Positioning of UHF PD Sensors on Power Transformers Based on the Attenuation of UHF Signals

Prolonged partial discharge (PD) activities can deteriorate the insulation of a power transformer leading to its complete failure. Therefore, PD monitoring is essential to ensure that PD sources are identified early. Ultra-high Frequency (UHF) PD measurement has gained attention because of some advantages compared to the conventional electrical method, such as low sensitivity to external disturbances. CIGRE recommends providing dielectric windows in newly manufactured transformer tanks for installing UHF sensors. However, there is no general recommendation on the placement of UHF sensors on a transformer tank, yet. This contribution provides knowledge about this discussion based on experimental data obtained from a 300 MVA, 420-kV grid-coupling transformer. UHF signals were measured using antennas at 17 different positions on the transformer tank and were used to provide a general evaluation of signal attenuation depending on distance and different types of propagation paths. Two approaches of signal evaluation, namely, signal power and peak-to-peak amplitude, were compared and found to have a high qualitative correlation, which implies that either method can be used to draw similar conclusions. Additionally, certain sensor positions were analyzed for any inherent benefits and finally, prospective sensor positions were suggested based on their performance and relative distances.