Showing papers on "Partial discharge published in 2000"

Fundamentals of partial discharge in the context of field cable testing

Abstract: An article providing background on fundamentals of partial discharge, especially as related to field PD testing of cable systems.

PD source recognition by Weibull processing of pulse height distributions

Abstract: The problem of identification of partial discharge (PD) phenomena occurring in an insulation system is addressed in this paper. PD distributions coming from different sources, such as internal voids, as well as surface and corona discharges, are compared. In particular, the investigation focuses on the Weibull probability function applied to pulse charge-height distribution. It is shown that the different discharge sources can be identified on the basis of the value of the shape parameter of the Weibull distribution and that identification holds even when two PD sources are combined, i.e. are active simultaneously. In this case, the application of the 5-parameter Weibull function permits separation of PD phenomena, and recognition by means of the shape parameter value associated to each phenomenon. Finally, the proposed procedure is applied to practical objects, i.e. insulation systems of rotating machines, with real insulation defects, showing promising on-field application prospective.

Advanced partial discharge diagnostic of MV power cable system using oscillating wave test system

Abstract: To obtain a sensitive picture of discharging faults in power cables the PD should be ignited, detected, and located at power frequencies that are comparable to operating conditions at 50 or 60 Hz. In this way, realistic magnitudes in [pC] and reproducible patterns of discharges in a power cable can be obtained. PD measurements during service as well as on-site continuous energizing at 50 (60) Hz of MV cables are not always economically realistic for on-site inspections. Different energizing methods have been introduced and employed during recent years. Therefore, based on the assumption that sensitive detection of critical PD sites occurs by a method mostly similar to 50 Hz energizing conditions, a method as introduced for on-site PD diagnosis of MV cables is discussed: oscillating wave test system (OWTS).

Digital signal processing applied to the detection of partial discharge: an overview

Abstract: This article is the first of a four-part series by the authors that treats various aspects of digital signal processing applied to partial discharge detection. It is shown that manufacturers and users of high-voltage apparatus are interested in on-site, off-line and on-line analysis of partial discharge to identify and locate defects in insulation systems.

Fuzzy logic applied to PD pattern classification

Abstract: A procedure is described for the application of fuzzy logic systems to the classification of partial discharge pulse patterns in terms of cavity or void size. The features employed in the pattern recognition task are those related to the form or shape of the partial discharge pulses and their associated apparent charge transfer. Preliminary results, obtained with the fuzzy logic system on simple partial discharge sources, indicate a performance approaching that attainable with artificial neural networks.

Phase relationship of PD pulses in dielectric liquids under ac conditions

Abstract: Partial discharge pulse phase analysis carried out on a mineral oil and a perfluoropolyether with point-to-plane electrodes provided further and additional evidence that discharge epochs tend to be concentrated in the vicinity of the alternating voltage peaks at discharge inception in lieu of the voltage zeros characteristic of normal cavities occluded in solid and liquid-impregnated solid insulating systems. Relatively large positive polarity discharge pulses, having an associated charge transfer of /spl ges/15 pC with a recurrence rate of at least one per every ten consecutive cycles, appear initially on the positive half-cycle; with further increases in applied voltage both their number and magnitude continue to exceed those of negative polarity over the negative half-cycle. This behavior is to be distinguished from that commonly observed with high sensitivity measurements, which clearly indicate that early discharge onset in liquids is characterized by the occurrence of minute highly intermittent negative polarity pulses. The increase in charge transfer of the discharge pulses with applied voltage is suggestive of the development of longer more intense streamers in the dielectric liquids at the more elevated electrical fields. Also the results infer that discharges or streamers form and propagate more readily in the mineral oil than in the electronegative perfluoropolyether.

Partial discharge monitoring system for transformers

Abstract: An apparatus for detecting partial discharge in on-line high voltage electrical equipment containing a dielectric, such as high voltage transformers. The apparatus includes an ultrasonic transducer (16) and an annular capacitive plate (15) for detecting, respectively, the ultrasonic pulse and the radio frequency pulse generated by the occurrence of a partial discharge in the equipment. The outputs of the transducers are analysed by signal processing means to allow determination of occurrences of true partial discharge from the plethora of electrical noise normally present in on-line high voltage equipment.

Detection of wide-band E-M signals emitted from partial discharge occurring in GIS using wavelet transform

Abstract: Recently, diagnostic techniques have been investigated to detect a partial discharge (PD) associated with a dielectric material defect in a high-voltage electrical apparatus. Gas insulated switchgear (GIS) is an important equipment in a substation, it is highly desirable to measure a partial discharge (PD) occurring in GIS which is a symptom of an insulation breakdown. As it is important to develop a noncontact method for detecting the insulation fault, this paper proposes a new method to detect the wide-band electromagnetic (E-M) wave emitted from PD using the Wavelet transform. The Wavelet transform provides a direct quantitative measure of spectral content, "dynamic spectrum", in the time-frequency domain. This paper experimentally shows the "dynamic spectrum" of the wide-band E-M wave emitted from PD in the time-frequency domain. This method is shown to be useful for detecting the symptom of the insulation breakdown occurring in GIS.

Self-consistent modeling of electrical tree propagation and PD activity

Abstract: The self-consistent model of electrical tree propagation and partial discharge (PD) activity within growing tree channels is presented. The local electric field and the damage accumulation in the dielectric material surrounding the channels govern the tree growth. The damage increment is proportional to the energy release in the channels due to PD. The electric field distribution is determined by the charge deposition within the tree structure and the electrode geometry. The charge distribution changes within the channels during PD. PD starts when the electric field along the channels exceeds threshold inception value and stops when the field falls below the threshold quenching value. The numerical three-dimensional realization of the model has been used for simulation of electrical treeing with sinusoidal and triangular voltages in a needle-plane geometry. The spatial-temporal dynamics of the tree growth and phase-resolved characteristics of the PD have been studied for various magnitudes of the applied voltage. The simulation results have been compared with experimental data given in the literature.

Novel frequency domain based technique to detect incipient stator inter-turn faults in induction machines

Abstract: Traditionally, for medium and high voltage motors and generators, condition based monitoring (CBM) of stator faults are performed by measuring partial discharge (PD) activities. For low voltage machines, negative sequence impedance or currents are measured for the same. Such prognosis schemes should be carefully implemented as supply voltage unbalance, manufacturing related asymmetry, etc. also produce negative sequence voltages. A few motor current signature analysis (MCSA) based approaches have already been proposed to detect stator inter-turn 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 the 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 and time harmonics do not influence the measurements except as initial conditions; 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.

A modified Paschen law for the initiation of a dc glow discharge in inert gases

Abstract: Breakdown of inert gases in a homogeneous dc electric field is studied experimentally and theoretically at various distances L between the electrodes and radii R of the discharge tubes. It is shown that, for arbitrary geometric dimensions of the discharge chamber and cathode materials, the ratio of the breakdown electric field strength to the gas pressure holds constant at the breakdown curve minimum. A modified Paschen law is obtained, according to which the breakdown voltage is a function of both the product of the gas pressure by the distance L and the ratio L/R.

Partial Discharge Location in Power Transformers

Abstract: In a power transformer, high levels of partial discharge activity may lead to an insulation failure. If a partial discharge (PD) could be detected and located before the insulation fails, the transformer could be repaired or replaced before any consumers connected to the transformer lose supply. A technique is described in the paper that can be used to locate a PD in a power transformer with a continuous disc type winding. The technique requires knowledge about the physical geometry and design parameters of the transformer and also information about the characteristics of how the pulse or pulses caused by the PD propagate through the winding. The latter is difficult to obtain and consequently numerous simulation studies with slightly different assumptions were necessary before the technique could be considered validated. This involved analysing the data available at the terminals when a PD occurred in the winding and trying to extract the features related to the PD location. An algorithm using feature template matching has been used to locate the PD source in a 110 kV continuous disc type winding. Experimental tests carried out on this type of winding were extremely important in verifying the validity of the PD location algorithm. Results indicate that a location accuracy of better than 5% of the winding length is normally achieved.

Partial discharge tests using CIGRE method II

Abstract: In this paper, the results of an experimental project on insulating material aging, performed in both Denmark and Italy, are reported. This study was concerned with partial discharge (PD) behavior at temperatures between 30 and 80/spl deg/C using CIGRE method II. The material tested was a commercial polymethylmethacrylate (PMMA) which was chosen not for its good dielectric properties but rather because much of its discharge resistance data at ambient temperature is already well documented. A description is given of the theoretical and experimental methodology followed in this work. Mixed Weibull analysis techniques in terms of the PD amplitude and phase distribution characteristics were employed to distinguish the presence of different aging mechanisms. Such a difference was observed at 30 and at 80/spl deg/C. At 30/spl deg/C the analysis inferred a single discharge aging process acting until breakdown, while at 80/spl deg/C the results suggested the predominance of a single PD aging mechanism for the first half of the insulation lifetime; however, for the remainder of the insulation life time, an additional degradation mechanism was evinced.

Degradation of epoxy resin by partial discharges

Abstract: Epoxy resins are used in the production of high-voltage insulation components. Partial discharges, inherent in high-voltage installations, degenerate the resinous material and cause degradation of the insulation thus reducing the working life of the component, An understanding of the nature of the chemical changes taking place under partial discharge stressing should allow the production of improved insulation systems and/or identification of diagnostic methods for monitoring the state of existing equipment. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) microspectroscopy is used to determine the chemical changes to the surface of a bisphenol-A based epoxy resin resulting from partial discharge stressing in an air environment. Differences in stressed resin surface chemistry are found to be dependent upon the humidity of the stressed air. The variation in degradation products occurring in dry and in moist air are discussed and possible reactions to explain these differences are presented.

Electrical system with capacitance tap and sensor for on-line monitoring the state of high-voltage insulation and remote monitoring device

Abstract: A sensor and associated circuits are provided for on-line monitoring of the state of the high-voltage insulation in electrical equipment with capacitance tap. In this arrangement, both the power frequency signal and the radio frequency signals associated with partial discharge activity are sensed. These signals are transmitted from the sensor to remote monitoring instrumentation via one connecting cable. The sensor contains a surge arrester in parallel with a capacitor shunt, a radio frequency current transformer and a connecting circuit. The polarity terminal of the primary winding of the radio frequency current transformer is connected to the tap output. The non-polarity terminal is connected to the common connection point of the surge arrester and the capacitor shunt while the second terminals of these components are connected to the local ground. The non-polarity terminal of the secondary winding of the radio frequency current transformer is connected to the non-polarity terminal of its primary winding, and the polarity terminal-to the signal conductor of the connecting circuit. The second conductor of the connecting circuit is connected to the local ground. Both the power frequency signal and the radio frequency signal are transmitted on the same cable from the sensor to the remote monitoring instrumentation.

Dielectric barrier discharge lamp light source

Abstract: A dielectric barrier discharge lamp light source comprises a dielectric barrier discharge lamp (1) filled with a discharge gas that emits light by dielectric barrier discharge and provided with dielectric bodies (7, 8) interposed between at least one of negative and positive electrodes (5, 6) and the discharge gas and a feeding device (9) for feeding an AC high voltage to the electrodes, wherein the feeding device has an abnormal discharge detecting circuit (14) for detecting abnormal discharge inside a transformer (81 or 114) of the feeding device and the portions from the secondary-side output terminals of the transformer to the electrodes so as to stop the feed of the AC high voltage when abnormal discharge is detected. The abnormal discharge detecting circuit has no special abnormal discharge sensor for detecting light, sound, heat, or any chemical substance and detects abnormal discharge only from the electrical behaviour of the feeding device.

Using finite difference time domain techniques to model electrical discharge phenomena

Abstract: This paper investigates the use of finite-difference time-domain (FDTD) models in the study of electrical discharges, particularly through the measurement of radiated signals. Support for electrostatic fields within the model is demonstrated. Signals radiated by a partial discharge pulse are simulated and found to be in good agreement with experiment. Improvements required to create more sophisticated models are identified.

Charge and voltage characteristics of corona discharges in a coaxial geometry

Abstract: An engineering model is proposed to evaluate the variation of corona charge and corona current as a function of applied voltage for coaxial geometry (q-v curves). The two main assumptions of the model are the following: for streamer discharges to propagate continuously, the background field must be at, or just above, a critical level; and the streamers maintain a constant voltage gradient in the corona sheath. The model simulations are compared with the experimental q-v curves and a reasonable agreement is obtained between them.

Advancements in partial discharge analysis to diagnose stator winding problems

Abstract: On-line partial discharge (PD) testing has been used for over 50 years to assess the condition of motor and generator stator winding insulation systems. In the past decade, advanced hardware and software have enabled superior methods of converting the PD data into practical information about the relative condition of the insulation, as well as determining the root causes of any insulation deterioration. This helps machine owners plan appropriate maintenance. By performing a statistical analysis of about 19,000 test results in our database, it is clear the type of measuring system, the rated voltage of the stator, as well as if the machine is air or high-pressure gas cooled, has a significant impact on what PD readings may be considered high. Tables are presented which allow, through gross comparisons to similar machines, for a single measurement on a stator to be classed as having high or low PD, In addition, a case study is presented which shows new methods to identify phase-to-phase insulation problems.

A noise reduction technique for on-line detection and location of partial discharges in high voltage cable networks

Abstract: Partial discharge (PD) measurements taken on-site are frequently affected by noise due to external disturbances. This paper describes a noise reduction method that may be used to isolate the PDs from the noise. The basic principles of wavelet de-noising analysis are presented. The PDs are then processed to allow accurate location of the PD. This method of location will also be presented.

Electrical conduction and breakdown in insulating polymers

Abstract: This paper describes our theoretical and experimental approaches to the mechanisms responsible for electrical conduction at low and high fields, electrical aging, partial discharge and breakdown phenomena in insulating polymers. Electrical conduction always involves carrier injection from electrical contacts and subsequently dissociative trapping and recombination. When injected carriers make a transition from one energy state to a lower one, an energy equal to the energy difference between the two states will be evolved, and this energy could be of the order of 4 eV and it will be dissipated in the breaking of the polymer bonds and the creation of free radicals or low weight molecules and hence new traps. It is this gradual degradation process that leads to electrical aging. Depending upon the band structure and the potential barrier profile of the carrier injecting contracts, the normally electron-dominant conduction may change to a hole-dominant conduction at high fields, such as in polyethylene films. Electrical conduction is filamentary at high fields. Internal discharges such as electrical treeing and breakdown are initiated by impact ionization within the low-density regions created by electron (or hole) trapping or recombination, and then followed by the development of thermal instability in the high current density region of the main conduction filaments (or channels). This process then leads to a rapid increase in carrier multiplication and final destruction of the material inside the filaments (or breakdown channels). Methods for suppressing carrier injection and for reducing energy release during carrier trapping or recombination are also briefly discussed.

An algorithm for partial discharge location in distribution power transformers

Abstract: Partial discharges (PDs) are a major source of insulation failure in power transformers. Techniques for locating a PD source are of major importance in both the maintenance and repair of a transformer. A technique is described for locating a PD in a distribution power transformer. To study the propagation characteristics of a PD, a simulation model was developed for the transformer winding and the PD pulse. The terminal signals caused by PDs at different locations were analysed and the features related to the location extracted. An algorithm using feature template matching was developed for locating the PD source. Experimental tests carried out on a 110 kV continuous disc-type winding have verified the validity of the location algorithm. Results indicate that a location accuracy better than 5% of the winding length is normally achieved.

Assessment of insulation condition of large power transformers by on-site electrical diagnostic methods

Abstract: This contribution describes an off-line partial discharge (PD) detection technique for large power transformers in the substation environment. It is demonstrated that off-line PD-analysis is one of the most meaningful method to assess the insulation condition of large power transformers. Details of an advanced PD-detection technique and on-site measuring procedure are described and illustrated with practical examples. The interpretation of results from PD-tests needs complementary diagnostic methods, such as dissolved gas-in-oil-analysis or integral dielectric measurements.

Broadband measurement of the conductivity and the permittivity of semiconducting materials in high voltage XLPE cables

Abstract: Because of several technical and economical advantages XLPE (cross-linked polyethylene)-insulated power cable systems are increasingly used in the high voltage (HV) and extremely high voltage (EHV) range. High voltage XLPE-insulated cables consist of a copper or aluminium conductor (inner conductor), a semiconducting layer extruded over the inner conductor, an XLPE-insulation, an outer semiconducting layer, a metallic wire screen or aluminum sheath and an outer polyethylene sheath. The extruded semiconducting layers provide a well bonded and smooth interface between the conductors and the dielectric, avoiding electric field strength enhancements, which would lead to partial discharge and a premature breakdown. However, the XLPE-insulation is very sensitive to partial discharges (PD). To ensure the reliability of the whole cable system, sensitive laboratory PD tests before installation and on-site verification are required. One common method to detect PD is the measurement of the radiated fields of the PD with sensitive field sensors. These sensors couple PD signals through the outer semiconducting layer and therefore the properties of the semiconducting layer have a significant influence on the coupling mechanisms. Knowing the conductivity and permittivity and their dependence on frequency and temperature is important to optimise PD field sensors. Those sensors are usually placed above the outer semicon layer close to or within cable joints. This paper presents a measurement set-up to determine the frequency dependent conductivity and permittivity of small samples taken from the outer semicon layer of different medium and high voltage cables. The measurement set-up can handle extremely high, frequency dependent dielectric constants, which are typical for semiconducting materials (carbon black filled polymers). Besides, the temperature of the sample can be adjusted from 10/spl deg/C to 85/spl deg/C to study the temperature dependence of the properties of the semicon sample.

Breakdown of solid insulating materials under high frequency high voltage stress

Abstract: This paper presents the variation of breakdown voltages experimentally obtained under high frequency high voltage stress on different insulating materials viz., PTFE, polypropylene and paper. An experimental facility using a vacuum tube oscillator circuit has been developed for the generation of high frequency high voltages. The breakdown voltages at high frequencies are found to be much lower than the breakdown voltages at 50 Hz. The cause for lower breakdown voltages could be due to the increased internal heating as well as the intense partial discharge activity taking place within the material.

Acoustic emission techniques for partial discharge detection within cable insulation

Abstract: This paper summarises the latest developments of the proposed acoustic emission technique. Acoustic emission signals have been processed using the three-dimensional /spl phi/-q-n distribution and the influence of electric stress, void size, void shape and test time studied. The attenuation of the AE signal through different cable materials and interfaces has been investigated. Test results have been obtained from a 132 kV prefabricated commercial cable joint that contains a known defect.

Method of diagnosing partial discharge in gas-insulated apparatus and partial discharge diagnosing system for carrying out the same

Abstract: The present invention provides a method of diagnosing a partial discharge in a gas-insulated apparatus and a partial discharge diagnosing system for carrying out the same to achieve highly accurate diagnosis through the detection of a partial discharge signal and diagnosing the partial discharge signal through multiple conditional branching. The partial discharge diagnosing system of the present invention comprises: a partial discharge signal detector for detecting a partial discharge signal representing a partial discharge produced in the gas-insulated apparatus, a signal analyzer for receiving the partial discharge signal detected by the partial discharge detector and analyzing the partial discharge signal, a diagnostic unit for determining a cause of the partial discharge on the basis of results of analysis made by the signal analyzer, and a display unit for displaying results of diagnosis. The diagnostic unit is provided with a diagnostic means to make multiple conditional branching to a plurality of second diagnostic means according to result of diagnosis made by a first diagnostic means, and obtains a result of diagnosis made by the second diagnostic means, thereby achieving highly accurate diagnosis by detecting the partial discharge signal and subjecting the partial discharge signal to multiple conditional branching.

Effect of dielectric surfaces on the nature of partial discharges

Abstract: A mathematical model for a short gap breakdown in air was employed to examine the nature of the initiating discharge on the dielectric surface of a dielectric/metallic electrode gap of 0.500 mm separation in a uniform electric field at atmospheric pressure. The calculations demonstrate that in the absence of field enhancing surface asperities, the incipient discharge, occurring on a dielectric surface, exhibits the pronounced breakdown channel expansion characteristic of pulseless glow discharges.

Detection of partial discharges in stator windings of turbine generators

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Method and system for identifying cause of partial discharge

Abstract: A method and system for identifying the cause of a partial discharge occurring in an electric system, which method comprises the steps of measuring a variable of the electric system, such as voltage or current, to which partial discharges occurring in the electric system cause pulses, separating the pulses caused by partial discharges, ie partial discharge pulses, and occurring in the measured variable, defining and storing pulse parameters depicting the partial discharge pulses, or information from which the pulse parameters can be derived, defining (301) one or more pulse group graphs by means of the pulse parameters after at least a predefined number of partial discharge pulses, ie a pulse group, has been obtained, defining (302) one or more characteristic parameters from said at least one graph, and determining (304, 305) the cause of the partial discharge by means of the defined characteristic parameters and a reference library (600) formed in advance, which library describes one or more partial discharge causes, the reference library comprising one or more descriptions which correspond to partial discharge causes described by means of one or more membership functions of fuzzy logic, and for each characteristic parameter to be used, there is a corresponding membership function, and the step (304, 305) of determining the cause of the partial discharge comprises the steps of placing (304) the defined characteristic parameters into the membership functions corresponding to the characteristic parameters of each description in the reference library and calculating the values of the membership functions, and defining (305) the cause of the partial discharge on the basis of the values of the membership functions