Presentation on Electrical Equipment

8/4/2019 Presentation on Electrical Equipment

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Presentation on Transformer


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MAJOR TOPICS


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INTRODUCTION

Transformer is a device that

Transfer electrical power from one circuit to

another

It does so without a change of frequency

It accomplishes this by electromagnetic

induction


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Working Principle of

Transformer


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Classification of Transformer

1.Step Up Transformer

2.Step Down Transformer


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Step Up Transformer

If the secondary winding has more

no. of turns than primary winding,

then secondary voltage is higher

than primary voltage and

transformer is called step uptransformer


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Step Down Transformer

If the secondary winding has lesser

no. of turns than primary winding,

then secondary voltage is lowerthan primary voltage and

transformer is called step up

transformer


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CONSTRUCTION


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Type of Transformer

Core Type Transformer

Shell Type Transformer


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Core Type Transformer

Core type transformers the winding

surrounds a considerable part of the

winding. The coils used are formwound and are of the cylindrical type.


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Shell Type Transformer

In Shell type transformers the core

surrounds a considerable portion of

the winding. In this type the coils arealso form wound but


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Major Parts of Transformer

ransformer Tank

rimary & Secondary Winding

agnetic Core

ap Changer

onservator

adiators

ilica Gel Breather


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Primary & Secondary Windings

Primary Winding: -

The winding to which the input voltage is

applied is called the primary winding.

Secondary Winding: -

The winding to which the load is connected orwhich delivers output is called secondary

winding.


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Magnetic Core

Magnetic core made of thinsilicon steel laminations . Inorder to reduce the eddycurrent losses, these

laminations are insulatedfrom each other by thin layersof varnish. The verticalportions of the core are calledlegs or limbs and the top andthe bottom portion are calledthe yokes


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Conservator

It is a large cylinder connected bypipe to the transformer main

tank. Transformer oil is filled up

to the certain level in the

conservator. Remaining upperportion is filled with air. The

conservator volume is used for

expansion of oil during higher

loads. Communication of the airbetween top of the conservator

and outside air is through the

breather.

Conservator tank


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Conservator

Conservator are of two type: -

Conventional Conservator

Conservator with Rubber Air Bag


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Silica Gel Breather

It is installed in a pipe from the

conservator. Air is breathed

during the load cycle through

the breather. Silica gelbreather provides moisture

free air to the conservator

tank. Silica gel crystal are dark

blue in color when dry.Silica Gel Breather


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Tap Changer

Tap Changer is generally

provided on the High

Voltage side of the

winding of transformer. Tap changers are used

to maintain thesecondary voltage

reasonably constant atthe users end.


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Typpes of Tap Changer

Off Circuit Tap Changer: -

Designed to operate with Transformer out ofcircuit.

Used for seasonal voltage variations.

Consists of three parts: Live part connected to

winding ; operating handle drive brought out oftransformer for operating the switch & linkconnection between the above two parts.


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On Load Tap Changer: -

Designed to operate with Transformer in circuit.

Used for daily or short period voltage variations


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Buchholz Relay

Gas & oil operatedinstrument which detects

low oil level, formation of gas

or development of sudden

pressure inside the oil filledtransformer.

Gives both the alarm &

tripping signal depending

upon the oil level or amount

of gas formed.


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Bushings

The Function of abushing is to provideinsulating support to aconductor passing

through earth tank.

Bushing is used when aconductor is taken outthrough metallic tank ora wall.

Is made of single pieceporcelain.

Bushings


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Marshalling Box

This Box is provided with eachlarge transformer and placedon the side of the transformerand this enclosure is

completely weather proof. All control cable and power

cables for fan control and andauxiliary supply switchgear etc.

are via marshalling box and italso houses the oil andwinding temp. indicators.


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Magnetic Oil Level Indicator

This is direct oil level indicating

device provided on conservator. The center of the dial is normally

marked with a temp. 25 degreeCelsius. High & low level points are

also marked to follow level changesas oil expands & contracts withtemp. changes.

The low oil contacts provided can

be used for automatic alarm ortripping when the oil level in theconservator falls to a low level.


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Oil & Winding Temp. Indicator

Oil Temp. Indicator: -

The thermocouple is placed in the pocket

provided with the tank near hot oil &thermocouple leads are connected to oil

temperature indicator placed in marshalling

box.


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Winding Temp. Indicator: -

winding temperature indicator placed in themarshalling box gets input for measurement

from

1. Thermocouple placed in the pocketprovided with the tank near hot oil

2. CT secondary which measures the current

in windingThe indicator is provided with alarm &

tripping contacts


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Radiators

Provides cooling to the

transformer oil. Oil is

circulated through the

radiators in whichcooling can be assisted

by a blast of air

provided by fans. This

cycle is continuouslyrepeated


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Types of Cooling of Power

Transformer

O.N.A.N Oil Natural Air Natural

O.N.A.F Oil Natural Air Forced

O.F.A.F Oil Forced Air Forced

O.F.W.F Oil Forced Water Forced


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DOUBLE DIAPHRAGM EXPLOSION

VENT Prevents damage to transformer tank by releasing

any excess pressure that is generated inside thetank.

Consists of a bent pipe with aluminum diaphragms atboth ends. A protective wire mesh is fitted at theopening of the tfr. Also a wire mesh is provided atupper end to prevent upper diaphragm frommechanical damage.

Near the lower end there is a small oil level indicator


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OPERATION


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LOSSES IN A TRANSFORMER

Mainly two types of losses- Copper loss & Iron

loss.

Copper loss is directly proportional to square

of load on the transformer.

Iron loss consists of

Eddy current loss.

Hysteresis loss


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Eddy Current Losses


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Why Transformer rating in KVA

Because Copper loss of Transformer depends

upon current and Iron loss on voltage. Hence,

total transformer loss depends on volt-

ampere (VA) and not on phase angle betweenvoltage and current i.e. it is independent of

load power factor. Hence rating of

transformer is in KVA and not in KW.


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VOLTAGE REGULATION

It is defined as change in magnitude of the secondaryterminal voltage, expressed as a percentage of thesecondary rated voltage, when load at a given powerfactor is reduced to zero, with primary applied

voltage held constant.

If V1 is secondary terminal voltage at no load

V2 is secondary terminal voltage at full load %regulation = (V1 -V2 )*100/ V1


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This change in secondary terminal voltage

with load current is due to leakage

impedances of the transformer. Themagnitude of this change depends on the load

power factor, load current, total resistance &

total leakage reactance of the transformer. Distribution transformers should have good

voltage regulation so that voltage at the

consumers premises doesnt vary widely asthe load changes.


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EFFICIENCY

Efficiency of Transformer is defined as Ratio of

output power to input power.

Efficiency = Output/Input

= (Input-Losses)/Input

Efficiency of power & distribution transformers isvery high( 95% to 99% ).


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Condition of Maximum Efficiency

Efficiency of a Transformer is maximum when

Copper Losses = Iron Losses

The above condition occurs at

About half the rated load in distribution

transformers

Near rated load in power transformers


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All Day Efficiency

Transformers used for supplying lighting have theirprimary energized all the twenty four hours,although their secondary supplies no load much ofthe time. Hence the Iron losses occurs whole of the

day and copper losses when loaded. Theperformance of such transformer is judged by all dayefficiency (for a 24 hours)

%efficiency(All Day)

= Output in KWH/Input in KWH (For 24 Hours)


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Difference Between Distribution

Trf & Power Trf.Distribution Trf.

The maximum efficiency of adistribution trf may occur atabout half the rated load oftrf.

The efficiency of distribution

trf is usually measured as Allday efficiency as load on

these trf varied b/w widelimits during 24 hours of aday.

Power Trf.

The maximum

efficiency of a power trf

occur at or near its fullload rated KVA.

Efficiency of power trf is

measured as

output/input

DISTRIBUTION


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DISTRIBUTION

TRANSFORMERSPOWER TRANSFORMERS

Distribution trf are thosewhich change the voltage

to a level suitable for

utilization purposes at the

consumers premises.

Distribution trf are

designed to have very low

iron losses.

Distribution trf have less

efficiency than power trf.

Power trf. are those which areused at sending & receiving

ends of a long, high voltage

power transmission line for

stepping up or stepping downthe voltage.

Power trf have more iron

losses.

Power trf have more efficiency.


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Maintenance


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Maintenance Schedule

Sr.

No

Frequency of

operationItem to be inspected Inspection

1. Hourly

Winding temperature.

Oil Temperature

Load in Ampere

Voltage

Check that

temp. rise is

reasonable

Check against

the rated.


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Sr.

No

Frequency of

operation Item to be inspected Inspection

2. Daily

Oil level in

transformer and on

load tap changer

Oil level in

transformer bushing

Dehydrating Breather

Check against

the oil level

Do

Check the colourof the silica gel

& oil level in oil

cup


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Sr.No

Frequency ofoperation

Item to be inspected Inspection

3. Quarterly

Bushing

Oil in transformer

and OLTC

OLTC driving

mechanism &

Automatic control

Examine for any

crack or dirtdeposit

Check the

dielectric strength

of the oil

Lubricate the

bearing & check

the all circuits

including Limit

switches


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Sr.No



4. Yearly

Oil in transformer

Insulation Resistance

Value

Temperature

Indicator

Check for acidity

and sludge.

Compare with the

value at the time

of commissioning

Pocket holdingthermometer

should be checked


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Sr.No



4. Yearly

4. Dial type oil Gauge

Relays, alarms, their

circuits etc.

Earth resistance

Check pointer for

freedom

Check the relays

and alarm contactsand their

operation.


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Testing


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Ratio Test

This test is conducted by applying a single

phase 230 V supply on the High Voltage Side

winding and measure the voltage on Low

Voltage Side winding. The Ratio should bechecked at all tap positions.

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Presentation on Electrical Equipment