Analysis and comparison of aging-trends of cellulose for transformers with oil-cellulose-insulation

Abstract

The lifespan of a transformer with oil-cellulose-insulation is, in the end, dependent upon the aging of the cellulose. While the insulating oil can be reprocessed or renewed, the aging of the cellulose is an irreversible process. The current condition of aged cellulose is described by the degree of polymerization (DP). When it is new, the insulating paper has a DP-value in the range of between 1000 and 1200. This value decreases over the course of time due to aging processes such as hydrolyses, pyrolysis and oxidation. A degree of polymerization of about 200 to 300 is regarded as the “Cend-of-life” of the paper. Aging models defined by engineering standards only refer to temperature and time as factors which impact the aging of cellulose. On the other hand, influential variables such as moisture, cellulose quality (kraft paper / thermally upgraded paper) and oxygen (air) are, as yet, not considered in these models. The water, in particular, accrued as part of the aging process splits up the cellulose chains of the solid insulation. The water content thus acts to continuously accelerate the aging process. In this paper, measured and calculated DP-time-trends of the cellulose are analyzed and compared. Furthermore ideas for an aging model with a new physical approach are presented.