| With the development of industry, various kinds of power electronic equipments'access produce a variety of effects on power systems and it also threatens the operation of reactive power compensation equipments. So various kinds of compensation devices based on power electronic devices have been put into use. Modular multilevel converter (MMC) has been focused on by many scientific researchers for its modular, highly integration and extension. Several key issues of MMC were analyzed and verified through theoretical analysis and simulation results.In addition, in order to vertify the feasibility of MMC used to improve the quality of the power system, a lot of simulation experiments and research experiments have done. And the relevant research contents and conclusions can be summarized as:Firstly, the equicalent circuit of MMC is set up according to the structure of it. In addition, two kinds of modulation strategies and the Sub-Modules'capacitor voltage control strategy are extracted according to the existing liturature. Then the simulation platform is set up to compare the results of MMC using different modulation strategies and the Sub-Module capacitor voltage control strategy.Secondly, according to the existing liturature, the formula of the arm's power and the circulating current are derivated from the aspects of the same power on the AC side and DC side of MMC. And then, the direct control of the arm's current is selected to restrain the circulating current according to the existing liturature and the simulation platform to validate whether it is rightly is established.Thirdly, the Modular Multilevel Converter's arms consist of a large number of the Sub-Modules, so the reliability of the Sub-Modules must be considered. Therefore, the redundancy Control Strategy about how to deal with the fault of the Sub-Modules is put forward. It can quickly put the redundant Sub-Module into use when the Sub-Modules in the same bridge goes wrong. Through this way, the normal operation of the device can be ensured. Lastly, A simulation is built to verify whether the propsed method is feasible.Forthly, the principle of the reactive power compensation based on MMC is introduced. And the mathematical model of MMC-STATCOM is established in abc and dq coordinate system separately. Lastly, the control system for generateing the reactive current quantitatively is set up.Fifthly, the principle of power quality compensation on MMC-STATCOM is introduced in detail. Then the extraction method of instruction current and the design of controller is given.Sixthly, in order to verify the validity of the device in the field of power quality comprehensive compensation based on the Modular Multi-level Converter, the simulation model is bulit on the electromagnetic transient simulation software PSCAD/EMTDC. And the simulation results show that the device in the field of power quality comprehensive compensation based on the Modular Multi-level Converter is feasible.Finally, overall hardware circuits and control circuits are conformed in accordance with the control algorithm parameters and simulation platform. And the related experiments are done to verify whether the device based on the Modular Multilevel converter is feasible. Then the expertimental results verified the feasibility of the MMC in improving the power quality. |
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