Modeling and control of a cascaded multilevel converter-based electronic power transformer

Control strategy of a cascaded multilevel converter based electrical power transformer (EPT) in a distribution system with capabilities of low voltage ride-through and unbalanced load current management is investigated in this study. The mathematical model and decoupled control schemes of the system, including a high-voltage side control scheme, an isolation-stage control scheme, and a low-voltage side control scheme, are presented in detail. A dual current control scheme is introduced to control both positive and negative sequence currents for enhancing the low voltage ride-through capability of the high-voltage side cascaded H-bridge converter. Positive, negative, and zero-sequence voltages are controlled for the low voltage side three-phase four-wire converter in the decoupled control scheme, respectively, for unbalanced load current management. A proportional resonant controller (PRC) is utilized to control the zero-sequence voltage, while the root locus method is applied in the PRC design. Three-dimensional space vector pulse width modulation (PWM) switching strategy is then used for the low voltage side converter. Simulation studies were conducted with MATLAB/Simulink to validate the coordinated control strategy. (c) 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.