P. von Glahn

Importance of unraveling memory propagation effects in interpreting data on partial discharge statistics

The significance of memory propagation in controlling the stochastic behavior of partial-discharge phenomena is demonstrated by determination of various conditional amplitude and phase-of-occurrence distributions for both measured and simulated discharge pulses. A system that can be used to measure directly a set of both conditional and unconditional pulse amplitude and phase distributions needed to reveal memory effects and quantify the phase-resolved stochastic properties of partial-discharge pulses, is briefly described. It is argued that not only is an unraveling of memory effects essential in any attempt to understand the physical basis for the observed stochastic behavior of partial-discharge phenomena, but also that the data on conditional distributions provide additional statistical information that may be needed to optimize the reliability of partial-discharge pattern recognition schemes now being considered for use in insulation testing. >

Continuous recording and stochastic analysis of PD

We describe the design and use of a digital partial discharge (PD) data recording system capable of continuous real-time recording of PD pulse trains. The recording system consists of a custom two-channel PD digitizer coupled to a personal computer via a 10-bit parallel interface. The digitizer is under software control with the resulting data being stored in binary files on the computers hard disk. The stored data subsequently are subjected to stochastic analysis using appropriate computer software. Because all data are retained and the computer provides the desired stochastic analysis of data files, the new system is well suited to investigate non-stationary PD behavior such as encountered in aging studies. By way of illustration, the new system was used to determine the time-varying stochastic behavior of ac-generated PD from point-to-dielectric gaps in air where the insulation material was cast epoxy with aluminum oxide filler. The results confirm and extend previous measurements made with an analog stochastic analyser. With these sample results, we demonstrate how the system allows detailed stochastic analyses not possible with data obtained from existing conventional PD measurement systems. >

Variations in the stochastic behavior of partial-discharge pulses with point-to-dielectric gap spacing

The stochastic behavior of AC-generated partial-discharge (PD) pulses in a point-to-dielectric air gap has been thoroughly characterized from direct measurements of various conditional and unconditional phase-restricted pulse-height and phase-of-occurrence distributions. A stochastic analyzer was used to examine changes in the stochastic behavior of PD pulses that occur when the gap distance between a point electrode and a solid planar dielectric surface is varied. The results reveal significant pulse-to-pulse and phase-to-phase memory propagation at all gap spacings. The observed memory effects are seen to be important in controlling the initiation and growth probabilities of partial-discharge pulses at any given phase of the applied voltage.<<ETX>>

Anomalous stochastic behavior of partial discharge on aluminum oxide surfaces

The stochastic properties of pulsating partial discharge (PD) generated by applying a low-frequency sinusoidal alternating voltage to a point electrode touching an aluminum oxide (Al2O3) surface in air have been investigated. The time dependence of such statistical characteristics as mean numbers of positive and negative PD pulses per half cycle and the amplitude and phase distributions of individual positive and negative PD pulses selected according to their order of occurrence in a cycle were extracted from records of the amplitudes and phases of all PD events that occurred while the voltage was applied for times up to 40 min. The discharge characteristics exhibit a dramatic sensitivity to the impurity content of Al2O3. In the case of high-purity (99.9%) Al2O3, the positive-PD pulses cease within 30 s after application of the voltage from which time the negative-PD pulses persist indefinitely in a relatively stationary pattern. The cessation of positive PD was not observed for Al2O3 samples of lower pur...

Correlations between electrical and acoustic detection of partial discharge in liquids and implications for continuous data recording

Simultaneous measurements were made of electrical and acoustic signals from partial discharges (PD) produced by applying an alternating voltage at a frequency of 70 Hz to a point-plane electrode gap immersed in transformer oil. Both internally and externally mounted acoustic sensors were tested, and in all cases the intensity of the acoustic PD signal was found, on average, to increase with the amplitude of the electrical PD signal. The correlation between acoustic and electrical PD signals was found to be consistent with results reported from previous investigations. It is shown here that, because of this strong correlation, it is possible to extract statistical information from continuously recorded acoustic PD data, such as pulse phase (time) and amplitude distributions that are in agreement with those obtained from the electrical data. It is demonstrated that by having continuous records of all PD events that occur during a test, it is possible to uncover new statistical information that is useful in attempts to understand the physical basis for the phenomenon.

Performance evaluation of a new digital partial discharge recording and analysis system

We describe the design and performance evaluation of a new digital partial discharge (PD) recording system capable of real-time recording of PD pulse trains for later off-line computerized stochastic analysis. The new recording system consists of a custom two-channel PD digitizer coupled to a new 16-bit parallel digital interface installed in a personal computer. The digitizer is under software control with the resulting data being stored in binary files on the computers hard disk. Since post-test analysis software run on the computer provides the needed stochastic analysis of the data files, the new system offers a unique capability to perform stochastic analysis on non-stationary PD data such as found in aging studies. By way of illustration, measurements were made of the time-varying stochastic behavior of ac-generated PDs in point-to-dielectric gaps in air where the insulation material was cast epoxy with aluminum oxide filler, extending the work reported previously. Sample analysis results are presented, demonstrating that the new system provides analysis results comparable with the results achieved by the existing NIST analog PD stochastic analysis system. Sample stochastic analysis results are presented demonstrating the additional insights possible with the new system.<<ETX>>We describe the design and performance evaluation of a new digital partial discharge (PD) recording system capable of real-time recording of PD pulse trains for later off-line computerized stochastic analysis. The new recording system consists of a custom two-channel PD digitizer coupled to a new 16-bit parallel digital interface installed in a personal computer. The digitizer is under software control with the resulting data being stored in binary files on the computers hard disk. Since post-test analysis software run on the computer provides the needed stochastic analysis of the data files, the new system offers a unique capability to perform stochastic analysis on non-stationary PD data such as found in aging studies. By way of illustration, measurements were made of the time-varying stochastic behavior of ac-generated PDs in point-to-dielectric gaps in air where the insulation material was cast epoxy with aluminum oxide filler, extending the work reported previously. Sample analysis results are presented, demonstrating that the new system provides analysis results comparable with the results achieved by the existing NIST analog PD stochastic analysis system. Sample stochastic analysis results are presented demonstrating the additional insights possible with the new system. >

Partial discharged-induced aging of cast epoxies and related nonstationary behavior of the discharge statistics

The authors provide evidence for PD (partial discharge)-induced changes in epoxy surface resistivity and relate these changes to observed nonstationary behavior in the statistical characteristics of the PD with the aid of a Monte-Carlo simulator. The experimental results are obtained for an alternating voltage of variable frequency applied to a point-dielectric gap configuration. The stochastic properties of a PD generated by applying an alternating voltage to a point-dielectric gap are predicted using a Monte-Carlo computer simulation based on a model similar to that described by R.J. Brunt et al. (1992, 1993).<<ETX>>

Behavior of surface partial discharge on aluminum oxide dielectrics

Partial discharge (PD) was generated with alternating voltage applied to a point electrode touching the surfaces of aluminum oxide (Al/sub 2/O/sub 3/) dielectrics of different purity in air. The amplitudes and phases of all pulsating PD events that occurred during voltage application periods up to 35 minutes were recorded and the results were analyzed to reveal details of the time- and voltage-dependent stochastic behavior of the discharge. It was discovered that the characteristics of PD exhibit rapid changes with time that depend significantly on voltage and the purity of the Al/sub 2/O/sub 3/ sample. Examples are shown of statistical data for individual positive and negative pulses in a voltage cycle that are much more refined than data that can be obtained from conventional PD measurement systems. The results presented here illustrate the advantages of using continuous data records in the investigation and characterization of pulsating PD phenomena.

Phase behaviour in AC generated partial discharges: electro-convection and the moderation of discharges

Summary form only given, as follows. The temporal (phase) behaviour of partial discharges generated in transformer oil is examined. The system evaluated consists of a point-plane electrode gap energized by an ac potential oscillating at a frequency of 70 Hz; the experimental methods employed provide a complete phase-history of all discharge events. Analysis of these data show that the discharge events are highly correlated at short phase-separations, in that the likelihood of discharges with phase-separation 4//spl pi/ is greatly diminished and is in marked contrast with the behaviour predicted for random, independent events. We suggest that convective flow plays a predominant role in moderating the initiation of further discharges for a phase-separation of 4//spl pi/ under the described experimental conditions. The conditions for electro-convection and the characteristics of the fluid flow are discussed.

Improved Monte-Carlo simulator of partial discharge

A previously introduced Monte-Carlo simulator of partial discharge (PD) has been extended and made more versatile to allow simulation of a wider range of observed discharge behavior. The version of the simulator described here allows simulation of pulsating PD that can be represented as a point process and covers such properties as nonstationary behavior associated with PD-induced modifications of the discharge site and statistical characteristics of multi-site discharges. In the present work, it is shown how the simulator can be applied to gain insight into the physical basis for the previously reported anomalous stochastic behavior of PD generated by applying low-frequency alternating voltages to point electrodes that touch the surface of pure Al/sub 2/O/sub 3/.A previously introduced Monte-Carlo simulator of partial discharge (PD) has been extended and made more versatile to allow simulation of a wider range of observed discharge behavior. The version of the simulator described here allows simulation of pulsating PD that can be represented as a point process and covers such properties as nonstationary behavior associated with PD-induced modifications of the discharge site and statistical characteristics of multi-site discharges. In the present work, it is shown how the simulator can be applied to gain insight into the physical basis for the previously reported anomalous stochastic behavior of PD generated by applying low-frequency alternating voltages to point electrodes that touch the surface of pure Al/sub 2/O/sub 3/.