Power And Energy Management Strategy For Hybrid Energy Storage System-solid State Transformer-based Dc Microgrid

With the integration and utilization of renewable energy sources,battery energy storage,fuel cells and other power devices with DC output characteristics,together with the large-scale grid connection of various DC driven loads,DC microgrid is gradually gaining widespread attention as an advanced microgrid technology.The stable operation of DC microgrid is highly related to its internal power balance,therefore,it is important to study the power and energy management strategies such as coordinated control of DC microgrid multiple sources and AC-DC microgrid power exchange strategies.In this paper,a DC microgrid structure based on solid-state transformer and distributed hybrid energy storage structure consisting of battery energy storage and superconducting magnetic energy storage is proposed,and the power and energy management strategy of DC microgrid is designed for this structure.The purpose is to fully utilize the advantages of superconducting magnetic energy storage and battery energy storage respectively to ensure the long-term reliability of DC microgrid when facing with various types of power fluctuations.The detailed contributions of this thesis are listed as follows:(1)To ensure the long-term power stability of DC microgrid,a hierarchical control strategy for distributed battery energy storage is designed.The control strategy is able to proportionally distribute the unbalanced power between the source and the load to multiple distributed battery energy storage devices;the adaptive droop control based on the battery SOC is proposed to avoid the deep discharge or overcharge of the battery,and contributes to improve the lifetime of batteries;the automatic AC-DC microgrid power exchange control strategy based on the DC bus voltage is proposed to enable the AC microgrid to automatically provide power support when the severe power imbalance occurs in DC microgrid.(2)To improve the response speed for transient power fluctuation of energy storage devices in DC microgrid and reduce the battery charging and discharging frequency,a hybrid energy storage structure based on superconducting magnetic energy storage and distributed battery energy storage is built and a filtration-based power decoupling control strategy for hybrid energy storage system is proposed.The control strategy decomposes the transient power fluctuation into high frequency component and low frequency component,which will be compensated by superconducting magnetic energy storage and distributed battery energy storage respectively,effectively improving the response speed of DC microgrid to transient power fluctuation.(3)To cope with large-scale power fluctuations in DC microgrid,a mode switching strategy is proposed for superconducting magnetic energy storage.This control strategy removes the limitation of the filter control on the superconducting magnetic energy storage when large-scale power fluctuations occur,allowing the superconducting magnetic energy storage to provide transient power compensation more quickly and accurately.(4)To enable the superconducting magnetic energy storage to work uninterruptedly in the DC microgrid,the SOC self-recovery control strategy is proposed.It allows superconducting magnetic energy storage to absorb energy automatically when its SOC is not enough.And the whole process will not affect the stability of the DC microgrid.