Research On Topology And Control Technology For AC-DC-AC Traction Substation Based On Series And Parallel Of Converters

The single-phase,nonlinear and impact characteristics of the traction loadresult in the power quality problems,such as negative sequence current,reactive power and harmonics.These problems will effect the normal operation of power grid,and even degrade its supply voltage.In addition,the neutral sections do not allow the train to maintain the durative current collection,which results in the loss of traction force,and restrict the speed of the train.Thus,it will affect the transport capacity of the train.These problems have existed since the birth of electric railways,but with its rapid development today,the problems of power quality and neutral sections become more prominent.The traditional scheme does not show nice performance,and the neutral sections can not be removed.The compensentor-based co-phase power supply system can solve the power quality problems,but only the neutral sections in the outlet of the traction substation can be removed,so the whole traction network can not be linked up completely.With the development of power electronic technology,ac-dc-ac traction substation provides a solution to the power quality problems and neutral sections,and shows good prospect.AC-DC-AC traction substation can solve the power quality problems comprehensively,and supports the continuous co-phase power supply,which can remove neutral sections totally.Moreover,it can dispatch the power flow of the load to improve the energy utilization.The scholars and engineers all over the world paid much attention on the topology and control of the ac-dc-ac traction substation,and Germany and Japan are the most representative among them.However,all the researches are based on the characteristics of electric railways in their respective countries,which can only provide references to our research.Combining with the situation of electric railways in our country,this paper proposes a traction substation based on cascade-parallel of converters to solve the power quality problems and neutral sections in electric railways,and conducts the study and exploration on the topology,control strategy,parallel operation and so on.Moreover,the feasibility and correctness are validated by the simulation and experiment.Firstly,combining the characteristics of traction power supply systems(TPSSs)in China and the defects of the existing schemes,the topology of ac-dc-ac traction substation based on cascade-parallel of converters is proposed.This paper discussesthe topology of traction substation in detail and introduces the structure and function of each part,as well as analyzes the performance of the power module.The topology has many advantages in the electric railway application.The topology is symmetric characteristic is very suitable for solving the negative sequence problem.The cascade-parallel structure at the output side enables adoption of the switching devices with low-voltage and low-current stress in the high-voltage and high-current occasions,so the cost is saved.The modular design makes the system have redundancy and fault-tolerance ability,which is easy to maintain.Fruthermore,the control is simple and convenient for engineering application.We design the main circuit,and compare this topology with traditional power supply systems,railway static power conditioner(RPC)and co-phase power supply system based on active power compensator(APC),as well as the modular multilevel converter(MMC)-based scheme in cost,economic benefit,overall performance,number of the switching devices,cost and the complexity of the control system to highlight its superiority.Secondly,the control scheme of the ac-dc-ac traction substation is studied,which mainly solves the special problems of the application for the ac-dc-ac system in electric railways.Combined with characteristics of traction load and the system control targets,the overall control scheme is designed.Aiming at the double-frequency fluctuation of the dc-link,this paper analyzes the defects of the traditional digital filter in the application of traction power supply,and designs a digital filtering scheme for dc-link which is suitable for the traction load.According to the proposed control targets of the input side,the working principle of the single-phase PWM rectifier is analyzed,and the mathematical model is established.The control parameters of the dual-loop which consist of the current inner loop and the voltage outer loop are designed,and the performance of the system is analyzed with bode diagram.Combined with the topology of the input side,the control strategy of the input side based on the characteristics of traction load is proposed.For the series-parallel structure at the output side,the nonlinearity and impact of traction load are considered.The control strategy of the cascaded inverters and the circulating current suppression method for the parallel structure are determined,and the control scheme for the output side is proposed.The simulation results show that the scheme can solve the power quality problems of electrified railway,and it has good output performance.Then,the control strategy for the parallel operation of the ac-dc-ac traction substations is studied.In view of the particularity of traction power supply systems,weimprove the traditional drooping control model,and the parallel operation model of ac-dc-ac traction substation based on traction network impedance characteristics is proposed.Combining with the main circuit,the parallel operation strategy considering the circulation performance at the output side is proposed.The simulation results show that the system can achieve the “plug-and-play” for the source and load,and there is no impact during the paralleled operation.It has good dynamic performance,and can automatically distribute the load power flow.In addition,the system can improve the traction network voltage level,and the output side of the substation has a good current sharing performance.Finally,a experiment prototype is built based on the proposed scheme.The circuit and control system of the prototype is described in detail,and carry out the experimental verification.Based on the low-power platform,the performance of the power module and the machine are debugged.The experimental results generally match the simulation ones and the system can connect the power grid friendly.In addition,the system can respond to the abrupt change of the load rapidly,and has good dynamic performance.