Buchholz Relay:
It providing protection against all kinds of incipient faults, such as – insulation failure of winding, core heating, fall of oil level due to leaky joints etc. It placed in the connecting pipe between main tank and conservator. The upper elements consists of a mercury type switch attached to a float to alarm circuit and the lower element contains a mercury switch mounted on a hinged type flap to trip the circuit breaker in case of internal faults.
Earth-fault (core-balance leakage) Protection:
This relay has low current setting and operates under earth or leakage faults only. Under normal conditions (i.e. no earth-fault), the vector sum of the 3-phase currents is zero. N earth-fault usually involves a partial breakdown of winding insulation to earth and the vector sum 3-phase is no longer zero. The resultant current sets up flux in the core of the CT which induces e.m.f. in the secondary winding.
Over-load (Combined leakage) Protection:
This relay has high current setting and are arranged to operate against phase-to-phase faults and over-loading. Because, faults or leakage occurs between phases, the core-balance relay will not operate. Here, two over-load relays are sufficient to protect phase-to-phase faults. Another one is leakage or earth relay. Three relays are connected in parallel and will operate with either over-load relay or earth relay.
Differential Protection:
Also called Merz-price circulating current scheme. It providing protection against both earth and phase faults (However, short-circuits are better taken care of by Buchholz relays). For a 3-phase delta/delta transformer protection against phase-to-phase and phase-to-ground faults CT’s on the two sides are connected in Star.The protection zone is limited between high-voltage and low-voltage CT’s of the transformer.
