Research On Resonance Instability Suppression Strategy For The Multi-Inverters Gridconnected System Based On Reshaping Of Global Admittance At PCC

Multi-inverters modularized parallel connection technique not only realizes large-scale gridconnected consumption of renewable energy but also makes the grid-connected system more and more electronic.The system controllability and flexibility are improved,but the mutual interactions between the current loops of the inverters and the grid-side passive impedances may bring in harmonic resonance instability,seriously threatening the safe and stable operation of the system.Therefore,this paper studies the resonance instability suppression strategy for the multi-inverters grid-connected system,and the research work is carried out in the aspects of stability analysis,inverter output admittance reshaping and global resonance suppression unit design,details are as follows:1.In terms of the grid-connected system modeling and the stability analysis based on resonant point: The full admittance model of the grid-connected system is established by using the Norton equivalent model of the inverter current loop,and the current response expression in the coupled environment is derived.The stability criterion based on the global admittance is improved,the global admittance is divided into two dimensions of the real part and the imaginary part,the concept of the resonant point is clarified,furtherly the stability analysis method based on the resonant point is proposed.By adopting this method,the resonance instability mechanism of the grid-connected system is revealed,and the reshaping directions of the global admittance are explored from the perspective of controlling the resonant point.2.In terms of the inverter output admittance reshaping: Firstly,the impact of the controller types and parameters on the damping characteristics of inverter output admittance is analyzed,on this basis,a tuning method for the controller parameters is proposed to avoid the inverter output admittance and the grid admittance interact in the negative-damping frequency band.Secondly,the PCC voltage proportional feedforward technique is used to reshape the inverter output admittance by connecting a paralleled virtual admittance,and a reasonable range of voltage feedforward coefficient is calculated to ensure the inverter output admittance behave passive.Lastly,the capacitor current feedback technique is used to reshape the inverter output admittance by modifying the filter structure,and the optimal feedback coefficient of the capacitor current is obtained based on quantitative calculation.The simulation models are built,and the simulation results show that the proposed methods of reshaping the inverter output admittance are correct.3.In terms of the global resonance suppression units design: A topology of the resistive passive damper is proposed,and the sub-resistor input scheme is designed to reduce power loss.An active damper is implemented with a power electronic converter,and a correction function is introduced to make its output characteristics behave resistive in a wider frequency range.With reference to the active damper,an active inductor is proposed and designed to suppress the resonance instability from the perspective of shifting resonant point,and a biquad filter is adopted to improve the output characteristics of the active inductor in the low-and middlefrequency bands.Aiming at the resonance information which is necessary for the operation of the active resonance suppression units,a resonance detection strategy based on the secondorder generalized integrator is proposed.The frequency-domain analysis results and timedomain simulation results prove that the active resonance suppression units can effectively improve the grid-connected system stability.