Diagnostics on the Basis of the Frequency-Temperature Dependences of the Loss Angle Tangent of Heavily Moistured Oil-Impregnated Pressboard

Abstract

The aim of this study was to perform precision measurements of the frequency-temperature dependences of the loss angle tangent of the liquid-solid composite with the FDS Dirana meter. The composite consisted of heavily moistered oil-impregnated pressboard. The moisturization of the pressboard occurred in a manner as close as possible to the process of wetting the insulation in power transformers to a moisture content of (5.0 ± 0.2) wt. %. This value of moisture content was chosen because exceeding this value can lead to transformer failure. The measuring temperature range was from 293.15 K (20 °C) to 333.15 K (60 °C), with a step of 8 K. The measuring frequency range was 0.0001 Hz to 5000 Hz. It was observed that the shape of the frequency dependence of the loss angle tangent for a moisture content of 5.0 wt. % does not depend on the value of the measuring temperature. An increase in temperature leads to a shift of the waveforms into the higher frequency region. This is associated with a decrease in the relaxation time, and its value depends on the activation energy. It was found that a good fit of the waveforms, simulated by Dirana, to the actual tgδ waveforms obtained at temperatures between 293.15 K (20 °C) and 333.15 K (60 °C) requires the introduction of temperatures, higher than the actual insulation temperatures, into the program. It was found that estimating the moisture content for different temperatures using Dirana soft-ware for insulating an oil-impregnated pressboard produced large discrepancies from the actual content. Better results were obtained after an adjustment requiring manual temperature correction towards higher, compared to measured, temperatures. The moisture content estimated after correction by the Dirana meter ranges from of 4.5 wt. % to 5.7 wt. % and increases almost linearly with increasing measuring temperature. The average moisture content estimated by the Dirana meter for all measuring temperatures is 5.1 wt. % and is close to the actual content (5.0 ± 0.2) wt. %. The uncertainty of the estimate is ±0.43 wt. % and is more than twice as high as the true value.

Analysis of the Influence of Insulation Moisture during Long-Term Exploitation of Transformers on Their Reliability as Determined by Alternating Current Electrical Parameters

“…Firstly, a precise determination of the so-called model characteristics-the frequency-temperature dependencies of the oil-impregnated pressboard moistened to a given water content-must be carried out. In a previous study [28], model characteristics were determined for water contents ranging from 1 wt.% to 4 wt.% while model characteristics were determined for a critical content of 5 wt.% in the papers [29][30][31][32]. Secondly, transformer insulation measurements should be carried out under conditions of thermodynamic equilibrium between the pressboard, insulation oil and water [5,33].…”

“…This means that determining the insulation reliability status of transformers based on software dedicated to FDS meters does not accurately detect the risk of failure. In [31], it was found that in order to match the frequency dependence of the loss angle tangent simulated by the meter to the waveform obtained by measurement, a temperature, greater than the actual measurement temperature, must be entered into the software. Even after manual temperature correction, the result given by the meter is overestimated by approximately 1 wt.% in relation to the actual moisture content.…”

“…For example, as reported in [29], in the ultralow-frequency region, DC conductivity was established to be equal to the AC conductivity of a moist oil-impregnated insulating pressboard. The studies, presented in the publications [29,31,32,34], show that the models used to date in the FDS meter software have resulted in large errors in the determination of the water content of the cellulose component of transformer insulation, the oil conductivity and the cellulose-oil volume ratio. This means that in order to estimate moisture correctly, a real model of conductivity and polarization found in composites should be developed and used in FDS meter software.…”

Research on Changes in the Phase Shift Angle and Admittance of the Cellulose–Bio-Oil Composite under the Influence of Increasing Moisture during the Long-Term Operation of Power Transformers

“…For this purpose, pressboard plates were moistened to various moisture contents, impregnated with mineral oil, and then temperature-frequency characteristics were determined [24][25][26][27]. Recent publications [28,29], have determined that FDS meter software provides moisture values inconsistent with actual ones. This implies that the models assumed in the meter software do not consider the real phenomena of conductivity and dielectric relaxation occurring in transformer insulation.…”