Insulation Diagnostics on Power Transformers Using the Polarisation and Depolarisation CurŽ .rent PDC Analysis

Abstract: A lot of power transformers currently in operation have been installed in the 60’s and 70’s. Now, they might have reached a critical age with an operating time of 30 years and more. Utili-ties have to make a decision either continuing to use these trans-formers or replacing them. As a basis for this decision informa-tion about the condition of the currently operating power trans-formers 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 mois-ture 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. Recently, new methods of dielectric spectroscopy have been developed to overcome this implication. The PDC-Analysis (Polarisation and Depolarisation Current Analysis) is a non-destructive 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 measure-ments on new and aged transformers. 1