Distributed Hierarchical Control Strategy Of Bidirectional Converters In AC/DC Hybrid Microgrids

Under the dual crisis of both the energy and the environment,distributed generation(DG)of renewable and clean energy becomes important to seek a way out of human development.In order to solve the serious problems of grid-connected distributed power generation,microgrids technology is introduced.However,the pure AC or DC microgrids cannot avoid the low power conversion efficiency and reliability problems caused by frequently absorbing the renewable energy sources.In addition,AC loads and new kinds of DC loads in a pure AC or DC microgrid have a similar defect.Taking into account the economic operation of DG,assigning them to different reasonable buses,along with energy storage equipment and load's to form the AC/DC hybrid microgrids has become an inevitable choice.The research object of this paper is the DC-side grid-connected bidirectional converter interlinking the AC subgrid and the DC subgrid in AC/DC hybrid microgrid.Through literature review,this paper first introduces the AC/DC hybrid microgrids research status and development trends,summarizes the characteristics of hybrid microgrids topologies and gives the traditional control strategies of several topologies of hybrid microgrids,and then introduces intelligent distributed control strategies in detail,which lays the theoretical foundations for the study of the bidirectional converter.Secondly,based on distributed hierarchical control thought,this paper designs a distributed hierarchical control strategy of hybrid AC/DC microgrids bidirectional converter.Firstly,the traditional grid-connected control methods are introduced,based on which the DC-side distributed grid current secondary control methods is designed.The bottom level is using a DC voltage and AC current based closed-loop control in ?? coordinate system,the upper is PI control based on distributed grid current secondary control.The consistency of the system is analysed.On the basis of the establishment of the system current source model and the system dynamic impedance model characteristic,the current secondary controller is designed.Again,based on the distributed hierarchical control theory,a distributed hierarchical control strategy of the hybrid AC/DC micro-grid island operation bidirectional converter is designed.First,the existing control methods are introduced,based on which the DC-side distributed voltage secondary control method is designed.The bottom control layer is based on DC voltage droop control and current closed-loop control in the dq coordinate system,the upper is PI control based on distributed voltage secondary control.On the basis of the establishment of the system voltage source model,and the analysis of DC bus voltage drops,the secondary voltage controller is designed.This thesis also focuses on the power quality special problems in hybrid AC/DC microgrids.First,micro-grid power quality problems are reviewed.The DC ripple serious problem of Bidirectional converter for grid-parallel systems is analyzed based on the harmonic transfer mechanism,which is resolved by the use of a ripple power DC/DC structure-based active snubber circuit and the ripple compensation solution based on SPWM.And then,the common mode current problem caused by parasitics of bidirectional converter is analyzed based on the system common mode equivalent model.This problem is resolved by using an improved LLCL filtering.