R. Bartnikas

Spark-to-glow discharge transition due to increased surface conductivity on epoxy resin specimens

The effects of long exposure of epoxy resin surfaces to partial discharges under AC field stress were examined. The use of electrical pulse measuring techniques in conjunction with light emission measurements using a photomultiplier tube, which allowed simultaneous detection of both spark and glow discharges, permitted the establishment of a definite transition from spark to glow discharges. Spark discharges were found to be responsible for the initial surface modification leading to increased surface conductivity, which favored the transition to a pulseless glow type discharge within the cavity. The latter form of discharge was found to prevail over the major portion of the remainder of the exposure period. >

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.

On the character of different forms of partial discharge and their related terminologies

Experimentally obtained partial discharge pulse forms are compared with those derived theoretically, using a mathematical model of a short gap. The calculations indicate that, with overvolted short gaps, the apparent absence of the protracted ion current tail in the overall pulse form is due to the large excess of the electron current component generated by strongly enhanced cathode emission by the photoeffect and other phenomena. Under conditions of high overvoltage the ion-impact-induced emission, the ion fluxes and, consequently, the ion current component are greatly reduced as compared to the electron current. The long ion current tail, representing not more than a few percent of the total current, passes undetected. Pulseless and pseudo-glow discharges are considered. Preferred terms to designate the different forms of partial discharges are suggested. >

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).

Some Observations on the Character of Corona Discharges in Short Gap Spaces

Corona discharge results, obtained with a number of gases under atmospheric pressure using plane-parallel metallic and dielectric electrodes placed between 0.05 and 9.00 mm apart, are discussed. With short gap spacings, deviations from the pulse or spark-type discharge mechanism are observed to occur in particular when dielectric surfaces are involved; the deviations are characterized usually by the appearance of pseudoglow discharges, which exhibit properties that are common to both pulse and glow discharges. The practical significance of pseudoglow discharges in relation to corona measuring techniques, utilizing conventional pulse discharge detectors, are considered both on a short-and long-term test basis.

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.

On the spark to pseudoglow and glow transition mechanism and discharge detectability

The transition mechanism for spark to pseudoglow and glow discharge is examined and the attending question of discharge detectability in an electrical apparatus is considered. The effect of gap spacing or cavity diameter and overvoltage is discussed in terms of experimental observations and theoretical calculations using a two-dimensional mathematical model. >

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.

Performance characteristics of dielectrics in the presence of space charge

The short and long-term dielectric behavior of a number of representative electrical insulating systems is compared in the presence of space charge. Dielectric materials, used both in the communications and power application areas, are considered. In this overview, particular attention is given to thin inorganic films, organic solid-liquid and solid polymer systems as regards to the manner in which space charge affects their dielectric loss, voltage breakdown, treeing and electrical aging characteristics.