Understanding impact of copper sulphide diffusion into pressboard insulation in transformers

Surface discharge inception voltage (SDIV) in copper sulphide diffused Oil Impregnated Pressboard (OIP) material reduces with increases in harmonic content in the supply voltage, irrespective of Total Harmonic Distortion (THD). The rise time of current signal injected during Surface discharge process is about few ns and the Fast Fourier transform (FFT) analysis of Ultra High Frequency (UHF) signal formed due to surface discharge process have dominant frequency of 0.9 GHz. The characteristics are the same irrespective of level of thermal aging of OIP and amount of DBDS present in transformer oil. The amplitude and width of the current pulse increases with increases due to aging. Atomic Absorption Spectroscopy (AAS) studies results indicate that the amount of copper content in pressboard material have increased with increasing in concentration level of DBDS. Energy dispersive spectroscopy (EDS) conform the amount of copper and sulphur is high with thermally aged OIP due to aging of OIP in transformer oil with DBDS concentration. Attenuated Total Reflectance - Fourier Transform Infrared (ATR-FTIR) spectroscopy studies indicates the formation of aldehydes and carboxylic acids, caused due to degradation of cellulose structure. Phase Resolved Partial Discharge (PRPD) analysis indicates that the surface discharge activity occurs near the peak of the applied AC/harmonic AC voltages at inception voltage and at rising portion of the applied AC/harmonic AC voltage at higher voltages. The tensile and flexural strength of the material reduces with thermal aging of OIP material, irrespective of concentration of DBDS in oil.