Bus-bar’s Protection

Differential Protection:

The bus-bar is fed by a generator and supplies load to two lines. Secondaries of CT in the generator lead, in line 1 & 2 are all connected in parallel. The protective relay is connected across this parallel connection. When a fault occurs, the fault current upsets the balance and produces a differential current to operate a relay.

Fault Bus Protection:

The metal supporting structure or fault bus is earthed through a CT. A relay is connected across the secondary of this CT.

Transformer’s Protection

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.

Protection

Alternator’s Protection

Transformer’s Protection

Bus-bar’s Protection

Alternator’s Protection

Differential Protection:

It is also called Merz-Price circulating current scheme Protection.〖CT〗_1 and 〖CT〗_2 are placed on either side of each phase of the stator winding. Secondaries of each set of CT are connected in Star.Two neutral point and corresponding terminals of Star groups are connected together by means of four-cable Pilot cable.Relay coils are also connected in Star, neutral to common neutral and outer three ends across equi-potential points which naturally be located at the middle of the three Pilots.
N.B: In some case, alternator is located at a considerable distance from relays near circuit breaker. Therefore, it is convenient to connect relay coils to the actual mid-points of pilots. So, balancing resistances are inserted (Located in box) in the shorter lengths of pilots to divide the whole secondary impedance of two sets of CT’s into equal portion.

Earth-fault Protection:

In small-size alternators, neutral ends of 3-phase windings are often connected internally to a single terminal. So, it is not possible to use “Merz-Price circulating current scheme”. Then three line of CT’s, one mounted in each phase, having their secondaries connected in parallel with that of a single CT in the conductor joining Star point of alternator to the earth. This scheme provides no protection against phase-to-phase faults and is limited to earth-faults only between neutral and line current.