A comparative analysis of loss current obtained by measuring circuits used in studies of degradation power cables

Abstract

Abstract-It is widely recognized that water tree degradation would cause serious problems and it is generally agreed that is one of the most hazardous factor in the life of XLPE power cables and hence a major cause of MV power insulation premature failure. Such accidents affect the power supply system operation and reliability. Therefore, many techniques have been developed and employed in order to detect water tree at the early stages. In this context, the loss current technique is a suitable method for investigating slight degradation, since the cable degradation by water tree gives rise to harmonics components in the loss current. A comparative analysis of the values of various performance parameters that has been used in studies to evaluate the water tree degradation in power cables, obtained from the different loss current measuring circuits, is presented. This comparative analysis is made through a mathematical procedure and confirms the possibility to identify, with a very good accuracy, the loss current only from the measured leakage current without the need for specific measurement circuit used to distinguish the loss current that flows in the insulation layer. Furthermore opens the possibility to use the Fryze's equivalent conductance to eliminate in real cases the fundamental capacitive current of the total leakage current and perform online diagnosis only from the applied voltage and total leakage current measurements.

A Contribution for the Diagnosis of Insulation Cables due different conditions of degradation, voltage stress and frequency.

“…This procedure does not consider the possibility of harmonic capacitive current, because only the fundamental frequency is considered in the capacitive current [17].…”

“…6. θ ଷ variation due to degradation This relation will be represented by a polynomial of order 11, to eliminate the capacitive harmonic components in the current as shows in the adopted reference in this paper [17]. To eliminate these harmonics components, the methodology presented in [18], will be useful in the current signal identification delay, which ensures the value of θ ଷ expected.…”