1. The zig-zag transformer is used to provide a ground reference on a delta system. They are used where continuous processing is done where a single ground would not cause the system to shut down. In the event of a ground fault an alarm of other indication is given so that the ground can be found and cleared before the next ground appears causing a double phase to ground fault.
2. The main bus protections is the differential relay, it is connected to measure the current entering the bus versus the current leaving the bus. if there is a difference between them, the relay operates. These are usually low set, high speed relays. The differential relay's zone of protection is between the current transformers in the differential circuit.
An overcurrent relay is used in the input part of the differential circuit to provide back-up protection in the event that a circuit breaker fails to open during a feeder/equipment fault. This relay is coordinated with the feeder portection to provide this protection.
