Bus bar protection relays depends mainly on khirchoff’s current law, which states that the sum of current inter to any node must be equal to the sum of the currents leaving it.
Figure (50) shows current direction of two equipments connected to the same bus bar in the normal condition (without faults), In this example one of the equipments acts as a source to the bus and the other is a load and they have equal current magnitude and opposite in direction. In this case, the differential current through the differential relay is zero and the relay is stable and dose not operate.
External Faults
This case is similar to the pervious healthy case in that the two current will increase to very high values but still equal in magnitude and opposite in direction and the relay will not operate due to the zero current in the differential relay coil as shown in figure (51).
This case is similar to the pervious healthy case in that the two current will increase to very high values but still equal in magnitude and opposite in direction and the relay will not operate due to the zero current in the differential relay coil as shown in figure (51).
Internal Fault
In the internal fault case shown in figure (52), There are two expected situation:
- Non redial system, then I2P has a non zero value. In this case the differential current is equal to (I1S+ I2S) and this value is sufficient to operate the relay and trip all the equipments connected.
- Redial system, then I2P= 0.0
In this case the differential current is equal to i1S and this value is also sufficient to operate the relay and trip all equipments connected to the bus bar.
Note
All equipment’s current transformers must have the same transformer ratio, if different ratios are exist Aux current transformers (matching current transformers) must be used to compensate these difference. These matching may be internally in the relay (Taps) or externally by separate matching current transformers.
There are different types of bus differential protection relays. Low impedance and high impedance relays may be considered there is some difference between them, but it is not considered in this study.