Dielectric and mechanical assessment of cellulosic insulation during transformer manufacturing

Abstract

Due to the impact of cellulose of paper insulation on transformer life, it is imperatire to remove moisture from the oil and the solid insulation. Several techniques have been implemented during manufacturing of power transformers to reduce water content in transformers. These drying processes can involve different costs and time, and they can damage the insulation paper. In this work, a drying process has been implemented in the laboratory trying to simulate the most aggressive conditions that can be suffered by the paper in transformer manufacturing in a real industry. Then the effect of the drying process on paper was evaluated using the analysis of mechanical and dielectric properties and the degree of polymerization. Different commercial papers were studied to quantify the possible degradation induced by the drying process. The results of the mechanical strength study showed a reduction on the degree of polymerization from 1100 to 850 after 4 days of drying. The dielectric analysis of the samples showed different behavior in one of the solids evaluated and it was also found a decreased hygroscopic capacity of degraded samples in comparison with new samples.

Evaluation of the Aging of Oil‐Paper Insulation Using an Improved Fractional Poynting‐Thomson Model

“…Degradation of solid insulation can occur due to several factors, the primary agents of paper degradation being water (hydrolysis), acids resulting from the decomposition of oil when combined with water (acid hydrolysis), oxygen (oxidation), and temperature (pyrolysis) [17,18]. Pyrolysis promotes cellulose depolymerization, producing gases, namely hydrogen and carbon monoxide (CO) and carbon dioxide (CO 2 ), as well as liquids, such as water (H 2 O) and furan compounds [18].…”

“…The coincidence index can more intuitively express the deviation between the calculated and measured values. Equation (17) will be used to determine the coincidence index [11]:…”

“…Tanδ m and tanδ c represent the measured and calculated dissipation factor values, respectively. Figure 6 The results of the coincidence index of the traditional P-T, improved P-T model 1 , and model 2 obtained by (17) are presented in Table IV. It can be observed that the improved P-T model 2 has a better coincidence index than the traditional P-T and P-T model 1 .…”

“…Nevertheless, these models were proposed based on the Debye model. Thus, these models can be represented by the same general function (17), the H-N model [9]:…”

Numerical and experimental evaluation of dielectric properties of thermally aged insulating paper used in power transformers

“…Based on the previous works, this paper analyses the dielectric properties of several samples of Kraft paper with different ageing levels. Contrary to previous studies, where the relationship between ageing and dielectric response was studied under non-controlled conditions of cellulose moisture content, [7], samples with the same moisture content are compared. The objective is to discriminate the effect of the degradation from the one of moisture.…”