I have been involved in a number of transformer failures. I am taking this opportunity to

show you these failures of transformers and how an expert can be of service to you.

Transformer

Transformers are used to step up or down supply voltages. Essentially, there are two types

of transformer, dry and liquid immersed. A dry transformer is most suitable for installation in

a commercial building because of its small size, light weight and simple maintenance.

A liquid immersed transformer, in particular, the oil filled type, is versatile as it can be

installed indoor or outdoor environments.

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Of the range of oil-filled transformers, there are three general types: distribution, power and

generator step-up (GSU). Distribution transformers are typically installed in distribution

networks, such as, at consumers’ premises. They are generally not more than 200MVA,

with operating voltages ≤11kV. Power transformers can be installed in electricity

transmission systems or at large commercial premises where there is high demand for

electricity consumption. They are generally rated from 200MVA, with operating voltages

≥33kV. GSU transformers are installed at electrical power stations. Their capacity generally

starts from several hundreds of MVA. The transformers are used to step up voltages of

turbine generators to that of transmission systems, which may reach 800kV.

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In a typical oil-filled transformer, each of the three phase (R, S and T) windings has four

layers of winding coils: two are low-voltage (LV), comprising an inner and an outer coil; and

two are high-voltage (HV), comprising a main and a coarse regulation (CR) / fine regulation

(FR) coil. Each of the coils comprises many turns of copper conductors, each coil is

wrapped with paper insulation, for example, kraft or nomex. The LV coils are housed

concentrically within the HV coils and wound onto a laminated sheet steel core.

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Figure 1 above shows a plan view of the R, S and T winding phases.

Figure 2 above shows the internal construction of a typical phase winding.

There are insulating spacers separating each individual concentric layer of LV and HV coils,

constructed in cylinder form from pressboard sheets, and vertical strips from wood.

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The top and bottom of each of the three phase windings are stacked with wooden blocks,

and there are wooden structures compressing the blocks and the windings.

In most large transformers, there is an on load tap changer (OLTC), which automatically

regulates the voltage of the transformer in the event of load variation.

The purposes of investigating transformer failures are,

- To determine compliance with regulatory body or insurance policy’s conditions.

- To identify any future risk in operating similar equipment.

- To identify any potential for recovery of loss.

- To identify possible fraudulent insurance claims.

In addition to physical damage to the transformer and its resulting downtime, failures of

transformers can lead to power interruption to the premises and significant business

interruption losses. It is usual for there to be preventative maintenance programmes in place

for operational transformers. Despite this, failures can sometime occur.

The leading cause of transformer failure is breakdown of electrical insulation of the windings,

which can result from poor installation, inadequate maintenance, defective material or

deteriorating insulation due to aging or mechanical damage associated with vibration.

Potential causes of failures external to the transformer unit include over-voltages involving

lightning activity or switching transients in the transmission lines.

During a preliminary investigation, exterior damage is recorded and appropriate testing is

performed. The tests may comprise some or all of the followings:

Dissolved gas analysis (DGA)Testing a sample of the insulation oil for the quantity of particular gases that could be related

to certain failures in the windings of an oil-filled transformer. Typically these gases are

hydrogen (H2), methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6),

carbon monoxide (CO) and carbon dioxide (CO2). With the exception of CO2, these gases

are combustible.

Oil insulation test,

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To test for signs of oil deterioration, such as, reduced dielectric strength, increased moisture

content and change in colour of the oil, in accordance with the applicable standards.

Polarization index (PI)It indicates the moisture content and possible deposition of conductive dusts on the surface

of insulation material.

Applied voltage (Hi-Pot) testTo test the insulation between two windings, and between the windings and the earthed

chassis.

Frequency response analysisTo evaluate whether there is any displacement of the transformer core, windings and

holding structures.

Partial discharge (PD) measurementThis is to detect any localised dielectric breakdown or deposits of conductive dusts on solid

insulation of the windings.

Excitation current test

This test detects winding problems including incipient damage associated with any

unintended movements in the windings that were caused by external short-circuit events or

mechanical impact during transportation.

Winding insulation testTo determine the condition of the winding insulation by measuring the resistance between

windings, and between each winding and the earthed chassis.

Winding resistance testThe test results may indicate a disconnected winding conductor, an inter-turn shorting or

a problem associated with high electrical resistance at a connection.

Coil ratio testTo detect for an inter-turn shorting within a winding.

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Following inspection of the equipment damage on site and the collection of background

information pertaining to the equipment, it is often necessary to inspect the internal windings1

of the transformer for any physical evidence of the failure. The internal inspection is best

performed in the facility of the manufacturers or compatible contractors, where hoisting

equipment, tools for dismantling and test equipment are available in the facility.

Findings are then tested rigorously against the evidence available, which would include

reviewing of all history of the condition of the equipment and interpretation of the test results.

Once the cause of the failure has been established it may be possible to review the existing

maintenance programme, and make any recommendations to prevent a recurrence of the

failure.

1 A winding comprised an outer set of high voltage coils and an inner set of low voltage coils, both of which were concentrically wound.

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Below are two examples of failures resulting from a manufacturer’s defect, involving a dry

and an oil-filled transformer.

Case I - Failure of a dry transformer, in the cabin of a newly installed wind turbine generator

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A transformer inside the cabin of a newly erected 80m tall wind turbine generator abruptly

failed during a site commissioning test, resulting in a severe and rapid spread of fire that

completely consumed the fibre reinforced plastic cabin walls. There was an unusual burn

pattern on the transformer surface. Following a detailed examination at the facility of the

equipment manufacturers in Spain, it was determined that there was a manufacturing defect

within the internal layers of the windings, where a conductive turn was displaced and had

almost overlapped on another, separated by a very thin layer of insulation. This led to

localised arcing following a breakdown of the thin insulation upon initial energisation on site

and the subsequent electrical discharge from the windings to the electrical chassis. The

findings were unanimous and were accepted by all parties. Further findings revealed that

during the assembly stages at production, the windings of the transformer were not secured

when lowered to the moulding machine, for encapsulation in epoxy resin.

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Case II - Failure of an oil-filled transformer

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The buckling or deformation of the inner windings in this core-form transformer was caused

by inadequate mechanical strength that held the winding turns and the spacer supports.

Electromechanical force produced during an external electrical fault, for example, short-

circuiting, can lead to movements in the winding turns. In some severe situations,

the winding displaces, and abrasion to the windings’ paper insulation occurs, to the extent

that conductors of separate turns are exposed. Subsequently, localised arc damage can

occur at one or more points of the adjacent winding turns. In this instance, there were

several locations of arcing involving displaced turns. Although the initiating event was an

external electrical short circuit, the root cause was a manufacturing defect in the winding

assembly.

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