Understanding the effects of moisture equlibrium process on dielectric response measurements for transformer oil-paper insulation systems

Moisture present in oil and paper insulation is extremely damaging for healthy operation of a transformer. Moisture not only reduces electrical, chemical, mechanical and dielectric strength of the insulation, it also accelerates the insulation ageing process, thereby reducing its effective remaining life. It is thus of high interest for the utilities to have accurate quantitative estimates of moisture content in oil and cellulose insulation of a transformer. Oil moisture can be measured by chemical analysis of oil samples collected from running transformer. On the other hand, collecting paper sample for moisture measurement being extremely difficult, moisture content in paper are normally estimated from measured oil moisture values using certain mathematical equations, or equilibrium charts or recently, from dielectric response measurements. Such estimation of paper moisture content from measured values of oil moisture is largely dependent on the equilibrium status of moisture in the oil-paper insulation assembly. This equilibrium process is highly temperature dependent and thus poses serious accuracy issues in moisture estimation procedures. The present contribution reports results of experiments performed on the oil-paper insulation structure of a model transformer. Operating temperatures have been varied over a range of 40°C to 80°C to study the effects of moisture equilibrium process on measurement of oil moisture and estimate of paper moisture there from. Dielectric response analysis using polarization and depolarization currents (PDC) measurement have been used to further study the effects of moisture equilibrium at different temperatures. A comparative analysis of oil and paper moisture estimates using chemical tests, mathematical formulae, equilibrium charts and dielectric response measurements have also been reported in this article.