| Urban rail transit which is characteristic of low pollution, low energy consumption and high efficiency plays an important role in relieving the jamming situation of the urban traffic, expanding urban space and so on, but due to the distance between the train stations is relatively short in urban rail transit, and the train starts and breaks frequently, braking energy will be generated when the train is braking. As the general DC traction system adopts the diode rectifier, power generated by regenerative braking can not flow back to AC network side, as well as excessive regenerative braking energy will lead the increase of DC traction network voltage, the safe and stable operation of train will be influenced. Therefore dealing with regenerative energy is an urgent problem, in this paper, by using high-power energy feedback device in parallel to the DC traction network, regenerative energy can flow back into the medium-voltage AC network side, thereby stabilizing the direct voltage, ensuring safe and stable operation of train and save energy. The thesis is carried out from the following aspects:Firstly, the overall structure of current urban rail transit power supply system is analyzed, which is mainly composed of external AC power supply, DC traction network system and other components of urban rail transit power supply system. From the composition of power supply system it’s clear to know all the external conditions required for train operation, and then the operation status of the power supply system during the train breaking is drawn off, and it provides direction and conditions to solve the problem of regenerative braking energy.Secondly, the analysis of train braking mode gives first place to electric braking. The principle of train regenerative braking principle is elaborated, mainly the analysis of regenerative braking principle in DC traction motor and AC traction motor. Regenerative energy is analyzed by taking the train of AC motor traction braking as an example. In view of dynamics and regenerative braking characteristic curve, a train braking energy of different initial speed is calculated theoretically. The current three scenarios, energy dissipation type, reservoir can type, energy feedback type, for dealing with regenerative braking energy are compared, so the thesis chooses energy feedback type to feed regenerative energy back into AC medium voltage network side, and selects NPC three-level high power converter as the core part of the energy feedback device.Then, by considering the size of feedback energy and the harmonics effect on the AC side, the thesis chooses diode clamped three-level PWM converter with four quadrant operation function, and mainly researches composition, control, effect of energy feedback and the influence on AC/DC net side etc. of feedback device. This paper analyzes the core part, NPC three level converter, on structure, working principle and builds mathematical mode. Through modeling and analysis control system of main converter is designed, in order to make the control strategy simple and ensure control effect, the paper adopts traditional double closed loop control scheme and uses SVPWM for pulse width modulation to output the control signal of the main converter. SVPWM neutral point voltage balance control is designed based on the capacitor charge balance control in order to improve the quality of the feedback power.Finally, through MATLAB/Simulink software simulation model of energy feedback device is established and analyzed from the system’s accuracy, stability and balance, quality of system feedback electric energy, system response speed and voltage of stable DC network side. It fully verifies the function of feedback device. |
PDF Full Text Request