Characteristic of acoustic emission signals generated by electric arc in on load tap changer

Results of scientific research works considering technical condition diagnosis of on load tap changers (OLTC) by use of acoustic emission (AE) method are presented in the paper. OLTC are crucial part of electric power transformers. They enable for on load voltage regulation in the network. Reliable operation of these devices is important especially for distributive transformers but also for units working in mills and industrial plants vulnerable on occurrence of voltage value changes in the network. Operation of OLTC is connected with formation of electric arc which occurs on main contacts and also on resistor contacts. Intensity of this phenomenon depends among others on: scale of current flowing during switch over, wear and tear level of contacts, character of receptions etc. The aim of research works, results of which are presented in the paper, was to determine characteristic parameters, so called “finger prints”, for AE signals generated by ignition of electric arc and in particular determination of the corresponding time-frequency structures. The measurements were performed in a modeling assemble equipped with tap changer including separated selector and power changer that were supplied with single phase voltage, 50 V, forcing current flow, nominal value equal to 150 A. The measurements were performed for each phase separately each time forcing current flow through the contacts accruing to a single phase. Electric arc created during coupling and uncoupling of contacts generated AE signals which were registered with a measuring apparatus equipped with wide band transducer type WD AH-17. Analysis of achieved results was performed in the time-frequency domain by use of short time Fourier transform (STFT), continuous (CWT) and discrete (DWT) wavelet transform.