Avaliação dos fatores de envelhecimento dos cabos cobertos de classe 15 kv e seus impactos sobre a coordenação de isolamento

Abstract

Compact Distribution Network (CDN) provides a great reliability for the medium voltage electric system due to the polyethylene protection present in the cables, avoiding system interruptions caused by non-permanent contact between the cables and ground objects. Besides this, the reliability increase is also associated with the dielectric supportability increase of the system, considering the greater dielectric strength of the polyethylene in comparison with dielectric strength of the air. However, the polyethylene presents accelerated ageing in comparison with the glass and porcelain insulators, which are widely used in the conventional distribution network (without protection on the cables). The polyethylene degradation is even more accentuated in high temperature, humidity and pollution areas. Therefore, the polyethylene ageing may cause problems in the electric system, such as the increase of the maintenance, equipment replacement and short-circuits. On the other hand, there is a scarcity in the literature of laboratory methods capable of promoting the accelerated aging of the covered cables used in CDN. Most methods take into account the evaluation of other polymeric components and, often, the most effective methods are difficult to implement in the laboratory and have high costs. In this way, this work proposes two own methods to evaluate the aging of the covered cables, installed on polymeric insulators during the tests. The first ageing method consisted in the utilization of two factors: voltage application equal to the double of the system phase-ground voltage and pollution caused by salt fog. The second method added the thermic effect, due to the current induction in the cable, with the purpose to obtain 60ºC on its surface. To characterize the aging and evaluate the quality of the samples, were used in this work monitoring tests, divided into physicochemical tests and electrical test. The goal of the physicochemical tests is to determinate the ageing influence in the XLPE structure, analyzing the oxidation factor (IO), the activation energy (E) of the degradation process, the fusion heat (C) and the XLPE surface hydrophobicity. On the other hand, considering the electrical tests, this work evaluates the impact of the polyethylene (XLPE) ageing, present in the covered cables, in the CDN insulation coordination of 15 kV voltage class. The electric tests used were: partial discharge measurement, leakage current (CF) measurement and lightning withstand. Considering the results, the second ageing method caused material degradation, increasing the oxidation factors and reducing the system lightning withstand considering positive voltage impulses. On the other hand, regarding the first method, although the samples tested presented punctual damages during the test, it was not observed structural changes capable of causing reductions in the lightning withstand of the system. Analyzing the results, it was possible to equate the trend of the parameters IO, CF and Disruptive Voltage, in relation to the aging exposure time, using linear regression. Besides this, it was verified that, even aged, the lightning withstand of the system is higher than the Basic Impulse Level (BIL) of the system. The critic problem for the CDN is the failures events caused by superficial discharge on the material, in the form of erosion and tracking. Therefore, this work presents an accelerated ageing test method and evaluates the quality of covered cables.