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Transformers are essential components of power delivery and their failure can cause serious problems in electric utility operation.
Among the causes of transformer failures, those related directly or indirectly to the winding deformation and ageing of insulation system, are outstanding. Therefore, the study on methods of detecting the transformer winding deformation and insulation ageing is of practical value to avoid unexpected accidents. Among the different diagnostic methods, the Sweep Frequency Response Analysis (SFRA) gained popularity for its ability to evidence winding displacements and internal short-circuits between turns or coils, while the Frequency Dielectric Spectroscopy (FDS) and Polarization Depolarization Current (PDC) methods are nowadays in a very advanced experimental phase for the assessment of transformer insulation system (ageing evaluation), particularly the humidity content of the paper. In fact, the presence of humidity in the paper-oil insulation system can cause premature ageing, mechanical strength reduction and, in the case of abrupt temperature variations, formation of gas bubbles leading to potential internal discharges. The results of an extensive research program carried out both in laboratory and in the field with the purpose to obtain reliable diagnostic tools based on SFRA, PDC and FDS test methods, are reported in this paper. Since both methods are based on the comparison among the present results and those obtained in sound or known conditions (reference), significant efforts have been spent on the analysis of the relationship between the physical aspects behind the SFRA curve modification and the relevant impact in reliable diagnostics. Moreover, since the understanding of the comparison requires skilled operators and wide experience, an advanced method based on the “Fuzzy Logic” has been developed for the automatic analysis of the curve modification. The advantages obtained by the use of these modern techniques for transformer condition assessment are also discussed reporting examples of the results obtained applying the two methods on power transformers in service conditions.
31 Dicembre 2007
Tecniche di valutazione delle condizioni, della vita e delle funzionalità residue di componenti elettrici mediante metodiche sotto tensione (P2 (T&D))