| The track transportation security is a huge complex system, there are many factors affecting its running security, and the power supply is one of major factors. The direct current (DC) feeder cable is one of the key devices for the safety operation of mass transit system. The insulation of DC feeder cable is serious hidden trouble factor threatened the operation safety of mass urban transit. Therefore, carrying out research on the insulation diagnosis technology of the cable to getting the insulation status of the cable timely not also embodies the economic benefit but also adequate social benefit.At present, the testing methods for the insulation of DC feeder cable are mainly off-line measuring resistance of cable periodically and kinds of protections alarming. Firstly, the thesis reviewed the current diagnosis technology on the insulation of DC cable of mass urban transit. The results of analysis indicate the implemented diagnosis technology, off-line periodic testing and motion alarming of kinds of protection, didn't meet the demand of safety operation of urban transit, therefore the new fault predicting technology, on-line monitoring detection, shall be adopted.But at present the on-line monitoring technology was concentrated on AC cable, the relevant research on DC feeder cable was scarcely carried out around the world, and the on-line monitoring technology on AC cable can not be adopted. Then, in the thesis, the faults reasons were analyzed and fault models with real project were set up started with the faults of the DC feeder cable. To meet with the DC feeder cable without ground wiring, different DC current method was proposed to solve the problem of DC leakage curren on-line detection of the cable. The paper still studied in primary the feasibility of low frequency superposition method to detect the DC feeder cable. It not only explained the working theory of the system, but also presented some problems to be solved.For the DC leakage current could not effectively evaluate the state of DC feeder cable insulation layer with cut or crack, the discharge theory and its characteristic were studied in the DC feeder cable under special condition, and the DC discharge was proposed to detect the insulation cut or crack fault in DC feeder cable. The results of research show that the proposed method could effectively diagnose the cut or crack fault by detecting the discharge in the DC feeder cable.The thesis discussed and studied the potential interference signals in detecting discharge for the DC cable. The results of study showed that there were many interference signals in discharge detection signal for the DC feeder cable, which determined by the special electric wiring of the cable. The results of laboratory simulation and field measurement showed that the main interferences involved discharge pulse generated by other equipments of the traction substation, white noise, rectifying pulse generated by the rectifying equipments, current pulse generated by the braking of the locomotive,stochastic pulse signal, and narrow-band signal generated by electric equipment. Then the time-frequency characters of aforementioned interference signals were studied and the anti-interference steps were proposed and the effect was given.The thesis studied and constructed the diagnosis expert system of DC feeder cable insulation. The expert system, worked on rule, adopted the discharge signal and DC leakage current value as its characteristic parameters, and then diagnosed the insulation state of DC feeder cable by the values of the characteristic parameters.An on-line monitoring system was developed on the project background of Shanghai metro.The developed monitoring system evaluated the insulation state of DC cable by synthesizing the monitoring its discharge signal and DC leakage current signal. The results of laboratory experiment and filed operation indicated that the system operated stably and could credibly evaluate the insulation state of DC feeder cable.
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