A Circulating Current Suppression Control Method For Multi-Paralleled Bi-directional Power Converters Under Unbalance Operation

With the continuous depletion of primary energy and the rapid development of distributed generation technology,microgrid system has become one of the most effective ways to integrate new energy generation system and multiple types of load.However,due to the increase of power system capacity and the limitation of switching devices,the single-module converter structure has been difficult to meet the application of low voltage and high current.Therefore,a multi-module paralleled redundant configuration of common AC and DC buses is usually used to improve system capacity and operational reliability.The common AC and DC bus structure provides a path for the circulating current,and it will cause the distortion of the output current,generate a large current stress,increase the switching loss of the system,reduce the stability of the system,and even damage the switching devices.So this paper studies the circulating current suppression method of paralleled BPCs.Firstly,this paper introduces the topology,control method and development trend of AC/DC hybrid microgrid,and describes the current research situation of circulating current between paralleled BPCs under balanced and unbalanced conditions respectively.Based on this,a mathematical model of BPC is established and a dual closed-loop control strategy for the converter is designed.What’s more,the parameter selection process of the dual closed-loop is given,The effectiveness and feasibility of this strategy proposed in this paper is verified in MATLAB/Simulink.Secondly,in order to solve the problem of ZSCC between converters under balanced working conditions,the causes of ZSCC are first analyzed,the path of ZSCC between converters is given,and a mathematical model of ZSCC is established.A standardization of autonomous power control is designed,and the integral term is added to the reactive-voltage link to realize the tracking of voltage without static error,which can reduce the output voltage difference between converters.An adaptive virtual impedance circulating current suppression strategy that is dynamically adjusted over time with power fluctuations is proposed based on the analysis of the impact of virtual impedance on converters,and the parameter selection method and its dynamic analysis process are given.The effectiveness of the ZSCC suppression strategy proposed in this paper is verified in MATLAB/Simulink.It proves that the circulating current suppression strategy proposed in this paper has a better effect on circulating current suppression by comparing with the traditional virtual impedance control strategy.It improves damping characteristics and stability of the system.Thirdly,a mathematical model of circulating current under unbalanced working conditions is established for the circulating current between paralleled BPCs under unbalanced working conditions.The causes of NSCC and ZSCC are analyzed in details.By considering the previous power control,an adaptive virtual impedance control strategy based on the SOGI is designed.The NSCC and ZSCC control links are designed separately.This strategy can avoid the influence of the differential link on the system stability.The effectiveness of the circulating current suppression strategy proposed in this paper is verified in MATLAB/Simulink.It proves that the proposed circulating current suppression strategy is more effective by comparing with the traditional virtual impedance control strategy.Finally,with the help of d SPACE,an AC/DC hybrid microgrid experimental platform of paralleled converters is established to verify the effectiveness and feasibility of the circulating current suppression strategy proposed in this paper.It shows the superiority of the control strategy proposed by comparing with the traditional virtual impedance circulating current suppression strategy.