Showing papers on "Partial discharge published in 2008"

Partial discharge measurement in the ultra high frequency (UHF) range

Abstract: The paper provides essentially a summary of PD measurements applying the UHF range in order to increase the detection threshold, to improve the localization accuracy and to perform on-line measurements of Partial Discharge (PD) in noisy environments. The electromagnetic UHF technique offers good signal to noise ratios, because external PD signals and disturbances can be shielded effectively. A new developed method allows the localization of PD in gas-insulated substations (GIS) by frequency domain measurements. The basic idea is the displacement law of Fourier transformation. The interference phenomena of superposed signals from two sensors give information about the time delay of the sensor signals. On-site PD measurements are made at cable connectors by means of monopole antennas housed in a barrel sleave, while the cable is in service. Thus a sensitive PD measurement even in noisy environment is possible. PD measurements on several 72 kV cable connectors were performed in an unshielded laboratory. On-site measurements during operation showed the great potential for condition assessment. For decoupling sensitive UHF PD signals from the inner of a power transformer tank UHF sensors applied through drain/oil valves are used. Experimental studies indicate that all relevant types of PD possibly occurring within a transformer emit high frequency spectra to be detected with UHF sensors. Furthermore in laboratory experiments and on-site measurements very moderate UHF signal attenuations have been observed.

Experimental study on repetitive unipolar nanosecond-pulse dielectric barrier discharge in air at atmospheric pressure

Abstract: Dielectric barrier discharge (DBD) is a typical approach for producing non-thermal plasma at atmospheric pressure. In this experimental study, the DBD is excited by repetitive unipolar nanosecond pulses with a rise time of ~15 ns and a full-width at half-maximum of ~30 ns. With a unipolar pulse voltage of 35 kV, the measured discharge current across the DBD circuit has a positive and negative pulse, and the pulse peak value can be hundreds of amperes. The low-speed camera images of the discharge show that both diffuse 'glow-like' and filamentary discharges can be observed, and the air gap length, barrier variety and applied repetition rate are the important factors influencing the transition of the two discharge modes. According to a known equivalent circuit and measured data of a typical DBD, electrical parameters including voltage, current and instantaneous power across the air gap and dielectric layer are calculated. The calculated results indicate that there are two consecutive discharges in the air gap, and the secondary discharge fires immediately after the primary discharge extinguishes. The power consumption of the secondary discharge is provided by the dielectric layer and deposited during the primary discharge. With the increase in the repetition rate, the energy deposition in the air gap per second is greatly enhanced and can exceed one joule for a repetition rate up to 100 Hz, and the corresponding charge transfer is also increased. The electron temperature and density are estimated to be approximately 5.1 eV and 1.6 × 1012 cm−3, respectively.

Time-correlated force production measurements of the dielectric barrier discharge plasma aerodynamic actuator

Abstract: An atmospheric-pressure dielectric barrier discharge (DBD) plasma, operated as a surface-mode discharge with a single encapsulated electrode and an asymmetric electrode alignment, is known to couple momentum into the surrounding neutral fluid and through this coupling has shown considerable promise as an aerodynamic flow control device. Several different models, often with conflicting explanations, have been offered to explain the process of this momentum coupling. The DBD is known to proceed in two stages during the discharge cycle, one on the positive-going portion of the applied ac high-voltage waveform and the other on the negative-going portion. By using the actuator to drive a second-order mechanical system, we show here that the great majority (97%) of the momentum coupling occurs during the negative-going portion of the discharge cycle and we relate this behavior to dramatic differences in the structure of the discharge revealed with high-speed photography. This information is critical in evaluati...

Digital detection, grouping and classification of partial discharge signals at DC voltage

Abstract: In this paper, a digital dc PD pulse detection system with bandwidth of 10 kHz - 40 MHz is introduced, which was developed using some artificial intelligence methodologies. Focus is made on digital detection, grouping and classification of random pulse signals generated by PD phenomena at dc voltage. Digital detection is developed only resorting to a band-pass filter, a high-speed digitizer (100 MS/s) and a PC with data processing software. Grouping is realized with feature extraction of pulse waveshapes using equivalent time-frequency method (ETFM), making the 2D parameters plane or 3D parameters space, then using the unsupervised clustering Fuzzy C-Means (FCM) method to achieve fast separation for pulse sequence. And classification resorts to least square support vector machine (LS-SVM) based on a fingerprint, which is derivate form 2D histograms of basic parameters, the discharge magnitude q and the time between discharges Deltat of each sub-group. Field application is made for typical defects of oil-paper insulation under dc voltage. At last, several methods to improve separability of the grouping technique are also given for some special cases, including threshold value grouping, marginal coordinates grouping based on 2D parameters plane and grouping using ETFM preprocessed by wavelet denosing. Experimental results show that the dc PD detection system developed with artificial intelligence methodologies is practical and effective.

Cross-equipment Evaluation of Partial Discharge Measurement and Diagnosis Techniques in Electric Power Apparatus for Transmission and Distribution

Abstract: Partial discharge (PD) measurement allows detection of insulation defects in various electric power apparatus before an insulation failure occurs. It can also improve the reliability of the insulation performance of apparatus, and support rational and economical design, lifetime prediction, and nondestructive inspection. Historically speaking, PD detection methods have evolved in each category of apparatus and power cables. However, there does not seem to exist what we call universality in PD detection technology that is available to all kinds of power equipment. This article specifically describes features, similarities, and differences of partial discharge measurement and diagnosis technologies for electric power apparatus used in transmission and distribution. The main task of this article lies in not simply reviewing PD measurement and diagnosis techniques but trying a cross-equipment evaluation. First, this article deals with similarities and differences among partial discharge detection techniques for typical power apparatuses including gas insulated switchgear, oil-immersed transformers, and power cables. In particular electrical measurement technology and electromagnetic measurement technology are compared selectively. The second subject is to describe signal-processing technologies, and discuss similarities and differences among equipment in terms of noise removal, location identification, and foreign particle identification. Finally, judgment methods and judgment criteria for equipment diagnoses as a whole are analyzed and described. These discussions are expected to clarify the positioning of the partial discharge measurement technologies for various equipment among all electric power apparatus, and support technological development in the future.

Sensitivity of UHF PD measurements in power transformers

Abstract: The reliability of electrical energy networks depends on the quality and availability of electrical equipment like power transformers. Local failures inside their insulation may lead to catastrophic breakdowns and might cause high outage and penalty costs. To prevent these destructive events power transformers are e.g. tested for partial discharge (PD) activity before commissioning and currently also during service. The current work deals with the electromagnetic PD detection method, also known as UHF method. The disadvantage of the UHF method is still the missing possibility for a calibration or at least a verification of the sensitivity. The so-called Sensitivity Check might show in future a relation between unconventionally measured UHF quantities to the apparent charge level in pico Coulomb. An important aspect is the attenuation of UHF signals within power transformers which is investigated in this paper.

Streamers and partial discharge mechanisms in silicone gel under impulse and AC voltages

Abstract: Silicone gels are widely used to encapsulate power electronics modules. The objective of this paper is to study the partial discharge (PD) mechanism in a silicone gel, using electrical measurements and fast visualization. Experiments are carried out in a pointplane geometry, using either impulse voltage or ac. Under impulse voltage, the visualization shows that the first discharges recorded in a new sample are due to the initiation and propagation of "streamers", with features close to those previously reported in viscous liquids. Under ac, a stable PD regime rapidly establishes: a streamer creates a cavity with a long lifetime, and PDs occur in this cavity during the subsequent ac waves. Since slow streamers develop during long times (some ms), classical PD measurements do not provide an adequate evaluation of the actual discharge magnitude: very small PDs (1 pC) are recorded, whereas large cavities with a total charge in excess of 10 pC are actually present in the gel. When PDs are repeated permanent degradations of the gel rapidly occur, showing the limited self-healing capabilities of silicone gels.

A UHF Technique for Advanced PD Measurements on Inverter-Fed Motors

Abstract: This paper deals with partial discharge (PD) measurements on inverter-fed motors. They are performed by means of a broadband sensor-filter system that allows PD signal to be recorded, damping the noise generated by the commutation interference of electronic power supply. In this way, online or offline measurements can be carried out with the purpose of evaluating the presence of PDs in adjustable speed drives, which often constitute the main cause of winding insulation degradation and premature breakdown. An offline testing technique is also proposed to offer an insight into PD activity and a more precise location of PD sources. These techniques can allow improved quality control and diagnostics on power-electronic-controlled motors.

Optimisation of a sensor for onsite detection of partial discharges in power transformers by the UHF method

Abstract: Among the different solutions which allow onsite partial discharge measurement in energized power transformers, the UHF technique is gaining general interest. In order to apply this method in existing transformers, it is considered advantageous to design a UHF sensor to be fitted inside the transformer tank. This paper describes the constraints affecting the development of such sensor and the process followed to optimize its design. During this process, different solutions for broadband UHF antennas were analysed. The selection was based on computer simulation and experimental results. Computer simulation was used to evaluate differences in radiation pattern, antenna impedance, gain and effective area. Measurements of the antenna response to different partial discharge sources in oil were performed using an oil dielectric breakdown test set in an electromagnetic shielded laboratory. A group of selected antennas were then tested in a power transformer simulating the broadband signals generated by partial discharges by injecting controlled voltage pulses in the transformer. Finally, a sensor based on a conical monopole antenna structure was built and attached to a specially designed housing to be fitted inside the transformer tank through the drain valve. This final prototype was benchmarked during a high voltage factory test of a power transformer which showed a significant partial discharge activity.

Detection of harmful metallic particles inside gas insulated switchgear using UHF sensor

Abstract: Gas insulated switchgears (GIS) are widely used as major substation equipment because of their excellent insulation and interruption performance. However, while a GIS is highly reliable, it is a concealed device, meaning diagnostic technology using various sensors is required to detect any insulation abnormality. Among various potential defects inside a GIS, the most problematic is caused by the existence of fine metallic particles, and thus the ability to detect harmful particles in tests at a factory and on site as well as in operation is becoming an important subject. This paper examines, on an experimental basis, the UHF sensor's characteristics of detecting partial discharge (PD) signals generated from particles inside the GIS with those placed freely on the bottom surface of the tank, having adhered to the insulating spacer's surface, and fixed on the center conductor, respectively. Based on previous reports, breakdown characteristics with the existence of particles were arranged in terms of the particle length, and the harmful particle length was estimated. Both results were compared with each other to evaluate the possibility of detecting harmful particles in each location with the UHF sensor. Consequently, it emerged that free particles were detected comparatively easily. However, while those having adhered to the spacer's surface and fixed on the center conductor could be detected in insulation tests at 2 pu at a factory, detection was difficult in many cases in the field.

Partial discharges and breakdown at protrusions in uniform background fields in SF6

Abstract: The breakdown mechanism of compressed SF6 in gas insulation is known to be controlled by stepped leader propagation. This process is reasonably well understood for strongly non-uniform insulation gaps ('point-to-plane') and in the absence of pre-breakdown discharge activity ('corona stabilization'). Open questions still remain for weakly non-uniform insulation gaps with small electrode protrusions (particles, surface roughness), in which pre-breakdown partial discharge (PD) activity is present. This paper presents a first attempt to derive a consistent picture under these conditions, which are characteristic for practical gas insulation systems. Experiments were carried out in a uniform field gap with a short protrusion on one electrode. This configuration was studied at various pressures from 0.1 to 0.5?MPa and both polarities using electrical and optical diagnostics. The results are interpreted using a quantitative model and order-of-magnitude estimates. The emerging picture allows prediction of most of the technically relevant aspects of the discharge processes and their main parameter dependences. It comprises statistical time lags, formative time lags including pre-breakdown PD activity and breakdown fields as a function of gas pressure, protrusion length and polarity.

Study of Partial Discharge Localization Using Ultrasonics in Power Transformer Based on Particle Swarm Optimization

Abstract: The partial discharge source localization by ultrasonic measurements in power transformers is studied by the Particle Swarm optimization (PSO), and the signal parameter estimated model of ultrasonic localization has been established on the basis of the propagation properties of the ultrasonic wave. The experiment result shows that the PSO algorithm possesses high calculation efficiency and high convergence speed, and can efficiently prevent the result from falling into the local optimum, the localization error of the PSO algorithm is clearly better than that of the Least-Square algorithm obviously.

Electrical discharge across micrometer-scale gaps for planar MEMS structures in air at atmospheric pressure

Abstract: We examine electrical discharge current responses across atmospheric pressure air gaps in the 2 µm to 7 µm range between planar polysilicon microstructures to determine the physical process of electrical discharge. The effect on discharge response from shape-related field enhancement is investigated through simulated field distribution and experimental current response for devices with design variations in electrode shape. Initial Townsend discharges, between 50 pA and 400 pA, transitioned to a second stage of 2 nA to 20 nA when the applied voltage increased 13% to 21% above the initial breakdown voltage.

Uhf partial discharge and its location measuring device for high-voltage power devices

Abstract: Disclosed is a UHF partial discharge and its location measuring device for high-voltage power devices. The measuring device includes a partial discharge sensor, an external noise sensor, an analogue-digital converter, a peak detector, a partial discharge signal processor, an arrival time detector, a discharge location processor, and a display unit.

Energy Harvesting for Online Condition Monitoring of High Voltage Overhead Power Lines

Abstract: For electric power transmission, high voltage overhead power lines play an important role as the costs for power transmission are comparatively low. However, the environmental conditions in many geographical regions can change over a wide range. Due to the high voltages, adequate distances between the conductors and objects in the environment have to be ensured for safety reasons. However, sag of the conductors (e.g. due to temperature variations or aging, icing of conductors as a result of extreme weather conditions) may increase safety margins and limit the operability of these power lines. Heavy loads due to icing or vibrations excited by winds increase the risk of line breakage. With online condition monitoring of power lines, critical states or states with increased wear for the conductor may be detected early and appropriate counter measures can be applied. In this paper we investigate possibilities for monitoring devices that are directly mounted onto a conductor. It is demonstrated that such a device can be powered from the electric field around the conductor and that electronic equipment can be protected from the strong electric and magnetic fields as well as transient signals due to partial discharge events.

Principal Component and Hierarchical Cluster Analyses as Applied to Transformer Partial Discharge Data With Particular Reference to Transformer Condition Monitoring

Abstract: This paper analyses partial discharges obtained by remote radiometric measurements from a power transformer with a known internal defect. Since fingerprints of remote radiometric measurements are not available, the formation of clusters with similar features obtained from captured partial discharge data is crucial. Hierarchical cluster analysis technique is used as a method for grouping different signals. Investigation based on Euclidean and Mahalanobis distance measures and Ward and Average linkage algorithms were performed on partial discharge data pre-processed by principal component analysis. As a result of the analysis, a clear separation of partial discharges emanating from the transformer and discharges emanating from its surrounding is achieved; this in turn should enhance the methodologies for condition monitoring of power transformers.

Guide for Electrical Partial Discharge Measurements in compliance to IEC 60270

Abstract: rent standard IEC 60270, WG D1.33 “High-Voltage Testing and Measuring Techniques” decided the edition of a Technical Brochure, which is intended as a guideline for engineers dealing with conventional electrical PD measurements. In this context it should be noted that currently the new standard IEC 62478 is under preparation, which covers non-conventional electromagnetic and acoustical PD detection methods. These topics, however, are outside of the scope of this brochure.

Study of slot partial discharges in air-cooled generators

Abstract: Two laboratory experiments were set up in order to investigate the influence of different parameters on slot partial discharge (PD) activity. The goal of the first experiment was to establish the relative importance of temperature and gap size on slot partial discharges (PDs). The second experiment was used to determine how different stresses (e.g. electrical, thermal and mechanical) influence the discharge mechanisms of stator bars affected by slot PDs. Finally, the results obtained in the laboratory were confirmed when compared with actual field measurements on a generator.

Time variation of partial discharge activity leading to breakdown of magnet wire under repetitive surge voltage application

Abstract: This paper discusses partial discharge (PD) propagation, degradation and lifetime characteristics of magnet wires for inverter-fed motor under surge voltage application. Experimental results showed the time variation of PD activity, i.e. intermittent PD, successive PD, critical PD and breakdown (BD), under repetitive surge voltage application with a fixed peak value. Critical PD was a drastic change of PD activity and was identified as the partial BD of magnet wires. Since the final BD was confirmed to be always induced at the critical PD location, the critical PD was regarded as an important indicator to determine the lifetime of magnet wires. The experimental results also clarified that the lifetime of magnet wires could be determined by the total number of PD generation from its inception to breakdown. PD activity was interpreted by the time transition of PD generation frequency, and its mechanisms were discussed.

Partial discharge pattern classification using composite versions of probabilistic neural network inference engine

Abstract: A major requirement of any power apparatus is the reliable performance of its insulation. The incidence of minor flaws and irregularities such as voids, surface imperfections, in the electrical insulation is however inevitable and lead to partial discharge (PD). Since each defect has a unique degradation mechanism, it is imperative to ascertain the correlation between the discharge patterns and the type of defect in order to evaluate the quality of the insulation. Efforts to correlate discharge patterns with the type of defects have been undertaken by several researchers. Though encouraging attempts to recognize and classify simple PD defect sources have been reported, misclassifications still occur, which affect the assessment of the index of the insulating degradation. A Composite Probabilistic Neural Network Inference System has been devised and elucidated in this research using two versions of Probabilistic Neural Network. The inference is obtained based on the outcome to innovatively conceived fourteen unique characteristic vector inputs to enable an accurate and reliable decision in the classification of complex stochastic PD patterns thus obviating the necessity of skilled operators. Validation of the fingerprints of PD patterns has also been carried out using well-established techniques.

Electrical Aging Phenomena of XLPE and EPR Cable Insulation Energized by Switching Impulses

Abstract: During the operation of power system, the insulation of high voltage extruded cables will be placed under high electrical stress, including overvoltages caused by switching surges. In the study, 15 kV XLPE and EPR power cable samples are energized by switching impulses. The aging phenomena of cable insulation are observed by the measurement of partial discharge level and AC breakdown voltage during the aging process. The study helps to determine the reliability of power cables, as well as a better understanding of the aging mechanism. Many factors other than aging by switching surges have to be considered as an aging force for the insulation degradation.

Sulfur Hexafluoride-Filled Extrinsic Fabry–PÉrot Interferometric Fiber-Optic Sensors for Partial Discharge Detection in Transformers

Abstract: This letter presents a side-hole fiber-based extrinsic Fabry-Perot interferometric (EFPI) sensor whose cavity was filled with sulfur hexafluoride (SF6) for in situ power transformer partial discharge detection. The Fabry-Perot cavity was formed by a polished thin diaphragm and a cleaved side-hole fiber that were thermally fused onto a fused silica ferrule. The cavity was first vacuumed and then inflated with 300-kPa SF6 through the fiber side holes to increase its resistance to electric stress. Experimental results show that the sensor filled with SF6 was able to withstand electric stress of at least 10 kV/mm (ac root-mean-square value) in transformer oil, three times larger than that of a traditional air-filled EFPI sensor.

UHF technique for identification of partial discharge in a composite insulation under AC and DC voltages

Abstract: DC power transmission is gaining considerable importance and converter transformers form an important component in it. An insulation of these transformers is stressed by both ac and dc voltages. Particle contamination is one of the major problems in insulation structures as it generates partial discharges (PD). If a particle is moving between the ground electrode and barrier, the magnitude of partial discharges formed is less compared to that of its movement between two metal electrodes. On the other hand, when the particle is lying over the barrier insulation material, it does not lead to any partial discharge activities as observed under ac/dc voltages. Broadband measurement of UHF signals produced by partial discharges due to particle movement in transformer oil under dc voltages, at high applied electric fields, indicates that the frequency content of the signal lies in the entire UHF range of 300 MHz - 3 GHz. Narrow band measurements of UHF signal due to PD formed by particle movement, at 1 GHz, indicates that, in the presence of barrier, an increase in applied voltage results in increase in number of partial discharge pulses. This phenomenon occurs in converse in oil gap without any barrier. In the presence of barrier, the sequential movement of particle occurs and the magnitude of discharges increases with increase in applied voltage, irrespective of polarity of the dc voltage. The number of discharges that occur under ac voltage is high compared to the dc voltages.

Design-dependent slot discharge and vibration sparking on high voltage windings

Abstract: Partial discharge effects are widely acknowledged as a damage mechanism for deteriorating high voltage windings of electrical machines. Vibration sparking is another but scarcely known. Not only is vibration sparking significantly more damaging to electrical machine windings, unlike partial discharge deterioration it can occur at any position within the winding. Deteriorated winding insulation at the low voltage ends can only be detected via offline monitoring as online monitoring is not sufficient. Prevention of vibration sparking requires a reliably earthed outer corona layer of the winding bars or sufficiently large resistances of the outer corona layers. However the slot design needs to take account of both vibration sparking and partial discharge in order to mitigate these deterioration mechanisms.

Characteristics of an atmospheric pressure argon glow discharge in a coaxial electrode geometry

Abstract: An atmospheric glow discharge controlled by a dielectric barrier is realized in a coaxial electrode geometry in argon. The discharge characteristics are studied by the electrical method and optical emission spectroscopy. The experimental results indicate that there is only one discharge pulse per half cycle when the applied voltage is very low, and the rise time of the discharge pulses at the positive half cycle is much shorter than that at the negative one. With an increase in applied voltage, the width of the discharge pulse increases, while the inception voltage at which breakdown occurs decreases. The rise time at positive half cycle almost equals that at negative half cycle when the applied voltage is high enough. The research results pertaining to gas gap voltage indicate that the critical electric field for breakdown decreases with increasing applied voltage. The electron temperature is estimated from the Einstein relation, and the result indicates that the electron temperature and the electron density are functions of the applied voltage and the gas flow rate. The electron temperature is also studied by emission spectroscopy and a similar result is obtained.

Partial discharge detection for condition monitoring of covered-conductor overhead distribution networks using Rogowski coil

Abstract: OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Ghulam Murtaza Hashmi Name of the dissertation Partial Discharge Detection for Condition Monitoring of Covered-Conductor Overhead Distribution Networks using Rogowski Coil Manuscript submitted February 2008 Manuscript revised June 2008 Date of the defense August 22, 2008 Monograph Article dissertation (summary + original articles) Faculty Faculty of Electronics, Communications and Automation Department Department of Electrical Engineering Field of research Power Systems Opponent(s) Prof. Akihiro Ametani and Prof. Dr. Ömer Usta Supervisor Prof. Matti Lehtonen Instructor Prof. Matti Lehtonen Abstract Partial discharge (PD) is a small electrical avalanche caused by locally disrupted electric fields in dielectric materials, and is known to be one of the major factors which accelerate the degradation of electrical insulation. This thesis deals with a relatively new and challenging application of conducting on-line high frequency PD measurements for the monitoring of falling trees on covered-conductor (CC) overhead distribution lines. A measuring test set-up was arranged in the high voltage laboratory for real-time analysis. A pine tree was leaned against a 20 kV energized conductor and PDs were measured at different locations on the CC line using a Rogowski coil. The time domain reflectometry (TDR) measurement technique is presented to extract the frequency-dependent wave propagation characteristics (attenuation, phase constant, and propagation velocity) of CC overhead distribution lines. The theoretical modeling of the CC line based on its geometry is presented using two-wire transmission line theory and its frequency-dependent line characteristics are derived. The theoretical model is verified experimentally using TDR measurements taken on a certain length of the line. The entire single-phase on-line PD monitoring system including CC line and Rogowski coil is simulated in the electromagnetic transient program-alternative transient program (EMTP-ATP) simulation environment for detecting falling trees on CC overhead distribution lines. The model is confirmed by the measurement results taken in the laboratory. The model can be used to estimate the length of the CC line at which the PDs due to falling trees can be detected, thus deciding the number and positioning of the sensors over a particular length of the CC line. Moreover, the challenges in on-line condition monitoring of falling trees on CC lines using wireless sensors are also discussed. The wavelet transform technique is applied as a powerful tool to de-noise on-line PD signals, which are completely buried by electromagnetic interference. Automatic detection of falling trees will reduce visual inspection work after storms and it will improve the reliability and safety of the distribution system. The system can be planned to be integrated into the distribution automation system to reduce the overall costs of CC lines.Partial discharge (PD) is a small electrical avalanche caused by locally disrupted electric fields in dielectric materials, and is known to be one of the major factors which accelerate the degradation of electrical insulation. This thesis deals with a relatively new and challenging application of conducting on-line high frequency PD measurements for the monitoring of falling trees on covered-conductor (CC) overhead distribution lines. A measuring test set-up was arranged in the high voltage laboratory for real-time analysis. A pine tree was leaned against a 20 kV energized conductor and PDs were measured at different locations on the CC line using a Rogowski coil. The time domain reflectometry (TDR) measurement technique is presented to extract the frequency-dependent wave propagation characteristics (attenuation, phase constant, and propagation velocity) of CC overhead distribution lines. The theoretical modeling of the CC line based on its geometry is presented using two-wire transmission line theory and its frequency-dependent line characteristics are derived. The theoretical model is verified experimentally using TDR measurements taken on a certain length of the line. The entire single-phase on-line PD monitoring system including CC line and Rogowski coil is simulated in the electromagnetic transient program-alternative transient program (EMTP-ATP) simulation environment for detecting falling trees on CC overhead distribution lines. The model is confirmed by the measurement results taken in the laboratory. The model can be used to estimate the length of the CC line at which the PDs due to falling trees can be detected, thus deciding the number and positioning of the sensors over a particular length of the CC line. Moreover, the challenges in on-line condition monitoring of falling trees on CC lines using wireless sensors are also discussed. The wavelet transform technique is applied as a powerful tool to de-noise on-line PD signals, which are completely buried by electromagnetic interference. Automatic detection of falling trees will reduce visual inspection work after storms and it will improve the reliability and safety of the distribution system. The system can be planned to be integrated into the distribution automation system to reduce the overall costs of CC lines.

Conical DC Discharge in Ambient Air Using Water as an Electrode

Abstract: Conical dc discharge is generated between a copper cathode with a hemispherical head (2-7 mm in diameter) and a static water anode in ambient air. The discharge is hollow and is just like an umbrella if the electrical conductivity of the water electrode is higher than 17.31 musldrcm-1. The profile is composed of three zones with different luminosities. The effects of the discharge current, air gap length, cathode dimension, and electrical conductivity of the water electrode on the discharge are studied. Volt-ampere characteristics and images of the discharge under different experimental conditions are shown to characterize the discharge. A mechanism for stabilizing the discharge is discussed.