Electromagnetic Wave Radiation Due to Partial Discharges Inside Power Transformers in the Frequency Domain

摘要

Reliable operation of power systems depends highly on condition of power transformers, hence making their condition monitoring top priority. Partial discharge (PD) is among the most common and deteriorative faults in a power transformer. PDs cause progressive deterioration of the insulation system of the transformers, and may lead to catastrophic failures, if no countermeasure is taken. PD measurements in power transformers are among well-known tools for the evaluation of the insulation system's condition. For this purpose, the conventional method described in IEC 60270 is applied; however, the ultrahigh frequency (UHF) PD measurement technique, which is based on the radiation of electromagnetic (EM) waves due to a PD fault in very high frequency (VHF) and UHF range, has advantages, e.g., higher robustness against external noises, possibility of PD localization as well as its applicability for both offline and online PD measurements. In this contribution, UHF PD pulses are studied from an EM stand point. A specially designed PD fault model is placed inside a transformer tank model. The EM waves propagated due to PD are captured via UHF probes connected to a high sampling digital storage oscilloscope and transformed into the frequency domain using discrete Fourier transform (DFT). A Histogram of the frequency components is presented for different UHF probe positions. The results are verified by recording phase resolved PD patterns using both the conventional, and the UHF measurement method. Finally, a practice-oriented analysis of suitable and unsuitable center frequencies and bandwidths for UHF measurements is presented.