Comparison and performance evaluation of the mid-point voltage balance modulation schemes for a three-phase 2L-3L DAB

Capacitor voltage balancing is critical for neutral-point-clamped-based DAB converters. This paper focuses on the mid-point voltage balancing of a three-phase two/three-level dual-active-bridge dc–dc converter (2L-3L DAB3). The voltage imbalance leads to overvoltage stress on power devices and can cause saturation of transformers or device failure in extreme cases. Two modulation schemes are proposed in the literature for the DAB3: λ-modulation and -δ-modulation (Duty Cycle adjustment). In this paper, a comparison of the two modulation schemes is provided for the 2L-3L DAB3. The average mid-point current for the two modulation schemes is modeled. Different characteristics, such as polarity inversion, operational limits, and impact on power transferred, are compared for the two modulation schemes. Operating regions are identified where one modulation scheme is shown to be superior. Comprehensive simulation and experimental results on a hardware prototype are provided to support the analysis.

I. Introduction

The Dual Active Bridge (DAB) topology enables many desirable features such as galvanic isolation, bidirectional power flow, soft switching capabilities, and simplicity of control [1], [2]. They have been widely used for a range of applications such as EV charging applications, DC microgrids and solid-state-transformer applications [3]. A three-phase DAB (DAB3) presents a suitable solution for high-power applications over single-phase DAB in terms of higher power densities and efficiencies. DAB3 offers lower current stress and higher effective frequency ripple seen by the filter capacitors [1].