A differential relay is defined as the relay that operates when the phase difference of two or more identical electrical quantities exceeds a predetermined amount. The differential relay works on the principle of comparison between the phase angle and magnitude of two or more similar electrical quantities. Comparing two electrical quantities in a circuit using differential relays is simple in application and positive in action.
For example, consider the comparison of the current entering a protected line and the current leaving it. If the current enters the protected line is more than the current leaves it, then the extra current must flow in the fault. The difference between the two electrical quantities can operate a relay to isolate the circuit.
For the operation of the differential relay, it should have two or more electrical quantities, and these quantities should have a phase displacement (normally approximately 180). Any types of the relay can operate as a differential relay depends on upon the way it is connected in a circuit. In other words, it doesn’t depend on the construction of the relay it depends on the way it is connected to the circuit.
Differential protection provides unit protection. The protected zone is exactly known by the location of current and potential transformers. The phase difference is achieved by suitable connections of secondaries of CTs and PTs.
The differential protection principle is employed for the protection of generator, generator-transformer units, transformers, feeders, large motors, and bus-bars. The differential protection relay is mainly classified into four categories. These are
- Current Differential Relay
- Voltage Differential Relay
- Biased or Percentage Differential Relay
- Voltage Balance Differential Relay
Current Differential Relay
A relay which senses and operates the phase difference between the current entering into the electrical system and the current leaving the electrical system is called a current differential relay. An arrangement of overcurrent relay connected to operate as a differential relay is shown in the figure below.
The dotted line represents the element of the system that is to be protected by the differential relay. The system element might be a length of the circuit, a portion of the bus or a winding of a generator or that of a transformer. A pair of current transformers is fitted on the either ends of the section to be protected. The secondaries of current transformers are connected in series with the help of the pilot wires in such a way that they carry the induced current in the same direction. The operating coil of an overcurrent relay is connected across the current transformer secondary circuit shown in the figure below.
When there is no fault current or there is an external fault, then the current in the secondaries of the current transformers are equal, and the relay operating coil, therefore, does not carry any current. When the short circuit developed anywhere between the two current transformers, then the currents flow to the fault of both sides, and the sum of the current transformer secondary current will flow through the differential relay.
In other words, the differential relay current will be proportional to the phase difference between the currents entering and leaving the protected element. If the differential current exceeds the relay’s pick up value, then the relay will operate.
Biased or Percentage Differential Coil
This is the most used form of differential relay. Their arrangement is same as that of the current differential relay; the only difference is that this system consists an additional restraining coil connected in the pilot wires as shown in the figure below and current flows in both CTs flows through it.
The operating coil is connected to the midpoint of the restraining coil. The reasons for this modification in circulating current differential relay is to overcome the difficulty arising out of differences in current transformers ratio for high values of short circuit current.
Induction Type Biased Differential Relay
This relay consists of a pivoted disc free to rotate in the air gaps of two electromagnets. The portion of each pole of the electromagnet magnet is fitted with a copper shading ring. The ring can be moved further in, or out of the pole. The disc experiences two torques one due to operating element and other due to restraining element.
If the shading rings were in the position on each element, then the resulting torque experienced by the disc would be zero. But if the shading rings of restraining element were moved further into the iron core, the torque exerted by the restraining element will exceed than that of the operating element.
Voltage Balance Differential Relay
The current differential relay is not suitable for the protection of the feeders. For the protections of the feeders, the voltage balance differential relays are used. In this arrangement, the two similar current transformers are connected at either end of the system element under protection using pilot wires.
The relays are connected in series with the pilot wires, one at each end. The relative polarity of the current transformers is such that there is no current through the relay under normal operating conditions and under fault conditions. The CTs used in such protections should be such that they should induce voltages in the secondary linearly with respect to the current. Since the magnitude of the fault current is very large, so that the voltage should be a linear function of such large currents, the CTs should be aired cored.
When the fault occurs in the protected zone, the currents in the two primaries will differ from one another, and so voltage induced in the secondaries of the CTs will differ and circulating current will flow through the operating coils of the relays. Thus the trip circuit will be closed, and the circuit breaker will be open.