IEEE Std 3004.11-2019 pdf download – IEE E Recommended Practice for Bus and Switchgear Protection in Industrial and Commercial Power Systems.
In the fourth scheme described in Table 1 and shown in Figure 8, CTs associated with the tie circuit breakers are located on the main load buses and overlap. The tie interconnection is within both zones so a fault in fault locations 2, 3, and 4 will cause both ties and both load buses to be isolated from sources. An interlocking scheme using logic from one or both relays may be implemented that operates only the tie circuit breakers when both differential zones sense differential faults simultaneously, which occurs when a fault is located in the overlapping zone. This may be considered if a fault in location 3 is more probable than a fault in location 2 or location 4. In the fifth scheme described in Table 1 and shown Figure 9, four sets of CTs associated with the tie CB are used. Tie bus faults are always selectively isolated. However, faults at location 2 and location 4 can still unncessarily clear a load bus. If faults at locations 1 and 5 are deemed improbable, it may be advisable to implement a logic scheme that prevents the main bus protection differential from tripping the main CBs for a set delay if the tie differential protection is opening the tie CB. That may improve reliability but may delay protection if a fault occurs at location 1 or 5. Generally when using bus differential, the intent and expectation is fast bus protection. Delaying the opening of a source CB is not normal practice and should be carefully considered if contemplated as it significantly limits the benefit associated with bus differential protection. These examples demonstrate that there is no single perfect implementation of differential protection in sources with multiple sources and multiple ties in series. What is optimal for a situation will depend on various factors including perception of the most probable fault locations and the need for reliability. In addition to differential protection, interlocking logic can further improve a scheme to ensure minimum interruption of power to served loads.