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PRESENTATION

Transformer failure Failure analysis

Case studies

C.P.Deshmukh

(Sr. Specialist)BHEL

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INTRODUCTION

Transformers play vital role intransmission of Generated power.

Distribution of power to the utilities

of electricity. Operation demands reliability and

uninterrupted power supply.

Minimum outage and earliestrestoration.

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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.

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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.

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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.

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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.

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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

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ACCESSORIES FAILURES (BUSHING, TAP CHANGER).

OIP Bushing :

o Increase of tan delta of bushing.

o Leakage through porcelain / flange joints.

o Oil leakage from test tap connection.

o Oil loss visible from gauge glass.

Tap changero Oil contamination.

o Poor BDV value

o Ingress of moisture

o Imperfect joints

o Contacts pitting/ erosion

Conservator

o Air cell failure.

o Excessive oil rise due to thermal fault.

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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.

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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.