Showing papers on "Partial discharge published in 1989"

Degradation of polymeric insulation due to photoemission caused by high electric fields

Abstract: The mechanism of electrical tree initiation in polyethylene subjected at AC, DC, and half-rectified AC voltages is studied by an optical technique which is at least two orders of magnitude more sensitive than partial discharge detection. For DC voltage up to 46 kV, no light emission or tree inception was detected in the polymer containing the normal and decreased concentrations of air in its free volume. For AC and half-rectified AC voltages, tree inception only occurred if light, in the visible and ultraviolet ranges, was detected. Photodegradation of the polymeric insulation by ultraviolet radiation can occur when the voltage applied to the polymer exceeds the threshold voltage of light inception such as during switching and lightning overvoltages. This can account for the gradual degradation of the polymeric insulation which ultimately leads to the formation of an electrical tree. By comparing the light emitted from the polymer subjected to AC voltage having half-cycles of different amplitudes to DC superimposed on AC it is shown that the space charge injected into the polymer causes the zero-crossing of the local field in the polymer. This is required for light emission and subsequent tree inception. >

Application of digital filtering techniques to the determination of partial discharge location in transformers

Abstract: The capacitative network representation of transformer windings has long been used for the study of pulse distribution along the windings. However, the author's test results and those published in the literature show that theoretical analysis based on this equivalent network may not be applicable. It was found that the capacitive network approximations were valid only within certain ranges of frequency. The frequency ranges were determined through the analysis of terminal measurements. The capacitatively transferred components of the input pulse along the windings in the appropriate frequency range were then extracted by the aid of digital filtering techniques, and good agreement between measured and theoretical distributions was obtained. These techniques, in conjunction with a new straight-line interpolation method, were employed to locate simulated partial discharges in a 66 and 132 kV transformer winding. Measurement results showed that an accuracy of better than 5% of winding length was obtained. >

Partial discharge detection method and apparatus

Abstract: A method and device for detecting a partial discharge pulse in a high voltage conductor in the stator of a motor, turbine generator or the like, comprising a conductive plate having an insulated face, an insulated signal conductor spanning the insulated face of the plate, and means for transmitting an electrical pulse produced in the signal conductor to recording means for determining the location and severity of partial discharge activity within the high voltage insulation of the conductor.

Electrode for a corona discharge apparatus

Abstract: An electrode for a corona discharge apparatus is comprised of a conductor, which can be a stainless steel tube or filament, a metal bar or a rotatable roll, and which is covered by a dielectric, which can be an organic elastomer having a high dielectric constant or quartz or ceramic. The dielectric may be coated with a dielectric adhesive that is loaded with a granulated inorganic dielectric. In most applications a film resistor is inserted between the conductor and the dielectric to reduce any voltage spikes that can result if the corona discharge excites a resonant circuit.

Power supply system for negative impedance discharge load

Abstract: A power supply for negative impedance discharge loads such as discharge tubes used in gas laser systems, in which the discharge power is supplied not from the voltage source but from the current source through the current transformer so that stabilized and well balanced discharge currents can be supplied to a plurality of discharge loads.

Measurement of partial discharges in hexane under DC voltage

Abstract: Partial discharges are recorded by means of high-magnification, high-speed photography in effort to determine the relationship between the growth of the partial discharge and the current supplied to produce it. A needle-sphere electrode geometry is used to observe partial discharges at the tip of the needle for both positive and negative DC polarities. Observations of the initiation phase of the partial discharges are accomplished by the use of an image-preserving optical delay that permits photography of fast events. The current is monitored by a low-noise amplifier that also provides timing signals to operate the photographic system. The partial discharges are found to closely resemble the structure and growth of the slowest (subsonic) cathode events emanating from a tip under the application of negative impulse voltages. With DC, the current producing the partial discharge is in the form of a pulse train, which is in contrast to the linear growth observed in the streamer. >

Method for measuring the stochastic properties of corona and partial‐discharge pulses

Abstract: A new method is described for measuring the stochastic behavior of corona and partial‐discharge pulses which utilizes a pulse selection and sorting circuit in conjunction with a computer‐controlled multichannel analyzer to directly measure various conditional and unconditional pulse‐height and pulse‐time‐separation distributions. From these measured distributions it is possible to determine the degree of correlation between successive discharge pulses. Examples are given of results obtained from measurements on negative, point‐to‐plane (Trichel‐type) corona pulses in a N2/O2 gas mixture which clearly demonstrate that the phenomenon is inherently stochastic in the sense that development of a discharge pulse is significantly affected by the amplitude of and time separation from the preceding pulse. It is found, for example, that corona discharge pulse amplitude and time separation from an earlier pulse are not independent random variables. Discussions are given about the limitations of the method, sources of error, and data analysis procedures required to determine self‐consistency of the various measured distributions.

Process and apparatus for preparing the surface of a plastic molding by means of an electrical corona discharge

Abstract: Moldings having a thickness of 1 to 60 mm and made of flexible webs or rigid sheet of plastic are passed through a corona discharge treatment system consisting of high-voltage electrodes and a counter-electrode and a high-frequency alternating current voltage of 20 to 25 kHz and 20 to 70 kV is applied to the high-voltage electrodes by a generator. A corona discharge forms in the gap between the high-voltage electrodes and the counter-electrode. An aerosol formed by atomizing a liquid is blown into the corona discharge zone by means of an air or gas stream. The aerosol modifies the surface of the sheet-like molding in the desired manner.

Metallic particle movement, corona, and breakdown in compressed gas insulated transmission line systems

Abstract: A simplified computational algorithm has been developed to study the behavior of metallic particles in gas insulated transmission line (GITL) systems. The model provides an estimate of the effectiveness of particle trapping for a particular trap design at a given commissioning voltage. Results of measured breakdown probabilities of particle contaminated SF/sub 6/ show fair agreement with calculated values. A computational model was developed to calculate the corona inception voltage due to conducting particle motion in GITL systems. The computed results for various parameters were in fair agreement with the measured values. A corona pulse pattern is computed and plotted using the same algorithm. Oscillographic records of the transient corona show that particle discharge is larger for long particles, and its magnitude increases with voltage level. Corona patterns obtained in the laboratory indicate that conducting particles can be detected in GITL systems and that the particle size can be determined through partial discharge measurements with AC voltage at different gas pressures. >

On the electrical breakdown of gaseous dielectrics-an engineering approach

Abstract: Quantitative criteria are derived for the onset of breakdown in atmospheric air and similar weakly electronegative gases. A comparable criterion is then formulated for strongly electronegative gases such as SF/sub 6/. These criteria contain no quantities other than those which can be obtained directly from breakdown voltage measurements in a uniform field, i.e. from Paschen curve data. No specific data are required for the ionizing coefficients of the gas. The advantage, from an engineering point of view, is that it is much easier to perform reliable Paschen curve measurements than it is to measure ionization growth parameters. The price paid is that criteria formulated in this way cannot be used to yield unknown Paschen curves. The criteria can be applied to any nonuniform field configuration provided the electrostatic field distribution is known. Moreover, in many cases a simple analytical approximation to the field distribution can render an extensive calculation of the electrostatic field unnecessary. The discussion deals exclusively with situations in which the onset of breakdown in the system is synonymous with the onset of a corona discharge. >

Process and apparatus for pretreating the surface of a single-layer or multilayer molded material by means of an electrical corona discharge

Abstract: Pretreating the surface of electrically conducting molded materials, such as metal foil or plastic films in whose polymer matrix electrically conducting particles have been incorporated, by means of electrical corona discharge is provided. A generator subjects the high voltage discharge electrodes of the corona discharge to a high-frequency alternating current of high voltage. The metal core of a roller, over whose metallic circumference a film web or foil is transported, serves as the grounded metallic counter-electrodes. A housing encloses the high voltage electrodes of the corona discharge and is connected via a line to an atomizer apparatus, in which a liquid is atomized to give a suspendible aerosol by means of a piezoelectric ultrasonic vibrator system or by two-material atomizer nozzles operating at the speed of sound. A blower whose throughput can be regulated is connected to the atomizing apparatus and conveys the carrier gas, for example air, for the aerosol through the atomizing apparatus into the corona discharge.

High-voltage and partial discharge testing techniques for space power systems

Abstract: Partial discharge (PD) detection techniques are reviewed, focusing on application in the aerospace industry. The discussion covers dielectric withstand voltage test, causes of high-voltage failures in flight hardware systems and how PD testing can detect incipient problems, and high-frequency PD test methods and applications. >

Electrical characteristic degradation of ZnO varistors subjected to partial discharges

Abstract: Zinc oxide is a polycrystalline ceramic having nonlinear current‐voltage characteristics. This specific property allows it to be used as varistors. When these components are set in medium‐ or high‐voltage lightning arresters, they may be subjected to partial discharges. The objectives of this study were to determine the action of partial discharge parameters on the I(V) characteristic degradation. Varistor samples under test were made with a standard process. They were metallized on both ends and were either covered with an insulating layer or not. The results obtained from bare samples in dry and wet air and at room temperature showed an important modification of the I(V) characteristic in the ohmic region with a large increase of leakage current.

Partial discharge measurement on three-core belted power cables

Abstract: It is shown that the partial discharge (PD) characteristics of three-core belted-type cables determined using single-phase testing techniques may differ greatly from the actual discharge performance of the cable when subjected to three-phase voltage conditions in service. To achieve meaningful results concerning the voltage ratio and the life expectancy of the cable, careful considerations must be given to the way of energizing the cable and PD response characteristics of single-phase test configurations. An example of erroneous single-phase testing techniques is a circuit which gave an error of up to 88% lower discharge level as compared with three-phase detection. A novel technique was developed in which single-phase voltage was applied to one core, the other two cores were earthed, and the sheath was left floating. The PD detection used a coupling capacitor connected to the energized phase. >

Partial discharges in 3-core belted power cables

Abstract: It is noted that partial discharge testing of three-core belted-type cable using single-phase test voltage may lead to large errors as compared to the actual three-phase service conditions. The method of energizing the cable and the configuration of the discharge detection circuit largely affect the magnitude of the error. Eight different test circuits are discussed with respect to their usefulness in the laboratory as well as the field. Attention is paid to the similarity of the electrical stresses in the single and three-phase circuits. To this purpose field calculations were performed on different single-phase test circuits to determine a design that meets the three-phase field patterns at its best. Three circuits are selected as being suitable for a correct single-phase partial discharge measurement. >

Implications from the influence of electrohydrodynamic motion on breakdown with respect to the discharge mechanism in dielectric liquids

Abstract: Electrical discharge measurements with constant voltage pulses in a thin dielectric liquid layer between plane-parallel electrodes provide information about the dielectric strength as a statistical distribution of the time lags to breakdown. Under a large variety of circumstances, the time-lag distributions are composed of two main parts, termed the fast and the normal components. The nature of these components has been identified. The initial fast component lasts as long as the electrical charges, present or created in the liquid, have not yet been able to induce electrohydrodynamic motion. When the point of instability is reached, the probability of breakdown drops by a large amount as evidenced by the normal component of the time-lag distribution. The ions in the liquid, which are responsible for the electrohydrodynamic instability, also condition the electric-field distribution. The peak value of the local field strength is likely to determine the breakdown characteristics. >

Digital acquisition and processing of partial discharges during acceptance test of HV transformers

Abstract: A six-channel digital recording system has been developed for monitoring partial discharges (PDs) in the HV insulation of power transformers during acceptance tests. The analog PD signals are converted to digital words describing their apparent charge, polarity and the point on the wave (phase) of the test voltage at which the PD pulse appears. The data are recorded during a 1 s acquisition period, processed, displayed and stored during the time (up to 7 s) the system is preparing for the next acquisition. The complete record of PDs acquired during the test is stored on a hard disk, and specialized programs have been developed for post test evaluation of the transformer performance. Different presentation formats allow easy identification of interference signals induced in the test circuit by external sources, correlation of PD patterns recorded on different transformer windings, and evaluation of changes in PD activity during the test. >

Charge injector calibrating partial discharge measurement circuits - has charge storage capacitor supplied by recharge device and short duration switch

Abstract: A charge storage capacitor (Cs), supplied by a recharging device (NL) with a h.v. pref. greater than 100 volts, is connected at one end to an output port (K1) via a first high impedance resistor (RL1) and series capacitor (Ca) and at the other end to the other output (K2) either directly or via a further high impedance resistor (RL2). A short duration switch (SK) is connected between the second output and the resistor-capacitor junction. The outputs are connected to the discharge measurement circuit, with test object ports or, at a min. sepn. of 10 cm., to the metal surface of the specimen container or to the specimen potential and pref. to the conducting injector housing (GM) and/or an e.m. antenna radiator (AS). Both outputs can be bridged by a shunt resistor pref. the characteristic impedance of a connected cable. USE/ADVANTAGE - Extremely simple circuit eliminates disadvantages of conventional ones, including high maintenance internal battery supply and limitations of calibration. Charge pulses not only injected into terminals of test piece but also into metallic, usually earthed, housing of h.v. equipment such as large transformers, SF6 switchgear, generators or motors.

Frequency-changing series resonant breakdown voltage testing device

Abstract: The utility model relates to a frequency-changing series resonant breakdown voltage testing device, which is suitable for power equipment of power transmission and transformation engineering and components thereof and breakdown voltage and partial discharge index testing operation. The device is composed of an extra-high voltage flexible connecting lines made of a linear amplification type frequency changer whose frequency range is 30-300 hertz, an intermediate transformer, a hollow extra-high voltage reactor, a glass fibre reinforced plastics equalizing ring, and a conductive fiber cloth and an extra-high voltage resonant capacitor with the housing of glass fibre reinforced plastics. The characteristic is that waveform distortion is little, loading ability is strong, efficiency is higher, anti-flashover ability is strong, and higher partial discharge index is easy to be obtained.

Partial discharge measuring method

Abstract: PURPOSE: To easily diagnose insulation with high accuracy by specifying a frequency range and fitting high impedance which has a specific value to the cross-bond line of an insulated connection part. CONSTITUTION: In an underground long channel, noises are attenuated abruptly at frequencies above an intermediate wave (up to 1.6MHz), so the frequency range is set to 1.5 - 20MHz. High-frequency iron cores 3a - 3c which provide effective impedance at a measurement frequency are fitted to the cross-bond lines of insulated connection parts 1a - 1c which have sheath insulating cylinders 2 on a long three-phase AC power cable line laid in this underground channel. Their cross-bond terminals are connected to detection impedance elements 4a - 4c through signal lead-out lead lines (a) - (c) and their outputs are amplified 5a - 5c and detected 6a - 6c independently. The signals after the detection are supplied to a judgement circuit 7 to decide noises and outputted 8 only when judged to be partial discharge signals. COPYRIGHT: (C)1991,JPO&Japio

On a model for the comparative analysis of the ageing characteristics of epoxy resins subjected to internal discharges at various environmental temperatures

Abstract: Different epoxy systems typically used in HV gas-insulated systems were investigated to obtain a more global view of the materials' performance. An accurate scanning microscope analysis was carried out on the aged wall cavity and inside the breakdown channel to obtain information on which to base a model of material performance under high temperature. The time to spontaneous breakdown of samples of identical specimens having a calibrated enclosed void was used as the life criterion. Particular attention was focused on the scatter of the lifetimes. In many cases PD (partial discharge) amplitude distributions were detected during life tests by means of a computerized system, and emphasis was given to their time changes in relation to the testing temperature. This technique made it possible to obtain a more global view of material performance against PD activity for measurements carried out by means of a laboratory test cell. On the basis of the experimental results, a model that provides a deeper insight into resistance to PD, especially when comparative information between different materials is required at temperatures different from the ambient, is proposed. >

Defect detecting method of winding of rotary machine

Abstract: PURPOSE:To make it possible to detect the presence or absence and the position of a pinhole in a winding at a relatively low voltage, by reducing the pressure in a container under the state a constant voltage is applied to the winding of a rotary machine, and measuring the amount of the electric charge and the voltage when partial discharge occurs. CONSTITUTION:A constant voltage (e.g., 340-410VPEAK for air), which is slightly higher than the minimum breakdown voltage in a Paschen curve of a gas in a container, is applied to a winding 3 of a rotary machine, which is set in the container 2. It is desirable that the value of the applied voltage is as low as possible in a range wherein an enamel wire is not deteriorated. With the voltage applied, the pressure in the container 2 is reduced with a vacuum pump 11. Then partial discharge occurs at a certain pressure. When the pressure is further decreased, very large amount of discharge occurs at the intermediate part. The large amount of the discharge is the discharge from a pinhole. Therefore, whether the discharge is the partial discharge from the pinhole or the partial discharge from another part is judged based on the magnitude of the partial discharge. The pressure when the partial discharge occurs is measured with a pressure gage 12. The distance to the pinhole is obtained based on the characteristics of pressure/gap length, and the position is judged.

Dielectric breakdown and partial disc0harge in batio3 ceramics

Abstract: Barium titanate ceramics were subjected to dielectric breakdown and partial discharge tests under 60 Hz voltage as a function of sintering temperature (1300°C to 1450°C). The high temperature sintering resulted in large pore size. The large pores in over fired BaTiO3 ceramics, having relatively low inception voltage of partial discharge, contributed to the low breakdown field.

A discharge locating probe for rotating machines

Abstract: The Tennessee Valley Authority (TVA) probe permits location of partial discharge (PD) since it is sensitive to radio frequency signals produced by PD. However, the present TVA probe has limitations both in directionality and in providing quantitative measurements for comparison of different windings. The authors present the basic principles of detection of an improved TVA probe, known as the discharge locating (DL) probe, which was developed to be an evolutionary upgrade to the TVA probe. The principal features of the new device are very-high-frequency detection using a capacitive sensor and microprocessor-based signal processing. These features are intended to make the probe easier to use and less ambiguous in the interpretation of results. The coupling technique used permits significantly improved spatial resolution of the discharge source and immunity to radiated electrical interference. Field evaluation of the DL probe has generally been successful, with the results confirming theoretical and laboratory-based studies of the device. >

How useful are diagnostic tests on rotating machine insulation

Abstract: A critical review of commonly used diagnostic tests for the stator winding insulation of rotating machines is made from the perspective of one largely hydro utility. Relatively low voltage measurements of insulation resistance and polarization index are shown to be important indicators of winding dryness for all thermoplastic insulation systems as well as for those polyester systems which are prone to tape separation. The DC ramp test has proven to be very useful for nondestructively detecting cracks and fissures in both thermoplastic and polyester insulation systems. The TVA probe test measures both the internal PD (partial discharge) and the slot discharge associated with individual connected coils/bars. These data frequently correlate with online PD tests and/or other tests and inspections. The CEA PDA (PD analyzer) with permanent couplers has proven to be an important addition to available diagnostic test methods. Frequent monitoring allows PD-related problems to be detected before much damage has occurred, while extending the period between outages for other more costly tests and inspections. >

Dielectric phenomena in semicrystalline polymers

Abstract: For the application of polymers in insulation systems it is essential to understand the dielectric phenomena leading to distinct electrical properties such as conductivity, permittivity, or electrical strength. It is generally accepted that space-charge formation plays an important role in these complex interactions, causing distortion of the electric field determined by the electrode configuration. An assessment of the electrical behavior of partially crystalline polymers requires the knowledge of the DC and AC properties and the interrelations with space-charge processes as well as the dependencies of the electric properties from structural conditions and their relaxations. Sophisticated experimental techniques such as polarization and depolarization current measurements, on-line measurement of charge carrier motions in AC fields, and highly sensitive and time-resolved partial discharge inception measurements provide necessary data for the understanding of dielectric phenomena and permit first approaches in analytical and numerical modeling. >

The effect of high-voltage lightning impulses on the core properties of transformers

Abstract: Results of a preliminary investigation into transformer discharge measurements and core losses under high-voltage impulse conditions are presented. Attention is given to the voltage distribution through the transformer winding for high-voltage impulses, the effects of partial discharges, and the variation in core temperature. It is found that at certain positions in the winding the surge voltage can exceed the initial peak value, which in turn leads to an increase in partial discharge activity at the same monitoring position. It is concluded that there is a possible relationship between voltage distribution, partial discharges, and core losses during surge conditions. >

AC energized ferroelectric pellet bed precipitator for sterilization and gas clean up

Abstract: The performance of an electrostatic precipitator utilizing an AC-energized packed ferroelectric pellet bed has been studied. A stable partial discharge was formed inside the pellet bed due to polarization of the ferroelectric pellets. Dust particles were precharged by a DC corona discharge in front of the pellet bed and collected on the pellet surface. The minimum dust penetration P was observed at the certain voltage at which partial discharge occurred. The minimum value of P was about 0.001% for particles larger than 2 mu m in diameter. Using the partial discharge, sterilization can be effected and gas-phase plasma chemical reactions can be promoted for deodorizing. Survivability of yeast cells deposited on the pellet surface decreased with exposure to the partial discharge. When the cells were dispersed to air and fed to the precipitator, live cells could not be detected in the exhaust air. Performance was also measured using a mixture of ammonia with air, showing that 120 p.p.m. of ammonia could be removed with a detention time of 0.7 s inside the partial discharge. >

Gas-filled discharge tube for high voltage switch element

Abstract: PURPOSE: To enable the stable maintenance of discharge starting voltage at high voltage by giving values having a determined relation to the radius of discharge electrodes, the space between electrodes of the discharge electrodes, the space between the outer wall surfaces of the discharge electrodes and the inner wall of an enclosure. CONSTITUTION: Onto the both side ends of an enclosure formed of a ceramic cylinder of the same size as the outer diameter of the flanges of line electrodes 10a, 12a, the flanges of the electrodes 10, 12 are air-tightly brazed. Kr is sealed in this discharge tube. The radius (d) of the discharge electrodes 10, 12, the space D between electrodes of the discharge electrodes 10, 12, the space D' between the outer wall surfaces of the discharge electrodes 10, 12 and the inner wall surface of the enclosure 14 satisfy d

Space charge injection and partial discharge inception at the metal polymer interface

Abstract: Injection and transport of charge carriers occurring above a distinct threshold field are shown to play a dominant role in the stability of dielectrics under long-term electrical stress. Since the required threshold fields are well above the design fields of the usual types of insulation they can only be reached very locally at field enhancement tips (conducting defects) or at interfaces in composite insulators. From such tips treeing almost inevitably develops in a series of intermediate steps when applied AC voltage leads to local transgression of the threshold field. The time to failure is found to be strongly dependent on how large the transgression is. >