Showing papers on "Partial discharge published in 1997"

Failure mechanism of winding insulations in inverter-fed motors

Abstract: The failure of magnet wires under repetitive pulses as seen in inverter-fed motors cannot be attributed to a single factor but is a result of the combined effects of partial discharge, dielectric heating, and space charge formation. Voltage overshoots produced by PWM drives may be above discharge inception voltage. Partial discharge may therefore be present in inverter-fed motors. In addition to partial discharge, pulses with a fast rise time and high frequency enable the insulation to generate local dielectric heating, which increases the local temperature. The degradation rate of the insulation is therefore increased. Furthermore, the fast rise and fall of pulses make it possible for space charges to accumulate in the winding insulation and on its surface.

PD source identification with novel discharge parameters using counterpropagation neural networks

Abstract: Computer aided partial discharge (PD) source identification using different multidimensional discharge patterns is widely regarded as an important tool for insulation diagnosis. In this paper, a neural network (NN) approach to PD pattern classification is presented. The approach is based on applying variants of the counterpropagation NN architecture to the classification of PD patterns. These patterns are derived from physically related discharge parameters, different from those commonly used. It is shown that considerable improvements of the classification quality can be obtained when an extended counterpropagation network with a dynamically changing network topology is applied to patterns that employ the voltage difference between consecutive pulses instead of the phase of occurrence as the main discharge parameter. Furthermore, using a particular parameter vector that takes the correlation between consecutive discharges into account also allows to solve the rejection problem with this type of NN.

Gas analysis by infrared spectroscopy as a tool for electrical fault diagnostics in SF6 insulated equipment

Abstract: Infrared spectroscopy shows an enormous potential for the analysis of by-products generated from electrical discharges in sulfur-hexafluoride (SF6) insulated equipment. Since by-product composition can be related to the fault genesis (arc, partial discharge or corona), the analysis of contaminated SF6 provides a valuable diagnostic tool. The IR-spectrometric results from discharge experiments are presented, carried out with the application of SF6 pressures around 300 kPa and an alternating voltage up to 30 kV. Under the discharge conditions used, the main by-products found are the sulfuroxyfluorides SOF4 and SO2F2 with concentrations correlated to the discharge time. Due to its toxicity, special attention is also paid to S2F10. The experimental conditions and practical aspects for reliable quantitative analysis of reactive species are discussed.

New high temperature polyimide insulation for partial discharge resistance in harsh environments

Abstract: New power-inverter technology has made sharp voltage rises increasingly common in today's motor and magnet wire insulation applications. That is why voltage pulse performance is an important property of any high-voltage insulation material.

UHF PD-detection in a power transformer

Abstract: The detection of partial discharge (PD) in high voltage insulation is an effective way to determine dielectric faults in the early stages of development. Partial discharge detection in a power transformer was made possible by use of the acoustic detection technique and also by the UHF detection technique. Floating metal parts and partial discharges were detected in paper insulation in a 110/10 kV transformer. The discharge patterns and detection sensitivity obtained with several discharge sources in transformer oil were presented. The UHF technique is considered an ideal method for monitoring and for inspection because of the possibility for automatic classification of the type of discharge and because of its low sensitivity to external noise. It was shown that partial discharge in oil is short in duration and more irregular in time than partial discharge in gas. The UHF technique was also found to be more sensitive because of the lower wave attenuation and classification of the type of discharge. 3 refs., 5 figs.

Movement of particles in compressed SF/sub 6/ GIS with dielectric coated enclosure

Abstract: In compressed GIS (gas insulated switchgear), the control of wire particle contamination by dielectric coating of enclosure is demonstrated in this paper. Assuming that a particle acquires free charge through a partial discharge, the modeling of particle movement when the inside surface of a horizontal section of GIS/GITL (gas insulated transmission line) enclosure is coated is described. Under an applied 60 Hz ac voltage, the maximum height reached above the enclosure by a particle is much lower for a coated than for an uncoated enclosure. When the applied ac voltage varies, the maximum height reached depends on the magnitude of the applied voltage, phase and velocity at which the particle hits the enclosure. Typical results show that by coating the enclosure, the maximum height reached by a wire particle in a coaxial system can be reduced, thus inhibiting particle movement and minimizing the possibility of insulation breakdown.

On-line partial discharge detection in cables

Abstract: This paper describes an on-line partial discharge (PD) detection technique in power cables. The technique uses a digital spectrum analyzer with a high-frequency preamplifier and several types of high-frequency inductive sensors. In extruded and impregnated cables, the PD detection is made in the VHF range. The suitability and the sensitivity of the VHF technique were checked against two other conventional PD detecting methods (Pulse Phase Analyzer and Fast Digital Scope) by performing PD measurements in EPR-insulated cable under laboratory-controlled conditions. The suitability and sensitivity of the VHF method were as good as the two conventional PD detection method. The VHF method runs used to conduct on-site PD measurement in several power cables. These measurements were conducted while the cables were in service. Special care was given to distinguish between external electromagnetic noise and electromagnetic signals produced by the PD activities inside the cable insulation.

Partial discharge detector of gas-insulated apparatus

Abstract: Electromagnetic waves generated by partial discharge occurring when gas-insulated apparatus fails are detected by using a compact, wide band and high sensitivity antenna. Electromagnetic waves produced by partial discharge are detected over a wide range with a high sensitivity by a circular antenna receiver (4) which comprises a pair of semi-circular discs combined on a plane inside the metal vessel (3) of the gas-insulated apparatus. The detected signal is measured by a measuring instrument (10) through a coaxial enclosed terminal (7) and a coaxial cable (8). Since the area of the semi-circular antenna unit is large, the capacitance between the antenna unit and a potential measuring unit can be large, so that the high sensitivity potential measurement can be also performed.

Small partial discharges and their role in insulation deterioration

Abstract: It is a well known fact that PD (partial discharge) affects the lifetime and the deterioration rate of an insulation. Research work has shown that a mixture of magnitudes and frequencies of PD may appear, depending on the type of the applied voltage, its frequency, the electrode arrangement, the type of the insulation under test, and a number of other parameters, such as temperature, humidity and pressure. Various types of discharges also have been recorded, such as streamer like discharges, Townsend discharges, glow and pseudoglow discharges. A type of discharges not yet well understood is the so called SPMD (swarming partial microdischarges), which seem to appear just before breakdown. In this paper, only aspects of the latter type of PD are investigated. Some suggestions for further research regarding the SPMD are made.

Monitoring partial discharges on 4 kV motor windings

Abstract: Partial discharges are a symptom of most of the stator winding insulation deterioration mechanisms found in motors and generators rated 6 kV or more. Thus, partial discharge testing has been used for over 40 years as a means of determining the maintenance requirements of such high voltage stators. As the benefits of such testing have become more widely known, there has been an interest to apply partial discharge testing to 4 kV motor and generator windings. This paper discusses the theoretical reasons which may limit the testing of 4 kV stators. The results of an extensive series of offline partial discharge tests are presented which indicate that partial discharges indeed do occur. The results of a series of online partial discharge tests conducted on about ten 4 kV motors over the past two years show that partial discharges may only occur for a short time prior to failure with some types of failure mechanisms. Thus to prevent in-service failures, continuous monitoring is necessary. An inexpensive continuous partial discharge monitor is described. It seems that partial discharge monitoring will only be cost-effective for very critical process motors and generators where in-service failures may have large financial consequences.

Dielectric strength of transformer insulation at DC polarity reversal

Abstract: Among special insulation tests for DC power converter transformers, polarity reversal tests can be substituted by applied AC voltage tests from a stress similarity in oil ducts In order to investigate equivalent levels of other insulation tests to polarity reversal tests, the dielectric strength of a transformer at polarity reversals was examined by use of a model of typical oil/paper composite insulation From the comparison of polarity reversal and other insulation tests, it was found that dielectric strength of transformer insulation at polarity reversals is phenomenally similar to that under switching impulse stresses Equivalent AC test voltage to polarity reversal test was also estimated from the experimental results The breakdown characteristics of polarity reversal tests showed that partial discharges at polarity reversals impose less harmful stress on insulation barriers than those at usual AC or impulse tests

Power processor circuit and method for corona discharge pollutant destruction apparatus

Abstract: A solid-state power processor circuit and method is used to supply power to generate a discharge in a corona discharge pollutant destruction apparatus. The circuit uses field effect transistors (FETs) and integrated circuit devices based upon metal oxide semiconductor field effect transistor (MOSFET) technology to amplify low-voltage pulse signals to high voltage levels. A resonator in the power processor circuit generates sinusoidal oscillation from the high voltage pulses, and provides the high-voltage, high-frequency electrical power necessary for corona discharge.

Partial discharges in epoxy resin voids and the interpretational possibilities and limitations of Pedersen's model

Abstract: Partial discharges take place, among others, inside voids enclosed in insulations. The partial discharge behaviour of epoxy resin samples is studied with the aid of a recently developed streamer partial discharge model (Pedersen's model) and its relation to the modem Pulse Height Analysis and Phase Resolved Measurement techniques is investigated. Furthermore, some relations based on Pedersen's model are deduced. The advantages and disadvantages of the aforementioned model are analysed. Scanning electron microscope photographs show the nature of the deterioration of the void walls due to the partial discharges. The question whether initiation of the treeing phenomenon leading to breakdown from an enclosed spherical void is possible, is also investigated.

Dielectric barrier discharge device

Abstract: A dielectric barrier discharge device in which uniformity of the light irradiation energy density is achieved, even if dielectric barrier discharge lamps have variations with respect to the lamp material and the processing. The dielectric barrier discharge device has at least one dielectric barrier discharge lamps and a feeder device for applying an AC high voltage to the electrodes of the dielectric barrier discharge lamp, the dielectric barrier discharge lamp having a discharge gas-filled discharge gap which produces excimer molecules by a dielectric barrier discharge and one dielectric between at least one of the electrodes and the discharge gap for inducing the discharge phenomenon in the discharge gas. Uniformity of the light irradiation energy density is achieved is achieved in this dielectric barrier discharge device by the feeder device applying the AC high voltage such that an operating state for time interval (Ta) in which in the dielectric barrier discharge lamp produce a discharge emission is repeatedly alternated with an operating state for a time interval (Tb) in which essentially no discharge is produced, or in which a lower voltage is applied within a range that is only sufficient to maintain discharge in the dielectric barrier discharge lamps.

A physical model, describing the nature of partial discharge pulse sequences

Abstract: A description is given of a straightforward physical model, which is based on only a few physically meaningful parameters and that allows to describe the PD behaviour resulting from specific insulation defects. It is demonstrated that computer simulations of PD pulse sequences on the basis of this model correspond closely to experimental results. The comparison of simulated sequences and experimental results suggests that the discharge activity is not mainly a stochastic process but consists of periods, with different prevailing processes, each showing a nearly deterministic behaviour.

Development of partial discharge monitoring technique using a neural network in a gas insulated substation

Abstract: A study was carried out to detect partial discharge signals generated in a gas-insulated substation (GIS) subjected to an insulation spacer method (internal electrode method), and to determine whether or not partial discharges were generated and then locate them with a pattern recognition method using a neural network (NN). This paper describes the results, while also explaining the equivalence between simulation pulse injection and partial discharges as web as the determination using the NN mentioned above and then presenting applications using field data.

Investigation of the failure mechanism of insulation subjected to repetitive fast voltage surges

Abstract: Electrical rotating machines are subjected to nonsinusoidal wave shapes when supplied from high frequency inverters. PWM (pulse width modulated) voltage sources with steep wave fronts can be destructive to the insulation and criteria normally used under sinusoidal conditions cannot necessarily be transposed to this new case. In order to determine the influence of this electrical stress, laboratory experiments were carried out to determine whether such repetitive voltage surges can precipitate premature failure of high voltage motor windings. A fast voltage surge generator and laboratory cells have been built, to study partial discharge development and material degradation according to voltage magnitude, polarity, rise time and surge repetition rate. Partial discharges (PD) are strongly dependent on the voltage waveform, related to the statistical time lag which is necessary before PD inception. Physical and chemical analysis and surface potential decay measurements show that the ageing process is related to the voltage waveform (frequency, shape factor, etc.). It is suggested that under PWM operation, insulation ageing mechanisms are predominantly controlled through an electrostatic phenomenon, different from the 50/60 Hz sinusoidal case.

Analysis of transformer oil for transformer condition monitoring

Abstract: International experience shows that large transmission transformers can have service lives in excess of 50 years. For the operator the concern is to be able to identify the onset of unreliability as the end of life approaches. There are a variety of chemical, electrical and physical condition monitoring techniques which can be applied. For many utilities, the basic method is a regular analysis of an oil sample. Mechanical and electrical faults may arise following short circuits, local overheating at hot spots or leakage flux and eddy currents in the core, and partial discharge or arcing at areas of high stress. Decomposition products from breakdown of the oil, paper or insulating boards, glues etc. are transported through the transformer by the coolant oil. Some are low molecular weight gases dissolved in the oil and can be identified by gas chromatography. Others indicating solid degradation include furans, cresols and phenols and detected by liquid chromatography. Alternative technologies require outages and access to bushing terminals to measure mechanical electric characteristics of the winding and the magnetic circuit. For many these are used as secondary methods to be used for condition audits, or to investigate problems identified by oil analyses. (5 pages)

Activity of moving metallic particles in prebreakdown state in GIS

Abstract: The paper presents experimental results of measurements of prebreakdown activity, caused by free metallic particles in GIS, obtained using a phase resolved partial discharge (PRPD) analysis and acoustical measurements. The numerous parameters characterizing prebreakdown activity of particles were identified and analyzed. These included phase resolved distribution of charge, maximum charge, average charge per period, or per pulse, a balance of charge in positive and negative half-wave of voltage, pulse rate (number of pulses per period) for electrical and acoustic methods. The measurements were conducted in a compartment of real GIS, and Al as well as Cu particles of different shapes were investigated. The pressure varied from 150 to 550 kPa, however most measurements were taken at 450 kPa. A discharge model which takes into account the effect of a free metallic particle in GIS is presented.

Discharge characteristics under non-uniform electric field in He, Ar and air at low pressures

Abstract: We measured dc discharge inception voltage V/sub i/ for various electrode configurations in He, Ar and air in vacuum range from 10/sup 5/ to 10/sup -1/ Pa to determine discharge characteristics under a space vacuum environment. By quantitative consideration of the E/sub i//p distribution in the gap space at the discharge inception, the discharge inception mechanism under non-uniform electric field was investigated in vacuum. At the same time, we observed the discharge profile variation with residual gas pressure and quantitatively analyzed its characteristics, using an image processing technique. Based on the analysis, we found that the dependency of the discharge inception voltage and the length of the discharge path on the residual gas pressure under non-uniform field conditions agreed well with that previously obtained for uniform field conditions.

New method for measuring statistical distributions of partial discharge pulses

Abstract: A new digital detection system is described for measuring pulsating partial discharges (PDs). The PD detection system can continuously record all PD pulses that occur over extended periods of time, with a minimum inter-pulse time separation of 6 μs and a vertical amplitude resolution of 12 bits. Earlier PD detection systems detected PD pulse amplitude and time using complex custom-designed hardware while the present system continuously records the complete electrical waveform that carries the PD pulses using a commercial data acquisition board and extracts, in real time, the time and amplitude information of all PD pulses in software. The current approach considerably reduces the development and maintenance cost of the PD detection system, significantly increases the system portability, and may prove to be a crucial step for transferring the digital PD detection and analysis technology developed in laboratories to industry. The features of the new system are illustrated by the study of dc-excited PD pulses occurring in a point-to-plane gap in air. A new surface-mediated burst mode of PDs is discovered in which a PD pulse has a certain probability to induce another pulse. The probability is determined for several gap voltages and is found to vary strongly with the applied voltage.

Turn insulation aging of motors exposed to fast pulses of inverter drives

Abstract: The effects of fast pulses from PWM inverter AC drives on motor insulation are not well understood and their impact on long term reliability is not known. Turn insulation stresses have been simulated on small "layer-to-layer" specimens subjected to typical pulses of 120 ns rise-time at a repetition rate of 10 kHz. Voltage pulse overshoots reached 920 V. Under these conditions no partial discharge was detected. Measurements of capacitance, dielectric losses, relaxation currents, return voltage and thermally stimulated current (TSC) have been used to characterise insulation modifications resulting in the cumulative effects of multiple pulses. Preliminary results indicated that changes in the dielectric properties after a short exposure (1310 hours) to voltage alone were negligible. When specimens were exposed to both temperature and voltage, TSC results showed that some modifications of the dielectric properties could be initiated after short exposure time, even below the partial discharge inception voltage.

Influence of high voltage harmonics on partial discharge patterns

Abstract: This paper describes the influence of high voltage harmonics on partial discharge (PD) phase resolved images. The evolutions of these images as a dependency of total harmonic distortion (THD) parameter as an assessment factor, calculated by means of Fast Fourier Transform (FFT) are shown.

Glow discharge plasma processing method and device thereof

Abstract: PROBLEM TO BE SOLVED: To continuously generate uniform glow discharge plasma under pressure in the vicinity of atmospheric pressure regardless of a gas atmosphere in processing, and perform stable glow discharge plasma processing by setting voltage rising time not more than a specific valve, and setting electric field strength to a specific range in a device where an impressing electric field is converted into a pulse. SOLUTION: In glow discharge plasma processing, a pulse electric field on which electric field strength is 1 to 100kV/cm and rising time is not more than 100μs, is applied. In this device, a solid dielectric 16 is arranged on a lower electrode 15, and discharge plasma is generated in a space between the solid dielectric 16 and an upper electrode 14. Processing gas is supplied to a discharge plasma generating space 13 from a gas introducing pipe 18, and an upper surface of a base material 17 contacting with generating discharge plasma is processed. When both surfaces of the base material 17 are wished to be processed, the base material 17 is floatingly arranged in the discharge plasma generating space 13.

Partial discharge coupler

Abstract: A partial discharge (PD) coupler includes at least one high voltage capacitor; potting material surrounding the capacitor; and a fuse physically and electrically connected in series to the capacitor. The capacitor and the fuse are capable of passing high frequency partial discharge with minimal attenuation. The PD coupler can have potting material surrounding both the capacitor and the fuse. The PD coupler can include a housing surrounding the capacitor, the potting material, and the fuse.

Investigation of aging rate in polymer films subjected to surface discharges under distorted voltage

Abstract: This paper presents the first results of a research carried out with the purpose of investigating life performance and degradation mechanisms of electrical insulating materials subjected to distorted ac voltage. Accelerated life tests were performed until breakdown, measuring partial discharge evolution with time, by a test arrangement suitable to induce surface discharges on insulating material specimens. Harmonic voltages having different frequencies and phase, in the range of those usually recorded in electrical plants, were superimposed to the fundamental component. The tested material is polyethylene terephthalate. Either reduction or increase of life, with respect to sinusoidal conditions, were detected, dependently on the way as harmonic voltages are summed up. In particular, significant life shortening was obtained in the cases when voltage maximum is increased due to the contribution of harmonics. These result can be directly correlated with the partial-discharge pattern structure.

Method and device for controlling discharging current slope of wire cut electrical discharge machine

Abstract: It is a main object of the present invention to improve the drawback of a conventional wire cut EDM which controls the pulse width of discharging current to supply discharge energy under the state of constant current slope. The discharging current slope control method and device of the wire cut electrical discharge machine of the present invention comprises an electrical discharge detecting circuit, a timing controller circuit, a normal discharge driver circuit, an arc discharge driver circuit, normal discharge switch elements and arc discharge switch elements. The slopes of the output triangle waveform currents shall be variable due to the different results of detecting by the electrical discharge detecting circuit, depending on normal discharge or arc discharge condition, to render the current slope of the arc discharge smaller than the current slope of the normal discharge, so as to achieve the effect of keeping a high machining speed without easily breaking of the wire electrode.

Application of acoustic sensing and signal processing for PD detection in GIS

Abstract: The presence of free moving metallic particles and discharges deteriorates the insulation strength of a compressed SF/sub 6/ (sulphur-hexafluoride) gas insulated switchgear (GIS) and causes serious limitation in its practical application. Free moving metallic particles and other defects inside the GIS cause partial discharge (PD) which can degrade the insulating properties of the insulant gas SF/sub 6/ to such an extent that breakdown may occur in the GIS system. So, it is necessary to detect partial discharge in the GIS at an early stage before system failure and extensive damage to the equipment and the rest of the power system. Partial discharge emits acoustic signals which can be detected by applying an acoustic emission sensor (AE sensor) outside the GIS chamber. This paper initially describes some fundamental aspects related to PD detection. Finally, the development of an acoustic PD detection system and some experimental investigations are presented.