| The MMC-MTDC transmission system is a new type of high-voltage DC transmission technology.Compared with the traditional high-voltage DC transmission,MMC-MTDC has the advantages of convenient capacity expansion,strong fault redundancy handling capability,and low harmonic content.However,due to the unique structure of MMC bridge arm sub-module cascade,the dynamic characteristics of DC side of MMC converter are significantly different from those of two-level converter,especially in the case of asymmetric AC grid,the voltage and current of valve side of MMC converter are unbalanced,and the power of AC side will be oscillated,and the power oscillation will change the energy distribution of bridge arm sub-module,and the DC side of converter will have two-frequency voltage and current fluctuations.The power oscillation will change the energy distribution of the bridge arm sub-module,and the dc side of the converter station will have difrequency voltage and current fluctuations,while the interaction between MMC-MTDC converter stations may occur,which further adversely affects the stability of the whole MMC-MTDC system.Therefore,it is important for theoretical analysis and practical engineering application to study the inter-station interaction effects and system optimization control of MMC-MTDC when the AC side is asymmetric.Firstly,the dynamic characteristics of the converter station and the causes of DC-side fluctuations when using negative sequence current control and active power fluctuation control under AC asymmetric conditions are studied,and the simulation analysis of the two control strategies is carried out,and the characteristics of the two control strategies are analyzed to provide reference for the specific control strategy to be used in actual projects.Then,based on the basic topology and basic control strategy of the MMC-MTDC,the interaction effects between the receiving converter stations when the AC side of a single receiving converter station is asymmetric and the effects on the receiving converter station when the AC side of the sending converter station is asymmetric are analysed by means of joint controller modelling and two-fold frequency disturbance analysis.Different control strategies are proposed in order to suppress the fluctuations on the AC side of the MMC converter station and the interaction between the MMC-MTDC stations during the two different operating conditions.For the large fluctuations on the DC side during the AC asymmetric operation of the receiving converter station,negative sequence current control and energy balancing strategies are adopted,which can achieve multi-objective control to suppress the AC current imbalance,stabilise the DC side voltage and current and reduce the inter-station interaction.For the large fluctuation of AC active power in the AC asymmetric operation of the converter station at the sending end,the active power fluctuation control and the loop current suppression and DC side voltage fluctuation control strategy with Universal Controller（UC）are adopted,which can achieve the multi-objective control of suppressing the AC side active power fluctuation,stabilizing the DC side voltage and current,mitigating the DC current dip and reducing the inter-station interaction.Finally,based on the RT-LAB5600 real-time simulation platform,a four-terminal MMC-MTDC model is built to simulate and analyze the interaction influence theory and the proposed control strategy between the converter stations.The correctness of the interaction influence theory analysis and the effectiveness of the proposed improved control strategy are verified. |
