| Due to the extensive using of non-linear loads, harmonic current has been increased in power system. The serious harmonic wave will influence the quality of power grid and have interfere for communication system. Active power filter (APF) can compensate the harmonic wave and reactive power effectively with real-time performance. As the voltage increasing constantly, the traditional structure of APF is difficult to meet the requirement. Modular multilevel converter(MMC) can be well applied to the high voltage and scale system. This thesis contains the following research on the MMC-based APF.Firstly, the main circuit mathematical model of current and voltage has been established on the basis of main circuit structure of MMC. The MMC input current is decomposed into active and reactive power id、iq on the mathematical model. Coupling the control of DC-link voltage can make the sub-system control in fundamental frequency. Meanwhile, the characteristics of upper and lower arm of MMC circuit is researched, the relationship between the internal circulating current of MMC and the voltage of bridge arms are obtained. The average voltage control and voltage balance control of system have been studied. Through carrier phase shifted sinusoidal pulse width modulation (CPS-SPWM), the output of driving signals are obtained.And then, p/q harmonic detection method and id/iq harmonic detection method have been compared. Harmonic control theory is analyzed based on the predictive current control. The load current signal replaced by the grid current signal is adapted in the thesis, which can reduces the number of current sensors and enhances the effect of harmonic compensation. To realize the control of modular multilevel APF, the modified predictive current control is applied into modular multilevel circuit structure. Besides, the simulation is completed.Finally, an experimental prototype of five level MMC is built and the system control is designed by the Verilog hardware description language. The online monitoring waveform of Signal Tap II demonstrate the validity of module structure. By synthesizing each sub-modules, all the MMC algorithm for five level control system are designed in one chip of FPGA. Dynamic and static performance of the system are proved by experimental system. The THD of grid side current is reduced from 16.2% to 3.6%. After all, the compensation effect is obvious. |
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