Three-port dc-dc converters to Interface Renewable energy sources with bi-directional load and energy storage ports.

Abstract

Power electronic converters are needed to interface multiple renewable energy sources with the load along with energy storage in stand-alone or grid-connected residential, commercial and automobile applications. Recently, multi-port converters have attracted attention for such applications since they use single-stage high frequency ac-link based power conversion as compared to several power conversion stages in conventional dc-link based systems. In this thesis, two high frequency ac-link topologies are proposed, series resonant and current-fed three-port dc-dc converters. A renewable energy source such as fuel-cell or PV array can be connected to one of the ports, batteries or other types of energy storage devices to the second port and the load to the third port. The series resonant three-port converter has two series-resonant tanks, a three-winding transformer and three active full-bridges with phase-shift control between them. The converter has capabilities of bi-directional power flow in the battery and the load port. Use of series-resonance aids in high switching frequency operation with realizable component values when compared to existing three-port converter with only inductors. Steady-state analysis of the converter is presented to determine the power flow equations, tank currents and soft-switching operation boundary. Dynamic analysis is performed to design a closed-loop controller to regulate the load-side port voltage and source-side port current. Design procedure for the three-port converter is explained and experimental results of a laboratory prototype are presented. For applications where the load-port is not regenerative, a diode bridge is more economical than an active bridge at the load-side port. For this configuration, to control the output voltage and to share the power between the two sources, two control variables are proposed. One of them is the phase shift between the outputs of the active bridges and the other between two legs in one of the bridges. The latter uses phase-shift modulation to reduce the value of the fundamental of the bridge output. Steady-state analysis is presented to determine the output voltage, input port power and soft-switching operation boundary as a function of the phase shifts. It is observed from the analysis that the power can be made bi-directional in either of the source ports by varying the phase shifts. Design procedure, simulation and experimental results of a prototype converter are presented. The current-fed bi-directional three-port converter consists of three active full bridges with phase-shift control between them. Their inputs are connected to dc voltage ports through series inductors and hence termed as current-fed. The outputs are connected to three separate transformers whose secondary are configured in delta, with high frequency capacitors in parallel to each transformer secondary. The converter can be used in applications where dc currents at the ports and high step-up voltage ratios are desired. Steady-state analysis to determine power flow equations and dynamic analysis are presented. The output voltage is independent of the load as observed from analysis. Simulation results are presented to verify the analysis.