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Abstract
Large-scale photovoltaic power plants are becoming popular worldwide owing to the cost reduction in all parts of the solar power system, conversion efficiency improvement and the efforts from both governments and commercial companies to seek for new, clean, and sustainable energy sources.
Inverters are important parts that connect the photovoltaic modules to the grid. Compared to the conventional two-level inverters, multilevel inverters feature improved harmonic performance with a reduced switching loss. Among all multilevel conversion topologies, the cascaded H-bridge converter is considered as one of the most promising candidates for the next-generation large-scale photovoltaic inverters.
However, the solar irradiance and temperature conditions may vary significantly in a large-scale solar power plant. As a result, the bridges connected to different photovoltaic arrays are expected to generate different amounts of power. The thesis deals with the challenge to provide three-phase balanced grid currents when the power imbalance problem occurs.
To begin with, several new zero-sequence injection methods are presented, featuring superior power balance capabilities to guarantee balanced currents under severe power imbalance. The optimal zero-sequence injection, in terms of the power balance capability, is also presented and proved to be the optimal solution of the star-connected H-bridge converter.
Furthermore, the power balance capabilities of the presented zero-sequence injection methods are evaluated from both theoretical and practical perspectives.
Additionally, the fault-tolerant scheme allowing the cascaded H-bridge to continue the operation after one or more bridges are bypassed owing to faults is presented. Different to the fault-tolerant scheme of the same converter with other applications, the unique problem of power imbalance in the photovoltaic application is taken into consideration.
Finally, the delta-connected cascaded H-bridge converter is presented as an alternative to the star-connected converter. The converter overratings (both voltage and current), required by the star and delta-connected cascaded H-bridge converters to cope with all theoretically possible power imbalance cases, are compared to demonstrate that the delta-connected converter features significantly less converter overrating.
All the findings are analyzed theoretically and verified experimentally with a converter prototype built in the laboratory.