Abstract
High power Var Compensators are usually connected to the gird-side high voltage. Using Multilevel Converters as Var Compensators to connect to the gird side can save the cost of transformers. Due to the benefit of easy expansion, Multi-Level Cascaded H-Bridge Converters is often used. However, each H-Bridge has an independent DC capacitor voltage, but there exist the problems of unbalancing capacitor voltages due to different characteristic of device in each H-Bridge. Unbalanced capacitor voltages can damage devices when the converter operates over the rated voltage of devices. Therefore, it is important to analyze the reason of unbalance capacitor voltages. The control method is adopted to balance capacitor voltages, and therefore, the converter can be well-operated. Firstly, the thesis discusses the reason of unbalanced capacitor voltages by simulation. After that, voltage balancing control is used to solve the unbalancing problem. In the end, the test bench of Seven-Level Cascaded H-Bridge Converter is constructed and testified. At start-up, the converter injects an inductive 1kVAR into the power system and it only took 50 ms to set up the capacitor voltage to 70 V, in the meaning time, the converter also keep capacitor voltages in balancing. In addition, the injection of reactive power can change inductive operation to capacitive operation in 10 ms and capacitor voltages can still be balanced. The experimental result shows that the converter has fast response on reactive power injection. Capacitor voltages can keep balancing under transient state and steady state.