Advances in Bidirectional DC-DC Converters for Future Energy Systems

Abstract

The contemporary electricity grid is in the midst of a transformation in which decentralization of energy production is playing a key role. Spurred by the environmental concerns of traditional energy sources and the costs reduction of photovoltaic energy and energy storage systems (ESSs), energy decentralization is disrupting traditional models of energy generation. In addition, vehicle-to-grid technology has been presented as an opportunity to optimize the grid utilization. Considering that photovoltaic energy, energy storage systems and electrical vehicles operate in dc, the electricity network is following a trend of moving towards dc distribution in the form of multiple microgrids. Accordingly, technological advances that allow a simplified and flexible interconnection of microgrids with high energy efficiency are key enablers for the electricity grid transformation. In this regards, high efficiency power electronics interconnecting microgrids and integrating energy storage systems, constitute a main pillar for the development of microgrids and reaching high penetration of renewable energy sources. The state-of-the-art technologies in electrical power conversion are trending towards the utilization of dc solid-state transformers (SST) as interlinking converters between dc grids. Among the different power converter topologies implemented as SSTs, the series-resonant converter (SRC) has been extensively used, thanks to its load regulation characteristics in open-loop together and its soft-switching conditions for wide power ranges. This Ph.D. dissertation is divided into two parts. In the first part, the investigation of two-port and three-port SRCs in open-loop operation for dc SST applications is carried out and presented. The study focuses on the design considerations of distributed resonant tanks to improve the load regulations characteristics in open-loop operation at a fixed switching frequency and duty cycle. On this subject, the design criteria to operate multi-port SRCs in soft-switching at input and output ports is overviewed. The resonance frequency matching can pose a challenge in multi-port SRCs with distributed resonant tanks. Therefore, a resonance frequency matching process is proposed to address this issue. This methodology allows to remove the resonant inductors and solely use the stray inductances and leakage inductance of the multi-winding transformer as the inductive component of the resonant tank. As a result, the efficiency and power density of the converter can be highly increased. The SRC tends to have large rootsquare-mean (rms) currents due to the sinusoidal waveform of the resonant currents and the circulating energy required to achieve zero-voltage switching. So the conduction losses are usually high. The minimization of circulating energy by an optimal selection of dead-time and magnetizing inductance is also analysed. In this regard, wide bandgap semiconductors, which are widely known for their benefits in reduced switching loss, have a direct impact on the circulating energies. This introduces additional advantages into the SRC which have also been investigated. Some of these advantages are the reduction of conduction losses and turn-off losses. In the second part of this PhD dissertation, different power converters configurations to integrate energy storage systems into the dc microgrid are investigated. Each of the converters presented aim to solve different challenges in the integration of ESSs. Firstly, a dual-active-bridge (DAB) derived topology for high voltage gain operation is illustrated. The proposed topology features voltage and current stresses reduction as well as an additional degree of freedom to improve the DAB controllability. Secondly, a power conversion system which achieves a large reduction of the power processed by the dc-dc converter is presented. This solution focuses on the rearrangement of the dc-dc converter connection with the dc bus and the ESS. With this configuration, the system efficiency and power density can be largely increased, while the fabrication costs can be potentially reduced. Finally, a three port converter to integrate photovoltaic modules and the ESS into the microgrid is proposed. The converter is derived from conventional buck and boost topologies, hence its implementation is simple. High efficiency can be easily achieved since single energy conversion stages are required to transfer power between different ports.