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Dielectrics and Electrical Insulation, IEEE Transactions on

Issue 5 • Date October 2012

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  • IEEE Transactions on Dielectrics and Electrical Insulation [Table of contents]

    Publication Year: 2012 , Page(s): c1
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  • IEEE Dielectrics and Electrical Insulation Society executive committee officers

    Publication Year: 2012 , Page(s): c2
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  • Dielectric liquids [Editorial]

    Publication Year: 2012 , Page(s): 1475
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  • On partial discharge measurement in dielectric liquids

    Publication Year: 2012 , Page(s): 1476 - 1481
    Cited by:  Papers (3)
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    Partial discharge detection in dielectric liquids requires particular PD measurement techniques and instruments. Simultaneous partial discharge (PD) measurements that are employed on insulating liquids, using dual narrow/wide band detectors, are described. Narrow band PD detectors, which are of the integrating type are suitable for the measurement of the total charge transfer in pC of the overall discharge event. Wide band measurement techniques permit the acquisition of the discrete PD pulses in dielectric liquids, that comprise the overall PD pulse burst signal. These PD pulse bursts are comprised of discrete high frequency pulses of usually ascending magnitude charge transfers. The time position of these pulses within the pulse burst is determined by the times at which the Paschen's minima occur across the expanding cavity due to the small size of the microcavities. In order for them to undergo discharge requires enormously high internal pressures (ca 10 MPa) to ensure a sufficient number of charge carriers. The life duration of these cavities is of the order of several's, with the cavity collapsing, when its size reaches dynamic instability. View full abstract»

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  • A comment on the test methods for the breakdown voltage of dielectric liquids

    Publication Year: 2012 , Page(s): 1482 - 1484
    Cited by:  Papers (2)
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    The electrical breakdown at 50 or 60 Hz of an insulating liquid is of importance as a measure of the liquid's ability to withstand electrical stress. In national and international Standards (ASTM and IEC) different methods are reported for evaluating this fundamental property of an insulating liquid. Nevertheless, these test methods differ, depending upon the procedure, shape and dimension of the test specimen holders and types of electrodes. In the present communication an improvement of a test cell is described, which may be useful in a possible revision of Standards for improving the scatter of the test results in the measurement of the 50 or 60 Hz dielectric strength of different insulating liquids. View full abstract»

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  • Transformer oil and modern physics

    Publication Year: 2012 , Page(s): 1485 - 1497
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    It was demonstrated that using modern experimental methods of applied physics methods provides information important for understanding of the processes occurring in transformer oils. These methods could be useful for development of new systems of transformer oil quality control. View full abstract»

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  • Alternative method for the determination of the antioxidant content in transformer oil by electrochemical techniques

    Publication Year: 2012 , Page(s): 1498 - 1501
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    2,6-Di-tert-butyl-4-methylphenol (BHT), also known as 2,6-di-tert-butyl-p-cresol (DBPC), is an antioxidant widely used in transformer oil. The electrochemcial properties of BHT in transformer oil were investigated by using electrochemical techniques, and a new determination method of BHT in transformer oil based on differential pulse voltammetry was established. It was confirmed that the intensity of characteristic peak of differential pulse voltammetric corresponded for BHT exhibited a good linear relationship with the concentration of BHT within the concentration range from 0.01 to 0.5 %. After researching the accuracy of the experimental results by using spectrophotometry, liquid chromatography, and the electrochemical method, it was further demonstrated that the electrochemical method could be applied to the determination of the content of BHT antioxidant in transformer oils. View full abstract»

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  • Dielectric properties of nanopowder dispersions in paraffin oil

    Publication Year: 2012 , Page(s): 1502 - 1507
    Cited by:  Papers (1)
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    This work contributes to the study of nanofluids, by investigating polarization phenomena induced by nanoparticle inclusions in paraffin oil, an insulating organic liquid. Fine metal oxide powders and nanopowders of Al2O3, TiO2, CuO, Cu2O and Fe2O3 were dispersed at concentrations up to 5% w/v in the liquid matrix, using ultrasonic treatment. The relative dielectric constant and loss tangent (tanδ) of both pure oil and the prepared nanofluids were recorded in the 20 Hz-1 MHz frequency range. Results depend both on grain size and on the specific compound. In the case of alumina nanoparticles, dielectric behavior is dominated by grain surface polarization phenomena induced by adsorbed water. This effect can be partially cancelled out by the addition of titania nanoparticles. Titanium and (to a lesser extent) ferric oxide increased the dielectric constant at middle and high frequencies. Cupric and cuprous oxides exhibit a distinct relaxation mechanism at the high end of the examined frequency range. View full abstract»

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  • Comparative study of statistical breakdown in mineral, synthetic and natural ester oils under AC voltage

    Publication Year: 2012 , Page(s): 1508 - 1513
    Cited by:  Papers (1)
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    This paper reports on a comparative study of statistical Breakdown Voltage (BDV) in mineral oils, natural esters (vegetable oils) issued from different origins, and one synthetic ester through the measurements of 32 BDV using spherical electrode under AC voltage according to main lines of IEC 60156 specifications. The Normal and Weibull distribution functions are used to analyze and estimate the BDV probability of these oils. The hypothesis testing is then used to select the distribution functions suitability. We mainly analyze the histogram, frequency, mean, standard deviation and estimate the BDV probability of the sample population. The considered liquids are three different mineral oils (MO3', MO4' and MO5), one synthetic ester (SE1) and three different natural esters (VO2, VO3 and VO4). It's shown that the measured BDV data follow better Normal than Weibull distribution. Basing on the BDV at the levels of risk 1%, 10% and 50% probability for U1%, U10% and U50%, the dielectric withstand of ester oils, under AC, homogeneous field and small gap, is slightly better compared to mineral oils. PCA (Principal Component Analysis) method is also used to analyse the BDV variance of tested oils. View full abstract»

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  • Investigation of partial discharge activity by a conducting particle in transformer oil under harmonic AC voltages adopting UHF technique

    Publication Year: 2012 , Page(s): 1514 - 1520
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    A methodical experimental study was carried out to understand the partial discharges initiated due to particle movement in transformer oil under high frequency ac voltage and under harmonic ac voltages (with different THDs), by adopting UHF technique. It is also observed that irrespective of order of harmonics, increase in THD of the supply voltage shows reduction in levitation voltage. The rise time of the partial discharge current pulse formed due to particle movement is few ns. It is observed that the frequency content of the signal radiated due to partial discharges generated due to particle movement in the electrode gap lies in the UHF signal range, with its dominant frequency near 1 GHz. The supply voltage frequency and the time required for 100 UHF signals to form shows inverse relationship. When the applied voltage is increased above levitation voltage, the UHF signal magnitude formed due to particle movement increases, irrespective of supply voltage frequency. Ternary diagram clearly indicates that irrespective of frequency of the supply voltage, the UHF signal location in the ternary diagram is the same. View full abstract»

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  • Investigations on streamers phenomena in mineral, synthetic and natural ester oils under lightning impulse voltage

    Publication Year: 2012 , Page(s): 1521 - 1527
    Cited by:  Papers (5)
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    This paper presents the experimental results of a comparative study of mineral, synthetic and natural ester oils issued from different origins through the analyze of the initiation threshold voltage of streamers, their pattern and their stopping length (final length) Lf as well as the associated current and electrical charge, in a point - plane electrodes arrangement submitted to a standard lightning impulse voltage (1.2/50 μs). It's shown that the streamers are filamentary in the different tested oils whatever the polarity of voltage. The stopping lengths Lf are longer and the conductivity higher when the point is positive than when it's negative. Lf is generally longer in ester oils (natural and synthetic) than in some mineral oils. When the point is negative, Lf can be ten times higher in ester oils than in mineral ones. Different modes of propagation of streamers are also observed; the transition between these modes of propagation depends on the experimental conditions and the molecular structure of liquid. This transition is well pronounced with some mineral oils when the point is negative. View full abstract»

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  • Isothermal microcalorimetry as a tool for studying oxidation stability of insulating liquids

    Publication Year: 2012 , Page(s): 1528 - 1536
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    The application of an isothermal microcalorimeter for the investigation of the oxidation stability of insulating liquids has been studied. We propose that the total integral heat signal (area below the calorimeter curves) is a good indicator of the degree of oxidation taking place in the sample during the experiment. Known and unknown insulating liquids have been used in this study, and the amount of available oxygen in the samples has been varied. The results show that isothermal microcalorimetry is well suited for characterizing the oxidation stability of insulating liquids. View full abstract»

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  • Ester oils and flow electrification hazards in power transformers

    Publication Year: 2012 , Page(s): 1537 - 1543
    Cited by:  Papers (3)
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    In large power transformers where forced convection of the insulating liquid is the predominant heat transfer mean, the electrification risk can appear. This phenomenon has been suspected to be responsible for several failures. Studies have shown, in addition to the transformer design (shell or core type), that the type of liquid used is quite important in limiting these hazards. The goal of this paper is to present an analysis of ester oil behavior versus electrostatic hazard due to flow electrification. Two experimental studies were conducted on different ester oils: the first one is for measuring the Electrostatic Charging Tendency (ECT) by using ministatic charge tester, and the second one is made by mean of a test loop simulating the oil flow as in a real transformer and equipped with a capacitive sensor. This sensor is able to measure both the charge accumulation at the pressboard surface and the generating current. All these results are compared with those of mineral oils which are actually used for large power transformers. It was shown that even if ester oils increase the charge generation in comparison with mineral oil, the charge accumulation on the solid surface is not excessive thus limiting the potential at the solid/liquid interface and associated electrification hazards. One can assume that the higher conductivity of esters would enable a better evacuation of generated charges. View full abstract»

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  • Assessment of transformer mineral oil condition based on dc and ac conductivity

    Publication Year: 2012 , Page(s): 1544 - 1551
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (364 KB) |  | HTML iconHTML  

    Due to the increase of electrical energy consumption, the power transformers are loaded beyond nameplate rating. The degradation of their insulation systems is intensified and the dielectric properties are worsened. Therefore, the interest of asset managers, regarding the condition of their transformers is a very important topic. Electrical conductivity is strongly influenced by the high operating temperatures and catalysis products (water, gases, acids) that appear as a result of oil and paper degradation reactions during transformer operation. The present paper is focused on an experimental study regarding the influence of water, acids and gases (hydrogen, acetylene, ethylene, methane etc.) resulted from degradation processes on electrical conduction phenomena of mineral oil. For that, the frequency/time variation curves of ac/dc conductivity for different concentrations of water, acids and gases are drawn. Finally, the correlation between ac and dc conductivity and the possibility to assess the ageing state of the mineral oil based on these quantities are analyzed. View full abstract»

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  • Breakdown processes in gas micro-bubbles in liquids under electric stress

    Publication Year: 2012 , Page(s): 1552 - 1558
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    The present work is concerned with a theoretical analysis of the breakdown characteristics of gas-filled micro-bubbles formed in insulating liquids stressed with electric field. It is assumed that the gas inside these bubbles is air which allows the use of experimental Paschen curve data for air in this analysis. Two main discharge mechanisms have been considered, the Townsend discharge and impulse breakdown. The combination of bubble diameter, D, gas pressure, p, and duration, τ, of the field stress determines the type of breakdown. Parameters which are required for the Townsend mechanism of breakdown and impulse breakdown to occur inside gas bubbles have been obtained and these conditions have been represented as boundary lines in the (Dp, τp) coordinate system. It is shown that there are such combinations of these parameters which satisfy neither Townsend nor impulse breakdown conditions. Experimental data on breakdown in air for these intermediate values of (Dp, tp) between the Townsend and the impulse discharges are not available in the literature and the breakdown behavior under such conditions is not well defined. View full abstract»

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  • Impulse breakdown of water with different conductivities

    Publication Year: 2012 , Page(s): 1559 - 1568
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    The complexity of the impulse breakdown of liquid water is reflected by the dependency of pre-breakdown processes on the polarity, rise-time and wave-shape of the applied impulses as well as on physical properties, such as the electrical conductivity of water itself. Further understanding of the mechanisms of formation and propagation of impulse discharges in water and water solutions is therefore required to enable the development of pulsed power and plasma technologies. This paper presents a study of the dielectric behaviour of water stressed with positive and negative high voltage impulses in a point-plane electrode topology. Water with different conductivities including distilled water and tap water was investigated. The volt-time breakdown characteristic of water is discussed and the total pre-breakdown time has been obtained for both positive and negative polarity impulses for 2 mm, 5 mm and 10 mm inter-electrode distances. The Laue statistical analysis has been used in order to calculate the statistical and formative time lags. Using the formative time data the nominal breakdown velocity in water for positive and negative impulses has been obtained. It has been shown that the anode discharges propagate with supersonic velocities, up to ~20×105 cm/s, and the cathode discharges are sub-sonic with a maximal velocity of ~c1x105 cm/s. Discharge velocities are slightly higher in distilled water as compared with tap water. Potential breakdown mechanisms which can explain the observed polarity effects in the transient prebreakdown processes in liquid water stressed with high voltage impulses are discussed. View full abstract»

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  • Electro-acoustic detection, identification and location of partial discharge sources in oil-paper insulation systems

    Publication Year: 2012 , Page(s): 1569 - 1578
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (4855 KB) |  | HTML iconHTML  

    The detection of Partial Discharges (PD) is a reliable technique to analyze the status of electrical insulation in power transformers. Phase resolved partial discharge patterns are being complemented with high frequency pulse waveform analysis, in order to identify discharge sources. In addition to this, acoustic techniques are being implemented trying to locate PD sites in large power transformers. In this work, an inductive loop sensor will be used to identify two different PD sources by means of the energy distribution of the detected waveforms. Additionally, these PD sources will be located by means of acoustic measurements with an electrical reference, through the analysis of the acoustic activity detected for each sensor individually. View full abstract»

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  • Theoretical study of the initial stage of sub-nanosecond pulsed breakdown in liquid dielectrics

    Publication Year: 2012 , Page(s): 1579 - 1582
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    This paper presents a simple theoretical analysis of the initial stage of a sub-nanosecond pulsed high-voltage breakdown in liquid dielectrics. We show that a rapid breakdown is related to a rupture in the continuity of fluid induced by the electrostrictive forces in the inhomogeneous electric field in the vicinity of a needle electrode. View full abstract»

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  • Aged oils reclamation: Facts and arguments based on laboratory studies

    Publication Year: 2012 , Page(s): 1583 - 1592
    Cited by:  Papers (1)
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    Lifetime extension of power transformers is a subject of high importance for electric power systems utilities. The decision of replacing, refurbishing or repairing a service aged power transformer requires considering several factors, especially the cost and time to execute the work. The lifetime of the power transformer being related to the condition of the insulation system; one way of improving the situation is to reclaim insulating oil by Fuller's Earth treatment. This procedure is economically attractive because of increasing prices for both mineral and synthetic transformer coolants, effective cost and environmentally sounds. Reclamation rejuvenates the transformer oil by eliminating contaminants. In this paper, a series of experiments has been performed with service aged oils reclaimed in laboratory conditions. Fast, inexpensive and reliable laboratory testing procedures developed by ASTM (D 6802 and D6181) were also used to monitor decay products as traces impurities. The results obtained in laboratory conditions, indicate that a large number of reclamation passes (around 15 passes) are required to regenerate inservice aged oil to a grade close to new oil. It is also shown that not only the reclamation improves the gassing tendency of oil, but also the type of Fuller's Earth is very important for reclamation process. By upgrading the analytical chemistry of oil, the predictive maintenance of this non-renewable resource is modernized, its cost diminished and the service reliability of transformers enhanced. Currently, Fuller's Earth is only used once. After depletion it must be disposed of in a land fill. By using two organic solvents that are recoverable by atmospheric distillation, it is shown, in laboratory conditions, that this mineral absorbent can be reactivated and successfully reused many times. View full abstract»

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  • Influence of impurities and additives on positive streamers in paraffinic model oil

    Publication Year: 2012 , Page(s): 1593 - 1603
    Cited by:  Papers (4)
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    This paper presents an experimental study of positive streamers in paraffinic model oil in a point-plane gap. The investigation was performed concerning the influence of voltage, impurities and additives. It was seen how increased voltage changes streamer shape and speed resulting in different propagation modes. Carbon particles markedly reduce inception, breakdown and acceleration voltages, while dissolved gases/air have little effect on these voltages. A low ionization potential additive decelerates the streamers, while an electron scavenger shows an increase in streamer velocity. The correlation between propagation velocity and streamer's branching is discussed. View full abstract»

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  • Rapid determination of oxidation stability for transformer oils with antioxidant

    Publication Year: 2012 , Page(s): 1604 - 1608
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    The service life of transformer oil is directly influenced by oxidation stability of the oil. However, the standard method of ASTM or IEC to assess oxidation stability of transformer oil is very time-consuming. After 2 h of accelerated aging of transformer oil at specified conditions of temperature, oxygen flow rate and catalyst, the consumption rate of antioxidant was measured by high performance liquid chromatography. By researching the relation between the consumption rate of antioxidant and acid value after 164 h oxidation, the quick assessment method for oxidation stability of transformer oil with antioxidant was established, and the assessment criterion for this method was proposed. The feasibility of the method, with its advantages of rapidness and easiness, has been proved through experiments using samples of in-operation transformer oil. View full abstract»

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  • DGA comparison between ester and mineral oils

    Publication Year: 2012 , Page(s): 1609 - 1614
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    This paper reports on a comparative study between mineral and various ester oils based on Dissolved Gas Analysis (DGA). Electrical (breakdown of low energy) and thermal (stray gassing) faults were realized at laboratory scale, then gas formation as well as diagnosis methods were compared. Two extraction methods, described in IEC 60567 Ed.3 and independent from partition/solubility coefficients were used: multi-cycle vacuum degassing and stripping. Three different diagnosis methods were applied: IEC gas ratios, Duval's triangle and key gases. For electrical faults and especially for low energy discharges, the same main gases (Hydrogen and Acetylene) are created in relatively same ratios, thus confirming the efficiency of key gases method for ester oils. It is also shown that classic Duval's triangle (triangle 1) is well applicable for this type of fault confirming the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters. The new Duval¿s triangles can be applied: Triangle 4 for mineral oil and Triangle 6 for ester oils. For extraction methods and in both electrical and thermal faults cases, it can be noted that results are comparable for mineral oil whereas they are slightly underestimated with stripping method for ester oils. The higher viscosity of ester could interfere with the extraction. View full abstract»

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  • A new mathematical model of moisture equilibrium in mineral and vegetable oil-paper insulation

    Publication Year: 2012 , Page(s): 1615 - 1622
    Cited by:  Papers (1)
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    Moisture equilibrium characteristics of oil-paper insulation systems are important for insulation condition evaluation of oil-filled transformers. This paper presents moisture equilibrium curves of mineral and vegetable oil-paper insulation systems obtained by both direct and indirect methods. A new mathematical model is proposed to determine moisture content in oil-paper insulation systems. Experiments on moisture equilibrium in oil-paper insulation systems are conducted. The results estimated through the mathematical model agree with experimental results. The moisture equilibrium curves between vegetable and mineral oil-paper insulation systems are presented and compared. The moisture content in the vegetable oil-impregnated paper is significantly smaller than that in the mineral oil-impregnated paper, when both types of insulating oils have the same moisture content. View full abstract»

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  • Electrical conduction and dielectric breakdown characteristics of alkyl ester dielectric fluids obtained from palm kernel oil

    Publication Year: 2012 , Page(s): 1623 - 1632
    Cited by:  Papers (3)
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    Naturally occurring palm kernel oil (PKO) and its ester derivatives are being considered as sustainable alternatives to synthetic oils for use as dielectric fluids. This paper reports on their dielectric properties, which have been studied and compared to BS148 mineral oil. The low frequency complex dielectric response of the PKO and its derivatives are related to ionic conduction and electrode polarization phenomena. The purified PKO has an electrical conductivity of 3.04 × 10-12 Sm-1 at 30°C; this is 10 times lower than crude PKO but about 10 times greater than the BS148 oil. The bulk conductivity is thermally activated, activation energy = 0.47 eV, and influenced by viscosity. The ester derivatives have a higher conductivity than the PKO, which is related to ionic impurities introduced during processing. The breakdown field was measured in a bespoke cell enabling smaller volumes of oil (15 ml) than that used in ASTM D1816. The characteristic ac breakdown strength of purified PKO and is alkyl esters were found to be in the range, 41 to 43 kV/mm, which is significantly higher than the mineral oil (27 kV/mm). The results support the proposition that a dielectric fluid derived from palm kernel oil, once re-purified, may be a suitable replacement for mineral oil based fluids in HV electrical equipment. View full abstract»

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  • IEC 62697-2012: State of the art methods for quantification of DBDS and other corrosive sulfur compounds in unused and used insulating liquids

    Publication Year: 2012 , Page(s): 1633 - 1641
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (363 KB) |  | HTML iconHTML  

    This article describes state of the art methods developed by IEC TC-10 WG-37 for quantitative determination of a highly corrosive sulfur compound dibenzodisulfide (DBDS) and other corrosive sulfur compounds in unused and used insulating liquids. The methods permit an objective approach for assessing corrosiveness of insulating liquids rather than subjective assessment based on color perception that is prevalent in the current standard test methods for corrosive or potentially corrosive sulfur in insulating liquids. In addition, quantification of total corrosive sulfur (TCS) in insulating liquids permits an objective ranking of sulfur compounds according to their corrosivity towards copper. View full abstract»

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Aims & Scope

IEEE Transactions on Dielectrics and Electrical Insulation contains topics concerned with dielectric phenomena and measurements with development and characterization of gaseous, vacuum, liquid and solid electrical insulating materials and systems.

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Meet Our Editors

Editor-in-Chief
Reuben Hackam