Research On Control Strategy Of DC Distribution Network Under AC Side Imbalance

With the rapid development of power electronic technology,more and more new energy sources and DC loads are applied to the current AC distribution system,which brings many new challenges for the stable operation of the traditional AC distribution network.In contrast,DC distribution network based on power electronics technology is more suitable for new energy access.Besides,it has higher conversion efficiency,larger transmission capacity and stronger controllability,so it provides a new technical scheme for the future development of distribution network.However,when the three-phase unbalance occurs in the AC network or AC load connected to the DC distribution network,the AC pulsating power will affect the DC side and endanger the operation of all ends of the distribution network.For this reason,according to the unbalanced classification of DC distribution network,the DC voltage ripple suppression strategies for grid-connected converters and energy storage units are designed to improve the quality of DC voltage.The main work and achievements are as follows:(1)The models of each end of DC distribution network are established,including grid-connected converter unit,battery unit,wind power unit,photovoltaic unit and AC load unit and DC load unit.The basic control of each unit and the multi-mode hierarchical voltage regulation strategy used to coordinate the power in the network are analyzed.The simulation system is built to provide a platform for studying the impact of AC side imbalance on DC distribution network.(2)The influence on DC voltage caused by the unbalance of AC side is analyzed.Modeling and theoretical analysis of voltage source converters under unbalanced AC network and unbalanced AC load are carried out respectively.Through the modeling of AC/DC power exchange,the influence of AC pulsating power on DC voltage is analyzed and the calculation method of DC pulsating voltage is given.Finally,the correctness of the theoretical analysis is verified by simulation.It lays a foundation for the elimination of secondary fluctuation components of DC voltage.(3)Aiming at the unbalanced situation of AC network,a DC voltage ripple suppression strategy for grid-connected converter unit is proposed.Firstly,the characteristics of PI,PR and quasi PR controllers are analyzed and compared for the selection of controllers in control strategy design.Secondly,the principle of DC voltage secondary ripple suppression is analyzed,and the strategy of extracting positive and negative sequence components based on second-order generalized integral method is designed.On this basis,the control system design based on double PI current inner loop and the control system design based on quasi PR current inner loop with better performance are proposed respectively,and the effectiveness of the two control strategies is verified by simulation.(4)In view of the unbalanced AC load,based on the performance analysis of the controller,the control system design of battery side converter based on quasi-PR signal tracking is proposed,which can directly track and suppress the secondary fluctuation component of DC voltage.The feasibility of the control strategy is verified by simulation,the control effect under different parameters is compared,and the voltage quality of DC side is improved.