Techniques for Localization of Insulation Degradation along Medium-Voltage Power Cables
Sammanfattning: Time Domain Reflectometry (TDR) has been used for localization of transmission line discontinuities in diverse applications. Various challenges have to be addressed when applying TDR for localization of insulation degradation in power cables. The dominant problem is that TDR detects reflections coming from insulation changes, but it also records reflections related to geometrical irregularities along the cable length and, in some cases, surrounding medium variations. It is realized that one can compare TDR measurements before and after changes have occurred or have been induced in the aged sections. Thus the signals from the deteriorated insulation can be distinguished from other reflections that are constant.The measurement technique to localize water-treeing in cross-linked polyethylene (XLPE) insulation is based on the voltage nonlinearity of water trees. TDR measurements have been performed before, after and during the application of high AC voltages. The high stresses affect the cable characteristic impedance and speed of wave propagation in the water-treed region. The possibility to detect local changes in the velocity of wave propagation has been presented as a technique for localization of insulation ageing. Localization of impedance changes related to water treeing and water ingress in mass impregnated paper insulated cables has also been achieved.To characterize the power cables in the high frequency domain two parameter extraction methods have been developed. The first one uses TDR measurements and analyses short pulse propagation along the cable. The second one is based on the S-parameter measurements in the frequency domain. The methods extract the complex propagation constant and the characteristic impedance, as well as the LCRG Telegrapher's Equation parameters for the frequencies between 300 kHz and 300 MHz.The dependence of the high frequency power cables properties on the metallic screen design and cable surrounding medium has been studied. The cable characteristics are modeled using time- and frequency-domain numerical simulations. This is required due to the complex spiralized structure of the outer metallic screen. It is established that this screen design causes a dependence of the cable characteristics on the surrounding medium.
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