Showing papers on "Partial discharge published in 2005"

Partial discharge monitoring of power transformers using UHF sensors. Part I: sensors and signal interpretation

Abstract: Determining whether power transformers are suffering from internal arcing or dangerous levels of partial discharge (PD) is important because failure without warning can result in damage to neighboring equipment, customer dissatisfaction, disruption to economic activity, and the imposition of regulatory fines. This paper provides an overview of the excitation of UHF signals by PD inside transformers. The use of externally mounted sensors has been outlined, and the structure of a dielectric window that can be constructed on an inspection hatch has been outlined. Attenuation of UHF signals propagating inside a transformer tank has been shown to be relatively low. A new approach to locating PD sources in three dimensions has been presented, based on using a numerical model of the transformer materials, which defines electromagnetic propagation velocities on a mesh of 5-cm sub-cells. The concepts of the propagation-velocity matrix (PVM) and the propagation-time matrix (PTM) were thereby introduced. Once the PD source has been located to a specific region of the transformer, the PVM might be used to index a database of physical descriptions of the transformer that could provide additional information to assist with diagnosis.

Electrohydrodynamic force and aerodynamic flow acceleration in surface dielectric barrier discharge

Abstract: Surface discharges created in dielectric barrier discharge (DBD) configurations have been proposed as actuators for flow control in aerodynamic applications. We focus on DBDs operating in a glow regime, i.e., where the discharge is sustained by ion-induced secondary electron emission from the surface and volume ionization. After a brief discussion of the force per unit volume acting on the flow and due to the momentum transfer from charged particles to neutral molecules, we present calculations of this force based on a two-dimensional fluid model of the surface discharge. We show that this force is of the same nature as the electric wind in a corona discharge. However, the force in a DBD is localized in the cathode sheath region of the discharge expanding along the dielectric surface. While its intensity is much larger than the analogous force in a direct-current corona discharge, it is active during less than one hundred nanoseconds for each discharge pulse and the time-averaged forces in the two cases a...

Partial discharge diagnostics and electrical equipment insulation condition assessment

Abstract: Partial discharge (PD) measurement has long been used as a test to evaluate different insulation system designs, and as a quality control test for new equipment. However, in the past 20 years, PD measurement has been widely applied to diagnose the condition of the electrical insulation in operating apparatus such as switchgear, transformers, cables, as well as motor and generator stator windings. Improvements in the capabilities as well the lower cost of sensors, electronics and memory is partly the reason for the increased popularity of PD diagnostics. Another reason has been the development of methods-including the use of ultrawide band detection-to improve the reliability of the PD measurement in the presence of noise. In addition, rapid progress is being made in automated pattern recognition techniques that also helps to suppress noise. This paper reviews the various PD measurement technologies that have been specifically developed to improve PD diagnostic methods, and outlines how they have been implemented for stators, cables, transformers and switchgear. Areas for further research are also presented.

Asymmetric surface dielectric barrier discharge in air at atmospheric pressure: electrical properties and induced airflow characteristics

Abstract: The electrical properties of an asymmetric surface dielectric barrier discharge in atmospheric air have been investigated experimentally. The discharge is used for airflow production close to the dielectric surface, and the time-averaged flow velocity spatial profiles have been measured. Velocities of up to 3.5 m s−1 at heights of 1–2 mm are reached when filamentary discharges with current peaks up to 20 mA are produced along the surface. In terms of powers, mechanical powers (output) of a few milliwatts are obtained for electrical powers (input) up to 10 W. Variation laws or behaviour with several discharge parameters (applied voltage waveform, distance between electrodes, dielectric thickness and permittivity) have been experimentally determined.

Trends in partial discharge pattern classification: a survey

Abstract: Partial discharge (PD) detection, measurement and classification constitute an important tool for quality assessment of insulation systems utilized in HV power apparatus and cables. The patterns obtained with PD detectors contain characteristic features of the source/class of the respective partial discharge process involved. The recognition of the source from the data represents the classification stage. Usually, this stage consists of a two-step procedure, i.e., extraction of feature vector from the data followed by classification/recognition of the corresponding source. The various techniques available for achieving the foregoing task are examined and analyzed; while limited success has been achieved in the identification of simple PD sources, recognition and classification of complex PD patterns associated with practical insulating systems continue to pose appreciable difficulty.

Degradation of solid dielectrics due to internal partial discharge: some thoughts on progress made and where to go now

Abstract: The amount of literature on partial discharge (PD) and partial discharge induced degradation is vast. In the past 10-20 years significant progress has been made on research within partial discharge induced aging of dielectrics. Researchers now agree on the main mechanisms pertaining to this topic. With the advent of a new generation of dielectrics of which many properties now can be affected by the introduction of small amounts of nano-sized particles it seems to be a good moment to review the progress on the understanding of PD induced aging. Focusing on internal partial discharge in solid polymeric insulation this paper tries to identify achievements and at the same time challenges still to be solved.

Partial discharge signal interpretation for generator diagnostics

Abstract: Several common sources of discharges activity occurring on generators have been replicated in the laboratory under well-controlled conditions Each source was evaluated individually and recorded with a phase resolved acquisition system and with a spectrum analyzer The dominant features of each respective phase resolved partial discharge (PRPD) pattern are presented The frequency content of the discharge signal at the detection coupler was also investigated The association of each well-defined type of discharge source, with its specific PRPD pattern, constitutes the basis of our database used for the discharge source recognition during generator diagnostics The comparison of laboratory results with actual field measurements gathered over the last decade is summarized

Radiometric location of partial discharge sources on energized high-Voltage plant

Abstract: Partial discharges (PD) generate wideband radio frequency interference and, consequently, can be detected using radio receiving equipment. Due to the advances in ultra-high-speed sampling equipment, it is possible to accurately measure the propagation of the PD wavefront as it passes through a 4 element antenna array. From these measurements, the three-dimensional position of the PD source can be calculated using an iterative algorithm. The locating equipment is suitable for use within the vicinity of energized high-voltage plant and can locate sources up to 15 m from the array. Results are presented showing the location ability of the equipment under laboratory and field conditions. A significant advantage is the ability to detect PD sources in energized plant without the need for outages or electrical connections.

Partial discharge monitoring for power transformer using UHF sensors. Part 2: field experience

Abstract: Because of the large capital value of power transformers and their critical role in the electricity network, there is an increasing need for non-intrusive diagnostic and monitoring tools to assess their internal condition. The UHF technique is being applied increasingly to monitor partial discharges (PD) in power transformers and has been shown to be much more sensitive than acoustic techniques when the signal path passes through solid insulation. Field trials were performed on five power transformers ranging in size from 18 to 1000 MVA and in voltage from 25 to 400 kV. The results of PD tests on power transformers provide sufficient evidence to justify making provision for UHF sensors on new transformers to facilitate their monitoring when required during the service lifetime.

Electrical model and analysis of the transition from an atmospheric pressure Townsend discharge to a filamentary discharge

Abstract: This work is a contribution to the understanding of the mechanisms controlling the transition from a Townsend to a filamentary dielectric barrier discharge when the power increases. The approach consists in developing an electrical model of the discharge and the power supply to study the interaction between these two elements. The main components of the discharge model are (i) two Zener diodes whose characteristics depend on the power to take into account the effect of the gas density variation induced by the temperature variation and (ii) a RC circuit describing the memory effect from one discharge to the following one which is due to the metastables and the electrons trapped on the surface of the solid dielectrics. The calculated and measured currents are very similar over all the range of amplitude and frequency allowing to get an atmospheric pressure Townsend discharge. The model also describes the transition to filamentary discharge observed when the excitation frequency increases too much showing that it is due to a very fast variation of the load connected to the power supply. From this understanding, a solution is deduced to increase the maximum power dissipated in the discharge which consists in decreasing the solid dielectric capacitance.

Signal denoising techniques for partial discharge measurements

Abstract: One of the major challenges of on-site partial discharge (PD) measurements is the recovery of PD signals from a noisy environment. The different sources of noise include thermal or resistor noise added by the measuring circuit, and high-frequency sinusoidal signals that electromagnetically couple from radio broadcasts and/or carrier wave communications. Sophisticated methods are required to detect PD signals correctly. Fortunately, advances in analog-to-digital conversion (ADC) technology, and recent developments in digital signal processing (DSP) enable easy extraction of PD signals. This paper deals with the denoising of PD signals caused by corona discharges. Several techniques are investigated and employed on simulated as well as real PD data.

The transition from a filamentary dielectric barrier discharge to a diffuse barrier discharge in air at atmospheric pressure

Abstract: A transparent plasma discharge reactor using air was used to investigate the transition from a filamentary dielectric barrier discharge (FDBD) operation regime into the diffuse barrier discharge regime. Recent results of other researchers indicate that the stability of diffuse barrier discharges in nitrogen may be attributed to the lack of a fully formed cathode fall layer when the discharge operates in a regime between the Townsend discharge and a normal glow discharge. We have demonstrated that a diffuse barrier discharge in air exhibits an increased accumulation of electric charge on the electrode's dielectric plates as compared with the FDBD. This may provide a means of stabilizing the discharge in a Townsend-to-glow discharge transition. Unlike operation in nitrogen, a streamer mechanism is involved in the formation of a uniform air plasma, though in a different manner than is associated with the FDBD. Numerous diffuse streamer clusters were observed on pre-charged dielectric plates at the breakdown voltage. Our conclusion is that the macroscopically uniform atmospheric-pressure DBD in air is obtained by the numerous radially expanding streamers that are temporally overlapping.

A new methodology for the identification of PD in electrical apparatus: properties and applications

Abstract: Applications of a new methodology, aimed at the identification of defects occurring in insulation systems of HV apparatus and based on partial discharge (PD) measurements, are presented in this paper. This methodology relies upon the digital acquisition of a large amount of PD pulses and separates the acquired pulses into homogeneous subclasses. Signal processing tools recognize the presence of noise among the different classes. Identification of basic PD source typologies (i.e., internal, corona and surface discharges) is then achieved, resorting to fuzzy algorithms. The proposed procedure is applied to measurements performed on different HV apparatus, such as cables, transformers and rotating machines. The purpose of this paper is to show that the identification process is robust, regarding the measuring circuit, and flexible, so that it can constitute an advanced tool for condition based maintenance, guiding maintenance experts in making decisions on the condition of the insulation system under test.

Detection of partial discharge in sf6 gas using a carbon nanotube-based gas sensor

Abstract: For electrical insulation diagnosis of gas-insulated switchgear (GIS), detection of partial discharge (PD) generated in sulfur hexafluoride (SF 6 ) gas is important. This paper describes a new detection method of PD generated in SF 6 gas using a gas sensor composed of multiwall carbon nanotubes (MWCNTs). The gas sensor was fabricated by electrokinetic manipulation of semiconducting MWCNTs using positive dielectrophoresis. The MWCNT gas sensor and a point-to-plane electrode system were placed in a steel tank filled with SF 6 gas at atmospheric pressure. AC high voltage was applied to the point electrode in order to generate PD while the electrical impedance of the MWCNT sensor was monitored. When the PD was generated, the electrical conductance of the MWCNT sensor gradually increased. The sensor response was reversible and was influenced by the PD intensity as well as by the relative position of the sensor to the point electrode. It was suggested that the sensor conductance increase was caused by an electronic interaction between MWCNTs and non-identified oxidative decomposition products. The faint PD, which could not be sensed by gas detecting tubes, was successfully detected by the MWCNT sensor on a real time basis.

A novel technique for partial discharge and breakdown investigation based on current pulse waveform analysis

Abstract: A novel technique for partial discharge (PD) measurement and analysis (PD-CPWA; PD current pulse waveform analysis) is developed and introduced in this paper. PD-CPWA is expected to be utilized to discuss PD mechanisms and physics in electrical insulating materials, focusing on the PD current pulse waveform and its time transition from PD inception to breakdown (BD). In this paper, the concept and principle of PD-CPWA are described, and the applications of PD-CPWA to (1) epoxy spacer samples under thermal and electric combined stresses in GIS, (2) creepage PD on epoxy spacers in SF/sub 6/ gas and (3) liquid nitrogen/polypropylene laminated paper composite insulation system for high temperature superconducting cables are introduced and discussed.

On-line detection and location of partial discharges in medium-voltage power cables

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Partial discharge pulse sequence patterns and cavity development times in transformer oils under AC conditions

Abstract: Discrete partial discharge (PD) pulse occurrence times within the PD pulse bursts in transformer oils were found to extend from approximately 0.42 to 2.9 /spl mu/s, with the discrete pulse separation times evincing a weak dependence on the inverse of the oil viscosity. Many PD pulse burst patterns exhibited substantial deviation from the classical behavior, which is normally characterized by successive discrete pulses of ascending amplitude with well defined increases in the pulse separation times. The average elapsed time from the incipient formation for the cavity to the onset of the first partial discharge event extended downwards from 0.62 to 0.42 /spl mu/s with increasing oil viscosity.

An ultra-sensitive optical MEMS sensor for partial discharge detection

Abstract: A diaphragm-based interferometric fiber optical microelectromechanical system sensor with high sensitivity is designed and tested for on-line detection of the acoustic waves generated by partial discharges (PD) inside high-voltage power transformers. In principle, the sensor is made according to Fabry Perot interference, which is placed on a micro-machined rectangular silicon membrane as a pressure-sensitive element. A fiber-optic readout scheme has been used to monitor sensor membrane deflection. Sensor design, fabrication, characterization, and application in PD acoustic detection are described. Test results indicate that the fiber optical sensor is capable of detecting PD acoustic signals propagating inside transformer oil with high sensitivity.

Analysis of partial discharge phenomena in paper-oil insulation systems as a basis for risk assessment evaluation

Abstract: Partial discharge (pd)-based risk assessment in electrical insulation systems becomes effective only if the nature of the defect generating pd can be ascertained In oil/paper insulation systems this is a fundamental item since eg harmfulness of pd generated by gas bubbles is much smaller than that of pd occurring between paper layers For such reasons, partial discharge analysis of oil/paper insulated systems often lacks a clear correlation between cause (ie, defect type) and effect (partial discharge recording) Results relevant to partial discharge measurements performed on lab-scale objects having different electrode configuration are presented in the paper with the purpose to spotlight characteristic features aimed at identifying the basic nature of partial discharge-generating defects in oil/paper insulated systems

Separation of corona using wavelet packet transform and neural network for detection of partial discharge in gas-insulated substations

Abstract: It is essential to detect partial discharge (PD) as a symptom of insulation breakdown in gas-insulated substations (GIS). However, the accuracy of such measurement is often degraded due to the existence of noise in the signal. In this paper, a method using wavelet packet transform and neural network is proposed to separate the PD pulses from corona in air, which enables more accurate detection of insulation breakdown of GIS.

Partial discharge and dissipation factor behavior of model insulating systems for high voltage rotating machines under different stresses

Abstract: This article describes the change in partial discharge (PD) pattern of high voltage rotating machines and the change in the tan /spl delta/ as a function of the applied test voltage during the aging processes as caused by the application of different stresses on stator bars. It also compares the PD patterns associated with internal, slot, and end-winding discharges, which were produced in well-controlled laboratory conditions. In addition, the influence of different temperature conditions on the partial discharge activities are shown. The investigations in this work were performed on model stator bars under laboratory conditions, and the results might be different from those obtained for complete machines, as rotating machines are complex PD test objects, and for example, the detected PD signals in a complete machine significantly depend on the transmission path from the PD source to the measurement device.

Analysis of insulation failure modes in high power IGBT modules

Abstract: Increasing operating voltages of insulated gate bipolar transistor (IGBT) modules results in higher demands on electrical insulation capabilities as well as partial discharge resistance. This paper discusses the insulation failure modes observed in high voltage power modules; most critical are water trees, partial discharge (PD) and electrical trees. Besides the review of insulation failure mechanisms, an experimental analysis method is discussed in order to identify possible failure source. Presented is a PD setup that includes a light sensitive CCD camera for optical inspection. Within the optical measurements, electroluminescence maps are also recorded in order to identify critical regions of high electrical fields. This method permits an analysis even before PD or electrical treeing begins. Optical PD inspections allow identifying PD failure root causes like protrusions or irregular edge shapes. In addition, investigations of PD failures on the ceramic substrate level have been performed and the distribution of inception voltages for one substrate type analyzed.

Quantification of corona discharges on nonceramic insulators

Abstract: This paper attempts to establish a correlation of the visual images of corona obtained from a camera with discharge magnitude measured with conventional partial discharge equipment. A linear relationship is shown between the transformed image parameters and the discharge magnitude, thereby providing a means for quantifying corona observations made during routine maintenance inspections, of insulators from ground. Different insulator designs using silicone rubber (SIR) and ethylene propylene diene monomer (EPDM) housings were examined. The effect of fog has been examined by performing experiments inside a fog chamber. It is shown that this information can be used along with the corona degradation characteristics of housing materials to identify discharge patterns that can pose problems to the integrity of the insulator.

Procedure for identification and localization of dangerous PD sources in power transformers

Abstract: In case of a detected partial discharge (PD) in new power transformer during the delivery test in the high voltage laboratory, the most important question arises from both the customer as well as from the manufacturer: where is the PD source? In such a situation, additionally to the measurement of apparent charge with a conventional PD-system, detection of acoustical PD signals can be used to localize the PD source. In many cases, especially for hidden PD sources in the solid insulation, or in the main insulation, there are no acoustical PD-signals detectable. To open a transformer for visual inspection without a clear indication about the position of the PD source is not only time consuming but also in many cases without any findings. In this contribution a successful procedure for reliable identification and localization of dangerous PD-sources is described.

Water Treatment Using Discharge on the Surface of a Bubble in Water

Abstract: A new method for water treatment utilizing radicals produced by a discharge on the surface of a bubble in water is proposed and this method is applied to the de-color of indigo solution whose original color is blue. The discharge of this method begins at the crossing points of mesh electrode, acrylic resin spacer, and bubbling gas (i.e., called a triple junction where there is a weak point for the electrical breakdown in the electrical power devices). This weak point for the electrical breakdown is used very actively for the surface discharge. The discharge process on the surface of the bubble in water is also observed using a wire-to-plane electrode system to fix the observation region of discharge (type-A reactor). The streamer length estimated by photographs increased linearly with the applied voltage. The energy consumption of discharge for type-B reactor with a mesh-to-plane electrode in oxygen is about 0.09 J when the applied voltage is 20 kV. Production efficiency of the H 2 O 2 concentration of about 27 mg . L - 1 in oxygen (46 mg . L - 1 in helium) has been obtained when the repetition rate of discharge operated by a pulsed-power generator, the peak value of applied voltage, and the discharge period are 100 pps (pulsed power generator), 25 kV and 60 min, respectively.

Detection of electrical tree propagation by partial discharge measurements

Abstract: The partial discharge activity during propagation of electrical treeing in epoxy resin is described. The electrical trees grew in needle-plane samples without and with an internal barrier up to the final breakdown. The simultaneous taken tree growth and discharge activity show a correlation between the propagation state of the tree and the thereby measured PD. Especially the changes in the tree structure can clearly be detected. Based on these characteristics a new model describing tree growth will be presented. It appears that measurements from machine insulation can be interpreted in terms of the PD pattern.

On-line signal analysis of partial discharges in medium-voltage power cables

Abstract: Partial discharges are symptomatic of many degradation phenomena in power cables and may cause further deterioration of the insulation in many cases. Electrical im- pulses, generated by partial discharges, travel towards the cable ends, and can there be detected using appropriate sensors. Continuous monitoring of the insulation con- dition can be achieved by on-line detection and location of partial discharge (PD) signals. An important aspect of such a diagnostic is the analysis of on-line measure- ments. The research reported in this thesis is aimed at analysis of PD signals from on-line measurements and location of discharge sites. Signal analysis depends on knowledge of both signals and disturbances that are to be expected. To that end, characteristics of PD signals in medium voltage cables are studied in this thesis. The result of this study is a signal model of the propagation path between the discharge site and the sensors. The model accounts for cable sections with di®erent properties, and incorporates the propagation channel load impedances, i.e. the equipment to which a cable is terminated in an on-line situation. The exact propagation properties and load impedances depend on the speci¯c cable connection under test, and are unknown a priori. For this reason, research is conducted on meth- ods that enable experimental characterization of the parameters, by evaluating the response of the cable to applied transients. The presented methods rely on the ex- traction of pulses that are re°ected on impedance transitions within the cable system under test. On-line ¯eld measurements are corrupted by noise and interference, which impede PD signal detection and location. Generally, narrowband interferences resulting from radio broadcasts dominate the measurements, thus prohibiting data-acquisition trig- gered by PD signals. Broadband background noise is present within the entire PD signal bandwidth, and therefore poses a fundamental limit on PD signal analysis. Generally, existing extraction techniques for PD signals only partially exploit a priori knowledge of both signals and interference. In this thesis, matched ¯lters are ap- plied that are derived from the signal model, and are optimally adapted to the signals that can be expected. Besides signal extraction, matched ¯lters provide a means to estimate the PD magnitude and the signal arrival time. Likewise, discharge location methods based on the signal model are proposed, resulting in optimal location esti- mators. Computer simulations illustrate the e®ectiveness of the proposed algorithms and show that the attainable accuracy can be speci¯ed by theoretical bounds. Accurate PD location relies on estimation of the di®erence in arrival times of signals originating from the same discharge. In case of on-line detection, the cable is connected to the grid, and signals are not necessarily re°ected at the cable ends. Therefore signal detection at both sides is generally required for the purpose of dis- charge location. Synchronization of the measurement equipment is achieved using pulses that are injected into the cable connection. Finite-energy disturbances, such as PD signals that originate outside the cable connection under test, frequently occur in on-line situations. Since measurements are synchronously conducted at both cable ends, pulses originating within and outside the cable can be distinguished by examining the di®erence in time of arrival. Moreover, in many situations, the signal direction of arrival can be determined by detecting pulses in two di®erent current paths at a cable termination. This method is applied as an additional technique to discriminate PD signals and disturbances. Based on the results of research, a measurement system is proposed, which enables automated on-line PD detection and location in medium voltage cable connections. The conceptual design is validated by experiments, and the results demonstrate that the practical application is promising.