Nonunit Protection of Parallel Lines Connecting Solar Photovoltaic Plants

Fault current control by solar plant converters introduces different fault characteristics compared to conventional synchronous generator-based systems resulting in numerous issues to the available protection methods. In this article, the issues with conventional distance relaying is analyzed while protecting parallel lines connecting solar plant to grid and a new protection method is proposed using local voltage and current data. The proposed nonunit protection method derives positive sequence reactive powers for both lines for different solar plant operating conditions and uses their difference to ensure correct zone-1 protection. For faults during single circuit operation, the method includes an additional scheme comprising of instantaneous zero-sequence overcurrent check and delayed distance relaying to derive correct protection decision. Performance of the proposed nonunit protection method is tested for parallel lines connecting the solar plant in a 39-bus test system for different situations using PSCAD/EMTDC simulated data and found to be accurate. Comparative assessment reveals the high reliability of the proposed method.