Quickly Locate Equipment Faults! High‑Frequency Causes, Diagnosis & Step‑by‑Step Troubleshooting Guide for Dry‑Type & Oil‑Immersed Transformers

ntro Dry‑type and oil‑immersed transformers serve as core power‑supply equipment within power distribution systems. Their faults are often concealed, develop rapidly and can cause severe damage. Minor unresolved defects may trigger overheating tripping, insulation breakdown, winding burnout, large‑scale power outages and substantial economic losses.

According to operation and maintenance statistics, around 80 % of transformer faults can be identified via visual inspection, temperature measurement and electrical tests, without complete unit disassembly. In field practice, disorganized maintenance workflows and confusion over the difference between dry‑type and oil‑immersed transformer failures frequently lead to inefficient troubleshooting. You can find more technical analysis in our industry insights library.

Combined with structural differences and real‑world operation experience, this guide sorts out high‑frequency fault triggers, typical symptoms and standardized step‑by‑step inspection workflows to help maintenance crews rapidly detect and eliminate defects.

1. Fundamental Fault Differences Between Dry‑Type and Oil‑Immersed Transformers

Due to different cooling and insulating media, dry‑type and oil‑immersed transformers show distinct fault patterns and different inspection priorities.

Comparison Item	Dry‑Type Transformer (Epoxy‑Resin Cast / Air‑Cooled)	Oil‑Immersed Transformer (Insulating‑Oil Cooling & Insulation)
Key Weak Points	Dust accumulation & creepage discharge, blocked air ducts, loose connections, multi‑point core earthing, dampness & corrosion	Insulating‑oil degradation, oil leakage, gas relay faults, partial discharge & gas generation in oil, bushing dampness
Typical Fault Signals	Discharge buzzing / cracking noise, winding carbonization, fan failure, overtemperature alarm from temperature controller	Gas‑relay activation, sharp oil‑temperature rise, dark‑colored oil, oil seepage, excessive gas in DGA chromatogram
Primary Initial‑Check Methods	Infrared temperature measurement, visual cleaning, insulation‑resistance test	DGA oil‑chromatography analysis, oil‑quality test, gas‑relay signal verification
Common Operating Environments	Dust‑prone workshops, high‑humidity underground spaces, harmonic‑rich loads, summer overload conditions	Outdoor exposure, rainwater intrusion, short‑circuit caused by small animals, long‑term heavy‑load operation

Supplementary note: Dry‑type transformers rely on air cooling and are mostly deployed indoors; faults mainly stem from environmental conditions, heat dissipation problems, insulation damage and poor connections. Oil‑immersed transformers use insulating oil for both cooling and insulation, widely applied for high‑power outdoor distribution, and tend to develop latent internal defects. Review our case studies to see how these fault characteristics appear in real‑world project scenarios.

2. High‑Frequency Faults, Symptoms and Troubleshooting for Dry‑Type Transformers

Dry‑type transformers are highly environment‑sensitive. Most failures originate from environmental influences, abnormal heat dissipation and insulation deterioration.

2.1 Multi‑Point Core Earthing (Core‑to‑Earth Insulation Drops Near Zero)

Symptoms: Increased no‑load loss, local overheating on core housing; core‑to‑earth insulation resistance close to zero; protection devices prone to mis‑operation and tripping. Causes: Burrs on core laminations, bridging metallic debris; aged or damaged spacer insulating gaskets; conductive metal fragments connecting core and clamping plates.

Troubleshooting Steps:

1. Measure core‑to‑earth insulation with a 2500 V megohmmeter; values below 10 MΩ indicate a fault.
2. Power off and clear metallic debris inside coils.
3. Inspect and replace damaged insulating gaskets.
4. Retest insulation after finishing repairs.

2.2 Winding / Insulator Discharge Noise (Creepage & Partial Discharge)

Symptoms: Hissing or cracking discharge noise during operation; noise worsens under humid weather; carbonized creepage marks on insulation surfaces; discharge spots observable at night; large fluctuation of insulation resistance; tripping easily when load varies. Causes: Dust deposits absorb moisture and form conductive paths; mis‑aligned high‑voltage components cause concentrated electric‑field stress; gaps created by poor connector processing produce gap discharge.

Troubleshooting Steps:

1. Power off and check windings and terminals for burn‑in or creepage traces.
2. Measure high‑ and low‑voltage side insulation resistance; readings lower than 80 % of factory test values suggest insulation degradation.
3. Carry out partial‑discharge test: partial‑discharge level ≤10 pC for equipment ≤35 kV.
4. Purge dust with dry compressed air and perform dehumidification drying.
5. If faults persist, replace insulators and readjust component installation clearances.

2.3 Excessive Temperature Rise & Overtemperature Alarm (High‑Incidence Fault)

Symptoms: Overtemperature alarm triggered by temperature controller; winding temperature difference among three phases＞10 ℃; fan malfunctions; burnt smell detected; obvious hot‑spots captured via infrared thermal imaging. Causes: Overloading, unbalanced three‑phase loads; dust‑clogged air ducts, defective cooling fans; loose tap‑changer bolts; high ambient temperature and poor ventilation inside distribution room; faulty temperature‑sensing probes or temperature controller.

Troubleshooting Steps:

1. Check three‑phase current readings and control load‑unbalance ratio.
2. Remove dust from air ducts and test cooling‑fan performance.
3. Take infrared temperature readings and calibrate temperature‑sensing probes.
4. Locate hot‑spots by infrared thermal imaging.
5. Fasten tap‑changer bolts and re‑measure DC resistance.

2.4 Burned‑Out Connection Terminals & Delta‑Junction Connectors

Symptoms: Oxidized and blackened connectors; unbalanced three‑phase DC resistance; heating aggravates with rising load, resulting in contact‑related tripping. Causes: Low‑quality connector materials, oxidized contact surfaces; bolt loosening caused by vibration and thermal expansion‑contraction cycles.

Troubleshooting Steps:

1. Locate hot terminals by live‑line infrared inspection.
2. Power off and inspect bolts and gaskets; remove oxidation from conductor surfaces.
3. Test contact resistance.
4. Polish contact surfaces, fasten bolts according to specified torque and replace aged gaskets.

2.5 Excessive Vibration‑Noise & Malfunctioning Temperature‑Control System

1. Abnormal Roaring & VibrationSymptoms: Harsh noise, cabinet base resonance.Causes: Loose clamping‑plate and pressure‑plate bolts; harmonic resonance; missing or damaged shock‑absorbing pads.Troubleshooting: Fasten bolts; replace broken shock‑absorbing pads; install filter devices under severe harmonic conditions.
2. Faulty Temperature‑Control with False AlarmsSymptoms: Abnormal temperature readings, disordered fan start‑stop logic, missing or false overtemperature alerts.Causes: Displaced / aged probes, loose wiring connections, defective temperature‑controller hardware.Troubleshooting: Inspect probe wiring and reset parameters; cross‑verify readings with infrared measurement; replace defective components directly.

Supplementary note: Aged & damaged winding insulation: persistent discharge noise, excessive partial discharge readings, cracked or peeling insulation. Usually caused by high‑temperature corona, collision‑induced micro‑cracks or long‑term damp aging. Detect by partial‑discharge and ratio‑test. Minor defects can be fixed via reinforcement and dehumidification; severe cases require factory repair.

3. High‑Frequency Faults, Symptoms and Troubleshooting for Oil‑Immersed Transformers

Supported by DGA oil‑chromatography analysis, oil‑immersed transformer faults are divided into thermal faults, discharge faults and mechanical / leakage faults.

3.1 Internal Overheating Faults (Low / Medium / High‑Temperature Overheating)

Symptoms: Rising top‑oil temperature, darkened oil color; methane and ethylene concentrations increase in DGA without acetylene; hot‑spots appear externally on bushings and tap‑changers. Causes: Low‑temp: overload & three‑phase load imbalance; medium‑temp: worn tap‑changer contacts, loose connections; high‑temp: short‑circuited core laminations, multi‑point core earthing.

Troubleshooting Steps:

1. Readjust unbalanced loads; find external hot‑spots by infrared scanning.
2. Judge overheating severity level by DGA chromatogram.
3. If no external abnormality found, perform core lifting inspection on tap‑changer, leads and iron core.

3.2 Discharge‑Type Faults: Partial Discharge, Spark Discharge & Arc Discharge

Symptoms: Partial discharge: rising hydrogen content, faint abnormal noise; spark discharge: simultaneous rise of hydrogen and acetylene; high‑energy arc: acetylene＞5 μL/L, heavy gas‑relay activation, even oil ejection. Causes: Cracked bushing, water ingress and damp oil, inter‑turn winding damage, floating‑potential metallic components.

Troubleshooting Steps:

1. Run DGA analysis on oil sample.
2. Inspect bushing appearance and insulation performance.
3. If acetylene exceeds limit or heavy gas relay trips, power off immediately and lift core for overhaul.

3.3 Insulating‑Oil Degradation, Oil Leakage & Moisture Intrusion

Oil seepage at flange and bushing root, decreasing oil‑level; fast color‑change of breather silica‑gel; turbid oil with impurities; unqualified breakdown voltage, moisture content or acid‑value in oil tests. Causes: Aged sealing gaskets, weld‑joint leakage; rainwater entering oil tank; high‑temperature accelerating oil oxidation and degradation.

Troubleshooting Steps:

1. Locate all leakage points.
2. Test oil sample for moisture, breakdown‑voltage and chromatogram.
3. Replace sealing components and repair leaking weld points.
4. Apply vacuum oil‑filtration for mild degradation; replace oil completely under severe conditions.

3.4 Gas‑Relay Protection Activation

Symptoms:Light gas relay: internal gas generation, abnormal DGA readings. Heavy gas relay: internal short‑circuit arc, direct equipment tripping. Causes: Oil‑gas decomposed by overheating, arc‑related faults, air intake or oil leakage.

Troubleshooting: When light gas relay triggers, collect gas and oil sample for analysis; only reset protection after fault removal. Once heavy gas relay trips, never attempt forced energization; cut power and perform core‑lifting overhaul.

3.5 Multi‑Point Core Earthing Leading to Overheating

Symptoms: Higher no‑load loss and no‑load current, elevated oil temperature; methane and carbon‑monoxide rise in chromatogram. Causes: Degraded core insulation; multi‑point earthing caused by metallic foreign objects; over‑excitation operation.

Troubleshooting: Measure core earthing current and no‑load loss; judge fault severity via chromatogram; lift core to clear foreign objects and restore core insulation.

3.6 Winding Deformation & Inter‑Turn Short‑Circuit

Symptoms: Frequent protective tripping, abnormal noise; increased acetylene and ethylene; distorted FRA frequency‑response curve. Causes: Winding displacement caused by electromagnetic force from short‑circuit surges; insulation wear from long‑term vibration; inter‑turn insulation breakdown by over‑voltage.

Troubleshooting Steps:

1. Apply FRA frequency‑response test to detect winding deformation.
2. Use DC‑resistance and transformation‑ratio test to check inter‑turn short‑circuit.
3. Apply withstand‑voltage test to expose hidden insulation defects.
4. Reinforce for minor damage; send back to factory for winding re‑winding for severe damage.

3.7 Breather & Radiator Malfunction

Symptoms: Higher‑than‑normal oil temperature without overload; caked / discolored silica‑gel; fouled radiator surfaces. Causes: Failed moisture‑absorption and blockage; oil‑dust fouling on radiators; defective oil‑pump or cooling fans.

Troubleshooting: Replace silica‑gel; clean radiators; overhaul oil‑pump and cooling‑fan assemblies.

4. General Standard Multi‑Layer Troubleshooting Workflow (For Both Dry‑Type & Oil‑Immersed Transformers)

General principles: from outside to inside; from simple to complex; live‑line inspection first then power‑off work; prioritize data analysis before disassembly.

Layer 1: Emergency Preliminary Assessment (No Power‑Off, Rapid Screening)

Record fault timestamp, load current, voltage, protection alarms, ambient conditions and historical maintenance logs.

Four‑sense inspection:

• Observe: carbonization traces, oil seepage, fan working condition
• Listen: distinguish normal hum from discharge / impact noise
• Smell: burnt odor or insulating‑oil smell
• Touch: only touch housing surface to judge local overheating

Infrared temperature measurement: For dry‑type transformers focus on terminals, windings and iron‑core. For oil‑immersed units inspect bushings, radiators and connectors. Temperature difference＞15 ℃ within nearby areas indicates potential fault location.

Layer 2: Power‑Off In‑Depth Visual Inspection

Dry‑type: Purge dust with dry compressed air; fasten all bolts; inspect insulation for cracks or carbonization; verify fan and temperature‑controller performance.

Oil‑immersed: Mark oil‑leakage positions; check breather silica‑gel; look for action traces of gas relay and pressure‑release valve; clean and examine bushing porcelain insulators.

Layer 3: Professional Electrical Tests for Hidden Defects

Common tests for both types

1. Insulation‑resistance test (2500 V megohmmeter): winding phase‑to‑phase, phase‑to‑earth, core‑to‑earth. Reading ≥1000 MΩ at ambient temperature; values＜10 MΩ indicate dampness or insulation damage.
2. DC resistance test: Three‑phase unbalance ≤2 % for ≤1600 kVA units; ≤1 % for larger‑capacity transformers.
3. Transformation‑ratio test: Deviation within ±0.5 % across all tap positions.

Additional for dry‑type transformer: Partial‑discharge test. Additional for oil‑immersed transformer: DGA oil‑chromatography & oil‑quality analysis. If winding damage suspected: run FRA winding‑deformation test; adopt three‑ratio method / Duval triangle diagram to classify fault patterns.

Layer 4: Pinpoint Root Cause

• Core earthing fault: Remove foreign objects, replace insulating gaskets
• Discharge noise: Purge & dehumidify; replace insulating components if ineffective
• Overheating: Balance load, improve heat dissipation, fasten loose connections
• Gas generation in oil‑immersed transformer: Distinguish thermal / electrical faults by chromatogram; lift core for inspection if necessary
• Oil leakage: Repair sealing structure; replenish or replace insulating oil
• Winding deformation: Mechanical reinforcement or factory return‑for‑repair

Layer 5: Post‑Repair Verification

Re‑measure all technical parameters after defect elimination; perform no‑load and on‑load trial operation to confirm complete fault removal.

Before putting repaired equipment into operation, follow our transformer commissioning checklist to complete full‑scope acceptance checks.

5. Quick‑Reference Cheat‑Sheet for Emergency Repair

Dry‑Type Transformer

• Overtemperature alarm → Infrared scan → Inspect fan & air duct → Check three‑phase current → Measure DC resistance
• Discharge noise → Power‑off & purge dust → Verify component installation clearances → Partial‑discharge test
• Zero core‑to‑earth insulation → Clear foreign objects, overhaul spacer insulation
• Excessive vibration‑noise → Fasten bolts, check shock‑absorbing pads, install filter if harmonic is serious
• Abnormal temperature‑controller alarm → Calibrate probes, reset parameters, replace defective hardware

Oil‑Immersed Transformer

• High oil temperature → Inspect cooling system → Balance load → DGA oil‑chromatography test
• Gas‑relay activation: Light gas → Run DGA test; Heavy gas → Cut power off and lift core for overhaul (no forced energization)
• Oil leakage → Repair sealing, test oil quality, replenish oil
• Bushing discharge → Clean bushing surface, test insulation, replace damaged bushing
• Suspected winding damage → DC resistance + turns‑ratio + FRA winding‑deformation test

6. Critical Safety Operation Specifications

1. Strictly follow power‑off, voltage‑verification and earthing‑grounding procedures during power‑off maintenance.
2. Only use dry compressed‑air to clean dry‑type transformer windings; never flush windings with water.
3. Release pressure before sampling or overhauling oil‑immersed equipment to prevent oil‑spray‑ignition risks.
4. Release residual winding charges before and after insulation and partial‑discharge tests.
5. Never force re‑energize under conditions including acetylene over‑limit, smoking‑out or heavy‑gas‑relay tripping.

For full‑depth analysis of failure‑caused fire hazards, read our transformer explosion safety guide.

7. Daily Preventive Maintenance Recommendations

Dry‑Type Transformer

• Periodically purge winding dust deposits
• Deploy dehumidification devices for high‑humidity distribution‑rooms
• Regularly calibrate temperature‑controller system
• Keep three‑phase load‑unbalance under control

Oil‑Immersed Transformer

• Regularly inspect sealing condition and breather performance
• Take oil samples for oil‑quality and chromatogram analysis on schedule
• Carry out routine infrared thermal‑imaging patrol inspection
• Avoid sustained long‑term overload running

For more actionable guidance, read our 7‑maintenance‑tips‑dry‑oil‑immersed‑industrial‑transformers.

FAQ — Frequently Asked Questions for Transformer Troubleshooting

Q1: Can I skip DGA test for oil‑immersed transformer if there are no obvious external faults?

A: No. Many internal latent faults produce no visible external symptoms. DGA chromatogram is critical for detecting early thermal and discharge defects before severe accidents.

Q2: Is partial‑discharge test only needed after obvious discharge noise appears?

A: Not exactly. Partial‑discharge test belongs to preventive test. Even without audible noise, hidden partial‑discharge may already erode insulation, so it shall be included in routine preventive maintenance.

Q3: What should I do if infrared test finds large temperature difference but cannot locate exact fault point?

A: Perform power‑off comprehensive visual inspection plus standard electrical tests: insulation resistance, DC resistance, transformation‑ratio test, to dig out hidden contact‑looseness or insulation degradation.

Conclusion

Transformer troubleshooting should follow the workflow: visual preliminary assessment → power‑off inspection → instrument‑aided testing. For dry‑type transformers focus on dust contamination, heat‑dissipation performance and contact joints; for oil‑immersed units pay close attention to oil quality, sealing performance, core earthing and dissolved gas in oil.

Most transformer failures stem from long‑term accumulated hidden defects. Regular cleaning, bolt tightening, temperature monitoring and preventive electrical tests can effectively mitigate major accidents. This practical guide can serve as operation reference for factory plants, substations and distribution‑network maintenance teams.

These seven practical maintenance tips cover core dimensions including load control, environment management, heat‑dissipation component cleaning, terminal inspection, insulation testing, insulating‑oil care, and record‑keeping. They help operators capture early fault signals and slow insulation aging, maximizing the service life of both dry‑type and oil‑immersed transformers.

Dry‑type and oil‑immersed transformers adopt completely different structures, so generic maintenance standards cannot be applied to both. Many low‑cost, under‑configured transformers on the market lack sufficient insulation and heat‑dissipation design margins, and may suffer frequent overheating and short‑circuit failures even under regular maintenance.

All ZHONGXIN GENERAL dry‑type and oil‑immersed transformers adopt pure‑copper windings and high‑grade silicon‑steel sheets with redundant insulation structures, delivering stronger anti‑aging and anti‑failure performance under full‑load and harsh‑environment working conditions. Browse our full product portfolio on our products page.

We provide customized transformer solutions for photovoltaic power stations, industrial mines, charging‑pile projects and commercial buildings. Our engineers can offer free parameter matching and power‑distribution safety scheme suggestions. Please contact us via official channels for product quotations.