Study Of Flexible HVDC Based On Modular Multilevel Converter

Compared to traditional AC transmission systems, flexible HVDC transmission system has the incomparable advantages in solving the problem of the interconnection of new energy power grid, the remote isolated load power supply and interconnection of the asynchronous power grid. However, under the circumstance of traditional three-level VSC-HVDC technology, the voltage output level of the inverter is low, which leads a high harmonic content in the output voltage and requires a larger filter. Meanwhile, as each of the switching devices bears a high voltage, making it difficult to be applied to high voltage and high power applications. These shortcomings put a great limit on the development and application of the flexible HVDC technology. The modular multilevel converter is designed modularly. Different numbers of submodules can be connected in series according to the output voltage requirements without filter, which is suitable for the flexible HVDC.The working principle of the modular multilevel converter has been introduced in this paper. On the basis of analyzing the mechanism of the sub-module capacitor voltage fluctuation component and inverter circulating current component, this paper design a circulation suppression controller, then verify the correctness of the analysis and the effectiveness of the controller though the simulation.This paper introduces several common multilevel modulation strategies. By comparing these modulation strategies, APODPWM are selected as modulation strategy of this paper. The high computational complexity of APODPWM is improved in this paper. The traditional APODPWM pulse distribution has power and voltage imbalance of each module. To solve the imbalance this paper put forward a new pulse cycle distribution which making the sub module capacitor voltage in a more balanced state without increasing the switching frequency. Several common capacitor voltage balancing strategies are introduced in this paper. On the basis of introducing those strategies, this paper put forward a new capacitor voltage capacitor voltage equalizing strategy which based on sorting the sub module capacitor voltage. The new capacitor voltage capacitor voltage equalizing strategy can be balanced the capacitor voltage and applied to the previously proposed distribution at the same time. At last Through the simulation to verify the effectiveness of the pulse distribution and the strategy which balancing capacitor voltages.Since the flexible HVDC system is used for power transmission, it is essential for power transfer analysis. In this paper, the principle of Flexible HVDC is analyzed. The mathematical model and equivalent circuit are created for Flexible HVDC in 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. Finally, the simulation results show the effectiveness of the designed controller and the superiority of the modular multi-level inverter applications in flexible HVDC system.