Regional Backup Protection Scheme for Distance Relays Considering Network and Communication Uncertainties

Distance relays provide fast protection for transmission lines, but have limitations that require supplementary protection schemes to quickly clear main line faults detected beyond the first zone. This paper proposes a new protection scheme to enhance fault detection accuracy and prevent network instability when instantaneous operation of distance relays is not achieved. The proposed scheme can operate before conventional backup protection using local protection units implemented in all substations. It identifies the fault between connected lines to the substation by analyzing the detection status of distance relays in local and first adjacent substations. Local protection units in nearby substations communicate with each other and with local distance relays via a telecommunication platform to execute protection algorithms. The fault detection algorithm estimates the likelihood of faults between connected lines by calculating the line fault probability index. This index is calculated by weighting factors assigned to adjacent distance relays using operation and expectation functions. This method remains effective even with telecommunication link failure and is robust against various network and equipment uncertainties. The implementation of fault scenarios on the IEEE 9 bus test system demonstrates that the proposed method is more effective than similar approaches under uncertainty conditions.