Complex Vector Impedance Analysis And Stability Control Method For Three-phase Grid-tied Converter

With the rapid development of new energy generation technology,grid-tied converters have been widely used as the key components to the new energy generation grid-connected technology.The grid have weak grid characteristic when the new energy generation device is located in a remote area and the electric energy needs to be transmitted over a long distance.Because of the asymmetry of its controller,the three-phase grid-tied converter has a frequency coupling phenomenon.This phenomenon will not only affect the power quality of the system,but also affect the system stability under weak grid conditions.Therefore,the frequency coupling dynamics of the three-phase grid-tied converter,and the effect of the dynamics on the system stability under the condition of weak grid are studied in this paper.The admittance model for the three-phase grid-tied converter is first performed in the stationary coordinate system.A small signal Model based on the main Circuit of the Converter is established using complex vector representation;then considering the effect of the current loop,phase-locked loop and the DC voltage loop in the controller,a closed-loop equivalent admittance model for a three-phase grid-tied converter is established in the form of complex vector,and the model is verified by the frequency scan method.The frequency coupling dynamics of the converter is considered in the model,which can be used to analyze the frequency coupling phenomenon.The research on the stability and the frequency coupling of the three-phase grid-tied converter system is the focus of this paper.The open loop transfer function of the three-phase grid-connected converter system considering the grid impedance is deduced.The system stability is analyzed by bode diagram of the open loop transfer function.The dq asymmetry of the converter system in the synchronous rotating coordinate system will cause frequency coupling dynamics.Since only the q-axis voltage is control within the phase-locked loop and the d-axis current is control within the DC voltage loop will cause asymmetry in the dq axis.This paper focuses on the impact of phase-locked loop and DC voltage loop on the frequency coupling dynamics,and based on the system's equivalent transfer function block diagram,analyzes the effect of frequency-coupling components on system stability under weak grid conditions.According to the mechanism of frequency coupling dynamics caused by the phase-locked loop and the DC voltage loop,an improved control method is proposed by compensating the asymmetrical dynamics of the phase-locked loop and the voltage loop and suppressing the frequency coupling dynamics respectively.Before and after adding the suppression methods,the amplitude variation and admittance measurement results of the coupled admittance term in the model prove that the proposed method can effectively suppress the frequency coupling dynamics in the converter.The same simulation model was built and analyzed in Matlab /Simulink.Experiments were performed on the laboratory experiment platform of the converter.Both the simulation and the experiment verified the correctness and effectiveness of the stability analysis and frequency coupling suppression method in this paper.