The below graph is for a Busbar Protection System for Double Bus Double Breaker Scheme. The Feeder Bays CTs are connected in parallel in a loop based on the Bus Isolator status.

o If Q1 is close and Q2 is open, then the CT is connected to BB Zone_01 Loop

o If Q2 is close and Q1 is open, then the CT is connected to BB Zone_02 Loop

o If Q1 and Q2 are open, then the CT is shorted and isolated from any BB Loop.

o If Q1 and Q2 are close, then actually both BB Zone_01 and Zone_02 are merged and act as one zone only.

Bus Coupler and Bus Section CTs are connected directly to the loop, however it is recommended to include them in the loop with the status of isolators and CB to avoid false tripping of another Bus Zone while the CB is open and a fault occurred in the dead zone (between the CT and the CB).

The configuration is also achieved in the Low Impedance Relay, by configuring the Isolators Status as a Binary Input to the Bay unit so the relay can include or exclude the CT measurement from the BB Zones based on the Isolator Status.

Tricks

o The Isolator status shall be taken directly from the auxiliary contacts of the isolator. the auxiliary contacts of the isolator are

NO contact >> Early Make Type

NC contact >> Late Break Type

To avoid CT open circuit (in case of High impedance configuration) or CT exclusion from the measurement (in case of Low impedance configuration) during the isolator status change.

o It is also recommended to have a Bistable relay that simulated the Isolator status instead of using direct auxiliary contacts from the isolator to preserve the Isolator status in case of Isolator Auxiliary contacts mechanical operation stuck. Below is the logic diagram and table

The above logic can be executed in the BB Low impedance What about the BB High impedance?