Research On The Application Of New Clean Energy In Rail Transport Systems

As a basic national industry,rail transport plays an important and unique role in promoting economic development and improving people’s quality of life.The need for conventional energy in the traction system has grown significantly as a result of the rail transport industry’s quick expansion,and it is impossible to overlook the issues of high carbon emissions and energy scarcity brought on by its high energy demand and energy-intensive construction.The full exploitation of these new clean energy sources and their application in rail transportation can perfectly solve the problems of high energy demand and high carbon emissions in rail transport.However,there is less research on the application of new clean energy in rail transit systems,and there are problems with the power quality of the rail transit system after new energy access,the mismatch between clean energy generation and load,unreliable power supply,and low converter transmission efficiency.This paper takes rail transport systems as the research object and focuses on the application of new clean energy sources in rail transport systems,mainly including the integration scheme and topology of different rail transport types with adapted new clean energy sources,and the design of system optimization methods and control strategies.Depending on this,the thesis’s primary creative results are outlined below.(1)For the split-phase traction power supply system of the electrified railway,the three-phase access scheme of distributed photovoltaics along the railway line based on V/V transformer is designed;the relationships of the threephase currents and three-phase voltages between PV power generation units,the traction network and the utility grid are analysed;the conclusion is drawn that without phase sequence separation,the converter current references can be rationally configured directly in the three-phase static coordinate system;and it provides the basis for formulating the current control strategy of photovoltaic converter.For non-electrified railway power systems,the power structure and topology of the hydrogen-powered locomotive is designed as "traction network+hydrogen fuel cell+lithium battery";under the premise of considering reliability and economy,the hydrogen-electric dual operation mode is adopted.(2)Two current control strategies for photovoltaic power generation are proposed to solve the problem of unbalanced three-phase currents in the utility grid caused by negative sequence currents in the traction load.A single-phase current control strategy of the PV converter is proposed for the operation mode in which the PV power generation gives priority to supplying power to the locomotive load,which can realize the compensation of the negative sequence current of the locomotives.According to the relationship between the load current and the PV converter current,an asymmetric current referene is configured to supply power to the single-phase locomotive;according to the relationship between the utility grid current and the PV converter current,a symmetric current reference is configured to deliver the excess PV active power to the utility grid.For the operation mode in which the PV unit delivers the full amount of active power to the utility grid,a phase-separated current control strategy for the PV converter is proposed to achieve negative sequence current compensation for the locomotives.The positive sequence current coference is configured to ensure that the PV active power is fully output to the utility grid;according to the relationship between the negative sequence current of the locomotive and the PV converter current,the negative sequence current reference is configured to compensate for the negative sequence current generated by the locomotives;in order to prevent the superposition of the positive and negative sequence current references from causing over-current problems in the converter,a current limit control link is added based on the capacity constraint of the converter.Based on the RTLAB experimental platform,the accuracy and effectiveness of the proposed two control strategies are verified in terms of both grid connection of PV power generation and negative sequence compensation of locomotives.The effectiveness of the two suggested control methods for grid-connected PV and traction load negative sequence compensation has been shown in hardware-inthe-loop experiments on the RTLAB test platform.(3)A flexible switching control technique for Boost converters is proposed to achieve seamless switching of hydrogen fuel cells betweent the grid-connected operation and standalone operation.Firstly,the DC bus deviation from the rated value that will be caused by the instantaneous standalone of the traction network is used as the switching signal of the grid-connected/standalone,instead of the switching state identification signal in the traditional control,to make up for the defect of the transient process and overcurent in the switching that leads to the degradation of power supply reliability.Then,based on the droop control theory,a power loop with the DC bus voltage difference value as the control object is constructed for power redistribution,and it is combined with the traditional power outer loop as a new outer loop control to unify the control strategy in gridconnected and standalone operation modes.Finally,the corresponding hardwarein-loop test platform is built using RTLAB software,and the test is verified under the mixed operation conditions of standalone to grid-connected switching,gridconnected to standalone switching and step change of minimum power tracking value,which verifies that the proposed strategy has good performance in seamless switching and power tracking.The performance of the proposed strategy in seamless switching and power tracking is verified.(4)Taking a bidirectional full-bridge LLC resonant converter as the object,a variable frequency-duty cycle double-bridge arm asymmetric control strategy is proposed to improve the light-load transmission efficiency of the bidirectional full-bridge LLC resonant converter and to achieve the effect of efficient utilization of limited energy in the full-load range of lithium batteries.Firstly,starting from the proportionally significant switching losses and transformer losses,the duty cycle and switching frequency are analyzed as the two important factors affecting the light load efficiency.Secondly,the variable frequency-duty cycle double-bridge arm asymmetric control approach is suggested,and its underlying operating concept is examined.Thirdly,based on the time domain analysis method,the mathematical model of LLC under this control strategy is established,and the relationship between voltage gain,switching frequency and duty cycle is derived,and the optimal duty cycle is determined with the minimum loss as the operation target.Finally,the feasibility of the proposed control strategy in improving the light-load efficiency of the circuit is verified based on MATLAB simulation.