Why busbar main & ckeck relay is required for busbar protection?
In substations, we normally apply some form of busbar differential protection to quickly detect and clear a fault on the bus. The time-graded overcurrent protection used on other parts of the circuit cannot be configured to provide the necessary discrimination with fast clearing for such a critical piece of equipment. Realize that a busbar may have more than one incoming source, especially in a transmission grid, and the bus fault affects all connected circuits. The bus differential relays – either high impedance or low impedance type – are the primary protection, since they provide clear discrimination between a fault on the busbar vs. a fault just a few meters away on the other side of a breaker. These relays normally trip in no more than 1.5 – 2 cycles. Half-cycle tripping is not unusual. A ‘check’ relay is some form of backup protection relay that operates on different CT inputs, in case of a problem with the bus differential protection. It could be high-speed overcurrent elements
