Assessment of the effect of commercial vegetal oils on Kraft paper ageing through mechanical characterization

Vegetal oils are considered a suitable substitute for mineral oil that is widely used in power transformers as insulation and cooling medium. Due to the fact that more and more power transformers containing this alternative insulating fluid are being constructed a mathematical ageing models is needed to help predict transformer failures. The continued performance of power transformers depends on the condition of its paper insulation mainly. In this sense, this paper analyses Kraft paper degradation through the loss of its mechanical strength. Accelerated thermal ageing test of the paper in two different vegetal oils were carried out at three temperatures during diverse periods of time, in order to obtain information on the kinetics of the ageing degradation of the paper. The evolution of the mechanical properties of paper failure are analysed as a function of temperature and ageing time. In addition, the results obtained are compared with the traditional method of degradation analysis, based on the measurement of the degree of polymerisation.

1 Introduction

Power transformers play a critical role in transmission and distribution electrical networks [1], [2]. During their operation these machines suffer thermal, electrical, mechanical and chemical stresses which degrade insulation system [3]. This insulation system usually consists of dielectric oil and paper. The functions of the liquid are provide electrical insulation and absorb and transmit the heat produced by energy losses to reduce the rise of temperature in the windings and cores. On the other hand, the dielectric solid provides both electrical and mechanical support in power transformers, its immersion in insulating oil increases its electrical insulation strength [4].