Research On Key Technologies Of MMC For Multi-terminal Flexible DC Grid System

Multi-terminal flexible dc grid has become one of the effective technical means to solve the grid integration of large-scale renewable energy and interconnenction of multiple ac asynchronous power grids,as well as the important direction of the upgrade and transformation of the future power grid,due to its flexibility,economy and reliability.Modular multilevel converter(MMC)has become the key equipment to construct the multi-terminal flexible dc power grid,thanks to its high modularity,low power loss,no need of ac/dc side filters,and so on.However,MMC topology is essentially different from the traditional two-level converter topology,since MMC has extremely complex internal and external dynamic behaviors,which brings great challenges to the design of control systems.In addition,the conventional MMC(based on half-bridge sub-modules)has two main shortcomings.The first one is that the sub-module capacitance is very large,which greatly increases the volume and cost and reduces the power density of the converter.The second one is that it does not have the ability to deal with dc faults and indeed relies entirely on high-voltage dc circuit breakers with very high performance requirements and high cost.How to overcome these two shortcomings and how to optimally design the control system of the MMC are the key technical issues to be solved urgently.In this paper,these key technical issues are studied in depth.The main contents are as follows.1)Considering the complex internal dynamic behavior of MMC as well as the condition large dc-side inductance,a complete dynamic model of MMC is established.On this basis,the decoupling control strategy of the internal and external dynamics is designed systematically,and a complete controller parameters design is given.The proposed control solves the problems of the uncoupling of converter current control and poor dynamic performance of dc current.The correctness of theoretical analysis and the validity of the full dynamic control strategy are verified by simulations.2)The optimized control of MMC under asymmetric conditions is studied.Aiming at the asymmetric impedance of upper and lower bridge arms,the mechanism of abnormal phenomena is revealed,and an optimized control strategy is proposed,which eliminates the fundamental frequency oscillation component of dc side current as well as the dc component of ac side current,and ensures that the ac and dc port characteristics of MMC are not affected by the asymmetric impedance of bridge arms inside converter.In view of the asymmetry of ac power network,a precise dynamic model of MMC is established.On this basis,an optimized control strategy(compatible with symmetrical grid conditions)is proposed,which improves the internal and external dynamic and steady-state characteristics of MMC under asymmetrical ac grid condition,and enables MMC to act as a "firewall" between ac and dc grids.The correctness of theoretical analysis and the effectiveness of the proposed optimized control are verified by simulations and experiments.3)The optimal control method that is used to greatly reduce the sub-module capacitance is studied.A mathematical model of the capacitor voltage ripple of the sub-module is established.The method is proposed to transfer the capacitor voltage ripple from low frequency to high frequency by injecting double-frequency circulating current and triplefrequency common-mode voltage.On this basis,the capacitance of the sub-module is optimized,and the capacitance can be reduced by 38% without increasing the loss.In order to ensure that MMC with minimum capacitance can still meet the performance requirements under asymmetric power grid and transient fault conditions,a capacitance minimization control method(including ripple control,capacitance voltage peak control and disturbance power feed-forward compensation control)is proposed.The correctness of the theoretical analysis and the effectiveness of the optimized method are verified by simulations and experiments.4)The optimized design of hybrid MMC and dc fault-ride-through control strategy are studied.Comparing cost,loss,dc fault current limiting capability,control complexity and encapsulation complexity,a hybrid MMC topology with dc fault handling capability is obtained.Considering the transient and steady state conditions,the hybrid ratio of half-bridge and full-bridge sub-modules and capacitance capacitance are optimized.An optimized control method of capacitor voltages based on fundamental common-mode voltage and negative sequence current injection and disturbance power feed-forward compensation under dc fault is proposed,which can ensure that the capacitors does not overvoltage during the fault.In addition,based on the characteristics of both voltage source and current source in the dc side of MMC,a control strategy for fast clearance of fault current is designed,and an optimized control method(compatible capacitance minimization control)for the hybrid MMC under dc fault is proposed systematically.5)The dc fault-ride-through strategy of multi-terminal dc power grid based on the hybrid MMC with high power density is studied.The hybrid MMC is adapted to dc high-speed switch and multi-terminal dc power grid protection system.The sequence of dc fault detection,location,clearance,isolation and power recovery is designed.A complete dc fault-ridethrough strategy for multi-terminal dc power grid is proposed.The effectiveness of the proposed dc fault-ride-through strategy is verified by simulations.