Research On Resonance Suppressionstrategy Of Grid-Connected Inverter Considering The Influence Of Distributed Capacitance Of Transmission Line

As the key interface between photovoltaic,wind power and other distributed power generation units,grid-connected inverter’s safe and stable operation is directly related to the stability and reliability of distributed power generation system and power grid.This paper takes LCL grid-connected inverter as the research object,studies its stability under different working conditions such as weak power grid and grid-connected transmission line,and proposes corresponding control strategies to improve the quality of grid-connected current and improve the stability of grid-connected inverter.Firstly,the resonant characteristics of LCL filter are described,and six kinds of traditional passive damping are analyzed and compared.On this basis,the active damping method based on capacitive current proportional feedback is derived.Then,three common current control methods are analyzed,and the appropriate current control method is selected by comparing their open-loop characteristic curves.Secondly,the traditional single-loop current feedback control method is studied.The inverter side current feedback is widely used because of its lower cost and more in line with the actual situation.However,the digital control delay,grid impedance and LCL parameters will affect the stability of the inverter side current feedback system and limit the stable range of the inverter side current feedback.On this basis,the inverter side current control strategy based on capacitor voltage proportional feedforward is studied.Without considering the control delay,this strategy not only has a good suppression effect on the resonance of the LCL filter,but also has a good adaptability to the weak power grid.However,when the control delay is considered,there is still a reverse resonant peak in the loop gain of the system and it moves to the low frequency direction with the increasing impedance of the grid.Although the system can still maintain stability,the phase margin will gradually decrease,and the stability of the system will be challenged.The above analysis is based on the inverter side current feedback on the basis of analysis,but in practical applications,often pay more attention to the grid-connected current,can not be used to completely replace the inverter side current grid-connected current.Then,the output impedance model of the inverter is established from the perspective of impedance analysis,and the stability of grid-connected inverter and the quality of gridconnected current are further analyzed.The phase compensation method of the gridconnected LCL inverter based on high-pass filter is proposed.That is,a high-pass filter is added to the capacitor voltage feedforward link to improve the phase of the output impedance of the inverter,so that the phase difference between the output impedance of the inverter,the impedance of the grid and the impedance of the transmission line at the interaction point is less than 180°.At the same time,the combination controller of quasiproportional resonance controller and harmonic controller is used to suppress the background harmonics in the grid.Finally,the effectiveness of the strategy is verified by simulation and experiment.