Proceedings of the IASTED International Conference Power and Energy Systems November 8-10, 1999, Las Vegas, USA 301-145 -1- Insulation properties of composite dielectric arrangements F. V. Topalis I. F. Gonos I. A. Stathopoulos National Technical University of Athens Department of Electrical and Computer Engineering, Electric Power Division e-mail: topalis@softlab.ntua.gr, igonos@softlab.ntua.gr, stathop@power.ece.ntua.gr 9, Heroon Politehniou Str., Zografou, GR 157 80 Athens, Greece Abstract This paper examines the barrier effect on the dielectric strength of non-uniform electric fields, stressed by impulse voltages. The electric field is a rod-plane air gap with length up to 40 cm. The barrier is a sheet of craft paper that is placed to several positions between the grounded plane and the high voltage rod. The experimental results show that the position of the barrier influences substantially the dielectric strength of the gap. Another parameter that is also investigated is the electrode configuration. The diameter of the rod electrode and the shape of the tip affects the uniformity of the field and therefore the dielectric strength. The dependence of the breakdown voltage upon the shape of the impulse stress is also investigated. The experimental tests show that the strength of the gap decreases with the time to crest of the impulse. It becomes minimum under voltages with a time to crest of some tens of microseconds and starts increasing under slower impulses with time to crest of some hundreds of microseconds. The obtained results seem to be very useful for the design and the optimisation of the internal insulation of high voltage electrical equipment. Keywords: Air gaps, barrier effect, insulation, dielectric arrangements 1. Introduction The use of insulating materials in air gaps is often used because it has been noticed that their dielectric strength increases essentially after the insertion of a barrier in their electric field [1-6]. Technically and economically speaking, the quality and the thickness of the barrier is not the most important criterion for the insulation co- ordination. It was found out that variations in the thickness of the insulating material slightly affect the mechanism of the breakdown/puncture of the barrier [7]. Noticeable changes in the breakdown voltage have been observed only when the thickness is considerably increased [1, 3]. On the other hand, it should be mentioned that the breakdown voltage is not affected even the barrier is already punctured or even more, it possesses an opening [8]. The aim of the paper is the investigation of the parameters that affects substantially the breakdown mechanism and the strength of the gap qualitatively and quantitatively. Rod-plane air gaps with small and medium distances are tested experimentally under impulse voltage stresses. The investigated parameters are the gap length, the diameter and the tip radius of the high voltage electrode, the position of the barrier and the shape of the impulse voltage. 2. Experimental The test arrangement is a rod-plane air gap with length in the range of 10 to 40 cm. The rectangular plane is at the earth potential while the high voltage is applied to the rod electrode. Fig. 1: High voltage rod electrode D: diameter, R: radius of the tip. Five different rods (Fig. 1) are used, three of them with diameter D=12 and the other two with D=16 and 23 mm. The tip of the rod is terminated with a cone. The angle of the tip is 30°. The rod of the three electrodes with D=12 mm ends to a spherical surface with radius R=1, 2 and 4 mm respectively. The tip radius of the other rods (D=16 and 23 mm) is 1 mm. The material of the electrodes is brass. The insulating barrier is a sheet of paper, enriched with epoxy resins, which is used for the insulation of transformer windings. It has the following technical characteristics: thickness 0.39 mm, weight 481 g/m2, breakdown voltage in air 4.5 kV, pH in water solution 6 ÷ 8, moisture 8%, nitrogen 2% and ashes 1%. The dimensions of the barriers are always greater than the dimensions of the grounded plate in order to avoid a flashover. The barrier is placed perpendicular to the axis of the rod-plane gap (Fig. 2) at a distance x from the high voltage rod. The distance x is varying between 0 (barrier in touch with the high voltage rod) and G (barrier in touch with the grounded plane), in steps of 0.25G.