This circuit detects short-circuits in AC generator feeder lines or busbars; it is a method of protecting the generator from overheating and burning out.
Assuming a three-phase AC generator is installed, each phase has its own protection circuit. For illustration purposes, the circuit for a single phase is described. Two control transformers (CT) are located at either end of the distribution system. CT1 is located in the negative (earthed) connection of the generator’s output. CT 2 is located at the output from the busbar performing a monitoring function in a generator control unit (GCU). If a fault were to develop between the generator and busbar, a current I F flows to ground. The net current received at the busbar is therefore the total generator output current I T minus the fault current (I F). The fault current flows back through the earth return system through CT 1 and back into the generator; the
remaining current (I T _ I F) flows through CT 2 and into the loads. Current transformer CT 1 therefore detects (I T _ I F) _ IF which is the total generator current. Current transformer CT 2 detects (I T _ I F); the difference between control transformer outputs is therefore I F. At a pre-determined differential current, the generator control relay (GCR) is automatically tripped by the GCU and this opens the generator field.
