Examining the ageing of transformer insulation using FRA and FDS techniques

Abstract

Frequency response analysis (FRA) is a widely accepted testing technique to assess the mechanical condition of a power transformer. However, it has been reported that FRA is also sensitive to non-mechanical parameters such as the temperature and moisture content of the insulation. Therefore, further studies are required to investigate the ability of FRA on examining the transformer insulation condition. In this paper a study based on an insulation degradation of a single phase 5 kVA model transformer is presented. Throughout the accelerated insulation degradation process, FRA and Frequency Domain Dielectric Spectroscopy (FDS) measurements are performed to analyze the degradation condition. Two methodologies are proposed to analyze the FRA results based on the relative change of capacitance and change of inter-winding capacitance. Results obtained from these approaches are verified with FDS analysis.Index Terms -Power transformers, frequency response analysis, frequency domain spectroscopy, insulation ageing.

Electrical-Based Diagnostic Techniques for Assessing Insulation Condition in Aged Transformers

“…The frequency response of the dielectric materials is being widely used as a diagnostic tool for insulation systems [51,[110][111][112][113][114][115][116][117][118][119]. The monitoring of the complex permittivity and the DF of transformer insulation, as function of frequency provides inside information concerning the state of insulation within the components.…”

“…The same is true for the higher frequency range (>10 Hz). The central part of the response is, on the other hand, influenced by the properties of the oil, mainly by its conductivity [51,[110][111][112][113][114][115][116][117][118][119].…”

“…It is now well accepted that FDS measurement techniques can provide indication onto aging and moisture content of the transformer insulation. However, Saha et al [118] revealed that moisture content has a dominant influence on nearly all electrical based diagnostic techniques for assessing the condition of insulation, and indeed, masks their capability to Different parts of the response, in frequency axis, are separately sensitive to properties of oil and solid parts of the insulation, as illustrated in Figure 5. At the very low frequency range (<10 −2 Hz) the response is mainly influenced by the condition of the cellulose.…”

“…Theoretical and experimental results have been reported in many contributions to demonstrate the effects of temperature, electric field, ageing and moisture content of paper and oil on the FDS results (e.g., [51,[110][111][112][113][114][115][116][117][118][119]). It is now well accepted that FDS measurement techniques can provide indication onto aging and moisture content of the transformer insulation.…”

“…It is now well accepted that FDS measurement techniques can provide indication onto aging and moisture content of the transformer insulation. However, Saha et al [118] revealed that moisture content has a dominant influence on nearly all electrical based diagnostic techniques for assessing the condition of insulation, and indeed, masks their capability to determine the presence and extent of aging by-products of the insulation [121]. However, it must be emphasize that moisture and aging separation still constitute a challenging point in this domain.…”