Laboratory Model Studies on the Drying Efficiency of Transformer Cellulose Insulation Using Synthetic Ester

Abstract

<jats:p>This paper presents the results of laboratory tests of cellulose insulation drying with the use of synthetic ester. The effectiveness of the drying process was investigated depending on the initial moisture of cellulose samples (2%, 3%, and 4%), ester temperature (55, 70, and 85 °C), initial moisture of the ester (70, 140, and 220 ppm), drying time (48, 96, and 168 h), and the weight ratio of cellulosic materials to ester (0.067 and 0.033). A large influence of temperature and time of drying on the efficiency of the drying process was found. This is important information due to the application of the results in the transformers drying procedure. The heating and drying ester unit should provide the highest possible temperature. For the assumed experiment conditions the initial moisture of the ester had little effect on the drying efficiency. An ester with a moisture content below 140 ppm can still be considered as meeting the requirements for drying cellulose with significant moisture. The weight ratio of cellulose products to ester has no major effect on drying efficiency during cellulose drying by circulating dry ester.</jats:p>

Analysis of Polarization and Depolarization Currents of Samples of NOMEX®910 Cellulose–Aramid Insulation Impregnated with Mineral Oil

“…This phenomenon was already observed in earlier publications from a co-author of this article [7,19]. Figure 13 presents the effect of the degree of sample ageing on the activation energy E A value ( Figure 13a) and values of two dominant time constants of the relaxation processes (Figure 13b), which were determined on the basis of the depolarization current characteristics according to Equation (5). The most important change that was observed on the characteristics from Figure 13 is that the growing degree of thermal degradation of cellulose fibers resulted in a minor, but constant, decrease in the activation energy value, after which the low-frequency relaxation process of the insulation being examined was subject to change.…”

“…The above conclusion was confirmed for all the examined samples, i.e., regardless of the degree of their moisture or ageing. Figure 7a presents Debye regression functions according to Formula (5), which were used in order to fix time constants of two relaxation mechanisms based on the depolarization current of the cellulose-aramid insulation sample being examined. Certainly, the data presented in Figure 7a are only exemplary, while the regression functions being described were used for all the insulation samples, i.e., for each degree of moisture and ageing and for all temperatures.…”

“…The presented difference is probably related to a significantly higher water absorbability by cellulose than aramid fibers. Figure 12 presents exemplary characteristics of the polarization currents (Figure 12a) and the depolarization currents (Figure 12b Figure 11b presents the effect of the degree of sample moisture on the value of two dominant time constants of the relaxation processes, determined on the basis of the depolarization current characteristics according to Equation (5). Increased moisture resulted in a slight decrease in time constant τ 1 , which was correlated with the relaxation process of aramid fibers, and, on the other hand, a considerably more intensive decrease in time constant τ 2 , which was correlated with the relaxation of cellulose fibers.…”

“…Particularly for new power transformers, the technical condition of the electric insulation saturated with dielectric fluids is regarded as the basic hazard to safe operation, where the key factors are the level of moisture as well as the degree of ageing, defined as the degree of the materials' thermal degradation [1][2][3][4][5][6]. Despite decades of designing and manufacturing power transformers, cellulose electric paper saturated with insulating mineral oil continues to be used as the basic electric insulation system.…”

“…Despite decades of designing and manufacturing power transformers, cellulose electric paper saturated with insulating mineral oil continues to be used as the basic electric insulation system. However, various kinds of synthetic and natural esters have become more and more recognized as substitutes for mineral oil [4][5][6][7][8]. One of the key factors accelerating the process of departing from mineral oils as the impregnating fluid, apart from improved electric, physical and chemical parameters of esters as compared to mineral oil, is their considerably better biodegradability.…”