How are BDV, TAN, and moisture evaluated during testing of an oil transformer?

BDV is measured by applying increasing AC voltage across electrodes in a test cup until breakdown occurs, repeating several times and averaging results. TAN, meaning acidity or total acid number, is measured by titration to quantify oxidation products and acid formation. Moisture is measured using Karl Fischer titration in labs or portable moisture meters in the field. Together these values indicate dielectric margin, oxidation condition, and water contamination. Rising TAN and moisture typically reduce BDV and increase risk of partial discharge, overheating, and insulation aging.