Modeling And Stability Analysis Of Multi-inverter Parallel System Based On Floquet Theory

As an important interface between renewable energy and AC grid or load,parallel inverters are usually widely applied in renewable energy power generation systems.However,the interaction between modules in paralleled inverters system may make system suffering instability problem,even crash.Therefore,the modeling and stability analysis of parallel inverters system has become a key technical issue.In terms of system stability analysis,the traditional frequency domain stability analysis based on small-signal model need to simplify the closed-loop transfer function of system,which resulting the formulas obtained are often high-order polynomials.When analyzing the stability of more complex power electronic systems like parallel inverters system,not only does traditional frequency domain stability analysis brings tedious calculations,but also it is difficult to obtain the general model of the system.In terms of system stability improvement,the traditional method based on frequency domain improves system performance by adding passive or active damping control.It also need to derive the transfer function of the system,which has certain limitations in calculations.Compare with above all,this paper study the stability of parallel inverters system based on Floquet theory in time domain and obtain the general time domain model of system,which not only does ensure the accuracy of the theoretical analysis results but also reduces effectively system derivation complexity.Moreover,combining Floquet theory based on time domain with parameter sensitivity analysis to improve system stability,an stability-improvement method is derived through matrix transformation in this paper,which providing a new way for improving the stability of parallel inverters system.The main research work and innovation in this paper are as follows:Firstly,in order to make the traditional frequency domain control can be analyzed based on time domain stability analysis theory,a structural block diagram combination method is proposed.Taking the traditional proportional-resonant control as an example,the time domain expression of which is derived,and the time-domain model of the single-phase grid-connected inverter with proportional-resonant control is obtained.Further,the stable range of parameters are obtained based on Floquet theory.Simulation and experimental platforms are established to verify the correctness of the theoretical analysis results.Secondly,on the basis of above all,the general time-domain models suitable for N-parallel inverters system under island operation and grid-connected operation are derived separately.Taking two-parallel and three-parallel inverters system as examples separately,the stable range of parameters are obtained based on Floquet theory.Simulation and experimental platforms are established to verify the correctness of the theoretical analysis results.Finally,in order to improve the stability of the inverter system,taking a single-phase grid-connected inverter as an example,the combination of Floquet theory and parameter sensitivity analysis is applied to derive the inverter-side inductor current feedback control method.In order to verify its applicability,the general time-domain model of N-paralleled grid-connected inverters system with improved stability is derived.Taking three-parallel grid-connected inverters system as example,the stable range of parameters are obtained based on Floquet theory.Moreover,simulation platforms are established to verify the correctness of the theoretical analysis results.