An Investigation into the Initiation and Propagation of Discharge Channels in Polymeric Insulation

A test sample incorporating a divergent field electrode arrangement has been developed for the evaluation of the resistance of polymeric materials to discharge channel initiation and propagation. The sample is an injection moulded disc containing a conical pointed recess (between 3 and 5um radius) which is subsequently metallised to form a point electrode.
The initiation and growth of discharges in polyethylene samples have been monitored using a tuned circuit discharge detector and it is shown that:
1) provided the injection moulding conditions are carefully controlled, reproducible results are obtained when 50Hz is applied at 20°C,
2) the presence of small quantities of certain aromatic hydrocarbons and antioxidants affects the voltage necessary to initiate discharges, and the subsequent growth of discharge channels.
3) with negative 1/50us impulses, applied to the point, the breakdown voltage at 20°C is double the value with the opposite polarity. With several positive impulses of increasing amplitude the b.d.v. is greater than with a single positive pulse, but there was no significant reduction after a preceding negative impulse.