Study On Low Frequency Stability Of PV Connected To High Speed Railway Traction Power Supply System

With the vision of "carbon peak" and "carbon neutral" proposed by China,photovoltaic power generation has become a research hotspot of renewable and clean energy.At present,China’s high-speed railway industry is developing rapidly,and low-carbon railway transportation has become the development direction of the new era.The key lies in reducing the traction energy consumption of the traction power supply system.Connecting photovoltaic power to the traction power supply system can not only improve the photovoltaic absorption capacity in the western region,but also alleviate the carbon emission problem of the traction power supply system,in line with the development direction of national energy conservation and emission reduction.However,low-frequency oscillation is an inherent problem in traction power supply systems.Photovoltaic access will exacerbate the coupling and nonlinearity of the traction power supply system,forming a complex photovoltaic vehicle grid coupling system,which is more likely to cause electrical parameter mismatch issues.The research on low-frequency stability of photovoltaic access traction power supply systems is of great significance.Firstly,the topology and working principle of photovoltaic access to the traction power supply system through a back-to-back converter structure are described.According to the circuit structure and control strategy of each module,mathematical models of photovoltaic system,back-to-back converter,traction train,and traction network are established using small signal modeling methods.The equivalent impedance of each module is given,and the correctness of the output impedance of the back-to-back converter is verified using frequency sweep method.Then,the impedance based stability analysis method is introduced,and the loop ratio matrix of the photovoltaic vehicle grid coupling system is obtained using the system source load relationship and the equivalent impedance of each module,which lays the foundation for impedance based stability analysis;The principle of the forbidden region-based criterion is described,and the influence of converter controller parameters on the low-frequency stability of the photovoltaic vehicle grid coupling system is explored using the forbidden region-based criterion.The results show that using the forbidden region-based criterion to analyze the low-frequency stability of the photovoltaic vehicle grid coupling system has a certain degree of conservatism,which is not conducive to accurately judging the stable state of the actual system.Secondly,aiming at the errors caused by the Gail circle theorem in the forbidden region-based criterion,an extended forbidden region-based criterion is proposed and its form and equivalent form are given;The influence of parameters such as converter controller parameters,photovoltaic module parallel connection quantity,and train access quantity on the low-frequency stability of the photovoltaic vehicle grid coupling system was studied using the extended forbidden region-based criterion;Based on MATLAB/Simulink simulation platform and Star Sim HIL semi-physical platform,the effectiveness and accuracy of low-frequency stability analysis based on the extended forbidden region-based criterion were tested.The results indicate that the low-frequency stability of the photovoltaic vehicle grid coupling system is influenced by various parameters,,and compared to the forbidden region-based criterion,the stability analysis using the extended forbidden region-based criterion is same as the simulation results,reducing stability analysis errors,and reducing conservatism.This has a good effect on the low-frequency stability analysis of the photovoltaic vehicle grid coupling system.Finally,the low-frequency stability of the photovoltaic connection process under single arm light load conditions is discussed,the equivalent impedance of each module of the system and the loop ratio matrix of the system under light load conditions are derived,and the impact of the converter controller parameters on the low-frequency stability of the system when the voltage and current dual closed-loop control strategy is used for photovoltaic connection is analyzed using the extended forbidden region-based criterion;In order to improve the stability margin of the system under light load conditions,droop control is introduced based on voltage and current double closed-loop control.The equivalent impedance and loop ratio matrix of the constant voltage droop control system are derived.The influence of the converter controller parameters on the low-frequency stability of the system under constant voltage droop control is compared and analyzed using the extended forbidden region-based criterion,The effectiveness of stability analysis using dual closed-loop constant voltage control and constant voltage droop control for photovoltaic systems was verified using Star Sim HIL semi physical platform testing.The results show that the introduction of sag control improves the stability margin of the photovoltaic vehicle grid coupling system under light load conditions.