The Influence of Thin Dielectric Coatings on LI and AC Breakdown Strength in SF6 and Dry Air D. van der Born, P.H.F. Morshuis, J.J. Smit Intelligent Electrical Power Grids Delft University of Technology Delft, The Netherlands A. Girodet High Voltage Technology and Material Alstom Grid Villeurbanne, France Abstract—Electric power consumption has increased over the years while the available space for substations has become small- er. Newly designed gas insulated switchgears(GIS) have to be smaller and have to operate at a higher system voltage. Further- more, the urge to decrease the use of SF6 as an insulating gas in GIS is becoming larger because of the high production and han- dling costs and the high global warming potential. Unfortunately, the breakdown strength of alternative gases is usually lower than that of SF6. However, with the application of a coating on the electrodes in GIS the breakdown strength can be improved. In this paper, the performance of GIS with thin coatings is eva- luated with breakdown tests. The test setup consists of a rod- plane electrode arrangement with a coated rod electrode. The coatings vary in thickness between 40 micrometers and 2 milli- meters. The breakdown tests are performed at either 50 Hz AC or lightning impulse voltage. Keywords—GIS; thin coating; breakdown strength; dry air; SF6; lightning impulse; AC I. INTRODUCTION Gas insulated substations are generally in use in densely populated areas because of the small surface area with respect to open air substations. The increasing demand of electric power in large cities is changing the design constraints of gas insulated switchgear. Firstly, because of the increase in power consumption the operating voltage should be increased to limit the increase in losses. Secondly, the available space for the construction of a substation is decreasing resulting in a size limit for gas insulated switchgear. With an increase in system voltage the required size of the switchgear increases rapidly, which is therefore a problem in an urban environment. In GIS the most commonly used insulating gas is sulfur hexafluoride(SF6). SF6 shows excellent electrical breakdown behavior because of the electronegativity of the gas. Further- more, the gas has good cooling and arc-quenching capabilities. However, SF6 also has some disadvantages. Firstly, the gas has a high global warming potential(GWP) which is 23,000 times larger than that of CO2[1]. Secondly, the production costs of the gas are relatively high and the handling of the gas after use is also costly because of the environmental con- straints. With respect to costs and environment it would be favorable to reduce the amount of SF6 used in GIS. However, the electrical breakdown strength of most alternative gases is much lower. To improve the breakdown strength of GIS, for the effort of size reduction and usage of alternative gases, coatings can be applied to the surface of the electrodes[2-5]. For application in GIS thin and thick coatings can be used. Thick coatings have a layer thickness ranging from several millimeters to several centimeters. Thin coatings have a layer thickness in the range of a few microns to several millimeters. In this paper the influence of several types of thin coatings on the breakdown strength of a GIS is investigated with the help of breakdown tests. II. EXPERIMENTAL A. Set up The breakdown tests are conducted on a rod-plane elec- trode configuration which is shown in fig.1. The rod-plane setup consists of a fixed aluminum plane electrode and 12 retractable aluminum rod electrodes indicated by 1 and 3 re- spectively. The gas gap is indicated by 4 and the flat electrode, in which the rod electrodes can be retracted, is indicated by 2. The rod-plane arrangement is placed inside a pressurized test vessel which is filled with either dry air at 0.9 MPa abso- lute pressure or SF6 at 0.34 MPa. At these pressures and at the applied electric fields the effective ionization of both gases is comparable. The gas gap is set at 22 mm to acquire the desired electric field distribution which is comparable to the field distribution inside a real size 420 kV GIS. Fig. 1: Schematic drawing of rod-plane test setup[6].