The type of grounding employed and the system configuration determine the choice of protective scheme. If a high-impedance scheme is chosen, the ground fault current is limited to a value that will not cause mechanical or thermal stress on the generator. Protective schemes applied on high impedance grounded systems are focused on sensitivity and not on speed of operation. Therefore in this case suitable discrimination is provided easily.
The decision to use high-impedance grounding is based on the desire to minimize core damage by limiting the ground fault current below a damage threshold value. In theory, if the fault current is not damaging, then the protective relaying need not trip but can be applied to alarm only. This would allow generation to be manually shut down in an orderly manner, thus avoiding mechanical and thermal stresses that would accompany a trip and load rejection.
For systems with damaging ground fault current rapid tripping of the generator, field breaker and, shutdown of the prime mover are recommended. On high impedance grounded systems tripping is an option.
On the other hand, protection for grounding schemes that do not limit the ground fault current (low- impedance grounding systems) below damaging levels are less sensitive, but must operate quickly. An insulation fault inside the metal casing of a generating set may severely damage the generator of this set. The fault must be quickly detected and eliminated. Furthermore, if other generators are parallelly connected, they will generate energy in the fault and may cause overload tripping. Continuity of supply is no longer ensured. For this reason, a GFP device built-into the generator’s circuit allows: the fault generator to be quickly disconnected and service to be continued the control circuits of the fault generator to be stopped and thus to diminish the risk of deterioration.
This type of protection is called “restricted differential”. Stator restricted earth fault protection is high speed and very selective if its stability can be guarantee in operation conditions.
Discrimination rules between incoming and outgoing of busbar:
These two types of discrimination must be simultaneously implemented.
Current sensing discrimination
Threshold setting of upstream GFP device tripping is greater than that of the downstream GFP device. Because of tolerances on the settings, a 30 % difference between the upstream and downstream thresholds is sufficient.
Time graded discrimination
The intentional time delay setting of the upstream GFP device is greater than the opening time of the downstream device. Furthermore, the intentional time delay given to the upstream device must respect the maximum time for the elimination of insulation faults defined by the NEC § 230.95 (i.e. 1s for 3000 A).