Study Of Flexible HVDC Based On Modular Multilevel Converter

Compared to traditional AC transmission systems, Flexible HVDC transmission systems have some incomparable advantages in solving the non-synchronous AC system interconnection, supplying power to the remote passive loads. However, traditional Flexible HVDC systems adopt two-level or three-level voltage source converters. These converters need bulky filters, which increase the cost and the scale of HVDC. Flexible HVDC transmission system based on modular multilevel converter is adapted modular design. By changing the sub-module numbers, the HVDC transmission system could meet different voltage levels and the output levels can be any numbers, which means the bulky filter is eliminated and sub-module switching frequency is reduced. This typology is suitable for HVDC.Based on the traditional capacitor voltage balance strategy, a new type of voltage sort control strategy is introduced. This control strategy could efficiently keep capacity balance and reduce switching lose. By calculating the capacitor voltage fluctuating component, the circulating current is derived and the strategy of circulating current suppression controller is designed, which efficiently suppress the fluctuation without increasing arm inductance.Power control strategy plays a most important role in the operation of Flexible HVDC. In this paper, the principle of Flexible HVDC is analyzed. The mathematical model and equivalent circuit are created for Flexible HVDC in both three phase coordinates and dq two phase synchronous coordinates. According to the mathematical model, this paper innovatively classifies control strategy into three levels:the system level, the converter level and the IGBT level. The three control level is decoupled, which make control strategy more flexible. As to the coupling of dq component, a kind of decoupled PI controller is presented. This control strategy can eliminate coupling between dq components and achieve independent control of active and reactive power.Considering the imbalance of AC transmission systems, the mathematical model of Flexible HVDC is redesigned. The power balance principle is analyzed and phase detection method under fault condition is designed. Control strategy for MMC under unbalanced condition is proposed which can eliminate negative sequence components or ensure active power stable without fluctuation, thus enlarge the operating region of MMC.