High-frequency six pulse DC link based bidirectional three-phase inverter without intermediate decoupling capacitor

Abstract

Cascaded bidirectional dc-ac converters are commonly used in UPS application and battery chargers for electric vehicles (EV). A bidirectional dual active bridge cascaded three phase converter (DABCC) with six pulse dc link, high reliability and low cost is proposed for applications like line interactive UPS and EV. Power conversion units for such applications use large electrolytic capacitor at high voltage DC bus. Through a novel modulation technique Six Pulse Modulation (SPM), this electrolytic capacitor is eliminated that results in increased reliability, compactness and reduced cost. It needs only single H-bridge and one single-phase transformer at front-end for the required three-phase conversion. SPM technique also increases DC bus utilization and decreases the inverter average switching frequency to 33% when compared to traditional sine PWM. To effectively implement the SPM modulation, a fast dynamic control of dc link voltage is required. For that a coordinated control is implemented. And also the effect of dead time on the inverter terminal voltage for a SPM modulated converter is analyzed and its effect on terminal voltage is presented. To mitigate this effect, a compensation technique based on resonant control is proposed. RMS current stress for high voltage dc link capacitor is studied and it is shown that the capacitor requirement for SPM modulation is lower than the conventional Sine Pulse Width Modulation (SPWM). To validate the proposed control scheme, a 800 W prototype is fabricated. Hardware results are presented to show the effectiveness of the implemented control scheme.