Research On Resonance Mechanism And Suppression Strategy Of Distributed Grid-Connected Photovoltaic System

With the integration of large-scale renewable energy represented by distributed photovoltaic(PV)in low-voltage distribution network,the proportion of nonlinear power electronic devices such as grid-connected inverter is increasing,which may lead to wide-band harmonic disturbances in the system,so the risk of harmonic resonance in the grid-connected system cannot be ignored.At the same time,the weak grid environment is more obvious in the low-voltage distribution system,and the dynamic change of the grid impedance aggravates the harmonic resonance problem of the grid-connected system.If the resonance risk cannot be resolved,it may cause power quality problems such as voltage violation,and serious accidents such as large-scale offline of PV,equipment burning.Therefore,it is important to research the resonance mechanism,influence characteristics and suppression strategy of distributed grid-connected PV system for the safe and stable operation of power system.Based on above,the impedance model of distributed grid-connected PV system is derived in this paper,the influence characteristics of inverter and system parameter on resonance are investigated,and the resonance suppression strategy of distributed grid-connected PV system is proposed.The specific contents of this paper are as follows:(1)Aiming at the low-voltage distribution network scenario with the integration of large-scale distributed photovoltaic,the equivalent impedance models of single-phase and three-phase grid-connected system are derived respectively,and the influence of PV on system resonance under different aggregation structures is investigated.The equivalent impedance model of single-phase LCL inverter based on frequency domain analysis method is derived.Then,the difference of model and control principle of three-phase inverter between dq and αβ coordinate systems is analyzed.In order to reduce the complexity of the model,the Norton equivalent model of the three-phase inverter is derived in the αβ coordinate system.Furthermore,considering multiple types of photovoltaic aggregation topologies,the equivalent impedance models of grid-connected systems were established.Finally,the transfer function from multi-source harmonic disturbance to grid connected current was derived,revealing the harmonic interaction characteristics between photovoltaic and power grid under different operating conditions,as well as the resonant coupling mechanism of the system under different topology.(2)Aiming at the harmonic resonance problem of power system caused by distributed PV,a resonance evaluation and analysis method of distributed grid-connected PV system based on modal analysis is proposed.Firstly,the node admittance matrix model of grid-connected system with different number of inverters of the same type and different combinations of various type is established.The influence of the number and type of inverters on the resonant mode of the grid-connected system is qualitatively analyzed based on the modal analysis method.Then the influence mechanism of the parameters such as grid impedance,filter parameters,local load and current loop control on the harmonic frequency and modal impedance peak is analyzed.Based on the sensitivity analysis method,the participation degree of different nodes and different branch components in different resonance modes is quantitatively described;Finally,the harmonic resonance evaluation method for distributed grid-connected PV system is established,and the validity of the modal analysis method and the accuracy of its results are verified base on simulation.(3)Aiming at the harmonic resonance suppression problem of distributed grid-connected PV system,considering the excitation difference of different resonance modes,the resonance suppression strategy of grid-connected system is proposed from two aspects of optimizing inverter parameters and improving control strategy.Aiming at the resonance modal caused by harmonic current of inverter,the parameter joint adjustment strategy based on double-sensitivity is proposed to solve the problem that the parameter adjustment object is not clear.Based on the participation factor and Sobol sensitivity analysis method,the accurate location of the resonance dominant inverter and its dominant influence parameters is realized.Then,aiming at the low-frequency resonance of grid-connected current excited by grid background harmonic,a low-frequency resonance suppression strategy based on inverter impedance remodeling is proposed.In order to solve the phase lag problem in the quasi-resonant region of traditional voltage feedforward control strategy,a phase-lead compensation method based on Lead-link is proposed,which achieves the increase of impedance amplitude of inverter in low-frequency band and the enhancement of phase margin in the quasi-resonant region.Finally,the effectiveness of the proposed resonance suppression strategy is verified based on simulation.