Research On Regenerative Braking Energy Absorbing System For Urban Railway

Working as generator when generative braking, the traction motor of metro feedbacks power to DC traction network. Once the power cann’t be absorbed completely, it will lead to the voltage of the DC network increasing quickly even beyond the voltage rang, which will threaten the safety of DC electrical equipment. Compared with other power absorbing way such as Resistance absorption and Super capacitor energy storage, the manner of feedback with inverter not only can effectively absorb the regenerative braking energy with the achievement of the secondary use of energy, but also have the advantage of high degree of localization and low cost. Therefore, the study on the regenerative braking energy absorption system using feedback for urban railway was initiated.After introducing two kinds of metro external power supply scheme, which includes the concentration power supply scheme and the dispersed power supply scheme, the principle and operation of inverter feedback scheme in different power supply were analyzed and discussed. In consideration of the situation that the normal voltage fluctuations and the train suddenly existing will lead the existing generative braking judgment method which uses the traction supply outlet voltage to misjudgment, the existing method was improved with traction current which has different characteristic from acceleration. Then, the principle of LCL filter, plus with two ways including damping and active filter to restrain resonant peaks were introduced by comparison and analysis. After that, based on the presentation of the mathematical model of two levels inverter with LCL filter, the control strategy was introduced. By using the case of two grid-connection inverter parallel operation, the cause of circulation current was analyzed and following the accordingly introduction of the method to eliminate circulation current and balance output power with master and slave control and droop control strategy. Finally, after the introduction of the modeling of the DC traction system that adopts the concentration power supply scheme, the locomotive which consists of induction motor and diode-clamped PWM three-level inverter that modeling with SVPWM in 60° axis, and the grid-connected inverter which feeds back power to 35kV grid, with the simulation that generative braking and suddenly quit for single locomotive, the method of generative braking judgment was verified, after that the integrated simulation with all model and the judging method was finished.