Research Of Online Leakage Current Monitoring System For High Voltage Cables Of The High Speed Railway

Compared with normal train, the high-speed rail has the characteristics of heavy load and frequent load change; therefore higher requirements should be put forward for the traction power supply capacity and quality. As the connection between transformer and overhead line, the parallel cables connection mode is used as a common method in traction power supply system. However, cable faults will inevitably occur in the operation. Different types of faults have corresponding fault detection method. But the traditional detection methods are basically put forward on the basis of common power cable faults, which needs to extract the fault information from the grounding current of cables. Compared with the ordinary power system, there exists substantial harmonic disturbances in the traction power supply system, so it is difficult to extract the signal. Therefore this paper proposes the real-time detection of cable current in the head and tail. The cable leakage current is calculated by the difference of the two currents. And according to the growth trend of this current, it can be determined whether there is cable fault or potential safety hazard, such as local damage fault, overall aging fault and parallel gap fault etc. All the faults can be reflected by the growth trend of leakage current. The research includes the following contents:First of all, based on modeling and theoretical analysis with the single cable aging phenomenon, amplitude variation characteristics of cable leakage current with insulation aging is analyzed.The study suggested that leakage current is very small with slight increase in the early stage; but increase significantly in later stage. During the deterioration process, the phase difference between currents of cable head and tail will gradually increase, so the phase difference must be corrected in the detection process in order to calculate the reasonable value of leakage current. Next, theoretical analysis and calculation are carried out on three parallel operation of the current distribution imbalance of cable, the circulation of the protective layer and the induction voltage. The results show that the relative space position of cables are closely related. Therefore the cable laying arrangement must be reasonable.Secondly, the results of theoretical analysis are modeled and simulated by software ATP, including the cable, traction transformer, overhead line and locomotive.Simulation results show that with the aging of cable degradation, the leakage current will increase and the current phase difference of head end and the tail end will gradually increase.Again, according to the influence of locomotive harmonics on the cable current, the wavelet threshold filtering method based on EMD is used to filter the data. The method combines the advantages of wavelet and EMD algorithms. Through the comparison between simulation data and field data, it is concluded that this method is most effective in filtering.Finally, based on the above theory research, a set of online monitoring device for cable leakage current is developed, and do online test. It shows that this method can accurately detect cable fault.