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Transformer Failure

Failure of transformer in power substation is not a sudden phenomenon, for that matter each and every failure will take place only after alerting through some pre-signs. If they went unnoticed or unattended will results into a failure.

Troubleshooting Transformer In Power Substation And Detailed Failure Investigation
Troubleshooting Transformer In Power Substation And Detailed Failure Investigation (photo credit: ReinhausenTV via Youtube)

Therefore it is wise on the part of the maintenance personnel to act upon the pre failure signs noticed well in advance to keep the transformer failure free and serviceable at all times.

If new transformer is installed for which there is no past experience, it will be difficult to forecast the defects and probable failures.

Further if a failure occurred even though all the known precautions were observed, then it is necessary and more advantageous to investigate into the failure in such a manner to pull out the actual reasons of failure so that action shall be initiated to avoid recurrence.

Contents:


An approach to transformer failure investigation


1. On Failure Aspects

  1. Occurrence
  2. Date of occurrence
  3. Past similar occurrences if any
  4. Analysis of failure i.e. why did it happen?
  5. Whether the rate of failure is worse than other installations?

2. On Maintenance Aspects

  1. Whether scheduled maintenance and required testing have been carried out on the failed equipment as per norms stipulated?
  2. Does the frequency of maintenance require change?
  3. Was the work properly supervised?
  4. Is any modification possible to avoid failure?

3. About Staff

  1. Is the quality of work done satisfactorily?
  2. Is the skilled staff properly trained to carry out the work?
  3. Are proper tools available with the staff?

4. About Material

  1. Is the material received from approved source?
  2. Whether the material is as per approved specification?
  3. Can a better material be used?

5. About Testing

  1. Is the testing equipment available?
  2. Could testing procedure be improved to weed out the failures?
  3. Whether testing equipment are calibrated?

6. General Points

Whether following points were checked / performed properly?

  1. Proper contact
  2. Clearances
  3. Capacity
  4. Proper contact pressure
  5. Crack detections
  6. Cleaning
  7. Proper connections / alignment
  8. Cross checks / super checks

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Causes of failures and their remedies


Common Failures of Transformer

Some of the common failures/ defects occurred in transformer are as under:

1. Oil leakage
LocationPossible CausesRemedial Action
From screw jointsForeign material in threadsRemove the foreign material
Poor threadsCheck the threads & replace if required
Improper assemblyEnsure proper assembly
From gasket jointsInsufficient or uneven compressionTight gasket joints uniformly
Improper preparation of gaskets and gasket surfacesProvide proper gaskets
Old gasketsProvide new gaskets
From weld jointsShipping strains, imperfect weldRepair welds following proper procedure
From couplings & their jointsCracks in couplingsReplace couplings and secure the pipe lines near couplings properly
Defective coupling jointsMake proper couplings joints and tight screws
From drain plugsDefective thread portionCheck the threaded portion
Defective oil sealReplace the oil seal and tight the drain plug
2. Low break down voltage (BDV)
Type of FailurePossible CausesRemedial Action
Low BDVMoisture contamination in transformer oil due to inactive silica gel (pink color)Reactivate silica gel crystals or replace them. Purify the transformer oil to restore dielectric strength
Leaks around cover accessories, breathing air from leaksAttend leaks, replace gasket if necessary. Purify the transformer oil to restore dielectric strength
Humid atmosphere in rainy seasonPurify the transformer oil to restore dielectric strength and check the BDV & water content
3. Bushing failure
Types of FailurePossible CausesRemedial Action
HV Bushing flashoverLightning discharge or overvoltageIt may be a break in the turns or end lead, flash marks on the end coil and earthed parts close to it
Dirty bushingEnsure cleaning of porcelain bushing during each inspection
HV Bushing porcelain insulator petticoat broken / crackedExternal hittingEnsure proper cleaning and visual checking of porcelain bushing during each inspection
4. Winding failures
Types of FailurePossible CausesRemedial Action
Primary winding lead open circuited / earthedDue to overload or brazing failureCheck the winding in one or all phases would show signs of overheating and charring
Bulging and inter turn short, inter layer short or inter coils shortCoils shrink and in between insulation failureInvestigate for overloading and take corrective action accordingly
Shorting between LV and HV coilsInsulation failureDuring manufacturing / rewinding of the transformer, the coils should be pressed down, heated and cooled repeatedly until the coil height stabilizes
Flash mark on the core and supportDead short circuit due to lateral or displacement of the coilNomex paper insulation sheet should be provided between H.V. and L.V. coils so as to strengthen the insulation level. Ensure that this insulation sheet does not cause any obstruction in the passage of oil flow
Winding loose on the coreReplace the transformer and core to be lifted for thoroughly checking and take corrective action accordingly
Repair the winding if possible
5. Excessive overheating of oil
Type of FailurePossible CausesRemedial Action
Temperature rise of transformer oilAny internal fault such as short circuited core, core bolts/ clamps insulation failure etc.Replace the transformer and core to be lifted for thoroughly checking. Take corrective action according to observations and oil test report.
Low oil level in conservatorCheck the oil level in conservator and top up if required
Slugged oilCarry out purification of oil to remove sludge
OverloadingAdjust the load
6. Low IR value
Type of FailurePossible CausesRemedial Action
Low IR ValueMoisture in oilPurify the oil with high vacuum type oil purification plant and test the oil for electrical strength and water content
Insulation failure between winding and coreReplace the transformer. Lift the active part and check the winding thoroughly for insulation damage and take corrective action accordingly.
Internal connection leads insulation damageCheck the internal connection leads by lifting the active part and re-tape insulation paper of damaged portion
Weak brazingClean the joint and braze properly
7. Humming sound
Type of FailurePossible CausesRemedial Action
Humming soundLoose coreLift the active part and tight all the pressure bolts and clamping bolts.
Winding loose due to shrinkage of coilsDuring manufacturing/ rewinding of the transformer, the coils should be pressed down, heated and cooled repeatedly until the coil height stabilizes.
The winding pressure bolts and core clamping bolts should be tightened during the first periodical overhauling after commissioning to take care of shrinkage.

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Investigation into causes of failures of transformer

In most cases the causes of the fault can be surmised by careful observation of the condition of windings, e.g. displacement of the turns or coils, coil insulation (brittle or healthy), evidence of overheating, carbon deposit or flash marks on the core, supports, the inner surface of the tank or cover.

The following notes may be helpful in identifying the causes:


1. Failure due to Lightning Discharge or Overvoltages

This is characterized by break down of the end turns close to the line terminal. There may be a break in the turns or end lead, and also flash marks on the end coil and earthed parts close to it, but the rest of the coils will be found to be healthy.


2. Sustain Overloads

The windings in one or all phases would show signs of overheating and charring. The insulation would be very brittle and would have lost all its elasticity.


3. Inter-turn short, Inter-layer short, or Inter coils short

The same signs as for indicated for sustained overload would be noticed, but only on affected coils, the rest of the coils being intact.

This is likely if the differential relay or the Buchholz relay has operated.


4. Dead Short-circuit

This can be identified by the unmistakable, lateral or axial displacement of the coils. The coils may be loose on the core, some turns on the outermost layer may have burst outwards and broken as if under tension.

If, in addition to these signs, the windings are also completely charred, it is conclusive evidence that the short circuit has continued for an appreciable period, not having been cleared quickly by the protective relays.


5. Buchholz Relay Tripping

If the upper chamber of the Buchholz relay alone has tripped, check the insulation of core bolts, by applying a voltage of 230V to 1000V between the core and each bolt. If it fails, renew the insulating bush.

Observe also all the joints, and tap-changer contacts, for overheating and arcing.


6. Internal Flashover

If the oil shows a low break down voltage (BDV), it does not necessarily mean it has caused the breakdown. At high voltage ratings, excessive moisture content in the oil may result an internal flashover between the live parts and earth, which all leave corresponding tell tale marks.

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DO’s & DON’Ts

Do’ s

  1. Ensure all safety arrangement while working on electrical installation. Ensure that all tools and tackles are in good and working condition.
  2. Check and thoroughly investigate the transformer whenever any alarm or protection is operated.
  3. Check the protection system periodically.
  4. Ensure every employee is familiar with the instructions for restoration of persons suffering from electric shock.
  5. Trained the staff in operating the fire-fighting equipment.
    Always avoid unbalance loading on phase!
  6. Do earthing of all points before starting maintenance.
  7. Keep all spares away from dirt.
  8. Work with full confidence.
  9. Ensure thorough and full cleaning of insulators, since partial cleaning is worse than no cleaning.
  10. Ensure perfect isolation of supply before commencement of maintenance work. Put a caution board when on work.

Don’ts

  1. Don’t use low capacity lifting jacks on transformer.
  2. Don’t leave circuit tap switch unlock.
  3. Don’t leave any loose connection.
  4. Don’t meddle with protection system.
  5. Don’t allow conservator oil level to fall below minimum level mark of indicator.
  6. Don’t parallel transformers which do not full fill the necessary requirement.
  7. Don’t allow unauthorised entry in the sub-station.
  8. Don’t overload the transformer other than the specified limit mentioned.
  9. Don’t tight the nuts & bolts in excess to arrest any leakage.
  10. Don’t avoid any unusual noise / occurrence noticed in the substation.
  11. Never use fuses higher than the prescribed ratings on HV and LV sides.
  12. Earthing connections should never be done in loose manner.
    Simply twisting of galvanized iron (GI) wires would be dangerous! The earthing connections should as far as possible be done by using continuous wire or providing suitable connectors. It should be ensured that these connections are tightened rigid.
  13. Never keep the breather pipe open or exposed.
  14. Don’t ignore safety rules during maintenance work.

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Reference // Maintenance of general services substation by Camtech

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About Author

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Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

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