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 methodology 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 conventional equilibrium diagrams, for example it is unnecessary to wait for moisture equilibrium. For the new approach, combination of time and frequency domain measurements substantially shortens the measurement time, typically to 25 % compared to a pure frequency domain measurement. 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 byproducts. Conductive aging byproducts have similar dielectric properties as water; without compensation the analyzed moisture content will be too high, leading to unnecessary actions like drying. This paper also includes examples of practical onsite measurements in comparison to other measurement methods and a guide for assessing the obtained moisture content.