Resonance Mechanism Analysis And Its Suppression Strategy For Photovoltaic Multi-inverter Grid-connected System

Promoting the wide application of the micro grid,energy storage technology and distributed power generation technology and promoting the structural reform of energy supply side are the important content of energy development planning for the Thirteenth Five-year Development Plan.In this paper,the composition principle of the photovoltaic grid-connected inverter system is analyzed,and the impedance equivalent circuit models of the photovoltaic single inverter grid-connected system and photovoltaic multi-inverter grid-connected system are set up.Based on the models the resonance mechanisms of single inverter grid-connected system and multi-inverter grid-connected system are analyzed,and the corresponding resonance suppression strategies are proposed.The results of simulation experiments and the device platform experiments in different grid environment show that the proposed model and the resonance mechanism analysis method are all correct and effective.Paper main research work and innovation points are summarized as follows:By the conventional grid-connected control strategy and equivalent transformation of control block diagram,this paper builds the impedance equivalent circuit models of single inverter grid-conncctcd system and multi-inverter grid-connected system.The detailed calculation of model parameters is given.The models can accurately reflect the characteristics of impedance coupling between inverters and grid and between the inverters,and provides a theoretical basis for the subsequent resonance mechanism research of inverter grid-connected system.The resonance mechanisms of LCL filter interface circuit under the condition of single inverter grid-connected system open-loop and multi-inverter grid-connected system open-loop are analyzed with the different harmonic excitation sources.The change of the grid parameters and the number of grid-connected inverter will cause the resonant characteristics changes of filter interface circuit.Based on inverter impedance equivalent circuit model with the conventional PI control strategy,the resonant characteristics of the single and multi-inverter systems are studied.The change of the control parameters,grid parameters and the number of grid-connected inverter will affect the resonant characteristics of the system.The simulation experiments of the single inverter grid-connected system and multi-inverter grid-connected system in different grid environments verify that the impedance equivalent circuit models and the resonance mechanism analysis are the correctness.The analysis conclusion provides useful reference for the design of the inverter.On the basis of the conventional active damping methods,the improved active damping control method is proposed by increasing the power grid voltage feed-forward link and grid-connected current feed-forward link.The impedance equivalent circuit model of the system is deduced with the improved control strategy.Based on the models,the resonance suppression mechanism of the single inverter grid-connected system and multi-inverter grid-connected system is analyzed.The analysis results show that the capacitor current feedback link can inhibit the high frequency resonance peak effectively,but cannot inhibit the low frequency resonance peak,even make it be slightly enlarged under the condition of the part of the feedback coefficient values and expand the influence of same frequency harmonic from the grid;the grid-connected current feed-forward link can inhibit the low frequency resonance peak effectively,but cannot inhibit the high frequency resonance peak effectively;the power grid voltage feed-forward link can further reduce the influence of the power grid background harmonic sources.The improved active damping method containing the three control links can all inhibit the low frequency resonant peak and the high frequency resonance peak,eliminate sharply the influence of each harmonic source in wider frequency band,and suppress the occurrence of the system resonance.In order to further improve the adaptability of system in weak grid environment,the inverter output impedance adaptive control strategy is proposed.By the impedance equivalent circuit model of inverter grid-connected system the impedance ratio stability Nyquist criterion is given,the impedance interaction of interactive systems is analyzed,and the system instability phenomenon in a weak grid environment is judged.By the Nyquist criterion the system instability frequency band in which the system need operate stably in a weak grid environment is determined,and the phase angle compensation method based on grid-connected current differential feed-forward control is proposed.The adaptive algorithm of grid-connected current differential coefficient is given.The adaptive algorithm of differential coefficient is realized as follows:firstly the frequency spectrum of the grid-connected current is analyzed in real time and the grid impedance value range is estimated;secondly according to the grid impedance value range the differential coefficients of grid-connected current feedforward link adjust themselves and the inverter output impedance is reconstructed in order to improve the phase angle margin in weak grid instability frequency.The method meets the phase angle conditions of the system robust stability,inhibits the resonance and ensures the system operates stably.The results of grid-connected simulation experiments and device platform experiments show that the impedance adaptive control strategy is correct and effective.