Figure shows the single line diagram of Merz Price voltage balance system for the protection of 3-phse line. Identical current transformers are placed in each phase at both ends of the line. The pair of CTs in each line is connected in series with a relay in such a way that under normal conditions, their secondary voltages are equal and in opposition i.e. they balance each other

Under healthy conditions, current entering the line at one-end is equal to that leaving it at the other end. Therefore equal and opposite voltages are induced in the secondaries of the CTs at the two ends of the line. The result is that no current flows through the relays. When a fault occurs at point F on the line as shown in Fig 6. It will cause a greater current to flow through CT₁ than through CT₂. Consequently, their secondary voltages become unequal and circulating current flows through the pilot wires and relays. The circuit breakers at both ends of the line will trip out and the faulty line will be isolated. Fig 7 shows the connections of Merz Price voltage balance system for all the three phases of the line.

Advantages:

(i) This system can be used for ring mains as well as parallel feeders.

(ii)This system provides instantaneous protection for ground faults. This decreases the possibility of these faults involving other phases.

(iii)This system provides instantaneous relaying which reduces the amount of damage to over head conductors resulting from arcing faults.

Disadvantages

(i) Accurate matching of current transformers is very essential.

(ii) If there is a break in the pilot-wire circuit, the system will not operate.

(iii) This stem is very expensive owing to the greater length of pilot wire required.

(iv) In case of long lines, charging current due to pilot wire capacitance effects may be sufficient to cause relay operation even under normal conditions.

(v)This system cannot be used for line voltages beyond 33 kV because of constructional difficulties in matching the current transformers

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