Transformer comes in several sizes and shapes i.e smallest Dry Transformer to EHV Large Transformer. Various types of transformer are: 1) Generation Transformer
2) Power Transformer
3) Distribution Transformer
4) Zigzag Transformer
5) Instrument Transformer
6) Potential Transformer
Such a Transformer is needed with Transformer protection to prevent different incipient or in-turns faults. Transformer Protection is very necessary for Transformers, otherwise large fault current will flow which will burn out transformer.
Types of Fault in Transformer:
There are a variety of Faults in the Transformer. The most common one is winding to core faults because there is a weakening of insulation. Phase Faults are not common in the transformer but such faults may take place outside the transformer or on the terminal of the transformer which is in the Transformer protection zone.
Every Large transformer is filled with oil and sometimes the level of oil becomes low due to leakage. In such cases, Buchholz Relay operates for the transformer Protection.
Another problem is Large inrush currents which happens at the switching time of the transformer, these large inrush currents are not the abnormal condition nor as a fault condition but Special care is needed while designing a transformer. The transformer also has an over-fluxing problem.
Transformer Over Current Protection:
There are two phase-fault overcurrent relay and one ground fault overcurrent relay for providing overcurrent protection to Star-delta Transformer. Such protection is used as backup protection as well as primary protection.
In turn Faults in Transformer:
Such type of faults causes a heavy current to flow with the transformer turns or winding. When large current flows through transformer winding heat is produced and thus increases the temperature. Hence Buchholz relay is used for transformer protection in case of in turn faults.
Incipient Faults in Transformer:
In Transformer, there are some faults which are not significant in the beginning but can become serious Faults such are known as incipient Fault. Buchholz relay is used to protect such Faults.
The figure shows the basic construction of the Buchholz relay. When there is an incipient or in turn fault in a transformer there is a heating of the oil which causes gases to liberate at more than 400 Celsius. There is a build-up of oil pressure between the transformer and conservator. A drain vane is placed in the path of oil which is used to trip the contacts of Buchholz relay.
The Buchholz relay has also afloat that operates when there is a loss of oil due to leakage or decomposition. Normally float is connected with the alarm contacts, which raises the alarm in case of loss of oil.
Transformer Protection – Buchholz Relay
Click here for a single-phase transformer.