A Comparative Study of Positive and Negative Streamer Development along an Ice Surface I. Ndiaye, M. Farzaneh and I. Fofana NSERC / Hydro-Quebec / UQAC Industrial Chair on Atmospheric Icing of Power Network Equipment (CIGELE) and Canada Research Chair, tier 1, on Canada Research Chair on Icing Engineering (INGIVRE), at Université du Québec à Chicoutimi, 555, Boulevard de l’Université, Chicoutimi, Québec, G7H 2B1 Canada Abstract- Development of negative and positive streamers along an ice surface in short rod-plane gaps has been investigated experimentally and compared. Major characteristics of streamers including inception voltage, propagation velocity, breakdown voltage, time to breakdown and light emitted during their development are studied. Results showed that these characteristics are largely influenced by the voltage polarity under same experimental conditions. In negative polarity, it was found that the propagation velocity and the time to breakdown are significantly higher and the voltage required to initiate the streamer is lower. It was also found that the influence of the ice surface properties is more intense in negative polarity. High- speed camera recordings also showed different processes in the discharge development for negative and positive polarities. I. INTRODUCTION The study of streamer formation and its subsequent propagation is of a great interest in the electrical performance of the insulators dielectric strength because these phenomena are important precursors to electrical breakdown. However, if discharges in air have become now well understood, in the presence of an insulating surface, due the complexity of involved mechanisms, no satisfactory physical interpretation of streamer development has been provided yet [1,2]. As concerns ice surface discharge, those mechanisms have been revealed even more complex [3-6]. This additional complexity arises first from the combined effects of the temperature and the impurities present on the ice surface, both which can lead to the generation of a quasi-liquid layer on the surface, and then to the influence of some other parameters including gap geometry, effect of surface charge and applied voltage. In our previous work, the influence of these parameters was studied [3-6]. However the investigations were limited to positive polarity which leads to the generation of positive streamers. Since in many cases the dynamics of positive and negative streamers are different [7-9], our investigations are now extended to the case of negative streamers in order to compare the results with those of positive streamers and to furthering our knowledge of ice surface discharge. This paper summarizes recent investigations undertaken at CIGELE/INGIVREon the first nanoseconds of development of streamers along an ice surface and the effect of voltage polarity. Experimental investigations were performed using a rod-plane gap electrode. Streamers were initiated by applying a standard lightning impulse voltage, 1.2/50 μ s. Several parameters including streamer inception voltage, streamer propagation velocity, breakdown voltage, time to breakdown and the light emission from streamers were carried out. Results obtained under positive and negative voltage are compared and discussed. II. EXPERIMENTAL SETUP AND PROCEDURES The corona discharges were investigated using a simple rod– plane electrode arrangement with an adjustable electrode clearance. Two rod radii of 1.5 and 6 mm were considered. The electrodes were fixed into a rectangular Plexiglas box, which served as a mold to form the ice, allowing for adjustable gap distances of 35 and 70 mm. Varying distance and radius allow us to investigate the influence of applied electric field on the streamer characteristics. The ice mass was built in several steps as described in [4-6], in order to half- submerge the electrodes and obtain a flatter surface, as shown in Fig. 1. Fig. 1. Vertical section of the physical ice model. The conducting ice layer was formed with freezing water having a predetermined conductivity, adjusted by adding chloride sodium (NaCl) to de-ionized water at 20°C. The water conductivities considered in these investigations were 2.5 S/cm and 80 S/cm, assigned respectively to slightly contaminated ice surfaces and to the very contaminated ones. Negative and positive streamers were initiated by applying to the HV electrode, respectively the plane and the rod, a standard 1.2/50 μ s positive lightning impulse voltage while for generating the negative streamers the HV was applied to the plane electrode. A Hamamatsu R928 photomultiplier (PMT), used to detect the light emitted from the streamers and then to determine the time to first corona. An ultra sensitive high- speed framing camera, an Imacon 200 model, coupled to a CCD camera was used to record the light emitted by the streamer and to determine the its propagation velocity and its spatial extension. Fig.2 shows the complete experimental setup. All experiments were performed by placing the physical model inside a climate chamber kept at the investigated Conducting ice layer Plexiglas box de-ionized water bulk ice