Application of Frequency Domain Spectroscopy (FDS) in assessing dryness and ageing state of transformer insulation systems

Moisture in oil-paper insulation acts as a catalyst in transformer insulation deterioration and accelerates ageing. Thus it is crucial to detect accurately the presence of moisture within oil and paper insulation early as to avoid any pre-mature failures. Reliability of power transformers depends on the integrity of paper insulation due to its irreversible properties. Chemical analysis method available is impractical and difficult to be performed to estimate the moisture content in the transformer solid insulation. It has to be performed under laboratory conditions and requires accessibility to the internal insulation system. In contrast, electrical measurement such as Frequency Domain Spectroscopy (FDS) is preferred due to its simplicity and practicality. The aim of the paper is to study and analyse the application of FDS technique and its influencing factors under laboratory condition using oil impregnated insulation model and factory measurements with variations like temperature, ageing rate and oil/paper conductivity. To determine the reliability and accuracy of the FDS method, field measurements at a Malaysian Electricity Utility Company, TNB were performed on twin power transformers and analysed against chemical analysis coulemetric Karl Fischer Titration (KFT) method.