Modeling And Frequency Coupling Characteristic Research Of Voltage-type Grid-connected Converter Based On Harmonic State Space

In recent years,industries such as new energy distributed power sources,smart grids and electrified rail transit have been connected to the power grid on a large scale through power electronic equipment.During the interaction between the nonlinear grid-connected converter and the power grid,the undesired harmonics generated by the frequency coupling phenomenon bring unpredictable stability problems to the power system.The traditional linear timeinvariant modeling methods average the signal over one switching cycle,ignoring the nonlinear characteristics of the converter switching modulation process.Therefore,it is difficult to analyze the harmonic current characteristics and frequency coupling phenomenon,and can only be used to describe the harmonic instability phenomenon below half the switching frequency.The harmonic state space modeling method based on the LTP theory converts the model from the time domain to the complex frequency domain through Fourier transform,so as to realize the regularization of periodic time variables.While including all the harmonic components of the system,the complicated time domain operation process is avoided.Various frequency coupling phenomena inside the converter can be well described,with high modeling accuracy.In this thesis,the HSS method is used to model the three-phase voltage source converter,and the various frequency coupling phenomena in this system are studied based on the HSS model.The main work of this thesis can be summarized as:(1)Harmonic state space modeling and example analysis.The basic concepts of harmonic state space modeling are introduced.Taking a simple buck converter as an example,the detailed HSS modeling process is given.Based on the HSS model,the frequency coupling of various harmonics in the BUCK converter system is explored,and the relevant conclusions are verified by Simulink simulation,which lays a theoretical foundation for subsequent research.(2)The HSS model of three-phase voltage type grid-connected converter.The HSS method based on LTP theory is used to establish a mathematical model of the three-phase voltage-type grid-connected converter,and the harmonic transfer function matrix that can describe the multi-input and multi-output characteristics and frequency coupling phenomenon of the system is deduced.Comparative analysis of the advantages of choosing the LTP model:Compared with the traditional LTI model,the HSS model built in this paper considers both the LTI component and the coupled frequency component,so it has higher model accuracy.(3)Analysis of frequency coupling characteristics.Through detailed analytical modeling,it is found that there are two types of frequency coupling dynamics in the three-phase converter,which are the coupling with the fundamental frequency and the switching frequency.Using the established mathematical model,it is revealed that the asymmetric control of the controller in the dq-axis and the A/D sampling process in the digital control system are the causes of the two types of frequency coupling dynamics.In addition,through the simulation analysis,it is found that the two types of frequency coupling phenomenon often occur simultaneously in the actual system,and the secondary coupling will also produce mixed frequency coupling phenomenon.Based on the harmonic state space model,this thesis studies various frequency coupling phenomena in the grid-connected converter system,and the theoretical analysis results match the simulation data.An effective tool for analyzing frequency coupling and harmonic characteristics of converters is provided.The proposed model can be applied to analyze harmonic resonance and harmonic instability induced by unwanted harmonics generated by frequency coupling.