Research On The Mechanism Of Frequency Coupling Oscillation And Suppression Method Of Three-phase Grid-connected Inverter

In the context of the China’s carbon policy,distributed power generation systems have been widely adopted in order to alleviate the world’s resource scarcity.However,because the current renewable energy development projects are mostly concentrated in remote areas,a large-scale transmission line needs to be built between the grid-connected inverter and the grid,which in turn causes the line impedance to be introduced into the power transmission network.The grid at this time cannot be regarded as an ideal voltage source,which is called a weak grid.Under weak grid conditions,the d-axis impedance and q-axis impedance of the inverter will couple with each other because the controller of the inverter is asymmetric in the dq-axis.In addition,the inverter impedance and grid impedance will also couple with each other,which in turn generates frequency coupling oscillations and endangers the stability of the grid-connected system.In this regard,a three-phase grid-connected inverter under weak grid conditions is studied in this thesis.Firstly,the small-signal admittance models of single inverter grid-connected system and parallel multi-inverter grid-connected system are developed under dq-axis,and both small-signal admittance models consider DC voltage loop,phase-locked loop and current loop.Then,the mechanism and characteristics of frequency coupling oscillations in two gridconnected systems are analyzed based on the admittance model.The generalized Nyquist criterion is used to analyze the discrete stability of the two grid-connected systems,and it is concluded that the systems are unstable when the frequency coupling phenomenon occurs.In order to suppress frequency coupling oscillations,a suppression device is introduced in this thesis and an adaptive control strategy applied to the device is proposed.Compared with the conventional frequency coupling oscillation suppression method,this method does not require complicated design steps.The method can be applied to grid-connected systems without changing the device parameters when the operating conditions of the grid-connected system change so that the frequency coupling oscillations show a different mode.The device is also suitable not only for single inverter grid-connected systems but also for parallel multiple-inverter grid-connected systems.The device can be flexibly connected to the grid,which is highly advantageous in terms of convenience and has the feature of plug-andplay.In this thesis,the general design and working principle of the suppression device and its adaptive control strategy are explained in detail,and the small-signal admittance models of two grid-connected systems with the suppression device are developed.The discrete stability analysis results show that the stability of both grid-connected systems is improved after adding the suppression device.In order to verify the effectiveness of the proposed suppression device and its adaptive control strategy,this thesis uses Simulink to simulate and analyze the stability and dynamics of the two grid-connected systems,and adds a simulation comparison with other advanced suppression methods.Then the above simulations were experimented using RT-Lab real-time simulation system,and the experimental results proved the correctness of the simulation analysis.