Press the one of the two switches to create an internal transformer fault and the other for an external fault.
For the internal transformer fault the differential
protection should trip the primary side of the transformer and for the external fault should do nothing.
Press the NC button to release the tripping relay when the protection is activated.
Principle of Differential Protection scheme is one simple conceptual technique. The differential relay actually compares between primary current and secondary current of power transformer, if any unbalance found in between primary and secondary currents the relay will actuate and inter trip both the primary and secondary circuit breaker of the transformer (in the example we have only primary circuit breaker).
Suppose you have one transformer which has primary rated current Ip and secondary current Is. If you install Current Transformer of ratio Ip/1A at primary side and similarly, Current Transformer of ratio Is/1A at secondary side of the transformer. The secondaries of these both CTs are connected together in such a manner that secondary currents of both CTs will oppose each other. In other words, the secondaries of both CTs should be connected to same current coil of differential relay in such a opposite manner that there will be no resultant current in that coil in normal working condition of the transformer.
But if any major fault occurs inside the transformer due to which the normal ratio of the transformer disturbed then the secondary current of both transformer will not remain the same and one resultant current will flow through the current coil of the differential relay, which will actuate the relay and inter trip both the primary and secondary circuit breakers (in the example they trip the primary). To correct phase shift of current because of star – delta connection of transformer winding, the current transformer secondaries should be connected in delta and star.
Generally Differential Protection is provided in the electrical power transformer rated more than 5MVA.
