Practical Experience on Transformer Insulation Condition Assessment

Dielectric measurement methods are used for acquisition of moisture content in oil-paper-insulation of power transformers. Two equivalent principles exist: Time domain and frequency domain measurement. In this paper practical experience on measurement of Polarization and Depolarization Currents (PDC) on power transformers ranging from 100 to 800 MVA under different environmental conditions is described. Depending on the transformer type there are different strategies possible for cabling and grounding. Typical setups and resulting problems, e.g. the appearance of AC noise resulting from coupling of nearby energized lines or equipment and its reduction during measurements are discussed. On-site measurements on three coupled 110 MVA 400/220 kV single phase auto transformers before and after insulation oil reclaiming are presented