2012 International Conference on High Voltage Engineering and Application, Shanghai, China, September 17-20, 2012 978-1-4673-4746-4/12/$31.00 ©2012 IEEE 709 Barrier Diameter Effect on the Behaviour of Transformer Oil Submitted to AC Voltage F.Guerbas 1 , M.Zitouni 2 , 1 LSEI Laboratory, USTHB University BP 32 El Alia 16111, Bab Ezzouar, Algiers, Algeria 2 Université Ziane Achour, Djelfa, 17000, Algérie f_guerbas@yahoo.fr , med.zitouni@gmail.com A.Boubakeur 3 and A.Beroual 4 3 L.R.E. /, Laboratoire de recherche en Electrotechnique, Ecole Nationale Polytechnique, B.P.182, El-Harrach, 16200 El-Harrach.Alger, Algeria 4 Ecole Centrale de Lyon, Centre de Génie Electrique de Lyon, France ahmed.boubakeur@enp.edu.dz abderrahmane.beroual@eea.ec-lyon.fr Abstract-Dielectric interfaces can have a significant effect on the insulating performances of HV apparatus. The oil/solid interface represents a relatively weak component of an insulation system in power transformer. Therefore, when we are considering the effect of oil breakdown in oil filled transformer, we should consider the interactions which electrical discharges may have with solid surfaces. This work is devoted to the influence of insulating barrier on the behaviour of transformer oil under ac voltage in point-plane electrode configuration. The influence of the characteristics of barrier such as its position between electrodes, and its diameter on the pre-breakdown and breakdown phenomena is investigated. It is shown that the insertion of barrier near the point electrode increases the breakdown voltage and leads to decrease the pre-breakdown charge. The increase of breakdown voltage is due to the channel elongation of the disruptive charge. By determination of the pre- breakdown charge and breakdown voltage, we can estimate the optimal position of the barrier. The barrier diameter influences the breakdown voltage, the pre-breakdown charge and the trajectory of discharge. Keywords- transformer oil, breakdown voltage, pre- breakdown charge, insulating barrier I. INTRODUCTION The barriers are widely used in many high voltage devices, especially in power transformers. It is well known that the dielectric strength of long oil gaps is significantly increased by the insertion of an insulating solid material in divergent gaps [1]. The insulating structure of a transformer is exposed to different stresses and particularly to the discharge phenomena. The propagation of discharges at the solid /liquid interface leads to failure of the system [2]. Thus, the knowledge of the conditions of initiation and propagation of electrical discharges is of great interest to well understand the mechanisms leading to breakdown [3, 4]. The pre-breakdown charge is an important parameter which the determination can inform us about the physical phenomena implicated in pre- breakdown phase [5-7]. Indeed, in presence of the barrier, the pre-breakdown charge was decreased by the barrier. This is due to the fact that the barrier acts as a geometrical and electrostatic obstacle for the disruptive discharge [6, 8]. The barrier effect on the pre-breakdown charge depends on many parameters as its position between electrodes, its nature, dimensions and the applied voltage level. In this paper, we investigate the influence of the insulating barrier on the breakdown voltage and the pre-breakdown charge in long oil point-plane gaps. The effects of the barrier position and its diameter have been studied. A comparison between the pre- breakdown charges in the system with barrier to that one without barrier is presented. II. EXPERIMENTAL SET UP The experimental set up consists of a high voltage test transformer 300kV/50kVA/50Hz, a capacitive voltage divider and a transparent test cell of 175 litres volume (Figure 1). The test cell is made of Plexiglas (700mm×500mm×500mm). It contains a point-plane electrode arrangement mounted horizontally (Figure 2). The point electrode is made up of brass and has a radius of curvature of 6mm. The plane electrode is made up of steel and has a circular shape of 35cm diameter. The electrode gap varies between 1 and 12 cm. The insulating barriers have a circular shape and are made of Bakelite (ε r =5) or Presspahn (ε r =3). The barrier is mounted vertically between the electrodes and sustained by thin insulating threads. The barrier surface is cleaned with isopropyl alcohol. The mineral oil we used is of naphthenic type; it is subjected to rigorous conditions of storage in sealed drums for protection against degradation and humidity environment. The pre-breakdown charge is measured through a non capacity of 0.2 μF, inserted between the plane electrode and the earth; it is connected to a storage oscilloscope. The value of the pre-breakdown current reported on the different curves represents an average value on a series of six measurements [10].The position of the barrier is defined by the ratio ‘a/d’ (fig.2) where ‘a’ is the point-barrier distance and ‘d’ is the point-plane gap.