Better understanding of transformer failures and maintenance necessity
Better understanding of transformer failures and maintenance necessity (on photo: A Norwegian transformer station in Skien suffered an explosion in June 2000, causing a fire and a serious oil release; credit:

Transformer failures

Generally speaking, the transformer requires less care compared to other electrical equipment. But, as I already stated in some of the earlier technical articles, transformer failures can cause a huge problem in power system, since it’s one of the most critical link and it can take a while to replace if it fails.

The degree of transformer maintenance and necessary inspection for its operation depends on its capacity, on the importance within electrical system, the place of installation within the system, on the weather conditions, and the general operating conditions.

Let’s discuss now about what are the main causes of the transformer failures and how to discover them on time //

  1. Causes of the failure of a transformer
  2. Types of failures
  3. Early discovery of the failures
  4. Internal defects of the transformer
  5. How to detect internal failures?

1. Causes of the failure of a transformer

To track the cause of the failure is the first step to formulate its solution. The origin of the defects is not simple. Generally, it is the combination of many factors that can be classified in the following way //

  1. Imperfection on the specifications
    • Mistake in the selection of the type of insulation.
    • Not appropriate capacity.
    • Lack of attention to the conditions in the place of installation (dampness, temperature, dangerous gases, etc)
  2. Imperfection on the facilities
    • Wrong installation.
    • Wrong capacity and protection range of lightning rods
    • Switch and relay for protection is wrong.
  3. Imperfections on the operation and maintenance of the equipment
    • External conducting parts loose and heating up of the same.
    • Deterioration of the insulating oil
    • Excessive load or mistakes in the connection of the cables.
    • Mistake in the operation, and carelessness in the arrangement of the protection circuits.
    • Insufficient inspection of the gaskets and valves.
    • Poor maintenance of the accessories. Abnormal voltage
  4. Normal wear and tear
  5. Natural disasters

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2. Types of failures

Transformer failures produced by the causes mentioned above, create secondary failures and of a third type, hindering their tracking.

However, the operating conditions in the moment of the failure, the inspection records of the relays of protection of the various parts, as well as the transformer maintenance and the regular inspection, will help to detect the cause in many occasions.

The defects of a transformer can be classified in the following way //

  • Internal failures of the transformer: in core and coil
    • Dielectric interruption
    • Rupture and twist of the winding
    • Mistake on the grounding
    • Open connection of tap changer
    • Insulating oil
  • External defects of the transformer: In the tank
    • Due to oil leaks in the gasket, valve, or weld cord
    • Due to the bushings of the breathers, over pressure valve, thermometers, oil level gauge, etc.
    • Defects on the forced cooling fans, Buchholz Relay, exit of the current transformers of the bushings, etc.

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3. Early discovery of the failures

It is unnecessary to say that the sooner a failure is detected, the better for the transformer, and it requires a careful and detailed maintenance and inspection. There are procedures made for the regular inspection and of routine.

By means of this inspection a failure can be detected before it becomes serious, and reduce any damage in whatever possible.

Some defects are caused by reasons beyond human control, such as sudden transformer failures and defects that develop slowly. Let’s describe each of these non-human related reasons.

3.1 Sudden transformer failures

Most of the dielectric interruptions occur suddenly, especially due to lightning or to an abnormal tension, causing a direct failure. Excessive current by an external short circuit or by a mechanical hit also happen suddenly.

Disturbances by earthquakes and fires can accidentally damage the transformer.

3.2 Defects that develop slowly

3.2.1 Sudden defects are related generally to totally external or foreign factors to the transformer of such form that it is outside of our scope the power to foresee them and to prepare us to face these.

The objective of transformer maintenance and inspection is to discover the defects that occur and that may develop slowly. These defects are described below //

3.2.2 Deformation of the insulation materials and of the transformer windings, due to mechanical blows caused by an external short circuit. The transformer generally is designed and is manufactured to resist the heat and the mechanical blows. However, if it is exposed to frequent and intensive mechanical blows, even a small deformation can be converted into a serious internal defect.

3.2.3 Insulation of the core. There can be poor insulation between the sheets of the core, between the tightening screw of the core and the insulation pipe, etc. The poor insulation cause a short circuit in the magnetic flow, producing a constant short circuit current flow in this place and generating excessive heating up which can lead to serious defects.

3.2.4 Poor insulation due to a harsh operational condition such as excessive load. According to what was mentioned in the instruction manual, the insulation of the transformer deteriorates by the increase in the temperature and this deterioration over the years worsens and is converted into a serious failure when the transformer suffers an excessive load.

3.2.5 Deterioration of the insulation materials such as oil, bushings, etc. due to moisture absorption, to oxidation and to formation of a partial discharge, etc.

3.2.6 Deterioration of the external insulation of the transformer due to wind, snow, salt and dust. This can be prevented with the correct inspection and maintenance.

3.2.7 Defects in the accessories, oil leak, gas leak, etc.

Leaking oil from corroded transformer
Leaking oil from corroded transformer (photo credit:

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4. Internal defects of the transformer

4.1 Defects in the winding

4.1.1 Short circuits – There are short circuits between the turns, between the phases and between the windings. Most short circuits failures are caused by abnormal tensions on the surge arresters and others because of the deterioration of the insulation oil and to the penetration of rain.

Also, some short circuits are caused by the deterioration because of heat, caused by an electromagnetic mechanical force or by an abnormal excessive load.

Generally, as secondary effect, internal short circuits cause serious deformations to the windings.

4.1.2 Breakage of the terminals of the winding – The terminals of the spooled suffer damages by an excess of current (external short circuit, etc) or by a lighting strike. Also, the short circuit accidents of the system that accumulate, cause damages on the supports of the windings, by their repetitive mechanical destructive force, that finally break the terminals.

4.1.3 Short circuit to ground – The impulse voltage or the deterioration of the insulation can cause a short circuit to the grounding of the winding or of its terminal to the core or to the tank. The mentioned defects can be detected easily through an external diagnosis or an electrical monitoring.

4.2 Defects in the core

There are defects due to poor insulation of the tightening screws of the core, or an oil-cooling duct obstructed, that cause excessive heating of the core. The defects on the core develop slowly.

The insulation and the poor grounding contact already mentioned, cause a partial short circuit current, a deterioration of the oil of the insulation materials in their surroundings, which gradually are converted into serious failures.

A poor or loose tightening between the core and the clamps of the windings can cause damaging vibrations.

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5. How to detect internal failures?

As additional protection, use the different relays that the transformer has to detect and be protected of failures. Next you can find which parts are used to protect the transformer from internal failures:

  • Those devices that are attached directly to the transformer, and detect failures mechanically:
  • Those devices that are indirectly joined to the control cabinet which detect failures electrically:
    • Differential relay,
    • Overcurrent relay,
    • Ground current relay.

Go back to Transformer Failure Topics ↑

Reference // Operation and Maintenance for Power Transformers – ABB’s User’s Manual

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

One Comment

  1. Manuel Bolotinha
    Mar 31, 2016

    1. Introduction

    Transformer being vital equipment, its protection is equally important. The subject of transformer protection can be categorized under two major headings:
     – Protection of the transformer against the effects of faults occurring on any part of the system beyond the transformer.
     – Protection against the effects of faults arising in the transformer.
    Transformers faults may occur in the dielectric (oil) and in the windings.

    2. Oil & Insulation Faults
    Transformer oils are designed to provide electrical insulation under high electrical fields; any significant reduction in the dielectric strength may indicate that the oil is no longer capable of performing this vital function.
    Some of the things that can result in a reduction in dielectric strength include polar contaminants, such as water, oil degradation byproducts and cellulose paper breakdown.
    Faults may occur in the oil, due to ageing, contamination with air, gas formation and lack of pressure and level.
    In the event of a minor fault like damage to core bolt insulation, local overheating, etc., the arcing causes slow generation of gas in the oil.
    All faults in transformer core and windings result in the localized heating and breakdown of oil.
    When the fault is of very minor type such as hot joint, gas is released slowly and rises towards conservator.
    A major fault where severe arcing takes place causes rapid release of large volume of gas and oil vapor.
    This violent evolution of gas and oil vapor does not have time to escape and instead builds up pressure and bodily displaces the oil, causing surge of oil to the conservator.
    Faults may also occur in the windings insulation material, as a consequence of oil failure, ageing, overheating and insulation breakdown.

    3. Core and Windings Faults

    Core Faults
    If any portion of the core insulation becomes defective or the laminated structure of the core is bridged by any conducting material which can permit sufficient eddy current to flow, it will cause serious over-heating. The insulated core bolts are used for tightening the core. If the insulation of these bolts fails and provides easy path for eddy current, this will lead to over-heating
    Common windings faults are:
     Faults between primary and secondary windings (short circuit) of the same phase.
     Short circuit between the turns of the winding.
    These faults usually are a result of dielectric failure, both between windings and between the turns of the same winding, due to ageing of insulation material, which may increase due to overloads.
    It also must be considered that the windings are subject to both radial and axial forces related to the current and flux interactions. Radial forces in the inner winding (normally the LV winding) are in compression while the outer winding (normally the HV winding) forces are in tension.
    Design of the windings and bracing must consider the magnitude of these forces and provide adequate strength to withstand them without significant mechanical deformation which could result in a dielectric failure.

    4. Overloads and Overheating

    The loading of transformer is decided by permissible temperature rise of windings and oil. Permissible oil temperature is 65°C and hot spot temperature of the winding is 80°C at rated load.
    As the load on the transformer does not remain steady and varies according to load curve, the loading of transformer becomes an important operating problem.
    The rated output of a power transformer is mentioned on its name plate with reference to specified temperature rise under specified test conditions.
    The output which can be obtained from a transformer without causing undue deterioration of the insulation may be either more or less than the name plate rating depending upon the operating conditions. Such as ambient temperature, initial loading, cooling provision, life expectancy etc.
    Overheating may be caused by overloads above the permissible overloads specified by the manufactures, according to standards (IEC 60354 for oil-filled transformers and IEC 60905 for dry type transformers), and external faults, such as short-circuits on installations downstream.
    This overheating may cause a breakdown of the oil and of the insulation of the windings

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