1,721,065 research outputs found
Gas Formation in Transient Cavities Undergoing PD Pulse Burst Discharges in Transformer Oils
No full textPartial discharges in mineral oils take place in the form of pulse bursts within minute microcavities, which are formed at high-electric field charge injection sites. The lifetime of these transient microcavities is determined by the interval between their creation and their ultimate collapse due to dynamic instability. The vapor pressure within these microcavities has been estimated to be as high as 10 MPa and lately there has been considerable speculation as to the nature and type of gases within these microcavities. Currently, carried out experiments indicate that these gases consist principally of hydrogen, CH 4 (methane), and C2H6 (ethane) with smaller traces of other lower molecular weight hydrocarbons
On partial discharge measurement in dielectric liquids
No full textPartial 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
A comment on the test methods for the breakdown voltage of dielectric liquids
No full textThe electrical breakdown at 50 or 60 Hz of an insulating liquid is of importance as a measure of the liquid's ability to withstand electrical stress. In national and international Standards (ASTM and IEC) different methods are reported for evaluating this fundamental property of an insulating liquid. Nevertheless, these test methods differ, depending upon the procedure, shape and dimension of the test specimen holders and types of electrodes. In the present communication an improvement of a test cell is described, which may be useful in a possible revision of Standards for improving the scatter of the test results in the measurement of the 50 or 60 Hz dielectric strength of different insulating liquids
Corrosive sulphur in mineral oils: its detection and correlated transformer failures
No full textThe presence of corrosive sulphur in current insulating oils is discussed and typical winding failures in transformers and shunt reactors, due to the resultant formation of copper sulphide films and particulate matter, are described. Different standard test methods for the detection of corrosive sulphur in oils are compared and ASTM D1275 is shown to be the most sensitive
Phase relationship of PD pulses in dielectric liquids under ac conditions
No full textPartial 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 greater than or equal to 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
Corrosive Sulfur Effects in Transformer Oils and Remeidial Procedures
No full textThe 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
Testing, evaluation and standardisation of transformer oils
No full textA number of activities within the IEC on the standardization on transformer insulating oils are discussed. Among these, it is of great importance the present recent (2003) of the standard IEC 60296 that introduces only one class of mineral oil to be employed in transformers with a reduced viscosity. The importance of sulphur compound level control in transformer oils is emphasized and more stringent sulphur level control tests are proposed; environmental concerns inherent with the use of mineral oils are also indicated. A proposal is made for an improved standard on partial discharge measurements on transformer insulating oils
Simultaneous ultrawide and narrowband detection of PD pulses in dielectric liquids
No full textSimultaneous 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 ∼1 pC
Simultaneous measurements of PD in oil using conventional narrow band and ultra wide band detection systems
No full textPartial Discharge (PD) measurements were carried out with conventional narrow band and ultra wide band detector circuits on a transformer oil under ac conditions, employing point-to-plane electrodes having separations from 3.0 to 8.0 mm. The rapid rise time partial discharge events in the oil were readily detected by the conventional narrow band measuring system in the form of a single integrated apparent charge pulse. This pulse represented generally the single response to a PD pulse burst involving a number of pulses in the oil. The apparent charge values of the integrated pulse were found to range from approximately 10 to 16 pC, representing the lower range of PD apparent charge values normally encountered in oil-filled electrical apparatus when conventional narrow band PD measuring circuits are employe
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