Abstract
Cable manufacturing industries are constantly trying to improve the electrical performance of power cables. During the years, it was found that one of the most relevant degradation factors influencing the cable lifetime is the presence of space charge in the insulation layer. To detect the accumulated charge, the pulsed electro-acoustic (PEA) method is the most used technique. Despite the wide use of the PEA cell, several issues are still present. In particular, the PEA output signal is strongly disturbed by the acoustic waves reflections within the PEA cell. This causes the distortion of the output signal and therefore the misinterpretation of the charge profiles. This, in turn, may result in an incorrect cable characterization from the space charge phenomenon point of view. In 2017, due to the proved degradation effect of the space charge accumulation phenomenon, the IEEE Std 1732 was developed. This standard describes the steps to be followed for the space charge measurement in cables specimens during pre-qualification or type tests. Therefore, cable manufacturing industries started to take a particular interest in these measures. In the light of this, the aim of the present work is to highlight that the enacted standard is not easily applicable since various problems are still present in the PEA method for cables. In particular, in this work, the effect of multiple reflected signals due to the different interfaces involved, but also the effect of the signal attenuation due to cable dielectric thickness, as well as the effect of the PEA cell ground electrode thickness in the output charge profile, are reported. These issues have been demonstrated by means of an experimental test carried out on a full-size cable in the Prysmian Group High Voltage laboratory. To better understand the PEA cell output signal formation, a PEA cell model was developed in a previous work and it has been experimentally validated here. In particular, simulations have been useful to highlight the effect of the reflection phenomena due to the PEA cell ground electrode thickness on the basis of the specimen under test features. Moreover, by analyzing the simulation results, it was possible to separate the main signal from the reflected waves and, in turn, to calculate the suitable ground electrode thickness for the cable specimen under test.
Highlights
Energy efficiency in both high voltage alternating current (HVAC) and high voltage direct current (HVDC) transmission systems strictly depends on the cable design
Cross linked polyethylene (XLPE) material was found to be the best solution for these systems, as it has a high dielectric strength, low dielectric permittivity, and low electrical resistivity compared to other dielectric materials [7]
Based on its satisfactory characteristics for HVAC applications [8], in the last decades this dielectric material has been used in HVDC systems [9,10,11,12]
Summary
Energy efficiency in both high voltage alternating current (HVAC) and high voltage direct current (HVDC) transmission systems strictly depends on the cable design. It is important to evaluate before or after a space charge measurement the acoustic wave behavior with the aim to establish the temporal location of both main peaks as well as their reflections For this aim, in a previous work, a PEA cell model for flat specimens has been developed by the authors [27,28]. The changes essentially concern the introduction of the three layers specimen representing the cable (one dielectric and two semiconductors layers), as well as the different PEA cell component dimensions In this way, the spurious signals can be separated from the original one and the evaluation of the accumulated charges becomes more accurate. This phenomenon is influenced by several factors, such as the voltage level, the duration of the applied voltage, the contact between dielectric material and semiconductor layer, as transmission [7].Charge
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