Understanding Moisture Dynamics and Its Effect on the Dielectric Response of Transformer Insulation

Dielectric response measurement has recently been adopted by utilities for evaluating moisture content in cellulose insulation (paper and pressboard) of transformers. Moisture distribution is highly dependent on temperature. Since the temperature inside a transformer may change during the dielectric response measurement, the moisture in the transformer´s cellulose and oil insulation can hardly attain an equilibrium state. Instead, moisture dynamics exist inside the transformer: 1) cellulose absorbs (desorbs) moisture from (to) oil with the changes in temperature and 2) moisture migrates inside cellulose due to a moisture gradient. This paper investigates moisture dynamics and its effect on the dielectric response of a transformer´s cellulose insulation. It proposes a distributed parameter model to reveal the correlation between moisture distribution (under nonequilibrium conditions due to thermal transients) and dielectric response parameters (dielectric losses and permittivity) of cellulose insulation. It then estimates these parameters under moisture nonequilibrium conditions. The accelerated ageing and moisture diffusion experiments are conducted on a prototype transformer to verify the proposed model. The methodology developed in this paper can help the proper interpretation of dielectric response measurement of field transformers under thermal transients.