The Development of Electrical Discharges in Simulated "Tree" Channels

The development of spark discharges in 5 x 10-3 cm diameter cylindrical tubules of 0.5 cm length bounded by solid dielectric has been studied by making the conventional electric strength measurements and by using a self-triggered high speed camera which possesses a temporal resolution of 5 ns and a spatial resolution of about 50 line-pairs per mm to obtain sequential photographs of the prebreakdown luminescence accompanying voltage collapse. The photographs verify the deductions made from the information obtained from the electric strength measurements: the walls of the tubules interfere with the breakdown processes in the confined gas and collect considerable charge. This results in an electrostatic field which appreciably distorts the applied field. It is evident that the electric strength of tree channels under unidirectional voltage waveforms can be very high, approaching that of the solid insulation. It is concluded that the mechanisms of trapping and decay of charge in a solid insulation are important factors in assessing its resistance to tree penetration, and that the channels of electrical trees cannot be an effective extension of the electrodes into the insulation as is almost universally assumed.