitted with a diaphragm of mica, Bakelite or glass. 

When heavy internal fault occurs gases is generated by the transformer in large volume.  In case

diaphragm bursts with the force of the pressure generated by these gases, operating personnel

should immediately isolate the transformer and summon the maintenance crew for investigating

the cause.  Once the defect has been located remedial measures should be taken, and after its

removal, diaphragm should be replaced by new one and transformer may be energized. 

Transformers protected by buchholz relay are provided with a paper diaphragm at the bottom of

the explosion vent which serves as a deflector to ensure that the gases generated in the

transformer pass through buchholz relay instead of collecting in explosion vent.  The diaphragm

as provided in such cases is set to blow at a pressure of 5 pounds per square inch.

FAULT AND PROTECTION

A power transformer is subjected to the following fault:

1.                   Overload and external short circuits

2.                   Terminal Faults

3.                   Windings faults

4.                   Incipient fault 

All four conditions produce mechanical and thermal stress with the transformer windings overload

can be sustained for long periods being limited only by the permissible temperature rise in the

transformer windings and oil excessive overloading results in deterioration of insulating and

subsequent failure. It is used to provide winding and oil temperature indicators with alarm and trip

contacts.  As soon as the temperature of winding and/or oil exceeds the predetermined values of

the contacts and bridges completing the circuit of an alaram bell and if the temperature reached

the hot spot value the transformer is tripped.  The external faults subject the transformer to

electromagnetic stress and over heating.

It is therefore, desirable to limit the duration of such fault currents.  Terminal faults on the feeding

side of the transformer have no adverse effect on the transformer but those on the load side do

have such fault can not operate the gas and oil pressure relay more commonly known as

"Buchholz relay".  These no fall in the protective zone of the restricted earth fault or differential

relays.  The majority of internal fault is either earth fault or inters turn faults.  The severity of faults

depends on the location of fault transformer design and methods of system earthing, phase faults

within a transformer are rare.

Incipient faults are internal faults, which constitutes no immediate hazard.  However, if they are

left undetected.  They may develop into a major fault.  The main fault in this group are core faults

due to isolation failure between the core lamination and oil failure due to either loss of oil or

obstruction in the circulation due of the oil.  In that case overheating will occur the protective

system applied to a power transformer are as follows:

1.         Gas and oil pressure relay (Buchholz relay)         

2.                   over current and earth fault

3.                   differential

4.                   frame leakage

Simple over current earth fault protection is applied against external short circuits and excessive

over loads.  These over current relays and earth faults relays were inverse and definite minimum

time types.

The over current relay fails to distinguish between conditions of external short circuits over load or

internal faults of the transformer.

The operation is governed primarily by currents and time ratings and the characteristics curve of

the relay.  To permit the use of overload capacity of the transformer and co-ordination with either

similar relay at about 125 to 150 percent of the full load currents of the transformer but below the

minimum secondary current.

The simple over current relays therefore seldom serves the function of primary and relays

protection of the power transformers.  They serve only as the back up protection both for the

transformer, internal and external faults.  It is a common practise to provide the over current and

earth fault protection on the infeed side of the transformer and is made to trip both H.V. and L.V.

breakers over current and earth faults relays are provided on the other windings which are not

connected to any power sources but to serve primarily as back up protector to the circuit of load

of the respective voltage.  In the latter case no interruption of primary side breakers is provided

with the over current relays on the secondary side.

The over current relays have three elements one for each phase and earth fault has a single

element. The normal large of current setting available on IDMT over current relays is 50-200%

and on earth fault element 20 to 80 per cent on the latter another range 15 to 40 per cent is also

available and may be selected where the earth fault current is restricted. Due to insertion of

impedance in the neutral grounding. In the case of transformer winding with neutral earthen

unrestricted earth fault protection is obtained by connecting an ordinary earth fault relay areas a

neutral current transformer.

The unrestricted over current and earth fault relays should have proper time lag to coordinate with

the protective relays of other circuit to avoid indiscriminate tripping.

It is a common practice to provide differential relays on all transformers above 5 to 10 MVA.  The

advantages over other scheme of protection are -

(i)                               The buchholz can defect faults caused under oil only.  The flash over  the bushings

are not adequately covered by other protective scheme defects such faults and also on the feeds

between the current transformer and power transformer provided the current transformer are

separately mounted and not in the transformer bushings.  In case of very serve internal faults

differential relays operate faster than the buchholz relay they control the external damages.

(ii)                             The differential protection responds to phase to phase faults and the protective

zone. This generally comprises all equipment and connection between the current transformers

on all sides of the transformer.

(iii)                            A transformer differential relay operates on circulation current principle, the currents

in various branches windings of transformer is through the media of current transformers.  The

ratio and connections of various winding of all the secondary to earth through the tank and signal

earth connection thus energizing the current transformer and operating the relay.

This protective scheme is externally sensitive in detecting earth fault in the transformer zone, the

application is independent of normal load through fault conditions and to ratio variations due to

two transformer tapping it is possible to set the relay at very low value of fault current an order to

obtain successful results.

It is essential that the flow of fault current is correspondingly (that the flow of fault current) to the

path of transformer earth connection.  Precaution should be taken to ensure that there is no

possibility of a portion of the fault current finding its way to the transformer frame circuit of the

other adjustment transformer filled with the same type of protection else false tripping of healthy

transformer might occur.

The local distribution systems are of shorter length and operate on comparatively lower voltage. 

The transmission lines operate on considerably longer distance and at much higher voltages

more over the over head transmission lines or network or susceptible to more frequent and

numerous fault conditions.  The requirements of the protective gear equipment are different in the

two cases particularly in the high speed and unit type protection.

The relay used for feeder protection are commonly time graded over current and earth fault

instantaneous directional over current and earth fault in input ends only or instantaneous non-

directional.

Instantaneous pilot wire protection with time graded over current and earth fault acts as back up

at input .

The time graded over current relay is simple and versatile in application.  However, to achieve

proper selectivity the operating times have to be increased to each step.  To minimise the

damage due to fault and improve systems, stability faster schemes are necessary ,the pilot  can

be set independently to operate at high speeds such schemes often need to be start by protection

which is provided by time graded over current and earth fault relays.

On 66 kv and upward the specifically developed protective schemes are often economically

justified and are recommended.  The schemes may be classified as :

1.                   Distance relaying

2.                   Pilot relaying

3.                   Differential relaying.

INSTRUMENT - TRANSFORMER

The transformer are used in a.c system for the measurement of current , voltage, power and

energy, the actual measurements being done by measuring instruments. Transformer used in

conjucation with measuring instrument for measurement purposes are called as "instrument

transformer". The transformer used for the measurement of current is called "current

transformer". Transformer used for voltage measurement are called as "voltage transformer" or"

potential transformer"

Current and voltage transformer insulate the secondary (Relay, instrument and meter) circuits

from the primary (power) circuit and proceed qualities in the secondary winding which are

proportional to those in primary.  The role of a transformer in protective relays is not a readily

defined as that for metering and instrumentation whereas the essential role of a measuring

transformer is to deliver on its secondary a quantity accurately representative or that which is

applied to the primary side, a protective transformer varies in its role accordingly to the type of

protective gear it serves.

Failure of a protective system to perform its function currently is often due to incorrect application

of transformers. Hence current and voltage x-mer must be regarded as constituting part of the

protective system and carefully noticed with the relays to fulfill the essential requirements of the

systems.  There is no great distinction between a protective voltage transformer and a measuring

voltage transformer. The difference only being nature of the volt transformer. Quite often the

same transformer can serve between purpose and provided. The protective voltage transformers

reasonably, accurately its duty will have been fulfilled.

 

Voltage transformers which step-down systems voltage to sufficiently low values are necessary

on every system for:

1.                   Induction of the voltage conditions

2.                   Metering of the supply (or exchange of energy)

3.                   Relaying and

4.                   Synchronizing

On account of cost and voltage the indicating instruments meters and relays are designed for the

voltage as obtainable from the secondary sides of the voltage transformer.  The calibration of the

indicating instruction and meters is however done accordingly to the primary voltage of the V.Ts.

The voltage transformers are classified as under:

(a)                 Capacitive voltage transformer or Capacitive type

(b)                 Magnetic type

Capacitive voltage transformer is being used more and more for voltage measurement in high

voltage transmission network, particularly for systems voltage of 132 and above where it become

increasingly more economical.  It enables measurement of the line to earth voltage to be made

with simultaneous provision for carrier frequency coupling which has reached wide application in

modern high voltage net work for tale-metering remote control and telephone communication

purposes.  The capacitance type voltage transformers are of two types.

1.                   Coupling capacitor type; and

2.                   Pushing type.

Fig. On proceeding page shows a line diagram of coupling capacitor type voltage transformer. 

The capacitor C1+C2 are made of oil impregnated paper and Aluminum foil.  Each capacitor is

composed of a multiage of element provided with special contacts for series connection and

assembled in such a way that the capacitor inductance remains low.  A tap is taken in between to

contact the magnetic voltage transformer across the capacitor and earth.  This point is fixed in

consideration of the system voltage between line and earth, this is total capacitance of the

coupling capacitor and the primary voltage of the magnetic voltage transformer.  It is a usual

practice to diagram the magnetic transformer for a standard primary voltage of 5, 10, 15 or 20 kV

depending on the requirement of burden and accuracy special circuit (auxiliary) element are:

1.                   Comparating inductance coil,

2.                   Damping impedance,

3.                   Resistor (R)

4.                   Spark gap (F)

The compensating Inductance Coil in series with the primary or the intermediate transformer

compensates the voltage increase on capacitive voltage divider. The damping in the secondary

circuit avoids the Ferro resonance.  The resistor and spark gap provide necessary protection.

The condenser type bushings are primarily rolls of varnished, impregnated paper and laid under

heat and pressure with metal sheet lain in between the paper layers.  The sheet may consist of

the aluminum foil or a coating of graphite.  The voltage distributors between the various layers is

properly designed and predetermined.

A tapping across these can be proper calibration give a replica of the supply voltage.  The low

capacitance imposes restriction on the supply voltage.  The out put power of such capacitor

voltage x-mers and therefore limits the application to synchronizing and voltage indications.  The

following table shows the maximum output for various systems voltage that is obtainable with

typical bushings.

Line to line kV Output power (in watt)

66 kV 5 Watt

110 kV 12 Watt

132 kV 15 Watt

220 kV 25 Watt

Since, however, in a sub-station there are other requirements which needs a greater burden. 

These types of current transformer are not very commonly used.

These magnetic type transformer work on the same principle as the power transformer.  The

design is, however, different because of different requirement of two used cases.  The load to be

transmitted through a voltage transformer is quite limited depending on the purpose for which this

is to be used.  This is generally limited to a few hundred VA at the most.  The main object in the

design of a voltage transformer is to minimize possible errors in measurement made with its

help.  These errors are due to:

1.                   Voltage drop in the primary winding caused by exciting current and

2.                   Voltage drop in both the winding caused by the load current.

The former accounts for the errors at zero burden and the latter for the stop of the ratio and

phase angle curves.  Since the load current is fixed for a given burden the drop which it cause

can be reduced only by reducing the resistance and reactance of the transformer. This is done by

using relatively few turns and a large cross sectional of both iron and copper.  The low primary

impedance thus brought out, causes a small no load drop that may be still further reduced by

running the iron at relatively low induction.

In the lower voltage drop of voltage transformer the active part is contain in a steel housing and

the primary terminal is brought out through a bushing on higher systems voltage above 66 kV.  It

is generally practiced to contain the active part in porcelain housing.  Whether the voltage

transformer is contained in a steel tank or in a porcelain housing, the secondary terminals are

made and brought out in the steel housing provides as a base in the case of voltage transformer

housed in porcelain bushing.

Another significant distinction between the single-phase voltage transformer and single-phase

power transformer is that in the case of former only one terminal of the primary winding is brought

out for connection to the tank.  The secondary terminal is earthen alone the core and the steel

housing.

There are many factors to be considered before the choice between magnetic type voltage

transformer and capacitive type V.T.  can be made.

The important amongst these are

(i)                               Purpose

(ii)                             Layout

(iii)                            Price

We should first know the purpose for which voltage transformers are needed.  In case we need

voltage supply for voltmeter, synchronizing, energy meters, distance relays (without carrier)

magnetic type voltage transformer done case serve the purpose.  It is required to adopt carrier

protection.  It is that coupling capacitors are used on each phase along with the voltage

transformers. In such a way we can use either capacitive V.T. with coupling capacitor. If only

carrier communication is required the purpose can be served with only one capacitor coupling per

circuit and magnetic voltage transformer or capacitive voltage transformer only.

Only in 132 kV line's the desirability of providing career protection has to be checked.  It may or

may not be necessary generally, if the highest system voltage connecting the various

powerhouses is then greater than 132 kV.  The requirement of one coupling capacitor or three

with the magnetic voltage transformer can influence the comparison of price with capacitive

voltage transformers.

As present pricing in the magnetic market the price may work out almost equal. It is therefore,

more a matter of individual preference.

Test for V.T.:

The test for VT are classified as:

(i)                               Type test

(ii)                             Routine test

Difference between Instrument and Protective transformers

It should be appreciated by now that the current transformers can be classified depending on the

operational requirements or instrument transformers and protective x-mers.  The characteristics

of the two types of the different and recognized as much.  It is necessary to understand the

salient features, which distinguish the instrument transformers from protective transformers.

Instrument Transformers

Precision measurement and metering has assumed increasing importance as of the growth of the

supply system and particularly where energy interchanging between different power systems is

concerned with large quantities of energy transferred of the financial effect of measuring error

assume considerable importance.  It is also important and required that small fraction of the rated

primary current should be measured with adequate accuracy.  Some of them enters beyond rated

current is also necessary to make consideration of the normal system overloads.  The instrument

transformers is therefore, required to maintain the accuracy class within says  50% to 120% of

the rated current and small measuring errors with the range as evident.

Construction of Current Transformer

On construction basis that the current transformers may be subdivided as:

(a)                 Bar type: a current transformer in which the primary winding consist of a bar of suitable

size and material forming an integral part of transformer.

(b)                 Wound type: a current transformer having a primary winding of more than one full turn

wound on core.

The use of one of the other is determined by the rated current of the apparatus and the rated

burden required.

Bar type C.T.

For large primary current the bar type construction is ideal because it can meet with the burden

and accuracy requirement and the same time can have high thermal and dynamic short time

factors.  This type of construction is very sturdy.  This may be further sub-divided.

(i)                               Separately mounted type

(ii)                             Bushing type

The bushing type is mainly employed on the bushing of transformers on bulk oil circuit breakers. 

It has serious laminations with regard to the burden and accuracy.  There is an upper limit to the

rated output of the expected form a bushing current transformer generally 15 KVA in class 0.5 or

P15 or 10p20 for current of 400 amp.  The difficulty to keep the measuring errors within limits is

overcome either with the use of nickel Iron alloy.

Wound Type C.T

Where the primary currents are low on the burden and accuracy requirements are high.  Primary

winding consist of a number of turns normally not exceeding 5.  The primary number of turns

depends on the primary current.

The greater the number of turns lesser the thermal and dynamic short time current factors.

Selection of C.T.

The following points need to be considered while selecting a C.T.

1.                   Type

2.                   Number of Secondary

3.                   Accuracy class of each secondry

4.                   Rated burden

5.                   Accuracy limit factor

6.                   Short time current rating

7.                   Insulation valves e.g. power frequency dry and wet withstand valves; impulse withstand

valves.

Test for C.T.

The Indian standard IS: 2705 Lays down the following for the C.T.

(a)                 Type test

(b)                 Routine test

ISOLATOR

INTRODUCTION

When carrying out inspection to disconnect reliably the unit or section on which the work to be

done from all other live parts on the in-station in order to ensure completely safety of the working

staff.

To afford against minute mistakes it is desirable that it should be done by an apparatus which

makes a visible break in the circuit such an apparatus is the isolating switch (for insulator).  It may

be defined as a device used to open (or use) a circuit either when negligible current is interrupted

(or established) or when no significant charge the voltage across the terminals of each pole of the

isolator will result from the operation.

Isolator may be classified as single pole and 3-pole isolator i.e. according to number of poles. 

According to the service type these are:

(i)                               Indoor type and

(ii)                             Outdoor type

The doubling break rotating centre part isolating switches has

three isolator parts per phase mounted on a base of fabricated

construction.

The centre part carries the moving contacts arms tabular or fault with the intact assembles at the

extremities.  The moving contacts engage the fixed contacts on the outer fixed insulator parts. 

The designs of moving and fixed contacts vary from manufactures to the other.  The variants are

generally simple one of the contacts is the male contacts with the other is contacts.

The rotating centre part of the three phases are inter connected by operating rods 50 that

simultaneous movement of each part, connected by the operating rods and driven form one post

by operating mechanism through an adjustable lever drive rod and torque shaft supporting

structures.

The design of a contact could be different with different manufacturers for closing or both the

isolator parts rotate causing moment of contact arm.  The insulator shown is pneumatic operates

but is provided with emergency hand drive mechanism, also.

The contact at extremely which engares with the isolator contacts the line side.  The earthing

blade when provided are so inter locked with the main line blade that there can be closed only

when the main blade are in fully open positions.  Similarly it is possible to close the main blades

only when the earthing blades are fully in open position.  The earthing blade of a line switch have

a separate operating mechanism as well as gallery switch indicate on contact room the open or

close position of the earthing blades.

OPERATION

The operation of an isolator may be manual i.e. by hand without using any other supply or

storage of energy meter power operated isolates during the cause of operation utilize energy

which is not supplied by the operator.  The energy may be electrical pneumatic or the energy

previously stored in spring or counter weight.

Control

In case power operated isolators are purchased for any installation it may be worth while to

examine further weather control should be local in switchyard or remote in the control room.

The extra cost enrolled in the isolated is quite substantial particularly at voltage 132 kV and

below.  It should therefore considered in detail whether any installation really instifies the

procurement of remote operated isolators keeping in view the past that the frequency of operation

of isolators is rather low.

 

AUXILIARY SWITCH

This is an operating and important accessory and is designed as a switching device working in

conjunction with an isolator for controlling a circuit for auxiliary device such as trip coils indicators

or indicating lamps. The number of normally closed and normally open contacts should be

specially worked out particularly if electrical interlocking between breakers and isolators is

chosen.

Make before and Break after contacts

These are provided in series with the main contacts so that in case of load isolators, the arcing is

taken and whenever necessary only the arcing contacts are replaced.

ARCING HORNS

These are provided on each stack of post Insulator for the purpose of insulation co-ordination

some time confusion is created in the function of these arcing horns vis a vis (make before and

break after contacts. These may be fixed or adjustable types).

The use of arcing horns is avoided where insulation strength between poles or phases and

between higher than that of earth.  This is necessary for safety and security.  Any travelling wave

meeting an isolator is the closed position should causes of it must a flash over to earth rather

than between phase or between terminal of the same pole where the design of the isolator itself

provides for this.  It is necessary to use arcing hours on the insulator stacks.

INTERLOCKING

In correct operation of an isolating switch may be accidentally harmful effects and may cause

distribution of part of the plant as well as costly service interruption for preventing such incorrect

operation inter locks are used i.e. isolating switches.  The mechanical interlocking between

isolating switches and it is earthing switch consists of a rod linkage between isolating switch and

its earthing switch shafts of the respective switches.

The mechanical interlocking between isolation switches and circuit breaker and different isolating

switches is generally in the form of lock and key arrangement.  There is usually a common key for

a number of locks mechanical interlocking is generally provided on hand operated are isolating

switch only. Electrical interlocking is achieved with blocking magnets, these magnets are

arranged on the isolating switch on the hand drive or in the value controlled.  The pneumatic drive

and are controlled by pilot switch contents.  The requirement of interlocking may be summed as:

(i)                               The isolator can't be operated unless the association breaker is worked in the

position.

(ii)                             The earthing switch shell close only when the line isolator is open and locked and

net in its stroke.

(iii)                            The isolator shall close only where the corresponding circuit breaker and the

earthing switch of the corresponding line or open.

(iv)                            The circuit breaker shall close only after all the isolator associated with it have been

locked either in closed or open position

(v)                              When one bus bar isolations so that bus is open when on bus isolation of that bus

expecting the bus coupler bus is closed, the other small close only when bus coupler circuit

breaker and both the bus isolation are closed.

(vi)                            The bus isolator of bus coupler buy shell operates only when all the bus coupler

circuit breaker is open.

(vii)                           The bypass isolator is provided of the feeder shall also close manually irrespective

of the fact whether the feeder circuit breaker and it is adjoining isolator are open or closed. 

 

CIRCUIT BREAKER

 

INTRODUCTION

A circuit breaker is equipment, which can open or close circuit under all condition viz. No load, full

loads an fault conditions.  It is so designed that it can be operated manually under normal

conditions and automatically under fault conditions, for the later operation, relay circuit is used.

Circuit breaker can be defined as an electrical device, which protects the system from short

circuits or overloads with the help of relays.  In case, circuit breaker is not of adequate capacity,

its failure may result into interruption of power, shut downs, injury to personals and damage to

property.  Installation of over rated circuit breakers or extra sensitive and costly protective devices

will mean un-warranted expenditure.  It is therefore necessary that calculations in respect of short

circuit currents for the concerned system be made before correctly rated circuit breakers are

selected or steps are taken to improve the existing system.

OPERATING PRINCIPLE

A circuit breaker consists of fixed and moving contacts under normal operating conditions, these

contacts remain closed.

In this condition, the emf in the secondary winding of current transformer (CT) in sufficient to

operate the trip coil of the breakers but the contacts can be opened by manual or automatic

control.

When a fault occurs on any part of the system the resulting overcurrent in the C.T. primary

winding increases the secondary winding EMF and hence the current through the relay operating

coils.  The relay contacts are closed and the trip coil (tripping coil) of the breaker is energized. 

The moving contacts are pulled apart by some mechanism thus operating the circuit breaker.

When the contacts of the circuit breaker are separated under fault conditions, arc is produced

between them (male and female contact).  The current is thus able to continue until the arc

ceases.  This arc generates enormous heat, which may cause damage to the system or to the

breaker itself.

Therefore, the main problem in a circuit breaker is to extinguish the arc within the shortest time so

that heat generates by it may not reach a dangerous value.

Classification of circuit breakers for various voltages

1.       Bulk oil circuit breaker.

2.       Air blast circuit breaker.

3.       SF-6 circuit breaker.

4.       Minimum oil circuit breaker.

5.       Vacuums circuit breaker.

 

Bulk oil circuit breaker

In such circuit breaker transformer oil is used for arc extinction. The contacts are opened under

oil, which absorbs the heat of arc, and decomposed into gases as hydrogen, which have excellent

cooling propeties due to high heat conductivity.

Circuit breaker compresses of three-pole contact assembly housed in a circular welded steel tank.

Circuit breaker is mounted on an angle iron frame grounded in cement concrete base the breaker

is provided with spring or solenoid operating mechanism. However, provision for hand operation

is there. The contacts are of but type the stationary portion comprises of two contacts pivoted at

the base of the explosion pot. The cross jet assembly is made of blocks of insulating materials,

which together form a chamber of irregular shape. The throat block and single block have circular

holes located centrally through which moving contact passes. The barrier plates are shaped to

form nozzle outlets through which the oil and arc glasses are projected from the explosion pot.

When the breaker is tripped on load or on fault, the moving contact breaks circuit with the

stationary contacts and the resulting arc is drawn downward through the throat hole. The gas

produced from the oil by the arc accumulated in the expulsion a pot at high pressure. This

pressure acts downward on the end of the moving contact and so accelerates its movement. As the

moving contacts descends, the flush of oil and gas sweeps through the arc an passes out through

nozzle outlets thereby producing powerful quenching effect and causing disruption of the arc

before the contact leaves the cross jet assembly. The combination of explosion pot and cross jet

chamber confines the high pressure of the arc produced gases to the interior of the explosion pot

and thus prevent the heavy shocks to the outer tank from sudden movement of masses of oil. The

small hole near the top of the explosion pot allow the accumulation of the gases to escape so that

the explosion pot fills up again with oil after the arc is extinguished.

The disadvantages of oil as an arc excitation medium for an arc: -

It is inflammable and there is a risk of fire.

The quality of oil deteriorates, due to increase of carbon in oil with the excessive use of breaker.

This needs periodic checking and replacement of oil.

In B.O.C.B. the increase in carbonization weakness the dielectric strength of the oil of breaking

strength of oil.

 

VACUUM CIRCUIT BREAKER

 

PRINCIPLE

When the contacts of the breaker are opened in the vacuum (10 -7 toor and 10 -5 torr) an arc is

produced between the contacts by the ionisation of metal vapours of contact and it is is quickly

extinguished in the vacuum because it has excellent suspension arc quenching properties than any

other medium.

 

WORKING

When the breakers operates, the moving contact operates from the fixed contact and an arc is

produced between the contacts.  The production of arc is due to the ionization of metal ions and

depends very much upon the material of contacts.  The arc is quickly extinguished because the

metallic vapours i.e. electrons and ions produced during arc diffused in a short time.  Since

vacuum has very fast rate of recovery of dielectric strength, the arc extinction in vacuum breaker

occurs with a short contact separating (say .625 cm) vacuum.

 

ADVANTAGES

They are compact, reliable and have long life

There are no fire hazards

They require little maintenance and are quiet in operations

 

APPLICATIONS

They are widely used from 33 kV up to 66 kV voltages.

 

Air Blast Circuit Breaker

In such circuit breaker, high-pressure air blast is used for arc extinction.  The contacts are opened

in the flow of air blast.  The air blast cools the arc and removes the arcing products (mainly

composed of carbon) to the atmosphere.  This rapidly increases the dielectric strength of the

medium between contacts and prevents the arc restriking.  Consequently the arc is extinguished

a flow of current is interrupted.

For the interruption of current on load, air blasts C.B. are used. In 220kv A.B.C.B. is used. This

type of circuit breaker has four interrupter terminals while in 132kv there are three interrupter

terminal of A.B.C.B. A pole chiefly consists of number of identical column each are standing on it

compressed air receiver supporting interrupter.

The C.B. can be operated both electrically either from relay station or with a separate master

switch and a puss button operate valve in the central controlling cabinet AB-5.

The controlling impulse are transmitted from the controlling cabinet AB-5 electrically or

pneumatically to the control again in the panel box and from these to the intermediate value AB-5

which in turn determines the condition in air insulators and position of C.B.. The controlling

cabinet has alarm type pressure gauge for the pressure in the receiver of the pole pressure

switches and mechanically lode pressure blocking device.

Electrical operation of tripping.

The electrical impulse from the central controlling cabinet the opening coil releases the armature.

The pressure fills the wave below and the valve chamber is pressured. The controlling valve

changes its Position. The outlet is connected with the intake, thereby closing the intermediate

valve its position gets changes.

The controlling isolators are pressured and the interrupter carries out the operation. The control

oil from intermediate valve pressurizing the resulting unit via the inlet. Any pressure loss occurring

through leakage from the cylinder part below the piston is replaced via the outlet and the

interrupter carry out the opening operation. At the same time the resulting unit is pressured to

move the control piston towards the armature but this doesn't effect the cycle. The organs in the

operating reles don't particulars participate in the operation.

During the closing operation on receipt of an impulse the closing coils open the auxiliary control

(closing valve) which send controlling air to the piston. The piston lifts the pappet valve on other

side of the opening valve. The pressure on the upper part of the control piston closes the

controlling valve. The outlet is cut off from the intake and is joined to the atmosphere.

Hence the intermediate valve becomes pressure less and changes position, which causes to

carryout closing operation. The necessary unit is also pressurized and the control piston returns

to the circuit breakers upper position but this doesn't effect the cycle. The circuit breaker is also

provided with a facility of pneumatic closing operation. It's only difference is the armature coil has

been substituted by manually operating valve.

Compressed air system for A.B.C.B.:    

The EHV A.B.C.Bs are out door equipment's. The air pressure in the receiver of the circuit

breaker is of the order 20-30kg/cmsq for 220kv A.B.C.B. The local receivers are of the order of

(air receiver) 4 to 12replaces operations. When the pressure in the receiver of the C.B. is of the

order of 20-30kg/cmsq for 220kv A.B.C.B. the local receiver is of order of 4 to 12. Thus it repeats

the operation when the pressure in the receivers maintained at desired value.

While comparing the main type of pipeline, compressor and other types of equipment for the

measurement we find that the whole system is automatic. The pressure in the main receiver is of

35kg/cmsq.i.e. Higher than that the auxiliary receiver and in air receiver when the pressure

reduces below certain value the compressor motor starts automatically when the desired

pressure also stoops. Usually two identically compressor sets are provided. These are driven by

2-2.5 H.P. induction motor one set act like stand by set.

In air receivers when pressure reduce below certain value the compressor also stops. The

automatic compressors are installed in compressor room in the yard. The compressed air is taken

through trendies in individual breaker receivers, which are connected, to the pipelines.

DISADVANTAGES

(i)                               Considerable maintenance is required for compressor plant, which supplies the air

blast.

(ii)                             Air blast circuit breakers are very sensitive in the rate of rise of restriking voltage

(PRRV).

ADVANTAGES

(i)                               The risk of fire is eliminated

(ii)                             The arcing products are completely removed by blast whereas in the oil circuit

breakers oil deteriorates with successive operations.  The expense of regular oil replacement is

avoided.

(iii)                            The arcing time is very small due to the rapid build up of dielectric strength between

contact, therefore arcing energy is only of friction of that in the oil circuit breaker thus resulting in

less burning of contacts.

TYPES

Depending upon the direction of air blast in relation to the arc classified into:

(i)                               Axial blast type

(ii)                             Cross blast type

(iii)                            Radical blast type

 

SF6 circuit breaker:

The arc excitation process in SF6 gas removes the heat from the arc by axial convention and

radial dissipation. As a result the arc dia. reduces during the decreasing made of the current zero

and arc is extinguished due to its Electromagnetic and low arc time constant, the gas remains its

dielectric strength rapidly after the final current zero. The rate of rise of dielectric strength is very

high and time constant is very small. The arc extinguished properties of SF6 gas has pointed out

in 1983.

Minimum oil circuit breaker.

Oil circuit breaker uses dielectric oil (transformer oil) for the purpose of arc extinction. In bulk oil

circuit breaker the arc extinction takes place in the tank where as in M.O.C.B. the current

interruption takes inside interruption. The enclosure of the interpreter is made of insulating

material like porcelain. Hence clearance between the live part and the enclosure can reduce and

layer quality requires of internal insulation.

Construction:

In M.O.C.B. there are two chambers separated to each other but both are filled with oil. Their

upper chamber is the circuit breaking while lower one is supporting chamber. The oil from one

chamber does not mix with the oil in the lower chamber but it acts as dielectric support. This

arrangement permits two advantages, firstly, the circuit breaking chamber require a small volume

of oil. Secondly the amount of oil to be replaced is reduced as the oil in the supporting chamber

not get contaminated by the arc.

(i)                  Supporting Chamber

It is a porcelain chamber mounted on a metal chamber.  It is filled with oil, which is physically

separated from oil in the circuit-breaking compartment.  The oil inside the supporting chamber

and the annular space formed between the porcelain insulation and bakelized is employed for

insulation purpose only.

Arc extinction device is filled to the upper fixed contacts. The lower fixed contacts are ring

shaped. The moving contact makes a sliding contact with the lower fixed contacts. A region

bounded with glass fiber cylinder encloses the contact assembly. This cylinder is also fixed with

oil porcelain cylinder enclose the glass fiber cylinder.

The M.O.C.B. may be self-blast type or blast type or combination of both. According to the

principle two types of venting are used in the design i.e. axial venting and radial venting.

Axial venting has the advantages that it's generates high pressure and also directional strength. It

is used where low currents are to be tripping at high current and low voltage braking purpose.

 

(ii)        Circuit Breaking Chamber

It is a porcelain enclosure mounted on the top of the supporting compartment.  It is filled with oil

and has the following parts.

(a)                                         Upper and lower fixed contacts

(b)                                         Moving contacts

(c)                                         Turbulator

The moving contact is hollow and includes a cylinder, which moves 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.

(iv)                           Top Chamber

It is a metalchamber and is mounted on a circuit-breaking chamber.  It provides expansion space

for the oil in the circuit breaking compartments.

The top chamber is also provided with a separator, which prevents any loss of oil, by centrifugal

action caused by circuit breaker operations during fault conditions.

 

OPERATION

Under normal operating conditions the moving contacts remain engaged with the upper fixed

contact.  When the fault occurs, the moving contact is pulled down by the tripping springs and an

arc is struck.  The arc energy vapourises the oil and produces gases under high pressure.  This

action constrains (compels) the oil to pass through the central hole in the moving contact and

result in forcing series of oil through the respective passages of the turbulation.  The process of

turbulation is orderly one in which the sections of the arc are successively quenched by the effect

of the separate streams of oil moving across each section in turn and bearing away its gases.

ADVANTAGES

(i)                               It is requires lesser quantity of oil compare to the bulk oil circuit breaker.

(ii)                             It require smaller space

(iii)                               There is reduced risk of fire

(iv)                            Maintenance problem are reduced

Disadvantages

These chambers suffers from the disadvantages that a very low current, they have long arcing

period so a separate oil reflection device has to be used. The contacts are operated by pull rod or

by the spring operating mechanism.

Advantages

1.       It requires less oil in comparison to bulk oil circuit breaker.

2.       It requires lesser space.

3.       There is reduced risk of fire.

4.       Maintenance problems are reduced.

 

  CONTROL ROOM

 Synchronizing - Panel

There is a hinged synchronized panel mounted at the end of a control board.  To take out new

supply on the bus bar supply so the panel handles put to cuts synchronizing and then see the

synchronies scope. There is also two voltmeter one-give busbar voltages.  Second in coming

voltage when the syncronoscope stop zero we close the C.B. and the supply is taken on bus bar.

Syncrono scope

A syncronoscope is used to determine the current instance of closing the switch, which connects

new supply to bus bar.

The current instance of syncronising is when the bus bar and the incoming voltage.

1.                   Are equal in magnitude

2.                   Are in phase

3.                   Hare the same frequency

4.                   The phase sequence should be same.

Introduction

In order to generate the electric power and transmit it to consumer millions of rupees must be

spent on power system equipment.  These equipment are to work under specified normal

conditions.  However a short circuit may occur due to failure of insulation called by:

(i)                               Over voltage due to switching

(ii)                             Over voltage due to direct and indirect lightning strokes

(iii)                            Briding of conductors by birds

(iv)                            Break damage of insulation due to decrease of it's di-electric strength.

(v)                              Mechanical damage of the equipment.  The fault takes place in following

properties.

1.                                           Phase to phase 20 - 25%

2.                                           Single phase short circuit 50-60%

3.                                           Double phase S.S. 3-5% 20-25%

4.                                           Three phase short circuit 3-5%

5.                                           Phase to phase and phase to guard 10-15%

Fault may be defined as the rise of current in the several times to normal current resulting the

high temperature rise which can damage the equipment.

It reduces the voltage immediately and considerably.

Basic Equipment or Requirement of Protective Relays

Basic requirements of protective relays are as follows

Speed

Protective relaying should do's connect a faulty element as quickly as possible.

Selectivity

The ability of the protective relay to determine the point of which have the fault occurs and select

the nearest circuit breaker tripping of which will lead the clearing of fault with min-or so damage to

the system.

Sensitivity

It is the capacity of the relaying to operate relay under the actual condition that produces the last

operating condition tendency.

Depending upon the method of element connected primary relay (series element connect directly

on the circuit of protective element) and secondary relay (sensing element connected through a

current and voltage transformer).

Depending upon the time action

Depending upon the kind of contacts

These are called normally opened, normally closed in Heerapura sub station control room there in

panel in which the relays are set and there are many type of relays.

1.                   Over current relays

2.                   I.D.M.T. fault relay

3.                   Impedance relay

4.                   Earth fault relay

5.                   Bucheloz's relay

6.                   Differential relay

7.                   Auxiliary relays

Over current relay

It is used in over current protection scheme over current protection is the name given to protected

relay scheme devised to rise in current in a protected circuit of to a safe value inherent simplicity

of operation and reliability in operation has resulted in over current protection having obtained the

widest application in short circuit protection scheme and a mean of protection against abnormal

condition's of operation etc in power x mission circuit as here is Heerapura grid station when the

short circuit occurs the fault current which is very much higher than the normal current flow

through the relay i.e. from proportional due to C.T. and the over current relay because operations

(because flow is more than the present value) i.e. is more than Ix where Ip is relay picking up or

operating currents now due to close of C.B. the signal is go to trip coil of C.B. trip.

(a)                 Electromagnetic relay

(b)                 Induction over current relays

Inverse time characteristics relay

The relay using here having the inverse time characteristics having the time delays dependent

upon current value.  This characteristic is being available in relay of special design.  There are

(1)                 Electromagnetic Induction type

(2)                 Permanent magnetic moving coil type

(3)                 Static type

Earth fault relay

The earth fault relay and over current relay resembles because when the conductor break or by

any reason it is earth ment it is short circuited and fault current which flow in many times to

normal current, so there is always over current fault so now we have the over current relay and

both are same.  These relays can also be Electro-magnetic induction and static relay.

Directional Relay

The non-directional relay discussed above can operate for fault in either direction in order to

achieve operation for the fault current flowing in a specific direction.  It is necessary to add an

additional element, such a relay which corresponds to fault current flow in a particular direction is

closes called a directional relay.  These relays are added in the panel.

When a fault takes place, the fault current flows through the current coil of relay which produces a

flux in the lower magnet of the directional while the current in the voltage coil produces another

flux in the upper magnets.  The flux produces torque tending to close it's contact (directional

element contacts).  The relay also flows through the windings over the magnet of the non-

directional elements.  Since this winding provides a closed path the induced emf circulates a

current, which therefore produce another flux.

ANNUNCIATOR AND METER SECTION

In the control room the 'annonciator' is most control box as when the fault across and relay trip by

which we mean that the fault is cleared.

In this there is a box type thing in which probable fault at different feeder and different zone have

written and in front of them there is a bulbs.  There is also alarm systems.

When same fault is occurred the relay is trip and is given two signal one for two circuit breaker

and second to annuncitor auxiliary relays.  This relays first signal trip the C.B. and signal when

goes to relay i.e. auxiliary type trip, that relay this relay i.e. seeds the signal to annunciator which

give alarm and the bulb is lightning in the front of the type of the fault, which is occurred.   The

shift engineer can receives this signal and sees the annunciator at which feeder at which zone

and which type of fault is occurred

Meter Section

Panel at which the C.B. is open it trip again to see that whether it is instantaneous fault (like

monkey made short circuit or bird made a short circuit ) The closes the circuits breaker and reset

the relay is trip or not.  It is not other the system coil leak as much as it again announce that the

circuit is still faulty then as we know that suppose fault is at RPS Feeder zone earth fault send a

maintenance party to the fault.

Measuring Instruments

These are certain panel boards, which have the energy meter for differential feeder, wattmeter

and maximum demand indicator.

Energy Meter

The energy meter is the meter, which measures the energy.  These are filled to different feeders

and we note hourly reading how much a amount we are importing/exporting.  These meters read

in MW.

Watt Meter

This meter is also attached to energy feeder and we can note by the watt meter how much

amount energy power is exporting or importing .

Maximum Demand Indicator

This is also mounted on panel board.  The chief requirement of these indicators is that they shall

record the maximum power taken by the feeder during a particular period.  The maximum

demand indicator shall be a so designed that any sudden momentary increase in loads such as

due to short circuit not account.  Therefore, a maximum demand indication is to record the

average power over successive pre-determined period.

LIGHTNING ARRASTER

Introduction

 Lightning Arresters are installed in power houses and sub-stations to safeguard the major

equipment like power-transformers, switch gear and to ensure the flow of power un-interruptedly. 

It is true that lightning arresters require minimum post-installation care, but their importance as a

critical equipment can hardly be disputed.

Lightning Strokes and Over-voltages

The overhead transmission lines and connected electrical apparatus i.e. Power Transformers,

Switch gear etc. are subjected to over voltages on account of lightning discharges caused by

atmospheric disturbances and or by switching operations.  Abnormal voltages are caused by

atmospheric disturbances as a result of:

(a)                Direct Strokes

Direct stroke to the phase conductor or ground wire or to supporting structure results into

abnormal transient voltage, which gets super-imposed on the power net work.

(b)                Indirect Strokes

Direct stroke in the vicinity of the line or the equipment or charged cloud over the power line

induces abnormal voltages.

Abnormal transient over voltages super-imposed by direct or indirect strokes travel along the

conductor in both the directions with the speed of light i.e. 186,000 miles per second or 1000 feet

per micro second.  These waves are steep fronted in case of direct strokes and travel till the

surge voltage is attenuated or neutralized by reflected waves of opposite polarity from the earthed

object.  E.H.V. transmission lines and sub-stations are designed to take care of direct strokes by

providing:

 

(1)     Higher impulse level

(2)     Shielding and lower footing resistance

(3)     Lightning  arresters for draining undesirable voltage to the ground.

Type of Lightning Arresters

Ground wires or shielding wires generally of steel are fixed over the phase conductors in case of

transmission lines and sub-stations and are solidly grounded.  The ground wire when solidly

grounded through a very small resistance reduces the magnitude of voltage induces upon the line

conductors due to electrostatic field produced by charging cloud.  The ground wire is in a general

sense is preventive device, but it does not entirely prevent the formation of travelling waves on a

line.  Surges produced by direct strokes or by induced strokes must be drained to the ground

through low impedance ground to protect power transformers and other costly equipment and to

reduce outages in the system.  Lightning arresters are the devices to provide the necessary path

to the ground for such surges.  An ideal arrester must therefore have the following properties:

1.                   It should be able to drain the surge energy from the line in a minimum time.

2.                   Should offer high resistance to the flow of power current.

3.                   Performance of the arresters should be such that no system disturbances are

introduced by its operation.

4.                   Should be always in perfect form to perform the function assigned to it.

5.                   After allowing the surge to pass, it should close up so as not to permit power current to

flow to ground.

Lightning protective devices, which are in market, are of the following type:

(a)                Rod Gap or Sphere Gap

It is a very simple protective device i.e. gap is provided across the stack of insulators to permit

flash-over when undesirable voltages are impressed on the system.  It does not fulfil the function

of ideal lightning arresters i.e. it does not cut off power voltage after it has flashed over by a

surge, in other words a short circuit will be caused on the system every time a surge causes a

flash-over. Flash over conditions are also affected by rain, pollution, humidity temperature and

polarity of the incident waves.  In view of these disadvantages it can be only used as "back up"

protection in case main lightning arrester gets damaged.

(b)                Expulsion type Lightning arresters

Expulsion type lightning arresters are also called "expulsion protector tubes", "de-ion tubes" and

"line type expulsion arresters." Constructional details and salient features of expulsion type

lightning arresters are shown in fig.

It consists of an insulating tube, which has got an electrode at each end and discharge hole at the

lower end.  The length of the tube is such that spark over occurs in the tube between the two

electrodes.  While installing lightning arresters it is ensured that there is external series gap

between the cap and the line.  Series gap prevents constant application of system voltage and

thus leakage corona is avoided.  Whenever undesirable transient voltages occur, two gaps i.e.

external and internal breakdown due to flash over and provide a conducting path in the form of

arc for drainage of the voltage to the ground.  They are produced inside the tube by "follow up

current" produces gas which drives out ionized air (air is ionized by the arc) through the bottom

vent.  The "follow up current" at its zero finds the arc path de-ionized and space between the

electrodes fully insulated to prevent the flow of "follow up current."  The rapid expulsion of the

gases in the tube normally interrupt the short circuit power follow current within the first or second

half cycle.

(c)                 Valve Type Lightning Arresters

Valve type Lightning arrester consists of number of spark gaps in series with non-linear resistors,

the whole assembly being rigidly housed inside a hermetically sealed bushing. Under normal

conditions, power frequency system voltage does not cause break down of series spark gaps and

thereby insulate the line from ground for the highest system voltage.  When undesirable transient

voltages due to lightning are super-imposed over the system, the series gap assemblies spark

over at a pre-determined value.  After the breakdown of the gaps, the non-linear resistors conduct

the surge current to the ground offering very low resistance and limit the power frequency current,

to a value, the gaps can interrupt at the first current zero.  During the flow of the discharge current

the non-linear resistor limit the voltage drop across the arrester to a value far below the BIL of the

equipment.

The valve type lightning arresters are generally classified as station type and line type.  Station

type lightning arresters are very robust and efficient and are installed in sub-stations and power

houses.  Line type lightning arresters are similar to station type lightning arresters but are smaller

in cross-section and are less costly.  Line type arresters allow higher surge voltages across their

terminals and have low surge current capacity.

General rating recommendations of lightning arresters

(i)                               10 KV rated lightning arresters:  Arresters of this rating are used in case of power

stations and E.H.V. sub-stations.

(ii)                             5 kA rated lightning arrests: Arresters of this capacity normally are used in case of

high voltage sub-stations having system voltage as 66 KV or less.  These are also used in case

of small power houses.

(iii)                            2.5 kA rated lightning arresters:  Arresters of these ratings are used in case of

system upto 11 KV

(iv)                            1.5 kA rated lightning arrests: arresters of these ratings are normally used in case

of distribution system.

Location of Lightning Arresters

In order to ensure effective protection of the equipment lightning arresters should be located :

(a)                 Very close to the equipment to be protected and connected with shortest leads on both

the line and ground side to reduce the inductive effects of the leads while discharging large surge

currents.

(b)                 In order to ensure the protection of transformer windings it is desirable to inter-connect

the ground lead of the arrester with the tank and also the neutral of secondary.  This

interconnection reduces the stress imposed on the transformer windings by the surge currents to

the extent of the drop across the earth resistance and the inductive drop across the ground lead.

 

Power Line Carrier Communication

Introduction

Power Line Carrier Communication (PLCC) provides for signal transmission down transmission

line conductors or insulated ground wires.  Protection signalling, speech and data transmission

for system operation and control, management information systems etc. are the main needs

which are met by PLCC.

PLCC is the most economical and reliable method of communication because of the higher

mechanical strength and insulation level of high voltage power line which contribute to the

increased reliability of communication and lower attenuation over the larger distances involves.

High frequency signals in the range of 50 kHz to 400 kHz commonly known as the carrier signal

and to result it with the protected section of line suitable coupling apparatus and line traps are

employed at both ends of the protected section.  Here in 'Heerapura' and also in other sub-station

this system is used.  The main application of power line carrier has been from the purpose of

supervisory control telephone communication, telemetering and relaying.

PLCC Equipment

The essential units of power line carrier equipment consists of (a) Wave trap; (b) Coupling

Capacitor; and (c) LMU and protective equipments.

Wave Trap

Rejection filters are known as the line traps consisting of a parallel resonant circuit (L and C in

parallel) tuned to the carrier frequency are connected in series at each end of the protected line

such a circuit offers high impedance to the flow of carrier frequency current thus preventing the

dissipation.

The carrier current used for PLC Communication have to be prevented form entering the power

equipments such as attenuation or even complete loss of communication signals.  For this

purpose wave trap or line trap are used between transmission line and power station equipment

to -

(i)                               Avoid carrier power dissipation in the power plant

(ii)                             Reduce cross talks with other PLC Circuits connected to the same power station.

(iii)                            Ensure proper operating conditions and signal levels at the PLC transmit receive

equipment irrespective of switching conditions of the power circuit and equipments in the

stations. 

 

Coupling Capacitor

Coupling of high frequency transmitter receiver units to the power line is done through high

voltage capacitor.  The high voltage capacitor, which has a capacitance of about 0.000, 1 MF is

earthen through the drainage coil.  This provides insulation of the terminal equip from line by

providing a very high impedance to carrier frequency current.

Coupling Capacitor is used for coupling the carrier current or voltage to the power line.  It blocks

the power frequency current to flow into the PLC equipment.

The coupling is usually designed for mounting of the wave traps on it.  A corona shield is always

provided.  Arcing rings are usually not provided as they may give resistance to corona

discharges, which result in higher noise level in PLCC circuits.

The coupling capacitor is used as a part of filter network which allows a fairly wide band of radio

frequency to pass through to the PLC terminals to be connected in parallel to the coupling unit

and hence results in a saving in the use of installation.

Use of Capacitor Voltage Transformer (CVT) as a Coupling Capacitor

The matching transformer is used for matching the impedance.  The two capacitors and voltage

driver circuit.  The carrier signals pass through these capacitors.

Signal is transmitted to PLC from the point through co-axial.  The voltage drop across C2 is

applied to auxiliary transformer to get potential required for live line indication, metering and

synchronizing circuits.

Driving Coil

Function of this coil is to prevent high frequency signals from transmitter to ground and also

preventing the coming signals from earth the receiver.  Its value of order of 100 MH

 

Line Matching Filter & Protective Equipments

For matching the transmitter and receiver unit to coupling capacitor and power line matching

filters are provided.  These flitters normally have air corral transformers with capacitor assumed in

Heerapura.

The matching transformer is insulated for 7-10 KV between the two windings and perform two

functions.  Firstly, it isolates the communication equipment from the power line.  Secondly, it

serves to match the characteristic impedance of the power line (400-600 ohm).  The vacuum

arrester, which sparks over at 250 volts, is provided for giving additional protection to the

communication equipment.

 Cables

The connection of transmitter receiver units to the connecting fitters is done through co-axial

cable.  There is mesh shield with is grounded.

Transmitter

The transmitter consists of an oscillator and a amplifier.  The oscillator generates a frequency

signal with in 50 to 500 Hz frequency bands the transmitter is provided so that it modulates the

carrier with protective signal.  The modulation process usually involves taking one half cycle of 50

Hz signal and using this to create block to carrier.

 

Receivers

The receivers usually consist of an alternate matching transformer band pass filter and amplifier

detector.

The amplifier detector converts a small incoming signal in to a signal capable of operating a

relatively intensive carrier receiver relay.  The transmitter and receiver at the two ends of

protected each corresponds to local as far as transmitting.

Transmission System

The range of freque3ncies used for PLCC communication is generally between 10-500 kHz.  In

partial situation, frequencies above 25 kHz are used due to the following reasons:

(1)                 Harmonics, switching, lightning and corona, which are generally present on HV lines,

have component in the frequency band between 100 HZ and 24 kHz which course considerable

noise in communication circuits.  If frequency below 25 kHz are employed, the signal to noise

ratio in such circuit will be quite poor.

(2)                 Difficulty in separating power frequency and ratio frequency component below 25 kHz.

(3)                 Cost of coupling equipment becomes prohibitive owing to the size and complexity of

equipment required for operating efficiency at low frequency.

The upper limit of 500 kHz is used because -

(a)                 Radiation losses are high above 500 kHz

(b)                 Interference problems are encountered

The following methods can be employed for the speech in PLC Communications.

(i)                               Amplitude modulation with carrier and double side band transmission

(ii)                             Amplitude modulation with single side band suppressed or reduced carrier

transmission

(iii)                            Frequency Modulation

Almost all-modern PLC transmission equipments for speech use single side band suppressed or

reduced carrier transmission.  Single side band AM transmission has the following advantages

over double side band AM transmission.

(a)                                                                 The band width requirement channel is exactly half that of double

side band transmission

(b)                                                                 As the receiver accepts half the band of frequencies the noise input

to the receiver is correspondingly reduced

(c)                                                                 As the carrier and one side band are not transmitted, the power

required for these is saved.

 

MERITS AND DEMETIS OF PLCC

Merits

1.                   The severity that a power line can withstand is much more than that odd communication

line due to higher mechanical strength of transmission line

2.                   Power lines generally provide the shortest route between the Power Station and the

Receiving Stations.

3.                   The carrier signals suffer less attenuation, owing to large cross sectional area of power

line

4.                   Larger spacing between conductors reduces the capacitances which results in lesser

attenuation of higher frequencies.

5.                   Large spacing also reduces the cross talk to a certain extent.

6.                   The construction of a separate communication line is avoided.

Demerits

1.                   Utmost care is required to safeguard the carrier equipment and persons using them

against high voltage and currents on the line.

2.                   Noise introduced by power line is far more than in the case of communication line.  This

is due to the discharge across insulators and corona etc.

3.                   Induced voltage surges in the power line may affect the connected carrier equipment.

 

 

BATTERRY ROOM

Introduction

Storage battery is the most dependable source of supply of D.C. power required for closing and

tripping of circuit breakers, operation of automatic protective devices; signaling equipment,

remote control apparatus, telephone service and emergency lighting in case of power plants and

sub-stations.  Correctly selected and properly maintained battery will withstand heavy stresses

and strains during service without causing much headaches to the maintenance Engineer. D.C.

Auxiliary power supply is provided from storage batteries maintained continuously charged by

some type of supply set or a charger.  The voltage of the auxiliary supply is maintained at

110/220 volt.

Advantages of Storage Batteries

         High Reliability

         Independances of A.C. power circuit conditions of existence of the faults.

D.C. Earth Fault

All D.C. auxiliary supply circuits must have their insulation resistance maintained at an adequate

level, as any breakdown in the insulation with respect the earth may lead to false tripping due to

formation of a path for bypass of the current round the control devices.  Because of this danger

every D.C. auxiliary supply installation must include a unit for constantly monitoring the condition

of the insulation.

As fig. Shown, when the insulation is healthy the voltage of each power relative to earth V1 and

V2 will be equal and half the voltage between both the poles.

In case the insulation of one pole drops in value with respect to earth, the voltage to earth of this

pole will also drop, but the voltage to earth of the other pole of the circuit will increase by the

some amount.

Stationary Storage Batteries

Batteries are bring used for the supply of direct current to various types of equipment and

appliances and are broadly divided into two categories i.e. stationary and transportable type.  The

stationary type battery, once installed is never moved during the course of its service life.

Lead Acid Storage Battery

Basically, lead acid storage cell consists of electrodes i.e. anode and cathode in the form of

plates immersed in diluted sulphuric acid, placed in acid resistant container.  Acid resistant

containers are usually made of vulcanized rubber, glass, plastic, ceramic and good quality of

wood lined with lead.  Glass and plastic containers are normally used in case of stationary

storage batteries.  Where weight of the Battery does not present problem and simple space is

available wood containers lined with lead may be used.  The container is provided with the vent to

facilitate the escape of gases as well as it provides opening for the addition of distilled water or

electrolyte.

Plates as used are in the shape of grids made of an alloy of lead and antimony and active

material.  The use of plates in the shape of the grid is helpful in providing support to the active

material, conduction of electric current and maintenance of uniform distribution of the current

throughout the mass of the active material.  Heavier grids are used in case of stationary batteries,

while light grids are used in case of batteries designed for heavier discharges when these are of

short duration.  Lead non-oxide (lead power and Litharge) is normally used as an active material. 

Lamp black, or barium sulphate or organic extract is added as an expander to the active material

in case of active material.  An expander helps in decreasing the possibility of shrinkage and

hardening of the negative plates.   The positive and negative plates are separated from each

other by means of porous separators made of wood vanear, perforated and slotted hard rubber,

glass fibre, etc. etc.

The Electrolyte

Sulphuric acid of very high purity diluted with distilled water is used as an electrolyte in case of

lead acid batteries.  Since specific heat of the water is higher than sulphuric acid as such when it

is mixed with water abnormal increase in temperature takes place, it is, therefore, necessary to

get the solution cooled down before it is poured into the battery, to avoid damage to the plates.

Specific gravity and viscosity of the electrolyte has got great bearing on the condition and

capacity of the battery.  The rate of diffusion of the solution through the pores of the plates,

depends upon its viscosity.  The viscosity increases with the decrease in temperature, thereby

affecting the capacity of the battery at low temperatures.  The value of specific gravity is the

indicator regarding the condition of the battery.  It is defined as the ratio between equal volumes

of the liquid and the water at specific temperature and is measured by means of hydrometer.  As

the life of the battery is greatly influenced by the chemical reactions taking place in the cell itself

i.e. formation of sulphates, effect of concentrated solution on the separators, temperatures, and

working conditions, it is extremely important to keep careful watch over the specific gravity of the

electrolyte.  The range of the concentration of the electrolyte for different types of batteries as

suggested are as follows:

Specific gravity of the Electrolyte for various types of Batteries

Type of Battery Sp Gravity

Stationary batteries 1.200 to 1.225

Truck and tractor batteries 1.260 to 1.280

Starting and lighting batteries (in-topics) 1.200 to 1.233

Aviation Batteries 1.260 to 1.285

Operating Principle of Lead-Acid Cells

1.                   The decomposition of molecules in to ions due to action of water is called electrolytic

dissociation

H2SO4            2H + + SO4-2 

2.                   Since the sum of the electric charges carried by the positive H2  ions is equal to the sum

of electric charge carried by the negative acid redical ions, the solution remains electrically

neutral.

3.                   As a result of interaction of electrolyte with the atoms of lead of the (-)ve plate, some of

the lead atoms become ionized and two charge (+)ve ions of lead pass over in to the electrolytic

solution.  This leaves the surface of the (-)ve plate becomes charged negatively in relation to the

electrolyte and the positive plate positively.

Battery Room

The battery room should be ready in all respects by fulfilling the following minimum requirements.

1.                   The walls and the ceiling of the battery room should be well black washed and should

remain clean and dry.

2.                   The flooring of the battery room shall be acid resistant tiles and material.

3.                   The battery room should be well lit.  there should be no direct sun light on the cells.

4.                   Suitable exhaust fans shall be fixed to provide a minimum of six air charges per hour.

5.                   The exhaust fans shall be suitably distributed and placed on the wall, which open to

atmosphere, equally sufficient air inlet should be provided to prevent any negative pressure

developing in the room.

6.                   Necessary blowers are to be provided to maintain sufficient air inlet in to room

7.                   Never the entrance door should be kept closed which will lead to a negative pressure

developing in the batter room due to the continuous operation of exhaust fans.

8.                   Inlet air should be free from effluents (such as chlorine, acetic acid).

IMPORTANT INSTRUCTIONS

1.                   Float charger should always be kept on even when the battery set is being boost

charged.

2.                   In case of failure of A.C. supply to the battery charger, the battery set should be boost

charged at the equalizing current rate for the same number of hours for which A.C. supply

remained failed.

3.                   The door of battery room should always be kept open when the exhaust fan is running. 

Door can be kept closed if sufficient inlet for air is provided in the door itself.

4.                   Add only distilled water to maintain cell electrolyte level, never add acid.

5.                   Temperature correction of sp. Gravity

S.G. (27 C)  =  S.G. (t) + 0.7 ( t - 27)

                        where  t  =   actual temp. of cell electrolyte

6.                   If inspite of repeated charging the specific gravity or voltage of some cell(s) does not

improve, these are to be considered weak cells.

7.                   Premature gassing in any cell is due to (-ve) plate of low capacity because of

sulphation.  Such cells should be removed from the battery set.

8.                   Adjustment of specific gravity: (a) Replacement of electrolyte with distilled water in

cells having high gravity (b) Replacing electrolyte in cells having low gravity with electrolyte from

cells having high gravity.

9.                   Never store acid i.e. electrolyte of sp. Gravity higher than 1200.

Life of the Lead Acid Battery

Factors, which influence the life of battery, are as follows:

(a)                 Overcharging of the battery reduces the life of battery due to the following reasons:

(i)                                                       Water present in electrolyte is decomposed into hydrogen and Oxygen

due to excessive charging.  Gas bubble thus formed remove active material from the plates.

(ii)                                                     Removal of water from the electrolyte on account of decomposition makes

the electrolyte more concentrated.  Concentrated acid is harmful to the separators and negative

plate material at high temperatures.

(iii)                                                    Due to excessive heat, positive plates, separator negative plates get

damaged.

(iv)                                                    Over charging results into buckling and warping and of the positive plates.

(v)                              Under charging results into the formation of sulphates which cannot be converted

electrochemically into active material and thereby causes strain on the positive plates.  Strained

positive plates are likely to get buckled.

(vi)                            If the battery remains idle for longer periods and is undercharged lead sulphates

are formed and may result into "shorts".  When such battery is recharged, there is likelihood of

excessive self-discharge.

(vii)                           Undercharged battery will not feed the required power for which it has been

installed.  Under serveres right conditions Electrolyte may freeze.

 

Safety Precautions

It is in the interest of the Undertaking to provide protective equipment to maintenance crew and

also keep them properly instructed about the precautions, which should be observed while

working on batteries.  Protective equipment and recommended safety precautions are listed

below:

(a)                 Protective equipment

(i)                                                       Goggles

(ii)                                                     Acid proof Gloves and Aprons

(iii)                                                    Water

(iv)                                                    Bicarbonate of Soda

(b)                 Precautions

(1)                                         While preparing electrolyte for lead acid batteries never pour water into the

acid.

(2)                                         Handles of tools required for tightening the bolts should be insulated.

(3)                                         Smoking, presence of naked flame should prohibited in the battery room

(4)                                         Battery room should be well ventilated and provided with exhaust fans

(5)                                         Acid should be stored in separate rooms

(6)                                         Cells should be installed on wooden racks painted with acid resisting paint

(7)                                         Cells should be insulated from the racks by placing insulators in between

(8)                                         Cells should be levelled during installation by using lead shims

(9)                                         Sulphuric acid containers are normally enclosed in wooden crates.  These

crates should be handled by two men without any regard to its weight and each crate should be

clearly marked with the label that it contains acid.

(10)                                     Never use metallic vessels while handling acid or distilled water.

 

General Care

(a)                Lead Acid Batteries

(1)                                         Electrolyte level must be maintained 10 to 15 mm above the plates.

(2)                                         Terminal voltage of the cell must not be allowed to fall below 1.85 volts.

(3)                                         Battery should be charged to its rated capacity.

(4)                                         Battery should not be allowed to remain in semi-discharged condition

(5)                                         Commercial sulphuric acid should not be used

(6)                                         Distilled water should be used for topping of the battery.

(7)                                         Excessive charging should be avoided

(8)                                         Bare and insulated leads should be painted with recommended paint

(9)                                         Battery should be kept clean and dry and battery room should be well

ventilated

(10)                                     Should acid be spelt or if any parts are wet, wipe over with wet rag and dry

thoroughly

(11)                                     Terminal posts and connectors should be clean and free from corrosion

(12)                                     Nuts and bolts of cell connectors should be kept tightened and smeared with

Vaseline.

 

B.                SYNCHRONOUS PHASE MODIFIER

 

There are ways to correct poor power factor, through capacitors or

synchronous condensers. The choice will be dependent upon the

application and budget. There are two main reasons to correct poor

power factor. The first is you are being charged a power factor penalty

by your local utility and this penalty is of such a size that you

want/need to do something about it. The second reason is that your

existing load capacity is or shortly will be at capacity and you are

staring at a large bill to increase it. Depending on the size of the power

factor correction (number of kVARs that need to be injected into the

electrical system) and the dynamic nature of the load, capacitors or a

synchronous condenser may be the solution. If the load is less than

500kVAR, then capacitors are easier to justify. Anything over 500kVAR,

a synchronous condenser should be considered. If there are large,

rapid and random swings in kVAR demand throughout the day, a

synchronous condenser is a better solution. If the changes in kVAR

demand are small and made in regular increments, then capacitors are

a better solution. When your capacity becomes a problem, the choice

of a solution will be dependent upon the size of the increase needed.

Again the 500kVAR rules holds true. Anything less than 500kVAR,

capacitors should be considered anything over, a synchronous

condenser. Like all power quality solutions, there are many factors that

need to be considered when determining which technology is best to

solve power factor problem.

The electrical power requirement by bulk consumers like heavy industries due to various loads in their

premises is of generally lagging pf in nature.  Lighting loads, heaters, furnaces etc. consume electrical

power almost at UPF.  Whereas a major share of their requirement is for Induction motor loads which

always works on lagging power-factors. Thus a Synchronous motor used to change the phase angle or

power-factor in the supply lines is called Synchronous Phase Modifier.

But more specific requirement, imposed by power supplying authority, is to reduce the lagging reactive

current and power drawn by the load.  To meet this requirement the Synchronous motor is to be operated

on over - excitation so as to draw leading reactive current and power from the supply lines.  The leading

reactive current and power drawn by the motor compensates the lagging current and power requirement of

the load.

A Synchronous motor operated at no-load with over-excitation condition to draw large leading reactive

current and power is called a Synchronous Condenser.

 

 

CONSTRUCTION AND WORKING

Motor has 6 salient poles and it is fitted with damper winding to start it as a 3 phase induction motor at

reduced voltage of 3-4 kV.  since the starting of synchronous motor is like an induction motor, so as to

reduce the initial in rush of current, a reduced voltage of 3-4 kV is applied at the starting.

A large oil cooled auto-transformer suplies the starting voltage of 3-4 kV to the motor, one the motor gains

sufficient speed full voltage of 11 kV is applied to make the machine run at rated speed of 1000 rpm. 

The bearings are lubricated by means of forced air or jetting of oil.   cooled air acts as a cooling medium for

the motor and water is used to cool oil.  Various protective relays i.e. high oil temperature, high winding

temperature, over heating are provided for the system.

 Since the current rating is 1050 A, so buses are coming from switchyard power transformer instead of

cables.  These buses are open (bare) and kept underground.

 

MOUNTING OF SYNCHRONOUS CONDENSER

The condenser is mounted on a cemented R.C.C. foundation for the purpose of economy there is an angle

latticework around the condenser for the purpose of repair and maintenance.

 

COOLING: The lower half of the condenser is in a closed cemented room in which forced air phenomena

is used which acts as cooling medium for condenser.

 

Cooling arrangements includes :

(1)Heat exchanger               (2) lubricating oil

(3)Radiators                   (4)Water cools the stator winding of machine.

 

PROTECTION

Various protective relays are provided for system oil high temperature,

winding high temperature, lubricating oil pump failure, starting

transformers heating are some of the most common factor for which

protective relays are provided in the unit.

 

WORKING

The working of the synchronous condenser can be divided into

following steps:

Jacking: It is a phenomenon of the synchronous condenser bearing with the help of forced or jetting oil

operation, the operation makes the main shaft of the motor float in bearing with a thin film of oil.   This

operation reduces friction at effects considerably and hence the starting current is reduced.

Starting: Starting operation is accomplished with the help of an auto-transformer designed to give start

within an hour starting transformer is put into circuit by closing the neutral of three windings. A voltage of

3.4 kV is fed into the damped winding and the motor starts, drawing a current of 3000 ampere (starting)

from the auto-transformer.

Running: Once the motor has gained the speed nearly equal to the synchronous speed, the neutral switch is

opened and the bridge circuit is then closed.  During the operation the connection between the network and

machine is maintained through the X'mer winding which now acts only as a reactor, the whole operation is

automatic and is provided with interlocking arrangements to overcome any fault or non-correct switching

of apparatus.

Power factor adjustment: Power factor adjustment can be carried out by controlling the exciter field

current.  Hence a stepless variation of capacitive or reactive power upto the rated power can be achieved.   It

should be noted that synchronous condenser has internally inheritly, sinusoidal waveform and the

harmonics in the waveform do not exist. 

 

 

 

 

Power Factor Improvement Using Synchronous Condensers

When the KVAR requirement is small, it can be met through static capacitors.   However when requirement

exceeds 10,000 KVAR it is generally more economical to use the synchronous condensers.

A synchronous condenser is essentially an over excited synchronous motor.  Generally it does not supply

any active mechanical power.  The excitation of the machine is varied to provide the necessary amount of

the leading kVAR.  The advantages and disadvantages of using synchronous condensers as compared to

static capacitors are as under :

A synchronous condenser can supply kVAR equal to its rating and absorb kVAR upto 50 per cent of its

capacity.  Thus a synchronous condenser of certain kVAR is equal to a static capacitor of that kVAR and a

shunt reactor of 50 per cent kVAR.

By the use of synchronous condenser a finer control is possible than by the use of static capacitors.

A synchronous condenser can be overloaded for short periods but a static capacitor cannot be overloaded

A momentary drop in voltage causes the synchronous condenser to supply greater kVAR to the system

whereas in the case of static capacitor, the kVAR supplied is reduced.

The inertia of the synchronous condenser improves the system stability and reduces the effect of sudden

changes in load.      

The power loss in a synchronous condenser is much greater than that in a capacitor.

For small kVAR requirements, static capacitors are preferable and economical.  For requirements above

10,000 kVAR or so synchronous condensers are more economical.

Static capacitor installations can be distributed in the system.  Thus capacitors can be located near the loads

and are more effective.  However small size synchronous condensers are very uneconomical.  As such the

synchronous condensers have to be installed at one point only.

The rating of static capacity bank can be changed very easily as per requirements.   Capacitor units can be

added to the bank or taken away from it.  This is not possible with synchronous condensers.

Installation of a static capacitor bank is easy.

A failure of one unit of capacitor bank affects that unit only.  The

remaining units continue to do their job.  However failure of a

synchronous condenser means loss of total condenser capacity.  On

the other hand the failure rate of a synchronous condenser is very

small as compared to the failure rate of a capacitor bank.

Synchronous condenser adds to the short circuit currents in the system and increases the circuit breaker

ratings.

Synchronous condensers are mostly used by utilities at large sub-

stations to improve the power factor and voltage regulation.  Machines

upto 100 MVAR ratings or even higher have been used.  The field

current is regulated automatically to give a desired voltage level.  A

typical instance is of 150 MW to be transmitted over a distance of 240

km.  If the receiving end power factor is 0.85, the sending end power

factor is 0.68 and sending end voltage 1.5 times receiving end

voltage.  Addition of 75 MVAR synchronous condenser at receiving end

improves the sending end power factor to 0.88 and reduces the

voltage drop in transmission line by 50 per cent. 

 

 

 

 

 

 

 

 

 

 

APPENDICES Appendix 1

 

Definition of the terms used

 

1 Admittance: Admittance is the reciprocal of impedance.

The ratio of R.M.S current to the R.M.S

electromotive force which produces it.

It is given as: -

Y=I/V or Y=1/Z if Z=V/I

Where Y is admittance, I is current, V is

voltage and Z is the impedance.

2 Conductance It is the component of the current in phase

with the applied voltage divided by applied

voltage or the resistance divided by the

square of the impedance.

3 Susceptance: It is the component of current in quadrature with the

applied voltage divided by the voltage or the reactance

divided by the square of the impedance.

4 Phase sequence: It is the order in which the phase of polyphase system

reach a maximum voltage in the same direction.

5 Power factor:

 

It is defined as the cosine of the angle of lag of the

current, because it is the factor by which the apparent

power must be multiplied in order to obtain the true

power.

6 Synchronous speed: It is the speeds of an A.C. machine which corresponding to

the speed of rotation of the magnetic flux. It is related to the

frequency in cycle per second and number of poles in the

machine.

7 Primary relay: Primary relay are those which are connected directly in

the protected circuit.

8 Secondary relay: These are connected to the protected circuit through

current transformer and potential transformer.

9 Back-up relay A relay, which operates, usually after a slight delay, if

the normal relay does not operate to trip concerned

circuit breaker.

10 Instantaneous relay It is that relay that operate in less than 0.1 second, and

does not have intentional delay.

11 Time delay relay A relay which is equipped with delaying elements.

12 Bulking relay It is the device, which prevents the protecting relay

from tripping, either due to its own characteristics or to

an additional relay.

13 Operating current The minimum value of current at which the relay picks

up is called as operating current.

14 Drop out current The maximum value of current at which the relays drop

out is called the drop out current

15 Re-set ratio It ratio of drop out current to the pick up current

16 Recovery voltage It is the voltage, which appears across the terminal of

each poles of circuit breaker immediately after the

braking of the circuit.

17 Rate of rise of recovery

voltage (R.R.R.V.):

The rate of rise of recovery voltage is a rate expressed

in volts per micro second representative of the increase

of the recovery voltage, the assessment being made in

accordance with a prescribed method.

18 Spinning reserve Reserve generating capacity connected to the bus and

ready to take load.

19 Demand factor= maximum demand

    Connected Load

20 Group Diversity Factor =   Sum of individual maximum demand

    Maximum demand of the group

 

21 Peak Diversity Factor =  Maximum demand of consumer group

     Demand of the consumer group at the  time of

system peak demand

22 Load Factor =  Average Load

     Peak Load

23 Capacity Factor =   Average annual load

    Rated Plant Capacity  

              or

=   Maximum Load     X  Load Factor     

     Plant Capacity  

             or

= (Utilization factor) X(Load Factor )

24 Utilization Factor =  Maximum Load  

    Rated Plant Capacity

 

 

 

 

 

 

 

 

LIST OF ABBREVIATIONS

                   A.B.C.B.            Air Blast Circuit Breaker

A.C. Alternating Current

A.En. Assistant engineers

C.B. Circuit Breaker

C.T. Current transformer

Ckt. Circuit

CVT Constant Voltage Terminal

D.C. Direct Current

dia. Diameter

E.H.V. Extra High Voltage

Engg. Engineer

Ex.En. Executive engineer

G.S.S. Grid sub-station

H.P. Horse Power/ High Pressure

H.V. High voltage

Hz Hertz

I/P Input

J.En. Junior engineer

KV Kilovolt

KW Kilowatt

L.P. Low Pressure

L.V. Low voltage

M.O.C.B. Minimum Oil Circuit Breaker

MVA Mega volt ampere

O.B. Air blast cooling

O.C. Open Circuit

O.F.B. Forced oil air blast cooling

O.F.W. Forced oil water-cooling

O.N. Natural cooling

O/P Output

p.f. Power factor

P.L.C.C. Power line carrier communication

P.T. Potential transformer

R.M.S. Root mean square value

R.R.R.V. Rate of rise of recovery voltage

R.S.E.B. Rajasthan state electricity board

S.C. Short Circuit

S.E.B. State electricity board

S.G. Specific Gravity

S.R.B.P. Synthetic resin bounded paper

Temp. Temperature

U.H.V. Ultra High Voltage

V.T. Voltage transformer

VA Volt Ampere

W.T.I. Winding temperature indicator

wdg Winding

x-mers Transformer

 

 

 

                                                           

 

 

 

 

 

  

 

CONCLUSION 

The power sector of the country has been set on a path of vigorous and sustainable growth with

self-reliance in all technological aspect. The country has aim at energy self-sufficiency. The

growth in power sector helped millions of people in the country to secure employment. It is

anticipated that a large part of the nation's wealth would be in power sector.

A necessary and important aspect of engineering course is practical training. It gives an engineer

to face and tackle actual problems in the industries and field. The institute where the student

studies can't provide him that practical knowledge on all always of learning. Until and unless the

student has been exposed to its practical aspects, the study of the subject is incomplete. The

artificial studies built the engineer in him by providing him pools of knowledge where as the

practical application makes him agile and complete. If theoretical knowledge teaches the principal

and policies, the practical exposure tells how to use those policies and principal in practical

atmosphere. During training session the student learn to work in human laboratory. 

 As the case of practical training there is a major difference between

theoretical and practical knowledge, hence practical training is taken at a

company or industry to get familiar with engineering organizational and

technical practices in the field. This made a relation between theory being

taught in the class and application in the field along with the organizational

and financial aspect of industry. theoretical knowledge without any

practical implementations is just like a bird with one fan.

So, I thought to take my practical training at 132kv G.S.S.  R.R.V.P.L. Jawahar Nagar Jaipur. I

learnt there a lot. I came to know about CT and PT, distribution system, various transformers,

relays, circuit breakers, conductors, LA etc. etc.

In Jawahar Nagar G.S.S. every event added a lot to my practical knowledge during my training

session. As during the practical session, maintenance period was going on thus it made a right

direction to understand and make a clear and absolute approach about various equipment's and

their fittings and features installed there. This made a perfect relation between theoretical and

practical knowledge.