Control Of Power Supply VSC Converter In AC/DC Hybrid Distribution Network Under Fault Condition

With the application of high-power power electronic devices,DC transmission technology is gradually entering the practical engineering application.AC/DC hybrid distribution network based on traditional AC power grid is gradually changing the development form of power grid.As a key component of AC/DC transmission technology,voltage source converter plays a very important role in practical research.Modular multilevel converter(MMC)has many advantages,such as small harmonic content,high equivalent switching frequency,easy to upgrade voltage level and replace sub module,etc.In this paper,the control of VSC converter in AC/DC hybrid distribution network under fault condition is studied based on MMC.Firstly,the analytical mathematical model of MMC is deduced based on successive approximation method,and the detailed analytical expressions of each electrical quantity of MMC are obtained.Compared with the actual detailed model established in PSIM software,the accuracy of the mathematical model is proved.In order to accelerate the simulation speed,two kinds of MMC fast simulation models based on sub module DC voltage control closed-loop and bridge arm DC voltage control closed-loop are proposed.Compared with the detailed switch model,the proposed two kinds of models have greatly improved the simulation speed while maintaining the original accuracy of electrical quantities,which provides a model basis for the follow-up study of AC/DC fault characteristics based on MMC.For AC side fault of AC/DC hybrid distribution network,this paper studies two aspects of AC side bus three-phase open circuit fault and AC side three-phase unbalanced fault combined with fast simulation model.When AC side bus three-phase open circuit fault islanding operation,this paper mainly uses VF control to maintain AC side bus voltage by changing control strategy,which is verified by simulation The results show the effectiveness of the method.For the AC side three-phase imbalance problem,firstly,the mathematical expression and power fluctuation equation of MMC are derived.According to the power fluctuation equation,the corresponding control strategies are adopted for the negative sequence current and DC side double frequency fluctuation problems.Finally,the effectiveness of the control strategy is verified by the fast simulation model proposed in this paper.For the DC side fault of AC/DC hybrid distribution network,this paper studies the DC fault characteristics based on the simulation results,establishes the mathematical analysis model of two pole short circuit fault,and gives the control and protection method of converter when DC fault occurs.The rationality of the proposed control and protection method is verified by simulation,and the proposed control and protection method is verified by control and protection measures The results show that the fast simulation model is consistent with the detailed model in DC fault characteristics.