Voltage THD Minimization in Multilevel Cascade Inverters Using Repetitive Quadratic Programming

The multilevel cascade inverter is one of the most widely used power-electronics based interfaces in electrical distribution systems. Due to high losses and harmonics, the switching frequency of the inverter should be low in medium and high power applications. For this reason, the conventional carrier wave-based sinusoidal pulse modulation (PWM) and space vector PWM that have high switching frequencies cannot be used in these applications. The optimal PWM methods for inverters with step modulation result in lower total harmonic distortion (THD) in output voltage than other common modulation methods. However, one of the major disadvantages of these methods is that the optimal switching angles should be determined using the switching table, limiting the application of the optimal PWM. This paper proposes a method for determination of switching angles by using the iterative quadratic programming method. In each iteration, the proposed method calculates the switching angles by solving the quadratic sub equations with equality constraints and linear equations. Also in the appropriate conditions, global and asymmetric convergences are faster, more accurate, and more efficient, and there is no need for much time and memory for switching angles determination. The optimum switching angles minimize the switching frequency, switching losses, and THD in voltage and current of a three-phase cascade multilevel inverter with step modulation. Also, the power circuit breakers are switched on and off only once in each period. The effectiveness of the proposed method is evaluated through simulation case studies in MATLAB environment.