Troubleshooting Transformer In Power Substation And Detailed Failure Investigation

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 result in failure.

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

Contents:

• Transformer failure investigation
• Causes of transformer failures and their remedies
• Investigation into causes of failures of transformer
• DO’s & DON’Ts

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

2. Low break down voltage (BDV)

3. Bushing failure

4. Winding failures

5. Excessive overheating of oil

6. Low IR value

7. Humming sound

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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