Over-Current Relay 49, 50 & 51

Every Electric Circuit breaker is provide with over current relay to protection the bus transfer and the equipmnet. In the large power plant extrem over-current potentially happen when do to start a large equipment 10kV.

The biggest starting current potentially happen when change over Boiler Feed Pump at a unit power plant full load. If we have two Boiler Feed Pumps in service and one pump stand-by, we can start the stand-by pump anytime. The board conductor and circuit breaker should be has enough capacity for handle a large starting current.

Picture above for example the unit running with 75% load with two Boiler Feed Pumps in service (BFP-A & BFP-B). The planning is start BFP-CA and stop BFP-A, so that 10kV unit board A should be enough to handle starting current while BFP-CA initiate to start.

For protection for that over current from starting current of BFP, the CB provide with three of relay (49, 50 & 51)

Relay 49 for over thermal.

Relay 50 for Instantaneous overcurrent protection with settable time delay. (50TP/N/Q) (T)ime delay (P)hase (N)eutral and negative se(Q)uence overcurrent protection with settable time delay.

Relay 51 time overcurrent protection. (51P/N/Q) provide (P)hase (N)eutral and negative se(Q)uence overcurrent protection with settable time delay.



For example that before start BFP-CA, 10kVunit board current is 1500A and while start BFP-CA got starting current 3600A. And BFP-CA motor starting current 1280A and rate current normal 250A. It takes below 1 second.

Picture below is Relay 49, 50 & 51 setting on 10kV unit board incoming CB.

Picture below is Relay 49, 50 & 51 setting on 10kV incoming CB for BFP-CA.

The starting current of BFP-CA still save due to still far below the setting relays