DefinePK

Abstract

The High voltage high frequency (HVHF) oscillations caused by the power electronics and non-linear loads, such as converter transformers, arc furnaces, welding machines, photocopiers, microwave ovens, etc, can pollute the supply voltage by generating the high frequency oscillations thereby causing surface discharge formation on the insulation of transformer. This work deals with the effect of high frequency oscillations in the surface discharge formation on oil impregnated pressboard used in the converter transformer. Converter transformers are a part of transmission system located near the converter station, where the occurrence of high frequency oscillations is high due to the presence of power electronic devices in it. These high frequency oscillations, if enters into converter transformer, can cause formation of surface discharges on the insulation, which is one the major causes for failure of transformer. The insulation materials in converter transformers are mainly oil impregnated pressboard and mineral oil. During operation, the oil/pressboard structure undergoes multi stress ageing which results in surface discharge activity and cause gradual physical and chemical degradation. Surface discharge activity at the oil impregnated pressboard interface is studied by conducting tracking studies with high frequencies such as 85KHz and 113 KHz and observed the surface degradation due to tracking. After the test, it is concluded that the tracking paths are formed due to surface discharges, which can produce degradation by-products which contaminate and degrade the transformer oil. The current and voltage patterns formed during the surface discharge process under tracking studies are recorded. Degraded zone of oil impregnated pressboard insulation due to surface discharges was analyzed using Attenuated Total Reflectance Fourier Transform Infra-red (ATR-FTIR) spectroscopy, which can provide a specific wavelength spectrum of any functional group formed due to characteristic changes that occur due to ageing. The tracking study and pressboard material characterization at the tracked zone give a clear inference about the byproduct formation and degradation mechanism of converter transformer insulation system.