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A significant amount of lightning impulse breakdown and withstand testing has been accomplished in mineral oil to understand this important electrical characteristic for transformer insulation system design. To be considered as viable insulating fluids for high voltage equipment, alternative dielectric liquids should have similar lightning impulse characteristics as compared to mineral oil to provide the clearances necessary for common dielectric design. This paper reviews the testing for establishing the lightning impulse breakdown characteristics of natural ester fluids relative to mineral oil test results. The key variables of the testing included: various oil gap and solid insulation creep distances, electrode configurations, electrical stress characteristics, and solid insulation surfaces. The fluid gaps ranged from 3 mm to 55 mm. The electrode configurations included quasi-uniform in oil gaps, with some attached to pressboard, and nonhomogeneous contacts in oil gaps combined with a phenolic interface. The range in gap distance was selected to reflect the range commonly used in liquid insulated transformer core/coil designs. The electrical stresses used included 1.2 x 50 μs lightning impulses of both positive and negative polarity. The solid insulation materials consisted of high density pressboard, Kraft paper and a high density phenolic composite. it is concluded that the impulse breakdown voltage of the natural ester fluid is similar to mineral oil for the oil gaps and electrode configurations tested. [ABSTRACT FROM AUTHOR]

This study was designed to examine methanol as a new aging marker of insulating paper. An analytical GC-MS method coupled to headspace injection was developed, followed by a validation. To further validate the possible applicability of methanol for different transformer conditions some laboratory studies were performed such as a stability and an aging test. Oil samples of power transformers of several Belgian nuclear and coal-fired power plants have been analyzed since mid 2009 and are followed-up periodically to correlate the results with the analysis of the furanic compounds which have been used since mid 1980s as oil soluble degradation products of the insulating winding paper. It seemed that while no 2FAL was present some considerable amount of MeOH was detected which revealed some paper degradation. A second real case study was started to further correlate MeOH and 2FAL results. From these laboratory and real case studies it seemed that MeOH is a very promising marker for paper degradation in power transformers. [ABSTRACT FROM AUTHOR]

Wavelet network based approach for identification of fault characteristics of dynamic insulation failure during impulse test has been proposed. The network identifies the fault characteristics using the significant features extracted from cross-correlation sequence of winding currents of no-fault as well as impulse faulted winding insulation. The required winding current waveforms to extract significant features for identification of various fault characteristics are acquired by emulating different dynamic insulation failures in the analog model of 33 kV winding of 3 MVA transformer using developed analog fault simulator. The results show that the wavelet network using cross-correlation features has successfully identified the dynamic insulation failure characteristics, viz. fault type, condition and location of occurrence of failure along the length of the winding with acceptable accuracy. The efficacy of extracted features and developed wavelet network for fault characteristics identification is also compared with artificial neural network classifier. The concept of emulation of dynamic insulation failure, cross-correlation based feature extraction and wavelet based fault characteristics identification methods are explained. [ABSTRACT FROM AUTHOR]