Project source 2 with 1.5

A Mini Project report

On

PROTECTION OF TRANSFORMER (132/33 KV SUBSTATION, CHINTAL)

Submitted in partial fulfillment of the

requirement for the award of degree of

BACHELOR OF TECHNOLOGY

in

ELECTRICAL & ELECTRONICS ENGINEERING

by

M.SHIVA KUMAR 11611A0217

N.KARTHIK 11611A0220

T.PRASHANTH KUMAR 11611A0228

V.BHARATH 11611A0232

DEPARTMENT OF ELECTRICAL & ELECTRONICS

ENGINEERING

P.R.R.M ENGINEERING COLLEGE (Affiliated to Jawaharlal Nehru Technological University, Hyderabad)

Shabad,R.R.Dist – 509217,T.S.

Page | 1

1.INTRODUCTION

1.1 TS Transco

The erstwhile Andhra Pradesh State Electricity Board which came

into existence in 1959 was responsible for Generation, Transmission and Distribution

of Electricity. Under Electricity Sector Reforms Agenda, Government of Andhra

Pradesh promulgated Andhra Pradesh Electricity Reforms Act,1998.The erstwhile

APSEB was unbundled into one Generation Company (APGENCO),One

Transmission Company(APTRANSCO) and Four Distribution Companies

(APDISCOMs) as part of the reform process.

APTRANSCO came into existence on 1.02.1999.From Feb 1999 to

June 2005 APTRANSCO remained as single buyer in the State-Purchasing power

from various Generators and selling it to DISCOMs in accordance with the terms and

conditions of the individual PPAs at Bulk Supply Tariff (BST) rates. Subsequently, in

accordance with the Third Transfer Scheme notified by GOAP, APTRANSCO ceased

to do power trading and has retained powers of controlling system operations of

Power Transmission.

As per AP Reorganization Act 2014, APTRANSCO was divided

into TSTRANSCO and APTRANSO. Accordingly TSTRANSCO was established as

a Company w.e.f 02.06.2014 for the State of Telangana.

1.2 Substation and Layout

In modern power system to have a normal operation of the system

without electrical failure and damage to the equipment two alternators are available

with the designer, one is to design the system so that faults cannot occur and other is

to accept the possibility of faults and take steps to guard against the ill effects of such

faults. The main objective of our mini project work is to study the protection of

transformer in 132/33kv substation. Protective scheme required for the protection of

power system components against abnormal conditions such as faults etc., consists of

circuit breakers. In 132/33kv substation they used SF6 circuit breaker at HV side and

VCB at LV side. The auxiliaries like isolators, lightening arrestors, CT’s, PT’s,

control panel and indicating instruments are used.

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The assembly of apparatus used to stabilize the voltage from all

the components like harmonics, transients etc .,is called sub-station. Substation is

important part of power system. The continuity of supply depends to a considerable

extent upon the successful operation of substations. It is, therefore, essential to

exercise utmost care while designing and building a substation.

Fig. 1.1 Overview of Substation

We did the project at 132/33 KV substation which is located at Chintal near HMT

Tractor manufacturing company. The main feature of this substation is that it is very

near to the main load center. It is well designed, such that it consists of requisite area

for feature expansion.

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Fig. 1.2 Single Line Diagram

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2. POWER TRANSFORMER

2.1Construction

A transformer is an electrical device that transfers energy between

two or more circuits through electromagnetic induction without change in frequency.

A varying current in the transformer's primary winding creates a

varying magnetic flux in the core and a varying magnetic field impinging on the

secondary winding. This varying magnetic field at the secondary induces a varying

electromotive force(emf) or voltage in the secondary winding. Making use of

Faraday's Law in conjunction with high magnetic permeability core properties,

transformers can thus be designed to efficiently change AC voltages from one voltage

level to another within power networks.

Fig. 2.1 31.5 MVA Fig. 2.2 50 MVA

In 132/33kv substation we use the power transformer of rating 31.5

MVA and 50 MVA.

For the construction of transformer different types of cores are used.

They are :-

1) Laminated steel cores

2) Solid cores

3) Toroidal cores

4) Air cores

Page | 5

In this we use laminated steel type of core. The core, which

provides the magnetic path to channel the flux, consists of thin strips of high-grade

steel, called laminations, which are electrically separated by a thin coating of

insulating material. The strips can be stacked or wound, with the windings either built

integrally around the core or built separately and assembled around the core sections.

Fig. 2.3 Schematic of three-phase core form construction

Thickness ranges from 0.23 mm to upwards of 0.36 mm. The core

cross section can be circular or rectangular, with circular cores commonly referred to

as cruciform construction. Rectangular cores are used for smaller ratings and as

auxiliary transformers used within a power transformer. Rectangular cores use a

single width of strip steel, while circular cores use a combination of different strip

widths to approximate a circular cross-section. Just like other components in the

transformer, the heat generated by the core must be adequately dissipated.

While the steel and coating may be capable of withstanding higher

temperatures, it will come in contact with insulating materials with limited

temperature capabilities. In larger units, cooling ducts are used inside the core for

Page | 6

additional convective surface area, and sections of laminations may be split to reduce

localized losses.

The core is held together by, but insulated from, mechanical

structures and is grounded to a single point in order to dissipate electrostatic buildup.

The core ground location is usually some readily accessible point inside the tank, but

it can also be brought through a bushing on the tank wall or top for external access.

This grounding point should be removable for testing purposes, such

as checking for unintentional core grounds. Multiple core grounds, such as a case

whereby the core is inadvertently making contact with otherwise grounded internal

metallic mechanical structures, can provide a path for circulating currents induced by

the main flux as well as a leakage flux, thus creating concentrations of losses that can

result in localized heating.

The maximum flux density of the core steel is normally designed as

close to the knee of the saturation curve as practical, accounting for required

overexcitations and tolerances that exist due to materials and manufacturing

processes.

For power transformers the flux density is typically between 1.3 T

and 1.8 T, with the saturation point for magnetic steel being around 2.03 T to 2.05 T.

2.1.1 Winding

The conducting material used for the windings depends upon the

application, but in all cases the individual turns must be electrically insulated from

each other to ensure that the current travels throughout every turn. For small power

and signal transformers, in which currents are low and the potential difference

between adjacent turns is small, the coils are often wound from enamelled magnet

wire, such as Formvar wire. Larger power transformers operating at high voltages

may be wound with copper rectangular strip conductors insulated by oil-impregnated

paper and blocks of pressboard.

Power-frequency transformers may have taps at intermediate points on

the winding, usually on the higher voltage winding side, for voltage adjustment. Taps

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may be manually reconnected, or a manual or automatic switch may be provided for

changing taps. Automatic on-load tap changers are used in electric power

transmission or distribution, on equipment such as arc furnace transformers, or for

automatic voltage regulators for sensitive loads.

2.1.2 Insulation Drying

Construction of oil-filled transformers requires that the insulation

covering the windings be thoroughly dried of residual moisture before the oil is

introduced. Drying is carried out at the factory, and may also be required as a field

service. Drying may be done by circulating hot air around the core, or by vapor-phase

drying (VPD) where an evaporated solvent transfers heat by condensation on the coil

and core.

For small transformers, resistance heating by injection of current into

the windings is used. The heating can be controlled very well, and it is energy

efficient. The method is called low-frequency heating (LFH) since the current used is

at a much lower frequency than that of the power grid, which is normally 50 or 60 Hz.

A lower frequency reduces the effect of inductance, so the voltage required can be

reduced. The LFH drying method is also used for service of older transformers.

2.1.3 Bushings

Larger transformers are provided with high-voltage insulated bushings

made of polymers or porcelain. A large bushing can be a complex structure since it

must provide careful control of the electric field gradient without letting the

transformer leak oil.

Name Plate Details :- 31.5 MVA

MVA 31.5\19 STANDARD REF.IS 2026

VOLTS AT H.V NO LOAD

L.V

132000 FREQUENCY 50HZ

33000 TYPE OF COOLING ONAF/ONAN

AMPERES H.V L.V

137.78\83.80 MAX TAP

%IMPEDANCE NOR TAP

MIN TAP

12.60

551.10\332.4 12.48

10.33

PHASES H.V

L.V

3 WEIGHT OF

COPPER kg.

9000

3

VECTOR GROUP YNyn0 WEIGHT OF CORE kg.

16500

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DIAGRAME DRG. NO. TA3/01377 SHIPPING WEIGHT (OIL FILLED) kg.

50000

HV-BIL 550 KVP SHIPPING WEIGHT (GAS FILLED) kg.

39000

LV-BIL 170 KVP CORE & WINDING

kg.

28000

YEAR OF MANUFACTURI

NG

2010 VOLUME OF OIL ltr.

17500

OLTC MAKE CTR WEIGHT OF OIL kg.

14500

BUSHING

HV/LV MAKE

AREVA/CJI TOTAL

WEIGHT OF TRANSFORMER kg.

58000

UNIQUE NO. TA-31500\16

CUSTMER’S REF : CE/C/400 KV/P&MM 11/e-EHVT-30/2009/TAL/210-67 Dt.08-03-2010

GUARANTEED MAXIMUM TEMPARATURE RISE OF OIL : 50 oC

GUARANTEED MAXIMUM TEMPARATURE RISE OF WINDING :

55 oC

Name Plate Details :- 50 MVA

K.V.A 50 FREQUENCY HZ 50

RATED 30000/50000 IMPEDANCE AT 75oC 13.66

VOLTS H.V

L.V

132000 VECTOR GROUP YNyno

33000 CORE & WINDING

Kg.

41000

AMPERES H.V

L.V

131.22/218.7 WEIGHT OF OIL kg.

14000

524.8/874.8 TOTAL WEIGHT 75000

PHASES H.V

L.V

3 TRANSPORT MASS 65000

3 OIL IN LITERS 16600

TYPE OF

COOLING

ONAF/ONAN YEAR OF

MANUFACTURE

2011

OPERATING PRESSURE OF PRESSURE RELIEF DEVICE kg/cm2

0.42-0.49

GUARANTEED MAXIMUM TEMP RISE IN OIL/WDG oC 50/55

CUSTOMER

REF

P.O.NO. PM-0209/CE/LI/P & MMII/e-LIPT-16/2011

Page | 9

2.1.4 Breather

Fig. 2.4 Breather

The insulating oil of transformer is provided for cooling and insulating

purpose. Expansion and contraction of oil during the temperature variations cause

pressure change inside the conservator. This change in pressure is balanced by the

flow of atmospheric air into and out of the conservator. Transformer breather is a

cylindrical container which is filled with silica gel. Insulating oil reacts with moisture

can affect the paper insulation or may even lead to some internal faults. So it is

necessary that the air entering the tank is moisture free. It consists of silica gel

contained in a chamber. For this purpose breather is used. When the atmospheric air

passes through the silica gel breather the moisture contents are absorbed by the silica

crystals. Silica gel breather is acts like an air filter for the transformer and controls the

moisture not to enter into a transformer. It is connected to the end of breather pipe.

2.1.5 Conservator

Conservator conserves the transformer oil. It is an airtight metallic

cylindrical drum which is fitted above the transformer. The conservator tank is vented

to the atmosphere at the top and the normal oil level is approximately in the middle of

the conservator to allow expansion and contraction of oil during the temperature

variations. It is connected to the main tank connected from inside the

transformer(internally) which is completely filled with transformer oil through a

pipeline. The main function of conservator tank of transformer is to provide adequate

space for expansion of oil inside the transformer.

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Fig. 2.5 Conservator tank

2.1.6 Cooling System

The main source of heat generation in transformer is its copper loss or

I2R loss. Although there are other factors contribute heat in transformer such as

hysteresis & eddy current losses but contribution of I2R loss dominate them. If this

heat is not dissipated properly, the temperature of the transformer will rise continually

which may cause damages in paper insulation and liquid insulation medium of

transformer. So it is essential to control the temperature with in permissible limit to

ensure the long life of transformer by reducing thermal degradation of its insulation

system. In electrical power transformer we use external transformer cooling system to

accelerate the dissipation rate of heat of transformer.

Fig. 2.6 Radiator and Fans

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Radiators are used to cool the transformer oil. The transformer oil is

circulated through the them. The circulation of the oil may either be natural or forced

circulation. In natural circulation, when the temperature of the oil raises the hot oil

naturally moves to the top and the cold oil moves downwards. Thus the oil keeps on

circulating through the tubes. In Heat dissipation can obviously be increased, if

dissipating surface is increased but it can be make further faster by applying forced air

flow on that dissipating surface. Fans blowing air on cooling surface is employed.

Forced air takes away the heat from the surface of radiator and provides better cooling

than natural air. The full form of ONAF is "Oil Natural Air Forced". As the heat

dissipation rate is faster and more in ONAF transformer cooling method than ONAN

cooling system, electrical power transformer can be put into more load without

crossing the permissible temperature limits.

A) Oil Temperature Indicator or OTI

This device is used to measure the top oil temperature. An oil temperature indicator or

OTI is also used for protection of transformer.

Operating principle of Oil Temperature Indicator :

This device measures top oil temperature with the help of sensing bulb immersed in

the pocket by using liquid expansion in the bulb through a capillary line to operating

mechanism. A link and lever mechanism amplifies this movement to the disc carrying

pointer and mercury switches. When volume of the liquid in operating mechanism

changes, the bellow attached to end of capillary tube expands and contracts. This

movement of bellow is transmitted to the pointer in temperature indicator of

transformer through a lever linkage mechanism.

B) Winding Temperature Indicator or WTI

This device measures the LV and HV winding temperature. A winding temperature

indicator or WTI is also used as protection of transformer.

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Fig. 2.7 Winding Temperature Indicator

Operating Principle of Winding Temperature Indicator :

The basic operating principle of WTI is same as OTI. But only difference is that the

sensing bulb pocket on transformer top cover is heated by a heater coil surrounded

it.This heater coil is fed by secondary of current transformer associated with

transformer winding. Hence the current through the heater coil is directly proportional

to the current flowing through transformer winding. This is because there is no scope

of direct measuring of temperature inside a winding. When load of transformer

increases, the winding temperature is also increased and this increased temperature is

sensed by sensing bulb as the heater coil surrounds it. Rest of the working principle of

winding temperature indicator is same as principle of oil temperature indicator.

2.1.7 Faults in Power Transformer

Causes of faults in power transformer

Transformers are prone to variety of faults :

1. The most common type of fault being the winding to core faults because of

weakening of insulation. Phase faults inside the transformers are rare. However,

such faults may occur on terminals, which fall within the transformer protection

zone.

2. Power transformers are generally provided with on-line tap changing (OLTC) gear.

This is another major area of occurrence of fault.

3. All large transformers are oil immersed type. There is a possibility of oil leakage.

4. Transformers experience large inrush currents that are rich in harmonic content at

the time of switching if they happen to be unloaded.

5. A transformer may develop inter turn faults giving rise to local hot spots within the

winding.

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6. Transformers may suffer from over fluxing due to under frequency operation at

rated voltage. Over fluxing may also be caused when the transformer is subjected

to over voltage at the rated frequency.

7. In case of sustained overload conditions, the transformer should not be allowed to

operate for long duration.

(A) Restricted Earth Fault Protection A percentage differential relay has a certain minimum value of pick up

for internal faults. Faults with current below this value are not detected by the relay.

Winding-to-core faults, which are single phase to ground type, involving high

resistance, fall in this category.

Therefore for such type of faults RESTRICTED EARTH FAULT

PROTECTION is used. The reach of such a protection must be restricted to the

winding of the transformer; otherwise it may operate for any ground fault, anywhere

in the system, beyond the transformer, hence the name of this scheme.

Fig. 2.8

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Fig. 2.9 Earth fault protection for the delta side of delta star transformer

(B) Over Current Protection

Over current protection is used for the purpose of providing back up

protection for large transformers. (above 5MVA).Two phase fault and one ground fault relay is sufficient to provide OC protection to star delta transformer.

Fig. 2.10 Over-current protection of a transformer

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(C) Protection Against Overfluxing

The magnetic flux increases when voltage increases. This results in

increased iron loss and magnetizing current. The core and core bolts gets heated and

the lamination insulation is affected. Protection against overfluxing is required where overfluxing due to sustained overvoltage can occur. The reduction in frequency also increases the flux density and thus has the same effect of overfluxing.

The expression for flux in a transformer is given by

Φ = K E/f

Where Φ = flux, f = frequency, E = applied voltage and K is a constant.

To control flux, the ratio E/ f is controlled. When the ratio exceeds a threshold value,

it has to be detected. Electronic circuits with suitable relays are available to measure this ratio. Overfluxing does not require high speed tripping and hence instantaneous

operation is undesirable when momentary disturbances occur. But the transformer should be isolated in one or two minutes at the most if overfluxing persists.

(D) Protection Against Overheating

The rating of a transformer depends on the temperature rise above an assumed maximum ambient temperature. Sustained overload is not allowed if the

ambient temperature is equal to the assumed ambient temperature. The maximum safe overloading is that which does not overheat the winding. The maximum allowed

temperature is about 95°C. Thus the protection against overload depends on the winding temperature which is usually measured by thermal image technique.

In thermal image technique, a temperature sensing device like silicon resistor is placed in the transformer oil near the top of the transformer tank. A CT is employed on the L.V. side to supply current to a small heater. Both the temperature sensing

device and the heater are placed in a small pocket. The silicon resistor is used as an arm of a resistance bridge supplied from the stabilized dc source. An indicating

instrument is energized from the out of balance voltage of the bridge. Also the voltage across the silicon resistor is applied to a static control circuit which controls cooling pumps and fans, also gives warning of overheating ,in case of failure of cooling

system and ultimately trips the transformer circuit breakers.

(E) Protection Against Incipient Faults

Incipient Faults: Faults which are not serious at the beginning but which slowly develops into serious faults are known as incipient faults.

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2.2 Protection

2.2.1 Buchholz Relay

It is a protective device container housed over the connecting pipe

from main tank to conservator tank. It is used to sense the faults occurring inside the

transformer. It is a simple relay which is operated by the gases emitted due to the

decomposition of transformer oil during internal faults. It helps in sensing and

protecting the transformer from internal faults

Fig. 2.11 Buchholz Relay

Buchholz relay in transformer is an oil container housed in the

connecting pipe from main tank to conservator tank. It has mainly two elements. The

upper element consists of a float. The float is attached to a hinge in such a way that it

can move up and down depending upon the oil level in the Buchholz relay Container.

One mercury switch is fixed on the float. The alignment of mercury switch hence

depends upon the position of the float.

The lower element consists of a baffle plate and mercury switch. This

plate is fitted on a hinge just in front of the inlet (main tank side) of Buchholz relay in

transformer in such a way that when oil enters in the relay from that inlet in high

pressure the alignment of the baffle plate along with the mercury switch attached to it,

will change.

In addition to these main elements a Buchholz relay has Gas Relief Cock

(GRC) on top. The electrical leads from both mercury switches are taken out through

a molded terminal block.

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Fig. 2.12 Circuit Diagram

Operation:

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

relay chamber. When a predetermined amount of gas gets accumulated, it exerts

sufficient pressure on the float to cause it to tilt and close the contacts of mercury

switch attached to it. This completes the alarm circuit to sound an alarm.

If a serious fault occurs in the transformer ,an enormous amount of gas is generated in

the main tank. The oil in the main tank rushes towards the conservator via the

Buchholz relay and in doing so tilts the flap to close the contacts of mercury switch.

This completes the trip circuit to open the circuit breaker controlling the transformer.

Advantages:

It is the simplest form of transformer protection.

It detects the incipient faults at a stage much earlier than is possible with other forms

of protection

Disadvantages:

It can only be used with oil immersed transformers equipped with conservator tanks.

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The device can detect only faults below oil level in the transformer. Therefore,

separate protection is needed for connecting cables.

2.2.2 Differential protection

This scheme is employed for the protection of transformers against

internal short circuits. It provides the best overall protection for internal faults.

However in case of ungrounded or high impedance grounding it cannot provide

ground fault protection.

The following factors affect the differential current in transformers and

should be considered while applying differential protection.

These factors can result in a differential current even underbalanced power in & out

conditions:

1.Magnetizing inrush current– The normal magnetizing current drawn is 2–5% of

the rated current. However during Magnetizing inrush the current can be as high as

8–30times the rated current for typically 10 cycles, depending upon the transformer

and system resistance.

2.Overexcitation–This is normally of concern in generator–transformer units.

Transformers are typically designed to operate just below the flux saturation level.

Any further increase from the max permissible voltage level (or Voltage/Frequency

ratio), could lead to saturation of the core, in turn leading to substantial increase in

the excitation current drawn by the transformer.

3.CT Saturation – External fault currents can lead to CT saturation. This can cause

relay operating current to flow due to distortion of the saturated CT current.

4. Different primary and secondary voltage levels, that is the primary & secondary

CT’s are of different types and ratios

5. Phase displacement in Delta-Wye transformers.

Transformer Differential Relay

To account for the above variables less sensitive Percentage Differential Relays with

percentage characteristics in the range of 15 to 60% are applied to transformers.

Additionally, in modern microprocessor and numeric relays harmonic restraints can

be applied.

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Transformer Differential Relay Connections:

Fig. 2.13 Percentage Differential Relay Connections

Harmonic Restraint:

The percentage differential scheme tends to maloperate due to

magnetizing inrush. The inrush current waveform is rich in harmonics whereas the internal fault current consists of only the fundamental component. So to solve the

problem of inrush current, which is neither an abnormal condition nor a fault, additional restraint is developed which comes to picture only during inrush condition and is ineffective during internal faults.

2.6 Pressure Relief Valve

Defination

Pressure relief devices are specially designed to release pressure inside

the transformer developed during the incipient faults to reduce the risk of explosion of

the transformer itself.

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Fig. 2.14 Pressure relief valve

In case of sudden and uncontrolled increase in pressure inside the

transformer, the pressure relief device allows the discharge of insulating fluid in

milliseconds time facilitating the decrease of the pressure. It is highly recommended

to convey the outlet insulating fluid in order to preserve the environment and to

reduce the risk of fire.

This device can be used in:

Power oil insulated transformers with oil volume from 3000 to 25000 dm3. We

recommend using multiple pressure relied devices when insulating fluid exceed this

level.

There are smaller sizes available of pressure relief devices for distribution

transformers.

Description and general specifications:

Aim of the safety valves on electric transformers

The transformer tank filled with cooling liquid is a container subject to

internal pressure and then has to be provided with one or more safety valves suitably

calibrated for the maximum allowed pressure, so that over pressure caused by internal

faults can be instantaneously relieved through the valves, thus avoiding greater

damages such as the deformation or even the burst of the tank and the spraying of hot

oil with subsequent fire risks. It is necessary to protect the transformer tank with a

suitable equipment capable of almost instantaneously discharging overpressure can be

overcome by using special equipment called pressure relief valve.

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General features

The safety valves consist schematicaly of :

a valve base comprising the valve opening venting area with its special y profiled

gasket and a seat for an O-ring gasket on the flanged end towards the transformer’s

tank.

a valve cap pressed against the profiled gasket by calibrated helical spring, thus

making the valve completely tight up to the rated pressure.

a splash diverter to avoid damages caused by hot oil sprinkles (on request).

a single or a double electrical contact.

Mechanical protection degree : IP 65

Insulation : 2000V 50Hz between terminals and

earth for a 60 seconds time

Cable gland : PG 13,5

Microswitch breaking capacity : 10 A 250V AC

1 A 125V DC

Safety valves - types

The safety valves are built with different Major Diameters and rated

pressure to satisfy the requirements of the various applications.

TYPE MAJOR

DIAMETER

RATED

PRESSURE

PREVAILING

USE

T125 - VS 150

125mm. 0,3 /1 bar big power transformers

VS 100 100 mm 0,3 /1 bar Medium power

transformers

T80 - VS 80 80mm 0,3 /1 bar small power transformers

T 50 - VS 50 50mm 0,3 /1 bar cable boxes - small

tanks

Safety valves series VS : The advantage of safety valves series VS consists in the total

absence of projecting parts in the transformers tank making the mounting point

choose easier. These valves can be mounted, with regards to their base plane, both

horizontal y on the cover and vertical y on the transformer walls at the points where

their safety action is presumed to be more necessary.

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Safety valves series T : The advantage of safety valves series T is that, showing a

good effectiveness and reliability of valves series VS, their simplicity consent o

obtain very competitive prices.

Operating instruction and maintenance:

Instruction for mounting safety valves :-

The data about the transformer point where a short circuit is most

likely to occur, the preferential direction the resulting shock wave may have, the

intensity this one can reach, all depend on the transformer power, its transformation

ratio, its construction characteristic and the behaviour of the other installed protection

equipment.

Therefore, is not possible to give strict rules about safety valves application. It is the

manufacturer who must decide each time and on his own experience the valve type

and its position.

Mounting and maintenance :-

The safety valves mounting is carried out by means of the suitable

fastening holes of the flange, after the splash diverter removal and after the insertion

of the O-ring gasket supplied with the valve. After the transformer filling, the air

developed under the valve must be breathed by unscrewing the suitable breathing

screw. This breathing screw shall be tightened again as soon as the oil starts to come

out. During operation the safety valves do not ne d a particular maintenance.

Nevertheless , it is convenient to regularly check the electric contact go d operation

and to verify if there is no gas accumulation.

Instruction for ordering safety valves :-

The exhaust rated diameter shall be connected with the transformer oil

quantity and with the number of mounted valves. When a single valve is mounted the

barycentric posit on, with regards to the points where a failure is most likely to occur,

must be chosen.

Page | 23

3. LIGHTNING ARRESTER

Lightning arrester is a device used on electrical power systems and

telecommunications systems to protect the insulation and conductors of the system

from the damaging effects of lightning. The typical lightning arrester has a high-

voltage terminal and a ground terminal. When a lightning surge (or switching surge,

which is very similar) travels along the power line to the arrester, the current from the

surge is diverted through the arrestor, in most cases to earth. Here we used the latest

revolutionary type of Lightning Arrester i.e., metal oxide arrestor (MOA).

Smaller versions of lightning arresters, also called surge protectors, are

devices that are connected between each electrical conductor in power and

communications systems and the Earth. These prevent the flow of the normal power

or signal currents to ground, but

provide a path over which high-

voltage lightning current flows,

bypassing the connected

equipment. Lightning that strikes

the electrical system introduces

thousands of kilovolts that may

damage the transmission lines,

and can also cause severe damage

to transformers and other

electrical or electronic devices.

Lightning-produced extreme

voltage spikes in incoming power

lines can damage electrical home

appliances. Lightning arresters built Fig. 3.1 Lighting Arresters

for power substation use are impressive devices, consisting of a porcelain tube several

feet long and several inches in diameter, typically filled with disks of zinc oxide. A

safety port on the side of the device vents the occasional internal explosion without

shattering the porcelain cylinder.

Page | 24

4. ISOLATORS

In electrical engineering, a disconnector, disconnect switch or isolator

switch is used to ensure that an

electrical circuit is completely

de-energised for service or

maintenance. Such switches are

often found in electrical

distribution and industrial

applications, where machinery

must have its source of driving

power removed for adjustment or

repair. High-voltage isolation

switches are used in electrical

substations to allow isolation of

apparatus such as circuit breakers,

transformers, and transmission

lines, for maintenance. The

disconnector is usually not intended Fig. 4.1 Isolators

for normal control of the circuit, but only for safety isolation. Disconnector can be

operated either manually or automatically (motorized disconnector). Unlike load

break switches and circuit breakers, disconnectors lack a mechanism for suppression

of electric arc, which occurs when conductors carrying high currents are electrically

interrupted. Thus, they are off-load devices, intended to be opened only after current

has been interrupted by some otherontrol device. Safety regulations of the utility must

prevent any attempt to open the disconnector while it supplies a circuit. Standards in

some countries for safety may require either local motor isolators or lockable

overloads (which can be padlocked).

In high-voltage or complex systems, these padlocks may be part of a

trapped-key interlock system to ensure proper sequence of operation.

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5. CIRCUIT BREAKERS

A circuit breaker is an automatically operated electrical switch in

combination with relay designed to protect an electrical circuit from damage caused by

overload or short circuit. Its basic function is to detect a fault condition and interrupt

current flow. Unlike a fuse, which operates once and then must be replaced, a circuit

breaker can be reset (either manually or automatically) to resume normal operation.

Circuit breakers are made in varying sizes, from small devices that protect an

individual household appliance up to large switchgear designed to protect high

voltage circuits feeding an entire city.

In this substation we use two types of circuit breakers for high voltage

side and low voltage side. They are:-

1. Gas Circuit Breaker

2. Vacuum Circuit Breaker

Operation:-

All circuit breaker systems have common features in their operation,

although details vary substantially depending on the voltage class, current rating and

type of the circuit breaker.

The circuit breaker must detect a fault condition; in low voltage circuit

breakers this is usually done within the breaker enclosure. Circuit breakers for large

currents or high voltages are usually arranged with protective relay pilot devices to

sense a fault condition and to operate the trip opening mechanism. The trip solenoid

that releases the latch is usually energized by a separate battery, although some high-

voltage circuit breakers are self-contained with current transformers, protective relays

and an internal control power source.

Once a fault is detected, contacts within the circuit breaker must open

to interrupt the circuit; some mechanically-stored energy contained within the breaker

is used to separate the contacts, although some of the energy required may be

obtained from the fault current itself. Small circuit breakers may be manually

operated, larger units have solenoids to trip the mechanism, and electric motors to

restore energy to the springs.

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The circuit breaker contacts must carry the load current without

excessive heating, and must also withstand the heat of the arc produced when

interrupting (opening) the circuit. Contacts are made of copper or copper alloys, silver

alloys and other highly conductive materials. Service life of the contacts is limited by

the erosion of contact material due to arcing while interrupting the current. Miniature

and molded-case circuit breakers are usually discarded when the contacts have worn,

but power circuit breakers and high-voltage circuit breakers have replaceable

contacts.

When a current is interrupted, an arc is generated. This arc must be

contained, cooled and extinguished in a controlled way, so that the gap between the

contacts can again withstand the voltage in the circuit. Different circuit breakers use

vacuum, air, insulating gas or oil as the medium the arc forms in.

Arc interruption:-

Gas (usually sulfur hexafluoride) circuit breakers sometimes stretch the

arc using a magnetic field, and then rely upon the dielectric strength of the sulfur

hexafluoride (SF6) to quench the stretched arc.

Vacuum circuit breakers have minimal arcing (as there is nothing to

ionize other than the contact material), so the arc quenches when it is stretched a very

small amount (less than 2–3 mm (0.079–0.118 in)). Vacuum circuit breakers are

frequently used in modern medium-voltage switchgear to 38,000 volts.

5.1 Gas Circuit Breaker

A sulphur hexafluoride circuit breaker uses contacts surrounded by

sulfur hexafluoride gas to quench the arc. They are most often used for transmission-

level voltages and may be incorporated into compact gas-insulated switchgear. In cold

climates, supplemental heating or de-rating of the circuit breakers may be required

due to liquefaction of the SF6 gas.

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Working:-

In closed position of the breaker, the contacts remain surrounded by

SF6 gas at a pressure of about 2.8 kg/cm2 . 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 synchronized with the opening of a valve which permits SF6 gas at

14 kg/cm2 pressure from the reserviour 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 strengths and causes

the extinction of the arc. After

the breaker operation(i.e.,

after arc extinction ), the valve

is closed by the action of set

of spring.

SF6 gas has high dielectric

strength which is the most

important quality of a material Fig. 5.1

for use in electrical equipments and in particular for breaker it is one of the most

desired properties. Moreover it has high Rate of Rise of dielectric strength after arc

extinction. This characteristics is very

much sought for a circuit breaker to

avoid restriking.

SF6 is colourless, odour less and non

toxic gas.

SF6 is an inert gas. So in normal

operating condition the metallic parts in

contact with the gas are not corroded.

This ensures the life of the breaker and

reduces the need for maintenance.

SF6 has high thermal conductivity which

means the heat dissipation capacity is

more. This implies greater current carrying Fig. 5.2 Gas Circuit Breaker

capacity when surrounded by SF6 .

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SF6 has high thermal conductivity which means the heat dissipation

capacity is more. This implies greater current carrying capacity when surrounded

by SF6 .

SF6 being non-flammable so there is no risk of fire hazard and explosion.

5.2 Vacuum Circuit Breaker

A vacuum circuit breaker is such kind of circuit breaker where the

arc quenching takes place in vacuum. The technology is suitable for mainly medium

voltage application. For higher voltage vacuum technology has been developed but

not commercially viable. The operation of opening and closing of electric current

carrying contacts and associated arc interruption take place in a vacuum chamber in

the breaker which is called vacuum interrupter. The vacuum interrupter consists of a

steel arc chamber in the centre symmetrically arranged ceramic insulators. The

vacuum pressure inside a vacuum interrupter is normally maintained at 10 - 6 bar.

Working :-

The main aim of any circuit breaker is to quench arc during electric

current zero crossing, by establishing high dielectric strength in between the contacts

so that reestablishment of arc after electric current zero becomes impossible. The

dielectric strength of vacuum is eight times greater than that of air and four times

greater than that of SF6 gas. When the breaker operates, the moving contact separates

from the fixed contact and an arc is struck 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, electrons and

ions produced during arc are diffused in a short time and seized by the surface of

moving and fixed members and shields. Since vacuum has very fast rate of recovery

of dielectric strength, the arc extinction in a vacuum breaker occurs with a short

contact separation(say 0.625 cm).

They are compact, reliable and have longer life.

There are no fire hazards.

There is no generation of gas during and after operation.

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They can interrupt any fault current. The outstanding feature of a VCB is that it can

break any heavy fault current perfectly just before the contacts reach the definite open

position.

They require little maintenance and are quite in operation.

They can successfully withstand lightning surges.

They have low arc energy and low inertia and hence require smaller power for control

mechanism.

Fig. 5.3 Vacuum Circuit Breaker

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6. CURRENT TRANSFORMER

The Current Transformer ( C.T. ), is a type of “instrument

transformer” that is designed to produce an alternating current in its secondary

winding which is proportional to the current being measured in its primary.

Current transformers reduce high voltage currents to a much lower

value and provide a convenient way of safely monitoring the actual electrical current

flowing in an AC transmission line using a standard ammeter. The principal of

operation of a current transformer is no different from that of an ordinary transformer.

Fig. 6.1 132kv Fig. 6.2 33kv

The basic principle of current transformer is same as that of the power

transformer. Like the power transformer current transformer also contains a primary

and a secondary winding. Whenever an alternating current flows through the primary

winding alternating magnetic flux is produced, which then induces alternating current

in the secondary winding. In case of current transformers the load impedance or

“burden” is very small. Therefore the current transformer operates under short circuit

conditions. Also the current in the secondary winding does not depend load

impedance but depends on the current flowing in the primary winding.

The current transformer basically consists of an iron core on which

primary winding and secondary winding are wound. The primary winding of the

transformer is connected in series with the load and carries the actual current flowing

Page | 31

to the load while the secondary winding is connected to a measuring device or a relay.

The number of secondary turns is proportional to the current flowing through the

primary. (i.e.) larger the magnitude of current flowing through the primary, more the

number of secondary turns.

The ratio of primary current to the secondary current is known as the

current transformation ratio of the CT. Usually the current transformation ratio of the

CT is high. Normally the secondary ratings are of the order 5 A, 1 A, 0.1 A whereas

the primary rating vary from 10 A to 3000 A or more.

The CT handles very less power. Rated burden can be defines as the

product of current and voltage at the secondary side of the CT. It is measured in volt

ampere (VA).

The secondary of a current transformer should not be disconnected

from its Rated burden while current is flowing in the primary. As the primary current

is independent of the secondary current, the entire primary current acts as a

magnetizing current when secondary is opened. This results in deep saturation of the

core which cannot return to normal state and so the CT is no longer usable.

Current transformers can reduce or “step-down” current levels from

thousands of amperes down to a standard output of a known ratio to either 5 Amps or

1 Amp for normal operation. Thus, small and accurate instruments and control

devices can be used with CT’s because they are insulated away from any high-voltage

power lines. There are a variety of metering applications and uses for current

transformers such as with wattmeter’s, power factor meters, watt-hour meters,

protective relays, or as trip coils in magnetic circuit breakers, or MCB’s.

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7. POTENTIAL TRANSFORMER

Potential transformers are instrument transformers used to feed the

potential coils of indicating and metering relays. These transformers make the

ordinary low voltage instruments suitable for the measurement of high voltages and

isolate them for high voltage.

The primary winding of the transformer is directly connected to the

high voltage power circuits between two phases or between a phase and ground

depending on the transformer rating and its application. The secondary of the

potential transformer is connected various measuring devices and relays. The primary

winding has a large number of turns and the secondary winding has lesser number of

turns than the primary winding. These two windings are magnetically coupled. The

number of secondary turns depends upon the purpose for which the potential

transformer is used.

Fig. 7.1 Potential Transformer

Operation:-

The theory of operation of a potential transformer is essentially same

as that of the power transformer. The main difference between a potential transformer

and a power transformer is that the load current of the potential transformer depends

purely on the exciting current and its secondary impedance. The secondary impedance

Page | 33

of the potential transformer will be resistive in nature. The potential transformers are

rated in terms of their maximum burden it delivers without exceeding specified limits

of error, whereas the power transformer is rated secondary output it delivers without

exceeding a specified temperature rise. The output of PTs is limited is usually limited

to few hundred volt amperes while the output of a power transformer varies from

several KVA to several MVA.

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8. WAVETRAP

Line trap also is known as Wave trap. What it does is trapping the high

frequency communication signals sent on the line from the remote substation

anddivertingthemto thetelecom/teleportationpanelinthesubstationcontrol room(throug

hcouplingcapacitoraTisrelevant in Power Line Carrier Communication

(PLCC) systems for communication among various substations without dependence

on the telecom company network. The signals are primarily teleportation signals and

in addition, voice and data communication signals. Line trap also is known as

Wave trap. What it does is trapping the high frequency communication signals sent on

the line from the remote substation and diverting them to the

telecom/teleportation panel in the substation control room (through

coupling capacitor and LMU). This is relevant in Power Line Carrier Communication

(PLCC) systems for communication among various substations without dependence

on the telecom company network. The signals are primarily teleportation signals and

in addition, voice and data communication signals. The Line trap offers high

impedance to the high frequency communication signals thus obstructs the flow of

these signals in to the substation bus bars. If there were not to be there, then signal

loss is more and communication will be ineffective/probably impossible.

Fig. 8.1 Wave Trap

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9. CAPACITOR VOLTAGE TRANSFORMER

Capacitor Voltage Transformer

A capacitor voltage transformer (CVT), or capacitance-coupled voltage

transformer (CCVT), is a transformer used in power systems to step down extra high

voltage signals and provide a low voltage signal, for measurement or to operate

a protective relay.

Components :-

In its most basic form, the device consists of three parts:

two capacitors across which the transmission line signal is split, an inductive

element to tune the device to the line

frequency, and a transformer to isolate and

further step down the voltage for the

instrumentation or protective relay. The

tuning of the divider to the line frequency

makes the overall division ratio less sensitive

to changes in the burden of the connected

metering or protection devices. The device

has at least four terminals: a terminal for

connection to the high voltage signal, a

ground terminal, and two secondary

terminals which connect to the Fig. 9.1 Capacitor Voltage Transformer

instrumentation or protective relay. CVTs are typically single-phase devices used for

measuring voltages in excess of one hundred kilovolts where the use of wound

primary voltage transformers would be uneconomical. In practice, capacitor C1 is

often constructed as a stack of smaller capacitors connected in series. This provides a

large voltage drop across C1 and a relatively small voltage drop across C2.

The CVT is also useful in communication systems. CVTs in

combination with wave traps are used for filtering high-frequency communication

signals from power frequency. This forms a carrier communication

network throughout the transmission network.

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10. EARTH PITS

In electricity supply systems, an earthing(grounding) system defines

the electrical potential of the conductors relative to that of the Earth's conductive

surface. The choice of earthing system has implications for the safety and

electromagnetic compatibility of the power supply. Note that regulations for earthing

systems vary considerably between different countries.

Fig. 9.1 Earth pits

A functional earth connection serves a purpose other than providing

protection against electrical shock. In contrast to a protective earth connection, a

functional earth connection may carry a current during the normal operation of a

device. Functional earth connections may be required by devices such as surge

suppression and electromagnetic-compatibility filters, some types of antennas and

various measurement instruments. Generally the protective earth is also used as a

functional earth, though this requires care in some situations.

Page | 37

11. CONCLUSION

The project work is on protection of two transformers(i.e.,50MVA and

35.5MVA) in substation many devices have been employed for protection. The

external protection is given by using modern well developed metal oxide type

lightning arresters for protection over voltage surges, line isolators for the on load

operations,SF6 and vacuum circuit breakers, potential transformers for voltage

measurement, current transformers for current, capacitor voltage transformer and

wavetrap combinely for the auditing of incomming voltages and frequencies and the

internal protection is involved in cooling of oil by using radiators, pressure relive vent

and buchholz relay.

By this arrangement the protection of transformer from many faults

that occurred due to transients or extra high voltages with circuit breakers and relays,

current is measured using current transformer and the tripping action takes place.

The internal problems like oil heating due to transformer working

which increase core losses and shrinks the age of transformer is cooled by using

Radiators, Pressure balancing in the transformer is done by using pressure relive vent

and the Buchholz relay is for clearing dielectric failure. This protection is gone

through economic way not to exceed the cost of operation doesn’t effect the charges

to transmit power to customers.