gss herapura report

INDEX

LAYOUT pg. no

1. INTRODUCTION [SUB STATION] -1

2. BUS BARS -4

3. ISOLATORS -7

4. PROTECTIVE RELAYS -11

5. CIRCUIT BREAKERS -16

6. POWER TRANSFORMER -25

7. CURRENT TRANSFORMER -31

8. CAPACITIVE VOLTAGE TRANSFORMERS -33

9. TRASNFORMER OIL AND ITS TESTING -36

10. LIGHTENING ARRESTORS -38

11. CONTROL ROOM -41

12. EARTHING OF THE SYSTEM -44

13. POWER LINE CARRIER COMMUNICATION -46

14. CORONA -47

CONCLUSION

REFERENCES

INTRODUCTION

Electric power is generated, transmitted and distributed in the form of alternating

current. The electric power produced at the power stations is delivered to the

consumers through a large network of transmission & distribution.

The transmission network is inevitable long and high power lines are necessary to

maintain a huge block of power from source of generation to the load centers to

inter connected. Power house for increased reliability of supply greater.

The assembly of apparatus used to change some characteristics (e.g. voltage, ac to

dc, frequency, power factor etc.) of electric supply keeping the power constant is

called a sub-station.

Depending on the constructional feature, the high voltage sub-stations may be

further subdivided:

a) Out door substation.

b) Indoor substation.

c) Basement or Underground substation.

Dept. of Electrical Engg. 1

fig 1

220 KV G.S.S. SANGANER

1) It is an outdoor type substation.

2) It is primary as well as distribution substation.

3) One and half breaker scheme is applied.

The power mainly comes from HIRAPURA-1 and HIRAPURA-2 & KOTA

THERMAL .


Out going feeders

1) One feeder of 220kv to KTPS

2) One feeder of 220kv to Sakatpura.

3) One feeder of 220kv to KTPS2

4) One feeder of 220kv to Phulera

5) One feeder of 400kv to Merta

6) One feeder of 220kv to Sanganer

At this substation following feeders are established.

1. TIE FEEDERS.

2. RADIAL FEEDERS.

TIE FEEDERS:

There are 220KV tie feeders as follows.

1.220 KV KOTA-JAIPUR 1st & 2nd

2. Inter state 220KV KOTA –DELHI

3. Tie from 220 KV Heerapura.

4. 220KV KTPS first & second.

RADIAL FEEDERS

1. 220 KV JAIPUR –KOTA 1st & 2nd feeders

2. 132KV KOTA –BUNDI 1st

3. 132KV KOTA –SAWAI MADHOPUR 1st & 2nd

4.132 KV KOTA –SANGOD

5. 132 KV KOTA –MORAR


BUS BARS

Bus Bars are the common electrical component through which a

large no. of feeders operating at same voltage have to be connected.

If the bus bars are of rigid type (Aluminum types) the structure heights are low

and minimum clearance is required. While in case of strain type of bus bars

suitable ACSR conductors are strung / tensioned by tension insulator discs

according to system voltages. In the widely used strain type bus bars stringing

tension is about 500 - 900 kg depending upon the size of conductor used.

Here proper clearance would be achieved only if require tension is achieved. Loose

bus bars would effect the clearances when it swings while over tensioning may

damage insulators. Clamps or even effect the supporting structures in low

temperature conditions.

The clamping should be proper, as loose clamp would spark under in full load

condition damaging the bus bars itself.

BUS BAR ARRANGEMENT MAY BE OF FOLLOWING TYPES WHICH

ARE BEING ADOPTED BY R.R.V.P.N.L

1.) Single bus arrangement.

2.) Double bus bar arrangement.

a) Main bus with transformer bus.


b.) Main bus-I with Main bus-II.

3.) Double bus bar arrangement with auxiliary bus.

DOUBLE BUS BAR CONTAINING MAIN BUS I WITH MAIN BUS II:

1. Each load may be fed from either bus.

2. The load circuits may be divided in two separate groups if needed from

operational consideration. Two supplies from different sources can be put on each

bus separately.

3. Either bus bar may be taken out from maintenance and cleaning of insulators.

This arrangement has been quite frequently adopted where the loads and continuity

of supply is necessary. In such a scheme a bus coupler breaker is mostly provided

as it enables on load change over from one bus bar to other.

The normal bus selection isolators cannot be used for breaking load currents. The

arrangement does not permit breaker maintenance without causing stoppage of

supply.

DOUBLE BUS BAR ARRANGEMENTS CONTAINS MAIN BUS WITH

AUXILIARY BUS:

The double bus bar arrangement provides facility to change over to either bus to

carry out maintenance on the other but provide no facility to carry over breaker

maintenance. The main and transfer bus works the other way round .It provides

facility for carrying out breaker maintenance but does not permit bus

maintenance. Wherever maintenance is required on any breaker the circuit is


changed over to the transfer bus and is controlled through bus coupler breaker.

fig 2


ISOLATORS

Isolators which are also called disconnect switches or air break

switches after the assembly as per drawings on the leveled structures the

adjustment of connecting pipes, moving and fixed contacts is done so that all the

three phase of the isolator close and open simultaneously and there is a full surface

contact between moving and fixed contacts. Such switches are generally used on

both sides of equipment in order that repairs and replacement of the equipment can

be made without any danger. They should never be opened until the equipment in

the same circuit has been turned off and should always be closed before the

equipment is turned on.

The adjustment of the tendon pipes leveling of post insulator, stop holts in the

fixed contacts etc. is done for smooth operation of insulator. Following type of

insulator are being used in R.S.E.B-

a) Isolator without earth blades.

b) Isolator with earth blade.

c) Tendon isolator.

INSULATORS

The insulators for the overhead lines provide insulation to the power conductors

from the ground so that currents from conductors do not flow to earth through

supports. The insulators are connected to the cross arm of supporting structure and


the power conductors passes through the clamp of the insulator. The insulators

provide necessary insulation between line conductors and supports and thus

prevent any leakage current from conductors to earth. In general, the insulators

should have the following desirable properties:

1. High mechanical strength in order to withstand conductor load, wind load

etc.

2. High electrical resistance of insulator material in order to avoid leakage

currents to earth.

3. High relative permittivity of insulator material in order that dielectric

strength is high.

4. The insulator material should be non porous, free from impurities and cracks

otherwise the permitivity will be lowered.

5. High ratio of puncture strength to flash over.

These insulators are generally made of glazed porcelain or toughened glass. Poly

come type insulators [solid core] are also being supplied in place of hast insulators

if available indigenously. The design of the insulator is such that the stress due to

contraction and expansion in any part of the insulator does not lead to any defect. It

is desirable not to allow porcelain to come in direct contact with a hard metal

screw thread.

TYPES OF INSULATORS:

There are three types of insulators used for overhead lines:


1. Pin type- pin type insulator consists of a single or multiple shells adapted

to be mounted on a spindle to be fixed to the cross arm of the supporting

structure.

When the upper most shell is wet due to rain the lower shells are dry and

provide sufficient leakage resistance. These are used for transmission and

distribution of electric power at voltage up to voltage 33KV. Beyond

operating voltage of 33KV the pin type insulators thus become too bulky

and hence uneconomical.

Fig 3.1

2. Suspension type- suspension type insulators consist of a number of

porcelain disc connected in series by metal links in the form of a string.

Its working voltage is 66KV. Each disc is designed for low voltage for

11KV.


Fig 3.2

3. Strain insulator- the strain insulators are exactly identical in shape with

the suspension insulators. These strings are placed in the horizontal plane

rather than the vertical plane. These insulators are used where line is

subjected to greater tension. For low voltage lines (<11kV) shackle

insulators are used as strain insulator.

Fig 3.3


PROTECTIVE RELAYS

A Protective relay is a device that detects the fault and initiates the operation of the

circuit breaker to isolate the defective element from the rest of the system.

The relays detect the abnormal condition in the electrical circuits by constantly

measuring the electrical quantities i.e. voltage, current, frequency, phase angle

which are different under normal and fault conditions. Having detected the fault,

the relay operates to close the trip circuit of the breaker, which results in opening

of the breaker and disconnection of the faulty circuit.

Relay circuit connections can be divided in three parts:

1.) Primary winding of a C.T. that is connected in series with the line to be

protected.

2.) Secondary winding of C.T. and the relay operating coil.

3.)Third part is the tripping circuit, which may be either a.c. or d.c. . It consists of a

source of a supply, the trip coil of a circuit breaker and the relays stationary

contacts.

When a short circuit occurs at point F on the transmission line the current

increases to enormous value. This results in a heavy current flow through the relay

coil, causing the relay to operate by closing its contacts. This in turn closes the trip

circuit of the breaker, making the C.B. open and isolating the family section from


the rest of the system. In this way, the relay ensures the safety of the circuit

equipment

from damage and normal working of the healthy portion of the system.

Fig 4

Basic qualities that a protective relay must possess are:

1.) Selectivity

2.) Speed

3.) Sensitivity

4.) Reliability


5.) Simplicity

6.) Economy

DIFFERENTIAL RELAYS

A differential relay is one that operates when the phasor difference of two or more

similar electrical quantities exceeds a predetermined value.

Thus the current differential relay is one that compares the current entering and

current leaving the section. Under normal operating conditions, the two currents

are equal but as soon as fault occurs, this condition is no longer applied.

The difference between the incoming and outgoing currents is arranged to flow

through the operating coil of the relay. If this differential current is equal to or

greater than the pick up value, the relay will operate and open the C.B. to isolate

the faulty section.

BUCHHOLZ RELAY

It is a gas-actuated relay installed in oil immersed transformers for protection

against all kinds of faults. it is used to give an alarm in case of incipient (i.e. slow

developing)faults in the transformer and to disconnect the transformer from the

supply in the event of severe internal faults. it is usually installed in the pipe

connecting the conservator to the main tank. It is a universal practice to use

BUCHHOLZ relay on all such oil immersed transformers having ratings in excess

of 750kVA.


CONSTRUCTION

It takes the form of a domed vessel pipe between the main tank and the

conservator. The device has two elements. the upper element consists

of a mercury type switch attached to a float. The lower element contains

a mercury switch mounted on a hinged type flap located in the direct path

of the flow of oil from the transformer to the conservator. the upper element

closes an alarm circuit during incipient faults whereas the lower element is

arranged to trip the circuit breaker in case of server internal faults.

OPERATION

The operation of Buchholz relay is as follows:

(i)In case of incipient faults within the transformer, the heat due to fault

causes the decomposition of some transformer oil in the main tank the

products of decomposition contain more than 70% of hydrogen gas. the

hydrogen gas being light tries to go into the conservator and in the process

gets entrapped in the upper part of the relay chamber. when a pre determined

amount of gas gets accumulated, it exerts sufficient pressure on the float to

cause it tilt and close the contacts of the mercury switch attached tom it.

This completes the alarm circuits to to sound an alarm.

(ii)If a serious fault occurs in the transformer, enormous amount of gas

is generated in the main tank. The oil in the main tank rushes to the

conservator via the Buchholz relay and in doing so tilts the flap to close


the contacts of the mercury switch. This completes the trip circuit to open

the circuit breaker controlling the transformer.

ADVANTAGES

(i) It is the simplest form of transformer protection.

(ii) It detects the incipient faults at a stage much earlier than possible with

other forms of protection.

DISADVANTAGES

(i) It can only be used with oil immersed transformers equipped with conservator

tanks.

(ii) The device can detect only faults below oil level in the transformer. therefore

separate protection is needed for connecting cables.


CIRCUIT BREAKERS

Thus circuit breakers are used for switching & protection of various parts of power

system. Circuit breaker is a piece of equipment, which can

1) Make or break a circuit either manually or automatically under normal

condition.

2) Break a circuit automatically under fault condition

3) Make a circuit either manually or by remote control under fault conditions.

OPERATING PRINCIPLES

A C.B. consists of fixed and moving contacts called electrodes. Under

normal operating conditions, these contacts remain closed and will not open

automatically until and unless the system becomes faulty. When a fault occurs on

any part of the system, the trip coils of the circuit breaker get energised and the

moving contacts are pulled apart, thereby opening the circuit.

When the contacts of the C.B. are seperated under fault conditions, an arc is

struck between them. The current is thus able to continue until the discharge

ceaeses. The production of arc not only delays the current interruption process but

it also generates enormous heat which may cause damage to the system or to the

C.B.

It is thus necessary to extinguish the arc within the shortest possible time so that


the heat generated by it may not reach a dangerous value.

ARC PHENOMENON

When a short circuit occurs, a heavy current flows through the contacts of the C.B.

before they are opened by the protective system. At the instant when the contacts

begin to separate, the contact area decreases rapidly and large fault current causes

increased current density and hence rise in temperature. The heat produced in the

medium

between contacts is sufficient to ionize the arc or vaporize and ionize the oil. The

ionized air or vapour acts as conductor and an arc is set between the contacts. The

potential difference between the contacts is quite small and is sufficient to maintain

the arc. the arc provides a low resistance path and as a result the current in the

circuit remains uninterrupted so long as the arc persists.

During the arcing period the current flowing between the contacts depends

on the arc resistance. The greater the arc resistance, the smaller the current that

flows between the contacts. The arc resistance depends upon:

(i) Degree of ionization.

(ii) Length of arc.

(iii) Cross section of arc.


CLASSIFICATION OF THE CIRCUIT BREAKERS:

There are several ways of classifying the circuit breakers. However, the most

general way of classification is on the basis of medium used for arc extinction.

The medium used for arc extinction is usually oil, air, sulphur hexafluoride (SF6)

or vacuum. Accordingly, circuit breakers may be classified into:

They are generally classified on the basis of the medium used for arc elimination

(i) Oil circuit breakers, which employ some insulating oil for arc extinction.

(ii) Air-blast circuit breakers in which high pressure air blast is used for

extinguishing the arc.

(iii) Sulphur hexa fluroide C.B. in which SF6 gas is used for arc extinction.

(iv) Vacuum C.B. in which vacuum is used for arc extinction.

SULPHUR HEXAFLOURIDE (SF6) CIRCUIT BREAKER

In such breakers, sulphur hexaflouride (SF6) gas is used as the arc quenching

medium. The sf6 is an electro-negative gas and has a strong tendency to absorb

free electrons. The SF6 circuit breakers have been found to be very effective for

high power and high voltage service.


CONSTRUCTION

The cylindrical large size steel tanks are mounted horizontally parallel to

each other. Each tank consists of SF6 under pressure. The interruption is of multi

break type & is placed along the axis of each tank. The interruption assembly is

supported inside the tank by the vertical bushing, which are mounted near the end

of each tank. Gas at high pressure is supplied to the interrupter from a gas

reservoir.

The bushing are also insulated with SF6 the conductor is in the from of

copper tube supported at both end by porcelain shields. SF6 gas is supplied from

the high pressure tanks. Shields are provided with gasket seals to eliminate leakage

of gas from beginnings.


Fig 5

WORKING

In the closed position of the breaker the contacts remain surrounded by SF6 gas at

a pressure of about 2.8 kg/sq cm. When the breaker operates, the moving contact is

pulled apart and an arc is struck between the contacts. The

movement of the moving contact is synchronised with the opening of a valve

which permits SF6 gas at 14kg/sq cm pressure from the reservoir to the arc

interruption chamber. the high pressure flow of SF6 rapidly absorbs the free


electrons in the arc path to form immobile negative ions which are ineffective as

charge carriers. The result is that the medium between the contacts quickly builds

up high dielectric strength and causes the extinction of the arc. After

the breaker operation the valve is closed by the action of a set of springs.

400 KV SF6 C.B. [RATINGS]: -

Manufacture: BHEL Hyderabad.

Type: HLR245/2503 B.S.

Rated voltage: Normally 420 KV, maximum 440 KV.

Rated frequency: 50 HZ.

Rated power frequency: voltage: 520 KV

Rated Impulse withstand voltage:

Lightning: 1425KV

Switching: 1050KV

Normal current rating

At 50 c ambient: 2240Amps

At 40 c ambient: 2500Amps


Short time current rating: 40 KA for 3 sec.

Rated operating duty: 0 to 0.3 sec. c-0-3min-mb.

Rated short circuit duration: 1 sec.

BREAKING CAPACITY [BASED ON SPECIFIED DUTY CYCLE]:

(a) Capacity at rated voltage: 29000MVA [440KV].

(b) Symmetry current: 40 KA.

(c) Asymmetry current: 49 KA.

Making capacity: 100KA [peak]

Rated pressure of hydraulic operating (gauge): 250-350bar.

Rated pressure of SF6 gas at degree: 7.5bars.

Weight of complete breaker: 11700 Kg.

Weight of SF6 gas: 76.5Kg.

Rated trip coil voltage: 220 V. AC.

Rated closing voltage: 220 V. DC.

First poll to clear factor: 1.3

ADVANTAGES OF SF6 CIRCUIT BREAKER:


1. Due to the superior arc quenching property of SF6, such circuit breakers

have very short arching time.

2. Since the dielectric strength of SF6 gas is 2 to 3 times that of air, such

breakers can interrupt much larger currents.

3. The SF6 circuit breakers gives noiseless operation due to its closed gas

circuit and no exhaust to the atmosphere unlike the air blast circuit breaker.

4. The closest gas enclosure keeps the interior dry so that there is no moisture

problem.

5. There is on risk of fire in such breakers because SF6 gas is not inflammable.

There are no carbon deposits so that tracking and insulation problems are

eliminated.

6. The SF6 breakers have low maintenance cost, light foundation requirement

and minimum auxiliary equipment.

7. Since SF6 breakers are totally enclosed and sealed from atmosphere they are

particularly suitable where explosion hazard exists e.g., coal mines.

DEMERITS OF SF6 CIRCUIT BREKER:

1. Sealing problems arise due to the type of the construction used.

2. The presence of moisture in the system is very dangerous to SF6

circuit breaker.

3. Arced Sf6 gas is poisonous & should not be let out.

4. The double pressure SF6 CB is cost liner due to complex gas system.

5. The internal parts should be cleaned thoroughly during periodic maintenance

under clean dry environment.


6. Dust of Teflon & sulfide should be removed.

7. Special facilities are needed for transporting the gas.

APPLICATIONS

SF6 C.B. have been developed for voltages 115 KV to 230 KV, power ratings 10

MVA to 20 MVA and interrupting time less than 3 cycles.

S.NO I.E. MAKE TYPE VOLTAGE CURRENT STC SF6/HYD

1 552A 3AT3 3AT3 420/520 2000A 40KA/S 7.5/350

2 552T DO DO DO DO DO DO

3 552B MG FAR2 DO 3150A DO 7/300

4 452T NGEF S2M420 420/610/1425 2000A DO 8/35

5 252A BHEL 3AT3 420/520/1050 DO DO 7.5/350

6 252B ABB EL(V) 420/1050 3150 40KA/3S 7/31.5


POWER TRANSFORMER

The transformer is a static apparatus, which receives power/energy at it, one circuit

and transmits it to other circuit without changing the frequency. With this basic

conception we can use the voltages at our desired level while utilizing the power.

As, the voltage used to generate at modern power houses at 11 KV or so and

afterwards we get it step up at a level of 33 KV, 66 KV, 132 V, 220 KV or 400

KV, 750 KV for transmission to minimize the distribution losses. Again we get it

step down with the help of transformer to use at our wishes at 11 KV, 6.6 KV or

even 415, 230 volts at our houses.

BASIC PARTS OF TRANSFORMER

The following are the inherent parts of a modern day transformer:

1. Primary and secondary coils (circuit) or windings.

2. Core

3. Main Tank

4. Conservator

5. Breather

6. Radiator

7. Buchholz relay

8. Explosive vent


9. Bushings (HT & LT) (Primary or secondary)

10. Cooling fans

11. Tap changer (on load and off load)

12. NGR (Neutral Grounding Resistance) to minimize the earth fault current

Fig 6.1

DESCRIPTION OF PLANT:

The three transformer are oil immersed with rating of 250 MVA

& one with 315 MVA. However a synchronous loading of 100MVA at 0.8 power

factor (lag) and 18 MVA 0.8 pf (lag) on the tertiary can also be loaded to 20MVA

loading with 100MVA 0.8 pf on LV without exceeding the generated temperature

rise.


The transformer is also provided with a separate bank of

radiation, fans, and associated control equipments. The control equipments are

housed in a tank mounted miscalling.

Fig 6.2

RATING DATAS.

Type of cooling: ONAN / ONAF/ ODAF

MVA

HV: 189 / 252 / 315

IV: 189 / 252 / 315

LV: 63 / 84 / 105

VOLTS


HV: 400 KV

IV: 220 KV

LV: 33 KV

LINE AMPERES

HV: 273 / 364 / 455

IV: 497 / 662 / 828

LV: 1104 / 1471 /1839

IMPEDANCE VOLTAGE

HV to IV 12.65% on 315 MVA Base

HV to LV 39.16 % on 315MVA Base

IV to LV 26.66 % on 315 MVA Base

NUMBER OF PHASES

Three HV, LV, IV

FREQUENCY IN Hz

50 Hz

YEAR OF MANUFACTURE: 1985

Mass of Core & Windings: 1,32,000 kg

Mass of Oil: 65,150 kg


Total weight: 261,200 kg

Oil in tank: 73,200 kg

Oil in radiator: 8400 kg

Oil in tap changer: 83,850 kg

Transportation mass: 168,000 kg

Unmaking height: 7760 mm

Unmaking mass 18000 Kg

Guaranteed maximum temperature rise of:

Oil 45ºC

Winding 50ºC

COOLING FANS:

Rating: 2000 m3 of air per minute.

Type: 915 mm dial GEC (India) make.

Numbers per transformer: two

Fan motor: direct on line starts weather proof.

Squirrel cage IM 1400 W 400/440

Volt 3- , 50 Hz 720 rpm

PUMPS:

Rating: 1818 liters per minute.

Type: a landless A to 8c sentiment.


Number of pump per transformer: one working, one standby.

Pump motor: direct on line starts weather proof.

Squirrel cage IM


CURRENT TRANSFORMER

These transformers are used with low range ammeter to measure currents in high

voltage alternating current circuits where it is not practicable to connect

instruments and meters directly to lines. In addition to insulating the instrument

from the high voltage line, they step down the current in the known ratio. The

current (or series) transformers has a primary coil of 1 or more turns of thick wires

connected in series with the line whose current is to be measured. The secondary

consist of a large number of turns of fine wire and is connected across the ammeter

terminals (usually of 5 amp bracket should be removed or 1 amp range)

Fig 7


POTENTIAL TRANSFORMER

These transformers are extremely accurate ratio step down transformers and are

used in conjunction with standard low range voltmeter (usually 150 volt) whose

deflection when divided by voltage transformation ratio, gives the true voltage on

the high voltage side. In general, they are of the shell type and do not differ much

from the ordinary two winding transformer, except that their power rating is

extremely small. Up to voltage of 5000 potential transformers are usually of dry

type, between 5000 and 13800 volts, they may be either dry type or oil immersed

type, although for voltage above 13800 they are oil type. Since their secondary

windings are required to operate instruments or relays or pilot lights, their ratings

are usually 42 to 100 watts.


CAPACITIVE VOLTAGE TRANSFORMERS (CVT)

Capacitive voltage transformers are special kind of power

transformers using capacitors to step down the voltage.

DESCRIPTION:

The capacitive voltage transformer comprises of a capacitor divider with

its associated electromagnetic unit. The divider provides an accurate

proportioned voltage, while the magnetic unit transforms this voltage, in both

magnitude and phase to convenient levels suitable for measuring, metering,

protection etc. all WSI capacitor units has metallic bellows to compensate the

volumetric expansion of oil inside. The porcelain in multi unit stack, all the

potential points are electrically tied and suitably shielded to overcome the effect

of corona RIV etc. Capacitive voltage transformers are available for system

voltages of 33 KV to 420KV.


Fig 8

APPLICATION:

1. Capacitive voltage transformers can be effectively as potential sources for

measuring ,metering, protection, carrier communication and other vital

functions of an electrical network.

2. CVT are constructed in single or multi unit porcelain housing with there

associated magnetic units. For EHV systemcuts are always supplied in multi

unit construction.

3. In case of EHV cuts the multi unit system has many advantage easy to

transport and storing, convenience in handling.

RATING OF CVT

Voltage: 22/sqrt 3 KV

Total o/p: 500MVA


Operating voltage: 400/sqrt 3 max.

Voltage factor: 1.5/30 sec.

Test voltage: 630 KV for 1 min

Impulse withstands voltage: 1.2/ 50 s. 1425KV max.

Frequency : 50Hz

High frequency capacitance: 4400pF

Primary capacitance: 4657pF

Secondary capacitance: 80000 pF

S no Ie Make Ratio Burden Class Sec cap

1 Bassi Wsi/cve/420

/1425

400 200,200,

100

3p,3p,0.5 80000pf

2 Bassi 2 Wsi/cve/420

/1425

400 200,200,

100

3p,3p,0.5 80000pf

3 Bus 1 Wsi/cve/420

/1425

400 200,200,

100

3p,3p,0.5 80000pf

4 Bus 2 Wsi/cve/420

/1425

400 200,200,

100

3p,3p,0.5 80000pf


TRANSFORMER OIL & ITS TESTING

The prime function of oil is to convey the heat from the core and winding to the

tank where it can be dissipated. Besides these, the oil provides additional insulation

between primary and secondary windings. So, the oil must be completely free from

dirt, moisture and other un-wanted solid matter. The oil used in the transformer is

natural mineral oil and should undergo the following tests if required:

BREAKDOWN VOLTAGE:

The voltage at which the oil breaks down when subjected to an electric field.

FLASH POINT:

The temperature, at which the oil gives off so many vapors, when mixed with air

forms an ignitable mixture and gives a momentary flash with small pilot flame.

For checking above values, various tests are done. These are

categorized as:

1. Physical test.

2. Chemical test.

3. Electrical test.


The results must be close to standard results that are follows-

S.N TYPE OF TEST STD. RESULTS

1. Density (gm/cubic cm.)at 27C .85 to .89

2. Flash point >125C

3. B.D.V Test K.V (rms.) >50 KV

4. Tan delta at 90C < 20%

5. Water content (PPM.) 25(max.)above 145KV

6. Gas contents (PPM.)

(a) Hydrogen 100 to150

(b) Methane 50 to 70

(c) Ethane 30 to 50

(d) Ethylene 100 to 150

(e) Acetylene 20 to 30

(f) Carbon dioxide 3000 to 3500

(h) Carbon mono-oxide 200 to300

LIGHTENING ARRESTORS


An electric discharge between cloud and earth, between cloud and the charge

centers of the same cloud is known as lightening.

The earthing screens and the ground wires can well protect the electrical system

against direct lightening strokes but they fail to provide protection against

travelling waves which may reach the terminal apparatus. The lightening arrestors

or the surge diverters provide protections against such surges.

THYRITE TYPE:

Ground wire run over the tower provides an adequate protection

against lighting and reduce the induced electrostatic or electromagnetic voltage but

such a shield is inadequate to protect any traveling wave, which reaches the

terminal of the electrical equipment, and such wave can cause the following

damage.

1 the high peak of the surge may cause a flashover in the internal wiring

thus it may spoil the insulation of the winding .

2 the steep wave front may cause internal flash over between their turns of

transformer.

3 The stop wave front resulting into resonance and high voltage may cause

internal or external flashover causing building up the oscillator is the

electrical operation.

Lightening arrestors are provided between the line and earth provided the

protection against traveling wave surge the thyrite lightening arrestor are provided


at GSS. This type of LA has a basic cell made of thirties, which is a particular type

of clay, mixed with carborendum. Thirties has a particular property of being

insulator one voltage

At high voltage It will behave like a conducting material the electrical resistance of

thyrite depends upon the voltage each time the voltage is made twice the resistance

decrease in such a manner as to allow an increased current of 12.5 times the

change in current is independent of rate of application voltage and its instantaneous

value.

The above law is followed by this material without any limit on the voltage

increase and after the surge has passed the thyrite againretain its original property

A standard cell is rated for 1KV and is formed into a disc, which is sprayed on

both the sides of to give good contact with each disc. The dimensions of the discs

are stacked i.e. 16 cm in diameter and 17.5 cm thick these discs are stacked one

upon each other and they are further placed in to a porcelien container with a

suitable arrangement of gap between them.

These gaps serves as the purpose of preventing any current flow during

normal operating voltage in case of any transients the gap are punctured. The

Thyrite type arrestor will discharge several thousands ampere without the slightest

tendency of flashover on the edges of most important of the advance is that there is

absolutely no time lag in its performance.

400KV LIGHTNENIG ARRESTOR


manufacture: English electric company

no of phases: one

rated voltage: 360 KV

nominal discharge current (8×20µs) 10KA

high current impulse(4×110 µs ) 100KA

long duration rating(200 µs) 500KA

Sno Ie Make Type Current Voltage

1 Bassi1 Wsi Cpl 10KA 360KV

2 bassi2 Elpro Alugard2 10KA 360KV

3 ILT1 Elpro Alugard2 10KA 360KV

4 ILT2 Elpro Alugard2 10KA 360KVh

5 ILT3 WSI CDV303 10KA 398KV

6 ILT4 WSI CDV03 10KA 398KV

CONTROL PANEL


The diagram made on the control panel is known as mimic diagram.

COLOUR CODING

* 33KV GREEN

* 132 KV BLACK

* 220KV BROWN

* 440 VOLTS VOILET/INDIGO

* 110 VOLTS ORANGE

REACTOR

It is used to lower the over excited capacitor. Capacitor bank is connected in shunt

over the reactor. Capacitors main purpose is to boost up the voltage. so when we

want to lower the voltage we use reactors. it is also use to stop the sudden change.

the commonly used reactor is NGR(Neutral ground reactor).

CIRCUIT BREAKER

There is a one and half breaker scheme i.e. 3 breakers for 2 buses used in 400 KV

G.S.S.

BUS COUPLERS


It is used to equalize the load on both Bus bars.

DISTURBANCE RECORDER

It records the distance & fault on graph with voltage w.r.t time.

EVENT LOGGER

it monitors as well as provides the details as a printed material.

These details may contain the sequence of operation, switching time, closing time

etc.

ON LOAD TAP CHANGER (OLTC)

In this method a number of tappings are provided on the secondary of the

transformer. The voltage drop in the line is supplied by changing the secondary

emf of the transformer through the adjustment of its number of turns by using

transition resistor

which are placed in between each tapping.

In supply system, tap changing has to be performed on load so that here is no

interruption to supply. By using transition resister therefore shut down is not

required.


Fig 11

NO LOAD TAP CHANGER (NLTC)

in this we change the tap manually for which we have to shut down the

transformer.

When the load increases the voltage across the primary drops but the secondary

voltage can be kept at the previous value by placing the movable arm on to a

higher stud. Whenever a tapping is to be changed in this type of transformer, the

load is kept off and hence the name off load tap-changing transformer.

SYNCHRONOSCOPE

A synchronoscope is used to determine the correct instance of closing the switch

with connect the new supply to bus bar the correct instance of synchronizing is

indicated when bus bar and incoming voltage

* are equal in magnitude

* are equal in phase

* have the same frequency

the phase sequence is same


EARTHING OF THE SYSTEM:

The provision of an earthling system for an electric system is necessary by the

following reason.

1 In the event of over voltage on the system due to lightening discharge or

other system fault. These parts of equipment, which are normally dead, as

for as voltage, are concerned do not attain dangerously high potential.

2 In a three phase, circuit the neutral of the system is earthed in order to

stabilize the potential of circuit with respect to earth.

The resistance of earthling system is depending on

1 Shape and material of earth electrode used.

2 Depth in the soil

3 Specific resistance of soil surrounding in the neighborhood of system

electrodes.

PROCEDURE OF EARTHING:

Technical consideration the current carrying path should have enough capacity to

deal with more faults current. The resistance of earth and current path should be

low enough to prevent voltage rise between earth and neutral. The earth electrode

must be driven into the ground to a sufficient depth to as to obtain lower value of

earth resistance. To sufficient lowered earth resistance a number of electrodes are

inserted in the earth to a depth they are connected together to form a mesh. The

resistance of earth should be for the mesh in generally inserted in the earth at 0.5m

depths the several point of mesh then connected to earth electrode or ground

conduction. The earth electrode is metal plate copper is used for earth plate.

Neutral Earthing:

Neutral earthing of power transformer all power system operates with

grounded neutral. Grounding of neutral offers several advantages the neutral point


of generator transformer is connected to earth directly or through a reactance in

some cases the neutral points is earthed through an adjustable reactor of reactance

matched with the line. The earthling is one of the most important feature of system

design for switchgear protection neutral grounding is important because:

1 The earth fault protection is based on the method of neutral earthling.

2 The neutral earthling is associated switchgear.

3 The neutral earthling is provided for the purpose of protection arcing

grounds unbalanced voltages with respect to protection from lightening

and for improvement of system.


POWER LINE CARRIER COMMUNICATION

As electronics plays a vital role in the industrial growth, communication is also a

backbone of any power station, communication between various generating and

receiving station is very essential for proper operation of power system. This is

more so in case of a large interconnected system where a control load dispatch

station has to coordinate

the working of various units to see that the system is maintained in the optimum

working condition, power line communication is the most economical and reliable

method of communication for medium and long distance in a power network.

PLCC system in Rajasthan: -

1 HEERAPURA: JAIPUR, AJMER, BYAWAR, BHILWARA, PALI, JODHPUR

2 HISSAR: KHETRI, HEERAPURA, KOTA, RAPP

3 HEERAPURA: KOTS, JSP, RPS, GSD

4 BHILWARA: RPS

5 PALI: FALANA

6 HEERAPURA: ALWAR, BHARATPUR

7 NEEMUCH: DEBARI

8 DEBARI: SIROHI

9 DEBARI: ZAWAR MINES

10 HEERAPURA: SIKAR, RATANGARH, BIKANER

11 HANUM, ANGARH: HISSAR, SHRIGANGANAGAR

12 HEERAPURA: BADHERPUB


CORONA EFFECT

When an alternating potential difference is applied across two conductors whose

spacing is as large as compared to their diameters, there is no apparent change in

the condition of atmospheric air surrounding the wires if the applied voltage is low.

However when the applied voltage exceeds a certain value called critical

disruptive voltage, the conductors are surrounded by a faint violet glow called

corona.

The phenomenon of corona is accompanied by a hissing sound, production

of ozone, power loss and radio interference. The higher the voltage is raised, the

larger and higher the luminous envelope becomes, and greater are the sound, the

power loss and the radio noise. If the applied voltage is increased to breakdown

value, a flash over will occur between the conductors due to the breakdown of air

insulation.

The phenomenon of violet glow, hissing noise and production of ozone gas

in an overhead transmission line is known as corona.

If the conductors are polished and smooth, the corona glow will be uniform

throughout the length of the conductors, otherwise the rough points will appear

brighter. The positive wire has uniform glow about it, while the negative

conductors has spotty glow.


FACTORS AFFECTING CORONA

The phenomenon of corona is affected by the physical state of the atmosphere as

well as by the conditions of the line. The following are the factors on which corona

depends:

1. Atmosphere. In the stormy weather, the number of ions is more than normal

and as such corona occurs at much less voltage as compared with fair

weather.

2. Conductor size. The rough and irregular surface will give rise to more

corona because unevenness of the surface decreases the value of breakdown

voltage.

3. Spacing between conductors. Larger space between conductors reduces the

electro-static stresses at the conductor surface, thus avoiding corona

formation.

4. Line voltage. If the line voltage is low, there is no chance in the condition of

air surrounding the conductors and hence no corona is formed.

ADVANTAGES AND DISADVANTAGES OF CORONA

Corona has many advantages and disadvantages. In the correct design of a high

voltage overhead line, a balance should be struck between the advantages and

disadvantages.


Advantages

1. Due to corona formation, the air surrounding the conductor becomes

conducting and hence virtual diameter of the conductor is increased. The

increased diameter reduces the electro-static stresses between the

conductors.

2. Corona reduces the effect of the transients produced by surges.

Disadvantages

1. Corona is accompanied by a loss of energy. This affects the transmission

efficiency of the line.

2. Ozone is produced by corona and may cause corrosion of the conductor due

to chemical action.

3. The current drawn by the line due to corona is non-sinusoidal and hence

non-sinusoidal voltage drop occurs in the line. This may cause inductive

interference with neighboring communication lines.


CONCLUSION

A technician needs to have not just theoretical but practical as well and so

every student is supposed to undergo a practical training session after III year

where I have imbibed the knowledge about transmission, distribution, generation

and maintenance with economical issues related to it.

During our 30 days training session we were acquainted with the repairing of

the transformers and also the testing of oil which is a major component of

transformer.

At last I would like to say that practical training taken at 220KV GSS has

broadened my knowledge and has widened my thinking as a professional.

REFERENCES:Principles of Power System-by V.K.MEHTA


Electrical Power System-by C.L.WADHWA

REPORT BY-

Kapil Kumar

SKIT,JAIPUR


gss herapura report

Education