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Contamination of paper tapes by corrosive sulfur in insulating oils may cause shorting faults between turns. Typically, this occurs at higher temperature in the upper portions of the windings of shunt reactors and power transformers. In many of the tested oils, high amounts of dibenzyl–disul?de (DBDS) were found

The nature and causes of corrosive sulfur induced failures are examined in oil-filled transformers and shunt reactors. Copper sulfide, which is formed when the corrosive sulfur in a mineral oil reacts with the copper conductors, is likely to diffuse into the paper tapes insulating the conductors. Since copper sulfide is partially conducting, the dielectric losses of the contaminated oil-impregnated-paper tapes are markedly increased; paper tapes in close proximity to the copper conductors are found to attain tan delta values > 1.0 even at room temperature. It is highly likely that thermal instabilities develop at those sites at operating temperatures, leading to increased loss currents and, ultimately, short circuits between the turns. This sequence of events is substantiated by evidence from the field, which indicates large areas of thermally degraded insulations and charred breakdown regions along the coils, the extent of which becomes more pronounced at higher operating temperatures (toward the top of the windings)

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Discrete partial discharge (PD) pulse occurrence times within the PD pulse bursts in transformer oils were found to extend from approximately 0.42 to 2.9 mu s, with the discrete pulse separation times evincing a weak dependence on the inverse of the oil viscosity. Many PD pulse burst patterns exhibited substantial deviation from the classical behavior, which is normally characterized by successive discrete pulses of ascending amplitude with well defined increases in the pulse separation times. The average elapsed time from the incipient formation for the cavity to the onset of the first partial discharge event extended downwards from 0.62 to 0.42 mu s with increasing oil viscosity

The asset management of any Transmission System Operator (TSO) cannot ignore the evaluation of the power transformers fleet. Even in the absence, to date, of any specific international guide or standard, every big electrical utility is adopting a home-made 'Health Index'. An HI is developed to summarize in numerical form the transformers' reliability for the purpose of evaluation, ranking and comparison. In the present paper the author will elucidate a new model that merge the evidences of periodic tests (DGA, furans, acidity, inductance, FDS, etc.) with the keraunic properties of a substation

In this article, a robust Lyapunov demodulator (LD) based on orthogonal signal generation (OSG) approach for single/three-phase application is proposed. Conventionally, LD is not capable of rejecting the DC-offset in the grid signal. Thus, an additional estimation loop is required which may affect the dynamic performance. Nevertheless, an application of harmonically polluted grid voltage signal to LD may severely affect the steady-state performance of the estimated parameters. To address these issues, an enhanced LD based-OSG is proposed, wherein a moving average filter is incorporated in the LD-OSG structure. Thus, rapid rejection of DC-offset and harmonics is easily achieved without any additional loop. The proposed structure may accurately estimate the fundamental in-phase and the quadrature components. However, these orthogonal components may suffer from amplitude imbalance and errors in the phase information under off-nominal frequency conditions. Additionally, the errors in the amplitude and the phase information are eliminated using an open-loop frequency deviation detector and a feed-forward curve fitting approach. The dynamic performance of the proposed scheme has been validated by numerical and hardware studies. It is reported that, with the less sensitivity toward phase angle jump and good immunity to fundamental negative sequence, the proposed scheme is a potential technique for synchronization of single/three-phase grid connected power electronic equipment.

Partial discharge detection in dielectric liquids requires particular PD measurement techniques and instruments. Simultaneous partial discharge (PD) measurements that are employed on insulating liquids, using dual narrow/wide band detectors, are described. Narrow band PD detectors, which are of the integrating type are suitable for the measurement of the total charge transfer in pC of the overall discharge event. Wide band measurement techniques permit the acquisition of the discrete PD pulses in dielectric liquids, that comprise the overall PD pulse burst signal. These PD pulse bursts are comprised of discrete high frequency pulses of usually ascending magnitude charge transfers. The time position of these pulses within the pulse burst is determined by the times at which the Paschen's minima occur across the expanding cavity due to the small size of the microcavities. In order for them to undergo discharge requires enormously high internal pressures (ca 10 MPa) to ensure a sufficient number of charge carriers. The life duration of these cavities is of the order of several's, with the cavity collapsing, when its size reaches dynamic instability