Understanding surface discharge activity in copper sulphide diffused transformer insulation under harmonic AC voltages

In the present study, the surface discharge injected current were measured by using High Frequency Current Transformer (HFCT). It is observed that the rise time of the current pulse is about 800 ps. It is confirmed that the injected current pulse radiates Ultra High Frequency (UHF) signal and the dominant frequency of the UHF signal formed due to the surface discharge injected current is about 0.9 GHz. The characteristic frequency contents of the UHF signal formed due to surface discharge activity is the same with the virgin and copper sulphide diffused pressboard material. The Atomic Absorption Spectroscopy (AAS) results indicate that the amount of copper content in pressboard material have increased with the increase in the concentration of Dibenzyl disulphide (DBDS) in transformer oil. The Phase Resolved Partial Discharge (PRPD) analysis indicates that at the point of surface discharge inception, the discharge occurs at the peak of the applied AC voltage and when the applied voltage magnitude is increased the discharge occurs at the rising portion of the applied AC voltage. It is noticed that with increase in DBDS concentration, there exists an increase in the discharges.