Transformer Protection - Overcurrent and Earth-fault
Power Projects 
This post describes the Protection principle and pickup setting calculations for the backup protection of Transformers.
Transformer Protection:
Transformer Protection – Transformers are static devices, totally enclosed and generally oil immersed. Therefore, chances of faults occurring on them are very rare. However, the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessitates to provide adequate automatic protection for transformers against possible faults.
Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay is required. However, the probability of faults on power transformer is undoubtedly more and hence automatic protection is absolutely necessary.
Common Transformer Faults:
- Open Circuits
- Overheating
- Winding Faults or Internal Faults
- Over fluxing
- Through faults
The principle relays and systems used for transformer protection:
- Buchholz device – Providing protection against all kinds of incipient faults.
- Overcurrent relays – Providing protection mainly against phase-to-phase faults and overloading.
- Earth fault relays – Providing protection against earth faults.
- Differential System – Providing Protection against both earth and phase faults.
- Harmonic Restraint and Harmonic Blocking.
Here the blog covers the Overcurrent and Earth Fault Protection of Transformer.
Over Current Protection:
The inherent time delay of the IDMT element provides backup for the LV side. High-set instantaneous overcurrent is also recommended on the primary side mainly to give high-speed clearance to HV bushing flashovers. However, to ensure that these elements do not pick up and trip for faults on the LV side as discrimination is important.
51- IDMT Over Current Protection
Pickup Current (I >):
Categories: : Protection
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