Bus bar protection is generally differential protection for which zone of protection is defined by location of CTs. However, location of isolation is defined by CBs. The area between CB and CT is always an issue for protection schemes.
Bay-1: CT location on line side
Case 1.1 When CB is open: Current flowing through 1CT (feed from remote end) is not included in busbar differential relay calculations. Therefore, busbar relay will not operate for fault between 152CB and 1CT. End fault protection relay has one additional over current element, which will operate in this condition when current through 1CT is above set value. This will send direct trip command to remote end through communication channel so that fault is cleared without any time delay.
Case 1.2 When CB is closed: Current flowing through 1CT is included in busbar differential relay calculations. Therefore, busbar relay will operate for fault between 152CB and 1CT. Depending on scheme, Busbar protection will send direct trip command to remote end through communication channel so that fault is cleared without any time delay or end fault protection will operate and send direct trip after CB is open.
Bay-2: CT location on bus side
Case 2.1 When CB is open: Current flowing through 2CT (feed from local end) is not included in busbar differential relay calculations. This causes extension of bus bar protection zone upto CB. Therefore, busbar relay will operate for fault between 252CB and 2CT.
Case 2.2 When CB is closed: Current flowing through 2CT is included in busbar differential relay calculations. Since area between 252CB and 2CT is outside differential zone, busbar relay will not operate for this fault. This fault will be detected by line protection. After opening of CB, End fault protection relay will come in picture as mentioned in Case 2.1.
