The Single-phase Virtual Synchronous Machine Control Strategy Of The Advanced Co-phase Traction Substation

The traction power supply system is a significant part of the railway system,which is responsible for supplying energy to the electric locomoti ves,and is a kind of special but important load of the three-phase power grid at the same time.The characters of the traction power supply system have deep influence on safe and stable operation of the railway system and on power quality of the three-phase power grid.The advanced co-phase traction power supply system can reduce all the neutral sections which exist in the traditional power supply system,and realizes the integral connection of the traction network.Therefore,the advanced co-phase traction power supply system can decrease the bad impact on negative sequence and harmony of the three-phase power grid,and is flexible to be used in different power supply systems and easy to access the interface of the distributed energy as well.The virtual synchronous machine(VSM),the output characters of which are similar to the synchronous generator,is an important control strategy to connect the inverters with power grid in present study,but it mainly used in the three-phase power grid.The single-phase VSM control strategy,which can be applied in the advanced co-phase traction substation,is studied in this thesis in order to improve the stable and reliable operation with the traction network.Firstly,the topology of the advanced co-phase traction substation is analyzed in this thesis.To improve the withstand voltage capability of the three-phase to single-phase converter in traction substation,the neutral point clamped(NPC)topology is adopted,and the working principle is introduced.Furthermore,the control and modulation strategies of three-phase rectifier and the single-phase cascaded inverter based on the NPC topology are elaborated respectively.For reducing the output voltage harmonic content of the traction substation and the need of large filter inductor,the LCL filter is chosen to connect with the output port of the advanced co-phase traction substation.Based on the actual requirement,the parameters of the LCL filter are calculated.Secondly,the math model of the synchronous generator and the working principle of the three-phase VSM control strategy are analyzed deeply.So as to apply the VSM method into the advanced co-phase traction power supply system,the single-phase VSM control strategy is deduced,based on the research of the three-phase VSM.And each model of the single-phase VSG strategy is analyzed.And in order to achieve fast and accurate tracking of the traction network voltage phase,a concise phase tracking method is designed in this thesis.Combined with the actual need,the control model of single-phase VSM is detailedly analyzed in the process of connecting with traction network and following the goal of the active and reactive power,and even the situation that the voltage amplitude or frequency of traction network is changed.Based on these analyses,it can be ensured that the single-phase cascaded inverter which used the single-phase VSM strategy as the control method has the similarly output characters,for example the frequency and amplitude control as the synchronous generator.And the comparison between the droop control and the single-phase VSG control strategy is also carried out in this thesis.Thirdly,the simulation model is built based on the Matlab/Sinulink to verify the theoretical analysis.The operation results of the three-phase rectifier and the single-phase cascaded inverter make sure that the topology,control strategies and modulation strategies are correct.Especially,the working characters of the single-phase VSM are verified under various conditions.The results of the simulations prove the correctness and validity of the single-phase VSM control strategy.Lastly,based on FPGA,the low power experiment platform is built and the controller program is written with reference to the mathematical model of the single-phase VSM control strategy.The experiments of the single module and three modules are finished,and the correctness and effectiveness of single-phase VSM control strategy is proved by the results of experiments.The research of this thesis can devote to the further engineering application of the advanced co-phase traction power supply system.