Showing papers on "Partial discharge published in 2002"

Partial discharges. Their mechanism, detection and measurement

Abstract: Different partial discharge (PD) detection and measurement procedures suitable for use on cables, capacitors, transformers and rotating machines are examined and compared. Both narrow and wide bandwidth PD detectors are considered; particular attention is given in regard to their suitability to different types of electrical apparatus and cable specimens under test as well as their applicability to discharge site location and their capability to detect different forms of PD. A rather substantial portion of the discussion is devoted to the use of intelligent machines as applied to PD pattern recognition in terms of either PD pulse-height/discharge epoch (phase) distributions or discharge pulse shape attributes.

Novel frequency-domain-based technique to detect stator interturn faults in induction machines using stator-induced voltages after switch-off

Abstract: Traditionally, for medium- and high-voltage motors and generators, condition-based monitoring of stator faults is performed by measuring partial discharge activities. For low-voltage machines, negative-sequence impedance or currents are measured for the same. Such diagnostic schemes should be carefully implemented as supply voltage unbalance, manufacturing-related asymmetry, etc., also produce negative-sequence voltages. A few approaches based on motor current signature analysis have already been proposed to detect stator interturn faults. However, little or no physical insight was provided to explain the occurrence of certain harmonics in the line current or the influence of voltage unbalance on these harmonics. Also, in at least one of these papers, a large portion of the stator winding was shorted to emulate the faults. The method proposed in this paper monitors certain rotor-slot-related harmonics at the terminal voltage of the machine, once it is switched off. In the absence of supply voltage, issues such as voltage unbalance, time harmonics do not influence the measurements except as initial conditions, which is a very desirable feature when the machine is fed from an adjustable-speed drive. Satisfactory simulation and experimental results have been obtained with only about 1.5% (5/324) of the total number of turns shorted.

Comparison of on-line partial discharge detection methods for HV cable joints

Abstract: The capacitive coupler, acoustic emission (AE) sensor and radio frequency current transducer (RFCT) have been used to detect partial discharge (PD) activity within a 132 kV prefabricated cable joint containing a known defect. Although each of the three methods has been applied individually under different situations, a comprehensive investigation and comparison of these three methods has not yet been made. Results obtained were compared to measurements made using the conventional electrical detection method. The quantification of the capacitive coupler measured signal in mV with a discharge apparent quantity in pC has been investigated and an on-line quantification method proposed. The capacitive coupler has good detection sensitivity and PD location can be realised by studying the time of Right between signals from two sensors. The difference in pulse shape, spectrum and time of flight between an internal discharge and external interference has been investigated. The AE approach has the advantage of being free from electrical interference. However, investigation indicated that AE signals were significantly attenuated within the cable joint. RFCTs were used to detect the discharge current flowing through the cable sheath. Where the detection sensitivity was low, a wavelet de-noising method was applied to process the RFCT signals and proved to tie effective in increasing the measurement signal to noise ratio.

Pulse Sequence Analysis - a diagnostic tool based on the physics behind partial discharges

Abstract: The analysis of partial discharge signals is a well-known tool to monitor the degradation of electric equipment. In contrast to the common practice of the analysis of data sets (phase angles of occurrence and discharge magnitudes) accumulated during a predefined time interval, the Pulse Sequence Analysis (PSA) is based on the evaluation of data sets in which also the individual sequence or partial discharge events are registered. The classical analyses are mainly based on the frequency distributions of the pulse heights of discharge events and the phase angles of the applied voltage at which they occur. Detailed analyses of partial discharge sequences reveal that - if liquid or solid dielectrics are involved - the phase angle of the externally applied voltage is not a meaningful parameter in the partial discharge process. The ignition of the partial discharge is determined by the local electric field at the defect site and not by the absolute value or the phase angle of the externally applied voltage. Experiments with different model defects as well as measurements of commercial apparatuses show that there are characteristic sequences of partial discharge events. In many cases a systematic shift of the phase angles of occurrence occurs, which produces an apparently `statistic scatter' of the phase angles unless the correlation between consecutive discharges is taken into account. Sequence correlated parameters such as the voltage differences of the applied voltage or time differences between consecutive discharges are far more decisive parameters in the process. By means of the pulse sequence analysis, local defects can be characterized more precisely and single and multiple discharge sites can be clearly distinguished. A physically based model to describe discharge phenomena will be discussed and experimental results will be compared with results of numerical simulations of partial discharge processes.

Determination of neural-network topology for partial discharge pulse pattern recognition

Abstract: A time-series approach has been employed to devise neural-network topologies for time dependent partial discharge pulse pattern recognition applications. The cascaded output neural-network structure was found to provide the highest success rate in differentiating between two different partial discharge patterns. This was accomplished by utilizing the indexed feature of the first stage output as one of the inputs into the second stage of the cascaded neural network.

Transfer function-based partial discharge localization in power transformers: a feasibility study

Abstract: Statistical studies have shown that failures of bushings, winding insulation, and online tap changers are the main causes for long-duration outages of transformers. This article investigates the development of an instrument for supervising the conditions of transformer units. The use of sectional winding transfer functions (SWTFs) for online PD evaluation in power transformers has several advantages: localization of PD sources, discrimination between PDs inside or outside of the transformer, and evaluation of actual PD amplitudes along the winding. Direct measurement of SWTFs-when different points along the coil are accessible-and use of these SWTFs for PD evaluation and localization has shown excellent results. In order to use this method at transformers on site, an appropriate modeling method is needed for the computation of the SWTFs using only measurements at the transformer terminals. A detailed modeling approach based on discrete RLC circuit elements has been studied and different algorithms used for parameter estimation and optimization. This model is applicable in practical cases for a limited frequency range if genetic algorithms (GAs) are used for parameter optimization. A new method based on traveling wave theory has been investigated using genetic algorithms to search for the optimum parameters of a partial differential equation that describes the transient behavior of the coil. This method has shown potential in solving SWTF calculation problems for different transformers.

Applying UHF partial discharge detection to power transformers

Abstract: A new technique for analyzing partial discharge (PD) signals based on ultra-high frequency (UHF) measurements is presented, using results from site tests. Signals from simultaneously active discharge sources can be mapped according to their point of origin in a manner that will simplify the interpretation of phase-resolved discharge patterns. The scheme has the potential to allow automated location and tracking of multiple discharge sources in power transformers and other metal-clad high-voltage plants.

The relationship between partial discharge current pulse waveforms and physical mechanisms

Abstract: Partial discharge (PD) measurements have long been recognised as an important tool for detecting and predicting dielectric breakdown (BD). However, as a result of their small amplitude and wide bandwidth, the relationship between the physical mechanisms of gas breakdown and PD signal generation has not been clarified fully. We have investigated the relationship between PD current pulse waveforms and physical mechanisms to provide a basis for improved PD-based diagnostics.

Ultrashort pulsed barrier discharges and applications

Abstract: Atmospheric pressure nonequilibrium plasmas have made a recent remarkable progress in formation techniques including atmospheric pressure glow discharge (APG), dielectric barrier discharge (DBD), corona discharge, surface discharge, ultrashort pulsed discharge, etc., and are expanding their applications into the field of energy and environment as well as material conversion processes. This paper will especially focus on a large improve- ment of DBD by combining it with squared ultrashort high voltage pulses and various appli- cations.

Intelligent condition monitoring and asset management. Partial discharge monitoring for power transformers

Abstract: Condition monitoring of power transformers is a significant issue for electrical utilities. Research has demonstrated the efficacy of employing ultra high frequency measurement of partial discharge in the monitoring of transformers. A condition monitoring architecture is described that supports the capture and interpretation of diagnostic data, and provides engineers with meaningful diagnostic advice using intelligent system technologies.

Simulation of a transformer winding for partial discharge propagation studies

Abstract: A simulation model of a continuous disc type 6.6 kV transformer winding was used to study the propagation behaviour of partial discharge (PD) pulses. The model based on multi-conductor transmission line theory uses a single turn as a circuit element with the capacitance, inductance, and losses calculated as distributed parameters. Transfer functions that describe how the location of the PD source affects the current signals measured at the terminals of the winding were calculated. The paper shows how the position of the zeros in the frequency response of the measured current signals can be used to locate the source of the discharge. Sensitivity studies on the parameters of the model were used to investigate the effect of inaccuracies in the model on the position of the zeros and hence the location of the discharge.

Application of infrared spectroscopy to monitoring gas insulated high-voltage equipment: electrode material-dependent SF6 decomposition

Abstract: Sulfur hexafluoride is a chemically inert gas which is used in gas insulated substations (GIS) and other high-voltage equipment, leading to a significant enhancement of apparatus lifetime and reductions in installation size and maintenance requirements compared to conventional air insulated substations. However, component failures due to aging of the gas through electrical discharges may occur, and on-site monitoring for risk assessment is needed. Infrared spectroscopy was used for the analysis of gaseous by-products generated from electrical discharges in sulfur hexafluoride gas. An infrared monitoring system was developed using a micro-cell coupled to an FTIR spectrometer by silver halide fibers. Partial least-squares calibration was applied by using a limited number of optimally selected spectral variables. Emphasis was placed on the determination of main decomposition products, such as SOF2, SOF4, and SO2F2. Besides the different electrical conditions, the material of the plane counter electrode of the discharge chamber was also varied between silver, aluminum, copper, tungsten, or tungsten/copper alloy. For the spark experiments the point electrode was the same material as chosen for the plane electrode, whereas for partial discharges a stainless steel needle was employed. Complementary investigations on the chemical composition within the solid counter electrode material by secondary neutral mass spectrometry (SNMS) were also carried out. Under sparking conditions, the electrode material plays an important role in the decomposition rates of the gas-phase, but no relevant material dependence could be observed under partial discharge conditions.

Analysis of phase-resolved partial discharge patterns of voids based on a stochastic process approach

Abstract: A method is presented for the determination of physical discharge parameters for partial discharges (PDs) of voids in solid insulation. Based on a recently developed stochastic theory of PD processes, a statistical analysis of a measured phase-resolved partial discharge (PRPD) pattern allows the determination of the relevant physical parameters like first electron availability or decay time constants for deployed charge carriers. These parameters can be estimated directly from the measured patterns without the need of performing simulations. Furthermore, error bounds for the parameter values can be given. The parameter estimation algorithm is based on the analysis of a contiguous region of the PRPD pattern where this region can be chosen nearly arbitrarily. Thus, even patterns with several active PD defects or patterns which are corrupted by noise can be analysed. The method is applied to a sequence of patterns of a void in epoxy resin. The change in first electron availability in the course of a day can be determined quantitatively from the data while the other physical parameters remain constant.

Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films

Abstract: Recently, porous or cellular polymer films have received a lot of interest as materials for sensor and actuator applications. Films of cellular polypropylene have shown superior electromechanical thickness response when compared to conventional non-porous piezoelectric polymer materials. The electromechanical effect in cellular films originates from electrical discharges across internal gas-filled cells during preparation. The resulting charge separation is different from the charge-separation and dipole-orientation processes in non-cellular piezoelectric films. The aim of this work is to study the relationship between the partial discharges inside the cellular PP and its piezoelectric response. The focus is on the physical effects in the film at the threshold voltage that leads to a piezoelectric response. We studied the effects of charging voltage and charging time on partial discharges, surface potential and piezoelectric coefficient for a few cellular-film grades with different thicknesses. Also, the visible electroluminescence from numerous spots across the film surface during corona charging has been studied.

Laminated bus bar assembly

Abstract: A laminated bus bar assembly comprises alternating layers of a conductive layer and a foam dielectric layer. The foam dielectric layer has a dielectric constant greater than a solid non-porous dielectric constant of the same material, the use of which increases the partial discharge inception and extinction voltage levels, provides vibration resistance, decreases inductance and the like. The bus bar assembly of laminar construction is useful for power distribution systems, high power transistor modules, and the like.

Electric fields and electrical insulation

Abstract: The adoption of a field-theoretical approach to problems arising in the framework of electrical insulation is discussed with reference to six main topics, which have been addressed over the last 30 years. These include uniform field electrodes, Green's differential equation, electrode surface roughness, induced charge, electrostatic probes, and partial discharge transients, together with several follow-on aspects. Each topic is introduced and thereafter the progress achieved through the use of a field-theoretical approach is reviewed. Because the topics cover a wide spectrum of conditions, it is amply demonstrated that such an approach can lead to significant progress in many areas of electrical insulation.

The effect of voltage frequency on partial discharge activity

Abstract: The knowledge of partial discharge (PD) as a physical phenomenon has made considerable progress in the last decade(s). These efforts have led to a validated statistical-mathematical model able to simulate the PD behavior in dielectric bounded spheroidal cavities. Recent studies demonstrated that an approach based on simulations can give both qualitative information on the PD process and a good estimation of the main PD quantities (PD magnitude, PD inception/extinction voltage, PD pattern). This theoretical operating procedure was until now restricted to discharges ignited at the power stress frequency of 50 (60) Hz. In this paper the authors describe the use of a procedure to simulate partial discharges in dielectric bounded spheroidal cavities at frequencies in the range of 0.1 Hz....1000 Hz. Information on the effect of voltage frequency on the PD process has been derived from the simulation results. The results were validated by measurements at a range of test frequencies on test specimens containing spherical cavities.

Numerical simulation of partial discharge propagation in cable joints using the finite difference time domain method

Abstract: In this second of a series of three papers, the authors investigate partial discharge (PD) detection and propagation in cable joints. The complex nature of cable joints leads to errors when PD analysis is carried out using conventional equivalent circuits. The authors use the finite difference time domain method to determine the transient electromagnetic fields caused by simulated PD in model cable joints.

Comparative investigations on UHF and acoustic PD detection sensitivity in transformers

Abstract: This publication deals with UHF and acoustic partial discharge (PD) detection sensitivity assessment of paper/oil insulation configurations which are typical of power transformers. Even though the basis of this topic has been developed some time ago, the knowledge of the accuracy and limits of the sensitivity is of increasing importance for large power transformer test engineers who are confronted not only of reliable measurement of PD but also locating the source of discharges. Discharges representative of that occurring in a transformer were recorded by simulating them in a 220 kV test tank. The models simulated the basic insulation configuration of a transformer with multiple pressboard barriers and oil gaps. The influence of the effect of barriers on the sensitivity of UHF and acoustic PD detection were studied. Classical phase-resolved PD patterns were recorded alongside for analyzing the results of experiments. Experimental outcomes on these models were further verified on a 220 kV winding in a 400 kV test tank. Studies reveal that acoustic. sensitivity is seriously impaired by the presence of barriers, i.e. sensitivity of discharges shielded by insulating barriers is low as compared to those discharges in open oil volumes. For comparable distances, this sensitivity limitation is absent for UHF PD detection. Furthermore, merits and limits of individual methods are discussed that may lead to future research.

Partial discharge behavior in switching-surge-aged oil-paper capacitor bushing insulation

Abstract: Partial discharge (PD) diagnostics are utilised to examine the aging process of mineral oil-impregnated-high density kraft paper capacitor bushing models, subjected to repetitive voltage switching surges. It is demonstrated that the PD measurements of cumulative apparent charge transfer and discharge inception stress constitute an effective means for assessing aging and its reversibility due to gas evolution and re-absorption or solubility of a mineral oil-impregnated-paper insulation system in the presence and absence of switching surges. While the experimental results indicate that electrically overstressed solid-liquid insulation can recover in part its initial electrical properties following an electrical stress-free period, long term stability is only effectively achieved by a reduction in the switching surge overstress through an increase in the overall insulation thickness. The electrical stress levels, employed in the experiments, are found to be not sufficiently elevated to lead to any detectable partial discharge formation within the oil phase itself; the observed discharge behavior is rather characteristic of the type that is normally associated with macroscopic cavities occluded within the oil-paper insulation systems.

Electrical insulation characteristics of cold dielectric high temperature superconducting cable

Abstract: For the optimization of electrical insulation design for high temperature superconducting (HTS) cable, evaluation of electrical insulation characteristics especially for butt gap of LN/sub 2/ impregnated cold dielectric (CD) which consists of the wrapped tape insulation impregnated with LN/sub 2/ plays an important role. This paper presents partial discharge (PD) inception and breakdown characteristics in LN/sub 2/ impregnated butt gap model which modeled a weak point of the wrapped tape insulation impregnated with LN/sub 2/ and cable model with short length with polypropylene laminated paper (PPLP/sup /spl reg//), Nomex/sup /spl reg// paper and cellulose paper. PD current pulse was found to have a steep rise time of /spl sim/ ns and amplitude of /spl sim/ tens /spl mu/A at PD inception voltage region. Little dependency of breakdown stress on the insulating material is found. PD inception stress is almost independent of insulation thickness of 1 to 3 mm. The requirement insulation thickness for 66 kV class HTS cable is estimated to be /spl sim/ 5 mm under PD-free condition from viewpoint of long-term reliability.

Polarity effect of applied pulse voltage on the development of uniform nanosecond gas breakdown

Abstract: The spatio-temporal evolution of electric parameters of plasma and emission produced by high-voltage periodic pulsed discharge has been studied. The discharge was initiated by voltage pulses of 13.5 kV amplitude, 25 ns duration and 6 ns rise time with a repetition frequency of 40 Hz in molecular nitrogen at a pressure of 5 Torr. Differences between discharge characteristics for positive and negative polarities of the pulse has been investigated. The net electrical charge was measured by a calibrated capacitive gauge moving along the discharge tube. Time-resolved electric fields and electron densities at different distances from the high-voltage electrode were obtained. On the other hand, the electric field was derived from relative time-resolved emissions of second positive and first negative systems of molecular nitrogen. The spatial and temporal behaviour of the emission correlates with the behaviour of the electric field obtained from electrical measurements. A comparison of electric field values determined by different methods demonstrates a significant dependence of the results on the voltage polarity and leads to the conclusion that their differences, when working with positive polarity of voltage, is due to the non-local character of the discharge development.

High-Voltage on-Site Commissioning Tests for Gas-Insulated Substations Using UHF Partial Discharge Detection

Abstract: This paper describes the practical application of the ultra-high-frequency (UHF) partial discharge (PD) detection method which VA TECH Reyrolle has routinely applied to high-voltage onsite commissioning tests on gas-insulated substations (GISs) for more than 10 yr. The location of the PD sensors has a significant effect on the sensitivity of the UHF method, and research has been undertaken to quantify the parameters which affect this (i.e., attenuation, GIS geometry, etc.). Verifying the correct operation of the detection system and its sensitivity is a crucial step to prepare for a high-voltage (HV) test. In order to do this, special test equipment has been developed to ensure that all of the cabling, connections, etc., attached to the detection equipment are correct and that the detection level of the couplers is confirmed. Finally, the paper discusses IEC 60517 (general guidelines for GIS HV testing), its most usual interpretation, and how the application of the UHF technique can transform this interpretation.

Monitoring system and method for wiring systems

Abstract: A monitoring system includes at least one partial discharge (PD) sensor. The PD sensor is configured to monitor a component of an aircraft wiring system and to acquire a monitoring signal. A method embodiment for monitoring an aircraft wiring system includes acquiring a number of monitoring signals for a number of components of the aircraft wiring system using a number of partial discharge PD sensors. The method further includes conveying the monitoring signals from at least one of the PD sensors to a data acquisition system.

Fundamental Study on Locating Partial Discharge Source using VHF-UHF Radio Interferometer System

Abstract: The electromagnetic waves emitted from partial discharge (PD) due to material defects in high-voltage lectric power apparatus and facilities are broadband signals including VHF, UHF (Very High Frequency, Ultra High Frequency). A VHF-UHF radio interferometer system (VURIS) for locating PD source has been originally designed, constructed and tested. The system is based on the idea extracting the phase differences at different frequencies between a pair of the received signals by the Fourier transform and computing the direction of the electromagnetic waves emitted from PD source. It can be small-sized, more suitable for compact and portable system, because UHF half-wavelength dipole antennas are used to receive the electromagnetic waves and the baseline between the antennas is very short. And it requires fewer antennas to achieve equivalent angular resolution than a narrow band interferometer system, which requires multiple baselines corresponding to a few times of half-wavelength of the observation frequency. Therefore, it's able to reduce the operating time at the measurement site because it's not needed to arrange many antennas at the many points. The indoor test results indicate that the system is able to estimate the arrival angle of the electromagnetic waves emitted from PD source.

Gaseous electrical discharge characteristics in air and nitrogen at cryogenic temperature

Abstract: In designing superconducting electrical power apparatus, the knowledge of cryogenic gas and liquid insulation characteristics is essential. The authors have studied the discharge characteristics of relatively long-gap configurations in air and nitrogen at a cryogenic temperature. A sphere-to-sphere electrode with a gap length of 20 to 150 mm is used for measurements in uniform electric field. The breakdown voltage characteristics basically obey Paschen's law at cryogenic temperature for 50 Hz, AC, DC and lightning impulse voltage applications. A rod-to-plane electrode with a gap length of 20 to 330 mm is used for measurements in nonuniform electric field. In air at cryogenic temperature and nitrogen gas at both room and cryogenic temperature, streamer-like corona discharge appears near the tip of the rod electrode before the breakdown, and the breakdown voltage increases linearly with gap length. In air at room temperature thin film-like corona discharge, however, appears near the tip of the rod electrode before breakdown, and the breakdown voltage becomes higher than the other case. In order to examine the variation of corona discharge characteristics, some additional experiments are conducted. As a result, it becomes clear that thin film-like glow corona discharge appears when electronegative gas is contained and sufficient electrons are supplied from the cathode.

Characterization of partial discharge signals using wavelet and statistical techniques

Abstract: Study of partial discharge (PD) behavior in electrical apparatus is important to know the degradation of insulating materials. The characterization of these pulses in the form of cavity discharge, corona discharge and surface discharge is important to identify the faulty location and to quantify the degree of deterioration. A laboratory study is done by making the models of these discharges. Both time and frequency domain measurements were done. The necessary interfacing electronics to minimize the 50 Hz and harmonics from the laboratory power supply is developed. Wavelet signal processing is used to recover the PD signal by eliminating the noises of many natures. Furthermore, different wavelet filters and windowing techniques are used to improve the efficiency of PD signal extraction. With the fabricated models to create only a type of discharge, the statistical characteristic of that type of discharge is identified. A significant number of indicators are got to identify the type of discharge.

Partial discharge detection in power modules

Abstract: Today, most railway applications use hybrid power modules. These IGBT modules are constituted of several dielectric materials to achieve chip packaging. Due to the presence of high voltage, new phenomena which could be previously ignored for lower voltage ratings have now to be taken into account. This is the case of partial discharges which may appear in this dielectric material stack. Up to now, the only test carried out to detect partial discharges in such modules is the application of a normalised test (IEC270). The proposed test configuration only allows testing of a part of the dielectric insulation. For this reason, we propose and discuss a novel method for determining partial discharges existence in power modules.