Failure Of Transformers

PRESENTATION  Transformer failure  Failure analysis  Case studies

C.P.Deshmukh (Sr. Specialist)BHEL

INTRODUCTION  Transformers play vital role in transmission of Generated power.  Distribution of power to the utilities of electricity.  Operation demands reliability and uninterrupted power supply.  Minimum outage and earliest restoration.

FAILURE OF TRANSFORMERS         

External reasons for transformer failure Moisture Tap changer Bushings Hard synchronization Hot spot inside the transformer Short circuit faults Ageing/ poor maintenance Fire & others.

FAILURE ATTRIBUTION          

System faults, transients & line short circuits. Erection deficiencies. Poor maintenance . Accessories failures (bushing, tap changer). Incipient fault. Repeated back charging Hard synchronization Over fluxing Ageing & continuous operation/ inadequate maintenance. Fire and other reasons.

SYSTEM FAULTS & SHORT CIRCUITS          

Adequacy of protection against external faults. Analysis of transformer suitability. Testing of transformer for all low voltage test. Monitoring of oil parameters. Internal inspection Overhauling and maintenance Retightening of all the joints and terminal gear leads. Thorough checking of on load tap changer Healthiness of bushings. Re-commissioning & close monitoring.

PREVENTION OF ERECTION DEFICIENCIES           

N2 Pressure maintenance during storage. Internal inspection. Impact recorder analysis for jerks during transportation. Corrective action for internal anchoring of parts. Thorough cleaning. Ensure core and core clamping isolation. Purging with dry nitrogen and heating if necessary. Thorough testing of all the parameters including bushings etc. Details study of test results Commissioning of transformer by experts. Clearance for charging from OEM.

MAINTENANCE REQUIREMENT        

Deterioration of oil and ingress of moisture. OLTC oil drastic contamination. Healthiness of bushings. Isolation of core and clamping structure. Removal of sludge like material from bottom tank. Incipient fault diagnosis. Periodic monitoring of temperature parameters. Thermal scanning of all the external joints and transformer tank pockets.  Checking of gases collection in B/R.  DGA monitoring

ACCESSORIES FAILURES (BUSHING, TAP CHANGER).  OIP Bushing : o o o o

Increase of tan delta of bushing. Leakage through porcelain / flange joints. Oil leakage from test tap connection. Oil loss visible from gauge glass.

 Tap changer o o o o o

Oil contamination. Poor BDV value Ingress of moisture Imperfect joints Contacts pitting/ erosion

 Conservator o Air cell failure. o Excessive oil rise due to thermal fault.

INCIPIENT FAULT.      

Decomposition of oil due to local heating. Isolation failure of core & core clamping. Failure of yoke shunts insulation. Multiple earthing of wall shunts. Sludge deposit on core and moisturization. Improper earthing strips of shunts and core clamping structure.  Heating of joints ( brazing & crimped) & hot spot.  High discharges in critical areas in transformers.

      

REPEATED BACK CHARGING &HARD SYNCHRONIZATION Switching surges during operation. Decomposition of oil in critical areas. Physical movement of windings in the tank. Discharges in critical areas. Incipient fault development. Unbalance of amperes turns in transformer . Looseness of winding and leads due to physical movement during these operations.