Faults in Transformers:
- Some of the faults in the transformers are likely to be over-voltages which resulting from the atmospheric phenomenon (lightning) transmitted by overhead lines.
- Switching in the power system (especially high voltage switching more than 400kV) can produce over-voltages of less steep but longer duration surges stressing both liquid and solid dielectrics (insulation). These over-voltages should be restricted in amplitude to a value below the transformer insulation breakdown withstanding level.
- Short circuits in the power system subject the transformer to currents of 10 to 20 times the rated currents (short circuit current level will be severe when fault occur close to transformer). Power transformer is generally designed to withstand tens of short circuits, lasting not more than 2 sec duration in its life time. If there are more short circuits than the designed limit special construction is required. Short circuits should eliminated (by isolating faulty power system by opening circuit breakers) as quickly as possible to limit the short circuit intensity on transformer.
- Overloads can arise in transformers from planned or fortuitous (unexpected) circumstances. In the first case, temperature increase in transformer insulating material should not exceed the standard value. In the second case, certain time limit can be tolerated but this will have certain cost in reduction in the life of the transformer.
Internal Faults in Transformer:
- Electrodynamic faults: which occurs between insulation and current carrying conductors, HV and LV winding due to external and internal short circuits
- Electromagnetic faults: Which occur due to eddy currents induced in the magnetic circuits or the clamping structure.
- Electrical faults which occurs due to bad contacts in the leads or bad contacts in the tap changer Dielectric faults: Which occur due to shorting between windings or between live parts and earth, partial discharges
- Thermal faults: Which occurs due to abnormal temperature rise, hot spot, thermal ageing or pollution in transformer oil
- Mechanical faults: Which occur due to vibrations, leakages or defective operation of the tap changers
Different types of defects originated in transformers will have degree of gravity depending on the amount of damage it can do on transformers and their consequences. Some of the defects (vibrations, partial discharges) will not immediately endanger the equipment but care must be taken before causing major damage. On the other hand, defects such as (over-voltages and short circuits, and initial breakdown) requires immediate attention.