EFFECT OF ELECTRODES ON SPACE CHARGE IN CROSS-LINKED POLYETHYLENE UNDER DC FIELD Fatiha ROGTI Laboratoire de matériaux diélectrique, Département de Génie Electrique, Université Amar Tlidji, Laghouat Route de Gardaia BP 37, Algeria Abstract— Space charge behaviour at physical interfaces in cross-linked polyethylene (XLPE) films submitted to DC field has been investigated as a function of the nature of the interfaces using the pulsed electro-acoustic technique. Aluminium, gold and carbon black-loaded polyethylene were used as electrodes to investigate space charge built- up in a single dielectric layer. Charge injection rather than ionic species migration is seen to control space charge distribution in agreement with the storage conditions of the XLPE films. It is shown that the sign and distribution of space charge depend drastically on the nature and polarity of the electrodes Index Terms — Space charge, dielectric interface, pulsed electro acoustic technique. 1 INTRODUCTION THERMOPLASTIC materials are more and more used in Electrical Engineering as high voltage insulation. They are highly insulating media and exhibit excellent physical and chemical properties, with the possibility to be processed to comply with complex system geometry. Among them, polyethylene is the most well-known and is widely used as insulation in power cable industry. Although its formulae appears very simple, polyethylene is a generic term to name insulating materials with very different characteristics through different base resins (resin type, chemically modified or not, cross-linked or not) and formulation (antioxidants, additives, etc.) [1]. Severe constrains still limit its use in HVDC applications where electrical charges can be injected at electrode/insulation contact or generated within the bulk by dissociation of species and other electro- chemical reactions. These charges disturb the field distribution and could lead to critical field enhancement especially when the polarity of the line needs to be inverted [2]. Another important point is the need of junctions and terminations in cable systems. They have been identified as weak points with a failure probability much higher than the cable itself due to the presence of dielectric/dielectric and electrode/dielectric interfaces [3,4]. Whatever is the nature of interfaces, they play a fundamental role in the dynamic of space charge because they have specific electrical properties due to the physical and chemical disorders inherent to their nature. In the case of metal/dielectric interfaces, one generally considers that charge carriers of both signs can be injected and/or extracted from the contact depending on its nature [5, 6]. However, a correct description of charge injection from a metal into insulation using the classical injection laws (Schottky emission for example) has not been provided yet [5, 7]. Our objective in this study is to investigate the time dependent spatial distribution of space charges at metal/dielectric interfaces submitted to DC field, using the pulsed electro-acoustic method. 2 MATERIALS AND EXPERIMENTAL 2.1 SPECIMEN CHARACTERISTICS Peelings of cross-linked polyethylene of the series described in reference [8] were used for this study. They were peeled from a medium voltage (63/90 kV) cable having a 14 mm thick insulation using a lathe equipped with a special designed knife to get optimum surface smoothness. They have a nominal width of 8 mm and a thickness of 150 m. A thermal treatment was carried out at a temperature of 50 °C in order to get rid of the by-products of the cross- linking reactions. Investigations were made of the surfaces of the peelings since these might affect measurements. Because of the cutting procedure, the surfaces were not as smooth as if compression-moulded specimen had been used. Measurements using an atomic force microscope (AFM) indicated the two surfaces of the peelings have a different surface roughness. The typical and maximum peak-to-trough heights were measured and the RMS roughness estimated as