Search

Abstract: Grid-tied power converter system is the key link in many power conversion applications such as renewable energy power generation, changing of electric vehicle, and industry motor drives. The performance of the grid interfacing converter has a substantial impact on the overall performance of the entire system. One popular design concept of the grid-tied converter system is adopting modular parallel converters. With such design, system reliability can be enhanced and efficiency improvement is achievable. To further elevate system performances, interleaving technique is a promising approach to operate the parallel converters. In spite of the advantages of parallel modularity and interleaving technique, recent development trend has imposed further challenging requirements on converter system design.High power density is one of the key challenges. Specifically for modular parallel converters, the common mode circulating current (CMCC), as a side effect of interleaving technique, will increase the size of the common mode (CM) choke in each converter module. To fulfill the power density requirement, minimizing CMCC is a mandate.In parallel to the power density requirement, stable operation of grid-tied converters is another critical issue. With the increasing penetration of LCL-filtered power converters in the utility grid, the filter resonance issue has become a sever challenge. Eliminating the LCL filter, and thus the filter resonance related issues, while maintaining small filter size and grid code compatible current quality is highly desirable, but also a challenging task.The focus of this thesis is thus situated on the addressing above challenges and seeking potential opportunities for further improvements. To reduce CMCC while preserving parallel modularity, the first step is a comprehensive harmonic distribution analysis for generalized pulse width modulation (PWM) schemes. The analysis results in a two-degree-of-freedom (2DoF) interleaving scheme which can effectively minimize the CMCC in modular parallel converter system regardless the number or type of the converters. In addition, leveraging the interleaving technique, a new system design concept is proposed to eliminate the LCL filter in each converter module. A comprehensive system level design analysis indicates that only small L filter is required to meet the grid code when there are five or six interleaved converter modules in a system. As such, the filter resonance related issues can be completely eliminated. More importantly, the total filter size with the proposed design can be substantially reduced as compared to the LCL filter based design. To further enhance system performances, a novel multilevel converter topology concept, i.e. internal parallel converter (IPC), is proposed to integrate parallel modularity and interleaving technique into one multilevel converter. A high density Gallium Nitride (GaN) device based IPC prototype is also developed in this thesis as a demonstration.