A comparative study of physicochemical, dielectric and thermal properties of pressboard insulation impregnated with natural ester and mineral oil

Abstract

Natural ester is considered to be a substitute of mineral oil in the future. To apply natural ester in large transformers safely, natural ester impregnated solid insulation should be proved to have comparable dielectric strength and thermal stability to mineral oil impregnated solid insulation. This paper mainly focuses on a comparative study of physicochemical, ac breakdown strength and thermal stability behavior of BIOTEMP natural ester/pressboard insulation and Karamay 25# naphthenic mineral oil/pressboard insulation after long term thermal ageing. The physicochemical and dielectric parameters including moisture, acids and the ac breakdown strength of these two oil/pressboard insulation systems at different ageing status were compared. The permittivity and ac breakdown strength of these two oil/pressboard insulation systems at different temperatures were also investigated. And a comparative result of the thermal stability behavior of these two oil/pressboard insulation systems with different ageing status was provided at last. Results show that though natural ester has higher absolute humidity and acidity during the long ageing period, the lower relative humidity of natural ester helps to keep its ac breakdown strength higher than mineral oil. The pressboard aged in natural ester also has higher ac breakdown strength than that aged in mineral oil. The lower relative permittivity ratio of natural ester impregnated paper to natural ester is beneficial to its dielectric strength. Using natural ester in transformer, the resistance to thermal decomposition of the oil/pressboard insulation system could be also effectively improved.Index Terms -Natural ester, mineral oil, pressboard, physicochemical, dielectric strength, thermal stability.

Breakdown Voltage and Its Influencing Factors of Thermally Aged Oil-Impregnated Paper at Pulsating DC Voltage

“…All the above tendencies became inconspicuous when the OIP reached its end of life. It should be noted that the changing tendencies of the AC breakdown voltage at both temperatures were not in line with that obtained in reference [12], while they were similar to that in reference [21]. Nevertheless, the changing tendency of breakdown voltage versus aging time at DC and pulsating DC voltage with small ripple factors (r = 1/5 and 1/3) was opposite to the trend at AC voltage.…”

“…All the above tendencies became inconspicuous when the OIP reached its end of life. It should be noted that the changing tendencies of the AC breakdown voltage at both temperatures were not in line with that obtained in reference [12], while they were similar to that in reference [21]. …”

“…The papers used were all unaged insulating papers. There are few publications investigating the breakdown voltage of thermally aged paper at AC voltage [12,21,22], let alone investigation into that at pulsating DC voltage. Sun et al [12] found the AC breakdown voltage of thermally aged OIP increases with the increased duration of thermal aging.…”

“…The increase in breakdown voltage was attributed to the emergence of micro globules and the breakdown of the cellulose fibers, which improved the electric field distribution in the OIP. Liao et al [21] measured the AC breakdown strength of oil-impregnated pressboards thermally aged in both mineral and vegetable insulating oil at 110 • C. The breakdown voltage of the pressboard fluctuates with the increase of thermal aging duration and no clear correlation with degree of polymerization (DP) was found. Tang et al [22] investigated the AC breakdown voltage of thermally aged paper in both mineral and vegetable insulating oil at 90 • C, 110 • C, and 130 • C, and no pronounced correlation between AC breakdown voltage with the DP of the paper or thermal aging temperature was observed.…”

Streamer Propagation and Breakdown in a Very Small Point-Insulating Plate Gap in Mineral Oil and Ester Liquids at Positive Lightning Impulse Voltage

“…Besides, both esters are characterized by the ability to absorb water from the insulating paper, which is desirable from the point of view of paper aging. Additionally, higher electrical permittivity of esters (3.2-3.3) compared with that of mineral oils (2.2-2.4) results in a more uniform electrical field distribution in a paper-dielectric liquid insulating system [4,5,[8][9][10][11][12].…”