Research On Control Method Of Active Damper In Weak Grid With Multiple Grid-connected Inverters

Under weak grid conditions,multiple inverters form a complex multi-scale,high-order nonlinear network during the grid connection process,which is easy to couple with the grid impedance to generate resonance,causing system instability and affecting the power quality of the grid.Simulating a virtual resistor in parallel with PCC(Points Of Common Connection,PCC)through an active damper can effectively dampen the resonance generated by multiple grid-connected inverter systems in a weak grid.In order to improve the stability and applicability of the active damper,this article studies the control method of the active damper.The specific research content includes the resonant signal detection technology of the active damper,the adjustment method of the virtual resistance value and the port current Three parts of the control strategy.Firstly,this paper conducts modeling and stability analysis of multiple gridconnected inverter systems in weak grids.According to the impedance analysis method,the equivalent impedance model of multiple grid-connected inverter systems under weak grids is established,and the stability condit ions of the system are analyzed on the basis of this model.It can be seen from the stability conditions that when the grid impedance changes or the number of grid-connected inverters increases,it may cause the system to resonate and affect the system sta bility.The simulation results also verify that the system will resonate when the grid impedance changes or the number of grid-connected inverters changes.The simulation analysis results are consistent with the theoretical analysis results.Secondly,this article optimizes the resonance detection method of active dampers.The traditional resonant signal detection method composed of SOGI-FLL(Second Order Generalized Integtator Frequency Locked Loop,SOGI-FLL)with fixed parameters has poor anti-interference ability.This article improves SOGI-FLL based on the results of the local stability analysis of SOGI-FLL,that is,the gain parameters in SOGI-FLL can be dynamically adjusted according to the changes in the amplitude and frequency of the input signal.The simulation results verify that compared with the resonant signal detection method composed of SOGI-FLL with fixed parameters,the optimized resonant signal detection method composed of SOGI-FLL with variable parameters has superior dynamic performance an d antiinterference ability.Then,this article optimizes the adaptive adjustment strategy of the virtual resistance of the active damper.In order to make the active damper suitable for the system with a wider range of power grid impedance,this article adaptively adjusts the virtual resistance value according to the content of the resonant voltage at the PCC,and adjusts the threshold value in the method and the proportional parameter in the resistance adjuster.And integral parameters for design.The simulation results verify that the adaptive adjustment strategy after parameter optimization can make the applicability of the active damper better.Finally,this article improves the port current control strategy of the active damper.When the traditional PR controller controls the port current of the active damper,a phase jump occurs at the resonance point,which affects the stability of the resonance current control.In this paper,a VPI controller with adjustable bandwidth is used to replace the PR controller to control the port current of the active damper,which solves the problem of the phase jump of the PR controller at the resonance point and increases the phase margin of the current loop control.The active damper can dampen resonance more stably in a wider range.The simulation results verify the effectiveness and superiority of the VPI controller with adjustable bandwidth.