The Merz-price protection system gives protection against phase to phase faults and earth faults. But it cannot be used for the protection against inter-turn faults in a winding. The inter-turn faults i.e., the fault between the turns of the same winding of the stator cause to flow of a circulating current through the winding turns undergoing the fault.
So, it does not produce any imbalance between the currents entering and leaving the winding where the two CTs are located. Hence, the differential protection scheme does not respond to the inter-turn faults.
Split-phase relaying protection (also called transverse differential protection) is one of the protection schemes against inter-turn faults. The protection scheme is based on the cross-differential principle.
Not only against winding inter-turn faults, but it also provides protection against all the types of short-circuits within the stator winding. This protection scheme is applicable only for the alternators having more than one coil per phase i.e., multi-circuit winding.
The connection diagram of the protection scheme is shown in the figure above. The complete winding of each phase is divided into two groups. For an even number of circuits per phase, both the groups will be equal, and similar. CTs are placed in both groups. For an odd number of circuits per phase, both the groups will be different. In such cases, CTs with different primary current ratings but the same secondary current ratings must be used.
Under normal operating conditions and through fault conditions, the current flowing through each individual group of a phase will be such that the secondary current of the CTs in each phase will be equal. As the direction of currents in both the groups of a phase is opposite to each other due to the virtue of their connection, no current flows through the operating coils (O.C.) of the relay. Hence, the relay will not operate.
If an inter-turn fault occurs in one of the parallel windings, then the currents coming out of the two groups of the phase will produce an imbalance in the secondary current of the CTs connected in that particular phase.
As this out-of-balance current will be above the pick-up current of the relay, hence it operates and initiates a trip signal. Similarly, during internal phase to ground and phase to phase faults, the relay operates due to an out-of-balance current through its operating coil.
