Electrical Breakdown Strength of Paper-Oil Insulation under Pulsating Voltages

Electrical breakdown strength of paper-oil insulation samples was meas ured under dc, ac and pulsating voltages. The latter was obtained by superimposing ac upon dc voltage and provides an attractive method for a simultaneous testing and assessment of the state of insulation of the various parts of HV apparatus in service. The measurements were carried out over a wide range of the pulsation ratio defined as p=Eac/Edc. The results obtained under pulsating voltages follow closely an expression which relates the breakdown strength to the sum of arctangent and arccotangent function of the parameter p. The study was carried out using dry paper as well as paper containing various degrees of moisture. The presence of moisture showed a pronounced effect upon the breakdown strength which varied with the pulsation parameter p.