Study on the degradation of the compression strength of spacers made of high-density pressboard used in power transformers under the influence of thermal ageing

Abstract

The structural components inside power transformers are commonly made of high-density pressboard, due to its suitable mechanical and dielectric properties. One of these essential components are the spacers used in the windings of transformers, which are subjected to compressive loading during operation. The spacers are immersed in dielectric liquid and subjected to high temperatures and chemical reactions during the lifespan of the transformer, which result in the degradation of their dielectric and mechanical properties. The performance and reliability of the power transformer greatly depends on its mechanical integrity, so it is necessary to understand how the ageing degrades the mechanical response of the high-density pressboard. In this study, spacers made of high-density pressboard were immersed in uninhibited naphthenic oil and aged at 150 ºC for different periods of time. The evolution caused by the thermal ageing over some chemical parameters (acidity and moisture content) and dielectric properties (AC breakdown voltage, dielectric dissipation factor, resistivity and degree of polymerisation) of the oil and the high-density pressboard was studied experimentally. Compressive mechanical tests were performed on samples of the aged high-density pressboard, and the compressive stiffness during the ageing process was related with other chemical and dielectric parameters.

Experimental dataset on the compressive mechanical properties of high-density pressboard used in power transformers spacers

“…Moreover, this dataset provides a valuable supplement to the corresponding original research article, referenced as Ref. [6] , as it offers empirical evidence and precise quantitative measurements of the compressive loading curves. These curves serve as the basis for the stiffness values reported in the referenced article.…”

“…Adapted from Ref. [ 6 ]. High-density pressboard Paraffinic dielectric oil Property Unit Value Property Unit Value Thermal class °C 105 Density at 20 °C g/cm 3 0.839 Apparent density g/cm 3 1.2 Viscosity at 40 °C mm 2 /s 9.98 Nominal Thickness mm 2 Viscosity at -30 °C mm 2 /s 925.85 Tensile Strength (MD) MPa 124 Flash point °C 176 Tensile Strength (CD) MPa 92 Pour point °C -48 Elongation at breakage (MD) % 3.9 Interfacial tension mN/m 43 Elongation at breakage (CD) % 4.6 Dielectric dissipation factor, 90 °C – 0.00198 Compressibility % 4.8 Shrinkage (ZD) % 4.4 Acidity mg KOH/g <0.01 Shrinkage (MD) % 0.4 Water content mg/kg 15 Shrinkage (CD) % 0.5 Furfural content mg/kg <0.05 Oil absorption % 13

Graph 1 Degradation of degree of polymerisation in high-density pressboard with increasing duration of thermal ageing.

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“…Adapted from Ref. [ 6 ].

Graph 2 Compressive Test A: Load-displacement curves of the pressboard prism with three loading-unloading cycles at various ageing durations (a) 3 days, (b) 1 week, (c) 2 weeks, (d) 6 weeks, (e) 12 weeks and (f) 20 weeks.

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“…For each ageing period considered, a series of three compressive tests were conducted on the pressboard prism to obtain average values for the material's compressive stiffness which are presented in Ref. [6] . Pressboard, being a cellulosic polymer with a porous structure, exhibits a strongly non-linear behaviour in the thickness direction (ZD), see Graphs 2 and 3 .…”

“…10 and 11 from the related research article, see Ref. [ 6 ] only depict the third loading-unloading cycle for visual clarity, because compressive stiffness of the high-density pressboard was derived from this particular cycle. However, in this study, we present the complete set of compressive tests in Graphs 2 and 3 .…”