A systematic study in ambient air at high altitudes above sea level is done of the rod-plane DC corona discharge characteristics. The results show that the corona inception voltages VO are not dependently of 6, on the contrary the prebreakdown streamer voltages and the breakdown voltages are functions of 6. The propagation field of the prebreakdown streamers varies not linearly with the altitude.Tests with 10-100 cm gaps were carried out in open air and fair weather at a high-altitude laboratory (elevation: 2240 masl). Humidity was measured with a conventional hygrometer. An oscilloscope was used to detect current pulses.
Experimental Results and AnalysisIn non uniform fields on point-plane configurations, positive corona discharge has different modes as is illustrated in the figure 1. Introduction 3" 1 Standard EC-60 [l] establishes the correction factors for AC, DC and impulse high voltages. Also this standard adopted for ambient air the point-point arrangements as a device for measuring DC high voltage. The revision of the IEC-60 was strongly based on the research carried out by Pigini et al [2] whom assumed that the streamers propagation field at sealevel is about 500 kV/m, and they claimed that it varies linearly with 6 for higher altitudes. Recent researches have shown that the semi-empirical equations for correcting DC voltages are not appropriate at high altitudes above sea level, due to the combined effect of the relative air density and the humidity[3]. To understand this problem, it has been necessary to carry on studies related to determine the influence of the relative air density (6) and humidity (H) in the various modes of corona discharge.
Experimental ProcedureA rod-plane configuration was used with two Merent types of electrodes; 2 cm thick cylindrical rod with conical point (36') and spherical point, both were made of brass. Positive high voltage from a Cockrofi-Walton DC supply (with a ripple of 1.5 %) was applied to the rod electrode; with an 1.8 Mn current limiting resistor in series with the gaps, the plane electrode was connected to ground through a current detecting resistor. O L l b . ;o ' i o . 4b . 5il . €iI . : o &P bnglh (cm) Figure 1. Threshold curves for various modes of positive corona discharge. Spherical point-plane, H = 10.5 g/m3 and H = 12.4 g/m3. The corona discharge starts at the HV electrodes when their surface voltage reach a critical value EO. AbdelSalam and Allen for spherical point-plane gaps proposed the following relation [4]: being r the radius of the point electrode. For onset voltages Vo they propose: Vo =E0.qd=22.8 1+--[kvl (2)
