Resonance Problem Study Of Parallel Multi-inverter Based On Modal Analysis

At present,with the global energy crisis and environmental problems increasing,wind power,photovoltaic power generation and other renewable energy grid-connected power generation attract more and more attention,they have become an important part of energy sustainable development strategy.In the photovoltaic power plant,wind farms and other new energy-generating micro-grid,usually using multiple inverters and network system structure.With the extensive use of LCL filter in the practical engineering and the power grid impedance and the nonlinear factors in the system,the multi-inverter grid system occur to resonance problems.Harmonic resonance side will lead to the power quality of the power grid decline,on the other hand can induce harmonic instability,and further can lead to system instability,threatening the system safe and stable operation.In this paper,a harmonic resonance method for multi-inverter grid-connected systems is proposed.At First,a method of determining the resonant frequency and resonant characteristics of the system based on modal analysis is proposed.The effectiveness of the modal analysis method is verified by comparing the resonant frequency and the characteristic of the system with the traditional frequency function of the transfer function.Secondly,for the multi-inverter grid-connected resonance to determine the degree of participation in the system components,harmonic resonance sensitivity analysis.Finally,a resonant suppression strategy based on inverter output impedance remodeling is proposed.The effectiveness of the strategy is verified by theoretical analysis and RT-LAB experiment.And specific studies are as follows:(1)A method based on modal analysis is proposed to solve the problem of resonant frequency and resonant characteristics of multi-inverter grid-connected system.Respectively,the same type of inverter system composed of different numbers,different parameters of different types of different inverters constitute the grid system was analyzed.The results of the analysis are consistent with those obtained by the transfer function analysis method,which proves the effectiveness of the method.(2)Based on the sensitivity analysis theory,the sensitivity analysis of harmonic resonance of multi-inverter grid-connected system is carried out on the basis of modal analysis.The influence of the parameters of the components on the harmonic resonance is determined by analyzing the resonant frequency.It provides a theoretical basis for adjusting the modal impedance of the system components and reducing the harmonic resonance.Harmonic resonance sensitivity analysis,not only conducive to the treatment of system resonance,but also for the design of system components to provide a basis for parameters.(3)Aiming at the problem of harmonic resonance suppression,a suppression strategy based on inverter output impedance remodeling is proposed.First,a variety of remodeling forms are listed,and the equivalent control topology of remodeling is analyzed.Taking the parallel impedance remodeling as an example,the realization form of the control is analyzed concretely.Through the loop splitting and optimization,an equivalent control realization method is found.The effect of parallel impedance remodeling is revealed by theoretical analysis,PSCAD/EMTDC simulation and RT-LAB experiment.