Large Power Transformer Faults: Diagnosis & Analysis (100 MVA, 132/22 kV) – Part 3

Why Transformer Faults Matter in High-Voltage Systems

100 MVA 132 kV power transformers were delivered by Scotech Electrical to Malaysia in December 2025 for a TikTok data center project. This article, prepared by Scotech, is specifically designed for engineers, operators, and decision-makers dealing with large power transformer faults in 100 MVA class equipment, focusing on real-world transformer fault diagnosis, transformer troubleshooting, protection behavior, and transformer failure analysis. Related reference for installation stage context: Pre-Installation Checklist Guide

Large power transformers, such as 100 MVA 132/22 kV units, are critical infrastructure components. Power transformer faults in such systems can have system-wide consequences, including power interruption, data center downtime, and severe equipment damage. In extreme cases, failures may involve fire hazards, electrical faults, thermal overload conditions, insulation breakdown, and mechanical defects within the active part of the transformer.

This article explains how to identify, analyze, and respond to typical transformer failure modes based on real protection signals and field operation practices.

Common Types of Transformer Faults

Transformer failures can be grouped into four main technical domains for transformer failure analysis and transformer fault detection.

Internal Electrical Faults

These faults are usually severe and require immediate transformer troubleshooting.

Typical causes include:

• Winding short circuit (inter-turn or phase-to-phase)
• Core grounding faults
• Partial discharge inside insulation system

These are closely related to power transformer failure causes and are often detected by differential protection systems.

Thermal Faults

Thermal issues develop gradually but can escalate quickly under load, especially in high-capacity units where transformer overheating is a key operational risk.

Common problems:

• Local hotspot overheating
• Cooling system inefficiency or failure
• Excessive load operation

In data center applications, continuous loading makes thermal faults especially critical.

Insulation Failures

Insulation degradation is one of the most common long-term failure mechanisms in transformer fault diagnosis.

Typical causes:

• Oil-paper aging
• Moisture ingress
• Electrical stress accumulation

These conditions often lead to transformer fault detection alarms via DGA systems.

Mechanical and Structural Faults

Mechanical issues are often caused by transportation, vibration, or long-term operation.

Examples:

• Winding displacement
• Clamping loosening
• Structural vibration abnormalities

These are also part of transformer failure modes observed in field transformer troubleshooting.

Protection-Based Transformer Fault Diagnosis

Protection systems are the first and most critical defense layer in transformer fault diagnosis. When a protection signal occurs, engineers must determine whether it is an internal or external fault.

Differential Protection Operation

When transformer differential protection operates, it is generally considered a high-confidence internal fault indication.

Typical causes include:

• Inter-turn or phase-to-phase winding fault
• Severe insulation breakdown inside transformer tank
• Core grounding or abnormal core circulation current

In most large power transformers (such as 100 MVA units used in data center substations), differential protection operation is treated as a critical shutdown condition.

Field response logic:

• Verify protection signal consistency (relay + monitoring system)
• Check for external short-circuit evidence (to exclude miscoordination)
• Inspect for oil leakage, deformation, or abnormal noise
• Review fault records and waveform data

In most cases, immediate transformer isolation is required.

Buchholz Relay (Gas Relay)

The Buchholz relay is a key protection device for oil-immersed transformers, designed to detect internal faults at an early stage through gas accumulation and oil flow behavior.

For light gas alarm conditions, field diagnostic actions typically include collecting a gas sample for composition analysis, performing dissolved gas analysis (DGA), and checking oil level and temperature trends to assess whether the abnormal condition is developing or stable.

For heavy gas trip conditions, immediate shutdown of the transformer is strongly recommended. After isolation, a detailed DGA should be carried out, the relay chamber should be inspected for gas accumulation or abnormal mechanical behavior, and the fault gas composition should be thoroughly analyzed to determine the type and severity of the internal fault.

Pressure Relief Device (PRD)

The Pressure Relief Device operates when the internal tank pressure rises rapidly beyond safe design limits. This condition is typically associated with high-energy internal faults, such as internal arc faults, winding or lead short circuits, or rapid oil decomposition that generates a large volume of gas.

From an engineering perspective, PRD operation is usually a strong indication of a severe internal transformer failure, particularly in large MVA units where fault energy release is significant.

In field practice, the event should be treated as a potential critical failure. It is necessary to verify whether other protection systems such as differential protection or Buchholz relay, have also operated, and to inspect the transformer for signs of oil discharge, tank deformation, or mechanical stress. Immediate shutdown and electrical isolation should be prepared to prevent further damage or safety risks.

Typical Fault Scenarios and Field Troubleshooting

Transformer Fire Incident

Transformer fire is one of the most critical emergency conditions in power systems. It may be triggered by internal short-circuit faults with arc ignition, external ignition of leaked insulating oil, or severe bushing and terminal failures leading to sustained arcing.

In practice, transformer fire events must be treated as full-system emergencies. Immediate isolation of all transformer sides is required, followed by disconnection of the auxiliary power supply to the cooling system. Fire suppression systems such as spray or nitrogen injection, should be activated without delay to control flame propagation. Special attention must be given to preventing the spread of fire to adjacent equipment. In severe cases, controlled oil drainage may be evaluated depending on fire location and structural risk.

Bushing Failure or Explosion

Transformer bushing failure is typically caused by long-term insulation aging or electrical stress concentration, eventually leading to dielectric breakdown under high voltage conditions.

Typical symptoms include sudden transformer trip (often accompanied by differential protection operation), visible oil leakage, porcelain damage, and surface flashover or carbonization marks. From an engineering standpoint, bushing failure is classified as a high-risk insulation failure mode and can quickly escalate into internal transformer damage if not isolated.

Cooling System Failure

Cooling system failure directly affects the thermal stability of transformer windings and insulating oil. When cooling performance is reduced or lost, oil temperature rises rapidly, insulation aging accelerates, and under high load conditions, thermal runaway may occur.

Field response should first verify cooling system power supply and redundancy status, then confirm fan and pump operation, and continuously monitor winding temperature trends. If both cooling systems fail while the load remains high, immediate load transfer or shutdown should be considered to avoid transformer overheating.

Abnormal Noise

Transformer acoustic behavior is an important non-invasive diagnostic indicator. Changes in sound patterns often reflect internal electrical or mechanical abnormalities.

A continuous increase in hum is usually associated with overload or voltage fluctuation. Crackling or popping sounds may indicate partial discharge activity, while boiling-like sounds are often related to insulation overheating or moisture contamination. Irregular mechanical noise may suggest core looseness or structural vibration issues.

Acoustic monitoring is widely used in transformer fault diagnosis as an early warning method before electrical failure occurs.

Oil Temperature Abnormal Rise

Oil temperature trends must always be analyzed together with load conditions, ambient temperature, and cooling system performance. A gradual temperature increase usually indicates overload or reduced cooling efficiency, while a rapid temperature rise may suggest internal fault activity or severe cooling system failure.

Continuous monitoring and trend comparison are essential in transformer troubleshooting practice to distinguish between operational stress and developing fault conditions.

Oil Level Abnormality

Bushing Leakage and Partial Discharge

Partial discharge is a typical precursor of insulation deterioration in high-voltage systems and is closely related to bushing failure mechanisms. It may appear as surface corona, audible discharge noise, or UV detection signals.

In addition, dissolved gas analysis (DGA) often shows abnormal trends during early-stage insulation degradation.

Typical supporting indicators include:

• UV or infrared detection signals
• Increase in key DGA gases
• Localized heating or surface discharge traces

This condition indicates progressive insulation failure and may evolve into full breakdown if not properly managed.

Dissolved Gas Analysis (DGA) for Fault Diagnosis

Dissolved Gas Analysis (DGA) is one of the most important tools in transformer fault detection and transformer failure analysis. It provides insight into internal electrical and thermal fault types based on gas composition in insulating oil.

Key gases and interpretation

DGA trend analysis is more important than single-point measurements, as rapid gas growth rate often indicates active fault progression.

Conditions Requiring Immediate Shutdown

A transformer must be immediately removed from service if any of the following conditions occur:

These conditions indicate a high probability of serious internal failure and require immediate isolation of the transformer system.

Transformer Inspection and Maintenance Considerations

Under severe fault conditions, internal inspection may be required to determine root cause. This process must be carried out under controlled environmental conditions, including limited humidity, clean working environment, and strict grounding procedures.

Inspection typically includes evaluation of winding displacement, core condition, insulation resistance measurement, and identification of foreign objects or mechanical deformation inside the tank.

Environmental and Lifecycle Considerations

Throughout the transformer lifecycle, regular maintenance plays a critical role in ensuring operational stability. Noise, emissions, and insulating oil must comply with regulatory requirements, while end-of-life transformers should be properly recycled through certified processes to prevent environmental impact.

Project Installation Progress

Conclusion: From Fault Detection to System Reliability

Transformer faults cannot be fully eliminated, but they can be effectively controlled through proper protection system interpretation, DGA monitoring, thermal analysis, and structured maintenance strategy.

For high-voltage and data center applications, transformer reliability directly determines system continuity, making fault diagnosis and preventive maintenance essential components of operational safety.