Research On Cluster Resonant Mechanism And Suppression Method Of Photovoltaic Inverters

When a cluster of photovoltaic inverters is connected in parallel to the weak power grid,a complex high-order network will be formed by the coupling between multiple photovoltaic inverters and the power grid.This leads to the existence of multiple resonance points in the photovoltaic inverter cluster system.When the harmonic current frequency and resonance frequency of photovoltaic inverter output are the same,the harmonic resonance problem will occur in photovoltaic inverter cluster system.In serious cases,it may even endanger the stability of the control system and cause system disassembly.By connecting the active damper in parallel at the common connection points,the output impedance of the photovoltaic inverter cluster system can be reshaped to suppress the harmonic resonance problem of the system,reduce the voltage harmonic magnitude at the common connection points and improve the stability of the system.The main research contents are as follows:Firstly,the harmonic resonance of photovoltaic inverter cluster system is discussed.Aiming at the problem that photovoltaic inverter cluster is easy to lose stability due to harmonic resonance,impedance analysis method is adopted to analyze the problem.When the minimum gain of the system does not meet Nyquist criterion,self-stabilizing photovoltaic inverter connected in parallel to weak power grid may lose stability due to harmonic resonance problem.In view of the variable resonant frequency of photovoltaic inverter cluster resonant system,the analysis of frequency characteristics shows that the photovoltaic inverter cluster system usually has two resonant spikes.One of the resonant frequencies is fixed,and the other one will decrease with the increase of the parallelist units of the inverter.Secondly,an improved active damping control method is proposed to solve the harmonic resonance problem of photovoltaic inverter cluster.The impedance remolding of photovoltaic inverter cluster is realized by simulating the external resistance characteristics.The active damping control method can adjust the value of the virtual resistance adaptively according to the droop relation of the virtual resistance and achieve the goal of equal harmonic power sharing.Then,aiming at the problem of loss of the resonant frequency is bigger,the thesis further puts forward a kind of active damping control method based on frequency control,this method can make the damping resistance accurately access resonance frequency,thus more effectively inhibit the resonance problem of photovoltaic inverter cluster,and reduce the power loss at the resonant frequency.Finally,the cluster model of photovoltaic inverters is built with Matlab/Simulink platform and the simulation verification is carried out.The results show that the improved active damping control method is effective.