Bus differential protection in industrial systems with generators connected directly to the main distribution bus

In industrial facilities with on-site generation there are two major approaches to connect generators to the main distribution busbar. One is to connect generators through step up delta-wye transformers with low resistance grounded wye on the busbar side. The second common method is to connect generators directly to the main distribution busbar and use low resistance grounding or hybrid grounding in the generator neutral. A grounding resistor in the step up transformer's wye side neutral or in the generator neutral will limit the ground fault in the main distribution system. When busbar differential protection is used on the main busbar, sensitivity for bus ground faults has to be evaluated to ensure detection of limited low level ground faults. While step up transformer grounding resistors can be selected to allow ground faults of higher magnitudes detectable by bus differential protection, the choice of grounding resistor is more complex when generators are connected directly to the main busbar. In such systems, a grounding resistor in the generator neutral limits the generator fault to protect the alternator core from excessive damage. At the same time, busbar differential protection needs to detect a limited low magnitude ground fault on the bus. This paper discusses considerations to select the main distribution busbar differential protection type, CTs and relaying scheme to enable detection of busbar ground faults limited by the generator low resistance grounding resistor.