Frequency Domain Measurements for Diagnosis of Power Transformers: Experiences from Australia, Malaysia, Sri Lanka and UK

This is the author accepted manuscript. The final version is available from CIGRE via the link in this recordPower transformers and their performance and reliability in electric power networks are important research issues of power systems. Much information has already been collected and reported; however, it is rare to report comparisons of transformer service performance under different geographical locations and working conditions. CIGRE Working Group A2.37 collected information on 964 transformer failures from 58 utilities and 21 countries covering the period from 1996 to 2009. The main failure modes included failures in windings, tap changers, bushings, insulation, and core. As estimated, based on available literature, nearly 50% of the failures could be related to transformer windings and insulation. This collective paper summarizes experiences from the use of diagnostic tools based on frequency domain measurements at different geographical regions of the world. It starts with a short introduction to response measurements in frequency domain for condition assessment of transformers; then theoretical backgrounds of the two widely used response measurements, namely Frequency Dielectric Spectroscopy (FDS) and Frequency Response Analysis (FRA), are presented. Afterward details of tested transformers and measurement procedures are described for which selected set of FDS and FRA measurement results are analysed for respectively elucidating their potential in assessment of moisture content, ageing degree and fault identi cation

Dry-band discharges dynamic at the surface of SiR textured insulator during AC inclined plane test

Polymeric insulators are widely used for HV overhead lines and substations in polluted areas because of their hydrophobicity. However, it has been seen that the shank regions are vulnerable to thermal damage from partial arcs. Textured insulators have recently been proposed in order to reduce both the leakage current density and electric field strength in such regions, and by the same means to achieve an increase in creepage length. Inclined plane testing according to IEC 60587 is an indicator for the evaluation of the resistance to tracking and erosion properties of the silicone rubber formulation. In the case of conventional plane SiR insulators, it has been seen that dry bands were readily formed at or near the ground electrode, and these were often bridged by arc-like discharges causing deep erosion. By using IR camera, we measured the thermal activity of dry bands and the associated discharge in order to estimate the dynamic of the streamer/sparks with time. The observations show that the displacement of those discharges is not uniform and depends on the insulator surface state. Correlation between the temperature variation and the discharge position will made and a comparison with leakage current evolution will be discussed. A relationship is proposed also for the calculation of both of the insulator power loss and resistance

Image analysis and laser induced fluorescence combined to determine biological growth on silicone rubber insulators

High-voltage outdoor insulators made from silicone rubber are sometimes reported to be colonised by microorganisms. When the hydrophobic polymeric surface is covered by a hydrophilic biofilm, the electrical properties of the insulator are altered. In this work, mixed biofilms, similar to those formed on the surfaces of polymeric insulators in the field, were successfully grown on five types of silicone rubber substrates in the laboratory, using specially designed microenvironment chambers. Photography and digital image analysis were utilized to estimate the areas covered by the growth. It was found that direct UV-light exposure hindered growth of the biofilms. Further, growth was also hindered on samples where zinc borate had been added as flame retardant. In contrast, addition of ATH did not influence the growth. In parallel, LIF spectroscopy was explored as a tool for detection of biofilms on silicon rubber samples. Experiments revealed that even weak traces of growth, not visible to the naked eye, could be detected. Finally, it is believed that LIF spectroscopy in combination with image analysis can be used for field diagnostics of biological growth on insulators in service