Snrface Creepage and High Voltage Testing

Abstract

In the previous study of surface creepage in alternating field, (Surface Creepage and High-Voltage Insulation, J.A.I.E.E. Nov 1920, pp. 949-959) this phenomenon was found to be associated with the accurnulation of charge on the surface of dielectrics near the electrodes. During every half-cycle of alternating E. M. F., the residual charge accumulated during the preceding half-cycle is neutralised and fresh charge of opposite sign is accumulated, the latter being neutralised again in the following half-cycle Such a process goes on repeatedly sofar as the alternating field is held high enough to ionize air near electrodes.
The neutralization of residual charge produces damped oscillations of very complicated nature in the circuit. The maximum value of the current of such oscillation was found to be unexpectedly high, though the effective value is of the order of some hundredths the maximum, probably due to the considerable damping in the oscillation circuits. The frequency of such oscillation was found to lie in the range of radio frequency, though it was unable to be determined definitely. Another peculier feature of the damped oscillations is their unidirectional property, i.e. such oscillations are prodaced only when the corona starting electrode is in negative half-cycle.
In high voltage testing, if a surface corona is started anywhere in the circuit, the high frequency damped oscillation will be superimposed on the alternating E.M.F. of power frequency, it follows that:-
(1) In pressure test or dielectric strength test, the insulating material is liable to be over-stressed.
(2) A sphere gap for voltage measurement would discharge at apparently lower voltage.
(3) The flash-over value of insulators, bushings etc. would be indefinite in flash over test.