Critical Examination of the Dissipation Factor Tip-Up as a Measure of Partial Discharge Intensity

Bridge measurements of the dissipation factor tip-up at increasing voltage are generally assumed to give a correct measure of discharge intensity. It has been shown earlier that even when the tip-up is entirely caused by discharges it does not represent an unambiguous measure of discharge intensity, because Δtanδ depends on both the amplitude and the phase of the discharge pulses. The validity of theoretical expressions describing the dependence of Δtan δ and Δ C on pulse phase is confirmed by experimental results and by calculations of the effects of higher harmonics. The information from Schering bridge measurements can be improved by using simple functions, given in the paper, of measured values of the capacitance and dissipation factor. These functions permit separate estimations of discharge intensity and phase. Empirical examples of the variation of these functions with voltage are given. Finally, the high sensitivity of Δtanδ to harmonic distortion of the bridge voltage is discussed. This indicates that bridge measurements of partial discharge characteristics must be performed with a distortionless bridge voltage.