Flexible neutral point displacement overvoltage suppression method based on backstepping control in unbalanced distribution networks

Three-phase AC distribution networks are required to operate as symmetrically as possible for optimal performance, but the three-phase-to-ground parameters are asymmetric in the field due to network construction deviation, resulting in the three-phase voltage unbalance and the neutral point displacement overvoltage. The neutral point displacement overvoltage would endanger the distribution networks and even damage critical power equipment. Therefore, the flexible asymmetry suppression device (FASD) with the topology of a cascaded H-bridge (CHB) inverter and the backstepping control (BSC) method is proposed for limiting the neutral point displacement overvoltage in this paper. The FASD outputs the current to the grid for compensating the unbalanced distribution networks caused by the asymmetry of three-phase-to-ground parameters and realizing the suppression of the neutral point displacement overvoltage. A current-based BSC method is designed to improve the stability and reliability of the CHB inverter, and the effectiveness of neutral point displacement overvoltage suppression based on the BSC method is verified by experimental results, and the advantage of the proposed method is shown by comparing with proportional-integral control (PIC) method, sliding mode control (SMC), and proportional resonant control (PRC) methods.