Research On The Low-frequency Oscillation In Traction Network Of Xuzhou North Railway Hub

Low-frequency oscillation in a traction network is a new phenomenon as with AC-DC-AC locomotives utilised in railway transportation. Different from the former stable low-frequency oscillation cases, the one of Xuzhou North Railway Hub which occurred in January 1st,2014 is particularly extraordinary because it caused abnormal voltage fluctuation. It affects the quality of traction power supply, as well as the safe operation of electric locomotives to some extent. Therefore, it is necessary to analyze further the mechanism of this case. In addition, some practical suppression methods should be put forward. Based on the measured data and simulation analysis, the paper carries out some relative researches.This paper displays some typical waveforms of related electrical quantities in the former low-frequency oscillation cases. For all these cases, when the number of locomotives gets to a critical value, line-side voltage and current signal present a sustained and stable low-frequency oscillation state. The oscillation frequency is from 4 to 7Hz. The range of voltage fluctuation is positive-negative 4kV. Then, field tests were conducted in Xuzhou North Railway Hub. We obtained the voltage and current data which were in low-frequency oscillation. To process the tested data more efficiently, the paper compiles a software TDPS. We analyze tested data by TDPS. Unstable low-frequency oscillation appears in the measured low-frequency oscillation waveforms. What’s more, there is abnormal fluctuation in the line-side voltage. The fluctuation range is wide. For example, the root mean square (RMS) of the bus voltage in Xuzhou North Substation (XNS) reaches to the smallest value 8.94kV and the biggest value 36.1kV, while the bus voltage RMS in Xuzhou North Sub-feeder Switching Post (XNSP) reaches to the smallest value 5.29kV and the biggest value 37.8kV. Then, by calculating voltage loss, active power and inactive power of a certain fluctuating time, the paper make a conclusion that the unstabe low-frequency oscillation is caused by the current fluctuation of HXD2B locomotives.Traction network low-frequency oscillation is a locomotive-network matching problem. A united model is established in PSCAD/EMTDC, combining the traction power supply system and the line-side converter of HXD2B locomotives. The simulation model of the traction power supply system is simplified as Thevenin’s equivalent circuit. The model of the line-side converter controller is established by the single phase dq-decoupling control strategy. There are 6 paralleled single phase voltage source PWM rectifiers in one locomotive. It should be noted that the controller parameters are tuned to meet the technical demands of control system. After that, the paper does simulation for the single-locomotive model and the multi-locomotive model. The simulation results reproduce voltage and current oscillation, in which the voltage fluctuates abnormally.The FFT fitting method and the single phase dq-decoupling closed-loop control mathematical model can be used to explore the low-frequency oscillation mechanism. The FFT fitting result indicates that the oscillating voltage and current are made of the fundamental and inter-harmonics whose frequencies are near 50Hz. The PI paremeters impact the system stability differently, conluded from the eigenvalue trajectory of the Jacobian matrix of the closed-loop control mathematical model as the PI parameters change. The influence of the current-loop proportionality constant KiP is the strongest, and the voltage-loop integration constant KvI comes second, and the voltage-loop proportionality constant KvP comes third, and the current-loop integration constant KiI is the weakest.In the end, suppression methods to this problem can be proposed in respects of the traction power supply system and the locomotive. Finally, simulation is done on the suppression result of reducing the equivalent electrical distance between the locomotive and the substation (or sub-feeder switching post) as well as modifying the PI parameters of the controller line-side converter.