Abstract
<p>Prediction of withstand voltages in air-insulated systems are made on the basis of empirical models that are not sufficiently accurate for complex geometries. Better understanding of the spatiotemporal development of electrical discharges is necessary to improve the present models. Discharges in lightning impulse stressed 20–100mm rod-plane gaps are examined using a highspeed camera, photo-multiplier tubes (PMTs) and a highbandwidth current measurement system. The images and measurements of gaps larger than 20mm show a fast initial streamer discharge with a current rise time of some tens of ns, followed by a dark period of a few μs and a propagation of a slower leader-type channel leading to breakdown. The breakdown mechanisms in the shortest gaps are faster and geometry dependent, probably occuring by heating of initial streamer channels. Different light filters used with the PMTs indicate that all parts of the leader-type discharge development emit light over a spectrum from UV to IR. The initial discharges emit low amounts of warm light and IR compared to the leader-type channel. Finally, it is suggested that empirical breakdown voltage prediction models should be interpreted in light of the leader-type breakdown mechanism.</p>
Highlights
SF6, a very strong greenhouse gas, is often used as insulation in medium voltage (MV) equipment, insulating gases with lower global warming potential have recently been proposed [1], [2]
50 % breakdown (U50 %) and inception (Ui,50 %) voltages for the tested geometries are shown in Fig. 8. 50 % breakdown voltages fit well with the empirical streamer propagation criterion in (1) using U0 = 20 kV
Breakdown mechanisms in inhomogeneous rod-plane gaps have been studied with a high-speed camera, photo-multiplier tubes (PMTs) and a current measurement system
Summary
SF6, a very strong greenhouse gas, is often used as insulation in medium voltage (MV) equipment, insulating gases with lower global warming potential have recently been proposed [1], [2]. Air in combination with dielectrics is a feasible alternative to SF6 as insulation in MV switchgear [3], [4]. Meeting clearance requirements in air-insulated medium voltage (MV) substations requires accurate withstand voltage prediction models of the dielectric design. To understand how dielectric surfaces influence the breakdown voltage of an air-insulated gap, the breakdown mechanisms of the gap must be understood. The focus in this work is on positive lightning impulse (LI) stressed short (20–100 mm) inhomogeneous air gaps, applicable to MV switchgear insulation designs. Breakdown in these gaps can happen via a leader-type channel propagating around the space charge left by the initial streamer discharges [5].
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