Abstract
A well-defined condition-monitoring for power transformers is key to implementing a correct condition-based maintenance (CBM). In this regard, partial discharges (PD) measurement and its analysis allows to carry out on-line maintenance following the standards IEC-60270 and IEC-60076. However, new PD measurements techniques, such as acoustics or electromagnetic (EM) acquisitions using ultra-high-frequency (UHF) sensors are being taken into account, IEC-62478. PD measurements with antennas and the effect of their EM propagation in power transformer tanks is an open research topic that is considered in this paper. In this sense, an empty tank model is studied as a rectangular cavity and their resonances are calculated and compared with their measurement with a network analyser. Besides, two low cost improved monopole antennas deployed inside and outside of the tank model capture background noise and PD pulses in three different test objects (Nomex, twisted pair and insulator). The average spectrum of them are compared and can be found that mainly, the antenna frequency response, the frequency content distribution depending on the PD source and the enclosure resonances modes are the main factors to be considered in PD acquisitions with these sensors. Finally, with this set-up, it is possible to measure PD activity inside the tank from outside.
Highlights
Measurement of partial discharges using very-high frequency (VHF), 30–300 MHz, and ultra-high frequency, 300–3000 MHz, sensors has become an important tool when monitoring and assessing the state of the insulation system on gas-insulated substations (GIS) and power transformers [1,2]
The current pulses that occur as a consequence of partial discharges (PD) activity within this electrical equipment can generate the presence of electromagnetic emissions with bandwidths ranging from
The second is the frequency response of the antenna used, that must be matched for all the bandwidth required
Summary
Measurement of partial discharges using very-high frequency (VHF), 30–300 MHz, and ultra-high frequency, 300–3000 MHz, sensors has become an important tool when monitoring and assessing the state of the insulation system on gas-insulated substations (GIS) and power transformers [1,2]. One of the main drawbacks when using externally such sensors in electrical equipment with self-shielding, according with its constructive nature, is that it may modify and/or mitigate the EM generated by PD activity inside the enclosure [8] This happens in the case of power transformers, where part of the insulation system susceptible to the presence of PD is within this physical structure. In this regard, some studies have been addressed in order to establish the effect of shielding in the attenuation suffered by EM waves when pulses from inside are acquired from outside the transformer tank [8,9]. Background noise antenna outside 4.5 x 10 Amplitude [V] -7 x 10 Power [V2] FM TV GSM Wi-Fi
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