Merz Hunter Split Conductor Protection of Feeders:

This Merz Hunter Split Conductor Protection of Feeders system is another method of securing the benefits of a balanced method of protection without the necessity of using pilot wires. The principle of operation depends on the fact that two conductors of equal length and impedance, when connected in parallel will share the load equally, provided that the insulation of the system is sound. When a fault develops on any one conductor it will carry more current than the other, and this inequality of currents is arranged to operate a relay and thus isolate the faulty line.

In this Merz Hunter Split Conductor Protection of Feeders system of protection each phase of the line is split into two sections having equal impedances. The two sections are lightly insulated from each other. In this system, as shown in Fig. 14.15, a single-turn current transformer is inserted at each end of the split conductor. The current transformers consist of laminated iron rings on which a secondary winding is wound all round the periphery.

Under healthy conditions the current flowing along the two splits are equal and since these are threaded through the current transformers CT₁ and CT₂ in the opposite directions hence the voltage across the terminals of the evenly spread secondary winding is zero. In fault conditions one of the split takes more current than the other, thereby giving rise to an unbalance of the primary side of the current transformers. Due to unbalancing of currents on the primary side of current transformers resultant flux will be set up in the core of the one of the current transformers and so the current will be induced in the evenly spread secondary and the relay coil R will be energised. The relay contacts will be closed and the trip coil will trip the circuit breaker and isolate the fault.

In the best arrangement the splits are carried into the circuit breakers on both sides of the feeder so that the splits are opened by the breakers. This is explained as follows :

Let the splits be not carried into the circuit breakers and a fault develop at the receiving end of a long line. Under these conditions, the impedance of the differential current transformer at this end may be insufficient to cause unbalance between the currents carried by each split conductor. Hence such a fault will not be cleared by the circuit breakers since the relay will not operate. But when the splits are carried into the circuit breakers the fault current is confined to the faulty split after the sending-end circuit breaker has tripped. In the former case, although the sending-end circuit breaker trips, the fault current is not confined to the faulty split but it would divide practically equal between the two splits being solidly connected, so the receiving-end circuit breaker will not trip. But in the latter case, the fault current is confined to the faulty split, the opening of the receiving-end circuit breaker takes place.

From above discussion it is obvious that the split circuit breakers impart a high sensitivity of operation to the protection and switchgear.

Disadvantages:

The disadvantage of this Merz Hunter Split Conductor Protection of Feeders system is that we have to make use of a special type of cable with the lower limits for the voltages. In the case of overhead feeders, for each phase, a duplicate set of conductors, insulators, etc., have to be employed. The lines having step-up or step-down transformers or voltage regulators cannot be protected by this method.