Compensation of Defective Power Transistors of Cascaded High-Voltage Inverters in Dynamic Modes Using “Fractional” Cell Method

The use of high-voltage frequency converters is one of the most promising areas in modern high-power electric drives. Among such converters, cascaded multilevel inverters (CMI) with H -bridges are most widely used due to their increased reliability, maintaining the functionality in case of failure of one or more power modules, and the possibility of quick repairs due to the modular design. In this paper, the method of voltage balancing is analyzed and significantly developed; an exact analytical solution is given for all practically expedient cases when the neutral point is located inside the linear voltage triangle. Further analysis made it possible to propose the novel method of “fractional” cells according to which when the specified voltage is increased or decreased, the required voltage values (mostly fractional) and the corresponding shift angles of each phase are calculated, considering the number of undamaged power modules for interphase voltage balancing vectors, providing the least dynamic electrical and electromechanical loads in case of failure of power modules. Calculation methods, tables, graphs and models explaining the principles of implementation of these methods in electric drive control systems are provided.