Research On Propagation Mechanism Of AC-side Faults In AC/DC Hybrid Microgrid

Microgrid is one of the development directions of future power grid,which can solve the problems of unified grid-connected operation of distributed generation.Microgrid includes DC microgrid,AC microgrid and hybrid microgrid.DC microgrid and AC microgrid have their own advantages,and hybrid microgrid combines their advantages,which is an import part of the construction and development of microgrid.Hybrid microgrid is an AC-DC interconnected system,which has varied operation modes and complicated control strategies.When faults occur on AC side,fault disturbance will propagate to DC side.Influenced by the topology and control strategy,disturbance forms will change variously.It is of positive significance to analyze the fault propagation mechanism and design suppression measures for enhancing the security of hybrid microgrid operation.In this paper,the following aspects of research work are carried out:Firstly,according to the control principles of inverter distributed generation,the corresponding fault equivalent models are given.Based on the equivalent circuit of hybrid microgrid with DC-side simplified,a fault analysis method for AC side of hybrid microgrid is proposed,which can quantitatively analyze the fault characteristics of AC side of hybrid microgrid under various AC-side faults.The simulation results of PSCAD/EMTDC verify the validity of this method.Afterwards,the influence of asymmetric faults on DC microgrid is analyzed.Secondly,based on the general mathematical model of interlinked converter described by switching function,the fault response of DC side in three-phase short circuit of AC side is deduced.Besides illustrating that the negative sequence component will cause the doubling frequency fluctuation of power and voltage on the DC side,the bipolar power supply mode of the DC microgrid will provide a new path for fault propagation,so that the zero sequence component can act on the DC microgrid and cause the power frequency disturbance of the DC bus voltage.Then,by analyzing the typical control strategies of DC-side distributed generation,it is pointed out that harmonic voltage of DC bus will affect the output stability of DC-side distributed generation.The analysis of fault propagation mechanism between AC/DC microgrids is validated in PSCAD/EMTDC.Finally,according to the above analysis,the suppression measures for the impact of fault propagation are designed at the digital layer and the physical layer respectively.On the control layer,based on the "U-U" control strategy for interlinked converter,the positive sequence component improved control strategy is fitted and a negative sequence and zero sequence voltage compensation unit is added.On the control layer,a dual-tuned filter is designed to filter the disturbances caused by the power frequency and the second harmonic on the DC side.Taking the hybrid microgrid with bipolar power supply on DC side as an example,two methods are compared and verified in PSCAD/EMTDC.The results show that both methods can restrain the influence of fault propagation.