Inhibited electrical discharges in air

Abstract: Modem society requires a stable electricity supply, the reliability of which is partlydetermined by the dielectric strength of the outdoor high-voltage insulation systems.This thesis concentrates upon one particular discharge phenomenon that occurs whensuch a system is subjected to rain, namely the inhibited discharge. This is a dischargewhere the energy input into the discharge channel is limited by the presence of a largeseries resistor in the discharge circuit.Two experimental arrangements have been used: A quasi-uniform 10 mm electrodegap (simulating discharges between water drops) and a rod-to-plane 1 m electrode gap(simulating the flashover of a high-voltage post insulator).The presence of the series resistance caused no increase in the disruptive dischargevoltage for the 10 mm gap, but for the 1 m gap it induced an increase that was morepronounced for negative than positive polarity.The transition from a cold discharge to a hot plasma was significantly prolongedwhen a large series resistance was introduced in the discharge circuit. This is interestingfor engineering purposes since it increases the time window in which action can betaken before the end of the disruptive discharge.A five order-of-magnitude difference in the peak streamer-to-spark transition current(1 mA - 100 A) was found in the literature for 10 mm gaps. This difference has notbeen thoroughly investigated previously. Here an explanation is suggested for thisphenomenon.The experimental results revealed that the streamer propagation was not inhibited bythe presence of a large series resistance in the discharge circuit, but the final jump was.The effect upon streamer propagation was confirmed by calculations using a well-established streamer model. A final-jump model was derived as part of the work presented here and good agreement was obtained between the experiments and the calculations. This model should be useful in the design of high-voltage insulators.