Changes of new unused insulating kraft paper properties during drying - Impact on degree of polymerization

The degree of polymerization of kraft paper is one of the key properties of cellulosic insulation material used in power transformers, since it is providing information about paper mechanical properties and ageing condition. In this paper, a short introduction on cellulose insulation composition and changes during physical treatment of insulation drying is given, together with the results of round robin test of paper degree of polymerization (DPv) using viscometric technique. The DPv measuring method is used for quality control of new paper samples, evaluation of cellulose degradation under transformers operating conditions, and in recent decades for the evaluation of the condition of paper insulation after the transformer manufacturing drying process. The method is standardized as the standard IEC 60450 in 1974 and over time it has undergone several revisions. The initiative for this work were problems encountered with determination of DPv of some of the unused kraft papers after factory drying, as reported at the CIGRE SC A2 colloquium in Krakow in 2017.

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