Showing papers on "Partial discharge published in 2006"

Effect of supply voltage frequency on testing of insulation system

Abstract: This paper deals with the effect of supply frequency on partial discharge phenomena. Results of measurements performed on artificial and practical test objects are compared with computer simulations based on physical models. This comparative analysis points out and explains the differences in partial discharge phenomena observed in tests performed at different frequencies (from 0.1 to 300 Hz). The consequences associated to the use of different supply frequencies in practical applications are, eventually, outlined.

UHF and current pulse measurements of partial discharge activity in mineral oil

Abstract: Investigations are reported into a range of defects causing partial discharge (PD) activity in high-voltage oil-insulated equipment. The aim is to explore the relationship between radiated ultra-high frequency (UHF) signals and electric charge in a PD current pulse for PD sources in oil insulation. Experiments are performed under high-voltage AC conditions using electrode configurations that included a sharp protrusion, a bad contact, and a free metallic particle. A broadband current transformer is used to measure the PD current pulses with the UHF signals being detected using a pair of sensors mounted inside a metal tank surrounding the PD sources. Current pulses and UHF signals were recorded simultaneously using an oscilloscope with a bandwidth of 3 GHz. The level of correlation between the energy of the radiated UHF signals and the charge content of PD pulses is shown to vary depending on the defect type. Polarity-dependent UHF spectral content and ratio of UHF signal energies measured at the two sensors are shown to exhibit characteristics dependent on the discharge behaviour. These results and the analysis procedures introduced will contribute to the development of effective UHF monitoring systems for oil-insulated high-voltage plant.

Analysis and electrical modelling of a cylindrical DBD configuration at different operating frequencies

Abstract: A dielectric barrier discharge generated by flowing inert gas (helium) ionized by a high-voltage source through a cylindrical reactor working at atmospheric pressure has been studied and an electrical model characterizing this discharge is proposed. A sinusoidal voltage of up to 2 kV peak to peak with frequencies from 10 to 125 kHz has been applied to the discharge electrodes. The proposed model considers the geometry of the reactor and dielectric materials. From experimental and analytical results, a semi-empirical relation of the breakdown voltage is presented as a function of the operating frequency. The microdischarge regime is characterized by a dynamic equivalent capacitance.

Partial discharge location in power transformers using wideband RF detection

Abstract: This paper presents a method for partial discharge (PD) source location in power transformers based on detecting electromagnetic (EM) radiation emitted from PD. The key problem of PD location by EM waves is the difficulty in getting the accurate time arrivals of the signals from the source to multiple sensors. Several measures are taken to improve the time arrival resolution. Firstly, a measuring frequency band (1 GHz-5 GHz) within RF band is proposed to accurately pick up the fast rise time of the signals. Secondly, an antenna array with diamond-shaped configuration is designed to improve the correlation coefficient of the waveforms from multiple sensors. Thirdly, two antenna arrays are proposed to eliminate the influences of the EM wave scattering caused by the iron core. Fourthly, an algorithm is proposed to increase the signal to noise ratio and to measure the time arrivals automatically through cross-correlation and averaging operation. Lastly, a hyperboloid-genetic algorithm is developed to search the PD source. Experiments on a single-phase transformer model were conducted in the laboratory. The tested results proved the validity of the proposed method and showed that the location error in 16 cm, mostly is about several cm

Medium voltage cable defects revealed by off-line partial discharge testing at power frequency

Abstract: This paper describes typical cable defects uncovered while testing over 9,000 km of medium voltage XLPE insulated cables. After a brief review of the testing method, the procedure that led to the identification, localization, and characterization of typical defects found in operating cables is described. The paper also discusses the "conversion" of water trees (WT) to electrical trees (ET) which are associated with partial discharge (PD) activity in the context of laboratory research. Based on actual service performance of cables, several cases in which ETs associated with WTs did not lead to cable failure even after several years of service in very harsh operating environments are cited

Acousto-optical PD detection for transformers

Abstract: Partial discharge (PD) is one of the factors that could lead to failure of power transformers, leading to power outage and expensive repairs. The acoustic wave induced by PD can be measured and used for monitoring, diagnosing, and locating potential failures in the transformers. Fiber optic sensors have been shown to be attractive devices for PD detection because of a number of inherent advantages including small size, high sensitivity, electrical nonconductivity, and immunity to electromagnetic interference. A fiber optic sensor based on a Fabry-Perot interferometry is constructed by a simple micromachining process compatible with microelectromechanical system technology. The sensors are used in a transformer to measure PD acoustic waves. The experimental results show the sensor not only has an inherent high signal to noise capability, but is able to accurately localize the PD sources inside the transformer.

Partial discharge investigation of a power transformer using wireless wideband radio-frequency measurements

Abstract: The remote detection of a transformer internal partial discharge (PD) has been demonstrated using mobile wideband radio-frequency receiving equipment. The PD is externally detectable due to coupling within the transformer tank, causing impulsive signals to be radiated from external connections. A wideband direction-finding technique using a four-antenna array has shown the source of the radiation to be the tertiary winding connections; the radiated impulse has characteristics typical of this method of emission. No other external site of radiation from the transformer was detected. Due to the lack of coupling to the primary and secondary windings, it is believed that the PD is present between the tertiary windings and the core.

Objective methods to interpret partial-discharge data on rotating-machine stator windings

Abstract: Partial-discharge (PD) measurements have long been used to assess the condition of the electrical insulation in motors and generators rated 3.3 kV and above. There are many ways to measure PDs during normal service of the motor or generator. Unfortunately most of the measurement methods mix stator PD with electrical-interference signals from poor electrical connections, power tool operation, corona from transmission lines, etc. The result can be false indications of stator-winding problems, reducing confidence in PD measurements. Another issue with online PD testing is interpretation, i.e., identifying which machines are in good condition and which machines need maintenance. In the past decade, a database of over 60 000 test results has been assembled. In hundreds of machines, the condition of the insulation determined by a visual inspection has been compared to the PD levels. The result is a table that provides an objective means of determining the stator-insulation condition relative to other similar machines. An analysis of the results also shows that there are significant differences in PD activity between manufacturers. This paper gives a review of the methods that can reduce the risk of false indications, thus making the measurement less subjective.

A new approach to partial discharge testing of HV cable systems

Abstract: This paper describes the application of an innovative partial discharge (PD) detection and analysis approach to the diagnosis of HV polymeric cable systems. The results of PD measurements performed off-line on cable systems, rated from 220 to 400 kV just after laying or after some time in operation, are presented. PD generating defect identification and location are discussed, also in relation to forensic observations

Wavelet packet denoising for online partial discharge detection in cables and its application to experimental field results

Abstract: Partial discharge measurements taken online are severely corrupted by noise due to external disturbances. In this paper a powerful noise reduction technique, based on a wavelet packet denoising algorithm, is employed to isolate the signals from the noise. This methodology enables the denoising of partial discharges that are heavily corrupted by noise without assuming any a priori knowledge about the partial discharge features. A brief description of the wavelet packet theory as an extension of the multi-resolution analysis is given. Results of the application of this algorithm to simulated data of low signal-to-noise ratio are presented, demonstrating substantial improvement in signal recovery with minimum shape distortion. Finally, the capability of this technique is highlighted by applying it to experimental field data taken from three-phase 11 kV cables.

High voltage insulation monitoring sensor

Abstract: A sensor for condition monitoring the high voltage insulation of an electrical, generation, transmission or distribution system and/or an item of plant is described. The sensor comprises an impedance measurement unit that connects directly, via a single point of connection, to the electrical system to be tested. The electrical components of the impedance measurement unit are arranged so that high frequency partial discharge signals and low frequency power cycle information signal are preferentially transmitted through separate branches. Analysis of these separated signals thus enables extraction of information about the insulation of the electrical system being monitored. Safe operation of the sensor for users is also achieved through an opto-electronic circuit being employed to control the impedance measuring unit.

Partial-discharge-inception testing on low-voltage motors

Abstract: This is a review of one manufacturer's efforts toward testing National Electrical Manufacturers Association (NEMA) low-voltage motor insulation for compatibility with adjustable-speed drives (ASDs). Partial-discharge-inception-voltage (PDIV) measurements were selected as a tool to evaluate the future performance of the stator insulation system to resist deterioration by the drives. PDIV measurement is explained in terms of procedure and accuracy. Data are presented from tests made on motors from several prominent manufacturers.

Influence of wire mesh electrodes on dielectric barrier discharge

Abstract: A dielectric barrier discharge in a 2 mm air gap was studied. The experiments show that the mesh electrode and PET film really make the discharge looking homogeneous. The breakdown onset voltage in the case of a mesh electrode and PET film is 6.4 kV, considerably lower than 8 kV, the breakdown voltage in the usual case using two spherical electrodes. The field calculations show that even with this much lower voltage the gap field in the region very close to the PET film is enhanced to a value near the breakdown field in the usual case. It may be this field enhancement that initiates a corona discharge which provides seed electrons, leading to the breakdown of the whole air gap at a lower voltage. A lower voltage applied to the gap means a lower averaged field over the gap. Because the development of an electron avalanche is very sensitive to the electric field, a small decrease in the field will depress the avalanche significantly, preventing a homogeneous discharge from transiting to filament discharge. This may be the reason why a mesh electrode and PET film make the discharge look homogeneous.

Distortion of partial-discharge images caused by high-voltage harmonics

Abstract: Nowadays, one of the main methods in diagnostics of high-voltage insulating systems is registration of partial-discharge (PD) pulses and their visualisation in the form of phase-resolved images. The paper describes the influence of high-voltage harmonics on partial-discharge phase-resolved images, which is crucial for proper identification and classification of PD patterns. The evolution of these images, depending on the total-harmonic-distortion (THD) parameter being an assessment factor, calculated by means of fast Fourier transform (FFT), is shown. Numerical simulation which illustrates the harmonic influence on PD is reported. Different harmonic-distortion instances of test voltage have been analysed along with the corresponding PD phase distributions. The measurement results were obtained on thermosetting insulation of a stator bar with the controlled harmonic content of the test voltage. The basic principles of harmonic analysis, along with the measurement set-up, are presented. It is shown that THD may only be a rough and global assessment factor. For the practical analysis of PD images, as a tool for insulation diagnostics and monitoring, the test-voltage frequency spectrum will be required.

Detection, localization and interpretation of partial discharge

Abstract: In order to detect, localize and interpret a partial discharge occurring in a partial discharge site along an electrical equipment, two measurement probes and a synchronization probe are installed along the electrical equipment. The measurement probes detect pulses travelling in the electrical equipment while the synchronization probe detects a phase angle in the electrical equipment and is usable for calibration purposes. A control unit receives the signals sensed by the probes and conditions them. Digital processings applied on the conditioned signals, involving their correlation, a time-frequency distribution and a form factor estimation, allows establishing a diagnosis indicating a detection of a partial discharge and its localization along the electrical equipment.

On-line detecting and positioning device for local discharging of electrical insulated combined electrical appliance, and positioning method thereof

Abstract: This invention relates to gas insulation combination electric apparatus partial discharge online detecting and position equipment and its position method. It belongs to electric equipment insulation detecting technique field. This invention includes sensor array consisted by microtrip patch antenna, two groups multiple intelligence selecting switch; two amplify filter, high speed digital collector, computer and control module. The array in this invention is made up by the microtrip patch antennas fixed at GIS basin type insulator, its antenna is made up by rectangular metal patch, medium board, metal platter, coaxial probe type feed line and antenna shielding case. This invention is computer program control, it can online, actual time, quickly, actually poison of GIS inner discharging. This invention can be widely used to electric generator plant booster station and electric network GIS electric substation, it can auto detecting GIS inner partial discharge, and actually poison of partial discharging defect, also it can interconnection run with electric substation and electric generator plant auto monitoring system.

Streamer and leader discharge propagation characteristics leading to breakdown in electronegative gases

Abstract: This paper describes the propagation characteristics and mechanisms of partial discharge (PD) leading to breakdown (BD) in electronegative gases such as SF6/N2 and C2 F6/N2 gas mixtures for AC voltage application. An established PD measuring system of highly time-resolved PD current and light intensity waveforms, streak and light emission images enabled us to discuss the physical mechanisms of PD generation and propagation in terms of space charge behavior. Experimental results revealed that the time-resolved PD generation was divided into different groups composed of the first large PD and the subsequent small PDs; the former was identified as the leader discharge, and the latter corresponded to the streamer discharges. The successive generation of the streamer and leader discharges was interpreted by the change of critical volume size. The time-resolved PD generation characteristics leading to BD were also measured and discussed. The leader discharge would be obliged to take a detour along the periphery of the positive space charge area in the vicinity of the needle electrode, and finally result in BD in electronegative gases

X-ray radiation due to nanosecond volume discharges in air under atmospheric pressure

Abstract: The formation of nanosecond discharges in atmospheric-pressure air versus the applied pulse polarity and discharge gap geometry is studied. It is shown that the polarity of high-voltage nanosecond pulses and the electrode configuration have a minor effect on the volume discharges under a variety of experimental conditions. When the spacing between needle-like electrodes is large, the discharge is asymmetric and its glow is weakly dependent on the sign of the potential applied to the electrode. Negative voltage pulses applied to the potential electrode generate X-ray radiation from both the surface and volume. For a subnanosecond rise time of the voltage pulse and diffusion character of the discharge, the X-ray radiation comes from the brightly glowing region of a corona discharge. The average values of the fast electron velocity and energy in nitrogen are calculated. At field strengths E/p 170 kV/cm atm, fast electrons run away. Central collisions are the reason for X-ray radiation from the volume.

Phased-ultrasonic receiving-planar array transducer for partial discharge location in transformer

Abstract: Partial discharge (PD) location in transformers is very important, and many methods that have been brought forward in past decades have a limitation theoretically, namely, they cannot locate multiple PDs in electrical equipment. In this paper, a new PD location method based on UHF and ultrasonic-phased arrays receiving theory has been presented, which has a possibility to locate multiple PDs. According to the method, a phased-ultrasonic receiving-planar array sensor that possesses 16 x 16 elements is designed; and, based on the phased-array theory, the characteristics of the plane sensor are studied. The laboratory experimental tests on the plane sensor element indicates that it has a good performance within the frequency band of the main ultrasonic energy produced by PD in transformer oil. Location tests are conducted on one or two piezoelectric ultrasonic sources in oil, which are both simulated as PD sources and triggered by an electrical pulse whose front is considered as a time benchmark in the locating algorithm. The test results show locations to one and two PDs can be realized in a single measurement, which lays a foundation for locating PDs in a power transformer in service

Experimental and modelling investigations of a dielectric barrier discharge in low-pressure argon

Abstract: The discharge behaviour of a dielectric barrier discharge (DBD) in low-pressure argon gas was investigated by experiments and modelling. The electrical characteristics and light emission dynamics of the discharge were measured and compared with the results of a two-dimensional fluid model. Our investigations showed that the discharge consisted of a single, diffuse discharge per voltage half-cycle. The breakdown phase of the low-pressure DBD (LPDBD) was investigated to be similar to the ignition phase of a low-pressure glow discharge without dielectrics, described by Townsend breakdown theory. The stable discharge phase of the LPDBD also showed a plasma structure with features similar to those of a classical glow discharge. The presence of the dielectric in the discharge gap led to the discharge quenching and thus the decay of the plasma. Additionally, the argon metastable density was monitored by measuring light emission from nitrogen impurities. A metastable density of about 5 ? 1017?m?3 was present during the entire voltage cycle, with only a small (~10%) increase during the discharge. Finally, a reduction of the applied voltage to the minimum required to sustain the discharge led to a further reduction of the role of the dielectric. The discharge was no longer quenched by the dielectrics only but also by a reduction of the applied voltage.

Analysis of partial discharge signal using the Hilbert-Huang transform

Abstract: /sup T/he development of time-frequency and time-scale transforms has progressed rapidly over the last 20 years. The development and progression of numerical acquisition enable the utilization of these mathematical tools to analyze partial discharge in electrical equipment. This paper introduces a new transform, a Hilbert-Huang Transform to analyze partial discharge data. The mathematical model of a Hilbert-Huang Transform is described; and its application in a PD application shows its great advantage.

Relationship between PD-induced electromagnetic wave measured with UHF method and charge quantity obtained by PD current waveform in model GIS

Abstract: This paper deals with an estimation manner of charge quantity of partial discharge (PD) in GIS by PD-induced electromagnetic (EM) wave measured with UHF method. Charge quantity of PD is important to understand PD activity or condition for insulation diagnosis, and the charge estimation by UHF method could contribute to a simplification and high sensitivity of corona free test for electric power apparatus. In this study, we theoretically investigated the relationship between PD current waveform and PD-induced EM wave propagating in a model GIS, using FD TD method. As a result, we can find that the value of double integral of PD-induced EM wave's TEM mode component is basically proportional to the charge quantity of PD current pulse independently of the sensor distance from PD source and the PD current pulse's shape.

Improvement of plasma uniformity using ZnO-coated dielectric barrier discharge in open air

Abstract: The discharge behavior in atmospheric pressure dielectric barrier discharge using ZnO-coated dielectric layer is examined. ZnO thin film on alumina using rf magnetron sputter causes about a factor of a million higher surface conductivity than bare alumina surface. Experimental result shows that discharge uniformity is improved definitely in the case of ZnO-coated dielectric barrier discharge. Increase of surface conductivity stimulates charges to spread over the dielectric surface widely. These charge spreading enhances the uniformity and the stability of atmospheric pressure discharge in open air by initiation of consecutive streamers.

Detecting Partial Discharge in High Voltage Cables

Abstract: A method for detecting partial discharge events comprising analysing an output from the cable using one or more substantially shift invariant wavelet transforms, and using the analysed signal to identify one or more peaks associated with a partial discharge. Preferably, the shift invariant transform is a stationary wavelet transform (SWT).

Diagnosis of stator winding insulation of large generator based on partial discharge measurement

Abstract: The Stator winding insulation is aged by exposure to a combination of thermal, electrical, mechanical, and environmental stresses. Diagnosis of stator winding insulation is an important measure of ensuring the safe operation and extending the remaining life of large generator. In the paper, two new parameters are proposed for the diagnosis of stator winding insulation based on the test results of multistress accelerating aging test of stator bars. The test results show that the skewness of partial discharge phase resolved distribution and the high-frequency crest of partial discharge (PD) impulse change much more remarkably with the aging time than do the conventional parameters. Therefore, the two parameters could be considered as characteristic parameters for the diagnosis of stator winding insulation of the large generator. With these characteristic parameters, authors successfully assessed the insulation condition of actual stator bars removed from several large generators.

Methods and apparatus for analyzing partial discharge in electrical machinery

Abstract: Methods and apparatus for analyzing electrical insulation of an electrical machine are provided. The method includes receiving a first signal that includes a plurality of partial discharge pulses from the electrical machine and a plurality of noise pulses, receiving other signals that includes information relative to at least one process parameter associated with the electrical machine, determining the plurality of partial discharge pulses from the plurality of noise pulses, identifying characteristics of the plurality of partial discharge pulses relating to the location and character of partial discharges in the electrical machine, and determining a condition of the electrical insulation using the identified characteristics and the received information relative to at least one process parameter.

Propagation of partial discharge signals in stator windings of turbine generators

Abstract: We studied the propagation of partial discharge pulses in a stator winding by means of measurements on a dismantled 35-MW generator and found that a stator winding acts as a transmission line. Therefore, a partial-discharge (PD) signal manifests itself at the generator terminals after a transit time that depends on the location of the discharge. Due to capacitive and inductive couplings in the end-winding region, a second propagation mode is present for higher frequencies. This "fast mode" manifests itself at the terminals without appreciable time delay. The capacitive and inductive couplings also cause crosstalk between the phases. As a consequence, a signal measured in one phase does not necessarily originate from a discharge in that phase. The amplitudes of the fast mode and, to a lesser extent, the slow mode (or transmission-line mode) are heavily damped when the discharge occurs further away from the measuring terminal. The consequence is that only discharges close to the measuring terminal can be observed. The propagation of a PD signal is not only influenced by the construction of the generator but also by its external connections.

Simulation of Partial Discharges in Single and Double Voids Using SIMULINK

Abstract: Partial discharges (PD) in the insulation of power equipment have been a focal point in the field of electrical technology. Numerical simulation of PD is an important approach to knowing the PD mechanics. In this paper, the physical model of PD in voids was modified and the simulation circuit of PD in voids was built using Simulink. The simulations of the epoxy resin samples with a single void and double voids were investigated, respectively. The discharge voltage waveforms, discharge current waveforms, and the single discharge waveforms of voids were obtained by simulation. At the same time, we measured the PD pulse waveforms of epoxy resin samples in time-domain by an ultra-wideband PD measuring system. The test samples enclosing artificial voids have the same size as the simulation samples. The single discharge waveforms of a single void and double voids obtained by simulation are very similar to the measured ones in the same condition. Moreover, the different discharge rules between one void and two voids are found by simulation. The conformity of the simulation results and test ones indicates that the simulation model is effective and the simulation results are reliable.

A correlation method for detection of partial discharges in transformers

Abstract: A correlation method is proposed for the location of partial discharge within a winding. Theoretical simulations and experimental results on a lumped parameter model of a winding are shown to prove the feasibility of the method.