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Maintenance Aintenance Of Transformers

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Routine preventive maintenance/inspection shall be done by the various officers/officials at desire frequencies. Detailed procedure of checking is given below :-

Daily Maintenance:

Sr. No. Item to be inspected Inspection Procedure By whom to be insp. Action Required
10-01D Oil level in conservator Read magnetic oil level gauge on conservator JE (M) If indicator shows less, than take action for topping up.
10-02 D Oil level in OLTC year casing  Read the oil level in glass indicator after opening the OLTC control cubicle. '' If level is above or below the red zone find out the cause and rectify.
10-03 D Oil level in diverter switch Read magnetic oil level gauge on diverter switch oil conservator compartment on  main conservator tank. '' If less find out cause and rectify
10-04 D Oil in breather Check the oil in oil cup of breather. '' If colour of oil is brown or there is no oil, provide fresh oil in the oil cup.
10-05 D Oil level in bushing Check the oil level of MV bushings and oil level in glass indicator '' Check for any visual leakage of oil. If oil is leaking from the transformer, take action for stopping of leakage, replace bushing if necessary.
10-06 D Oil leakage Visually observe the oil leakage all ar4oun the transformer '' If oil leakage found, take action for stopping the oil leakage and top up oil.
10-07 D Silica jel colour Inspect the colour of silica jel in dehydrating breather of transformers '' If colour of silica jel is whit or pink replace the silica jel.
10-08 D AC Supply Check the visual indication provided on power receptacles near transformer '' If all phase indicators not proper, check fuses & MCB etc. rectify.
10-09 D Temperature indicator On the indicating type thermometers on transformers and on control panel. '' Ensure that the reading of the these two does not defter too much. In case of difference in temperature reading, should be recorded in Defect register.
10-10 D Crack & dust on bushings Visually check that there is no dust on bushings. Also visually observe any crack  on bushings. '' If dust deposited take shut down and clean the bushings. If cracks, repair of replace bushings.
10-11 D Noise and Vibration Observe abnormal sound from transformers If sound is more check the frequency and voltage ratio i.e. V/F if voltage is too high and frequency too low. Observe if sound is from a loose part. '' Take action for normalizing the voltage/frequency. If loose parts, tighten then.
10-12 D Relief vent Check the Relief vent diaphragm to be in tact. '' Investigate, replace if found broken.

Quarterly Maintenance

Sr. No. Item to be inspected Inspection Procedure By whom to be insp. Action Required
10-13 Q Oil in cup of dehydrating breather Check the oil in dehydrating breather '' If colour of oil is brownish
10-14 Q Cooling fans and pumps Open the cooler control cubicle and put the auto/test manual switch on test and observe the pumps running which can be ascertained by checking the flow indicator (NOTE) : Do not operate all the pumps, only one pump shall run at a time. put the switch on fan group (i) and (ii) on test and check visually that all fans are running. Normalize all switches in original position. '' If any fan/pump in not running check the control circuit and AC supply to fan/pump and take action as given in trouble shooing. Lubricate the fan bearings. Replace wornout contacts.
10-15 Q Heaters Check that heaters are working. These heaters are provided in control cubicles of transformer '' If not working check supply and rectify. If heaters are burnt, replace the heater,
10-16 Q Earth connections Check bolts-nuts in each and every clamp of earth circuits. Check copper band used to connect the neutral bushing terminal to the riser from the earth mat. ''
10-17 Q Check for dielectric strength and moisture contain. ''

Yearly Maintenance

Sr. No. Item to be inspected Inspection Procedure By whom to be insp. Action Required
10-18 Y Connections of HV, MV & LV Check the tightness of clamps on HV, MV & LV bushing top terminals. JE(M)
AE(M)
EE		 Tighten if loose.
10-19 Y Neutral and earth connection Check the tightness of neutral connection of neutral bushing top and all earth strips i.e. from neutral of transformer and also from transformer body. The earth resistance of the solidly grounded neutral should be checked form the top of the neutral bushing terminal itself so that any loose or bad connection in the earth circuit is defected in the form of high earth resistance. ''
10-20 Y Buchholz relay
(i) Main		 Open the cover of main buchholz relay and press the push button provided to observe alarm and tripping. " If alarm and trip does not occur, check circuits of the relay and rectify the defect.
(ii) OLTC -do- " -do-
10-21 Y Thermometer HVLV MV oil Open the cap of thermometer pockets on the top of transformer tank and see that the oil in the pocket is available. " If no oil or less oil, pour transformer oil in the pocket and replace the cap of thermometer.
10-22 Y Dial Glass of Thermo meter The Glass cover of dial type thermometer mounted on transformer main tank may be checked. JE(M)
AE(M)		 Replace the glass of AE(M) thermometer cover, if broken.
10-23 Y Contact testing of thermometer

In the dial type thermometer on one side of thermometer a knob has been given for manually operating the thermometer, Operate the thermometer and read the temp of various settings i.e. alarm, fan, starting, pump start, trip etc.

Note: Do not forget to reset the maximum temperature indicator.

 " If the setting are disturbed reset the temperature. If any contact elective, replace it.
10-24 Y Oil Take samples of oil about 1/2 lit. each from (i) bottom of tank (ii) bottom of radiator bank and (iii) from OLTC after opening the valves provided at these places.

Note : The samples should be taken in a glass flask having air tight cork and two Nos. glass tubes fitted. The oil during sampling should not come in contact with atmosphere. (Detail procedure can be seen in instruction manual of transformer.

 " if average BDV of the samples is less than 35 KV at 4.0 mm gap of 12.5 mm dia balls, then centrifuge the oil.
10-25 Y Oil level (o) The oil level in conservator tank should be checked by reading the indicator of magnetic oil level gauge. The scale has been marked with temperature and the reading must tally to the temperature of oil read by the thermometer. If the oil is less then check for fisual leakage of oil from transformer.

(o) Check oil gauge pointer for free movement.

 " If there is substantial oil loss and leakage, attend the leakage and top up the oil (tested oil) if no leakage is there, then check magnetic oil level gauge & repair/replace it, it found defective.
10-26 Y Oil in bushing Record the reading of oil level gauge provided on the top of the bushing (HV. MV. LV) " If needle or level in glass shows no oil in bushing, check the leakage of oil & if there is leakage take action to prevent it.
10-27 Y Cracks deposit on bushing Cracks and dirt deposit on bushing shall be checked thoroughly, cleaning of dirt deposit on bushings is essential. " Clean the dirt on bushings & check for crack Repair/replace the bushing if needed.
10-28 Y Radiator bank The dirt & foreign material deposited on the radiator fins & in between the radiator fins shall be checked Dust and dirt deposits on radiator banks effects the efficiency of the radiator. JE(M) Clean the Radiators Surface.
10-29 Y Fans and Pumps The cooling fans and pumps mounted on the radiators banks & below the radiators banks shall be checked cleanliness & running.

(o) Check IR value motor windings after rainy season.

 JE(M)
AE(M)		 Clean the fans and pumps, oiling & greasing must be done in fans & pumps. The defective bearings may also be changed. Replace the defective and pumps. (o) Dry out if found low.
10-30 Y Main tank & pipe connections Main tank, pipe connections to various accessories such as radiator, conservator buchholz relay etc. shall be checked for cleanliness & oil leakage. Gasket joints should be thoroughly checked. " Clean the main tank and repair pipe connections. Replace the defective joint and gaskets to attend oil leakage.
10-31 Y OLTC contacts The contacts of diverter switch should be cleaned. This may be done by carrying out operations of the tap changer from 1 to 17 & 17 to 1 when the transformer is under shutdown.

NOTE : During operation of OLTC, keep the transformer OLTC, master/follower/individual switch on individual, and do not forget to take the tap position back to the same of other T/F while energizing the Transformer.

 " Clean the contacts of OLTC.
10-32 Y Cooling system Check the starting of cooling fans and pumps by operating the WTI (Thermometer) and check the starting of fans pumps are running and no abnormal noise is there. JE(M)
AE(M)
EE		 If fans do not start check & rectify the control circuit. If pumps do not start check & rectify the control circuit. Repair/replace the defective fans and pumps if any fan/pump gives abnormal sound replace the bearings & properly lubricate.
10-33 Y OLTC Cubicle & control cubicle Open the OLTC cubicles power cubicles & control cubicle & check that all these are water tight & have rubber beading. Check the tightness of connection. Check operation of switches illumination and operation of heater circuits. " Replace defective rubber beading, defective wires and switches etc. Tighten all the connections. Replace the heater if defective.
10-34 Y OLTC Record the counter reading of the tap changer which is visible from outside the cubicle. Check the operation of OLTC. Check oil in gear casing. Check limit switch & cleanliness. AE(M)
JE(M)		 Find out number of operations from the last oil change or commissioning. If no. is more then 5000 replace the oil. If it has given the service of more than 3 years centrifuge the oil lubricate the gear, clean the finger areas, provide oil in gear casing if less, repair/replace the limit switch and clean the parts.
10-35 Y Fastenings & gasketed joints. Thoroughly check the jacketed joints on main tank, pipe connections, radiators, relays etc. for any oil leakage & loose/missing bolts and nuts. AE(E)
JE(M)		 Tighten all loose bolts and nuts & replace the missing bolt if any.
10-36 Y Valves & Stop Cocks The valves have been provided on pipes connecting the main tank and radiators. Stop cocks have been provided for smaller pipes on main tank, check these for operation and leaks. Also ensure the position of these valves and stop cocks. " Attend the leaks and put the valves in correct position.
10-37 Y Insulation resistance Disconnect the connection of HV, MV & LV terminals and record the IR values with 5 KV megger Compare the values with those at the time of commissioning. " If megger values are poor, find out the cause, and centrifuge the oil. Compare readings with previous values.
10-38 Y Pressure relief device Pressure relief device has been provided on a separate pipe connected to the main tank. Check the operation by manually operating the test switch and see it tripping occurs.

(i) For 100 MVA Transformer Yearly.

(ii) For 12.5 to 40 MVA Transformer once in 5 years.

 " Replace the pressure relief device if defective.
10-39 Y Dissolved Gas Analysis of oil Take out oil samples from valve provided in bottom of Main tank, radiator bank and OLTC and get the dissolved gas and chemical analysis done, Compare the results with those given below.

(1) Dissolved gas Analysis (Recommended) values as per SE 400 KV s/s Design Circle Lucknow letter No. 595/400 KV/ESDC/dated. 21.8.1985.

 "
 

Drying out of transformers is to be done if low iR value is observed. However,
10-40 TH
  • All power transformers upto 40 MVA capacity should be centrifuged to improve IR value and BDV of oil after every 3 years of service.
10-41 F
  • All power transformers more then 40 MVA capacity should be centrifuged after every 5 years of service.

Mizing of Transformer Oil :

  • Quality oil (know as EHV oil) is compatible with imported oils of paraffinic and naphthenic origin.
  • Quality oil (EHV oil) of inhibited type is compatible with inhibited type.
  • Oil conforming to ISS-335 is not be treated as EHV oil and not suitable for topping up purpose in EHV transformers.

Trouble Shooting :

In case of emergency and breakdowns take action as below :-

Sr. No. Trouble Problem Cause Action to be taken
1. Differential relay and Buchholz relay and/or pressure relief device operates simultaneously
  • Internal Fault in transformer
  • Electrical tests :
    • Insulation resistance.
    • Voltage ratio.
    • Exciting current.
    • Winding resistance.
  • Gas analysis (trapped in buchholz, contact the manufacturer.

2.

 Buchholz relay and pressure relief device operate simultaneously.
  • Internal fault
  • Electrical Tests :
    • Insulation resistance.
    • Voltage ratio.
    • Exciting current.
    • Winding resistance.
  • Gas Analysis (Trapped in Buchholz).

IDENTIFICATION AND SIGNIFICANCE OF GASES :

The nature of the fault could be judged with help of gas collected in the Buchholz relay.

Following checks should immediately be done after detecting that the gas has collected in the Buchholz relay.

(a) Colour of the Gas : This helps in finding materials being decomposed

Colour of Gas. Identification
* Colour less Air
* White Gas due to decomposed pap insulation.
* Yellow Gas due to decomposed wooden insulation.
* Grey Gas due to overheated oil due to heating of core.
* Black Gas due to decomposed oil due to electric arc.

(b) Testing of gases trapped in Buchholz to be done.

COMBUSTIBILITY :

A small qty. of gas may be drawn off through top petcock of the relay by syringe & flame tested. About 2 to 5 Cm. of gas is expelled into a flame. If the flame brightened the gas is combustible.

Incombustible gas indicates decomposed insulation & oil vapour.
3. Differential operates
  • Internal fault in Transformer
  • Electrical Test:
    • Insulation resistance.
    • Exciting current.
    • Winding resistance.
  • Dissolved Gas Analysis. consult the manufacturer.
  • In rush current at the time of energisation
  • Check setting of values of relays.
    • Value of in rush current is within limit then set right the defect of relays.
  • Surge Capacitor defective
  • Check capacitor visually.
  • Measure charging current of capacitor.
  • Lightening Arrestor on tertiary side defective
  • Check Lightening arrester visually.
  • Check insulation values of lightening arrester. Replace the defective LA.
  • Faulty operation of relays or devices.
  • Check wiring.
  • Check relays.
  • Check contact. Rectify the defect/Replace the relays.
4. Only Buchholz trip operates.
  • Internal fault.
  • Faulty operation of relay.
  • As at Sl. No. 1.
  • Check wiring.
    • Check contacts. Rectify the defect of wiring contact.
5. Only Pressure relief device operates
  • Internal fault
  • Faulty operation of device
  • As at Sl. No. 1.
  • Check wiring.
    • Check contacts. Rectify the defect of wiring/contacts.
6. Only Buchholz alarm operates.
  • Internal fault
  • Trapped air or evolution of dissolved gases.
  • Any sudden drop in temperature.
  • Repeat as Sl. No. 1.
  • Gas analysis of trapped gases.
  • Get dissolved gas analysis & rectify the defect on the inference of gas analysis.
7. OC / EF & Restricted E/F operates.
  • Internal fault in T/F
  • External fault on T/F such as short circuit fault in system.
  • As at Sl. No. 1.
  • Check for sign of heavy current circuited in windings and rectify.
    • Any sign or on load tap changer interrupting heavy short circuit current.
8.

Buchholz relay (pilot relay of on load changer and pressure relief device operates simultaneously.
  • Internal fault in Diverter switch.
  • As at Sl. No. 8.
  • Check for operation of relays.
  • Check & rectify external short circuit.
9. Incomplete tap changing relay actuates.
  • Diverter switch fault.
  • As at Sl. No. 8.
  • Check and rectify wiring.
10.

Either Buchholz relay of OLTC or pressure relief device operates.
  • Diverter switch
  • Faulty operation of relay or P.R. device.
  • Check and rectify wiring.
  • Check that pressure relief device is not malfunctioning, replace the pressure relief device found defective.
11.

Improper stopping positions due to incomplete to incomplete tap changing or out of step in tap position.
  • Driving Motor does not operate.
  • Driving motor operates but incomplete tap changing takes place.
  • Excessive operating torque.
  • Check for operation of mini circuit breaker for improper voltage, if MCB operated. Check & rectify defects in following-
    • Motor
    • Magnet brake
    • Pilot switches.
  • Check and rectify control circuit.
  • Check and rectify and slopping of magnet brake.
  • Check contact close and open positions of switches and rectify the defect.
  • Time setting value of relay.
  • Rectify the cause of excessive operating torque.
12. Alarm of AC supply failure to OLTC
  • Over current due to abnormal torque.
  • Insulation break-down of driving motor.
  • Certain phase of motor circuit has been out of connection.
  • Motor circuit has been faulty grounded.
  • Replace the motor if defective.
  • Check wiring and rectify.
  • Check and rectify defect of earthing.
13. Blowing out of fuse of AC supply.
  • The coil of motor/breaking contactor/step relay burst out.
  • The control circuit has been faulty grounded.
  • Replace the motor/breaking contact/step relay.
  • Defect & Rectify faulty Circuit.
14.

Driving Motor does not start up responding to a tap changing signal.
  • Certain phase of motor supply defective.
  • Disconnection or welding of contact of motor contactor.
  • The coil of motor contactor burnt or disconnected.
  • Poor contact of electrical limit switches.
  • Poor contact of inter lock switch.
  • MCB has been opened.
  • Fuse has blown off.
  • Poor contact of push button
  • Disconnection or looseness of wiring.
  • Very low supply voltage.
  • Connect all the three phase properly.
  • Repair/replace the contactor.
  • Replace outer lock switch.
  • Find out cause for MCB tripping & rectify.
  • Replace outer fuse.
  • Replace push button.
  • Rectify defect of wiring.
  • Increase supply voltage.
15. Continuous operation of tap changer.
  • Welding of a contact of motor contractor or sticking of its cores.
  • The signal circuit faulty & closed due to fault of the step relay.
  • Replace motor contactor.
  • Repair/replace step relay.
16. Faulty tap position indication in control room.
  • Disconnection of tap position indicator.
  • Poor contact of tap position transmitter.
  • Dislocation of contacts of tap position transmitter.
  • Bulbs fused.
  • Connect the tap position indicator.
  • Tighten the terminals.
  • Set right the contacts of tap position transmitter.
  • Replace fuse bulbs.
17. Oil temperature high alarm/winding temp high alarm.
  • Over loading.
  • Insufficient efficiency of cooler units due to dust and other foreign material.
  • Mal-operation of thermometer.
  • Defect in cooling fan starting.
  • Defect on pump starting.
  • Wrong position of valve.
  • Reduce the load.
  • Clean the cooler banks.
  • Repair/replace the thermometer.
  • Check starting of all fans and rectify the defects.
  • Check all Pumps & rectify the defects.
  • Check the position of all stop valves.
18. Low oil level alarm
  • Leakage of oil.
  • Abnormally low temperature in winter season.
  • Defective oil level gauge.
  • Attend all leakages and top us the oil.
  • Top up oil.
  • Replace oil level pumps.
19. Motor failure alarm
  • MCB of fan operated.
  • Fuse blown off.
  • Fan winding burnt.
  • Defective thermal relay.
  • Detect the cause of operation of MCB and rectify.
  • Replace fuse.
  • Replace winding.
  • Replace/repair thermal relay.
20. Fan failure Alarm
  • Defective MCB.
  • Fuse blown off.
  • Defective thermal - relay.
  • Pump motor winding burnt.
  • Replace MCB.
  • Replace fuse.
  • Replace/repair thermal-relay.
  • Repair/replace the pump motor.
21. Low dielectric strength of oil.
  • Leakage around cover accessories.
  • Defective breather or silicagel.
  • Regasket and filter the oil.
  • Repair/replace breather.