Research On Impedance Adaptive Dual-Mode Control Of The Grid-Connected Inverter For High-Penetration New Energy Generation

High-penetration wind power,photovoltaic and other types of new energy power generation are characterized by intermittent,random and output fluctuations,so the equivalent grid impedance often fluctuates greatly,which makes the grid presents weak grid or even extremely weak grid characteristics,bringing severe challenges to the stable and efficient operation of new energy grid-connected power generation system.Recently,the grid-connected inverter(GCI)used in new energy generation mainly adopts the current source mode(CSM)control.The CSM control can not only maximize the utilization rate of new energy,but also ensure the high quality of output power.However,the large fluctuation of grid impedance under the condition of high penetration will lead to the decrease of system damping and stability margin,resulting in the emergence of resonance even instability of the GCI.In order to solve this problem,this paper analyses the stability mechanism and characteristics of the GCI under CSM and voltage source mode(VSM),studies the method of identifying the rate of penetration of new energy generation system based on fundamental grid impedance estimation,and proposes the impedance adaptive dual-mode control of the GCI for high-penetration new energy generation,which realizes the stable operation of the system under the condition of the large fluctuation of grid impedance.The main research work and innovation are described as follows:1)Aiming at the stability problems of the CSM-GCI and the VSM-GCI in a weak grid,influences of the gird voltage feedforward(GVF),the phase-locked loop(PLL),the power transmission monotony and power loop damping characteristics on the stability of the two modes of the GCI are analyzed respectively,and it is concluded that the stability of the two modes is complementary.2)Aiming at the problems of improved control strategy of the CSM-GCI in a weak grid,the improved control strategy based on the GVF control strategy and the q-axis output impedance reshaping control strategy are proposed respectively based on the small signal model of the GVF and the PLL,which improves the stability of the CSM-GCI effectively.3)To solve the problem of the stability region and the impedance adaptive switching control of the dual-mode controlled GCI,a method of identifying the rate of penetration of the new energy generation system based on the fundamental impedance estimation is proposed,and the stability region of multi-parameters for the two modes are obtained based on the D-partition method respectively.On this basis,the stability boundary of the dual-mode controlled GCI under the condition of the large fluctuation of short circuit ratio(SCR)is analyzed,and the undisturbed switching control strategy between the two modes is given as well.4)In view of the stability of the grid impedance adaptation dual-mode control strategy and the minimum capacity ratio of the VSM-GCI in the multi-inverter system,the switching boundary between the two modes in the multi-inverters system and the stability under different capacity ratio of the VSM-GCI is obtained based on the modeling with Dpartition method and the state matrix method,respectively.Finally,a 2MW full power electronic high-penetration distributed generation experimental platform is built.Through this experiment platform,the stability of the CSMGCI and the VSM-GCI,the improved control strategy of the CSM-GCI,the grid impedance adaptation dual-mode control strategy of the single GCI and the impedance adaptation dual-mode control strategy of the multi-inverters system is verified,respectively.