M. Pompili

Corrosive Sulfur in Insulating Oils: Its Detection and Correlated Power Apparatus Failures

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-disulfide (DBDS) were found.

Corrosive Sulfur Induced Failures in Oil-Filled Electrical Power Transformers and Shunt Reactors

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).

Partial discharge pulse sequence patterns and cavity development times in transformer oils under AC conditions

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 /spl 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 /spl mu/s with increasing oil viscosity.

Corrosive sulfur effects in transformer oils and remedial procedures

The behavior of corrosive sulfur in mineral oils is examined in terms of the failures observed in transformers, the surfaces of the copper sulfide covered conductors and degraded paper insulating tapes. The role of dissolved gas analysis (DGA) in the evaluation of the risk of copper sulfide formation is described. The degree of corrosiveness of some sulfur compounds is examined and compared using a Kraft paper wrapped-copper test (standard IEC 62535). The occurrence of DBDS as the most relevant corrosive compound is compared with the presence of other corrosive species in insulating mineral oils. A number of mitigation techniques for corrosive sulfur are described and evaluated.

Phase relationship of PD pulses in dielectric liquids under ac conditions

Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of /spl ges/15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether.

Simultaneous ultrawide and narrowband detection of PD pulses in dielectric liquids

Simultaneous narrow band (300 kHz) and wide band (500 MHz) measurements were carried out to obtain estimates of the apparent charge transfer associated with discrete isolated discharge pulses and discharge pulse bursts, which are observed to occur in dielectric liquids when wide band detection systems are utilized. The integrated apparent charge transfers determined with the narrow band system ranged from 12 to 95 pC, which represents typically the lower range of values normally encountered with PD activity in dielectric liquid-filled or impregnated equipment. While the discharge phenomena in perfluoro polyether liquids was found to be typified by sporadic appearances of single isolated pulses involving charge transfers between 33 and 38 pC, the discharge events in mineral oils assumed most frequently the form of pulse bursts, displaying the usual pulse sequences of quasi-ascending amplitudes; the overall integrated apparent charge transfer of the observed pulse bursts extended commonly from 12 to 16 pC. The charge release, occurring with the first (initiating) smallest discharge pulse within the pulse bursts, was estimated to be of the order of /spl sim/1 pC.

PD pulse burst characteristics of transformer oils

Partial Discharge (PD) characteristics of four typical transformer oils, having respective viscosities of 3.5, 9.1, 13.0, 18.3 cSt at 40/spl deg/C, were examined under ac conditions, using a needle-to-plane electrode system. Wide and narrow bandwidth measurements were carried out to determine simultaneously the apparent charge transfers associated with the overall PD pulse bursts in the oil as well as that of the individual discrete PD pulses within the PD pulse bursts themselves. Based on the apparent charge transfer value, the size or depth of the PD initiating microcavities in the field direction within the oils, producing the first detected discrete PD pulse, were estimated to be in the order of 2 /spl mu/m. The recurrence rate of the PD pulse bursts was found to increase with voltage above the PD inception voltage; this increase was accompanied by both an increase in number of discrete PD pulses and their amplitude within the pulse burst itself. These increases were reflected by a substantial rise in the apparent charge transfer per PD pulse burst. The charge transfer levels of the PD pulse bursts associated with the highly pressurized gas microcavities were of the same order of magnitude as those produced within the normally much larger macroscopic cavities that exist under atmospheric pressure in conventional oil-impregnated insulating systems of power transformers.

Early stages of negative PD development in dielectric liquids

The pulse shapes in the early stages of partial discharge (PD) development, well before liquid streamer onset, and obtained with a 500 MHz wide band detection system using needle-to-metallic plane electrodes submerged in a mineral oil, were compared to those measured with needle to mineral oil plane electrodes in air under negative dc potential. All measurements were carried out /spl ap/50% below the breakdown field value, using gap separations between 3 and 25 mm. Whereas the needle to plane discharge behavior in the mineral oil was characterised by discrete sporadic pulses or burst of several pulses of increasing negative amplitude and oscillatory form, the needle to mineral oil plane gap in air gave rise to primarily dense bursts of unidirectional negative pulses with sequentially decreasing amplitude. The pulses had rise times typically of the order of 1 to 2 ns, indicating the involvement of rapid PD mechanisms. >

Partial discharge distributions in liquid dielectrics

Different dielectric fluids used in HV electrical components were subjected to AC stresses, and the partial discharge (PD) spectra produced under nonuniform and uniform field conditions were recorded with the help of an electrical detector and a multichannel analyzer (MCA) as a function of applied stress. The analysis of the collected data confirms that PD activity in a liquid is a random phenomenon as well as a breakdown process. The presence of PD may be related with the presence in the liquid of bubbles near the electrode interface and with a low density region (vapor phase), which is the first step in the breakdown development. This confirms that PD occurs only in a gas and not in a solid or liquid. In this paper, which is part of an ongoing research project, it is shown that MCA can be a helpful tool in PD measurements and analysis, allowing the demonstration of how the partial discharge inception voltage (PDIV) significantly changes according to the adopted inception criteria. >

Comparative PD pulse burst characteristics of transformer type natural and synthetic ester fluids and mineral oils

The partial discharge (PD) pulse burst behavior in natural and synthetic organic ester fluids and mineral oils of the type used in transformers has been examined, using simultaneous wide and narrow band measurement techniques. This permitted the determination and comparison of both the overall charge transfer of the PD pulse as well as that of the discrete PD pulses comprising the PD pulse burst in the evaluated dielectric fluids.