Research On Energy Management Of DC Microgrid Based On Solid State Transformer

With the development of society,the demand for electricity has grown exponentially.At the same time,with the continuous growth using of fossil energy,serious environmental problems have been caused,such as haze weather,extreme weather phenomena,and global sea level rise.In this context,renewable energy power generation has attracted worldwide attention.In order to make full use of renewable energy,microgrid technology came into being.Compared to AC microgrids,the advantages of DC microgrids are:high energy utilization,low investment and losses;no need to consider synchronization issues and reactive power issues;convenient access to renewable energy and DC loads;no harmonics and reactive issues.In order to better integrate the DC microgrid and the AC main grid,the solid-state transformer(SST)is appreciated by domestic and overseas scholars for its unique three-port structure.One side of the solid-state transformer is connected to the large AC power grid,and the other side provides a variety of AC/DC interfaces for the DC microgrid.In the DC microgrid system,the core device of the solid-state transformer is used to truly realize the flexible flow and energy management.The research object of this paper is a DC microgrid based on solid-state transformer.The DC microgrid system studied in this paper consists of SST,a photovoltaic system,a distributed energy storage system,and DC loads.The working principle of SST,boost converter and bidirectional Buck-Boost converter was analyzed separately,and the mathematical model was established.Then,the mathematical model was linearized to get the small signal model.The working principle of photovoltaic cell and battery was analyzed.Then,the mathematical model was established,and the output characteristics of the photovoltaic cell and the battery charge and discharge characteristics were got.Thers analysis laid the foundation for further research on the DC microgrid based on solid-state transformer.In order to realize the coordinated control of the DC microgrid,a voltage-classification control strategy based on bus voltage is adopted.This control strategy divides the DC bus voltage into several different intervals.Within each interval,there is a distributed unit controlling the DC bus voltage as voltage source.The remaining distributed units balance the system power as power sources;and each unit in the system can flexibly and automatically switches between the voltage source and the power source based on the bus voltage.Subsequent simulation analysis proves that this control strategy can balance the power to ensure the stability of the DC bus voltage.In order to realize the economical operation of the DC microgrid,an energy management strategy for DC microgrid based on solid-state transformers is proposed.This energy management strategy combines the voltage-classification control strategy and the hierarchical control strategy,and divides the control of the DC microgrid into three levels:the converter control level,the bus voltage control level,and the energy management level.Afterwards,the battery economical optimization algorithm that considers electricity prices is embedded in energy management level.When the electricity price is high,the power is sold to the AC main electricity grid.When the electricity price is low,the power is purchased from the AC main electricity grid.The stable cost of the system is taken into consideration to achieve the stable operation of the DC microgrid.Subsequent simulation analysis proves the effectiveness of this energy management strategy.In the research of the DC microgrid,the small signal stability problem is another important issue.Based on establishing the small signal models of each unit in the system,the small signal stability analysis of the DC microgrid is qualitatively analyzed using the small perturbation characteristic analysis method and the general impedance criterion.According to the small signal stability analysis,with the increasing of load power in the DC microgrid,the stability margin of the DC microgrid will decrease and even make the system unstable.At the same time,compared to the resistive load,constant power load is more possibly to cause system instability.Based on the stability analysis,an active damping control of SST is proposed to reduce output impedance and improve system stability margin.Subsequent simulations verify the correctness of the stability analysis theory.