Effects of moisture and temperature on the frequency domain spectroscopy analysis of power transformer insulation

Dielectric testing techniques such as frequency domain spectroscopy (FDS) is currently being widely used by the power utilities for the assessment of the condition of transformer oil-paper insulation (OIP) systems. However, it has been observed that the results of these tests are highly influenced by the operating temperature as well as the ingression of moisture into the paper insulation. The distribution, migration and equilibrium of moisture between oil and paper in transformer insulation system are also highly temperature dependent. It requires adequate experience and proper understanding to interpret the dielectric response results due to involvement of these factors. Proper analysis of the dielectric test result is only possible with a thorough understanding of the physical behaviour of the insulation system in response to temperature and moisture. To this end a series controlled laboratory experiments have been performed on a model transformer to study the effects of moisture and temperature on the FDS results. Later on, a possible analytical approach has been discussed to deal with situation in the field.