Analysis of Young’s Modulus, Zero- and Wide-Span Tensile Index and Degree of Polymerization of Insulating Paper for Power Transformers from Accelerated Aging Tests

Inhalt:
Power Transformer failures are often correlated with a breakdown of the cellulosic insulating system. Mainly the thin insulating paper wrapped around the conductors is affected by aging dependent degradation. There exists a multiplicity of publications dealing with the tensile index and degree of polymerization (DP) of aged insulating paper. However, this contribution presents some additional measurement results for the trend of the Young’s Modulus during the aging process. For this purpose, factory new insulating paper was aged in transformer oil at 105 °C and 115 °C for a period of 200 days. Paper samples were taken regularly at both temperatures to measure their degree of polymerization, tensile index, and Young’s Modulus. Both degree of polymerization and tensile index decrease with increasing aging time. In the beginning however, the Young’s modulus increases and first decreases for very low values of DP around 200. The mechanical parameters (tensile index, Young’s modulus) are correlated with the chemical parameter DP using analytic functions. Furthermore, the aged samples were analyzed under a laser microscope in order to visualize the aging dependent structural change of the cellulose fiber texture.