Research on high-frequency isolated bidirectional DC-DC converter for application in distributed energy storage systems

Abstract

In recent years, bidirectional DC-DC power conversion has become an important function of many modern power electronics systems, such as distributed energy storage systems, vehicle-to-grid (V2G) systems, and solid state transformers. Dual-active-bridge (DAB) converter is one of the most widely studied and applied bidirectional power converter topologies. It has the advantages of high power density, galvanic isolation, soft-switching capability, and flexibility to implement various power modulation schemes. However, conventional DAB converters often suffer from large circulating currents and hard switching under mismatched voltage gain and light load conditions, which considerably increase conduction and switching losses. To suppress circulating current and extend zero-voltage-switching (ZVS) range, several modulation schemes have been proposed. One common drawback associated with the existing modulation schemes is the dependence of soft-switching conditions on input-to-output voltage ratio. The optimal operating parameters of a given modulation scheme must be calculated for each input-output voltage ratio and power level in advance, which greatly increase the complexity of its practical application. In this thesis, a comprehensive review of DAB converters is presented. To address the inherent problems found in the existing topologies and modulation schemes, new topologies, modulation schemes, and resonant networks are proposed in three chapters to address the problems of the conventional DAB converter. The proposed converters utilize the attractive characteristics of immittance networks where alignment condition between the voltage and current at each port is independent of the input-to-output voltage ratio and depend only on the phase relationship between two port voltages. This simplifies the practical realization of full-range ZVS for different power levels and input-to-output voltage ratios. The thesis also introduces a reconfigurable immittance-based DAB converter. This proposed converter can switch between full-bridge and half-bridge configurations which leads to improved efficiency under wide-range load conditions, and Enhanced-Dual-Phase-Shift (EDPS) modulation scheme is also proposed for implementing full-range soft switching in all switches. Following this, the idea of reconfigurable DAB converter is further explored by proposing a multi-level immittance-based DAB converter utilizing reconfigurable operation mode for optimizing its performance over wide-load range. The proposed modulation scheme can achieve full-range ZVS, zero backflow power and wide-range zero circulating current (25% to 100% of rated output power). Finally, a DAB converter with reconfigurable resonant network and hybrid modulation scheme are further proposed. Dynamic frequency matching (DFM) and EDPS modulation schemes are presented for medium- to-full load condition and for light-load condition, respectively. Under EDPS modulation, all switches achieve zero backflow power and ZVS operation. Under DFM modulation, a switch-controlled capacitor (SCC) is used to adjust the resonant frequency of the immittance network for matching the switching frequency, which leads to minimum rms current due to unity power factor operation. A new modulation scheme for SCC is also proposed to minimize conduction loss which leads the further efficiency improvement of DAB converter under DFM modulation. These works have successfully demonstrated that reconfigurability of topology, operation mode and resonant network can be utilized to provide additional degrees of freedom (DoF) in shaping the efficiency performance of DAB converter over a wide range of operating conditions.