Research On Line Measurement Method Of The Insulation Fault Of XLPE Cable

As the widely used of XLPE cable in power grid , the using quantity and length of cable has greatly developed, but the number of fault has been increased. In order to be able to predict the developmental direction of the XLPE cable insulation, on-line monitoring of the cable insulation fault has come out, and has developed well.This paper, first, on-line monitoring methods of XLPE cable insulation fault over introduced(DC component method, DC superimposition method, AC superimposition method, Dielectric loss method, Harmonic component method, Earth line current method, etc). Next, the designing program which takes the DC component method as the main point, and the earth line current method as the supplementary aspect through research of the above different kinds of on line monitoring method and analysis the theory. Finally, the software system is designed which takes the microcomputer as the main point, including I/V Conversion, 3-stage amplifier, Band-stop Active Filter, Low-pass Active Filter, A/D Conversion and peripheral circuit (display, alarm, keyboard ) etc, and respectively designing corresponding Modularization program, display module, alarm module and data analysis and diagnosis module, etc.This insulation signal needs high gathering precision due to the main gathering of this on line monitoring system is nA current signal. Three-stage amplifier are adopted, the first stage is very important adopting differential amplifier, working under the amplifying model; The second stage adopts same- phase amplify and has some filter effect; The third stage adopts reverse phase amplifier, and has certain adjustable effect. Normally the noise signal focuses on the amplifier's first stage, in order to make sure the preamplifier working in linear region, the first–stage amplifier should be selected properly. So instrument amplifier AD620 is chosen,the amplifier is 7 times by adjusting the R x.Considering the power line interference is the main interference of XLPE cable insulation on line monitoring, although preamplifier electric circuit has great restricted effect of common mode interference, a part of power line interference enter the monitoring system as differential mode signal, we must filter it. Band-stop Active Filter and Low-pass Active Filter are adopted to filter it.The XLPE cable insulation fault on line monitoring system designed in this paper abides by modularization, hierarchical, having great scalability and small current measure reference function. XLPE cable insulation fault on line fixed-point is reserved for system expansion, In order to offer convenient factors for later development.