Research On Application Of Series Active Damper In Grid-Connected Inverter System

With the increasing scale of the distributed power generation,the grid which inverter connected to gradually turns out to be a weak grid.Even if the grid-connected inverter is designed to be stable,when connected to a weak grid through the point of common coupling(PCC),wide frequency range harmonic resonance even instability may be triggered due to the wide variation of the grid impedance.In order to improve the adaptability of the gird-connected inverter to the weak grid,this thesis studies the application of the series active damper(SAD)in grid-connected system.First,the cascaded-system impedance-based stability criterion is utilized to analyze the stability conditions of single and multi-parallel grid-connected systems.It is pointed out that the minor loop gain of the grid-connected inverter needs to satisfy Nyquist stability criterion under the weak grid.Based on this,the harmonic resonance mechanism of the grid-connected system with classic current control scheme under the weak grid is revealed.Further on,it is proposed that a series impedance can be connected to the PCC to reshape the minor loop gain of the grid-connected system.In this way,harmonic resonance can be damped and system stability can be enhanced.This series impedance can be emulated by a series active damper.Then,the topology and control strategy of the SAD which applied to the grid-connected system are introduced,and equivalent impedance model of the grid-connected system is established.In order to further enhance the damping capacity,an adaptive tuning method of the virtual impedance is proposed on the basis of utilizing the output impedance of the SAD itself.This method adaptively regulate the virtual impedance by detecting the resonant components in the grid current,which ensures the stability of the system and reduces the power loss.For the problem that the virtual impedance cannot accurately simulate the target resistance,a compensation function and a quadratic notch filter are proposed to optimize the characteristic of the virtual impedance.Moreover,the series and parallel active damper are compared from the aspects of suppressing LCL-filter resonance and attenuating the influence of grid voltage background harmonics.It is indicated that the grid-connected system with a SAD has better performance.Finally,the prototype of a 1kVA SAD is built and the design method of the main circuit,control loops and software is presented.The correctness of the proposed impedance model is verified by impedance measurement.Meanwhile,Saber simulation and experimental research are conducted.The results verify the feasibility of the proposed structure and control scheme of the SAD.Harmonic resonance can be effectively damped out and stability can be enhanced for the grid-connceted system.