1,720,986 research outputs found
Space charge measurement in polymer insulated power cables using the PEA method
No full textThe Pulsed Electro-Acoustic (PEA) technique has been widely used to measure space charge distribution in solid dielectrics. A considerable amount of work has been done on the charge dynamics in plaque samples. Recently, there have been several attempts to apply the technique to polymeric cables, but it has been hampered by experimental difficulties in achieving a universal electrode system to maintain an intimate contact between the curved ground electrode, transducer and absorber.In this research, a modified system employing a flat ground electrode, transducer and acoustic absorber has been introduced. By thoroughly analysing the propagation of the acoustic pressure in the new electrode arrangement, it is concluded that the revised electrode system will give the same detection sensitivity and spatial resolution as that of the old version. The new PEA system not only achieves an intimate contact acoustically but also has advantages of easy assembly of cable sample and is suitable for different sizes of cables without any modification.The basic principle of space charge measurement in cable geometry with the new system is fully obtained. To remove the influences of the geometry factor, the acoustic propagation attenuation and dispersion through the polymer material, a compensation algorithm has been developed to improve the measurement accuracy both on charge density and position.A comprehensive program with the functions of data acquisition, deconvolution of original signal, calibration and compensation both to geometry and propagation factors has been developed in the LabViewTM environment. The method to calculate the electric stress distribution from the results of the space charge profile in cable geometries has been proposed.For practical application, space charge distribution measurements were carried out on several prototype cable samples which adopt modified XLPEs (undegassed and degassed) as insulating materials. The comparisons were conducted on both the space charge accumulation and its influence on the electric stress distribution. The results showed that the modification to the XLPE material is profitable from the point of view of space charge suppression. Moreover, the thermal treatment to the sample showed that the removal of the volatile residue had a significant influence on the space charge behaviour.</p
Thermal ageing and its impact on charge trap density and breakdown strength in ldpe LDPE
No full textLow-density polyethylene (LDPE) has been widely used as power cable insulation, because of its good electrical performance and stable chemical characteristics. However, in recent years, with the rise of large-capacity and long-distance HVDC transmission systems, the effect of space charge has a significant impact on the insulation selection and design. Furthermore, the change in the electrical performance of insulation after ageing is also required to be understood. It has been reported that ageing leads to an increase in charge trap density. The increase of trap density in LDPE makes the transport of charge carriers between traps easier. Consequently, the electrical breakdown strength will also be affected. This paper focuses on the LDPE films with different degrees of thermal ageing and studies its impact on charge trap density and change in electrical breakdown strength. The ageing degrees of sample were characterized using Fourier-Transform Infrared (FTIR). Space charge dynamics were measured using the pulsed electroacoustic (PEA) technique. In addition, electrical breakdown strength of the aged samples was measured and breakdown data were processed using the Weibull distribution. The change in characteristic breakdown strength is related to the change in charge trap density. The results suggest that the change in charge trap density of an insulating material can be used to characterize electrical performance of the material, therefore, the ageing status
Space charge behavior in multi-layer oil-paper insulation under different DC voltages and temperatures
Full text linkOil-paper as a reliable insulation system is widely used in power transformers and cables. The dielectric properties of oil-paper insulation play an important role in the reliable operation of power equipment. However, the formation and dynamics of space charge can affect the performance of insulation material. In this paper, space charge in oil-paper insulation system has been investigated using the pulsed electroacoustic (PEA) technique. A series of measurements were carried out when the insulation system was subjected to different applied voltages at different temperatures. Charge behavior in the insulation system has been analyzed and the influence of temperature on charge dynamics was discussed. The test results show that homocharge injection takes place under all the test conditions, the applied dc voltage mainly effect the amount of space charge, while the temperature has greater influence on the distribution and mobility of space charge inside oil-paper sample
Space charge characteristics of multi-layer oil-paper insulation under different DC voltages
Full text linkIn this paper, space charge in oil-paper insulation system has been investigated using the pulsed electroacoustic (PEA) technique. A series of measurements were carried out when the insulation system was subjected to different applied voltages. Charge dynamics in the insulation system during the volts-on, volts-off and decay have been analyzed. It has been found that homocharge injection occurred both at the anode and the cathode. Positive charges are observed to accumulate in the layers, which indicate that the oil-paper layer interfaces act as a barrier for positive charges. The decay tests showed that after 30min, about a quarter of space charges remained in the sample, about 90% charges disappeared after 2 hours. Finally, total charge variation in these tests were analyzed
Space Charge Formation and its Modified Electric Field under Applied Voltage Reversal and Temperature Gradient in XLPE Cable
No full textThe results of space charge evolution in cross-linked polyethylene power cables under dc electrical field at a uniform temperature and during external voltage polarity reversal are presented in the paper. A mirror image charge distribution was observed in the steady state, but the pre-existing field altered the way in which the steady state charge distribution was formed from that obtaining when the cable was first polarized. Polarity reversing charge was generated in the middle of the insulation and moved towards the appropriate electrodes under the influence of a field in excess of the maximum applied field. Our results show that the mirror effect is a steady state effect that is due to crossinterface currents that depend only on the interface field and not its polarity. Measurements on cable sections with an elevated mean temperature and temperature gradient show that the interface currents are temperature dependent, and that differences between the activation energies of the interface and bulk currents can eliminate and possibly even invert the polarity of the space charge distributio
Space charge dynamics in pressboard-oil-pressboard multilayer system under DC voltages
Full text linkConverter transformers play a critical role in HVDC power transmission system. Space charge is a general concern for dielectric system under DC field. In this paper, the space charge behavior in a sandwiched oil-pressboard insulation system has been investigated using the pulsed electro-acoustic (PEA) method. The presence of homo-charges in the pressboards bulk and hetero-charges at the two interfaces between oil gap and pressboards leads to the electric field enhancement in the pressboards but reduction in the oil gap. The electric field enhancement is much larger than the value estimated by the Maxwell-Wagner polarization
An improved model to estimate trapping parameters in polymeric materials and its application on normal and aged low-density polyethylenes
Full text linkTrapping parameters can be considered as one of the important attributes to describe polymeric materials. In the present paper, a more accurate charge dynamics model has been developed, which takes account of charge dynamics in both volts-on and off stage into simulation. By fitting with measured charge data with the highest R-square value, trapping parameters together with injection barrier of both normal and aged low-density polyethylene samples were estimated using the improved model. The results show that, after long-term ageing process, the injection barriers of both electrons and holes is lowered, overall trap depth is shallower, and trap density becomes much greater. Additionally, the changes in parameters for electrons are more sensitive than those of holes after ageing
Trapping parameters estimation of fresh and thermally-aged low-density polyethylene by using an improved trapping/detrapping model
Full text linkIn the present paper, trapping parameters of normal and thermally aged low-density polyethylene (LDPE) samples were estimated using the improved charge dynamic model. The results show that, after long-term thermal ageing process, the injection barrier of both electrons and holes is lowered, the overall trap depth is shallower and electron trap density becomes much greater. The latter may indicate that electrons are more sensitive to ageing than those of holes
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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