Engineering World: Why do transformers fail?

Introduction

There are many initiators of transformer failures, but those which can potentially lead to catastrophic failure are:

1. Mechanical failure

2. Dielectric failure

In both cases, the transformer is no longer able to perform its intended function of carrying load and stepping down (or up) the voltage. Some of the most common reasons are listed below :

Most Common Causes of Failures

Lighting Surges	Line Surges/External Short Circuit
Poor Workmanship-Manufacturer	Deterioration of Insulation
Overloading	Moisture
Inadequate Maintenance	Sabotage, Malicious Mischief
Loose Connection

It can be a very catastrophic and dangerous situation when a transform fails as it can be seen in the below video:

1. So why does a transformer fail after all?

Transformer Failure

Failures appear in different ways, depending on the type of construction. Some modes of failure can occur regardless of construction type. These might include tap changer failures, bushing failures, tank failures, moisture ingress, and other forms of dielectric fluid contamination. Sometimes the failure could be purely due to lack of regular maintenance or lack of awareness. And sometimes it could be due to natural causes like lightning which causes electrical surge in the power lines.

1.1 Mechanical failures

Mechanical failures can be the result of shipping damage, seismic activity, and thru-faults. The obvious result of a mechanical failure is the displacement of winding turns or damage of the turns by the forces exerted during the damaging

Doble M5400 SFRA TEST KIT

event. Mechanical failure can result in scalloped conductors (beam failure), conductors which have been looped over adjacent turns by the hoop stress (hoop failure), or in rare cases, conductors which have been severed by the tension applied by the hoop force. That’s why it’s highly recommended to perform a SFRA (Sweep Frequency Response Test) test on site to observe any change in the SFRA test result in comparison to the factory results. Moreover change in low voltage excitation current, a change in impedance, and sometimes, the presence of partial discharge (PD) during an induce voltage test can also give valuable indications about a mechanical failure in the transformer. Mechanical failure is often discovered by electrical failures which are the result of mechanical deformation.

1.2 Electrical failures

Inter turn fault

Electrical failures are the result of insulation degradation. This can be caused by thermal degradation over the life of the transformer, by thermal degradation due to excessive or frequent fault current, or by dielectric breakdown due to high voltage stress. A dielectric breakdown can also be the result of mechanical forces tearing the insulation. The result of a electrical failure can be a turn to turn failure. The consequences can be arc from the energized winding to an adjacent winding or to ground. It is important to note that overloads rarely result in transformer failures, but do cause thermal aging of winding insulation. When a transformer becomes hot, the insulation on the windings slowly breaks down and becomes brittle over time. The rate of thermal breakdown approximately doubles for every 10°C. 10°C is referred to as the “Montsinger Factor” and is a rule of thumb describing the Arrhenius theory of electrolytic dissociation. Because of this exponential relationship, transformer overloads can result in rapid transformer aging. When thermal aging has caused insulation to become sufficiently brittle, the next fault current that passes through the transformer will mechanically shake the windings, a crack will form in the insulation, and an internal transformer fault will result.

Transformer oil leakage between the tank and the radiator

Transformer can also fail due to poor maintenance. Especially when there are leakages in the transformer tank and the transformer oil level drops below a certain level which gives rise to local heating in the windings which will eventually fail if there is no transformer oil to cool down the temperature. Thus it's very essential to carryout regular oil filtration of the transformer and leak arrests before it becomes very critical. Preventive maintenance is the key to avoid such kinds of disasters.

1.3 Other common failure nodes

Other failure modes can be the result of a grounded core or core clamping structures (such as through-bolts) that develop shorts. These result in a shorted turn (the core) and produce high currents which are often detected by dissolved gas analysis.

2.0 Conclusion

Maintenance is the key to avoid transformer failure: a planned program of maintenance, inspection and testing can significantly reduce the number of transformer failures, and the unexpected interruption of power. Gas-in-oil analysis should be performed annually to measure the dissolved gases in the oil that are created by developing faults in the transformer. Additional measures may include:

On liquid-cooled units, check the radiators for leaks, rust, accumulation of dirt, and any mechanical damage that would restrict the oil flow.
Keep the porcelain bushings and insulators clean.
Keep electrical connections tight.
Inspect tap changes on a regular basis.
The transformer windings, bushings, and arresters should have a Power Factor test on a three-year basis.
Check the ground connection on the surge arrester annually.