Aging rate of grade 3 presspaper insulation used in power transformers

Abstract

Paper materials are used as the insulation of power transformers. Over time these materials slowly degrade, until they reach a point when they no longer function effectively as transformer insulation, and the transformer has then reached its end of life. The aging rate of paper is affected by temperature, water, oxygen and acids. Investigations have been performed previously on Kraft and Kraft thermally upgraded types of paper. However, a different type of paper used in transformer insulation, Grade 3 presspaper (which contains cotton), has not been extensively tested and compared to Kraft paper. In these experiments we studied the aging rate of Grade 3 presspaper and compared it to our previous studies of Kraft paper. Traditionally, the aging rate of paper has been studied in sealed vessels. The problem with this approach is that the chemical environment within the vessel will change during aging, and so the aging rate will be affected. In our experiment setup we controlled the water and oxygen content to more accurately determine the aging rate. Similarly to Kraft paper, the aging rate of Grade 3 presspaper with the same water content increased with oxygen content in the oil. Life curves were developed based on the water content of the paper and the oxygen content of the oil.

Condition monitoring of biodegradable oil filled transformer

“…This study has also confirmed that NEs offer better performance concerning ageing of cellulose paper insulation than both mineral oil and synthetic ester oils. of wet insulation in BIOTEMP is 100ºC, confirming that an increase of moisture accelerates the ageing of paper insulation in NE in a way similar to mineral oil-paper insulation systems investigated in several studies [53,62].…”

“…Emsley and Stevens [5] have found that most of the published ageing data on cellulose paper materials is shown to be in good agreement with the pseudo-zero rate kinetic model developed by [53]. Factor (A) shows great dependence on the availability of reactants such as moisture, low molecular acids in paper and dissolved oxygen in oil.…”

“…Paper insulation is also produced from a mixture of unbleached Kraft wood pulp and cotton pulp. Mixing of Kraft wood pulp with cotton improves the dielectric and mechanical strength of paper insulation [17,53]. Moreover, Kraft-cotton composite paper shows a better oil absorption capability.…”

“…It causes to overestimate the expected lifetime of paper insulation in a transformer with IEEE loading guide criteria. Figure 2.8 compares the calculated lifetime curves of paper insulation using parameters obtained in [53,62] and eq (2.7) with IEEE lifetime curve. Here…”

“…Emsley [5], Lundgaard [62] and Lelekakis [53] have claimed that this linear relationship continues until the DP of paper insulation falls to about 200. Consequently, reaction rate (k) is the gradient of the plot of the number of chain scissions Chapter 4: Understanding the ageing behaviour of ester-paper insulation systems 77 | P a g e versus ageing time.…”