Multi-mode Operation Control Scheme For AC/DC Interfacing Converter Under Unbalanced Conditions In Hybrid Microgrid

Hybrid AC/DC microgrid could be well compatible with various AC/DC distributed generators and loads.It could greatly reduce the use of multiple AC/DC or DC/AC converters and cut down system operating cost and power loss.As a core component of hybrid microgrid,AC/DC interfacing converter(IFC)not only connects AC bus and DC bus,but also could operate as rectifier or inverter.It shoulders the responsibility of maintaining voltage stability and balancing power between AC subnet and DC subnet.Therefore,it is greatly significant to study the control strategy of AC/DC interfacing converter.At the same time,the actual hybrid microgrid is often under unbalanced conditions.This paper presents a multi-mode operation control strategy for AC/DC interfacing converter under unbalanced conditions.The proposed strategy could enable IFC to realize the unified control of power bidirectional transmission and power quality control.It improves the utilization ratio of IFC and reduces the operation cost of the system.The main research contents are as follows:(1)Firstly,the background and significance of this paper are briefly explained.The research status and development trend of AC microgrid,DC microgrid and hybrid AC/DC microgrid are introduced.The research status of IFC under balanced and unbalanced conditions is introduced.Based on that,the four quadrant working principle of IFC is described and the mathematical models of IFC in abc????dq coordinate system under balanced and unbalanced conditions are derived.(2)Secondly,based on the mathematical model of IFC in ?? coordinate system under unbalanced conditions,the overall control system of IFC is designed.It includs positive and negative sequence separation,power calculation,bidirectional droop control,voltage and current double loop control.In the bidirectional droop control,the amplitude of AC voltage and DC voltage are standardized and differentiated to produce positive sequence voltage reference value,which could make IFC operate as rectifier or inverter.And the simulation results of IFC under balanced and unbalanced conditions in MATLAB/Simulink verify the effectiveness of the proposed overall control system.(3)Thirdly,aiming at the problems of AC three-phase unbalanced current,AC three-phase unbalanced voltage and DC voltage with double frequency fluctuation under unbalanced conditions,multi-mode operation control strategy of IFC under unbalanced conditions is proposed.According to the instantaneous power transmission characteristics of IFC,it is divided into three main operation modes: unbalanced current compensation mode,unbalanced voltage compensation mode and non-oscillatory DC voltage mode.By introducing multi-mode control parameter,the negative sequence equivalent impedance of IFC and the flow direction of the negative sequence current source are controlled.So that IFC could operate in three modes.And the simulation results of three operation modes of IFC in MATLAB/Simulink verify the effectiveness of the proposed multi-mode operation control strategy.(4)Finally,a simple hybrid AC/DC microgrid experimental platform is built on the dSPACE.The experimental results show that three modes mentioned above could be achieved by controlling the multi-mode operation parameter.