Investigation of a High-Frequency Link Single-Stage Asymmetrical Multilevel Converter for Grid Integration of Renewable Energy Systems

Abstract

Different power electronic converter topologies have been investigated to integrate renewable energy systems to the grid. Cascaded multilevel converters with a high- frequency link have emerged as a viable candidate for such applications. Electrical isolation can be provided using a compact high-frequency transformer connected in the link thus avoiding a bulky line frequency transformer. The use of cascaded modules allows the generation of a multilevel voltage having low total harmonic distortion (THD) but increases the overall system size. In this thesis, a fifteen level high-frequency AC- link single-stage asymmetrical multilevel converter for grid integration is proposed. The single-stage conversion approach eliminates the DC-link capacitors, resulting in a reduced footprint. The asymmetrical module voltages are generated by the multi-winding transformer having unequal turns on each of the secondaries. This allows the generation of fifteen output voltage levels, using only three modules in each phase. A modified uni- polar modulation strategy is proposed to generate multilevel output voltage. A modified hybrid modulation technique using triangular level shifted carriers based on the high frequency link is also investigated for this topology. Unlike previously used technique, the proposed modulation shifts the dominant harmonics in the output voltage to the sidebands of multiples of twice the switching frequency, thereby reducing the output filter size. However, the incorporation of the switch non-idealities requires the implementation of a commutation strategy for the single-stage conversion. A detailed circuit analysis showing different modes of operation to generate a specific output voltage level, taking into account the switch non-idealites is outlined in the thesis. The analysis aids in the real-time implementation of the converter and, it also explains the distortion in the ideal output voltage profile. The analysis in general can be used for any isolated single-stage converter. The effect of switch non-idealities on the output voltage is analyzed and two compensation techniques are developed to improve the voltage profile. A multi-winding high-frequency transformer is a critical component in the proposed converter topology. A four-winding transformer is designed and characterized using Ansys 3-D Finite Element Analysis and Network Analyzer measurements. The presence of transformer leakage inductance will require a clamp circuit to dissipate the leakage energy during commutation. A detailed circuit analysis showing the various modes of operation considering both the switch and transformer non-idealities is presented. Comprehensive analysis is done for ensuring the generation of balanced currents in case of a module failure. A look up table is provided to operate the proposed topology for all possible cases. The presented concepts are verified by simulation and further validated experimentally on a three-phase fifteen level converter prototype.