A fast and reliable dielectric diagnostic method to determine moisture in power transformers

This paper describes a novel dielectric diagnostic method ldquoDIRANArdquo which features a fast measurement technique and an improved analysis algorithm for determining moisture in pressboard and paper of oil-paper-insulated transformers. Dielectric response methods provide substantial advantages compared to moisture determination by oil sampling and conventional equilibrium diagrams, for example the influence of aging can be compensated and it is unnecessary to wait for moisture equilibrium. For the new instrument, the novel combination of time and frequency domain measurements substantially shortens the measurement time. The data analysis algorithm compares the measured dielectric properties of the actual power transformer to modeled dielectric properties. The software features a compensation for the influences of conductive aging by-products, which have similar dielectric properties as water. Without compensation the analyzed moisture content will be too high and may lead to unnecessary actions like drying. After moisture analysis, the new software categorizes the moisture concentration according to IEC 60422.