Quantification of Kraft paper ageing in mineral oil impregnated insulation systems through mechanical characterization

Abstract

Most power transformers use Kraft paper as the main solid insulation between the winding conductors. Dielectric oil used in transformers as an insulating and cooling fluid typically has an operating temperature range of 60-90°C. These service temperatures can cause slow degradation of both the oil and the insulating paper winding, with a loss of mechanical and dielectric properties. In this sense, this paper proposes the possibility of analysing paper degradation through the loss of its mechanical properties. An accelerated thermal ageing of the paper in mineral oil was carried out at temperatures of 110, 130 and 150ºC over different periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties and micro mechanisms of paper failure are analysed as a function of temperature and ageing time. Finally, the results obtained are compared with the traditional method of degradation analysis, based on the degree of polymerisation (DP) measurement.

Accelerated Charge Dissipation by Gas-Phase Fluorination on Nomex Paper

“…Nomex paper has been recognized as an excellent insulation material due to its good thermal stability, stable mechanical and electrical properties and has been widely used in dry-type, converter and oil immersed transformers among others, in which the reliability of the oil-paper insulation system greatly influences the safety of the power transmission system [1,2]. Previous researches indicate that the surface charge accumulated on the oil paper interface and the space charge accumulated in the oil impregnated paper may accelerate the degradation process of the oil paper insulation system, distort the electric field distribution and lead to insulation failures such as flashover and breakdown [3,4].…”

Effects of aging on the mechanical and dielectric properties of transformer grade Kraft paper

“…Fig. 2a shows measurements of DP as a function of aging time for 3 independent sets of samples and as expected, the full range of DP values could be accessed within 500 h [4,7,9]. Elsewhere [2] an equation for the dependence of DP (DPt) on aging time (t) was derived from first principles;…”

“…For subsequent flow testing it was required to have aged paper samples of known DP and water content for insertion into a mineral oil of known condition. Consequentially, transformer grade Kraft paper (0.1 mm thickness) was cut into 10 x 10 cm pieces and hung on hooks in a fan oven maintained at 150 o C. This temperature was chosen to ensure that the complete range of DP values was accessible within a reasonable timeframe whilst avoiding pyrolysis [4,7,9]. The use of a fan oven, rather than aging samples under oil, has several advantages such as; removal of aging by-products [3], keeping the paper in a dry state during aging [9], ease of subsequent sample analysis and eliminating any aging of the mineral oil.…”

“…The moisture content was then inferred by drying to constant mass. Measurements of DP were performed according to ASTM D4243 [4,7]; 15 mg samples were dissolved in 20 ml of 0.5 M Bis(ethylenediamine) copper(II) hydroxide solution (Sigma Aldrich) through constant stirring in closed jars for 4 h. The viscosity of the resulting solutions was then measured five times using a glass capillary viscometer placed into a water bath maintained at 20.0 ± 0.5 o C and the DP was then calculated. To check for oxidation, infrared spectroscopy was performed using a Thermoscientific Nicolet iS5 spectrometer fitted with an iD7 ATR accessory.…”

Kraft And Diamond Dotted Paper Thermally Aged In Mineral Oil And Natural Ester: Mechanical Characterisation

“…Alghough, the parameters obtained in the test can be Young's Modulus (E), yield stress (σ y ), rupture strength (σ R ), strain under ultimate strength (εcm), and energy consumed per unit volume of the failure zone (E R ). However, in this work only σ R , ε cm and E R were evaluated because in a previous work [28] the experimental results showed that Young's Modulus and yield stress can hardly provide any information on the degree of degradation. In the case of Young's Modulus, their values were practically the same during the thermal ageing period.…”