The behavior of the Busbar system with one or two CTs at the Bus-Section or the Bus Coupler.

• In case of One CT

Faults on Bus Zone_02 are tripped immediately and discriminatively in Prot. Zone_02. Faults on Bus Zone_01 but not between the CTs and the Bus Section or Bus Coupler CBs are also tripped immediately and discriminatively in Prot. Zone_01.

A fault between the CT and the Bus Section or Bus Coupler CBs [On Bus Zone_01 but in Prot. Zone_02] trips initially Prot. Zone_02 (including the Bus Section / Bus Coupler CB) immediately. The Prot. Zone_01 will extend up to the Bus Section / Bus Coupler CB after the Bus Section / Bus Coupler CB being tripped and the reclaim time elapsed, hence Prot. Zone_01 will trip Bus Zone_01 after the Reclaim Time [Time Delayed Trip].

• Manual Close Command Effect:

In case of two set of CTs as shown below, and an existing fault is on BB.02 (Closed grounding isolator or missing grounding clamp). While closing the Buse Section / Bus Coupler CB on the existing fault, the Arc ignites before the CB reposts a close status:

o The measuring system for Bus Zone_01 is unable to measure the outgoing fault current towards Bus Zone_02 because the measurement of the Bus Section / Bus Coupler CT is still blocked. A differential current equivalent to the fault current is therefore created and the measuring system for Bus Zone_01 will trip BB.01.

o The measuring system for Bus Zone_02 is unable to measure the incoming fault current flowing from Bus Zone_01 because the measurement of the Bus Section / Bus Coupler CT is still blocked. Since However, there are no feeders connected to Bus Zone_02, the measuring system doesn’t trip.

Solution:

The measurement of the Bus Section / Bus Coupler CTs has to be reinstated before the breaker is actually closed (CB reports a close status through auxiliary contact). So, the manual close command of the CB has to be maintained by activating a prolongation timer (internally in numerical relays or externally for high impedance protection).