Bus-bar protection relays be determined by in the main on Kirchhoff’s current law, which states the aid of current lay to rest to any node should be equal to the aid of the currents going it.
The above figure shows that the current direction of 2 equipment’s connected to a similar bus bar within the traditional condition (without faults), In this illustration one among the equipment’s acts as a supply to the bus and also the different is a load & that they have the same current magnitude and conflicting in direction. In this case, the difference in current over the differential relay is nil (i.e.zero) and the relay is steady and dose not operates.
External Faults in Bus-bar Protection Relay
This case is comparable to the permeable healthy case in this the two current can increase to terribly high values however still the same in magnitude and reverse in direction and therefore the relay won't operate owing to the zero current in the differential relay coil as shown above.
Internal Fault in Bus-bar Protection Relay
Here internal fault case shown in figure, there are 2 expected situations:
Non radial system, then I2P incorporates a non-zero value. During this case the differential current is adequate (I1S+I2S) and this value is adequate to work the relay and trip all the equipment’s connected.
Redial system, then I2P= 0
In this case the differential current is adequate I1S and this value is also spare to work the relay and trip all equipment’s connected to the bus bar.
Note: - All equipment’s current transformers should have constant transformer magnitude relation, if completely different ratios are exist in matching current transformers should be used to recompense these distinction. These matching are also internally within the relay (Taps) or outwardly by separate similar current transformers.
There are differing kinds of bus differential protection relays. Low impedance and high electrical resistance relays is also thought-about there's some difference amongst them, however it's not thought-about during this study.