Multiport high frequency transformer coupled bidirectional dc-dc converters for hybrid renewable energy system

This dissertation proposes a new approach Harmonic Balance Technique (HBT) using converter switching functions to the modeling and analysis of the bidirectional actively controlled dc-dc converters. This modeling technique aims to effectively obtain the dynamic equations of the converter system and study the steady state performance of the converter. Also the minimization of the reactive power flow of the converter system based on the HBT is clearly discussed.;The full mathematical model of bidirectional dual active bridge (DAB) converter system using state space method is presented first. A complete insight of dynamic characteristics of the proposed converter system is given. The influence of the mutual inductance is analyzed. The state space method allows a switched nonlinear system to be approximated as a linear system after linearization by classifying in several time intervals. The power flow expressions of the DAB system based on different switching modes are also derived. The loss caused by semiconductor voltage drop is taken into account in the modeling of the DAB system. Also, the effect of dead time on the system under certain operation mode is studied and computer simulation results are given.;The complex closed form expressions gotten by state space method are not easily used to study the steady state performance of the DAB converter system. A novel approach HBT is used in which the linear and nonlinear components are decomposed. The time invariant components are extracted to study the steady state performance of the DAB system. The study of ripple quantities of the state variables can be used to the filter design of the system. Compared with the well-known averaging technique, HBT has a greater advantage when multiport bidirectional dc-dc converter system is studied.;A new control strategy aiming to minimize the system losses from a macroscopic point of view is presented. It is based on the state variables derived from the HBT. In order to minimize the system power losses caused by reactive current flowing inside the DAB converter system, the proposed control strategy can be used to determine the reference switching function command to achieve a high power efficiency. The prototype of the proposed system is designed and experimental results are discussed.