DC-link filter capacitors reduction in multilevel modular H-bridge converter structures

A three-phase multilevel modular H-bridge converter structure can be used to drive high power AC motors and has several advantages, if compared with the other most used multilevel converter structures. Often known as a cascaded multilevel inverter, this simple but effective technical solution is being used by several manufacturers. However, even if a three-phase AC motor is driven, each H-bridge converter is single-phase and the DC-link capacitance must be very large to reduce the DC-link voltage ripple. This paper shows how to reduce such capacitance and/or voltage ripple by using a new devised PWM strategy, performing also nonconventional filtering actions on the DC-link. The main analysis has been based on the structure with simple C filter in the DC-link, but the developed method can be applied also to classical L-C DC-link filtering systems, thus permitting the avoidance of resonant conditions, extremely critical in these applications. All the analytical computations and the most significant analyses and results are reported.