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Improved Ageing Characteristics of Insulating Papers Made from Thermally Upgraded Highly Crystalline Cellulose

IP.com Disclosure Number: IPCOM000238542D
Publication Date: 2014-Sep-03

Publishing Venue

The IP.com Prior Art Database

Abstract

During the development of an easy-to-dry layer insulation paper for distribution transformers, ageing benefits were discovered from a thermally upgraded blend of cotton and softwood Kraft fibers. As measured by tensile strength retention, the blended paper aged more slowly in mineral oil and in natural ester liquid, compared to softwood Kraft with an equivalent thermal upgrading content. Cotton’s highly crystalline cellulose structure is believed to be responsible for the slower ageing, and a mechanism is proposed explaining the results.

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Improved Ageing Characteristics of Insulating Papers Made from Thermally Upgraded Highly Crystalline Cellulose

  Authors: William Ernst, Aleksandr Levin, Kevin Biggie, Steven Ho, Jinesh Malde (WEIDMANN Electrical Technology, Inc. St. Johnsbury, VT USA) September 2, 2014

Executive Summary

During the development of an easy-to-dry layer insulation paper for distribution transformers, ageing benefits were discovered from a thermally upgraded blend of cotton and softwood Kraft fibers. As measured by tensile strength retention, the blended paper aged more slowly in mineral oil and in natural ester liquid, compared to softwood Kraft with an equivalent thermal upgrading content. Cotton's highly crystalline cellulose structure is believed to be responsible for the slower ageing, and a mechanism is proposed explaining the results.

Additional work is underway to confirm the thermal class of these cotton / Kraft papers in mineral oil and natural ester liquid, and to determine the ageing characteristics of other forms of highly crystalline cellulose when thermally upgraded: such as, but not limited to other forms of cotton, flax, hemp, sisal, abaca, jute, algeal cellulose, and nano-cellulose products, alone or when blended with Kraft.

The use of thermally upgraded highly crystalline cellulose papers may create new elevated temperature design options for both power and distribution transformers once their thermal class is confirmed.

Background

Transformer reliability and life extension have become even more important in recent years due to ageing transformer fleets, increasing electrical loads, and increasingly variable loads on the electrical grid. India's grid failure on July 30-31, 2012 affected roughly 600 million people, and is a recent example of the severe consequences of poor grid reliability.

It is well known that the cellulose insulation ages during the transformer's operating life and is generally considered to be the life-limiting factor for transformers. One of the most critical aspects of cellulose ageing is the loss of the paper's mechanical strength that helps to maintain the winding integrity and resistance to the mechanical forces experienced during short circuit, switching, lightning strikes, and other events. If the paper cannot withstand the mechanical forces in the winding during such an event, the winding conductors will move. Even if electrical breakdown does not occur during the event itself, the change in the winding geometry will increase future stresses and increase the possibility that a future event will result in failure.

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Since the 1960's, the thermal upgrading of papers and boards for use as electrical insulation has been known to slow the irreversible ageing degradation of cellulose [1,2,3]. A number of patents were filed describing various systems for thermal upgrading, although it does not appear that differences in the effectiveness of thermally upgrading
of highly crystalline cellulose were in...