On-site methods for reliable moisture determination in power transformers

This paper discusses present day and new on-site approaches to determine moisture in power transformers: dielectric response methods and equilibrium diagrams. In recent years dielectric diagnostic methods have been developed, which derive moisture concentration of the solid insulation from its dielectric properties. This paper presents a new method, which combines measurements in frequency and time domain, and which makes reliable diagnostics possible even for heavily aged insulations. The second part of this paper deals with equilibrium diagrams applied for moisture determination. Since the conventional application of these diagrams leads to erroneous results for aged insulations, a new representation of equilibrium diagrams based on moisture saturation in oil is introduced. Examples of onsite moisture determinations compare the new methods to conventional approaches and describe the assessment of moisture analysis results.