Discharge Developing Time of Surface Flashover on Solid Dielectrics around Impulse Test Voltage of Vacuum Interrupters

We have found that the developing time
$T_{\mathrm{d}}$
of surface flashover on solid dielectrics (alumina) in vacuum is dominated by the propagation time of the conductive channel, which is proportional to the discharge path length and inversely proportional to the square of the discharge inception voltage
$V_{\mathrm{d}}$
. Based on the experimental database on
$T_{\mathrm{d}}-V_{\mathrm{d}}$
characteristics below 30 kV, we have proposed a non-optical diagnostic method for the discharge type and its path length of vacuum interrupters (VI). In this paper, we extend the
$T_{\mathrm{d}}-V_{\mathrm{d}}$
characteristics up to 100 kV, which is close to the impulse test voltage of VI at 6.6 kV class. Experimental results for different experimental systems and electrode configurations have revealed that at low voltages
$(V_{\mathrm{d}} {<}$
30 kV), it is possible from the conventional
$T_{\mathrm{d}}-V_{\mathrm{d}}$
characteristics to identify the flashover discharge path length regardless of the experimental system and electrode configuration. On the other hand, at high voltages
$(V_{\mathrm{d}} > 40\text{kV}), T_{\mathrm{d}}$
is longer than the expected value based on the conventional
$T_{\mathrm{d}}-V_{\mathrm{d}}$
characteristics, which may be affected not only by the propagation time of the conductive channel but also by the propagation time of secondary electron avalanche and the developing time of gap discharge.