Research On Resonance Mode And Stability Region Of Multi-inverters Grid-connected System

The recent years have seen the installed capacity,the power generation and the penetration of renewable energy increase continuely.Grid-connected inverters feature in flexible operation mode and good controllability,which makes it become one of the main interfaces of renewable energy to the grid.The grid-connected system usually adopts the structure in which large-scale inverters parallel to the grid at PCCs.The potential factors such as long lines,transformers and changes in the operation mode of power grid will lead to the fluctuation of the grid impedance.A weak power grid with inverters connected risks in resonance and instability,which cannot assure the quality of grid-connected power and risks in destroying normal work of the system.Therefore,it is necessary to study the stability and resonance risk of multi-inverter grid-connected system under complex conditions.The main research and results of this paper are as follows:(1)Inverter control strategy is studied and impedance model of inverter and grid-connected system have been established.The passive and active damping methods of inverter are studied separately,both of which proves can eliminate resonance peaks from LCL filters.The control models of three-phase inverters in dq synchronous rotating coordinate system and ?? static coordinate system are deduced.Then the equivalent Norton model of three-phase inverters is established in alpha-beta static coordinate system.So that the inverter grid-connected current and output impedance(admittance)are obtained.It is proved that the abc phase models in three-phase inverters are independent from each other under the given conditions.In multi-inverters grid-connected system,the phase-separated impedance model of the system is established.It is found that the grid-connected current is related with inverters itself,other grid-connected inverters and grid voltage.(2)The stability criterion and its remodeling strategy of multi-inverter grid-connected system are proposed.Based on the impedance model of inverters,the stability criterion of multi-inverters grid-connected system is deduced in detail.Considering the control loop of inverters,method of rebuilding the system impedance criterion has been studied.Through the stability criterion and Nyquist curve,the stability parameter boundaries of the key variables of the system can be obtained.The stability criterion is also applied to study and analyze the influence of the factors such as grid impedance,LCL filter,line impedance,voltage feed-forward link on the system stability.The simulation is carried out on the PSCAD platform.The accuracy and applicability of the stability criterion before and after remodeling are compared and analyzed.(3)A method of harmonic resonance analysis is proposed to investigate the resonance problem in a multi-inverter grid-connected system based on resonance modal analysis(RMA)and participa-tion factor(PF)calculations.Initially,an impedance-based analytical approach is employed and expanded to an electricity dis-tribution network that is dominated by multiple inverters with LCL filters and proportional resonant controllers.Then,the RMA is introduced to investigate the resonant interactions among inverters and the grid.Specifically,harmonic resonance characteristics are addressed through transfer functions and the proposed method considering variation of inverter number,dif-ferent inverter combinations and critical system components.Finally,time domain simulations on the PSCAD/EMTDC platform are presented.The case study results validate the effective-ness of the proposed method.