Transformer faults and how can it be prevented

Transformer faults and how can it be prevented

Power transformers play a major role in power system so that high reliability of the transformer is needed for power transmission. A high-quality transformer which is properly designed and supplied with suitable protective relays and monitors is highly reliable. When a fault occurs in a transformer the damage would be normally severe and this will affect the power transmission system very badly


What are the transformer conditions which could lead to faults

  • Insulation breakdown
  • Aging of insulation
  • Over-heating due to over excitation
  • Oil contamination and leakage
  • Reduced cooling

Fault currents

  • Ground faults in solidly grounded star-connected secondary winding
  • Ground faults in delta connected winding
  • Turn to turn faults
  • Phase to phase faults

Insulation breakdown

Breakdown of insulation results in short circuits or ground faults. Breakdown of insulation between windings or between windings and core can be caused by

  • Aging of insulation due to over-temperature during a long time
  • Contaminated oil
  • Corona discharges in the insulation
  • Transient over-voltages due to thunderstorm or switching in the network

Aging of insulation

Insulation aging is the function of time and temperature, the winding part which is operated at the highest temperature will ordinarily undergo the greatest deterioration and it will only have the shortest length of life

In order to avoid this cooling of the transformer must be done when it gets too hot or the load must be reduced so that we can reduce the aging of the insulation

Over-heating due to over-excitation

When a transformer is operated at very high voltages or at very high frequency the transformer core will get over-excited

In order to prevent this over-excitation relays can be used the relay will operate when the ratio between voltage and frequency gets too high

Oil contamination and leaking

Oil in the transformer consists of an electrically insulating medium and a cooling medium. So the quality of the oil is important for the reliability of the transformer

Dielectric strength of the oil is an important factor if the dielectric strength is reduced by water or other impurities then breakdown will happen in order to avoid this an oil level monitor must be used and a silica gel breather

Reduced cooling

An alarm indicator must be used in case if the cooling system stops so that we could take proper action before the transformer gets over-heated

Fault currents

Ground faults in a solidly grounded star-connected secondary winding

Mostly magnitude of the fault current is controlled by the reactance and voltage between the point of the fault and ground. Reactance will decrease rapidly for the faults approaching the neutral. So the fault current would be higher for the fault close to the neutral than for a fault at the middle of the winding

Ground faults in a delta connected winding

Ground fault current magnitude depends upon the grounding of the power system, fault impedance is high for the faults at the midpoint of the winding. The current in the short circuit turns is several times greater than the rated current and it could cause overheating, arcing… so we can use relays for this a gas detector relay detects the turn to turn faults. Rate of rising pressure relay can also detect the fault

Phase to phase faults

Short circuit between the phases could create fault currents, only limited by the source impedance and the leakage impedance of the transformer

Turn to turn faults

Direct metallic contact or flashover between conductors within the same physical winding is called a turn to turn fault

It can be caused by the steep fronted surge voltages or corona discharges and also by high fault currents through a transformer during system faults which can damage the insulation which could create turn to turn fault. These faults are difficult to detect by current measuring relays because of the terminal current increases very little