Behavior of Acoustic Partial Discharge Signal in Oil-Pressboard Insulation System

Online monitoring of partial discharges (PD) can reduce the risk of catastrophic failure of power apparatus. Acoustic emission technique is advantageous for online PD measurement. PDs are classified according to the origin of discharges and their effects on insulation deterioration are also different. Acoustic emission (AE) signal produced by PD are detected by sensors mounted on the experimental tank surface, stored on an oscilloscope and fed to computer for analysis. PDAE signal propagates from source to sensor through oil and pressboard insulation. This paper discusses the effects of different thicknesses of pressboard insulation barrier on the propagation path of partial discharge acoustic emission signal. PD may occur at surface of insulation having different thickness at different voltages. This paper also discusses the effect of insulation thickness on surface discharge. The behavior of PDAE signal at different voltage level is also discussed. The AE signals are analyzed using FFT and DWT. The parameters calculated are frequency at peak magnitude & median frequency from FFT data and percentage signal energy in different frequency bands from DWT data. Effect of applied voltage beyond PD inception voltage is also examined. Acoustic PD magnitude (mV) and electrical PD magnitude (pC) relation for different electrode system is investigated.