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, current at the ends of the protected sections are compared. 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 becomes differ. This difference of current under fault conditions is made to flow through the relay operating coil.
The relay then closes it 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
There are two sets of identical current transformers each set is mounted on either side of the stator phase windings. The secondaries of the current transformer are connected in star, and their end point is connected to the pilot wires. The relay coils are connected in star, their neutral points are connected to the common point of the current transformers neutral, and the outer ends one to each of three pilot wires.
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 center, 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 of Differential Protection System
Consider an earth fault occurs on the phase R due to the breakdown of the insulation to earth. Due to the fault, the current in the secondary of the current transformers in phase R will become unequal. The difference of the two currents will flow through the corresponding relay coil, and it will give tripping command to the circuit breaker.
If the fault occurs between the two phases Y and B, then fault current will flow through the two phases. The current in the secondaries of the two CTs (current transformer) in each affected phases will become unequal, and the differential current will flow through the operating coils of the relay connected in these phases. The relay then closes its contact to trip the circuit breaker.
Problem Associated with Differential Protection System
Generally, 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 for phase fault protection and the third for earth fault protection only. The two-phase element 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 symmetrical overflow current from the circulating current 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.