Transformer - Inspecton and Maintenance#

7/28/2019 Transformer - Inspecton and Maintenance#

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1 of14Transformer Inspection and Maintenance

Transformer: Inspection and Maintenance

Information

1) General

Recently power transformer are getting larger in their capacity to meet increasing power demand. Soemergency or unexpected power ouage due to transformer trouble cause much loss of production aswell as inconvenience.

Therefore it is essential to assure trouble-free performance by a well planned maintenance program.Daily or periodical inspection will result in detecting abnormal condition a transformer and prts beforethey cause any more serious troubles. A regular program is inspection should be established andrigidly carried out for preventive maintenance of power transformer.

2) Recuirements for inspection and maintenance

The inspection items are calassified into three categories.

(a) Roution inspection

The roution inspection should be made, preferably daily, on every transformer in service.Especially, oil temperature and/or winding temperature, load current, ambient temperature and oil levelin transformer should be daily checked and recorded.

(b) Periodical inspection items

The periodical inspection should be made occasionally, one every six months to once every threeyears in order to ascertain the good performance os a transformer and its parts. Most of the periodicalinspections should be made in detail when the transformer is de-energised.

(c) Additional inspection items

Some additional inspections or measurement of electrical characteristics of a transformer arerecommended as preventive maintenance actions and when any transformer trouble should beinvestigated throughly.

Repaintaing of radiators and transformer tanh and exchange of parts, gaskets and bearings of motorshould be planned and prepared previously for preventive maintenance, which will result in continousgood performance aof transformer.

If any protective relays given alarm, investigate the trouble causes according to Table 5 and Table 6. Itis essential in investigating the right causes to check whenever differential relay, overcurrent relayand/or ground fault relay have operated or not, in combination with other relays such as Buchholz relaysudden pressure relay and pressure relief device, which have phisically operating mechanisms.

Transformers should be inspected internally whenever they have subjected to unusually severeoperating conditions such as overloads and frequent short-circuits aor outer bus ar transmission line.

Any symtoms such as unusual noises, high or low oil levels, rupturing or bursting palte, etc. should beinvestigated throughly.

3) Inspection record

(a) The establishment of the report and recording of the condition and repair of the transformers isrequired for a good maintenance program.

(b) A preventive maintenance system will operate satisfactorily with the following records.

i. An equipment record


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This may be simply a card which contains the basic information of a transfoemer itself such asthe serial number, the location, site etc.

ii. A reapair record card.This may keep a running record as to cost of maintening a transformer. It is essentialdiagnostic record for avoiding future difficaulties.

iii. An inspection check list or inspectors record.This may be simply a listing of the points to be checked on a transformer and the

establishment of the time that these check should be made.

Without these records it would be very difficult for a preventive maintenance program to work, becausethe knowledge from regular inspections would be quickly lost.


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4) Routine inspection

All least item should be checked daily and recorded.

Table 1

No Items Method Action

1. Oil temperature andwinding temperature

Read indication of dial and/oralcohol thermometers andwinding thermal relays, ifprovided.

Compare them with the datapreviously obtained.

When oil and/or windingtemperature are much higheror lower considerating loadcurrent and ambienttemperature, thermometers

should be inspected.

Check if indicate of thermometeris correct or not.

If the thermometer is not correct,exchange with a new one.

If oil temperature is too high dueto dust and other foreignmaterials on the finned area ofradiatos, or the inner surface ofcooling water tube clean upthem.

Load current andAmbient temperature

Load current and ambienttemperature should berecorded at the same time.

2.1

2.2

Oil level in transformer:See note.

Read indication of the dial oillevel gauge or the conservator.Compare it with the oil level-oiltemperature curve for thetransformer.

Oil level is deemed to be

normal when its difference iswithin one graduation of thegauge on the curve.

When the indicated oil level isconstant with the changing oiltemperature, inspect the dialgauge and check the actual oillevel.

If the oil level is abnormal, adjust

it according to instructions.

In bushing. Check the oil level with the oillevel gauge.

3. Oil leaks Check oil leaks visually fromradiators, flangepipe,transformer tank and so on.

If oil leaks are due to gasket,tighten bolts or exchangegasket.

In caes of oil leaks from weldedparts, apply adhesive material or

weld again on the leaking part.

4. Abnormal noise andvibration.

Abnormal or unusual noise,especially from oil pumpmotors and fan motors, shouldbe carefully listened to.

When oil pump or fan motor isgetting noiser, exchange thebearings, etc. with new one.

Abnormal vibration can bechecked comparatively easilyby hand.

When any supporters, pipes andother parts are vibrating due toloose bolts, tighten them.

5.1 Appearance boltconnection

Confirm tighness of bolts with atest hammer.

When any loose bolts are found,tighten them.


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5.2

5.3

5.4

Discoloration Check visually that allconnections are normal withoutany discoloration due to localheating.

When any connections showsigns of having been hot, cleanand tighten bolts and nuts.

Dust Inspect visually for dust,especially on radiators.

Clean up dust if it may reducecooling capacity of radiators.

Rust Inspect visually for rust,especilly on radiators.

Re-painting is recommendableonce every five years.

Anti-rust and final paint shouldbe applied after rubbing off rustand old paint.

5.5 Dew Inspect visually for dew in theterminal box, control cabinetsand protective relays.

If any dew is found, dry it ourand ensure space heaters are inuse if they are provided.

Note: In case of a self-cooled transformers, there is some difference between top and average oiltemperatures depending on loading conditions and ambient temperature. Strictly speaking, the averageoil temperature should be referred to for checking oil level.


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5. Periodical inspection

The recommended inspection frequency is described in parenthesis under each inspection item.

Table 2

No Items Method Action

1. Breather (Onceevery six months)

Check the discoloration ofsilicagel in breather due tomoisture in breathed air.

If the color of silicagel has turned topink by more than two thirds of totalquantity, dry out or exchange it.

If the color became to pink fromupper side, find out the air leaksand fix it up.

22.1

2.2

2.3

Insulating oil:Dielectric strength Measure dielectric strength with a

oil tester and confirm it is morethan 40kV/2.5 mm gap.

If any measured valus are notsatisfactory, filtering and/ordegassing of insulating oil arerecommended.

Moisture content Measure moisture content in oilwith Karl-Fisher method andconfirm that it is less than 30 ppm.

Acid value (Onceevery three years)

Measure acid value and judge itwith the criteria of Table 7.

3. Fan motors and/oroil pump motors(once every threeyears)

(1) Measure insulation resistanceof fan motor and/or oil pumpmotors with a 500V megger.

If insulation resistance is less than

2M, check balancing of loadcurrents of three phase and dry outthe interior of fan motors.

(2) Check if temperature rise of oil

pump motors cases, based on oiltemperature, is less tahan 10.

If it is more than 10, check itsinsulation, resistance, windingresistance and three phasebalancing of load current.

4. Cooler of water-cooled type (Onceevery three years)

(1) Analyse the characterities ofcooling water as shown in Table8.

(2) Check scale deposit on theinside of the cooling tube toreduce the water flow required,resulting in excessive heating of atransformer at normal load.

If any characteristics are notsatisfactory according to the criteriashown in Table 8, shorteninspection interval of coolers.If any heavy scalae is found on theinside of the cooling tubes, cleanthem up.

5. Control panel andterminal box andcables (Onceevery three years)

(1) Check the water-tighness of acontrol cabinet and a terminalbox.

If the rubber gasket it worn out,exchange it with a new one.

(2) Ascertain the tighness of allcontrol wiring connections.

Tighten the loose bolts.

(3) Operate all switches,annunciators and lamps toobserve proper functionsaccording to schematic diagram.

If any part maloperates, adjust orexchange them with new one.


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(4) Measure insulation resistanceof cable.

Check any crack or abrading oncable insulation.

If such defect is found, put tape onit or exchange it with new one.

6

6.1

6.2

Protective relays (1) dial thermometer(3) dial oil gauge(5) Buchholz relay(7) sudden pressure relay(9) bursting plate beakingdevice

(2) winding thermal relay(4) oil flow indicator(6) gas detector(8) differential pressure relay(10) leak detector

Note: (8) and (10) foroil/water cooler

Insulationresistance (Onceevery three years)

Measure insulation resistance ofprotective relays including theirwirings with a 500V megger.

When the insulation resistence isless than 2M, check dews in theterminal box.

Operation Operation test should be madeonly when protective relaysoperated even if no troubles of atransformer occured.

The cause of malopertion should beinvestigated by operation testaccording to instruction books forprotective relays.

7. Connections(Occasionally)

All connectors outside of atransformer should be inspectedto see whether they are in goodconditions without anydiscoloration, which indicateshot connection.

Any connection that shows signs ofhaving been hot should bethoroughly cleaned and boltedtogether tightly.


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6. Additional inspection

Sometimes the following inspection and measurings are recommended for a preventive maintenanceand in investigating a transformer trouble.

Table 3

No Items Method Action or remarks11.1

1.2

Insulating oil.Analysis of gasdissolved on oil.

After a transformer is put intoservice, a periodic analysis of gasdissolved in oil is recommendedevery one or two years.

Using an appropriate air tighcontainer, oil should be taken fromtransformer.

The dissolved gas should beextracted from the oil and analyzed.

When the quantities of combustiblegases are obtained, evaluationscan be made to a probable locationand type of trouble which may bepresent in a transformer. (Refer toTable 10)

If there is an indication of moisture,sludge formation, low dielectricstrength and low rsistivity, aninspection should be made on theinside of the tank for possible cause

of the trouble.Resistivity Measure resistivity of insulating oilwhen a periodic test shows that theoil is in bad condition.

2. Insulationresistance ofwinding

Measure the insulation resistancebetween a pair of winding andbetween each winding and groundwith a 2kV megger.

The megger test should be made tocheck if the transformer is insuitable condition for operation orapplication of the dielectric test

The insulation resistance is subjectto wide variation with temperature,humudity and cleanness of bushingporcelains.

It may be low due to the leakagecurrent through the weakest casegases dissolved in oil should beanaylyzed.

3. Ratio test Measure the ratio of a transformerby two voltmeter method or with aratio tester.

If the transformer has taps, the turnratio should be measured for alltaps as well as for the full winding.

The test voltage may be between100 and 200V at the ratedfrequency.

Compare the test results with thesein the test report.

If it is difficult to measure becauseof fluctuation of voltmeter pointer orbecause of unbalance of a bridgecircuit, more detailed investigationshould be made.

4. Windingresistance

Measure the winding resistance bybridge method or by drop-of-potential method.

If oil pumps are provided, theyshould be operated in measuringwinding resistance.

The oil temperature should be alsorecord at the same type.

If the winding resistance correctedto a specified temperature, isdifferent from the data obtainedpreviously, more detailedinvestigation should be made.


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5. Exciting currentat low voltage

Measure the exciting current at lowvoltage (100-200 volts) applied onthe lower voltage winding with otherwindings being open-circuited.

The voltage wave shape should be

sinusoidal. (Note (1))

If the measured exciting current ismuch larger than the original dataat installation, more detailedinvestigation should be made.(Note (2))

6. Impedancevoltage

Measure the impedance voltage atlow current (5-10 amperes) appliedon the higher voltage winding withlower voltage winding being short-cirduited. (Note (3))

If the measured impedance voltageis much different from the originaldata at installation, more datailedinvestigation should be made.

7. Bushing currenttransformer

Exciting current and windingresistance should be measured inthe same way as items 4 nad 5.

Same as above 4 and 5.

Note:

(1) the ammeter should be conneted inside of voltmeter as shown below to avoid error to highimpedance measurement.

(2) If the measured values satisfy an expression below, the transformer may not get seriousinjury such layer shorting.

( ) 10

: Rated current : Measured current : Rated voltage : Measured voltage

(3) The shorting cable should be more than 100 in its section and be as shorter aspossible in its length, to avoid measuring error.

Shorting cable


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7. Maintenance

Maintenance of parts and materials should be planed according to the following tables.

Table 4

No Material and parts Maintenance frequency Remarks

1. Grease of fan motors Grease them up once a year, withthe grease gun.

Refer to items No.6. It is nonecessary when a bearing is sealedtype.

2. Porcelain insulatorsuch as bushing,lightning arrester,supporting insulator.

Cleaning should be doneperiodically.

Interval depends on actualcircumstane condition at site.

According to instruction book.

3. Cooler:(1) Radiator type

Recommended cleaning frequncy isonce every two yares.

With compressed air and/or water.

(2) Forced air type Dust and other foreign materials,accumulated on the finned area,should be blown out to maintain theefficiency of the cooler.

(3) Water cooled type Internal compartment check andcleaning should be done onceevery two years.

4. Paint Repainting should be done onceevery five years.

After cleaning the painting surface.

5. Bursting plate of

transformer

Bursting plate is recommended to

be re[placed once every five years.

6. Bearing of fan motorsand oil pum motors.

Expected life is ten years.

Expected life is three years, whenbearing is sealed type. (Only fanmotors)

Exchange with new ones, if a motorbecomes noisy due to brokenbearings.

7. Gasket (cork-neoprene)Thermometer (oil,winding)Pressure relay(water, nitrogen,sudden pressure relay,water/oil differentialpressure relay)

Expected life is ten years. Exchange with new gaskets, if oilleaks from gasket are serious.


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8. Trouble shooting

If any protective relay gives an alarm, investigate its causes according to the following table.

Table 5

No Relay Function Cause

1. Dial type thermometer The dial type thermometer indicates thetop oil temperature and highest oiltemperature experienced. It gives analarm when oil temperature gets to thealarm stting (80 for example).

1. Overloading.

2.Insufficient efficiency ofcooler units due to dust andother foreign materialsaccumtlated on the finnedarea or due to heavy scale inthe water cooling coils.

3. Maloperation ofthermometer or thermal relaydue to their own defects.

2. Thermal relay for oiltemperature or windingtemperature.

The thermal relay detects and indicatesmaxikum oil or winding temperature of atransformer.

It also has protective functions to give analarm or tripping signal and automaticfunctions to control cooling system.

3. Oil flow indicator The magnetic oil flow indicator checks theoperating condition of an oil pump.

When an oil pumps stops, the pointerreturns to the stop position and the micro-switch contact closes to give an alarm

1. Trouble of oil pump motor.

2. Trouble of wiringconnection to oil pumpmotor.

3. Radiator valves are shutoff.

4. Oil level gauge-dialtype

The dial type oil level gauge indicates theoil level in a conservator of an oil-immersed transformer.

When the oil level comes down to thebottom of a conservator, its poiterindicates zero and give an alarm.

1. Shortage of oil.

2. Abnormally low ambient

temperature in winterseason.

3. Oil leakage.


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9. Relay function

If any protective relay operates, investigate the causes of a trouble according to the following relaysfunction.

Table 6

No Relay Function

1. Buchholz realy(1) First stage The first stage of a Buchholz relay detects the gas formation due to

minor troubles in transformer tank.

The type and location of trouble may be predicted by gas analysesof oil.

Note: Nitrogen gas dissolved in oil at high pressure couldaccumulate in the relay and actuate the relay because of the rapidoil-temperature drop on a cold day in case a gas-sealedtransformer.

(2) Second stage The second stage of a Buchholz relay detects the rushing oil due toa serious trouble in the transformer tank.

If other protective relays such as over-current relay or differentialrelay, operate at the same time, a serious internal damage issuspected.

2. Sudden oilpressure relay.

The sudden oil pressure relay detects the high rate of oil pressureincrease in a transformer tank due to the gas generation and oilvapour caused by serious troubles.

3. Sudden pressure

relay (Type SP-3)

The type SP-3 sudden pressure relay detects the high rate if

nitrogen pressure increase in a tank due to the gas generation andoil vapour caused by serious troubles in the transformer tank.

4. Pressure reliefdevice

The pressure relief device operates when the pressure in the reliefvent rises abnormally high enough to reach the pressure of

approximately 0.85 kg/ caused by serious failure in atransformer.

It also operates when the pipe of air breather is choked so as toincresse the pressure in the relief vent.

5. Differential relay The differential relay detects the differece between the input currentand the output current of a transformer converted by a current

transformer.

It also operates sometimes with the inrush current when atransformer is excited.

6. Overcurrent relayand ground faultrelay

These relays detect faults in the electrical system includingtransformer.


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Fig 1. Allowable value of transformer insulation resistance

Fig 2. Reaction between dielectric strength and amount of water in insulating (

)


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Table 7. acid value criteria

No Acid value (mg KOH/g) Action

1. Less than 0.2 Good

2. From 0.2 to 0.4 Filter or exchange with newoil.

In earlier stage

3. More than 0.4 immediately

Table 8. Criteria of cooling water characteristcs

No Test item Criteria

1 Hydrogen ion concentration (PH) 6.0 8.0 at 252 Electrical conductivity Less than 500 (/cm) at 253 Chlorine ion Less than 100 ppm

4 Total hardness Less than 150 ppm

5 Surfurid acid iron Less than 200 ppm

6 M-Alkaliniry 15 60 ppm

7 Sulfur ion Undetectable

8 Ammomium ion Undetectable

9 Total iron ion Undetectable less than 0.5 ppm

10 Silica Less than 30 ppm

Table 9. Resistivity criteria of insulating oil at 80

No Resistivity (ohm-cm) Judgement

1 More than 1 x Good2 From 1 x to 1 x Precaution3 Less than 1 x Poor

Table 10. Combustible gas and type of fault with dissolved gas analysis of insulating oil.

No Decomposed gases Type of fault

1 , , , , , , Local heating in the oil2 , , , , , , , Local haeting in the oil impregnated solidinsulation3 , , , , Discharge in the oil4 , , , , , , Discharge in the oil impregnated solidinsulation

Note: Underline means a significant gas.


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APPENDIX I: Action for safety and quality

Table 11

No Working condition Action for safety

1. Exterior check 1) Be careful not to approach live parts.

2. Electrical test andremedial work

1) De-energise the transformer by circuit breakers and line switch.2) Ground the line terminals and the transformer.3) Attach caution tags not to operate switches for circuit breakers

and line switches. See note.4) De0energise the control cabinets for coolerd and tap changer

by AC and DC switches.5) Attach caution tag on switch boxes.

3. When internalinspection is to bemade.

Same as above except the following additional items.1) Replaced nitrogen gas completely with dry fresh air, if it was

filled in the transformer.2) Make sure there is 18% or more oxygen to sustain life ia a

transformer tank.3) Make sure your pocket are empty.4) Take off a wrist watch and any other accessories on your body.5) List up name and quantity of all tools to be brought into a

transformer tank.6) Spread out clean cloth on coil groups when repairing.7) Protect lamps with guards not to break them in a tank.8) Be carefull not to drop any tool and foreign material into the

transformer.Secure all tools wiyh hand lines. Any metallic item dropped intoa transformer must be removed to prevent serious trouble infuture.

4. After inspection work. 1) Make sure all foreign materials are clear before closingmanhole and energizing.

2) Check the quantity of all tools brought out from tang.3) Remove the grounding wires on the line terminals of the

transformer.

Note: when test the remedial work must must be carried out under the live conditions for any specialreasons, pay attention to live parts to protect yourself against electrical shock.

Documents

Transformer - Inspecton and Maintenance#