Condition assessment of field aged transformer oils up to 40 years

Transformer oil is one of the essential insulation medium used in power transformers. During transformer operation oil may expose to various stresses such as electrical, thermal, chemical etc. As a result electrical properties of the oil may be degraded and hence leads to transformer failures. This study aims at monitoring condition of field aged oil and establishing any correlation among electrical properties. Twenty four oil samples (750 ml each) were collected from field aged transformers. The samples covered a wide spectrum of field ageing from one year to 40 years. First, colour of the collected oil samples were observed and categorized according to standard colour code (ASTM-D1500). Then electrical properties were obtained by frequency domain spectroscopy (FDS) measurements and breakdown voltage tests. The FDS measurements give the frequency dependent complex capacitance (C´ & C") and loss tangent (tanδ). Measurements were further utilized to estimate conductivities of the oil samples. As one may expect longer service life (i.e. longer ageing time) leads to darkening of the oil and lower breakdown voltages. The loss tangent and colour code has a reasonable good negative correlation with the breakdown voltage. The loss tangent and permittivity has a good positive correlation. However colour code and breakdown voltage does not have a correlation with the permittivity.