Very GOOD Condition Monitoring Transformers

8/13/2019 Very GOOD Condition Monitoring Transformers

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Working principle of a

transformer A transformer is static (or stationary) piece of

apparatus which:

1.Transfers electric power from one circuit toanother.

2.It does so without a change in frequency.

3.The principle is based on mutual inductionbetween two circuits linked by a commonmagnetic flux.


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Basic parts of a transformer Basically a transformer consists o f a :

1.A primary coil or winding.

2.A secondary coil or winding.

3.A core that supports the coils or thewindings


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Transformer construction

Main constructional elements of Transformers are -

A) Magnetic circuit Core & clamping structure

B) Electric Circuit Winding,Insulation, Bracing devices.

C) Terminals Tapping, Tapping switches, Terminal

Insulator, Leads , Bushings

D) Tank Oil, Cooling devices, conservator, piping,

Breather

E) Protective Circuit Buchholz relay, WTI, OTI, Oil surge relay

& Monitoring pressure relief device, MOG


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A)Magnetic Circuit:

The core provides closed path for flux. It is made up of CRGOinsulated laminations. (CRGO has iron loss of about 1.3 W / Kg at1.6 Tesla )

B)Electric Circuit:

Winding, insulation & bracing are constructional parts of electricalcircuit of transformers. This is the most vulnerable part oftransformer because of direct association with power system. Must

be designed to withstand voltage stress resulting from systemfault, transient over voltage and thermal stresses (lightening orswitching surges)


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Insulation:

Commonly used material are -

Paper or press board

Oil is used as insulating medium

Insulating varnish applied to make coil mechanicallystrong.

C) Terminal:

Leads:

Connection of winding are (copper rod or bus bar) taken tobushing.

Bushing:

Up to 33 kV porcelain bushing are used. Above 33 kV,condenser & oil filled terminal bushings are used.


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D) Transformer tank

Cooling:

Small transformers are air cooled whereas large transformers

are provided with oil or oil & air cooling .The transformer tank is designed to withstand full vacuum.

Types of Cooling:

Air insulated air cooledAN Air Natural

AF Forced Air Cooling

ANAN Natural Air cooling inside and outside transformer


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Oil immersed air cooled -

ONAN Natural Oil Circulation. Natural Air flow

ONAF Natural oil and forced air flow

OFAN Forced Oil & Natural Air CirculationOFAF Forced Oil and Forced Air Circulation

Oil immersed & water cooled

ONWF Natural Oil, water (internal) coolerOFWF Forced oil, water (external) cooler


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E) Protective devices :

Various protective devices mounted on transformer are asfollows:

Bucchholz Relay Gas actuated relay, Transformer

Internal fault.WTI/OTI Provided for alarm and / OR trip

against over load

PR Device To release internal pressure

generated in the transformer

during fault.MOG Alarm when oil level is low

Oil Surge Relay To release actual pressure

generated during fault in OLTC


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Accessories & their functions

Terminal & Bushing : Type of Bushing, terminal is selecteddepending on voltage, currents & operating conditions. Porcelain,condenser type of oil filled bushings are used as per requirement.

Cable Boxes : Cable boxes are primarily designed for receiving

and protecting cable ends and ensure effective sealing of cableagainst ingress of moisture.

Conservator : It is provided to accommodate change in oilvolume caused due to change in loads or ambient conditions.

Breather :Whenever there is change in ambient temperature orload , there is a change in oil temperature and hence the volume ofoil. Increase in volume causes the air above oil level in conservatorto be pushed out and decrease causes air to be drawn in. Thusthe transformer breathes.


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When air is breathed in, moisture and dust fromatmosphere is sucked in .

Silica gel crystals absorb moisture. Color of silica gel

is blue when dry and turns pink when absorbsmoisture. Oil cup at the bottom is filled with oil which acts as

coarse filter and removes dust form outside air. Magnetic Oil Level gauge:

This is a dial instrument operated by magneticcoupling from a float on oil surface. It is normally fitted with contact to give alarm for low

oil level.


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Oil temperature indicator:Bourdon tube with a pointer arrangement mounted in a casecomprising of a reading dial and a glass cover. There is atemperature sensing bulb which communicates to the bourdontube through the armored capillary.

Winding temperature indicator:It comprises of following:WTI pot :mounted at top of transformer tank. Oil in pot is temp. of topoil.Imae coil:Heater coil and develops additional hear raising temperature ofoil incide heater coil.

WTI CT:Mounted on one of the line lead with secondary connected to image coilWTI: The bulb of the WTI is immersed in oil inside image col.Temperature of this oil is dependent on top oil temperature and load ontransformer.


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Buchholz Relay : Gas & oil operated relay detects formation ofgas or development of sudden pressure inside the oil of transformer.

Any electrical fault inside the transformer is accompanied byevolution of gas.

Pressure Relief Device: This is provided to relieve the internalpressure in the event of major fault within the transformer.

Tapping Switch: To maintain secondary voltages reasonablyconstant at load end when incoming voltage and/or load ontransformer changes, it is necessary to change the voltage ratio (I.e,

turns ratio of the winding) of the transformer.This is achieved by changing the number of turns ( HT Side) byoperating a switch called as tapping switch.

Depending on the requirement, off circuit or on load tapchangeris installed in the transformer.


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Radiators:The function of radiator is to limit the temperature of oil and winding bydissipating heat that is generated due to losses within transformer while inservice. When transformer is in operation warm oil rises and enters the radiatorfrom the top valve cools and then descend to enter the bottom of thetank.

Other Accessories: Inspection cover, jacking lugs, Earthing terminals,Rating Plate, Filter valve, Drain Valve, Terminal marking, Rollers etc.


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DGA Insulating materials within transformer breakdown to

liberate gases.

The identity of these gases indicate the type of fault

and the rate of gas generation indicate the severityof fault.

Causes of fault gases can be divided into threecategories:

Corona or partial discharge


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DGA

Pyrolisis or thermal heating

Arcing

Arcing is the most severe fault (intensity of energythat is dissipated per unit time per unit volume ofthe fault) ,less with heating, least with corona.

One of the most important technique to indicate thehealth of a transformer.


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Major (Minor) fault gases undervarious fault conditions


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Sampling And Labeling ProcedureSampling

Dry Weather, avoid contamination

Clean, dry, leak proof glass or stainless steelcontainer.

Equipment operating normally

Take safety precautions.

Sample bottle must be full without any air trapcompletely sealed and should be properly labeled.


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FREQUENCY OF SAMPLING

NEW TRANSFORMERS

FIRST DGA TEST : BEFORE HEAT RUN TEST ONTRANSFORMER

SECOND DGA TEST : AFTER HEAT RUN TEST ONTRANSFORMER (2&24 HRS)

THIRD DGA TEST : BEFORE ENERGIZINGTRANSFORMER (B.M.)

FOURTH DGA TEST : WITHIN THREE MONTHS OFSERVICE


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FREQUENCY OF SAMPLING

IN-SERVICE TRANSFORMERS

ANNUALLY AS A ROUTINE CKECKING

BEFORE (LATEST DATA) & AFTER FILTRATION/TOPPING UP

TRANSFORMERS AFTER OVERHAULING,

REPAIRS, MAINTANANCE

BEFORE ENERGISING TRANSFORMER

WITHIN THREE MONTHS OF SERVICE


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Interpretation of Results

Ensure that gas concentrations are highenough to warrant further investigation.

Gas being present even in normal operatingcondition without any fault being present.

Gases might have formed on the occasion ofprevious faults or during repairs by brazing,

welding etc., and not completely removed. Since gases are produced in normal ageing

also, the service duration of the oil has to betaken into account.


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PERMISSIBLE LIMITS OF DISSOLVED GASES IN

OIL OF A HEALTHY TRANSFORMER

GAS 10 YRS (PPM)

H2 100-150 200-300 200-300CH4 50-70 100-150 200-300

C2H2 20-30 30-50 100-150

C2H4 100-150 150-200 200-400

C2H6 30-50 100-150 800-1000

CO 200-300 400-500 600-700

CO2 3000-3500 4000-5000 9000-12000

GAS 10 YRS (PPM)

H2 100-150 200-300 200-300

CH4 50-70 100-150 200-300

C2H2 20-30 30-50 100-150

C2H4 100-150 150-200 200-400

C2H6 30-50 100-150 800-1000

CO 200-300 400-500 600-700

CO2 3000-3500 4000-5000 9000-12000


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Condition monitoring oftransformer oil

Role of transformer oil: It is used as coolant. It is used as insulating material.

Reasons for deterioration of transformer oil

Physical contamination: Release of fibrous impurities by paper, pressboard, wood and

cotton tapes in contact with oil for longer period at elevatedtemperatures.

Due to dissolution of varnish . Due to foreign matters like dust, metallic particles and other

solid impurities Due to moisture


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Effects:

Life is reduced by high sludge formation.

Electrical properties of insulating oil get disturbed dueto conductivity of suspended particles.

Chemical deterioration:

It is due to oxidation.

Effects of oxidation:

Results in acids, sludge .Acid attack solid insulationand metal. Sludge causes poor thermal conductionand mechanical hindrance to proper oil circulation.


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Contamination of gases:

Gases are present in oil due to following:

Those which dissolve in the oil from atmosphere.

Those which are generated inside due to thermaldecomposition of oil, decomposition of oil by arcing.

Effects of gases:

The ignition of inflammable gases can be causes by

corona occurring in th air space or arcing.


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Transformer oil testing

Transformer oil testing is carried out to detectabnormalities in transformer and based on test results

corrective actins can be taken before actual failure takesplace.

To evaluate quality of oil

To decide periodic maintenance (filtration. reclamation

etc) To know health of transformer (by DGA)

To estimate remaining life of transformer


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Oil Testing

New oil (IS-335)- 15 tests

Oil in service (IS-1866)- 8 tests

Dielectric strength (BDV)

Dielectric dissipation factor (Tan delta)

Resistivity Neutralisation value (Acidity)

Flash point

Water content

Sludge

Interfacial tension


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1.Physical condition of the oil:Color ,clarity and odour gives information regardingquality of oil and presence of certain contaminants in

oil.2.Electrical strength:

Important parameter as used as insulating medium.This test gives conductive contaminants and moisturepresent in oil.

3.Water content:Reveals total water content ,leak or cellulosicdeterioration


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4.Specific resistance (Resistivity):This test provides a measure of the total soluble contaminants andageing products .It is numerically equal to the resistance betweenopposite faces of a centimeter cue of the oil and is expressed asohm com.

5.Dissipation factor:This test provides a measure of the total soluble contaminantsand ageing products.

6.Neutralisation value:This test gives acid present in the oil. It is the no. of milligrams ofpotassium hydroxide required to neutralise completely the acidsproduced in one gram of oil.


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7.Interfacial tension test:

This test provides a measure of sludge and

polar component present in oil. It isexpressed as molecular attractive forcebetween the molecules of water and oil atoil-water interface.

8.Flash point:Sudden drop in flashpoint is indicative ofunsafe working condition of transformer.


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Scheduled of oil characteristics for transformerin service as per IS : 1866-2000

Property Highest Voltage of equipment, kV

< 72.5 72.5 to 170 > 170

Breakdown voltage (kV),

Min.

More than

30

More than 40 More than

50

Water content (ppm),Max.

Max. 95 Max. 40 Max. 20

Neutralization value (mgKOH/g), Max.

Max. 0.3 Max. 0.3 Max. .0.3

Sediment & Sludge, %by mass

ND ND ND

Resistivity @ 90C x 1012

(ohm-cm), Min.0.1 x 1012 0.1 x 1012 0.1 x 1012


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Property Highest Voltage of equipment, kV

< 72.5 72.5 to 170 > 170

Dielectric dissipationfactor @ 90C, Max.

1.0 1.0 0.2

Interfacial tension(mN/m), Min.

15 15 15

Flash Point, (C), Min. 125 125 125


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Condition Monitoring - Oil testing

Sr. No. Test Remedial Action for

Deviations from

permissible limits

1 Electric strength 2.5

mm gap (Break downvoltage)

Oil filtration

2 Water content Oil filtration

3 Sediment and / or

precipitable sludge

Oil filtration


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Condition Monitoring - Oil testing

Sr.

No.

Test Remedial Action for Deviations

from permissible limits

4 Specific resistance

(Resistivity at 90 deg.C

Oil filtration if the tan value

permits other wise replace the oil

5 Dielectric dissipation factor (

tan )

Replace the oil

6 Neutralisation value (total

acidity) of the oil

Replace the oil

7 Inter facial tension of the oil

against water at room

temperature

Replace the oil


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Furanic compound test

When oil soaked paper is damaged by heat, some oilsoluble compound are released into oil called furans.

Paper is made of cellulose consisting of log chains ofglucose rings joined by glycosidic bonds.

During degradation bonds are broken and glucose

rings are opened. Glucose is unstable which furtherdegrades which are more stable and oil soluble calledfurans.


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Furanic Compounds

The most commonly found furanic derivative is 2-furfuraldehyde(2 FAL) and other derivatives are

2- furfuryl alcohol (2 FOL) ,

2- Acetyl furan (2 ACF),

5- hydroxymethyl furfuraldehyde,(5HMF)

5 methyl furfuraldehyde(5MEF).


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FURANS AND GASES

Cellulosedegradation=Glucose+H2O+CO+CO2+Organic acids.

With DGA and furan test extent of paper damage can

be seen. CO and CO2 are determined by DGA and are

considered as level indicator for cellulosicdegradation.

In case of severe localised paper damage ,high

furans and high gas content can be seen. In case general heating slow building of furans

without necessarily seeing an increase of gascontent.


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Degree of polymerization (DP)

Degree of polymerization (DP) is another way ofexpressing the molecular weight. Physical propertiesof the paper depends on the degree ofpolymerization of paper

M = Dp x mWhere

M= molecular weight of the polymer

Dp = the degree of polymerization

and m = the molecular weight of monomer.


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Degree of polymerization (DP)

DP value 1000 to 1500 i.e,1000 to 1500glucose units are present in cellulose

molecule. Degradation of paper is due to temperature,

water ad oxygen.

When DP value


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Correlation between Dp and Furan

The absolute correlation of Furan to DP isdifficult, but can be related fairly accuratelyextent through an empirical formula.

Dp = - 100 ln (2 FAL) +709


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INSULATION RESISTANCE ANDPOLARISATION INDEX TESTS

These tests are performed to verify state ofdryness if insulation.

These tests are intended to check overallcleanliness ,dryness ,localized defects and generalcondition of insulation system.

When DC voltage is applied across insulation, thecurrent flows is the resultant of three currents:

Capacitive charging current

Absorption current

Leakage current


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RL

RA

C

DC Voltagesource

S

Absorption currentConduction or leakagecurrent

Capacitance charging current


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Capacitance leakage current:

The current lasts for a few seconds as DC voltage isapplied and drops out after the insulation is chargedto its full voltage. The time depends on the size andcapacitance of the test object. Larger time for largercapacitance objects.


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Absorption leakage current: It is caused by polarisation of molecules within

dielectric material.

In low capacitance equipments the current is high forfirst few seconds and decrease slowly to nearly zero.

In high capacitance equipment or wet andcontaminated insulation ,there will be no decrease of

absorption current for long time.


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Conduction or leakagecurrent:

Conduction or leakage current

This is the current that normally flows through the

insulation ,between conductors or conductors toground.

It increases quickly and becomes stable. This current increases as insulation deteriorates and

becomes predominant after absorption current

vanishes. It is steady and time independent .Hence the

important current for measuring insulation resistance.


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Interpretation of results:

If IR value shows a decreasing trend it showsgradual deterioration of insulation quality due tohumidity ,dust accumulation etc..

A very sharp drop indicates insulation failure.


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IR MEASUREMENTS

THE TEST VOLTAGES FOR IR MEASUREMENT OF

TRANSFORMER ARE AS UNDER.RATING TEST VOLTAGE

415/440 V 500 V

3.3 kV 1000 V

6.6 kV 2500 V

11 kV & ABOVE 5000 V

THE MINIMUM ACCEPTABLE LOWER LIMIT FORIR VALUES IS GIVEN IN STANDARDS AS

V L-L+ 1 M


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Polarisation indexmeasurements

Polarisation Index=600sec IR value/60 sec IR value

PI


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STEP VOLTAGE TEST

The DC voltage is applied in various steps and ineach step the leakage current is noted. Stepduration:60 seconds.

The variation of this leakage current (or IR) with testvoltage gives the condition of insulation.

If insulation is dry, clean and with out physicaldamages shall show the same value at all voltagelevels.

If insulation value decreases at higher voltage levels,may be due to dirt, moisture, cracking, aging etc. The application of increased voltage creates electrical

stresses on internal insulation cracks. This can revealaging and physical damage in relatively clean and dryinsulation which would have not been apparent atlower voltages.

CAPACITANCE MEASUREMENTS


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CAPACITANCE MEASUREMENTS

THE CAPACITANCE VALUE IS DEPENDENT ON THE CHARACTERISTICS OF THE DIELECTRIC MATERIAL THE PHYSICAL CONFIGURATION OF THE ELECTRODES

A C =

d Void /impurities may discharge partially during a voltage apicationand the effective distance between eectrodes ncreases. HENCE C WILL INCREASE WITH INCREASE IN VOLTAGE,

WHICH INDICATES PRESENCE OF PD AND THE DETERIORATION

OF INSULATION

A

d

ELECTRODE

ELECTRODE

DIELECTRIC

TAN MEASUREMENTS


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TAN MEASUREMENTS

THE TAN VALUE DIRECTLY INDICATES THE POWERDISSIPATED BY THE INSULATION

IT INCREASES WITH INSULATION DETERIORATION ANDSERVES AS AN EARLY INDICATOR OF FAILURE HAZARDS

90

Ic

Ic

v v


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Concept of Tan

In an ideal capcitor the voltage and currentare phase shifted by 90 deg.and current

through insulation is capacitive. If there are impurities in insulation,the

resistance of insulation decrease resulting inincrease of resistive current.

Thus the total current I = Ic + Ir which leadsthe voltage by a phase angle < 90. Andlags the Ic by an angle .

2 0 .4

3 0 .6

4 5 .9


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The tangent of this angle directlyindicates the heat dissipation that takes

place inside the dielectric material.

The values obtained on new insulationforms the reference value for periodic

measurements.

P ti l di h


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Partial discharge:

It is an electrical discharge that occurs acrossa portion of the insulation between two

conducting electrodes without completelybridging the gap.

This results in localized, nearly instantaneousrelease of energy.

The most convention unit for quantifying thePD quantity is Pico coulomb

I t t ti f lt


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Interprtation of results:

High value of tan-delta at low voltage gives anindication of contamination and presence of moisturecontent.

Tan-delta tip up gives an indication of void content(variation of tan delta with applied voltage).Generally tan delta values shall not increase asapplied voltage increase.

A higher tan delta tip up at a applied voltageindicates presence of voids/moistures and the

inception of partial discharges at this voltage. Increase in tan delta above passing of time also

indicates deterioration of insulation


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Transformer protection


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Over load Capability: Working life of transformer dependent on life of insulation

Rate of deterioration of insulation increases with increasingwinding temperature W

Winding temperature dependent on loading

Transformer has substantial over load capability

IF W< 80 deg.c use of life negligible

If transformer is operated @ 104 deg. For every hour of operation= 2 hrs of life is lost.

If transformer is operated @ 116 deg. For every hour of operation8 hrs of life is lost

Rate of using transformer life doubled for every temperatureincrease of 6 deg.c

Transformer - Protections


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Transformer Protections


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Period1 Period2 Period3 Total LifeLost

LossHrs./Day

24 Hrs@80oC

24 X 0.125 3

24 Hrs@98oC

24 X 1 24

9 Hrs @80oC 7 Hrs @80oC 8 Hrs @80oC 9 X 0.125 +7 X1 +8 X 2

24

24 Hrs@104oC

24 X 2 48


T f P t ti


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Criteria for overload operation

For normal duty cycle, current shall not exceed 150% IRAT For emergency duty, current can exceed 150% IRATprovided

associated cables, switch gear, tap changers, bushings etc. aresuitable rated.

Under no circumstances,

Windingshall exceed 140 deg.c

Oilshall exceed 115 deg.c.


Transformer Protections


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Buchholz Protection: Relay installed in the pipe line between transformer tank and

conservator

Two Floats Upper float = responds to slow accumulation of gas due to mild or

incipient faults

for alarm Lower float (Vane) responds to oil surge caused by major internal

faults for trip

Relay mounting precautions Gas shall freely pass up the pipe work

Extra turbulence shall not be induced in oil stream

Relay shall be mounted on straight run of pipe line which shouldslope from transformer to conservator at an angle of 5 deg.

Operating time: 100 to 200 milliseconds

Petcock provided on top of housing to draw accumulated gas foranalysis


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Buchholz Protection: Gas actuated relay

Popularly used in all countries except USA

Used to detect incipient faults which may lead to major damage ifallowed to continue

Some Examples:

Hot spots on the core due to short in lamination insulation

Core bolt insulation failure

Faulty joints

Inter turn faults Loss of oil due to leakage

Depends for its operation on the fact that most internal faultsgenerate gases.


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Maintenance Schedule

Frequency ofInspection

Item to be inspected Remarks

Daily Amb. Temp.

Oil temp.

Winding temp.

Load currentVoltage

Check whether

temp. rise is

reasonable

Check against ratedvalues

Half yearly Oil level / leakageBushingCable boxes

BreatherTap changer operationWTI/OTI

IR value

Take corrective actionif abnormality isnoticed


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Maintenance Schedule

Frequency ofInspection

Item to be inspected Remarks

Yearly Oil testing Take correctiveaction (oil filtration

/ oil replacement)

as per the testreport

Yearly DGA analysis Over haul thetransformer ifabnormality is

indicated in the reportFive yearly orconditionmonitoringreport based

Over hauling oftransformer

Take corrective actionif abnormality isnoticed


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Very GOOD Condition Monitoring Transformers

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