Degradation analysis of insulation used in high power transformers by means of polarization/depolarization currents

One of the most important ageing indicators in transformers is the water content in the solid part of the insulation. The water reduces dramatically the dielectric strength of the solid insulation, accelerates ageing in paper and can cause water vapor bubbles. The paper ages too due to contaminants build up in the oil deposited in the paper. When the paper insulation is in high level of ageing, any winding movement caused by short circuit current or surge loading may result in short circuit between windings. The Polarization / Depolarization Currents (PDC) analysis is a nondestructive method in time domain for evaluation of dielectric response and conductivity of insulation. PDC measurements contain informations about ageing processes. The paper presents the physical principle and the mathematical description of measuring PDC technique. Measurements on two aged high power transformers are presented. The structure of the software program is presented and the water content in solid insulation is determined. On the basis of this analysis one can decide about actions like on-site drying of the active part of the transformer or replacement of the transformer.