Protection Coordination Using Mixed Characteristics of Directional Overcurrent Relays in Interconnected Power Distribution Networks

In this paper, protection coordination problem of directional overcurrent relays (DOCRs) is discussed by simultaneously selecting time-current characteristic curve and optimizing settings of the relays. For this, DOCRs are assumed to be associated with three setting variables which are time multiplier-setting (TMS), plug-setting (PS) and curve-setting (CS) to obtain their correct operating times. The TMS and PS of all the relays are considered as continuous variables which are possible with numerical/digital relays whereas the CS is considered as a discrete variable and are associated with the selection of the time-current characteristics of the relay among normal inverse, very inverse and extremely inverse. The proposed coordination studies have been investigated on the IEEE 14-bus test system.