Differential protection for a generator is mainly employed for the protection of stator windings of generator against earth faults and phase-to-phase faults. The stator winding faults are very dangerous, and it causes considerable damage to the generator. For the protection of stator winding of the generator, the differential protection system is used for clearing the fault in the shortest possible time for minimizing the extent of a damage.
Merz-Prize Circulating Current System
In this scheme of protection, currents at the ends of the protected sections compare. When the system is in normal operating condition, the magnitude of currents is equal on the secondary windings of the current transformers. On the occurrence of the faults, the short-circuit current flows through the system and the magnitude of current become differ. This difference of current under fault conditions is made to flow through the relay operating coil.
The relay then closes its contacts and makes the circuit breaker to trip and thus isolated the protection from the system. Such a system is called a Merz-Prize circulating current system. It is very effective for earth faults and faults between phases.
Connection for Differential Protection System
The protection system requires two identical transformers which are mounted on both sides of the protection zone. The secondary terminals of the current transformers are connected in stars, and their end terminals are connected through the pilot wire. The relay coils are connected in delta. The neutral of the current transformer and the relay are connected to the common terminal.
The relay is connected across equipotential points of the three pilot wires so that the burden on each current transformer is same. The equipotential point of the pilot wire is its centre, so the relay is located at the midpoint of pilot wires.
For proper working of the differential protection system, it is essential to locate the relay coils adjacent to the current transformer near the main circuit. This can be done by inserting the balancing resistance in series with the pilot wires to make equipotential points located near the main circuit breaker.
Working on Differential Protection System
Consider the fault occurs on the R phase of the network because of the insulation breakdown. Because of the fault, the current in the secondary of the transformer becomes unequal. The differential currents flow through the relay coil. Thus, the relay becomes operative and gives the command to the circuit breaker for operation.
If the fault occurs between any two phases, say Y and B then short-circuit current flows through these phases. The fault unbalanced the current flows through CTs. The differential current flows through the relay operating coil and thus relay trips their contacts.
Problem Associated with Differential Protection System
A neutral resistance wire is used in the differential protection system for avoiding the adverse effect of earth fault currents. When an earth fault occurs near the neutral, it will cause a small, short circuit current to flow through the neutral point because of small emf. This current is further reduced by the resistance of the neutral grounding. Thus, the small current will flow through the relay. This small current will not operate the relay coil, and hence the generator gets damage.
Modified Scheme of Differential Protection System.
To overcome the above problem, the modified scheme has been developed. In this scheme two elements are arranged, one for the protection of the phase fault and other for the earth fault protection.
The phase elements are connected in stars along with the resistor. The earth fault relay is kept between the star and neutral. The two-phase elements together with a balancing resistor are connected in star, and the earth fault relay is connected between the star and neutral pilot wire.
The star-connected circuit is symmetrical, and any balanced overflow current from the current circulating point will not flow through the earth fault relay. So in this system, the sensitive earth fault relay will operate at a high degree of stability.