Positive Streamer Propagation and Breakdown in Air: the Influence of Humidity P. N. Mikropoulos, C. A. Stassinopoulos and B. C. Sarigiannidou School of Electrical and Computer Engineering Aristotle University of Thessaloniki, Thessaloniki 541 24, Greece ABSTRACT The influence of humidity on streamer propagation at conditions from the threshold for propagation to those for streamer-induced breakdown was investigated in a uniform field in air at atmospheric pressure. Experiments were carried out in a three electrode arrangement consisting of a 12 cm long parallel-plane gap, with an auxiliary needle in the earthed anode. Positive streamers were initiated by applying at the needle electrode a pulse voltage which varied in amplitude. These propagated towards the upper plane electrode which was stressed by a negative dc voltage. Under natural atmospheric conditions, propagation and breakdown probability curves were obtained for several values of absolute humidity in the range between 5 and 22 g/m 3 . Thus, distributions of the electric field required for streamer propagation and breakdown were obtained and the associated velocity of propagation and time to breakdown were measured. Besides humidity, the amplitude of the voltage used for streamer initiation and the ambient electric field were considered as influencing parameters on streamer properties. Empirical equations are presented expressing the effects of the above parameters on the intrinsic streamer properties. A comparison with previous work in the literature is made and this leads to the conclusion that the influence of humidity on streamer propagation and breakdown can be placed in a sounder quantitative basis. Index Terms — Humidity, air gap, gas discharges, electrical breakdown. 1 INTRODUCTION THE detailed investigation of streamer propagation in air is of considerable fundamental and practical interest. Streamer development determines the breakdown characteristics of many electrode configurations in air, especially under lightning impulse and direct voltages. This fact is taken into account by IEC [1] where the procedures for correcting breakdown voltages in air for both air density and humidity variations are based on the electric field required for streamer propagation. Knowledge of streamer characteristics in air is thus important to the understanding of the air breakdown mechanism hence also for the design of insulating systems under various environmental conditions. Besides, there is always need for an accurate reference base for comparison with streamer propagation in other gases or along insulators. Much information is available on the conditions determining the propagation of positive streamers in air, especially on the dependence of the streamer properties on the atmospheric conditions. However, most of the research has been carried out in non-uniform field geometry where it is not certain whether the results are characteristic of single streamer propagation as they might suffer from the problem of space charge effects due to ample streamer branching. This is so since in non-uniform field geometry the values of streamer properties measured, namely the electric field required for streamer propagation and the associated velocity, were obtained either directly from measurements near the head of an advancing branched streamer corona or by assuming that the mean stress for breakdown in rod-plane gaps under lightning impulse is determined by the streamer gradient. In order to investigate the streamer properties in uniform field geometry, a plane-parallel electrode arrangement with streamers initiated at an auxiliary sharp point in an earthed anode plane has been found useful [2-10]. This arrangement allows the investigation of streamer propagation at conditions from the threshold of propagation to those for streamer- induced breakdown in the absence of significant space charge effects [7, 8]. Although some of these studies were aiming to determine the effects of atmospheric conditions on streamer propagation field, it was felt that a more detailed investigation is needed especially on the effect of humidity on the streamer propagation velocity which, as stated before [8], is as yet unknown. Thus, in the light of new measurements in uniform field and through comparison with previous studies, the present work allows for the influence of humidity on streamer propagation and breakdown in air to be modeled on a sounder quantitative basis than before. Besides humidity, the voltage amplitude used for streamer initiation and the ambient field were considered to influence the streamer propagation. Manuscript received on 16 October 2006, in final form 31 May 2007. 416 P. N. Mikropoulos et al.: Positive Streamer Propagation and Breakdown in Air: the Influence of Humidity 1070-9878/08/$25.00 © 2008 IEEE