Showing papers on "Partial discharge published in 2003"

Advanced PD inference in on-field measurements. II. Identification of defects in solid insulation systems

Abstract: Results of investigations performed to evaluate the effectiveness of a new inference method for the diagnosis of solid insulation systems, based on partial discharge (PD) measurements, are reported in this paper. Signal separation, noise recognition, and PD source identification are the main features of the proposed inference method. Techniques for signal separation and automatic noise rejection are reported in the 1st part of this paper, while the problem of the identification of PD phenomena, occurring in defects of insulation systems, is approached in this 2nd part. The identification is based on fuzzy logic and enables the recognition of PD generated from different basic sources, such as internal, surface and corona discharges. It is shown that the different source typologies can be identified by means of fuzzy rules applied to a selection of parameters derived from PD-pulse phase and amplitude distribution analysis, once PD phenomena have been clustered in homogeneous class through a fuzzy algorithm based on PD-pulse shape. The proposed identification procedure is finally applied to rotating machines and cables, affected by insulation defects, showing promising on-field applications.

Partial discharge characteristics of inverter-fed motor coil samples under ac and surge voltage conditions

Abstract: This work investigates the partial discharge (PD) characteristics of twisted pair samples for inverter-fed motor coil under ac and surge voltage conditions. It also discusses the difference in the PD inception characteristics of a twisted pair under 60-Hz voltage and surge voltages with a steep wave front. The PDIV under ac voltage was measured and was found to agree well with the theoretical value using electric field analysis and Paschen's law. The PDIV under surge voltage was measured for different repetition rates and the PDIV for 60-pps repetition was found to be lower than those for 6 pps and for the single shot, which suggested that the electrical insulation performance would be influenced by the repetition rate. The PDIV under surge voltages was 1.5 to 1.8 and 2.3 to 2.7 times higher than those under AC voltage application for the used and virgin samples, respectively.

A Deterministic Model for the Growth of Non-conducting Electrical Tree Structures

Abstract: Electrical treeing is of interest to the electrical generation, transmission and distribution industries as it is one of the causes of insulation failure in electrical machines, switchgear and transformer bushings. In this paper a deterministic electrical tree growth model is described. The model is based on electrostatics and local electron avalanches to model partial discharge activity within the growing tree structure. Damage to the resin surrounding the tree structure is dependent on the local electrostatic energy dissipation by partial discharges within the tree structure and weighted by the magnitudes of the local electric fields in the resin surrounding the tree structure. The model is successful in simulating the formation of branched structures without the need of a random variable, a requirement of previous stochastic models. Instability in the spatial development of partial discharges within the tree structure takes the role of the stochastic element as used in previous models to produce branched tree structures. The simulated electrical trees conform to the experimentally observed behaviour; tree length versus time and electrical tree growth rate as a function of applied voltage for non-conducting electrical trees. The phase synchronous partial discharge activity and the spatial distribution of emitted light from the tree structure are also in agreement with experimental data for non-conducting trees as grown in a flexible epoxy resin and in polyethylene. The fact that similar tree growth behaviour is found using pure amorphous (epoxy resin) and semicrystalline (polyethylene) materials demonstrate that neither annealed or quenched noise, representing material inhomogeneity, is required for the formation of irregular branched structures (electrical trees). Instead, as shown in this paper, branched growth can occur due to the instability of individual discharges within the tree structure.

A nonintrusive partial discharge measurement system based on RF technology

Abstract: Conventional electrical measurements of PD are performed with detectors connected to the plant. This paper describes research conducted in conjunction with the National Grid Company and the radiocommunications agency for the radiometric detection of partial discharge sources via a novel noncontact digital method. The equipment uses an antenna sensor and reference voltage inputs which are directly sampled at 2.5 GHz. Results for hv laboratory tests involving air, SF6 and oil discharges are presented. The results include the waveform, average frequency spectrum and pulse count distribution. A proposal is made for the characterisation of the recorded impulses from these different breakdown effects based on analysis of the spectra and pulse count distributions.

Partial discharge detection in cables using VHF capacitive couplers

Abstract: In order to detect and locate partial discharge (PD) activity within HV cables and in particular cable joints, the feasibility of using a very high frequency WHO capacitive coupler technique has been investigated. In addition, an alternative PD offline test method for cable joints or cable sections within long cable systems has been developed. By energising the joint or a short section of the cable screen only this method has the advantage of reducing the power supply requirement of the test source. The application of capacitive couplers for this PD offline test method has been investigated. In order to use this approach there must be an insulation gap between the coupler and the outer sheath of the test cable. The effect of this gap on measured sensitivity has been investigated experimentally and through the use of a simulation model. Obtained results are in good agreement. Cross-correlation techniques have been implemented to automatically estimate the signal time of flight from different-coupler outputs. Methods of calibrating the coupler applied to the alternative PD offline test method have also been investigated.

Electric field investigation in high voltage power modules using finite element simulations and partial discharge measurements

Abstract: The development of high voltage power module using IGBTs leads to serious problems concerning the electric field insulation. The values of these electric fields are very important regarding the capabilities of the dielectric gels used in the power packages. Solutions have to be developed to ensure limited field and good reliability. The aim of this paper is to show the interest of using finite element simulations in association with partial discharge measurements to determine and localize the maximum field values. Some solutions to lower them are also presented.

Experimental studies of free conducting wire particle behavior between nonparallel plane electrodes with AC voltages in air

Abstract: When a spherical conducting particle exists in an electrode system where electrical gradient force acts along the electrode surface, the particle tends to move towards the high field region. The possibility of appearance of this phenomenon is experimentally investigated with wire particles, which can easily produce the corona discharge on their ends resulting the action of a reactive force on the particle by a corona wind. The results show that a hovering particle travels deeper into the high electric field regions regardless of the corona discharge. Moreover, it is found that wire particle with corona discharge hovers near the negative electrode under DC voltage in atmospheric air due to different characteristics of corona discharge by the polarity. But under the AC voltage, the particle hovers near the bottom electrode only and invades in the high field region deeper than under the DC voltage.

Double discharges in unipolar-pulsed dielectric barrier discharge xenon excimer lamps

Abstract: Excitation of dielectric barrier discharge xenon excimer lamps by unipolar short square pulses is studied in this paper. Two discharges with different polarity are excited by each voltage pulse (double discharge phenomenon). The primary discharge occurs at the top or at the rising flank of the applied unipolar square pulse, which is directly energized by the external circuit. The secondary discharge with the reversed polarity occurs at the falling flank or shortly after the falling flank end (zero external voltage) depending on the pulse width, which is energized by the energy stored by memory charges deposited by the primary discharge. Fast-speed ICCD imaging shows the primary discharge has a conic discharge appearance with a channel broadening on the anode side. This channel broadening increases with increasing the pulse top level. Only the anode-side surface discharge is observed in the primary discharge. The surface discharge on the cathode side which is present in bipolar sine voltage excitation is not observed. On the contrary, the secondary discharge has only the cathode-side surface discharge. The surface discharge on the anode side is not observed. The secondary discharge is much more diffuse than the primary discharge. Time-resolved emission measurement of double discharges show the secondary discharge emits more VUV xenon excimer radiation but less infrared (IR) xenon atomic emission than the primary discharge. It was found that the IR xenon atomic emission from the secondary discharge can be reduced by shortening the pulse width. The energy efficiency of unipolar-pulsed xenon excimer lamps (the overall energy efficiency of double discharges) is much higher than that obtained under bipolar sine wave excitation. The output VUV spectrum under unipolar pulse excitation is found to be identical to that under sine wave excitation and independent of injected electric power.

Recognising multiple partial discharge sources in power transformers by wavelet analysis of UHF signals

Abstract: Partial discharges (PD) in a transformer generate transient UHF signals that resonate within the tank. Where more than one source of PD or interference is present, the ability to separate incoming signals depending on their point of origin would enhance the diagnostic process. A wavelet-based method for recognising UHF signals from PD sources at different positions in the transformer tank, even when the signal is received at only one UHF sensor, is described. Time-frequency characteristics of the UHF PD signals are established using the wavelet transform, providing a means of mapping the energy distribution of the signal. A similarity function is then defined to provide a measure of the similarity between the various wavelet coefficient distributions. The procedure is demonstrated by means of laboratory experiments and application of the technique to data obtained from a transformer in the field.

Partial discharge recognition using neural networks: a review

Abstract: Partial discharge (PD) recognition using neural networks is studied. The paper offers a description of the main neural networks used, their recognition rate as well as comments on the variety of PD parameters fed to neural networks. Problems regarding the stochastic nature of PD, the multiple defects and the influence of the voltage level on the recognition rate are discussed. Proposals for future work are made.

Tree growth monitoring by means of digital partial discharge measurements

Abstract: In this work results relevant to electrical treeing tests performed on several ethylvinylacetate EVA (with different vinylacetate percentage) and on cross linked polyethylene (XLPE) (of two different lots) polymer materials are presented. During the tree growth the partial discharges (PDs) were monitored by means of a digital PD acquisition system and the tree morphologies were optically evidenced by means of video recorder equipment. The PD acquired data, the outcome of the video monitoring and the time-to-breakdown were off-line processed and analysed. In this way, two main results were obtained: the first one, applicable to this kind of laboratory treeing tests, is related to the possibility of predicting the lifetime of the specimens, considering a specific PD measurement derived quantity; the second one demonstrates the possibility to detect the tree morphology considering the phase-number distribution obtained by the digital PD measurement. The presented test procedures are deemed to have a potential for diagnostic applications.

Fundamental aspects of excitation and propagation of on-line partial discharge signals in three-phase medium voltage cable systems

Abstract: On-line partial discharge (PD) detection of three-phase belted medium voltage cable connections results in a number of interpretation differences as compared to off-line measurements where only one phase is energized. The induced currents and charges in the phase conductors and earth screen upon a PD not only depend on the discharge site, but also become phase angle dependent. Furthermore, simulations show that the PD distribution itself varies with the amount of eccentricity of the rotating electric field and may differ from the off-line distributions obtained with a linear field. Finally, the PD propagation in a multi-conductor cable also alters the signals measured at the cable terminals. In this paper, induced charges and PD distributions are studied by means of computer simulation. The cable propagation characteristics are verified by measurements.

Application of dielectric response measurement on power cable systems

Abstract: Water treeing is one of the factors leading to failure of medium voltage XLPE cables in long-term service. Increased moisture content inside oil-paper insulated cable is not desirable. To identify water tree degraded XLPE cables or oil-paper cables with high moisture content, diagnostic tests based on dielectric response (DR) measurement in time and frequency domain are used. Review of individual DR measurement techniques in the time and frequency domains indicates that measurement of one parameter in either domain may not be sufficient to reveal the status of the cable insulation. But a combination of several DR parameters can improve diagnostic results with respect to water trees present in XLPE cables or increased moisture content in oil-paper cables. DR measurement is a very useful tool that reveals average condition of cable systems. However, it is unlikely that DR measurement will detect few, but long water trees. In addition, DR cannot locate the defect or water tree site within the cable system. Combination of DR and partial discharge (PD) measurements can improve diagnostic results with respect to global and local defects. However, it is doubtful whether PD test can identify the presence of water trees inside a cable in a nondestructive manner. Further research is needed for more detailed conclusions regarding the status of a particular insulation and for predicting the remaining life of the insulation system.

Modelling of partial discharge development in electrical tree channels

Abstract: A model of partial discharge development in electrical tree structures is presented. The model considers channel conductivity, charge transport, and electric field redistribution during propagation of the discharge along the channels. It has been used for numerical investigation of temporal-spatial and electric characteristics of the partial discharges in capillaries and electrical trees exposed to AC voltage in a needle-plane geometry. The simulation results have been compared with experimental data given in the literature.

Comparison of the AC barrier corona with DC positive and negative coronas and barrier discharge

Abstract: Results are presented from experimental studies of ac corona discharges between a point electrode and a dielectric-coated plate in nitrogen, argon, helium, and air in the voltage frequency range f=50 Hz–50 kHz. The characteristic features of this type of discharge are compared with the well-known features of dc positive and negative coronas and a barrier discharge between plane electrodes. It is shown that the presence of a dielectric barrier on the plane electrode significantly changes the electric characteristics and spatial structure of the corona, whereas the main phases of the discharge evolution remain unchanged as the voltage increases. With a point electrode, the breakdown voltage of the barrier corona decreases substantially as compared to the breakdown voltage of a barrier discharge with plane electrodes. This leads to softer conditions for the streamer formation in a barrier corona, which becomes more stable against spark generation.

Partial Discharge Detection and Localization in High Voltage Transformers Using an Optical Acoustic Sensor

Abstract: A partial discharge (PD) is the dissipation of energy caused by the buildup of localized electric field intensity. In high voltage devices such as transformers, this buildup of charge and its release can be symptomatic of problems associated with aging, such as floating components and insulation breakdown. This is why PD detection is used in power systems to monitor the state of health of high voltage transformers. If such problems are not detected and repaired, the strength and frequency of PDs increases and eventually leads to the catastrophic failure of the transformer, which can cause external equipment damage, fires and loss of revenue due to an unscheduled outage. Reliable online PD detection is a critical need for power companies to improve personnel safety and decrease the potential for loss of service. The PD phenomenon is manifested in a variety of physically observable signals including electric and acoustic pulses and is currently detected using a host of exterior measurement techniques. These techniques include electrical lead tapping and piezoelectric transducer (PZT) based acoustic detection. Many modern systems use a combination of these techniques because electrical detection is an older and proven technology and acoustic detection allows for the source to be located when several sensors are mounted to the exterior of the tank. However, if an acoustic sensor could be placed inside the tank, not only would acoustic detection be easier due to the increased signal amplitude and elimination of multipath interference, but positioning could also be performed with more accuracy in a shorter time. This thesis presents a fiber optic acoustic sensing system design that can be used to detect and locate PD sources within a high voltage transformer. The system is based on an optical acoustic (OA) sensor that is capable of surviving the harsh environment of the transformer interior while not compromising the transformer’s functionality, which allows for online detection and positioning. This thesis presents the theoretical functionality and experimental validation of a band-limited OA sensor with a usable range of 100-300 kHz, which is consistent with the frequency content of an acoustic pulse caused by a PD event. It also presents a positioning system using the time difference of arrival (TDOA) of the acoustic pulse with respect to four sensors that is capable of reporting the three-dimensional position of a PD to within ±5cm on any axis. This work was supported in part by NSF grant 9810688.

Hot electron and partial-discharge induced ageing of polymers

Abstract: Insulating polymers used in electrical engineering applications age under the combined effects of electrical, thermal, mechanical and environmental stresses. Of particular relevance is the electric stress that can initiate material degradation reactions through energetic processes such as partial discharges and hot electrons. Although the degradation can take specific form (microcavities, electrical treeing) depending on the time scale considered, similarities might exist in the aging mechanisms of polymers working in high voltage environment, and those exposed to plasma or radiation. Of particular interest is to identify the polymer excited states that are involved in the early stage of electrical ageing. Information on their nature can be obtained by analyzing the solid state luminescence stimulated by an electric field, UV source or fast electron beam.

Prediction of breakdown voltage of transformer oil from predischarge phenomena

Abstract: Complex experimental studies of the discharge mechanism in transformer oil under positive lightning impulse were carried out using alternative diagnostic methods. An attempt to establish a connection between the macroscopic features of discharge and the breakdown voltage was made.

Ozone generation characteristics by superimposed discharge in oxygen-fed ozonizer

Abstract: A superposition type ozonizer has been proposed to achieve relatively high ozone concentration and high ozone generation efficiency. A high-pulsed voltage and a 60 Hz ac voltage are simultaneously applied to the same reaction volume in order to improve the ozone generation yield in a wide discharge gap. The ozone concentration of the superimposed discharge consisting of surface discharge and pulsed corona discharge was approximately two times larger than that obtained by an individual discharge. It was observed that the cross-section of the superimposed discharge extended is observed to be approximately 20% at maximum. The emission intensity of the superimposed discharge shows about two times larger than that of the individual discharge at maximum due to the increased electron temperature. The increment of local electric field in the discharge region causes the superposition effect.

Ultraviolet-induced discharge currents and reduction of the piezoelectric coefficient in cellular polypropylene films

Abstract: Photostimulated discharge spectroscopy of cellular polypropylene films between 200 and 400 nm showed the existence of at least three distinct trapping levels at 4.6, 5.6, and 6.3 eV. The effects of UV irradiation on the piezoelectric d33 coefficient was studied by monitoring thickness-extension resonances in the dielectric spectrum. Prolonged irradiation at wavelengths below 210 nm led to a reduction of the piezoelectric coefficient, caused by partial discharge of the polymer foam.

Modeling memory phenomena for partial discharge processes in insulation cavities

Abstract: Partial discharge measurements performed at 0.1, 20, 50 and 300 Hz and at different voltage levels are compared with simulations based on models available in literature. It is shown that these models need to be improved to be able to fully extrapolate PD behavior at different frequencies. A solution based on a time-dependent increase of the work function when detrapping from negative charged surfaces is proposed and verified on the basis of experimental data.

Detection and wavelet analysis of partial discharges using an optical fibre interferometric sensor for high-power transformers

Abstract: The main problem in power transformers is the degradation of the isolation, and partial discharges are a major contribution to this shortcoming. Partial discharges inside medium-and high-power transformers generate ultrasonic pressure waves which are usually detected by using acoustic piezoelectric sensors placed outside the transformers. In this paper we describe an optical fibre interferometric sensor we are developing for the detection and location of partial discharges by measuring inside the power transformers. This non-intrusive optical fibre sensor allows a high level of sensitivity to be achieved for the typical small pressure variations created by partial discharges, specially if compared with the results obtained by acoustic piezoelectric external sensors. We present interferometrically detected levels of partial discharge. These results indicate that the developed optical fibre sensor is able to detect a partial discharge in the range of 103 pC that produces an optical phase shift of about 2 rad with a resolution of 30 pC in charge means. Finally, the wavelet analysis is used to identify the acoustic patterns instead of the more commonly used Fourier analysis. When transforming the measured signal to the frequency domain, the temporal information disappears. But the acoustic signals really contain numerous transitory pieces of information to be considered. By looking at a Fourier-transformed signal it is impossible to state when a particular event took place. The wavelet analysis helps to solve this problem, and some results are presented here to show this.

Method and electromagnetic sensor for measuring partial discharges in windings of electrical devices

Abstract: A method for measuring partial discharges in windings of electrical devices comprised by the following steps: Applying voltages having high frequency components to the winding of the electrical device, detecting partial discharge signals by means of a tuned VHF and/or UHF electromagnetic sensor located close to the electrical device and evaluating the detected sensor signals by means of electrical hardware or software. Further, a VHF and/or UHF electromagnetic sensor for measuring partial discharges in windings of electrical devices is described wherein an antenna made of a coaxial cable is provided as electromagnetic sensor. The present invention provides an improved measuring method and sensor device, which avoid the drawbacks of the prior art. The improved measuring method provides more detailed information about the status of the insulation system and clear short-circuits during the testing are not necessary any more. The proposed sensor provides a surprisingly simple and inexpensive solution.

Propagation characteristics of UHF signals in transformers for locating partial discharge sources

Abstract: This paper looks at the propagation characteristics of UHF signals in transformers for locating partial discharge sources

Device for detecting partial discharge in power equipment using radiated electromagnetic wave

Abstract: Disclosed is a device for detecting a partial discharge of power equipment. EM (electromagnetic wave) sensors detect EM signals from a partial discharge in a metal clad switchgear, a power cable, and a GIS. EM detectors amplify the signals from the sensors, and output only noise-removed IF signals. A pulse generator integrates the IF-processed EM to compare the integrated value with a previous value, and outputs a pulse according to a partial discharge. An EM level processor compares the IF-processed EM with reference voltages to output EM pulses of a plurality of levels. A waveform shaper shapes the pulses. A controller calculates the average number of pulses per 1 cycle by counting the partial discharge pulses for a predetermined time, receives the waveform-shaped pulses to calculate the partial discharge amount, and transmits the calculated amount, combined with the average number, to an external monitoring system.

A phase-related investigation of AC corona in air

Abstract: A computer-based partial discharge detector has been used to measure AC corona pulses in air in a point-cup electrode system. The pulse height distribution was measured as a function of phase angle and pulse height for a range of voltages. The onset voltages for both positive and negative half-cycles are the same, and significantly lower than for the DC case, due to space charge effects. In all other respects the patterns for the two half-cycles are different: pulses only occur at the onset voltage (rising) for the positive half-cycle, but continuously for the negative half-cycle, except for the higher applied voltages which exhibit a glow discharge transition near the (negative) peak. Differences between the distributions for avalanches and streamers increase as the breakdown voltage is approached. Qualitative explanations of the patterns and behavior are given, together with calculations showing that space charge from one cycle can linger in the gap between half-cycles.

Statistical pattern analysis of partial discharge measurements for quality assessment of insulation systems in high voltage electrical machinery

Abstract: In this paper, we present a new statistical analysis method of phase resolved partial discharge (PD) measurements for the quality assessment of the electrical insulation in high voltage machinery. The method is based on a supervised classification approach which utilizes histogram similarity analysis. The motivation for choosing histogram similarity analysis is twofold: first the phase resolved PD measurement itself is, in fact, a two dimensional histogram. Therefore a histogram matching based approach suits the very nature of the data. Second histogram similarity analysis combines the typical statistical parameters, such as mean, variance and higher order moments, used in the recognition of PD measurements in a statistically powerful and rigorous way. In our study, we utilize various histogram types and similarity analysis, including correlation, chi-square and Kolmogorov Smirnov tests. Our experimental study on the laboratory samples demonstrates that the method shows strong potential in defection and identification of insulation defects. The results from our study suggest that the proposed method provides a powerful, general and mathematically simple approach to the analysis of phase resolved PD measurements.

Hybrid type sensor for detecting high frequency partial discharge

Abstract: Disclosed herein is a hybrid type sensor for detecting high frequency partial discharge. The hybrid type sensor of the present invention can detect high frequency partial discharge at a high signal-to-noise ratio without being influenced by power noise and surrounding noise, and guarantee the safety of a test when breakdown occurs. The sensor of the present invention forms two or three signal paths with different impedances. A low frequency power signal is bypassed to ground through a first path, and a high frequency partial discharge current is allowed to flow through a second path and is detected as a resistance component through a resistor. Further, a surge voltage input to the sensor due to breakdown is input to the ground through a third path. Therefore, the present invention can precisely and safely detect the amount of high frequency partial discharge.