Power transformers are designed to operate reliably for several decades.

However, because of electrical faults, thermal stress, mechanical forces, insulation ageing, environmental conditions (or) improper operation, failures may still occur.

Failure analysis is the systematic procedure of identifying the root cause of a transformer problem using inspections, electrical tests, oil analysis, protection records and operational history. 

The objective is not only to repair the transformer but also to prevent similar failures in the future.

A proper analysis assists to improve system reliability, reduce downtime and optimize maintenance planning.

6 Stress Factors 

No.	Factor	Category it belongs to
1	Electrical faults	Electrical failures
2	Thermal stress	Thermal failures
3	Mechanical forces	Mechanical failures
4	Insulation ageing	Dielectric failures
5	Environmental conditions	Mainly external faults and dielectric failures (e.g. moisture, lightning, animal contact)
6	Improper operation	Can contribute to any category (e.g. overloading, re-energizing without confirming root cause)

Objectives of Failure Analysis

The objectives are to:

S.No	Objective	Analysis
1	Identify the actual cause of failure.	Distinguishes the root cause from the visible symptom.
2	Prevent recurrence of similar faults.	Turns one event into a lesson applied to the whole installation.
3	Decide whether repair, overhaul, or replacement is required.	Supplies the technical basis for the decision framework in Section 14.
4	Improve maintenance practices.	Feeds findings back into routine maintenance (e.g. cleaning and insulation testing, Section 11).
5	Enhance system reliability.	Reduces the chance of repeat outages.
6	Reduce downtime and operational losses.	Faster, better-targeted diagnosis shortens outage time.

Classification of Transformer Failures

Transformer failures can be classified into 4 categories.

Category	Failure modes listed in source
A). Electrical Failures	Winding short circuits; Earth faults; Inter-turn faults; Phase-to-phase faults; Flashover; Overvoltage damage
B). Thermal Failures	Overloading; Poor cooling; Hot spots; Insulation overheating; Excessive oil temperature
C). Mechanical Failures	Winding displacement; Loose core clamping; Damaged bushings; Gasket failure; Oil leakage; Broken leads
D). Dielectric Failures	Moisture contamination; Insulation ageing; Oil contamination; Partial discharge; Breakdown of insulation

External Faults

These originate outside the transformer but affect its operation. 

Examples:

• Lightning strikes.
• Short circuits in the switchyard.
• External line faults.
• Animal contact (birds, monkeys, snakes).
• Surge arrester failure.

These faults create high fault currents but do not always damage the transformer internally.

Internal Faults

These occur within the transformer. 

Examples:

• Winding insulation failure.
• Core faults.
• OLTC failures.
• Internal arcing.
• Loose internal connections.
• Insulation deterioration.

Internal faults are more serious and often require internal inspection or repair.

Comparison: External vs Internal Faults

Aspect	External faults	Internal faults
Origin	Outside the transformer	Within the transformer
Effect	Create high fault currents; do not always damage the transformer internally	More serious; often require internal inspection or repair
Examples (count)	5 listed	6 listed

Common Symptoms & their Possible Causes

Symptom	Possible Cause
High oil temperature	Cooling failure, overload
High winding temperature	Overload, blocked oil flow
Oil leakage	Damaged gasket, loose flange
Buchholz alarm	Gas generation, minor internal fault
Buchholz trip	Severe internal fault
PRD operation	Sudden internal pressure rise
Excessive humming	Loose core or magnetic vibration
Low BDV	Moisture or contaminated oil
High Tan Delta	Deteriorated insulation
Abnormal vibration	Loose mounting or winding movement
Burning smell	Insulation overheating
Unequal phase currents	Winding fault or loose connection

Root Cause Analysis (RCA)

Root Cause Analysis follows these steps:

Step	Action (as stated in source)	Phase (analysis)
1	Collect fault history.	Information gathering
2	Review relay indications.	Information gathering
3	Examine SCADA/event records.	Information gathering
4	Inspect the transformer externally.	Field examination & testing
5	Perform electrical tests.	Field examination & testing
6	Analyze transformer oil.	Field examination & testing
7	Conduct internal inspection (if needed).	Internal examination
8	Compare results with the factory records.	Evaluation and conclusion
9	Identify the root cause.	Evaluation and conclusion
10	Recommend corrective actions.	Evaluation and conclusion

Dissolved Gas Analysis (DGA)

DGA is one of the most powerful diagnostic tools for transformers.

Fault gases include:

Gas	Chemical formula	Indicates
Hydrogen	H₂	Partial discharge
Methane	CH₄	Low-temperature overheating
Ethane	C₂H₆	Moderate overheating
Ethylene	C₂H₄	High-temperature overheating
Acetylene	C₂H₂	Arcing
Carbon Monoxide	CO	Paper insulation degradation
Carbon Dioxide	CO₂	Cellulose ageing

Trending gas concentrations over time is more valuable than a single test result.

Buchholz Relay Case Analysis

Aspect	Alarm only	Trip
Possible causes	Small amount of gas generation; Air trapped after oil filling; Minor insulation degradation	Severe internal fault; Winding short circuit; Internal arcing; Major insulation failure
Action	Collect gas sample; Perform DGA; Inspect oil level; Monitor the transformer	Keep the transformer isolated; Perform detailed testing; Consider internal inspection

Pressure Relief Device (PRD) Operation

If the PRD operates:

Possible Causes

• Internal arcing.
• Rapid gas generation.
• Sudden pressure rise.

Actions

• Do not re-energize immediately.
• Inspect for oil discharge.
• Check relay records.
• Perform DGA & insulation tests.
• Open the transformer only if recommended by the manufacturer.

OLTC Failure Analysis

Common failures	Symptoms
Contact wear	Incorrect voltage regulation
Carbon deposits	Tap position mismatch
Motor failure	Failure to change taps
Mechanical jamming	Excessive arcing
Low OLTC oil level	—
Control circuit faults	—

Corrective action includes inspection, contact replacement (if required), oil replacement, and mechanism servicing.

Bushing Failure Analysis

Typical causes	Signs
Moisture ingress	Oil leakage
Cracked porcelain or composite housing	Corona discharge
Internal insulation breakdown	Flashover marks
Surface contamination	Abnormal heating
Loose connections	—

Routine cleaning and insulation testing help prevent failures.

Cooling System Failure

Symptoms	Possible causes
High oil temperature	Fan motor failure
High winding temperature	Oil pump failure
Fan or pump alarms	Closed radiator valves
Reduced transformer loading capability	Blocked radiators
—	Faulty temperature controller

Immediate repair is essential to prevent overheating.

Transformer Oil Problems

Common oil issues	Corrective actions
Moisture contamination	Oil filtration
Oxidation	Oil dehydration
Low BDV	Oil regeneration
High acidity	Oil replacement (when necessary)
Sludge formation	—
Particle contamination	—

Repair, Overhaul (or) Replacement

Decision	Suitable / Required for
Repair (suitable for)	Minor oil leaks; Faulty accessories; Fan or pump replacement; Gasket replacement
Overhaul (suitable for)	Winding repair; Core tightening; OLTC refurbishment; Internal cleaning
Replacement (required when)	Severe winding damage; Extensive insulation failure; Economically unrepairable condition

The analysis should be based on technical assessment, safety, reliability, and cost.

Safety Precautions during Failure Analysis

• Isolate the transformer from all power sources.
• Follow Lock-Out/Tag-Out (LOTO) procedures.
• Discharge any of the stored electrical energy.
• Wear appropriate Personal Protective Equipment (PPE).
• Handle transformer oil safely.
• Vent gas carefully prior to opening the transformer.
• Ensure adequate fire protection measures are in place.

Terms

Term	Meaning
BDV	Breakdown Voltage (of insulating oil)
CT / PT	Current Transformer / Potential (Voltage) Transformer
DGA	Dissolved Gas Analysis
FAT	Factory Acceptance Test
LOTO	Lock-Out/Tag-Out
LV	Low Voltage
OLTC	On-Load Tap Changer
ONAN / ONAF	Oil Natural Air Natural / Oil Natural Air Forced (cooling methods)
PPE	Personal Protective Equipment
PRD	Pressure Relief Device
RCA	Root Cause Analysis
SCADA	Supervisory Control and Data Acquisition
Tan Delta	Dielectric dissipation factor, an insulation condition test

Conclusion

Failure analysis is an essential engineering activity that goes beyond repairing damaged equipment. 

It is a structured procedure of understanding the sequence of faults, identifying the root cause, and implementing corrective and preventive actions to improve system reliability.

Combining visual inspection, electrical testing, transformer oil diagnostics, protection records and engineering judgement allows engineers to make informed decisions regarding repair, overhaul, or replacement.