Research On SVG Based On Modular Multilevel Converter

Reactive power compensation can improve the stability of power system. But the harmonics of the traditional reactive power compensators are so large that extra filter devices should be used. Because of the limitations of their topology, it’s difficult to apply the traditional compensators in high-voltage conditions. Modular Multilevel Converter(MMC), as a new topology converter, has high flexibility and is easy to extend. So it has been widely researched in high-voltage and large-capacity applications.Firstly, the paper introduces the operating principle of MMC, and analyzes its harmonic character. To limit MMC’s output harmonics, the paper compares the harmonic differences between the Carrier Phase Shifted Method(CSPWM) and the Nearest Level Modulation Method. And the CPSWM is chosen to produce the triggering pulses for the IGBTs of MMC.Secondly, the balancing method of MMC’s DC side voltage is studied to maintain the DC side voltage. The mathematic model indicates that the voltage unbalance between the MMC’s DC side and MMC’s phase side generates a loop current. In the purpose of suppressing the 100 Hz part of MMC’s DC side voltage and keeping sub-module(SM) capacitor voltage steady and consistent, the paper proposes the loop current suppression based on SM voltage-sorting method. Based on the positive sequence character of 1-Phase M-SVG, the paper proposes a MMC’s DC side voltage balancing method. The simulation result indicates the correctness of the control method.Finally, an experimental prototype is built. The software system is developed based on FPGA & DSP. The experimental results indicate that the stability of the DC side voltage is well maintained and the M-SVG can output the reference reactive current accurately and quickly.