Understanding incipient discharge activity in synthetic ester oil under Harmonic AC voltages

In recent times, with the increase in power electronic devices, the supply voltage profiles get severely distorted indicating the presence of harmonics with different total harmonic distortion (THD). Recently, it is identified that the incipient discharges formed due to various defects in transformer generate an electromagnetic wave of very high-rise time in the order of nanoseconds. These electromagnetic waves can easily be identified by the use of ultrahigh-frequency (UHF) sensors. Also, the mineral oil used in transformers, as an insulant, are slowly getting replaced by ester oils. The incipient discharge studies with ester oil, the databases are scanty. Having known all this, methodical experimental studies were carried out to understand the characteristics of UHF signal generated due to corona and surface discharges in synthetic ester oil-based insulation and to classify the type of defect. THD is one of the signal factors to show overall signal distortion but the harmonics voltages have different signal factors like crest factor, slew rate, shape factor, etc. It is realized that these signal factors have a high impact on phase at which the discharges occur, number of discharges and discharge repetition rate. Thus, an attempt has been made to classify the type of defect present in transformer insulation, by use of these signal factors and through the signal analysis of UHF signals. Analysis of the study enables one to use the parameters, for condition monitoring of transformers.