Mathematical model for selection of frequency range boundaries when recording acoustic signals of partial discharges in high-voltage transformers

Insulation damage is one of the main causes of faults in high-voltage transformers. Partial discharge characteristics are informative characteristics reflecting the state of insulation. One of the effective ways to monitor the technical condition of high-voltage transformers is to record and analyze acoustic signals of partial discharges. However, at present there are no generally recognized methods of transformer diagnostics based on the analysis of such signals. It is due to several understudied problems, one of which is the uncertainty of the boundaries of the frequency range for practical measurement of acoustic signals of partial discharges. Thus, the development of a mathematical model to select the boundaries of frequency ranges when recording acoustic signals of partial discharges in high-voltage transformers is an urgent task. The study of the impedance mathematical model, which considers the basic acoustic properties of an oil-filled transformer as a multilayer object, has been carried out using a characteristic parameter, i.e. the transmission coefficient. It determines the fraction of the amplitude of the acoustic signal that has passed through the object. The dependences of the transmittance coefficient on the frequency of the acoustic signal for different thicknesses of the steel wall of the transformer tank have been obtained. It is shown that these dependences for different thicknesses differ significantly only in the high-frequency region. When analyzing the frequency dependences of the acoustic signal transmission coefficient, it is possible to identify a rather narrow frequency range containing the maximum values of this coefficient. This frequency range can be recommended as an operating frequency band when selecting or creating measuring instruments for acoustic signals of partial discharges, as well as when searching for diagnostic signs of insulation in oil-filled high-voltage equipment.