Differential Current Protection

Differential protection can be used on generators, transformers, buses, transmission lines, and anywhere else you desire very fast protection. In the U.S.A., differential protection (called a ground fault current interrupter) is mandatory on 120V receptacles in wet locations.

Consider the protected zone as a point node. The differential protection system measures the current at all normal entry and exit points. The relay adds all of the currents, taking into account the direction of the current flow (into or out of the widing). If the algebraic sum of those currents is other than zero (plus a small tolerance), the relay calls for a trip. Each phase is metered separately, though most modern relays contain all 3 protection systems in one relay enclosure. In a generator, there is a CT on each end of each phase. Note: for a star-connected generator, the neutral-side CT must be between the phase winding and the star connection, so that only the current of that phase is measured.

During normal operation, the current through both CT's on the same phase will be identical. If they are not equal, there is power exiting the winding somewhere other than the terminal bushings (leakage current if a small value, fault current if a large value). The relay sees the inequality and trips the associated breakers to isolate the fault. In the case of a generator, the prime mover (engine, turbine, etc.) is also tripped, to remove the voltage source. Because no insulation system is perfect, there will always be some leakage current. The trip setting of the protection system includes a small tolerance value to minimize nuisance trips.

The setting of your particular protection system will primarily depend on the quality of your insulation, the phase relationship and turns ratio of any transformers included in the zone of protection, and the magnitude of any charging current at startup. If the auxiliaries load (pumps, fans, controls, etc.) is supplied from within the zone, you must either include each phase of the aux. transformer in the measured current values, or set the relay trip point high enough that the maximum auxiliary draw (including motor starting surge) will not trip the relay. Most engineers will measure the auxiliary load, because raising the trip point high enough to ignore the load would negate the advantages (low setpoint, rapid operation) of a differential protection system.

Further information can be found here.