What is a transformer
A transformer is a device which helps with the transmission of electrical power, it is a static device without any rotating part and is totally enclosed. So the transformer plays a major role in the electrical distribution so the protection of transformers is an important process. The transformer must be protected from faults in order to do effective power distribution.
What are the types of transformer faults
- Over heating
- Winding faults
- Open circuits
- External faults
- Over fluxing
- Earth faults
- Phase faults
- Inter turn faults
- Core faults
- Tank faults and cooling failure
- Tap changer faults
Over-heating is caused due to sustained overloads, short circuits and in case of failure of the cooling system. Over-heating is also caused by overexcitation if a transformer is operated at its maximum potential like high frequency or in high voltage the core will get excited.
Resistance temperature indicator can be used near the winding, thermal overload and temperature relays can be used. Bridge circuits can be used when the bridge balance gets disturbed more than the permissible duration circuit breaker trips.
The winding faults are internal faults, the over-heating or mechanical shocks deteriorates the winding insulation. If the winding insulation is weak there is a possibility of phase-phase faults, earth, and inter-turn faults. The insulation could be damaged due to a short circuit or ground fault. Insulation could be breakdown because of the aging of the insulation or the oil contamination or due to transient over-voltages due to lightning.
Differential and overcurrent protection can be used as backup protection for earth fault protection, restricted earth fault protection system, neutral current relays or leakage to frame protection system can be used. Electrical transients can be prevented by using a surge or lightning arrester.
An open circuit in one of the three phases causes the heating of the transformer. Compared to other faults open circuits are harmless, during such faults transformers can be manually disconnected from the system.
Through faults are external faults that occur outside the protected zone, these faults cannot be detected by differential protection. If an external fault occurs it could cause mechanical and thermal stress to the transformer and it could damage the transformer. Sustained external faults must be cleared before the thermal rating of the windings is exceeded.
Overcurrent relay with under-voltage blocking, zero sequence protection, and negative sequence protection are used to give protection. Overcurrent protection not only protects the transformer but also covers the station bus bar and a portion of the transmission line.
Over fluxing can be caused by poor regulation of voltage and frequency on the power system. The flux density in the transformer core is proportional to the ratio of voltage to frequency. Transformers work with a value of flux density in the core. If full excitation is applied in a generator transformer before it reaches its synchronous speed then over fluxing of the core could occur due to high V/f.
V/f relay is called volts/hertz relay is provided to give protection against over fluxing operation.
The flow of earth fault current is dependent on their being a presence of a zero sequence path. The ampere turn balance between the secondary and primary windings must be maintained. The magnitude of the transformer earth fault current is depended on the method of earthing, the impedance of winding, the transformer connection and position of the fault on the winding. Solidly earthed transformer have their winding impedance to limit the earth fault current. Earth faults are the most common type of faults in the transformer, to reduce the earth’s fault additional impedance can be added to the zero-sequence current paths by using earthing transformers and resistors.
Phase to phase faults or three-phase faults do not flow through the earthing arrangement and therefore are limited by winding impedance only. The value of winding impedance is deliberately designed into the transformer in order to limit the maximum short circuit current. It is achieved by designing the leakage reactance flux which does not link the primary and secondary windings to give the desired short-circuit level. To remove these faults Buchholz surge and differential relays can be used.
Inter turn faults
These faults occur in the high voltage winding because of the degradation of the insulation and it could be caused by the voltage doubling at the HV terminal. The voltage doubling can be caused by lightning or switching. The inter-turn faults cause localized extreme hot spots to occur as the faulted turns carry huge currents and this would badly affect the winding insulation. These faults can be detected by using the Buchholz gas device and differential relay.
These faults can be caused by two causes they are a breakdown in the insulation between the silicon steel core lamination and secondly by a second transformer earth fault point occurring due to the build-up of debris or a breakdown in core bolt insulation. This could cause the excessive flow of eddy currents and hot spots will be formed in the transformer core. This fault will badly affect the performance of the transformer and it can be prevented in the early stage by doing oil and gas analysis.
Tank faults and cooling failure
Transformer oil leakage could cause flashover between windings and end connections. In order to operate the transformer at the full rating, the cooling system must work properly. Mostly smaller transformers are naturally cooled but, large transformers have forced cooling. Failure in the cooling system will badly affect the performance of the transformer.
Tap changer faults
Tap changers are the most important part of the transformer, the tap changer provides automatic regulation of the voltage regardless of normal loading conditions by varying the ratio of primary to secondary turns. Tap changer can itself become faulty due to flashovers, selector or divertor failure, mechanical failure or burn out of divertor resistors. Tap changer faults could also occur due to inadequate maintenance.