5 Actual Reasons Why A Transformer Fails

• April 10, 2026
• Gaurav Joshi

Most transformer failures do not happen instantly. They build slowly over time. Weeks, months, or even years pass before failure occurs. Then suddenly, the transformer shuts down.

This is not just any equipment. In fact, a transformer is the most expensive asset in a substation. Therefore, when it fails, the impact is significant. Operations come to a halt, alarms start triggering, and losses increase rapidly.

In this article on the 5 Actual Reasons Why A Transformer Fails, we will break down the real causes behind these failures. Moreover, these reasons are based on industry data and actual case studies. By understanding them clearly, you can take timely action and prevent costly breakdowns.

Table of Contents

• Insulation Failure
• Winding Failure
• Bushing Failure
• OLTC Failure
• Thermal Overload and Cooling Failure
• Conclusion

1. Insulation Failure

Insulation failure is the most common cause of transformer failure. It develops slowly and often goes unnoticed.

A transformer has two types of insulation:

• Solid insulation like paper or pressboard
• Liquid insulation like oil or ester

Over time, this insulation weakens due to several factors:

• Heat from overloading
• Moisture from aging or leaks
• Oxidation due to sludge or acidic oil
• Contamination like carbon or metal particles

As degradation continues, oil quality drops and dielectric strength reduces. Eventually, the insulation cannot handle electrical stress.

Some warning signs may appear:

• Increasing moisture in oil
• Degrading oil condition
• Hotspot temperature alarms
• Repeated overload history

However, these signs are often ignored. Most failures are not bad luck. They result from missed monitoring and delayed action.

2. Winding Failure

Winding failure is the second major reason and, more importantly, it becomes dangerous because it escalates very quickly once it begins. Initially, the issue may seem minor, but it can rapidly turn into a serious fault if not addressed in time.

In general, winding faults can appear in several forms, such as:

• Turn-to-turn short circuit
• Phase-to-phase fault
• Winding-to-ground fault

Now, if we look at the root causes, there are mainly two factors responsible for these failures. First, weak insulation plays a significant role, especially as it degrades over time. Secondly, mechanical damage can also lead to winding failure, often triggered by external fault forces or operational stress.

Mechanical damage usually happens during external faults. When a short circuit occurs, current increases sharply. This creates strong electromagnetic forces.

These forces try to move the winding. If the structure is weak or aged:

• Winding deformation occurs
• Insulation gets damaged
• Failure follows soon after

Some signs may indicate winding issues:

• Differential protection operation
• Gas relay activation
• Sudden impedance change
• Abnormal excitation current

Failure databases show that winding faults are among the most common causes in real systems.

3. Bushing Failure

Bushing failure often looks sudden, but it develops over time. A bushing is a complex insulation system exposed to harsh conditions.

It faces:

• Environmental stress
• Thermal cycles
• Electrical stress
• Contamination

Common causes of bushing failure include:

• Moisture ingress
• Internal partial discharge
• Oil leakage
• Loose connections causing overheating
• Surface contamination

As these issues grow, insulation weakens. Eventually, failure occurs.

Some early signs can help detect problems:

• Increase in tan delta values
• Hotspots visible in thermal scans
• Audible noise
• Surface tracking marks

In worst cases, failure may lead to:

• Tank rupture
• Fire hazards

Utility data shows that bushing failures are one of the leading causes in many cases.

4. OLTC Failure

On-load tap changers (OLTC) are critical for voltage control. They operate while the transformer is energized.

This operation involves switching under load. As a result, arcing is unavoidable. Over time, this causes wear and degradation.

Common reasons for OLTC failure include:

• Contact erosion
• Carbon deposits in oil
• Mechanical misalignment
• Poor maintenance
• Improper oil handling

Since arcing is part of normal operation, regular maintenance is essential. Without it, failure becomes likely.

Some signs of OLTC issues are:

• Voltage regulation problems
• Irregular tap movement
• Mechanical abnormalities
• Temperature rise near tap selector

OLTC failures are frequently reported after winding and bushing issues.

5. Thermal Overload and Cooling Failure

Overheating is one of the most common and underestimated causes. Many failures start with small cooling issues.

At first, the problem may seem minor. However, it gradually becomes serious.

Common triggers include:

• Cooling fans not working
• Pump failures
• Blocked radiators
• Poor ventilation
• Incorrect control settings
• Low oil levels

As temperature increases, insulation aging accelerates rapidly. Even a small rise in temperature can reduce insulation life significantly.

Real-world conditions that lead to overheating include:

• Continuous overloading
• Dust accumulation in cooling systems
• Poor maintenance practices

In many failure records, overheating appears as a secondary cause. Still, it plays a major role in pushing insulation toward breakdown.

Conclusion

Transformer failure is rarely a sudden event. Instead, it usually develops over time due to continuous stress and overlooked maintenance. In most cases, multiple factors contribute gradually before the final breakdown occurs.

The five major reasons include:

• Insulation degradation
• Winding faults
• Bushing issues
• OLTC problems
• Overheating

Therefore, if you monitor key parameters regularly, you can identify these issues early. As a result, you can take corrective action in time and avoid costly failures and unexpected downtime.

For a clearer and more practical understanding, it is highly recommended to watch the full video explanation.