Surface charging of dielectric barriers under positive lightning impulse stress

Abstract

Abstract-The complex geometry of gas-insulated substations makes it difficult to predict withstand voltages. A key challenge is the characterization of the interaction between electrical discharges and dielectric surfaces. A 60 mm rod-plane air gap with a dielectric barrier is stressed with positive lightning impulse, initiating discharges that are characterized with a PMT, a current measurement system and a high-speed camera. The discharges do not lead to breakdown at the tested voltages. The residual potential on the barrier is measured with a potential probe. Depending on the gap distance, the potential distribution is either bell-shaped or saddle-shaped. The saddle-shape appears when back discharges are seen from the electrode to the barrier. Back discharges reduce the surface charge until the voltage between barrier and rod is lower than the rod inception voltage. Charge density distributions are estimated from the measurements using FEM simulations. In addition to streamer discharges, leadertype channels are sometimes observed. They are arrested close to the dielectric surface. Streamers from these channels charge the dielectric barrier additionally.

Surface charging of dielectric barriers in short rod-plane air gaps-experiments and simulations

“…This is mainly because of restrikes (also called back discharges), which create a "volcano" shape. The discharges have previously been observed by the authors as pulses with some 100 kHz at the tail of the lightning impulse [3]. Five other tested configurations, not included in table I, show similar agreement with the simulation model.…”

“…Leaders will charge the surface to a greater extent [6]. This was observed in experiments partly published in [3], see fig. 5.…”

“…The barrier was subsequently cleaned with isopropyl alcohol before the next experiment, resulting in a residual surface potential magnitude below 300 V. Local potential differences smaller than the surface area seen by the probe are not resolved [2]. This circular area can be assumed to be around 6-18 mm diameter [3]. Local charge peaks or sharp gradients are therefore smoothened.…”

“…Surface potential along the center line and saturation charge after a leader-less discharge. The measurements are taken from a previous publication where a high-speed camera was also used [3]. Image of the discharge with only streamers and image of the measured discharge with an arrested leader inset.…”

Breakdown mechanisms of rod-plane air gaps with a dielectric barrier subject to lightning impulse stress

“…With dielectric barriers in the discharge path, streamers typically propagate along and around the barrier to ground [11], [22]. Barriers can also inhibit secondary streamer development [23], cause leaders to propagate a longer path in the gas phase [24] or stop them [25].…”