Theory and practical performance of interlocked overcurrent busbar zone protection in distribution substations

Modern microprocessor based numeric overcurrent protection relays provide multiple overcurrent elements, including instantaneous elements, with tightly defined characteristics. Additionally such relays can provide internal timers with accurate independent pick-up and drop-off time delay settings, initiated from digital status inputs and capable of being mapped to output relays. Numeric relays also provide digital status inputs that can be user selected to internally inhibit the overcurrent elements. A distribution substation normally has overcurrent relays applied to all the incomers and to all outgoing feeders. The instantaneous elements and timers of these relays may be used not only for plain overcurrent protection but also as part of an efficient, cost effective, high speed, busbar zone protection, which protects not only for busbar faults but also internal faults within the circuit breakers. This paper discusses the protection theory and application issues to be considered in the design of busbar protection schemes utilising the instantaneous elements contained in numeric overcurrent relays and also reports on in-service performance.