Improved Residual Current Detection Method and Protection Scheme for Double-Circuit Bipolar LVDC System Based on Modal Transformation

Traditional residual current protection in meshed bipolar low-voltage direct current (LVDC) systems with metallic return lines (MRLs) is prone to maloperation due to circulating currents caused by line parameter asymmetry. To address this problem, dual-terminal schemes with communication and additional sensors have been introduced, but they inevitably increase system complexity and cost. To overcome these limitations, this study proposes an improved residual current detection method based on modal transformation. By constructing a six-dimensional modal transformation matrix, the coupled lines in the double-circuit bipolar-LVDC (B-LVDC) system with MRLs are decoupled, enabling precise fault detection by revealing the polarity characteristics and transmission paths of each modal current under different fault types. Furthermore, a novel single-ended residual current protection scheme without communication is proposed, in which the startup is triggered based on the 0-mode (0M) current, and precise fault conductor selection is achieved through comparative analysis of multiple modal current variations. The effectiveness of the proposed scheme is verified through simulation on the MATLAB/Simulink platform by building a model of the double-circuit B-LVDC system with MRLs. Besides, an experimental platform is constructed to demonstrate the validity of the method. The results demonstrate the robustness and reliability of the scheme in fault detection under different fault conditions, including system asymmetry and circulating current interference.