Differential Relays :
There are two fundamental systems of differential protection :
1) Current balance protection.
2) Voltage balance protection.
Current Differential Protection :
Fig. shows an arrangement of the differential relay.
A pair of identical current transformers are fitted on either end of the section to be protected
The secondaries of CTs are connected in series in such a way that they carry induced currents in the same direction.
The operating coil of the over-current relay is connected across the CT secondary circuit.
This differential relay compares the current at the two ends of the protected winding.
Under normal operating conditions suppose the current through the winding is 1000 A. therefore, the secondary current is 5 Amp.
This current will circulate through pilot wires.
If the ground fault occurs on the winding as shown in Fig. (b) the two secondary currents will not be equal and the current flows through the operating coil of the relay causing the relay to operate.
If some current (assume 500A) flows out of one side while a large current (assume 1000A) enters the other side as shown in Fig. (b).
Then the difference of the CT secondary currents i.e. 05 - 2.5 = 2.5 A will flow-through relay
Disadvantages of Current Differential Protection :
Pilot cable capacitance causes incorrect operation of the relay when a large through current flows.
The impedance of the pilot cable causes the slight difference between the current at the two ends of the section to be protected. If the relay setting is low then the small differential current flowing through the relay may cause it to operate even under normal conditions.
Accurate matching of current transformers cannot be achieved due to pilot circuit impedance.
Saturation of CT magnetic circuits during short circuit conditions. Due to these causes, the relay may operate even for external faults. The relay may loose its stability for through faults.
Tap changing causes a change in the transformation ratio of a transformer. There by CT ratios do not match with the new tap settings, resulting to current in pilot wires even during healthy conditions.
This difficulty is overcome by a biased differential relay.
Biased or Percentage Differential Relay :
The reason for using modification is circulating current differential relay is to overcome the trouble arising out of differences in CT ratios for high values of external short circuit currents.
In this relay, the operating coil is connected to the midpoint of the restraining coil.
The total number of ampere-turns in the restraining coil becomes the sum of ampere-turns in its two halves i.e I1N/2 + I2N/2 which gives the average restraining current of I1 + I2 /2 in 'N turns.
For External faults both the currents I1 and I2 increase and thereby the restraining torque increases which prevent the mal-operation.
The operating characteristics of such relay is as shown in Fig.
The ratio of differential operating current to average restraining current is a fixed percentage. Hence the relay is called 'Percentage Differential Relay'.
Voltage Balance Differential Relay :
Fig. shows the voltage balance protections. In this scheme of protection, two similar current transformers are connected at either end of the winding to be protected by means of pilot wires.
The secondaries of current transformers are connected in series with a relay in such a way that under normal working conditions, their induced e.m.f's are in opposition.
Under healthy working conditions, equal currents (I1 = I2) flow in both primary windings of CT1 and CT2.
Therefore the secondary voltages of the two transformers are balanced against each other and no current will flow through pilot wires and hence in the operating coil.
When faults occur in the protected zone, the currents in the two primaries will differ from one another (I1 ≠ I2) therefore their secondary voltage will no longer be the same.
These voltage differences will cause a current to flow through the operating coil of the relay, which closes the trip circuit.