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  • 1.
    Ariza, David
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Becerra, Marley
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Hollertz, Rebecca
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi, Fiberteknologi.
    Wågberg, Lars
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Pitois, Claire
    ABB AB Corporate Research.
    First Mode Negative Streamers along Mineral Oil-solid Interfaces2017Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, nr 4Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This document presents an experimental study on the propagation of first mode negative streamers along mineral oil-solid interfaces. Samples made of an oil impregnated kraft paper and a low-porosity paper made from cellulosic micro and nano fibrils, as well as different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)) are used as the solid. A comparison of the length, charge and velocity of streamers for all different mineral oil-solid interfaces is reported. It is shown that streamers propagate longer and faster along mineral oil-solid interfaces with low surface roughness, low porosity and higher electrical permittivity than mineral oil. Those streamers show a quasi-continuous injection of charge in the early stage of their propagation. This quasi-continuous charge injection consists of a sequence of small charge steps separated by few tens of nanoseconds in between. In comparison, the streamers that propagate along surfaces with similar permittivity to the mineral oil have lower injection of charge and higher stopping voltage conditions than streamers propagating free in the liquid without any solid barrier.

  • 2.
    Becerra, Marley
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    On the Estimation of the Charge of Positive Streamers Propagating in Air2014Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, nr 2, s. 627-634Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Streamer discharges are an important breakdown mechanism in air-based electrical insulation systems. This paper introduces a method to estimate the spatial distribution of the charge density of positive streamers in air, based on the solution of a Poisson inverse problem by optimization. In contrast to other methods, it does not require tuning parameters and can also be used in configurations including dielectric interfaces or preexisting space charge. Three different experimental datasets reported in the literature are used to validate the method. Good agreement between the measurements and the predictions of the method is found.

  • 3.
    Berg, Marcus
    et al.
    Uppsala University, Sweden.
    Thottappillil, Rajeev
    Uppsala University, Sweden.
    Scuka, Viktor
    Uppsala University, Sweden.
    Hydrophobicity estimation of HV polymeric insulating materials. Development of a digital image processing method.2001Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 8, nr 6, s. 1098-1107Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Image analysis of water drop patterns on an inclined flat polymeric insulator surface has been performed in order to find a simple mathematical function that indicates the level of hydrophobicity of the insulator surface. A simple function, given the acronym average of normalized entropies (ANE), seems to correlate well with hydrophobicity as defined by the Swedish Transmission Research Institute (STRI) hydrophobicity classification. It is a composition of three other functions, viz. the standard deviation, the Shannon information entropy and the 'fraction of small differences'. All these are in turn based on the histogram of horizontal nearest-neighbor pixel differences for a given digital greyscale image of a water drop pattern. ANE is fairly independent of illumination intensity (exposure) as well as total gain and offset in a camera system (linear sensor). The experimental results also indicate that ANE is fairly independent of limited changes in the surface inclination, although this needs further investigation. Some of the various pitfalls associated with the photography of water drop patterns and processing of images are identified, together with possible solutions for avoiding them.

  • 4. Castro, L. C.
    et al.
    Oslinger, J. L.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES).
    Wåhlander, Martin
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Dielectric and Physico-chemical Properties of Epoxy-Mica Insulation During Thermoelectric Aging2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 6, s. 3107-3117Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Three stator bars were subjected to 1142 hours of thermoelectric aging. At intervals during this time, the aging of the insulation was investigated by capacitance (C) and dissipation factor (DF) measurements and by the physicochemical techniques of infrared spectroscopy (FTIR) and Differential Scanning Calorimetry (DSC). Results indicated different degradation paths under discharge conditions, where the volume of voids within the insulation played a major role. Postcuring reactions were predominant during the first aging cycles. Such reactions were detected as reductions in capacitance and dissipation factor parameters, as a depletion of the chemical group epoxide at the absorption band 909 cm(-1) as measured by FTIR, and as an increase of the glass transition temperature (T-g) of the epoxy resin. Since the postcuring reactions occur globally in the insulation, they were detected by both dielectric and physicochemical techniques. With the advance of the aging program the epoxy resin underwent structural changes, seen as modifications of functional groups CH2, CH3 and C=O. The air atmosphere where partial discharges occurred also promoted the formation of reactive species. When the air volume was high enough, the findings of FTIR measurements suggested a local degradation mechanism of mica involving cation exchange reactions between partial discharge byproducts and potassium layers in the mica.

  • 5. Chen, Xiangrong
    et al.
    Murdany, Deni
    Liu, Dongming
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Andersson, Mattias
    Gubanski, Stanislaw M.
    Gedde, Ulf W.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Suwarno, S.
    AC and DC Pre-stressed Electrical Trees in LDPE and its Aluminum Oxide Nanocomposites2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, nr 3, s. 1506-1514Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Resistance of pure low density polyethylene (LDPE) and its aluminum oxide nanocomposites (up to 3.0 wt%) to degradation by electrical treeing under AC stress and DC pre-stress is analyzed. The experiments were carried out on wire-plane electrode specimens before and after exposure to thermal and DC electro-thermal ageing at 80 degrees C. The obtained results showed enhanced resistance of the nanocomposites to electrical tree inception under AC stress and the tree inception voltage (TIV) increased with nanoparticles content. It has been shown that there was an improved partial discharge (PD) resistance in the nanocomposites compared to the unfilled LDPE. The results also showed that the AC TIV in the nanocomposites consistently increased with the ageing and especially the DC electro-thermally aged specimens had about 30% higher the AC TIV as compared to the unaged material. This effect is attributed to significantly reduced mobility of charge carriers in the nanocomposites. The DC pre-stressed electrical trees generated in the investigated materials were of filamentary-branch structure and the branch channels content increases with the addition of nanoparticles. The mean tree number of the DC prestressed electrical trees decreased in the LDPE and its nanocomposites while the mean maximum tree length increased with the ageing treatments. It is postulated that material recrystallization and a very high electric field level on the wire electrode during the DC pre-stressed electrical tree test are the main reasons for the observed effects.

  • 6.
    Edin, Hans
    et al.
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Gäfvert, Uno
    Simultaneous measurement of phase resolved partial discharges and dielectric spectroscopyInngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Artikkel i tidsskrift (Annet vitenskapelig)
  • 7.
    Edin, Hans
    et al.
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Jayaram, Shesha
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Gäfvert, Uno
    influence of relative humidity on surface discharge over the frequency range 01 hz to 100 hzInngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Artikkel i tidsskrift (Annet vitenskapelig)
  • 8.
    Forssén, Cecilia
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Measured Applied Frequency Dependence of Partial Discharges in Disc-Shaped Cavities2007Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Artikkel i tidsskrift (Annet vitenskapelig)
  • 9.
    Forssén, Cecilia
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Partial Discharges in a Cavity at Variable Applied Frequency: Part 1: Measurements2008Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 15, nr 6, s. 1601-1609Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper partial discharges (PD) in disc-shaped cavities in polycarbonate are measured at variable frequency (0.01 - 100 Hz) of the applied voltage. The advantage of PD measurements at variable frequency is that more information about the insulation system may be extracted than from traditional PD measurements at a single frequency (usually 50/60 Hz). The PD activity in the cavity is seen to depend on the applied frequency. Moreover, the PD frequency dependence changes with the applied voltage amplitude, the cavity diameter, and the cavity location (insulated or electrode bounded). It is suggested that the PD frequency dependence is governed by the statistical time lag of PD and the surface charge decay in the cavity. This is the first of two papers addressing the frequency dependence of PD in a cavity. In the second paper a physical model of PD in a cavity at variable applied frequency is presented.

  • 10.
    Forssén, Cecilia
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Partial discharges in a cavity at variable applied frequency part 2: Measurements and modeling2008Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 15, nr 6, s. 1610-1616Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    In this paper partial discharges (PD) in a disc-shaped cavity are measured at variable frequency (0.01 - 100 Hz) of the applied voltage. The measured PD phase and magnitude distributions, as well as the number of PDs per voltage cycle, changed with the varying frequency. A charge consistent model is presented and used to dynamically simulate the sequence of PDs in the cavity. The simulation results show that the properties of the cavity surface, mainly the surface conductivity and the surface emission of electrons, change with the varying applied frequency. This is interpreted as an effect of the difference in time between consecutive PDs at different applied frequencies. This is the second of two papers addressing the frequency dependence of PD in a cavity. The first paper described how the PD frequency dependence changes with the applied voltage amplitude, the cavity size and the cavity location.

  • 11.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Janus, Patrick
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Wang, Xiaolei
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Kiiza, Respicius Clemence
    DIT Dar Es Salaam Institute of Technology, Tanzania .
    Partial Discharges in a Cavity Embedded in Oil-Impregnated Paper: Effect of Electrical and Thermal Aging2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 2, s. 1071-1079, artikkel-id 7076808Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, the rapid aging due to Partial Discharge (PD) activity was investigated in unaged and thermally aged oil-impregnated paper with a disk-shaped cavity between the sheets of paper. The PD inception voltage and the voltage for instantaneous breakdown were measured, and the time to breakdown was measured by applying voltages less than the instantaneous breakdown voltage. A comparison between time to breakdown of unaged and thermally aged paper shows no significant difference between thermally aged and unaged paper at sustaining PD activity. The time to breakdown as a function of applied electric field was plotted and compared for both cases showing points scattered around a line in log-log scale. Changes of the PD parameters, such as PD magnitude and PD repetition rate were analyzed from the beginning of PD activity up to the moment of breakdown. The results show that the number and magnitude of PD increase at the beginning of aging until they reach to a maximum value, then both quantities decrease slowly over time until the final puncture breakdown occurs. The results emphasize the importance of PD monitoring on real equipment with oil-impregnated paper as insulation system, such as power transformers, since a focus on the number and magnitude of PD at just the present time may mislead the interpretation. Dielectric spectroscopy measurements performed on unaged and thermally aged paper showed an increasing trend of epsilon '' after the sheets of paper were exposed to thermal aging for longer time. Dielectric spectroscopy measurements performed on samples before and after exposure to PD activity showed a big change of epsilon '', This change can be attributed to byproducts and ions produced by PD activity.

  • 12. Ghorbani, H.
    et al.
    Saltzer, M.
    Abid, Fahim
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Effect of heat-treatment and sample preparation on physical properties of XLPE DC cable insulation material2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, nr 5, s. 2508-2516, artikkel-id 7736807Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Power cables with extruded crosslinked polyethylene (XLPE) insulation are used in HVAC and HVDC applications. These cables usually go through a degassing process to remove the methane formed in XLPE during the cross-linking reactions. In case of HVDC cables, the general belief is that the polar peroxide decomposition products (PDP) content significantly influences the conduction and space charge behavior of XLPE. This belief is mainly based on the results of experiments made on thin XLPE samples with different heat-treatments; but since heat-treatment also influences the morphology of the polymer, it is necessary to consider this effect as well. It is common to use polyethylene terephthalate (PET) film as a protective layer during sample press molding. Studies on the influence of the pressing film on the electrical properties of the sample are rare. In this work, the results of a series of experiments performed on 0.5 mm thick XLPE plaque samples in reference to additive free LDPE samples with different heat-treatment times are presented. Beside the PDP content, the morphology, DC conductivity and polarization properties are studied and analyzed. It was found that the pressing film used during sample preparation has a significant effect on the results and if not corrected, it may lead to wrong conclusions about the influence of the PDP content. Eliminating the effect of the pressing film, no clear correlation between the DC conductivity, dielectric loss and the PDP content was observed. The relation between the PDP content and DC conductivity is not found to be obvious, hence this correlation may need to be further evaluated.

  • 13. Gustavsson, T. G.
    et al.
    Gubanski, S. M.
    Hillborg, H.
    Karlsson, Sigbritt
    KTH, Tidigare Institutioner                               , Polymerteknologi.
    Gedde, Ulf W.
    KTH, Tidigare Institutioner                               , Polymerteknologi.
    Aging of silicone rubber under ac or dc voltages in a coastal environment2001Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 8, nr 6, s. 1029-1039Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Material samples of silicone rubber with known differences in their composition, i.e. different filler content and extra silicone oil added, have been aged at the Anneberg field station on the west coast of Sweden. ac or dc voltage supplied to cylindrical samples at stress levels of 50 or 100 V/mm. The work includes laboratory examination of material changes together with on-site, visual observations and leakage current measurements. Material samplings for the laboratory tests were made after 18 months of electrical aging, which went on for one more year in order to gather further information on the long-term electrical performance of the material. The dominant aging factor was the level of the applied stress, independent of ac or dc voltage. The dc stressed samples showed a higher leakage current and exhibited larger surface degradation compared with samples exposed to ac voltage. The material parameter, an addition of extra silicone oil, initially lead to an increase in adhesion of pollutants, whereas the overall performance was improved by the higher suppression of the leakage current related to oligomer diffusion. Samples with lower level of alumina trihydrate (ATH) exhibited a delayed onset of degradation, but once damaged they degraded more rapidly than the specimens with a higher ATH content. Infrared spectroscopy showed that the ATH was completely consumed at the eroded surface regions. The aging of the surfaces was further assessed by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The low molar mass siloxanes present in the pollution layer were extracted and analyzed by size exclusion chromatography and gas chromatography-mass spectroscopy. The results indicated that the main degradation factor was thermal depolymerization activated by electrical discharges. Oxidative crosslinking of the silicone rubber, usually attributed to surface close corona discharges, appeared to have played a minor role.

  • 14.
    Hesamzadeh, Mohammad
    et al.
    Central Queensland University, Power Engineering Group, Rockhampton, QLD, Australia .
    Hosseinzadeh, N.
    Wolfs, P.
    An advanced optimal approach for high voltage AC bushing design2008Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, ISSN 1070-9878, Vol. 15, nr 2, s. 461-466Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This paper proposes a new and advanced methodology for finding the optimum electrical design of high voltage ac capacitive graded bushings using an improved genetic algorithm approach as an effective meta-heuristic method. A case study has been conducted on a 145 kV oil impregnated paper (OIP) bushing and the IEC 60137 tests have been performed to evaluate its performance. Condenser-bushings contain concentric conductive foils which are isolated from each other. The partial capacitances between conducting cylinders can be modified by adjusting the number, diameter, place and length of these cylinders as well as the thickness of insulating material between foils. As a result, the voltage drop and also the electrical stress in the core and along the surface will change. This paper finds optimal value of bushing design parameters to achieve well-distributed electric stress with the lowest possible maximum value and also a constant voltage drop for different layers by using an improved genetic algorithm optimization method subject to practical and technological constrains. The proposed method of this research work has been applied to a 145 kV OIP bushing. The performance of optimal designed 145 kV OIP bushing under IEC 60137 tests is very promising.

  • 15.
    Hollertz, Rebecca
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Wågberg, Lars
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi. KTH, Skolan för kemivetenskap (CHE), Centra, Wallenberg Wood Science Center.
    Pitois, C.
    Effect of Composition and Morphology on the Dielectric Response of Cellulose-based Electrical Insulation2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 4, s. 2339-2348Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Different wood-fibre based papers were characterized by dielectric spectroscopy, mechanical testing and microscopy. The data obtained were utilized to investigate the relationship between the chemistry, morphology and density of a paper and its permittivity and dielectric loss. The density strongly influences the dielectric response, but the response is not affected by the way the density has been achieved; by pressing the paper during drying or by mechanical treatment of the fibres before sheet preparation. The chemical composition of the pulp influences the polarization, dielectric loss and charge transport. It was found that paper-vacuum and paper-oil combinations can be represented by series-equivalent circuits. The permittivity of paper made from electrical grade kraft pulp, used in e.g. high voltage transformers, without any porosity is estimated to be 5.3 and tan delta to 0.01-0.02 at 50 Hz and 70 degrees C. The lignin and hemicellulose content of the kraft pulp do not affect the real part of the permittivity significantly, but the dielectric losses increase with increasing lignin and hemicellulose content in both oil and vacuum at 50 Hz and 70 degrees C.

  • 16.
    Hörnfeldt, Sven P.
    KTH, Skolan för elektro- och systemteknik (EES).
    Lifting force on metallic particles in GIS systems2006Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 13, nr 4, s. 838-841Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dielectric coatings have shown to reduce the activity of metallic particles in GIS systems and thus hinder the development of particle initiated electrical breakdown. In this report we present an experimental method to investigate the charge exchange between metallic particles and the coating and the conditions for particle lift off. A few experimental results are also presented. The system comprises a balance mechanism with which the force needed to lift the particle from the coating can be measured as a function of an applied electric field. An electronic servo system makes it possible to keep the particle on a given distance from the coating and at the same time determine the force needed to keep the particle in place. The report also contains suggestions of investigations that can be carried out with the device to clarify the desired properties of an optimal coating to suppress particle initiated electric breakdown.

  • 17.
    Jäverberg, Nadejda
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Nordell, Patricia
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Nawaz, Sohail
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Azhdar, Bruska
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Gedde, Ulf
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Dielectric Properties of Alumina-filled Poly (ethylene-co-butyl acrylate) Nanocomposites Part I - Dry Studies2012Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 19, nr 2, s. 383-390Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The dielectric properties of poly (ethylene-co-butyl acrylate) with 13 wt% of butyl acrylate (EBA) filled with alumina nanoparticles were studied as a function of particle size, particle coating, filler content (2, 6 or 12wt%) and temperature. The particles were either unmodified or surface-treated with either aminopropyltriethoxy silane or octyltriethoxy silane. The complex permittivity was measured with frequencies varying between 1 mHz and up to 1 kHz at different temperatures (25, 45 and 65 degrees C). Measurements were performed in dry conditions. From the dielectric spectroscopy measurements it can be seen that the tan delta for the dry samples is almost independent of the type of the particle coating used. It does, however, depend on the particle size, filler content and temperature.

  • 18.
    Jäverberg, Nadejda
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Nordell, Patricia
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Nawaz, Sohail
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Hillborg, Henrik
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Azhdar, Bruska
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Gedde, Ulf
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Dielectric Properties of Alumina-filled Poly (ethylene-co-butyl acrylate) Nanocomposites Part II - Wet Studies2012Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 19, nr 2, s. 391-399Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The influence of moisture on the dielectric properties of different types of poly (ethylene-co-butyl acrylate) filled with alumina nanoparticles was systematically investigated by varying the type of aluminum oxide, particle surface treatment and filler content (2, 6 and 12 wt%). The nanoparticles were either unmodified or surface-treated with either aminopropyl triethoxysilane or octyltriethoxy silane. The complex permittivity was measured with an IDA200 dielectric spectroscopy analyzer at applied voltage of 200 V-peak with frequencies varying between 1 mHz and 1 kHz. The measurements were performed at 25 degrees C and relative air humidities of 24, 54 and 86 %. In order to expand the frequency range the HP 4284A precision LCR meter was used for measuring sample impedance at the frequency range of 100 Hz to 1 MHz. From the frequency dependence of dielectric losses it can be seen that absorbed water plays a significant role in determining the dielectric properties of the nanocomposites. The magnitude of tan delta seems to be determined primarily by the particle size and filler content, while the peak frequency is mostly influenced by the amount of absorbed water and the type of particle coating.

  • 19.
    Jäverberg, Nadejda
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Venkatesulu, B.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Hillborg, H.
    Prebreakdown Current and DC Breakdown Strength of Alumina-filled Poly(ethylene-co-butyl acrylate) Nanocomposites: Part I - Breakdown Strength2014Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, nr 5, s. 2127-2134Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dielectric breakdown strength of poly (ethylene-co-butyl acrylate) (EBA) nanocomposites filled with spherical alumina particles was studied as a function of particle coating, moisture content and the effective specific surface area. Two kinds of alumina nanoparticles were used: NA (25 nm in diameter) or ND (45 nm in diameter). The particles were either unmodified or surface-treated with aminopropyltriethoxy silane or octyltriethoxy silane. Two different relative humidities of air were used for conditioning the samples prior to testing: 0 % RH (dry case) and 86 % RH (samples were saturated with moisture). The dielectric breakdown strength measurements were performed with a DC ramp of 1.2 kV/s. Twenty samples were tested for each material formulation - ten of which were tested in dry conditions and ten were tested after conditioning in humid environment. Results were compared to the reference unfilled EBA samples. Applying Weibull analysis to the measured breakdown voltages indicated that the material filled with amino-coated ND particles exhibited the highest breakdown strength under dry conditions. As expected, conditioning materials in a humid environment had a negative impact on breakdown strength. This effect was more pronounced for nanocomposite materials, compared to the unfilled reference material.

  • 20.
    Jäverberg, Nadejda
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Venkatesulu, B.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Hillborg, H.
    Prebreakdown Current and DC Breakdown Strength of Alumina-filled Poly(ethylene-co-butyl acrylate) Nanocomposites: Part II - Prebreakdown Currents2014Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, nr 5, s. 2135-2145Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Pre-breakdown currents through the poly (ethylene-co-butyl acrylate) (EBA) nanocomposites filled with spherical alumina particles were studied as a function of particle coating and moisture content. The nanocomposites were prepared from a mixture of EBA formulations (13 wt% butyl acrylate groups content) and alumina powder. Two kinds of alumina nanoparticles were used: NA (25 nm in diameter) and ND (45 nm in diameter). The particles were either unmodified or surface-treated with aminopropyltriethoxy silane or octyltriethoxy silane. Two different relative humidities of air were used for conditioning the samples prior to testing: 0 % RH (dry case) and 86 % RH (samples were saturated with moisture). The pre-breakdown current measurements were performed with a DC ramp of 1.2 kV/s. Twenty samples were tested for each material formulation - ten of which were tested in dry conditions and ten were tested after conditioning in humid environment. Results were compared to the reference unfilled EBA samples. The measured currents were analyzed under the assumption of space charge limited conduction. As expected, conditioning materials in humid environment caused an increase in pre-breakdown currents. This effect was more pronounced for nanocomposite materials, while the reference unfilled material was least affected.

  • 21.
    Jäverberg, Nadejda
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Venkatesulu, Bandapalle
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Hillborg, Henrik
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Polymera material.
    Prebreakdown Current and Breakdown Strength of Alumina-filled Poly(ethylene co-butyl acrylate) Nanocomposites: Part I - Breakdown StrengthInngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Artikkel i tidsskrift (Annet vitenskapelig)
  • 22.
    Jäverberg, Nadejda
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Venkatesulu, Bandapalle
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Hillborg, Henrik
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Polymera material.
    Prebreakdown Current and Breakdown Strength of Alumina-filled Poly(ethylene co-butyl acrylate) Nanocomposites: Part II - Prebreakdown CurrentsInngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Artikkel i tidsskrift (Annet vitenskapelig)
  • 23.
    Kiiza, Respicius Clemence
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Ghaffarian Niasar, Mohamad
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Nikjoo, Roya
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Wang, Xiaolei
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Change in Partial Discharge Activity as Related to Degradation Level in Oil-Impregnated Paper Insulation: Effect of High Voltage Impulses2014Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 21, nr 3, s. 1243-1250Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    High voltage (HV) transients in electrical power systems are mainly caused by lightning strikes and switching operations. The dielectric properties of a particular electrical insulation at the instant of interaction with a high voltage transient would determine a level of the degradation of an electrical insulation. This work investigates how high voltage impulses would change Phase Resolved Partial Discharge (PRPD) patterns in test samples consisting of a cavity deliberately introduced between the sheets of oil-impregnated paper used in power transformer bushings. It also investigates how the change in PRPD patterns is related to the degradation level of oil-impregnated paper. In order to accomplish these objectives, the experiments were set in such a way that the effect of HV impulses and an early stage PD activity at an elevated AC stress, HV impulses in combination with a prolonged PD activity at an elevated AC stress, and a prolonged PD activity alone at an elevated AC stress could be investigated separately. The experimental results presented in this paper indicate that HV impulses below the impulse breakdown stress following an early stage AC PD activity would not cause a significant change in PRPD patterns, and would not damage oil-impregnated paper to a level which can be noticed by a visual observation. On the other hand, a prolonged AC PD activity alone can cause the change in PRPD patterns, but cannot quickly damage the oil-impregnated paper as it would do when it is combined with HV impulses. However, the combination of both, HV impulses and a prolonged AC PD activity caused a high drop in the PD parameters (total PD charge and a repetition rate). The study found that the decrease of the PD parameters can be considered as a sign of severe degradation of oil-impregnated paper.

  • 24.
    Kiiza, Respicius Clemence
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Ghaffarian Niasar, Mohamad
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Nikjoo, Roya
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Wang, Xiaolei
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    The effect of PD by-products on the dielectric frequency response of oil-paper insulation comprising of a small cavity2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 5, s. 2923-2930Artikkel i tidsskrift (Annet vitenskapelig)
    Abstract [en]

    This work investigates the effect of alternating current partial discharge (AC PD) byproducts on the dielectric frequency response (DFR) of oil-paper insulation in the frequency range of 1.0 mHz to 1.0 kHz. The investigations were done by utilizing experimental results in combination with a Finite Element Method (FEM) model. The results presented in this paper show that AC PD by-products in the oil-paper insulation can result in a dissipation factor curve whose magnitudes increase with increasing volume of the aged zone. The change in a dissipation factor curve due to PD byproducts was observed to occur mainly in the middle and low frequency regions. The results indicate that the dissipation factor curves in the middle and low frequency regions depend on the level of PD by-products in oil and paper, respectively.

  • 25. Laihonen, Sari J.
    et al.
    Gustafsson, A.
    Gäfvert, U.
    Schütte, T.
    Gedde, Ulf
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Area Dependence of Breakdown Strength of Polymer Films: Automatic Measurement Method2007Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, nr 2, s. 263-274Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    An automatic method for determining the breakdown strength of thin polymer films was evaluated. The equipment consists of a scanning electrode arm with which the breakdown voltage over the film sample is measured. The measurement electrodes were of solid brass and the ground electrode was of thin aluminum foil. five different electrode areas between 0.045 and 9.3 cm(2) were used and 40-100 individual measurements per electrode area per sample were performed. All measurements were performed in air. The Weibull function parameters alpha and beta were fitted to the obtained data. Different features concerning the measurement system and conditions, e.g. the criteria for the automatic detection of the breakdowns, the effect of the electrode edge design, the DC ramp speed, partial discharges and humidity were analyzed. In order to control the reproducibility of the results, also another type of measurement setup was tested. It was concluded that the measured alpha-parameter values were stable and repeatable over a period of several years. The average beta-parameter values varied +/- 10-30%, more for the large than the small electrodes, and were also sensitive to changes in the sample, in the measurement conditions and the choice of the electrode system.

  • 26. Laihonen, Sari J.
    et al.
    Gäfvert, U.
    Schütte, T.
    Gedde, Ulf W.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    DC Breakdown Strength of Polypropylene Films: Area Dependence and Statistical Analysis2007Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, nr 2, s. 275-286Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The dc breakdown strengths of 4-15 μm thick capacitor grade polypropylene (PP) films were determined. The measurements were performed with an automatic measurement system using five electrodes with areas between 0.045 and 9.3 cm2. Some samples werestudied with larger electrodes. Samples were not aged but were measured as received.The measurements were performed in air at room temperature and typically 40-80 breakdowns per sample per electrode area were collected. The Weibull distribution parameters α and β were fitted to the data. Depending on the electrode area and the film grade, the α-parameter values obtained were between 450 and 850 V/μm. The breakdown strength values showed decreasing area dependence with decreasing electrode area and the Weibull scaling law was not valid for the smallest electrodeareas. The α-values for the large sample areas were extrapolated from the small areadata by area and Weibull extrapolation. The area extrapolation overestimated the breakdown strength at 4 m2 by 40-50% whereas the Weibull extrapolation gave anaccuracy of ±15%. The extrapolated breakdown strengths for the full-scale powercapacitors deviated from the capacitor manufacturers test data. This indicated that different defect distributions were responsible for the breakdown strengths in the oil impregnated power capacitors than in the small dry film samples.

  • 27.
    Li, Wangshu
    et al.
    KTH, Skolan för kemivetenskap (CHE).
    Gedde, Ulf W.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Hillborg, Henrik
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi. ABB Corp Res, Sweden.
    Structure and Electrical Properties of Silicone Rubber Filled with Thermally Reduced Graphene Oxide2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, nr 2, s. 1156-1163Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Graphene oxide (GO) was heat treated at different temperatures between 120 and 220 degrees C and the structural changes were assessed by thermogravimetry, infrared spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy (SEM). The resulting reduced graphene oxide (rGO) fillers showed markedly lower oxygen contents (primarily by reduction of epoxide and hydroxyl groups) than GO. SEM of silicone rubber composites containing 3 wt.% rGO or GO filler showed that the nanoparticles were uniformly distributed in the polymer. The rGO-filled composites exhibited electric field-dependent resistivity; the resistivity decreased from 10(14) to 10(11) ohm m as the electric field was increased from 0.2 to 6 kV (mm)(-1). The composites exhibited an increased resistivity after being exposed to a combined thermal cycling and electrical field. An increase in the resistivity of samples aged at 120 degrees C for more than 17 h was observed; the resistivity-electric field behavior and the dielectric constant of the aged composite resembled that of GO-filled composite. The composites exhibited dielectric constant values between 4.0 and 5.2 and a low tan delta (<= 0.015) at frequencies between 10(-2) and 10(4) Hz. The results suggest that the resistivity of the composites can be tuned by adjusting the degree of reduction of GO. The low rGO-filler content that was required to achieve this adequate property profile is attractive, which makes these composites potentially useful as electric field-grading material in HVDC cable accessories. However, this requires that the long-term stability problem can be sensible addressed.

  • 28.
    Li, Wangshu
    et al.
    KTH, Skolan för kemivetenskap (CHE).
    Hillborg, Henrik
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi. ABB, Sweden.
    Gedde, Ulf W.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Influence of Process Conditions and Particle Dispersion on the AC Breakdown Strength of Polyethylene-aluminium Oxide Nanocomposites2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 6, s. 3536-3542Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The voltage level in power transmission systems is increasing in order to transmit energy more efficiently and this requires efficient insulating materials. The dielectric breakdown strength of polymers used as electrical insulation can be increased by the addition of inorganic nanoparticles. A uniform particle dispersion is however important; large agglomerates may have a negative effect on the breakdown strength. The effects of temperature, screw rotation velocity and processing time in a micro-extruder on the dispersion of neat aluminium oxide nanoparticles in LDPE were studied, and it was possible to obtain an essentially agglomerate-free particle dispersion at optimum extrusion conditions. Smaller particle agglomerates (< 3 mu m) had no significantly negative impact on the AC dielectric breakdown strength. Composites with higher filler fractions (>= 3 wt.%) containing larger particle agglomerates (>15 mu m in diameter) showed a significant reduction in the breakdown strength compared to the neat polymer.

  • 29.
    Liu, Dongming
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Hoang, A. T.
    Pourrahimi, Amir Masoud
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Pallon, Love K. H.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Nilsson, Fritjof
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Gubanski, S. M.
    Olsson, Richard T.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Hedenqvist, Mikael S.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Gedde, Ulf W.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Influence of Nanoparticle Surface Coating on Electrical Conductivity of LDPE/Al2O3 Nanocomposites for HVDC Cable Insulations2017Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, nr 3, s. 1396-1404Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    LDPE/metal oxide nanocomposites are promising materials for future high-voltage DC cable insulation. This paper presents data on the influence of the structure of the nanoparticle coating on the electrical conductivity of LDPE/Al2O3 nanocomposites. Al2O3 nanoparticles, 50 nm in size, were coated with a series of silanes with terminal alkyl groups of different lengths (methyl, n-octyl and n-octadecyl groups). The density of the coatings in vacuum was between 200 and 515 kg m(-3,) indicating substantial porosity in the coating. The dispersion of the nanoparticles in the LDPE matrix was assessed based on statistics for the nearest-neighbor particle distance. The electrical conductivity of the nanocomposites was determined at both 40 and 60 degrees C. The results show that an appropriate surface coating on the nanoparticles allowed uniform particle dispersion up to a filler loading of 10 wt.%, with a maximum reduction in the electrical conductivity by a factor of 35. The composites based on the most porous octyl-coated nanoparticles showed the greatest reduction in electrical conductivity and the lowest temperature coefficient of electrical conductivity of the composites studied.

  • 30.
    Liu, Dongming
    et al.
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Hoang, A.T
    Chalmers University of Technology.
    Pourrahimi, Amir Masoud
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi, Polymera material.
    Pallon, Love
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Gubanski, Stanislaw
    Chalmers University of Technology.
    Ohlsson, Richard
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Hedenqvist, Mikael
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Gedde, Ulf
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Influence of nanoparticle surface coating on electrical conductivity of polyethylene/aluminium oxide nanocomposites for HVDC cable insulations2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Artikkel i tidsskrift (Fagfellevurdert)
  • 31.
    Mahidhar, G. D. P.
    et al.
    Indian Inst Technol Madras, Dept Elect Engn, Madras 600036, Tamil Nadu, India..
    Sarathi, R.
    Indian Inst Technol Madras, Dept Elect Engn, Madras 600036, Tamil Nadu, India..
    Taylor, Nathaniel
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknisk teori och konstruktion.
    Edin, Hans Ezz
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknisk teori och konstruktion.
    Study on Performance of Silica Nanoparticle Dispersed Synthetic Ester Oil under AC and DC Voltages2018Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 25, nr 5, s. 1958-1966Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Synthetic ester oil (SEO) is becoming a replacement for mineral oil in transformers due to its biodegradability and for its better insulation properties. Silica nanoparticles dispersed in SEO can increase its corona inception voltage (CIV) and breakdown voltage compared to pure SEO. Physico-chemical studies were carried out to understand the characteristics of nanoparticles and nanofluids. Methodical experimental studies were carried out to understand the corona discharge activity in SEO based nanofluid. A ultra-high frequency (UHF) sensor was used to identify corona activity and it is concluded that the UHF signal radiated from corona activity in SEO and nanoparticle dispersed SEO have a bandwidth in the range of 700 MHz to 2 GHz. A comparative study on the CIV of SEO and nanofluid under harmonic AC voltages has been carried out, which indicates a reduction in the CIV as the total harmonic distortion of the supply voltage increases. Phase resolved partial discharge analysis indicates that the magnitude of discharges formed in nanofluid is much lower than in the SEO. Comparative statistical analysis are made with the breakdown voltages of the synthetic ester oil and nanofluid. A comparative analysis on the energy of the UHF signal due to the corona discharges is performed.

  • 32.
    Mugala, Gavita
    et al.
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Eriksson, Roland
    KTH, Tidigare Institutioner, Elkraftteknik.
    Gäfvert, Uno
    ABB.
    Pettersson, Per
    Vattenfall.
    Measurement technique for high frequency characterization of semi-conducting materials in extruded cables2004Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 11, nr 3, s. 471-480Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Knowledge on the dependence of wave propagation characteristics on material properties and cable design is important in establishing diagnostic methods for cable insulation. In this study, a high frequency measurement technique to characterize the semi-conducting screens in medium voltage cross-linked polyethylene (XLPE) cables has been developed. The frequency ranges from 30 kHz to 500 MHz. The influence of the experimental set-up, sample preparation methods, pressure and temperature are investigated. A dielectric function is developed for the semiconducting screens and this is incorporated into a high frequency model for the cable. The propagation characteristics obtained from the high frequency cable model are compared with those obtained from measurements made on the same cables.

  • 33.
    Mugala, Gavita
    et al.
    KTH, Skolan för elektro- och systemteknik (EES).
    Eriksson, Roland
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Pettersson, Per
    Vattenfall.
    Comparing two measurement techniques for high frequency characterization of power cable semi-conducting and insulating materials2006Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 13, nr 4, s. 712-716Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Understanding the high frequency characteristics of the materials that make up medium voltage extruded cables is important in establishing diagnostics schemes based on electromagnetic pulse propagation methods. Two measurement techniques have been developed and used to characterize the high frequency material properties of semi-conducting screens and cross linked polyethylene (XLPE) insulation up to 100 MHz. The experimental details, parameter extraction and limitations of the two measurement techniques are presented.

  • 34.
    Mugala, Gavita
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Eriksson, Roland
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Pettersson, Per
    Vattenfall.
    Dependence of XLPE insulated power cable wave propagation characteristics on design parameters2007Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, nr 2, s. 393-399Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Propagation losses occur in medium voltage power cables as pulses propagate through them. Since cables have many components, these can make different contributions to the propagation losses. The relative contributions of the conductors, insulation and semi-conducting screens to the propagation characteristics of the cable are analyzed. The propagation characteristics of the cables are studied by a developed "approximate" model providing analytical expressions that can quantify the contributions to the losses by the different parts of the conductor and dielectric system. The model is compared with an "exact" model and is tested on four cables. © 2007 IEEE.

  • 35.
    Mugala, Gavita
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Eriksson, Roland
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Pettersson, Per
    Vattenfall.
    High frequency characteristics of water-tree degraded XLPE insulation in power cables2007Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, nr 5, s. 1271-1277Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The high frequency characteristics, the complex permittivity in particular, of water-tree degraded XLPE insulation in power cables are determined using a two port measurement method. The insulation samples are obtained from both laboratory made samples and those from ex-service cables. The electrical measurements are related to the water tree content of the samples which is estimated by an optical method. An effective materials model is derived to help understand the observed characteristics. From the model it is possible to estimate the water content of the water tree and the conductivity of the water. There is a considerable difference in the water tree characteristics between laboratory and service aged cables. The conductivity of the water in the water trees of service aged cables is much higher than that found in laboratory aged samples.

  • 36.
    Niasar, M. Ghaffarian
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Kiiza, Respicius Clemence
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Dielectric Frequency Response of Oil-impregnated Paper: the Effect of Partial Discharges Compared to other Influences2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, nr 3, s. 1769-1777Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, the influence of partial discharges (PD) on the subsequent dielectric properties of oil-impregnated paper (OIP) is investigated. Dielectric frequency response (DFR) of OIP is investigated at varied levels of moisture, thermal aging, temperature, oil conductivity and electrode material, to allow comparison of these influences. It is shown that PD activity inside OIP can significantly change the real and imaginary parts of permittivity of the paper layers, especially in the frequency range below 100 Hz. It is shown that even if the PD activity is concentrated on a few percent of the test sample, the real part of permittivity measured on the entire sample after a few tens of hours of PD activity can increase up to 10 times at very low frequencies (similar to 1 mHz) and the imaginary part of the permittivity can increase up to 100 times at low frequencies (similar to 1 Hz). The change of the real and imaginary parts of permittivity due to PD activity is permanent. It is very similar to the change caused by thermal aging, and somewhat similar to the change caused by moisture and higher oil conductivity. It is also shown that the dissipation factor of OIP in contact with copper and brass increases, even in the absence of PD, but contact with aluminum and stainless steel has no influence on DFR of OIP. The results suggest that the influence of PD can be significant when estimating the moisture content or the level of thermal aging of OIP with DFR method.

  • 37.
    Nikjoo, Roya
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Dielectric response measurement by impulse stimulus on AC: Measurement considerations, and laboratory testing on a bushing2017Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, nr 1, s. 511-518, artikkel-id 7873509Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Dielectric response (DR) measurement is commonly used for condition assessment of insulation systems of power components. A method for measurement of dielectric response using the stimulus of natural transients such as lightning and switching impulses was proposed in earlier work. Its desirable features include the ability to make measurements online over a range of frequencies, without requiring a voltage source. This article presents a laboratory demonstration of the method on a 150 kV service-aged transformer bushing, where the stimulus was a standard lightning impulse voltage superimposed on an AC voltage. Several aspects of the measurement and data processing that affect the results are studied experimentally and numerically. The results are compared with low-voltage frequency domain spectroscopy (FDS). Reasonable accuracy for monitoring changes in DR can be achieved by suitable choice of measurement circuit and data acquisition. The study suggests an approach for online monitoring of dielectric properties of power transformer bushings, and diagnostics of defects that affect the high frequency region of DR, such as moisture content.

  • 38.
    Nikjoo, Roya
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, nr 6, s. 3602-3611, artikkel-id 7823414Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics.

  • 39. Nikjoo, Roya
    et al.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Hollertz, Rebecca
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Wåhlander, Martin
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Wågberg, Lars
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Malmström, Eva
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknologi.
    Comparison of Oil-impregnated Papers with SiO2 and ZnO Nanoparticles or High Lignin Content, for the Effect of Superimposed Impulse Voltage on AC Surface PD2017Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, nr 3, s. 1726-1734Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Surface discharge behavior of modified oil-impregnated paper (OIP) with nanoparticles (NPs), has been investigated under AC voltage with superimposed impulses. Surface Partial Discharges (PD) can develop at an oil-paper interface and lead to its degradation. Modified paper, made from fibers with adsorbed nanoparticles, can affect the partial discharge behavior of a paper in combination with oil at the interface between oil and fibers. Papers with two different concentrations (2 wt% and 6 wt%) of silica (SiO2), and paper with silanized zinc oxide (ZnO) nanoparticles (1 wt%) have been studied. Papers with SiO2 NPs showed lower impulse-induced surface PD activity. However, thorough purification during the production of SiO2 filled papers was necessary to achieve a good performance. With less purification, paper with 2 wt% of SiO2 did not show such significant improvements. Paper with 6 wt% of SiO2 NPs showed a large number of AC surface PDs, but low influence of impulse voltage on subsequent PD. Papers containing 1 wt% of silanized ZnO showed reduced relative permittivity, but no significant difference in surface PD behavior. The effect of high lignin content in Kraft paper has also been studied. Paper with higher lignin content showed better surface PD characteristics under the impulse. Paper with low concentrations of pure SiO2 NPs, and paper with high lignin content thus appear good candidates for further studies to improve the surface PD behavior of OIP.

  • 40.
    Nikjoo, Roya
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Kiiza, Respicius Clemence
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Dielectric Response of Oil-impregnated Paper by Utilizing Lightning and SwitchingTransients2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 1, s. 335-344, artikkel-id 07033403Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    On-line monitoring of insulation systems in power components such as power transformer bushings has significant value for their reliability. Dielectric spectroscopy is known as a powerful tool in condition assessment of power components' solid insulation. However, it is difficult to apply it on-line. Therefore, a new approach of intended use for on-line diagnosis of oil-impregnated paper, as a major insulation component of power transformers and bushings, is presented. In this technique, natural transients such as lightning and switching impulses are used as stimuli for on-line dielectric response measurements. The wide frequency range of these transients is their advantage as stimuli for dielectric spectroscopy. The influence of moisture content and temperature on the dielectric response of oil-impregnated paper has been investigated for performance evaluation of the proposed technique. It is shown that this technique can track the changes caused by moisture content and temperature in the dielectric response of oil-impregnated paper. The results of both low and high voltage study cases have been verified by comparison with Frequency Domain Spectroscopy (FDS). According to the well-matched results with FDS, the proposed technique can offer a valuable approach for frequent monitoring of dielectric properties of oil-impregnated paper insulated systems such as those commonly encountered in power transformer bushings. However, the validity range of the results depends on the bandwidth of the applied transients and other discussed measurement considerations.

  • 41.
    Papazyan, Ruslan
    et al.
    KTH, Tidigare Institutioner                               , Elektrotekniska system.
    Eriksson, Roland
    KTH, Tidigare Institutioner                               , Elektrotekniska system.
    Calibration for Time Domain Propagation Constant Measurements on Power Cables2003Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 52, nr 2, s. 415-418Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A method has been developed for characterizingthe wave propagation properties of medium voltage (MV) powercables. It delivers the complex propagation constant of lossy cabletransmission lines as a function of frequency. The technique isbased on time domain measurements and frequency domain analysisof short pulse propagation. Particular attention is paid to thecalibration, as samples with arbitrary characteristic impedancescan cause multiple reflections in the test setup. The influence ofnumerical artefacts is also investigated. The developed techniqueis demonstrated on MV crosslinked polyethylene (XLPE) cables.The results are verified by comparison with frequency domainmeasurements of the cable S-parameters.

  • 42.
    Papazyan, Ruslan
    et al.
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Pettersson, Per
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Edin, Hans
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Eriksson, Roland
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Gäfvert, Uno
    KTH, Tidigare Institutioner, Elektrotekniska system.
    Extraction of the High Frequency Power Cable Characteristics from S-parameter Measurements2004Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 11, nr 3, s. 461-470Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A technique is developed for extraction of the wave propagation properties of power cables from S-parameter measurements. The method extracts the complex propagation constant and the characteristic impedance, as well as the LCRG Telegrapher's Equation parameters. The extraction process is developed after clarifying the effect of the connection between the measurement port and the power cable. It is concluded that treating the connection solely as a characteristic impedance change could lead to considerable errors in the parameter extraction. Furthermore, the method corrects for electrical lengths, which are not accounted for by the standard Network Analyzer calibration. The extraction is demonstrated for a medium voltage crosslinked polyethylene (XLPE) cable over the frequency range 300 kHz to 300 MHz. The results are compared to a time domain short pulse propagation method for cable characterization. Both measurement methods are evaluated against a cable model.

  • 43.
    Swati, Kumari
    et al.
    Indian Inst Technol Madras, Dept Elect Engn, Madras 600036, Tamil Nadu, India..
    Sarathi, R.
    Indian Inst Technol Madras, Dept Elect Engn, Madras 600036, Tamil Nadu, India..
    Yadav, Kandur Sahitya
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknisk teori och konstruktion.
    Edin, Hans Ezz
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknisk teori och konstruktion.
    Corona Discharge Activity in Nanoparticle Dispersed Transformer Oil under Composite Voltages2018Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 25, nr 5, s. 1731-1738Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Titania (TiO2) nanoparticles with cetyl trimethyl ammonium bromide (CTAB) surfactant dispersed in transformer oil has higher corona inception voltage than the normal transformer oil, under AC, DC, and composite AC and DC voltages. It is observed that corona inception voltage is lower under composite voltages compared to AC and DC voltages. The corona activity radiates ultra high frequency (UHF) signals with its dominant frequency near 1 GHz. Operating a spectrum analyzer in zero span mode, the UHF signals generated from the corona activity, the number of discharges is observed to be more under composite voltages than under AC voltages, both at inception and at higher voltages. A reduction in interfacial tension and an improvement in flash point is observed in nano-titania with surfactant dispersed transformer oil. The addition of surfactant reduces the turbidity of the nanofluid. Phase Resolved Partial Discharge (PRPD) analysis with UHF signals measured, indicates that corona discharge activity occurs around the peak and the pre-peak rising portions of the supply voltage. It is observed that more discharges occur in the positive half cycle for AC superimposed with positive DC voltage and in the negative half cycle for AC superimposed with negative DC voltage. The magnitude of UHF signals formed due to corona activity is less with nanofluid, irrespective of voltage profiles.

  • 44.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES).
    Measured and Modeled Capacitance, Loss and Harmonics in Stator Insulation with Nonlinear Stress Control2015Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, nr 6, s. 3133-3145Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Diagnostic measurements on stator insulation suffer disturbance from nonlinear resistive stress-control in the end corona protection (ECP). The extent of this effect is studied here, by measurements and modeling. Frequency-domain dielectric response measurements on bars with epoxy-mica insulation in new condition are presented as capacitance, loss and current harmonics, for the guarded insulation in the slot region, the end regions alone, and the combination. The voltages are from 0.3 up to 14.4 kV, at frequencies from 100 Hz down to 0.1 mHz, to cover the range relevant to common diagnostic methods. The ECP approximately doubles the change of capacitance with frequency, and has a much stronger relative effect on voltage-dependence and on current harmonics in an unaged insulation system. The ECP currents are modeled numerically, using material properties from further measurements on samples of the silicon-carbide based ECP tape. A nonlinear 1-dimensional model gives a fair fit to the measurements except at the highest frequencies. Reduction of the ECP disturbance in measurements by subtraction of modeled values is discussed. The main practical limitation of the model is likely to be uncertainty of the input parameters describing the geometry and material, rather than a need of more detail in the model.

  • 45.
    Taylor, Nathaniel
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Stator End-winding Currents in Frequency-domain Dielectric Response Measurements2010Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 17, nr 5, s. 1489-1498Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This is a study of the components of capacitance, loss and harmonic currents that the nonlinear 'corona protection' coating used on stator end-windings adds to frequency-domain dielectric measurements. The work is based on measurements on simple laboratory models, compared to one-dimensional numerical models having discrete or continuous parameters and linear or nonlinear resistance. The necessary components and parameters of the numerical models are discussed by comparison with the measurements. Commonly used models of the conductivity of the SiC-based nonlinear material are compared to dc measurements on samples. The studied range of the applied voltage has amplitudes from low values up to the levels of operation and testing of stator windings, and frequency from line-frequency down to millihertz.

  • 46.
    Theethayi, Nelson
    et al.
    Uppsala University.
    Thottappillil, Rajeev
    Uppsala University.
    Diendorfer, Gerhard
    Austrian Electrotechnical Association (OVE), Vienna, Austria.
    Mair, Martin
    Vienna University of Technology, Vienna, Austria.
    Pichler, Hannes
    Vienna University of Technology, Vienna, Austria.
    Currents in buried grounding strips connected to communication tower legs during lightning strikes2008Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 15, nr 4, s. 1153-1161Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    During a lightning strike to communication tower stroke currents are shared by the tower and by the shields of the cables along the tower. The currents in the tower proceed towards the grounding system (possibly a combination of counterpoises or ring conductors or ground rods or grounding grids) connected to tower legs’ foundation. In this paper, lightning strike to communication tower on mount Gaisberg in Austria is considered and measured currents at the tower top and those shared by an instrumented grounding strip connected to one of the tower leg’s are presented. The measured currents at different locations on the 70-m long ground strip are compared with the predictions of a frequency dependant lossy transmission line (TL) model and reasonably good agreement was found. From this validation it is claimed that the TL models are appropriate for lightning transient analysis of grounding systems.

  • 47.
    Theethayi, Nelson
    et al.
    Uppsala University.
    Thottappillil, Rajeev
    Uppsala University.
    Paolone, Mario
    University of Bologna, Italy.
    Nucci, Carlo Alberto
    University of Bologna, Italy.
    Rachidi, Farhad
    EMC Group, Swiss Federal Institute of Technology, Lausanne, Switzerland.
    External impedance and admittance of buried horizontal wires for transient studies using transmission line analysis2007Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 14, nr 3, s. 751-761Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The paper investigates the applicability of some closed form expressions for the ground impedance and ground admittance of buried horizontal wires (bare and insulated) for lightning or switching transient analyses based on transmission line (TL) theory. In view of the frequency contents that typically characterize such transients, the behavior of the ground impedance and admittance is studied for a wide frequency range up to 10 MHz. Low frequency approximation of the ground impedance is not always appropriate for transient analysis. Sensitivity analyses show that, unlike overhead wires, the ground impedance for buried wires is little sensitive to the ground conductivity. On the other hand, the ground admittance varies strongly with the ground conductivity. The paper also discusses the results of transient analysis of buried cables performed by means of electromagnetic transient programs (EMTP) that neglect the ground admittance. The limits of such an approximation are discussed in order to evaluate the applicability of EMTP-like programs to the transient analysis of buried conductors. Transient pulse propagation in time domain based on finite difference time domain (FDTD) method of solution of TL equations is also discussed for a future inclusion of non-linear phenomena, like soil ionization and arcing/breakdown mechanisms, in the soil. The analysis presented could be useful in estimating surge propagation characteristics of buried wires for appropriate insulation coordination and transient protection.

  • 48.
    Venkatesulu, Bandapalle
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Jonsson, B. Lars G.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Norgren, Martin
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Modeling of Insulating Nanocomposites-Electric and Temperature Fields2013Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 20, nr 1, s. 177-184Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper, simulation studies are carried out to enhance the understanding of the mechanisms for the reported improvements in breakdown strength of nanocomposites. Electric field driven partial discharges and temperature rise can lead to electrical breakdown and hence it is important to study the electric field distribution and temperature rise in nanocomposites. An FEM based software, COMSOL V4.1, has been used to carry out the simulations. The nanocomposite has been modeled as two cores around the nanoparticle in a polymer. The interface between the particle and polymer is called as 1st core and a shell around the 1st core is called as 2nd core. The mechanisms in the 2nd core are characterized by its electrical conductivity and dielectric constant. Various models are used for modeling the 2nd core. The electric field, resistive losses and temperature rise have been computed for static and quasi-static fields for the linear and nonlinear materials. For many models, it is tough to simultaneously get both vertical bar E vertical bar-fields and resistive losses and hence temperature rise lower than unfilled material both inside and outside the 2nd core region. Based on the magnitude of electric field, resistive losses and temperature rise under transient fields, two linear models of 2nd core support the argument of enhanced dielectric strength whereas most other models do not. Possible hypotheses have been proposed. The results reinforce the idea that interfaces are critical for the reported breakdown strength improvements.

  • 49. Wang, Jing
    et al.
    Gubanski, Stanislaw M.
    Blennow, Jörgen
    Atarijabarzadeh, Sevil
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik.
    Strömberg, Emma
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Polymerteknologi.
    Karlsson, Sigbritt
    KTH, Skolan för kemivetenskap (CHE), Fiber- och polymerteknik, Polymerteknologi.
    Influence of Biofilm Contamination on Electrical Performance of Silicone Rubber Based Composite Materials2012Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 19, nr 5, s. 1690-1699Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this research, three kinds of silicone rubber based composite materials, SIR, NSIR and DSIR, were prepared. SIR is the reference silicone rubber without addition of antimicrobial biocide. Two different antimicrobial agents at a concentration of 3 wt% were added to the reference silicone rubber to form the other two types of materials. The materials were inoculated with a fungal spore suspension containing nutrients. Fungal growth could be observed visually in form of spots by naked eye, and biofilm was formed and detected by SEM observation on the surfaces of SIR and NSIR samples, whereas DSIR samples were completely free of the fungal growth. Then the electrical performance of both clean and biofilm contaminated samples, including the surface and volume conductivity, surface flashover voltage and leakage current before flashover, were measured. The obtained results indicate that as compared with SIR base material, the biocides have not changed the surface conductivity and surface flashover voltage of NSIR and DSIR materials. At the same time, the volume conductivity of NSIR remains almost the same as SIR, while that of DSIR increases by about two orders of magnitude. Biofilm could increase the surface conductivity and decrease the surface flashover voltage significantly. The time domain and frequency domain analyses of the leakage currents flowing before flashover event provide useful information on the severity of contamination by biofilm and the eventual risk for surface flashover.

  • 50.
    Wang, Xiaolei
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Taylor, Nathaniel
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Edin, Hans
    KTH, Skolan för elektro- och systemteknik (EES), Elektroteknisk teori och konstruktion.
    Enhanced Distinction of Surface and Cavity Discharges by Trapezoid-based Arbitrary Voltage Waveforms2016Inngår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, nr 1, s. 435-443Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Identification of different partial discharge (PD) sources may be improved by the use of non-sinusoidal waveforms of the testing voltages. For instance, surface discharge (SD) and cavity discharge (CD) are not always easy to directly distinguish from the phase resolved PD pattern at traditional 50 or 60 Hz alternating sinusoidal voltage. This study compares PD patterns stimulated by sinusoidal voltage and by several forms of trapezoid-wave voltage, including the limiting cases of triangular and approximately square-wave voltages. Trapezoid-based voltage waveforms are considered as a potential new off-line diagnostic method for PD sources. Partial discharge measurements were performed with each different waveform in two test cells representing canonical cases of SD and CD, with polycarbonate plates as the solid insulating material. The results show that the applied voltage of arbitrary waveform could more clearly distinguish between these PD sources' behavior than normal sinusoidal voltage. The constant-voltage period of peak value in trapezoidal and square voltage waveforms played an important role in the distinction of the two discharge sources. Compared with the cavity discharge's symmetric features, surface discharge produced in the asymmetric test cell shows strong asymmetric behavior during the constant-voltage period between two polarities under trapezoidal and square voltage waveforms. A faster rise time and increased duration of the constant peak-voltage part of the waveform caused more obvious asymmetry of the surface discharge.

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