Common Causes of Failures of Power Transformers

As with most of electrical equipment, the key causes of failure of power transformers are either due to electrical failure or mechanical failure. These failures don’t occur that often as compared with other failures in other electrical equipment like switchgear. Common failures of power transformers and their causes are discussed in the following sections.

Typical Causes of Failures of Power Transformers

Lighting and Voltage Transients

These may cause external flashover of bushing insulations which may shatter or split oil-filled insulators, causing failures. If the voltage surge reaches a transformer winding it may cause a flashover above the oil and damage the winding insulation.

Internal Flashover above Oil

This may be due to the voltage transients as aforementioned but there are other causes that are more easily eradicated, for example arcing caused by fractured conductors, sparking at loose connections, and badly made joints that provide ionization above the oil level. Hot joints and connections may also be possible causes.

Inter-Turn Faults

These types of faults develop slowly. The volts per turn are typically rather low (i.e. 5-20 V for transformers below 2 MVA) and will not sustain continuous arcing until a number of turns become involved but this may not apply for example in modern transformers, where coils and turns are interleaved without spacers to enhance surge voltage distribution.

Overheating can be caused by short-circuited turns. These failures may go undetected by conventional protective relays as there is likely to be no significant change in through current. But some inter-turn faults can be detected by certain forms of monitoring and testing.

Failure of the Insulation of the Magnetic Circuit

This kind failure such as of core frame or clamping bolts or between laminations, can allow parasitic eddy currents and local heating. The former may cause sparking and the evolution of arc gases. Such faults do not lead to immediate danger however they contribute to the deterioration of insulation and oil. Furthermore, an accumulation of arc gas (especially H₂ and C₂H₂) in the airspace above or dissolved in the oil is a hidden explosion hazard.

Other faults which occur from time to time are caused by poor hygiene and untidiness during manufacture and maintenance. Nuts, bolts, cut-off ends of wires and even spanners may be mistakenly left wedged between windings by say maintenance personnel. These can cause local stress concentration and heating with possible mechanical damage to insulation.

Overloading and ‘through’ faults

These will cause overheating of the windings and cause cumulative damage to the insulation. These faults may not cause immediate danger unless they persist but the life of the transformer will be shortened.

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• Testing Power and Distribution Transformers
• Features and Applications of Instrument Transformers
• Ideal Transformers
• Overhead vs. Underground Power Distribution Systems
• Transformer Substations in an Electric Power Supply System