Partial discharges and breakdown at protrusions in uniform background fields in SF₆

Abstract

The breakdown mechanism of compressed SF6 in gas insulation is known to be controlled by stepped leader propagation. This process is reasonably well understood for strongly non-uniform insulation gaps (‘point-to-plane’) and in the absence of pre-breakdown discharge activity (‘corona stabilization’). Open questions still remain for weakly non-uniform insulation gaps with small electrode protrusions (particles, surface roughness), in which pre-breakdown partial discharge (PD) activity is present. This paper presents a first attempt to derive a consistent picture under these conditions, which are characteristic for practical gas insulation systems. Experiments were carried out in a uniform field gap with a short protrusion on one electrode. This configuration was studied at various pressures from 0.1 to 0.5 MPa and both polarities using electrical and optical diagnostics. The results are interpreted using a quantitative model and order-of-magnitude estimates. The emerging picture allows prediction of most of the technically relevant aspects of the discharge processes and their main parameter dependences. It comprises statistical time lags, formative time lags including pre-breakdown PD activity and breakdown fields as a function of gas pressure, protrusion length and polarity.

Inception level of partial discharges in SF<inf>6</inf> induced with short X-ray pulses

“…This is a statistical process. The time to first electron is described by the FowlerNordheim equation for negative polarity at the high field electrode [2,3] and detachment of electrons from negative ions for positive polarity [4], respectively. Both relations predict an exponentially increasing time to availability towards lower voltages.…”

SF6 high voltage circuit breaker contact systems under lightning impulse and very fast transient voltage stress

“…But the time scales of the negative formative time lag are larger than those at positive polarity [13]. The lower electric fields in the case of negative polarity (due to lower breakdown voltages), may further increasing the negative formative time lag [10]. At positive polarity, the formative time-lag is smaller and the time to breakdown is dominated by the statistical time-lag.…”

“…Such field stress can be reached at the electrode surface even in low inhomogeneous field configurations due to small surface defects or electrode roughness. The rate of electrons leaving the electrode due to field emission can be calculated by the FowlerNordheim equation as in [10].…”

“…Besides longer statistical time lags in the case of positive polarity, the time to arc formation is also influenced by the formative time-lag [10]. The time scales of this formative time-lag are larger for negative polarity [13].…”

An Efficient Procedure to Identify and Quantify New Molecules for Insulating Gas Mixtures

“…The above mentioned leader propagation model in SF 6 was presented in [58,59], experimentally validated for different cases in [60,61]. Experiments to test this model in other electronegative gases are currently in preparation.…”