Insulation diagnostics on power transformers using the polarisation and depolarisation current (PDC) analysis

A lot of power transformers currently in operation have reached a critical age with an operating time of 30 years and more. Utilities have to make a decision either continuing to use these transformers or replacing them. As a basis for this decision, information about the condition of the currently operating power transformers is required. Ageing processes are caused by dielectric, chemical, thermal and electrodynamic stresses. They are of very complex nature. As an example, water in the solid insulation material acts as a catalyst and enforces the ageing processes. Due to variable load there is a complex temperature dependent moisture dynamic behaviour of the oil paper insulation system. Well-known methods like Karl-Fischer-Titration and dew point measurement cannot be applied on transformers in operation since it is risky and difficult to open the transformer for getting a material sample. New methods of dielectric spectroscopy have been developed to overcome this implication. The PDC-analysis (polarisation and depolarisation current analysis) is a nondestructive method for determining the moisture content in the solid insulation material of a power transformer. On the basis of this reliable analysis one can decide about further actions like an on-site drying of the active part. This paper presents the PDC technique and results of measurements on new and aged transformers