Research On Port Characteristics And Stability Analysis Of MW Grid-connected Inverter

With the increasingly serious problems of environmental pollution,efficiency and sustainability of traditional power generation systems,it is an inevitable choice for the development of power resources to adjust the access to power resources with developing and utilizing renewable energy.At present,solar energy,wind energy and other new energy power generation systems have gradually matured and developed rapidly,and a large number of large and medium electricity generation systems with kilowatts and megawatts have emerged.The grid-connected inverter connects the power generation system with the grid,and the interaction between the grid and the grid-connected inverter will lead to oscillation of some frequencies,especially for high-power systems with low switching frequency,the oscillation frequency may be higher than the Nyquist frequency.This paper majors in MW grid-connected inverter,,and studies the grid-connected stability of the inverter.The main research contents are as follows:First of all,based on the characteristics of high power and low switching frequency of MW grid-connected inverter and the stability requirement higher than the Nyquist frequency,this paper makes a preliminary analysis of the grid-connected inverter system from the mathematical model,controller selection,delay link analysis and impedance modeling in each coordinate system of the grid-connected inverter,and analyzes the difference of establishing the equivalent model between the grid-connected inverter and the ordinary grid-connected inverter.The impedance models of inverter-side and gridside current control modes are also established and simplified to general impedance model,which lays a foundation for more comprehensive port characteristic analysis and stability research of stage-to-stage grid-connected inverter system.After that,on the basis of designing the parameters of the controller and verifying the system stability,when the grid-connected inverter is connected to different power grid environments,the impedance stability analysis method can be used to analyze the external stability of the grid-connected inverter.After interacting with a specific grid impedance,it will bring about oscillations in some frequencies,and in many cases,the oscillation frequency will be higher than the Nyquist frequency.Based on the theory of passivity in frequency domain,the passive interval distribution of the two control modes is analyzed and verified,and combined with the passive interval of output admittance,the reasons for the oscillation of some frequencies after grid connection are explained.Finally,in view of the instability of the system,the stability of the system is enhanced from the point of view of expanding the passive interval and increasing the positive real part of the output admittance.After using the CCF-AD based on proportional feedback and designing the proportional coefficient,it can be realized that the two current control modes can be passive in the range of 0～1500 Hz,and the real part of the output admittance of the two modes is exactly the same,but at the 500 Hz,the real part of the output admittance of the system is 0,so the passivity is fragile.Using the CCF-AD based on proportional feedback can increase the positive real part of the 500 Hz,but it can also weaken the low and high frequency passivity and reduce the passive interval.The lowpass filter can suppress the negative effect of CVF-AD on the high frequency band and keep a large passive interval while ensuring that the admittance of the 500 Hz has a positive real part of a suitable size.The final simulation results show that In the case of weak power grid,whether the oscillation frequency is lower or higher than the Nyquist frequency,the harmonic oscillation generated by the grid connection can be effectively suppressed by using the above methods;for the power grid environment with stronger power grid but not strong grid strength,the effect of the above methods is not good.On the whole,the above theoretical analysis is correct and can effectively restrain the harmonic oscillation after grid connection.