Control Strategy Optimization Of Energy Storage Converters Considering AC Constant Power Loads

Energy storage units could solve the intermittent power supply problem of distributed power supply,and they become indispensable parts of microgrid system.The bidirectional energy storage converters control charging and discharging processes of batteries,transform AC and DC energy,and also ensure the battery safe and stable operations.It is of great significance to study control strategies of the bidirectional energy storage converters.In microgrids,the closed-loop controlled converters connected to AC bus could be regarded as AC constant power loads(AC CPLs).Negative impedance characteristics are introduced by AC CPLs,and stable operations of the system are easily destroyed.On the contrary,appropriate controls for energy storage system may compensate dynamic characteristics of AC CPLs,and increase the system stability.However,the current research on energy storage converters is mostly focused on the optimization of system topology and control strategy,and there is no connection between the control strategy of energy storage converters and AC CPLs.In this paper,the stability control strategy of bidirectional energy storage converter is proposed,and dynamic performances of AC CPLs are considered to enhance the stability of the microgrid system.Firstly,DQ coordinate transformation is adopted,and simplified nonlinear models of AC microgrids are established separately considering the different control strategies of the microgrid in island mode and grid mode when the energy storage system in charging and discharging states.Then,large signal models are constructed depending on mixed potential theory in the charging and discharging state of the energy storage unit.Finally,stability control strategies for bidirectional energy storage converters are obtained in island and grid modes respectively,and AC CPLs are also taken into account.The derived control strategies give quantitative constraints on outer voltage loop control parameters of energy storage DC/DC converter and inner current loop control parameters of energy storage DC/AC converter in island mode.The derived control strategies give quantitative constraints on outer voltage loop control parameters of energy storage DC/AC converter and inner current loop control parameters of energy storage DC/AC converter in grid mode.Simulation and experimental results validate the validity of the proposed stability control strategies.The simulation and experimental results show that the system conforming to the derived stability control strategy can continue to run smoothly after large disturbance.On the contrary,the system which does not satisfy the stability control strategy cannot continue to run smoothly after large disturbance.It is proved that the stability control strategy of energy storage converter proposed in this paper is correct.