Power Allocation Strategy For Hybrid AC-DC Microgrid Energy Storage System With Solid State Transformer

In recent years,AC/DC microgrid has gradually formed a development focus due to its ability to better integrate various distributed power generation units and at the same time to work in conjunction with the larger grid.Power storage system is one of the most critical parts of microgrid technology,which can play the function of balancing the variation of grid energy and ensuring the stability of bus voltage.Hybrid energy storage technology is rapidly developing with its advantages of low cost and good performance.The research on the control and power distribution of hybrid energy storage system is a fundamental technical issue.In this thesis,an AC-DC microgrid with hybrid energy storage device based on solidstate transformer is studied and its energy management strategy in grid-connected mode is investigated.The details of the study are as follows:Firstly,the development process of solid-state transformer,AC-DC microgrid and energy storage technology and its control strategy as well as the current research status are introduced.For the solid-state transformer structure used in this thesis,the modeling analysis and control scheme design of each subunit are carried out.In order to accelerate the dynamic response of the solid-state transformer and improve the stability of the system,a coordinated control strategy is proposed to combine the front and rear stages to work in concert with the hydrogen energy storage.Subsequently,the effectiveness of the control method proposed in this thesis is verified by simulation,and the basis for subsequent power distribution and energy management is provided.Next,the advantages and disadvantages of each energy storage technology are analyzed and the energy storage system selected in this thesis is determined.A complete hybrid energy storage system is constructed by modeling and analyzing each subunit of the energy storage system and designing the corresponding control scheme.Then,the first-order low-pass filter is used for the initial allocation of the hybrid energy storage units,and the power response characteristics of different energy storage units are used to separate the high-frequency component from the low-frequency component of the unbalanced energy in the system.In order to extend the service life of hydrogen energy storage,optimize the power distribution and improve the utilization rate of super capacitors,and achieve stable operation and power balance among distribution network,AC/DC microgrid and hybrid energy storage units,an energy management strategy is proposed to redistribute the unbalanced power by designing a low-pass filter with automatic adjustment of filter coefficients,so that the hybrid energy storage units can obtain their respective required power status in grid-connected mode.The power state of charging/discharging in gridconnected mode.Finally,a simulation model of hybrid AC-DC microgrid based on solid-state transformer is built in Matlab/Simulink,and a variety of operating conditions under gridconnected mode are designed to verify the effectiveness of the energy management strategy proposed in this thesis.The simulation results show that the proposed control method and energy management strategy can achieve better power distribution and improve the system operation stability compared with the constant low-pass filter.