Transformer Fault Diagnosis and Solutions: Common Causes, Testing Methods and Repair Measures

In power systems, transformers are responsible for voltage conversion and power distribution, and their operating condition directly affects the reliability of the power supply. The problem is that after long-term operation, equipment failures are often unavoidable—not because manufacturing standards are insufficient, but because actual operating conditions are different from those under factory test conditions.

Some faults develop gradually. At the early stage, they may only appear as slight parameter abnormalities. If these changes are not identified in time, the problems can gradually worsen, eventually leading to unplanned outages or even complete transformer failure.

During after-sales inspections, Zisheng Electric has found that many faults could have been detected at an early stage. However, on-site maintenance teams often either fail to monitor data changes effectively or lack the experience to correctly interpret the abnormal signals.

This article summarizes Zisheng Electric’s accumulated experience in transformer manufacturing and project support services. Based on numerous fault analysis cases and operation & maintenance support projects across different application environments, we cover both oil-immersed and dry-type transformers, explaining the causes of failures, diagnosis methods, and corrective measures step by step.

I. Where Do Transformer Faults Occur? First Understand Their Location and Nature

The following are some common abnormal phenomena observed during actual operation and the corresponding inspection directions:

Through these operating conditions, the fault investigation scope can be quickly narrowed down.

For example, an increase in oil temperature does not necessarily indicate winding damage. It may simply be caused by reduced heat dissipation due to dust accumulation on the radiator. Similarly, abnormal noise does not automatically mean a core failure; further confirmation requires comprehensive analysis based on inspection data and test results.

II. Winding Faults: DC Resistance Unbalance Is an Important Indicator

Phenomena and Causes

The winding is one of the components with the highest failure rate in a transformer. Common faults include inter-turn short circuits, layer-to-layer short circuits, and winding deformation.

When an inter-turn or layer-to-layer short circuit occurs, a large circulating current is generated in the short-circuited section, causing the winding temperature to rise rapidly. Abnormally high oil temperature, increased current on the power supply side, and unbalanced DC resistance among phases are typical symptoms. In severe cases, abnormal sounds may also be heard from the oil tank.

What causes these problems? Mainly the following factors:

Insulation aging or moisture absorption — insulation strength decreases and can no longer withstand the normal operating voltage.

Manufacturing defects — the inter-turn insulation may already have damage that was not detected during factory testing.

Short-circuit current impact — short circuits at the system outlet or nearby areas generate electromagnetic forces that can deform the winding.

Winding deformation is a relatively hidden problem. After a transformer experiences a short-circuit fault, the winding may undergo irreversible deformation. The transformer may continue operating at that time, but the insulation has already been damaged, making future short-circuit failures much more likely. The hidden period of this process can be very long, but once it develops into a failure, the consequences can be serious.

How to Diagnose

The diagnosis of winding faults mainly relies on the following methods:

DC resistance testing.
Measure the DC resistance of each phase winding. If the three-phase unbalance rate exceeds 2%, it generally indicates possible inter-turn short circuits or poor contact. This is one of the essential items in every preventive test we perform.

Transformer ratio testing.
If the voltage ratio deviation at different tap positions exceeds the allowable range, it indicates that inter-turn short circuits may exist in the winding.

Short-circuit impedance testing.
Changes in short-circuit impedance can reflect whether the winding has experienced deformation. This is a necessary verification test after the transformer has suffered a short-circuit impact.

Dissolved Gas Analysis (DGA).
Overheating or partial discharge in the winding generates specific gases. By analyzing the type and concentration of dissolved gases in the oil, the fault type and severity can be determined.

How to Handle It

For minor inter-turn short circuits, oil filtration and drying treatment can be attempted first.

For serious winding faults, the transformer must be taken out of service for maintenance, with the active part removed, followed by replacement of damaged windings or rewinding.

For severe winding deformation, the only options are rewinding the winding or replacing the entire transformer.

Prevention Points:

• Avoid long-term overload operation
• After a short-circuit fault occurs in the system, perform winding deformation testing in time
• Carry out regular preventive tests and monitor changes in winding insulation condition trends

Key reminder:
DC resistance testing should not be performed only once during commissioning and then ignored. A historical database should be established, and each test result should be compared with previous data. Trend changes are often more meaningful than absolute values.

III. Core Faults: One Grounding Point Is Normal, Multiple Grounding Points Are a Problem

Phenomena and Causes

The transformer core is the magnetic circuit of the transformer. The main faults are multiple grounding points and local overheating.

During normal operation, the transformer core is only allowed to have one reliable grounding point. Once two or more grounding points appear, a closed circuit is formed. Under the alternating magnetic field, circulating currents are induced, causing local overheating of the core.

Minor multiple-point grounding may only increase the temperature by several degrees, while severe cases can burn the core.

Insulation damage between silicon steel sheets is also a common problem. External mechanical damage or long-term aging can both cause this failure. Once local short circuits occur between silicon steel sheets, eddy current losses increase and local temperatures rise rapidly.

How to Diagnose

Grounding current measurement.
Install an ammeter in series with the core grounding line. A significant increase in current indicates possible multiple grounding points.

Under normal operating conditions, the core grounding current is generally at the milliampere level. If it reaches the ampere level, the problem can basically be confirmed.

Dissolved Gas Analysis (DGA).
Characteristic gases generated by core overheating can be analyzed through DGA to determine whether overheating exists and its severity.

Partial discharge testing.
Multiple core grounding faults are often accompanied by partial discharge, which can be used as an auxiliary confirmation method.

How to Handle I

For multiple-point core grounding, a current-limiting resistor can first be connected in series with the grounding circuit to temporarily limit the grounding current.

Ultimately, the transformer must be taken out of service for maintenance and the active part removed to inspect the core grounding condition and eliminate unnecessary grounding points.

If insulation damage occurs between silicon steel sheets, the active part must also be removed for insulation repair or replacement of damaged silicon steel sheets.

IV. Insulation Faults: Temperature and Moisture Are the Two Biggest Enemies

Phenomena and Causes

The insulation system is the foundation of long-term reliable transformer operation. The main faults are insulation aging and insulation moisture absorption.

Insulation aging — this is an unavoidable physical process. Under the combined effects of electrical stress, heat, mechanical forces, and environmental factors, insulation performance gradually deteriorates in an irreversible manner.

The aging rate is directly related to operating temperature.

There is an engineering rule of thumb: after the operating temperature exceeds the design limit, every increase of 6–8°C approximately reduces the insulation material’s service life by half. This relationship is described in relevant sections of IEC 60076-14 and is worth emphasizing repeatedly during actual operation and maintenance.

Insulation moisture absorption — this is one of the most common causes of sudden insulation performance deterioration.

Moisture usually enters through three channels:

• Aging or damaged sealing components allow moisture to penetrate through gaps
• Low oil level increases the contact area between insulating oil and air, allowing moisture to gradually enter
• Breather failure prevents effective removal of moisture from incoming air

How to Diagnose

Insulation resistance testing.
Use a megohmmeter to measure insulation resistance between windings and ground, as well as between windings. A significantly reduced resistance value indicates possible insulation moisture absorption or aging.

During measurement, attention must be paid to temperature differences. Resistance values at different temperatures cannot be directly compared and must be corrected to the same reference temperature before comparison.

Dissipation factor (tanδ) testing.
An increase in tanδ indicates declining insulation performance. This parameter is highly sensitive to both moisture absorption and insulation aging.

Oil moisture analysis.
The moisture content in insulating oil is evaluated. For high-voltage equipment, it is generally required to be controlled below 15–25ppm (specific limits depend on voltage level). Excessive moisture indicates that the insulation system has become damp.

Oil quality testing.
Includes breakdown voltage testing, acid value measurement, and interfacial tension measurement to comprehensively evaluate insulating oil condition.

How to Handle It

For moisture-affected insulation, perform vacuum drying treatment to remove moisture from insulation materials.

At the same time, conduct pressure vacuum oil filtration to remove water and gases from the oil.

For severe insulation aging, oil replacement or oil regeneration treatment is required.

In extreme cases, aged insulation materials must also be replaced.

Key Prevention Measures:

• Regularly perform insulation performance tests and establish trend records
• Maintain the sealing system properly to prevent moisture ingress
• Ensure the breather operates correctly and replace desiccant regularly

V. Tap Changer Faults: Poor Contact Is the Main Cause

Phenomena and Causes

The tap changer is one of the transformer components with a relatively high failure rate.

Common problems of de-energized tap changers (DETC) include poor contact and contact erosion. On-load tap changers (OLTC) may additionally experience switching mechanism failures.

When contact resistance increases due to poor contact, severe local heating occurs. Over time, the contacts may burn or even weld together.

Symptoms include:

• Increased oil temperature
• Abnormal output voltage
• Increased DC resistance unbalance rate

Main causes include:

• Insufficient contact spring pressure causing poor contact
• Oxidation or contamination on contact surfaces increasing contact resistance
• Frequent switching causing contact wear
• Mechanical failure of the switching mechanism causing incomplete operation

How to Diagnose

DC resistance testing.
Measure DC resistance at each tap position. If the resistance at a specific position is significantly higher, it indicates poor contact at that tap position.

Infrared temperature measurement.
Use an infrared thermal imaging camera to inspect the temperature distribution around the tap changer during operation and identify local hotspots.

Switching test.
Operate the tap changer step by step to verify whether switching is smooth and whether each position is correctly reached.

How to Handle It

For poor contact, first shut down the transformer and operate the tap changer back and forth several times. The mechanical movement may remove oxide layers from the contact surface.

If the problem remains, the active part must be removed to inspect the contacts, polish burned areas, or replace damaged components directly.

Prevention Measures:

• Never operate a de-energized tap changer while the transformer is energized — this is the most basic and most frequently overlooked rule.
• Regularly inspect the operating mechanism and contact condition.

VI. Oil Quality Deterioration and Oil Leakage: Small Problems Can Become Major Failures

Phenomena and Causes

Oil-immersed transformers rely on insulating oil to perform two functions simultaneously: insulation and heat dissipation.

When oil quality deteriorates, both functions are affected.

Signs of oil deterioration include:

• Darkened oil color
• Increased acid value
• Increased dissipation factor
• Reduced breakdown voltage

The cause is oxidation of insulating oil during long-term operation. High temperature accelerates oxidation, while moisture and impurities act as catalysts, creating a continuous deterioration cycle.

Oil leakage is more common and may appear to be a minor issue, but it can cause serious problems.

Oil leakage does not only reduce oil quantity. More importantly, moisture and contaminants can enter together with the leakage path, accelerating oil deterioration.

We have seen many cases where slight oil leakage was ignored. One or two years later, oil test results exceeded acceptable limits across multiple indicators, requiring major maintenance.

How to Diagnose

Oil quality testing.
Regularly collect oil samples and test:

• Breakdown voltage
• Moisture content
• Acid value
• Dissipation factor
• Flash point
• Dissolved gas analysis (DGA)

Under normal conditions, testing should be performed once every one to two years. For heavily loaded equipment or harsh environments, the interval should be shortened to six months.

Oil level inspection.
Continuous oil level reduction indicates leakage. Do not simply assume it is caused by hot weather evaporation—first identify possible leakage points.

Visual inspection.
Inspect the oil tank, weld seams, valves, and sealing gaskets for signs of oil leakage.

How to Handle It

For deteriorated oil, perform filtration and regeneration treatment to remove moisture, impurities, and degradation products.

For severely deteriorated oil, replace the oil directly.

Leakage treatment depends on the leakage location:

• Replace leaking sealing gaskets
• Repair leaking weld seams
• Repair or replace leaking valves

Prevention:

• Regular oil quality testing should not be neglected
• Maintain proper oil level and avoid excessively low oil levels
• Ensure the breather operates properly

VII. Cooling System Faults: Once Heat Dissipation Stops, Temperature Rise Becomes Uncontrolled

Phenomena and Causes

The cooling system is responsible for dissipating the heat generated during transformer operation. Once a cooling system failure occurs, the transformer temperature rise exceeds the allowable limit, accelerating insulation aging.

Common faults include: radiator blockage (dust accumulation on the surface or internal blockage), oil pump failure (restricted oil circulation), fan failure (failure to rotate or insufficient speed), and cooler pipeline leakage.

In dusty environments such as the Middle East, dust accumulation on radiator surfaces occurs particularly quickly. Under normal wind conditions, the impact may be limited, but after a sandstorm, fine sand can completely fill the gaps between radiator fins.

Zisheng Electric encountered a similar situation in a Saudi Arabian project: the radiator surface was covered by a layer of dust and sand. The top oil temperature was nearly 8°C higher before cleaning. After cleaning, the temperature returned to normal levels.

How to Diagnose

Oil temperature monitoring.
Under the same load and ambient temperature conditions, if the top oil temperature is significantly higher than normal, it indicates that the cooling system may have a problem.

Cooling equipment inspection.
Check whether oil pumps and fans are operating normally—listen for abnormal sounds and check operating current.

Radiator inspection.
Check whether the radiator surface has excessive dust accumulation or blockage caused by foreign materials.

How to Handle It

If the radiator is blocked, perform cleaning or air blowing treatment.

If the oil pump or fan fails, repair or replace the faulty components.

If the cooler leaks, locate the leakage point and repair it.

Prevention:

• Clean radiators regularly — in dusty environments, cleaning should be performed at least once every quarter
• Regularly inspect the operating condition of oil pumps and fans
• Conduct comprehensive inspection and maintenance before the high-temperature season begins

VIII. Quick Comparison of Different Transformer Faults

IX. Preventive Testing: Do Not Wait Until Problems Occur

Tests That Can Be Performed While Energized (No Power Shutdown Required)

1. Dissolved Gas Analysis (DGA)

Currently, this is one of the most widely used transformer fault diagnosis methods.

When thermal faults or electrical faults occur inside a transformer, insulating oil and solid insulation materials decompose and generate specific gases. By analyzing the type and concentration of these gases, the fault type and severity can be determined.

Methods such as the three-ratio method can be used for preliminary diagnosis. The key point is to establish a trend of gas content changes—a single test result has limited reference value, while continuous data collected over multiple tests can show whether the problem is worsening or remaining stable.

Our approach in actual projects is as follows: DGA testing is normally performed twice a year. For heavily loaded transformers or important substations, the testing frequency can be increased to once every quarter. Accumulated data becomes the “health record” of the transformer.

2. Core Grounding Current Measurement

Used for monitoring multiple-point grounding of the transformer core.

Under normal conditions, the grounding current is at the milliampere level. A sudden increase should be treated as a warning signal.

3. Partial Discharge Detection

Ultra-high frequency (UHF), ultrasonic, and high-frequency methods can be used for online detection.

These methods are highly sensitive to internal insulation defects.

4. Infrared Thermal Imaging Inspection

Without shutdown or physical contact, infrared thermal imaging can scan the temperature distribution of:

• Bushings
• Lead connections
• Transformer tank
• Tap changers

Local overheating points can be identified immediately.

During on-site inspections, infrared thermal imaging is one of the most commonly used rapid screening methods.

Tests That Require Power Shutdown

Winding-Related Tests

• Insulation resistance, dielectric absorption ratio, and polarization index tests
  (between winding and ground, and between windings)
• Dissipation factor tanδ test
  (for windings together with bushings)
• Voltage ratio test (transformer ratio test)
  — All tap positions must be tested; measuring only one or two positions is not sufficient
• DC resistance test
  — All tap positions should be tested to evaluate winding condition and tap changer status
• Sweep Frequency Response Analysis (SFRA) winding deformation test
  — Especially important after the transformer experiences short-circuit impact

Core and Clamping Structure

• Core/clamp insulation resistance to ground test
  — Performed during shutdown to determine whether multiple grounding points exist

Oil Quality Testing

Tests include:

• Oil breakdown voltage
• Moisture content in oil
• Acid value
• Oil dielectric dissipation factor
• Interfacial tension
• Flash point

DGA samples can be collected during energized operation or together with shutdown maintenance tests.

Bushing Tests

• Bushing insulation resistance test
• Bushing dielectric loss and capacitance measurement

On-Load Tap Changer (OLTC), If Equipped

• Operating sequence test
• Switching time test
• Contact condition inspection

Other Tests

• AC withstand voltage test
  — Performed after major maintenance or during initial commissioning; generally not included in routine preventive testing
• Sealing and visual inspection
• Breather and sealing component inspection

X. Frequently Asked Questions

Q1: Does abnormal transformer noise always indicate a fault?

Not necessarily.

Minor sound changes may be caused by load variations. However, continuously increasing humming noise or discharge sounds require further inspection.

Q2: How often should transformers be inspected?

For normally operating transformers, inspection intervals should be determined according to operating environment and equipment importance.

For high-load transformers or equipment operating in harsh environments, inspection intervals should be shortened accordingly.

Q3: What are the differences between oil-immersed transformer faults and dry-type transformer faults?

For oil-immersed transformers, attention is mainly focused on:

• Insulating oil condition
• Oil leakage
• Oil temperature

For dry-type transformers, attention is mainly focused on:

• Winding temperature rise
• Insulation aging
• Environmental humidity

Conclusion

Most transformer faults actually show warning signs at an early stage, such as:

• Abnormally increased oil temperature
• Changes in gas content
• DC resistance imbalance

If these signals can be identified and addressed in time, fault escalation can generally be avoided.

However, the challenge is that on-site maintenance personnel may not always have sufficient time and technical resources to conduct systematic data analysis. Therefore, having a clear diagnostic method and troubleshooting process is extremely important.

For transformer users, three recommendations are particularly important:

1. Establish a standardized preventive testing system

Required inspections and tests should not be skipped.

2. Equip necessary diagnostic tools

Basic tools such as infrared cameras, DGA testing equipment, and DC resistance testers should be available.

3. Accumulate operating data and perform trend analysis

A single test result is only an isolated point; continuous trends provide the real early warning system.

Zisheng Electric specializes in the research, development, and manufacturing of power equipment. Its product portfolio includes prefabricated substations, oil-immersed transformers，dry-type transformers, amorphous alloy transformers, and related switchgear equipment.

In addition to manufacturing according to Chinese national standards, Zisheng Electric can also provide customized products based on international standards such as IEC 60076 and GOST, serving markets including the Middle East, South America, Africa, and Russia.

We provide more than just equipment, including:

• Operation and maintenance recommendations and fault diagnosis support
• Preventive testing plan consultation
• Emergency response solutions for abnormal operating conditions

Understanding diagnostic methods and preventive measures in advance is far more effective than waiting until a failure occurs and then figuring out how to repair it.

For questions related to transformer operation and maintenance, fault diagnosis, or equipment selection, please send your project parameters through Zisheng Electric’s official channels to receive technical support.

You can also leave specific fault symptoms in the comments section (such as oil temperature, gas analysis data, load conditions, etc.), and our engineers will provide diagnostic suggestions and respond accordingly.

About ZISHENG ELECTRICAL

Zisheng are a professional 19+ years manufacturer in producing Oil-Immersed Transformers, Compact Substation, Pad Mounted Transformers, Pole Mounted Transformers and Dry Type Transformers. We own the certificates of ISO/CE/IEC 60076 and TUV Rheinland.
Transformers undergo rigorous FAT and type testing, support voltage/capacity customization. Welcome to consult for Catalog and Product. you can contact us at email [email protected].