Water tree degradation phenomenon constitutes a serious problem in XLPE insulated power cables. The presence of water trees lead to harmonic components in the insulation loss current, of which analysis provides a powerful indication of the state of the degradation. The first goal of this study is to establish a procedure to recover the measured loss current waveform in XLPE cables with water tree degradation by numerical calculations, and the second is to identify the loss current component from the total current that flows in the insulation. A new methodology based on the measured loss current waveform and the parameters calculations of the suggested equivalent circuit for the insulation with water trees is presented. It allows getting the THD of the loss current, the dissipation factor (tan δ) and the power losses in the insulation (p). It is also possible to determine its equivalent capacitance (C) and equivalent resistance (R). It is shown that the proposed methodology reproduces with a very good accuracy the loss current waveforms obtained in experiments.

The spectral domain approach is employed to perform a theoretical investigation of tapered microstrip lines on magnetized ferrite substrates. A linear variation of the conducting strip width along the direction of propagation is considered. The analysis takes into account the effects of the applied dc magnetic bias field in the transmission characteristics of these structures. The properties of the taper are determined by a model based on a segmentation of the considered line into uniform microstrip line subsections. Normalized phase constants and characteristic impedances are obtained by using the Hertz vector potentials method and Galerkin numerical technique. Numerical results are presented to show the taper input parameters as a function of the load impedance, geometrical dimensions, operating frequency, and ferrite parameters, considering the orientation and magnitude changes of the biasing magnetic-field H0. The results agree fairly well with those available in the literature for tapered microstrip lines on isotropic dielectric substrates.

⎯ Questions related to power quality definitely challenges engineers all over the world. Amongst these, the matter of supply reliability emerges as an important factor towards the improvement of continuity indexes such as the duration and frequency of interruptions. In such a way, information associated to the operational conditions of equipment such as transformers, cables, etc. have a relevant role in the overall electrical complex behavior. Focusing the insulated cables operational status, the occurrence of physical degradation phenomenon such as the so called water tree appears as a common cause for cable life expectance reduction. Having in mind this target, this paper considers the basis related to this effect and describes a procedure to estimate the actual conditions of a given cable. A hardware and software structure is then presented and first experimental results are described to highlight the proposal potentiality at finding operational indexes about the component life expectance.

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.

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