Performance evaluation of a five-level flying capacitor converter with reduced DC bus capacitance under two different modulation schemes

A back-to-back HVDC converter system is typically used to connect two separate electrical grids or a remote large-scale renewable energy system with the electrical grid through a DC bus. The DC bus typically contains a large electrolytic capacitor giving a robust performance to the converter system. However, it is well known that the electrolytic DC bus capacitor is bulky and has a low reliability. Therefore, a possible solution to these drawbacks is to use a reduced DC bus capacitance, which will allow film capacitors to be used instead of electrolytic capacitors. This paper presents the performance evaluation of a five-level flying capacitor (FC) back-to-back converter system with reduced DC bus capacitance under different modulation schemes. Two carrier-based modulation strategies are studied, namely the phase shifted pulse-width modulation (PS-PWM) and the phase disposition pulse-width modulation (PD-PWM), along with their appropriate flying capacitor voltage balancing techniques. The classical synchronous reference frame control strategy has been implemented in order to provide DC bus voltage regulation and active/reactive power control. The performance of the two different modulation schemes is tested against the DC bus and FCs voltage ripples using the MATLAB/Simulink-PLECS software platform. The analyzed case studies demonstrate that the PS-PWM strategy is more suitable to operate the proposed FC converter system using reduced values for the DC bus and flying capacitors.