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ABSTRACT: Fully synthetic tetrasilisic mica from Topy Co., Ltd. and Cloisite<sup>®</sup> powder of Wyoming natural montmorillonite clay from Southern Clay Products were used to manufacture polypropylene-based nanocomposites with organoclay concentrations up to 8%-wt. Both types of nano-filler increase the AC breakdown strength by approx. 10% over the unfilled material. However, the resistance to partial discharges is significantly improved, with both nano-fillers yielding similar results. Nano-fillers increase dielectric losses of PP over broad frequency and temperature ranges. Specimens containing natural clay show higher losses than the specimens containing synthetic clay, especially at higher frequencies and temperatures.
Electrical Insulation (ISEI), Conference Record of the 2010 IEEE International Symposium on; 07/2010
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ABSTRACT: Polypropylene is extensively used as a dielectric in power capacitors, electrical machines and transformers. It has excellent mechanical, thermal, and electrical properties and provides outstanding resistance to moisture, grease, and oils. It does not present stress-cracking problems and at high temperatures it offers better thermal and chemical resistance than polyethylene. A thermoplastic such as polypropylene can be reinforced with small quantities of nanoparticles to not only achieve better chemical and thermal properties but also improved dielectric performance, such as high electrical breakdown strength, low surface and volume conductivity and resistance to partial discharges. The space charge properties of a structural nanocomposite based on layered synthetic silicate embedded in polypropylene (PP) are described in this paper. Electroluminescence emission, dissipation current measurement and pulsed electro-acoustic technique are employed to compare the properties of PP with and without the nanoparticles.
Electrical Insulation and Dielectric Phenomena, 2009. CEIDP '09. IEEE Conference on; 11/2009
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ABSTRACT: The incorporation of synthetic silica nanoparticles into polypropylene is shown to increase the ac breakdown strength compared to unfilled material. This breakdown strength stays unchanged during application of a 40 kV/mm dc field for up to 500 h at both, room temperature and 90°C. Dielectric spectroscopy shows an increase of dielectric loss factor, tan¿, with nanoflller concentration and a distinct relaxation around 60°C. The increase of tan¿ caused by nanoparticles is moderate and thus manageable in practical applications. Subjecting specimens to a dc field did not significantly change the dielectric loss spectra. The dc conductivity of the materials with nano-filler was found to be higher than for unfilled materials. This is believed to be caused by the overlapping of the diffuse double layers surrounding nanoparticles, which provide a path for the migration of electric charge. Aging nanocomposites in a dc field resulted in the increase of conductivity but the increments were significantly smaller than those observed in the material without organoclay.
Electrical Insulation and Dielectric Phenomena, 2009. CEIDP '09. IEEE Conference on; 11/2009
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ABSTRACT: Polymers are extensively used as insulating material in high voltage devices, such as underground power cables, power capacitors and transformers. During normal operation the polymeric insulation of a power device is not only subjected to electrical stresses, but could also be subjected to other stresses that can cause the degradation and ultimately lead to insulation failure. It has been well established that electroluminescence and charge injection, which gives rise to space charge in the polymeric insulation, occur at ac, dc and impulse field above a certain threshold value. Space charge can cause dissipative energetic processes such as photon and phonon emission, increase the local electric field and reduce the withstand voltage of the insulation. This paper describes the characteristics of electroluminescence in polymeric insulation subjected to ac voltage and shows its relevance to space charge injection in the material. It is shown that insulation subjected to high voltage could emit various types of light but electroluminescence emission which is related to space charge injection can be clearly distinguished from these other types of light emission. The electroluminescence technique is a valuable tool to evaluate the dielectric properties of novel insulating materials, such as nanodielectrics.
IEEE Transactions on Dielectrics and Electrical Insulation 11/2009; · 1.09 Impact Factor
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ABSTRACT: Laboratory tests on field aged and unaged cables were performed using DC depolarization current measurements. Aged cables with water trees showed high dielectric losses and depolarization currents as compared with the reference, unaged cable. Both the area under the depolarization current waveform and dielectric loss factor, tandelta, increased with DC poling and the applied AC voltage, respectively. A new indicator of the insulation aging derived from the measured current is proposed to rate the degree of aging of cable insulation. This indicator increases with the poling field, and a DC poling voltage less than 50% of the nominal peak value of the cable operating voltage is sufficient to obtain reliable values. The cable insulation aging indicator could be a useful tool for monitoring the operating condition of cables in service and help utilities to develop cable replacement programs based on the actual state of the cable insulation conditions.
Electrical Insulation and Dielectric Phenomena, 2008. CEIDP 2008. Annual Report Conference on; 11/2008
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ABSTRACT: The dc depolarization current measurement technique was used to test 20 medium voltage, underground XLPE-insulated cables in the local utility power distribution network between January and March 2006 and then in March 2007. The cables varied in length (30 m to ~1600 m), voltage rating (9 to 28 kV), and age (4 to 36 years). Unlike a typical depolarization current measurement where only a low frequency portion of the current is monitored with an electrometer, the described measurements have also included the high frequency component of the depolarization current, occurring right after switching off the poling voltage. The measurements in 2007 also included the dielectric loss factor measurements between 10 mHz and 500 Hz to correlate the tan delta values with the parameters of the depolarization currents. A reasonable correlation was found between cable age and other operating conditions and the depolarization current characteristics. A one year period between 2006 and 2007 produced noticeable changes in the depolarization current characteristics for some cable and almost no change in others. No consistent time effect on the insulation of various cables could be identified.
Power and Energy Society General Meeting - Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE; 08/2008
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ABSTRACT: Crosslinked Polyethylene (XLPE) has been extensively used as an insulating material in underground distribution and transmission class high voltage cables. During service, diagnostic techniques are often employed to determine the operating condition of the insulation. Some techniques use frequencies much lower than the power frequency because the required high voltage power supplies are much smaller and it is assumed that very low frequencies cause less damage to the insulation. In this paper it is shown that at low frequencies more space charge per cycle remains in the insulation. Electroluminescence (EL) and the newly developed Phase-Resolved-Pulsed-EIectro-Acoustic (PR-PEA) techniques are used to determine charge injection and space charge distribution in XLPE subjected to frequencies between 1 Hz and 50 Hz.
Electrical Insulation and Dielectric Phenomena, 2007. CEIDP 2007. Annual Report - Conference on; 11/2007
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ABSTRACT: Amodified technique of depolarization current measurements encompassing monitoring of high speed depolarization processes was used to perform diagnostic tests on polymeric cable insulation operating under diverse environmental conditions and voltage levels. The high frequency characteristics were compared with those of the slower depolarization processes commonly obtain during on-site tests. Both the high and low frequency measurements showed a consistent correlation with the operating conditions of the cable insulation. It was observed that the low frequency characteristics were dependent on the duration of the dc poling time but only if the insulation was deteriorated, especially with water trees. Unaged and relatively new insulation did not show such behavior. The majority of the field tests were performed with a relatively low dc voltage of only 1 kV or less and under very adverse environmental conditions, with temperatures as low as ¿30°C.
Solid Dielectrics, 2007. ICSD '07. IEEE International Conference on; 08/2007
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ABSTRACT: The PEA technique has been modified to study the space charge development in solid dielectrics subjected to AC electric field. Narrow (5 ns) electrical pulses are applied at various phase angles of the AC waveform. Special software, developed to precisely synchronize the pulse generator with the high voltage supply, applied the narrow pulses at 0deg phase angle and then in steps of 10deg till 360deg. By processing the PEA data at various phase angles of the AC waveform, without resorting to complex mathematical analysis, the electric field at which charges are injected into the polymeric insulation was determined. The phase resolved PEA technique can also provide the dynamics of space charge development under AC fields
IEEE Transactions on Dielectrics and Electrical Insulation 03/2007; · 1.09 Impact Factor
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ABSTRACT: DC field long-term breakdown characteristics of commercially available cable-grade cross-linked polyethylene (XLPE) and tree retardant XLPE (TR-XLPE) were investigated. At room temperature, under a unipolar field of 50 kV/mm, more than 90% of XLPE specimens survived 52, 000 h (~6 yrs) of aging, whereas all TR-XLPE specimens broke down within less than 1, 200 h. Reversing the polarity increased the life-time of TR-XLPE by more than 20 times but reduced significantly the life-time of the XLPE specimens. At 75 degC and with polarity reversals both materials survived almost 21, 000 h without any breakdown. These results are discussed in terms of the space charge accumulation and retention properties of the two materials with emphasis on the role of additives in determining insulation properties under DC fields.
Electrical Insulation and Dielectric Phenomena, 2006 IEEE Conference on; 11/2006
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ABSTRACT: A thermoplastic such as polypropylene reinforced with small quantities (<5% by weight) of nano-sized particles could show reduced flammability as well as improved dielectric properties. This paper describes short-term dielectric properties of polypropylene containing 0%, 2%, and 4% by weight of dispersed organosilicates. AC and DC breakdown strength, tanδ, electroluminescence emission, and space charge distribution due to AC poling of polypropylene with and without the nanoparticles are reported.
Electrical Insulation and Dielectric Phenomena, 2005. CEIDP '05. 2005 Annual Report Conference on; 11/2005
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ABSTRACT: The correlation between space charge accumulation and insulation failure has been investigated in two types of XLPE subjected to a DC field of 50 kV/mm. The specimens were aged for extended periods of time, up to 256 weeks at single polarity and up to 72 weeks with the polarity reversed every week. While all specimens of regular XLPE, without tree-retardant additives, survived 256 weeks of single polarity aging, all specimens having the additive broke down within 54 weeks. However, the latter material survived 72 weeks without a single breakdown when the polarity of the applied voltage was reversed every week at RT and 75 °C.
Electrical Insulation and Dielectric Phenomena, 2005. CEIDP '05. 2005 Annual Report Conference on; 11/2005
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ABSTRACT: Results of EL emission in flat-sheet samples of polymeric insulation (polyethylene), subjected to a uniform field, from two laboratories, are compared. The EL technique has been employed to detect space charge injection, trapping and recombination centers in the material. The voltage has been varied over a range often found in practice, and the frequencies used cover a part of the spectrum used in electrical and electronic apparatus. The aim of this collaborative project is to enhance the understanding of the aging mechanisms of dielectrics in service, determine the threshold field of space charge injection and provide the electrical and electronic industry with guidelines that would reduce the probability of catastrophic insulation breakdowns.
Electrical Insulation and Dielectric Phenomena, 2004. CEIDP '04. 2004 Annual Report Conference on; 11/2004
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ABSTRACT: The correlation between space charge accumulation and insulation failure has been investigated in two types of cross-linked polyethylene and ethylenepropylene rubber subjected to a positive DC field of 50 kV/mm for periods up to 3% years. One type of XLPE films was equipped with tree-retardant additives. A buildup of space charge in the films was monitored periodically during aging with Thermal Step Method, which measures space charge distributions under no voltage condition. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge density peaks that appeared initially near the electrodes moved closer towards one another during aging and the area under the space charge density peaks showed a noticeable increase shortly before insulation breakdown. These observed properties of the material behavior under dc fields could be used to screen new materials for practical applications.
Solid Dielectrics, 2004. ICSD 2004. Proceedings of the 2004 IEEE International Conference on; 08/2004
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ABSTRACT: The correlation between space charge accumulation and insulation failure has been investigated in four, polyethylene-based materials subjected to a DC field of 50 kV/mm. Two of the materials contained tree-retardant additives and all four materials were aged for extended periods up to 19,000 h. The specimens were removed periodically from the external field and subjected to space charge density measurements under no-voltage conditions using the thermal step (TS) technique. Not a single parameter that would consistently correlate with times to insulation failure could be derived from the space charge measurements. However, the space charge measurements provided valuable insight into a possible link between the dynamics of space charge development and the time of DC aging. In particular, a consistent surge in the amount of accumulated space charge shortly before insulation breakdown was observed in practically all samples. Also, a consistent relationship between time to breakdown and the position of the internal space charge density peaks in the sample was observed. Samples that had space charge density peaks further apart consistently survived longer than specimens for which the peaks were closer together. These phenomena, if confirmed, could be used to screen new materials for DC voltage applications.
IEEE Transactions on Dielectrics and Electrical Insulation 03/2004; · 1.09 Impact Factor
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ABSTRACT: Polarization currents obtained at a low dc field for two different brands of XLPE and EPR insulation subjected to water treeing were correlated with the corresponding time to breakdown characteristics of these materials. It is shown that although the RC time constant and the area under the polarization current curve changed consistently with the aging time for all three materials tested, the consistency and the diagnostic value of the information they provide about imminent insulation failure depends on the type of material.
Electrical Insulation and Dielectric Phenomena, 2003. Annual Report. Conference on; 11/2003
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ABSTRACT: A newly developed numerical analysis technique based on inverse matrix calculations is described. It is used to de-convolute electric current waveforms into the space charge and electric field distributions after subjecting solid dielectric samples to a thermal step and measuring the current induced in the external circuit by the perturbation of space charges in the material due to the heat flow. The technique gives significantly lower errors and requires less computation time than commonly used Fourier analysis.
Precision Electromagnetic Measurements, 2002. Conference Digest 2002 Conference on; 02/2002
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ABSTRACT: Polarization current measurements obtained from two different brands of cross-linked polyethylene subjected to water tree aging are discussed. It is shown that the RC time constant and the area under the polarization current curve consistently change with the aging time, i.e., with the extent of water treeing. In comparison, depolarization current measurements used in some diagnostic cable insulation assessment tests, did not produce consistent results.
Electrical Insulation and Dielectric Phenomena, 2002 Annual Report Conference on; 02/2002
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ABSTRACT: This paper describes the TSM current response of high voltage
cable grade XLPE with and without tree retardants subjected to a dc
field of 50 kV/mm. The evolution of the space charge distribution is
monitored with the time of aging. It is proposed that the changes
observed in the behavior of TSM current with the aging time could be
used to determine the difference in the operating conditions of various
polymeric materials. The advantage of this approach is that it does not
require the determination of the actual distribution of space charge in
the insulation and thus it could constitute a very simple diagnostic
tool
Solid Dielectrics, 2001. ICSD '01. Proceedings of the 2001 IEEE 7th International Conference on; 02/2001
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ABSTRACT: This paper describes the polarization and depolarization current
responses from XLPE cable insulation with and without water trees.
Comparison of the polarization current with the time of polymer aging
suggests that the analysis of the polarization current alone could be
used to determine the operating state of the insulation. The advantage
of this approach is that the polarization current provides consistent
results and eliminates the need to acquire and analyze the
depolarization currents as a function of the poling time
Electrical Insulation and Dielectric Phenomena, 2001 Annual Report. Conference on; 02/2001
