Differential Protection Of Transmission Lines
(a) Current differential protection
(b) Merz- price voltage balance scheme
(a) Current differential protection :
Two CT's are connected in each protected line, one at each end.
Under healthy/ external fault conditions the secondary currents of CT's at both ends are equal and this current circulates in pilot wires.
For external faults and normal conditions, the differential current I1-I2 of two CT's is zero and the relay does not operate.
During internal faults, this balance is disturbed and differential current flows through relay operating coil.
The circuit breakers at two ends are separated by a long distance, there is a need to have to relay going at each end associated circuit breaker.
In line protection, relaying point falls in the middle of the line. This creates some difficulties.
To solve this problem the circuit is modified by providing two relays, one at each end.
(b) Merz- price voltage balance scheme :
Fig shows the single line diagram of Merz price voltage balance system for the protection of 3 phase line.
For merz price protection system current transformers placed in each phase at both ends of the line should be identical.
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 opposite directions i.e they balance each other. Therefore there is no driving force for the circulation of current through relay coils.
Suppose a fault occurs at point 'F' on the line as shown in Fig. This will cause a greater current to flow through CT1 than through СТ2.
Therefore, 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. shows the connections of the Merz price voltage balance scheme for all three phases of the line.
Advantages of Merz price protection :
1. This system provides very fast protection against ground faults.
2. This system can be used for ring mains as well as parallel feeders.
3. Due to the very fast protection, the amount of damage to the system reduces.
Disadvantages of Merz price protection :
1. Current transformers at both ends should be exactly identical, which is very difficult.
2. If there is a break in the pilot-wire circuit, the system will not operate.
3. This system is very expensive during to the greater length of pilot wires required.
4. In the case of long lines, charging current due to pilot wire capacitance effect may be sufficient to cause relay operation ever under normal conditions.
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