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.
