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  • 1.
    Habib, Md Zakaria
    et al.
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Wang, Jianping
    Li, YouYi
    Taylor, Nathaniel
    KTH, School of Electrical Engineering and Computer Science (EECS), Electromagnetic Engineering.
    Phase Shift Compensation Method for the Line Differential Protection on UHV-AC Transmission Lines2018Conference paper (Refereed)
    Abstract [en]

    Line differential protection is popular for its good selectivity and simplicity as long as there is a dependable communication system between the two ends of the line. However, the sensitivity needs to be compromised when traditional line differential scheme is applied for UHV-AC lines because of the large charging current. This paper presents a study of the impact of UHV transmission line characteristics on line differential protection and a proposed solution based on compensation of the phase shift that exists between the sending and receiving end currents.

  • 2. Nikjoo, Roya
    et al.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wåhlander, Martin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Malmström, Eva
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Comparison of Oil-impregnated Papers with SiO2 and ZnO Nanoparticles or High Lignin Content, for the Effect of Superimposed Impulse Voltage on AC Surface PD2017In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, no 3, p. 1726-1734Article in journal (Refereed)
    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.

  • 3.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric response measurement by impulse stimulus on AC: Measurement considerations, and laboratory testing on a bushing2017In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, no 1, p. 511-518, article id 7873509Article in journal (Refereed)
    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.

  • 4.
    Chen, Feng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Balieu, Romain
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Kringos, Niki
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Dynamic application of the Inductive Power Transfer (IPT) systems in an electrified road: Dielectric power loss due to pavement materials2017In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526, Vol. 147, p. 9-16Article in journal (Refereed)
    Abstract [en]

    Inductive Power Transfer (IPT) technology is seen as a promising solution to be applied in an electrified road (eRoad) to charge Electric Vehicles (EVs) dynamically, i.e. while they are in motion. Focus in this study was placed on the dielectric loss effect of pavement surfacing materials on the inductive power transfer efficiency, induced after the integration of the technology into the physical road structure. A combined experimental and model prediction analysis was carried out to calculate this dielectric loss magnitude, based on which some preliminary conclusions as well as a prioritization of future focus needs were summarized in detail.

  • 5.
    Kande, Mallikarjun
    et al.
    KTH, School of Electrical Engineering (EES). ABB .
    Isaksson, Alf J.
    Thottappillil, Rajeev
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Rotating Electrical Machine Condition Monitoring Automation-A Review2017In: MACHINES, ISSN 2075-1702, Vol. 5, no 4, article id 24Article, review/survey (Refereed)
    Abstract [en]

    We review existing machine condition monitoring techniques and industrial automation for plant-wide condition monitoring of rotating electrical machines. Cost and complexity of a condition monitoring system increase with the number of measurements, so extensive condition monitoring is currently mainly restricted to the situations where the consequences of poor availability, yield or quality are so severe that they clearly justify the investment in monitoring. There are challenges to obtaining plant-wide monitoring that includes even small machines and non-critical applications. One of the major inhibiting factors is the ratio of condition monitoring cost to equipment cost, which is crucial to the acceptance of using monitoring to guide maintenance for a large fleet of electrical machinery. Ongoing developments in sensing, communication and computation for industrial automation may greatly extend the set of machines for which extensive monitoring is viable.

  • 6.
    Wåhlander, Martin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Nilsson, Fritjof
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Andersson, Richard L.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Cobo Sanchez, Carmen
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Carlmark, Anna
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Hillborg, Henrik
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials. ABB AB.
    Malmström, Eva
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Coating Technology.
    Tailoring Dielectric Properties using Designed Polymer-Grafted ZnO Nanoparticles in Silicone Rubber2017In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 5, p. 14241-14258, article id C6TA11237DArticle in journal (Refereed)
    Abstract [en]

    Polymer grafts were used to tailor the interphases between ZnO nanoparticles (NPs) and silicone matrices. The final electrical properties of the nanocomposites were tuned by the grafted interphases, by controlling the inter-particle distance and the NP-morphology. The nanocomposites can be used in electrical applications where control of the resistivity is desired. Hansen's solubility parameters were used to select a semi-compatible polymer for grafting to obtain anisotropic NP morphologies in silicone, and the grafted NPs self-assembled into various morphologies inside the silicone matrices. The morphologies in the semi-compatible nanocomposites could be tuned by steering the graft length of poly(n-butyl methacrylate) via entropic matrix-graft wetting using surface-initiated atom-transfer radical polymerization. Image analysis models were developed to calculate the radius of primary NPs, the fraction of aggregates, the dispersion, and the face-to-face distance of NPs. The dielectric properties of the nanocomposites were related to the morphology and the face-to-face distance of the NPs. The dielectric losses, above 100 Hz, for nanocomposites with grafted NPs were approximately one decade lower than those of pristine NPs. The isotropic nanocomposites increased the resistivity up to 100 times compared to that of neat silicone rubber, due to the trapping of charge carriers by the interphase of dispersed NPs and nanoclusters. On the other hand, the resistivity of anisotropic nanocomposites decreased 10–100 times when the inter-particle distance in continuous agglomerates was close to the hopping distance of charge carriers. The electrical breakdown strength increased for compatible isotropic nanocomposites, and the temperature dependence of the resistivity and the activation energy were ∼50% lower in the nanocomposites with grafted NPs. These flexible dielectric nanocomposites are promising candidates for low-loss high-voltage transmission cable accessories, mobile electronic devices, wearables and sensors.

  • 7.
    Niasar, M. Ghaffarian
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Frequency Response of Oil-impregnated Paper: the Effect of Partial Discharges Compared to other Influences2016In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 3, p. 1769-1777Article in journal (Refereed)
    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.

  • 8.
    Chen, Feng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kringos, Nicole
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Dynamic application of the Inductive Power Transfer (IPT) systems in an electrified road: Dielectric power loss due to pavement materials2016In: Construction and Building Materials, ISSN 0950-0618, E-ISSN 1879-0526Article in journal (Other academic)
    Abstract [en]

    It is well-known that the high cost and limited performance of existing energy storage systems have significantly constrained the commercialization of the Electric Vehicle (EV) at large scale. In recent years, attention has been given not only to the improved energy storage systems but also to develop appropriate charging infrastructures that would allow the EVs to be powered in an easier way. Inductive Power Transfer (IPT) technology, also known as a near-field wireless power transfer technology, is capable of delivering electricity wirelessly with large power and high efficiency at a given gap distance. It is therefore seen as a promising solution to be applied in an electrified road (eRoad) to charge EVs dynamically, i.e. while they are moving. Various technical aspects of this contactless charging solution have been studied actively by system developers, such as the charging power, its efficiency, the optimum gap distance as well safety issues. Focus in this study is placed on the effect of pavement surfacing materials on the wireless power transfer efficiency, after the integration of the technology into the physical road structures. Specifically, a combined experimental and model prediction analysis has been carried out to investigate this potential energy loss in a quantitative way, based on which some preliminary conclusions as well as a prioritization of future focus needs are summarized in detail. This work provides thus an important beginning for understanding the pavement materials’ influence on the IPT systems that may be used for dynamic applications in an eRoad.

  • 9.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of High Voltage Impulses on Partial Discharge Characteristics of Oil-Impregnated Paper for Online Diagnostics2016Conference paper (Refereed)
    Abstract [en]

    An important signature of degradation is usually linked to the onset of Partial Discharge (PD) activity within the insulation. The change in the PD pattern can concern the insulation degradation level. High voltage transients such as lightning and switching impulses are inevitable phenomena that happen in power systems. They can impose electrical stresses on the insulation system of power components such as power transformers and their bushings which may lead to their failure. In this paper, the effect of HV impulses on the change of surface and cavity discharges of oil-impregnated paper has been investigated. With resemblance to the real situation, superimposed impulse on the AC voltage has been applied to the samples. The probability of initiating partial discharges in oil-impregnated paper due to the impulses has been studied. The PD initiation depends on the status of the insulation, the impulse magnitude and the AC phase angle at which the impulse occurs. Behavior of the partial discharges before and after the impulse on the healthy samples has been compared to the samples with aging defects such as papers with moisture content or carbonization. The possible physical phenomenon behind each behavior has been discussed. The results show the correlation of PD characteristics with the insulation degradation level and HV impulses. Recording the voltage signals with incident of high voltage transients can be used for assessing the insulation condition of power transformers and their bushings. These measurements can be done through test tap of the bushings to be used for the online diagnostics.

  • 10.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of humidity on partial discharge in a metal-dielectric air gap on machine insulation at trapezoidal testing voltages2016In: Journal of Electrostatics, ISSN 0304-3886, E-ISSN 1873-5738, Vol. 83, p. 88-96Article in journal (Refereed)
    Abstract [en]

    The atmospheric relative humidity (RH) has a great impact on the partial discharge (PD) process which can damage the insulation in operating machines. This work investigates how the relative humidity would affect the PD activities in a metal-dielectric air gap on machine insulation, which consists of mica, epoxy resin and glass-fiber, with the application of periodic alternating trapezoidal voltage waveforms. The results show that the PD characteristics, such as discharge amplitude, the average number of discharge pulses, can be varied greatly with the increasing humidity. This is mainly due to the increased surface conductivity in humid air.

  • 11.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper2016In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 6, p. 3602-3611, article id 7823414Article in journal (Refereed)
    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.

  • 12.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Enhanced Distinction of Surface and Cavity Discharges by Trapezoid-based Arbitrary Voltage Waveforms2016In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 1, p. 435-443Article in journal (Refereed)
    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.

  • 13. Lyne, Asa Laurell
    et al.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jaeverberg, Nadja
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Birgisson, Bjorn
    Low frequency dielectric spectroscopy of bitumen binders as an indicator of adhesion potential to quartz aggregates using Portland cement2016In: Materials and Structures, ISSN 1359-5997, E-ISSN 1871-6873, Vol. 49, no 4, p. 1327-1336Article in journal (Refereed)
    Abstract [en]

    The purpose of this investigation was to interpret the bitumen-aggregate adhesion based on the dielectric spectroscopic response of individual material components utilizing their dielectric constants, refractive indices and average tangent of the dielectric loss angle (average loss tangent). Dielectric spectroscopy of bitumen binders at room temperature was performed in the frequency range of 0.01-1000 Hz. Dielectric spectroscopy is an experimental method for characterizing the dielectric permittivity of a material as a function of frequency. Adhesion data has been determined using the Rolling bottle method. The results show that the magnitude of the average tangent of the dielectric loss angle (average loss tangent) depends on bitumen type. The average loss tangent in the frequency range 0.01-1 Hz is introduced as a potential indicator for predicting polarizability and, thereby, adhesion potential of bitumen binders to quartz aggregates when using Portland cement. In order to obtain acceptable adhesion of 70/100 penetration grade bitumen binders and quartz aggregates when using Portland cement, it is suggested that the binder have an average tan delta > 0.035 in the frequency range 0.01-1 Hz.

  • 14.
    Chen, Feng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kringos, Nicole
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Birgisson, Björn
    KTH, School of Architecture and the Built Environment (ABE), Transport Science, Highway and Railway Engineering.
    A study on dielectric response of bitumen in the low-frequency range2015In: International Journal on Road Materials and Pavement Design, ISSN 1468-0629, E-ISSN 2164-7402, Vol. 16, p. 153-169Article in journal (Refereed)
    Abstract [en]

    From the current state of literature, the dielectric property of bitumen has not been understood extensively, nor its relation with other properties such as polarity and rheology. In this study, dielectric spectroscopy measurement in a low-frequency range (10−2–106 Hz) was performed on both pure bitumen in different grades and wax-modified bitumen (WMB). From the performed tests we found the following: (i) the dielectric response of base bitumen is strongly temperature and frequency dependent, which is also highly linked to the rheology of the system. (ii) No remarkable differences in the dielectric constant (Formula presented.) among different grades of bitumen from the same crude oil source can be seen. (iii) Regular changes of dielectric loss tangent (tan δ) among the different grades of bitumen can be observed, which can be a good indicator for the linkage between the dielectric and rheological responses. In addition, it can also be perceived that the dielectric spectroscopy may have the potential to become a new approach for the multi-scale characterisation of road infrastructure materials.

  • 15. Castro, L. C.
    et al.
    Oslinger, J. L.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES).
    Wåhlander, Martin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Dielectric and Physico-chemical Properties of Epoxy-Mica Insulation During Thermoelectric Aging2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 6, p. 3107-3117Article in journal (Refereed)
    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.

  • 16.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response of Oil-impregnated Paper by Utilizing Lightning and SwitchingTransients2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 1, p. 335-344, article id 07033403Article in journal (Refereed)
    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.

  • 17.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of High Voltage Impulses on Surface Discharge Characteristics of Polyethylene2015Conference paper (Refereed)
  • 18.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of High Voltage Transients on Surface Discharge Characteristics of Oil-Paper Insulation2015In: 2015 IEEE Electrical Insulation Conference (EIC), IEEE , 2015, p. 69-72Conference paper (Refereed)
    Abstract [en]

    Condition assessment of power components such as power transformers and their bushings depends on the interpretation of data obtained from the monitoring systems. Various defects in their insulation would behave differently under electrical stresses. Surface discharge on the oil-paper interface is one of the defects which can be responsible for the failure of those components. Power system transients can impose stresses on the insulation of the power transformers and their bushings. This study investigates the effect of transients such as lightning impulses on the surface discharge characteristics of oil-impregnated paper. Two configurations, needle and rod (IEC-b), are used here. Two distinctive situations occur on paper due to these different configurations. The corresponding effect is shown in the form of variation in the partial discharge numbers and phase-resolved patterns. In the needle electrode configuration, the paper remains un-aged and the applied impulses cause a prompt change in the total number of discharges which decays fast. However, with the rod electrode, the paper gets carbonized and the applied impulses cause a decrease in the total number of discharges. The influence of moisture content in oil-impregnated paper was also studied. For this case, the total number of discharges considerably increase after applying impulse.

  • 19.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of Partial Discharges on Thermal Breakdown of Oil Impregnated Paper2015In: IEEJ TRANSACTIONS ON ELECTRICAL AND ELECTRONIC ENGINEERING, ISSN 1931-4973, E-ISSN 1931-4981, Vol. 10, p. S14-S18Article in journal (Refereed)
    Abstract [en]

    In this paper, we investigate the effect of partial discharges (PDs) on the thermal breakdown voltage of oil-impregnated paper (OIP). A disk-shaped cavity in a stack of paper sheets formed the PD source. The paper sample was exposed to PDs for 18 h. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate the activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the OIP system was simulated by using the FEM software COMSOL Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the OIP insulation, PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

  • 20.
    Chen, Feng
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kringos, Nicole
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Electrification of Roads: Opportunities and Challenges2015In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 150, p. 109-119Article in journal (Refereed)
    Abstract [en]

    The Electrical Vehicle (EV) has become a potential solution for enhancing the sustainability of our road transportation, in view of the environmental impacts traditional vehicles have regarding emissions and use of fossil fuel dependence. However, the widespread use of EVs is still restrained by the energy storage technologies, and the electrification of road transportation is still in its early stages. This paper focuses on the technical aspects related to the ‘electrification of roads’ (called ‘eRoads’) infrastructure that aims to diminish the limitations for using EVs. A historical overview of the technology development towards the electrification of road transportation is presented, along with an overview of prospective technologies for implementing an eRoad charging infrastructure. Of these, the Inductive Power Transfer (IPT) technology is examined in further details. The main objective of this paper is to explore the potential knowledge gaps that need to be filled for a successful integration of IPT technology within actual road infrastructure. As such, this paper can be used as an overview of the current state-of-the-art of eRoad infrastructure and also as guidance towards future research directions in this domain.  

  • 21.
    Niasar, Ghaffarian Mohamad
    et al.
    KTH, School of Electrical Engineering (EES).
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, R. C.
    Impact of thermal and electrical aging on breakdown strength of oil-impregnated paper2015In: 2015 IEEE Eindhoven PowerTech, PowerTech 2015, IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    In this paper the impact of thermal and electrical aging on AC and impulse breakdown strength of oil-impregnated paper was investigated. Sheets of paper were thermally aged at 140 °C for 30 days. A comparison between AC and impulse breakdown strength of unaged paper layers, 10, 20 and 30 days thermally aged paper was performed. In order to investigate the effect of electrical aging, first the paper layers were exposed to partial discharges for 6 hours at accelerated electric stress. The partial discharge was created inside paper samples by introducing a disc shaped cavity on the center of the paper samples. The effect of partial discharges was then investigated on the impulse breakdown strength of oil-impregnated paper. The study shows that thermal aging has very small effect on AC breakdown strength. However, impulse breakdown strength of oil-impregnated paper may reduce up to 40% due to exposure to PD.

  • 22.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Insulation condition assessment of power transformer bushings by utilizing high voltage lightning impulses2015In: 2015 IEEE Eindhoven PowerTech, PowerTech 2015, IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    High voltage bushing forms an essential part of a power transformer and its insulation condition is of high importance in a transformer's reliability. Transformer failure statistics indicate considerable contribution from bushing failures, which makes the monitoring of power transformer bushings essential. Several off-line and on-line methods are available for condition assessment of these components. However, most of the on-line methods cannot provide as much information as off-line methods because of the performance limitations. Therefore, condition assessments of these components are mostly done through several on-line methods and decision-making must be done according to majority agreements between different techniques. This paper investigates the possibility of assessing the insulation condition of power transformer bushing through utilizing high voltage transients. They are used as stimuli to get the dielectric response of the insulation system. The investigation is done on two service-aged 150 kV power transformer bushings. The results are verified by comparison with Frequency Domain Spectroscopy (FDS). The proposed application seems promising for monitoring of dielectric properties of power transformer bushings.

  • 23.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES).
    Measured and Modeled Capacitance, Loss and Harmonics in Stator Insulation with Nonlinear Stress Control2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 6, p. 3133-3145Article in journal (Refereed)
    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.

  • 24. Selva, Kruphalan Tamil
    et al.
    Forsberg, Olof A.
    KTH.
    Merkoulova, Daniel
    KTH.
    Ghasemifard, Fatemeh
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Norgren, Martin
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Non-contact Current Measurement in Power Transmission Lines2015In: SMART GRID TECHNOLOGIES (ICSGT- 2015), Elsevier, 2015, p. 498-506Conference paper (Refereed)
    Abstract [en]

    This project deals with the technique of measurement of current in power transmission lines without the need for actual contact with the wires. Magnetic fields emitted by the wires are sensed using hall-effect based magnetic field sensors. Signals from these sensors are utilized for computing or 'reconstructing' the currents. Algorithms are developed in Matlab to facilitate the computation process. The algorithm also considers the challenge of interference due to Earth's magnetic field and stray magnetic fields. The study is performed on a scaled down model of a three phase three wire power transmission line configuration. This technology is aimed at improving the flexibility and portability of present day current measurement technology related to power transmission lines. They can be used as a secondary or temporary measurement system to complement measurements from the current transformers (CT) or be installed in places where CTs are absent. As the measurement system will be completely isolated from the power lines, they possess numerous advantages.

  • 25.
    Sanchez, Carmen Cobo
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wåhlander, Martin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Fogelström, Linda
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Malmström, Eva
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Novel Nanocomposites of Poly(lauryl methacrylate)-Grafted Al2O3 Nanoparticles in LDPE2015In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 7, no 46, p. 25669-25678Article in journal (Refereed)
    Abstract [en]

    Aluminum oxide nanoparticles (NPs) were surface-modified by poly(lauryl methacrylate) (PLMA) using surface-initiated atom-transfer radical polymerization (SI-ATRP) of lauryl methacrylate. Nanocomposites were obtained by mixing the grafted NPs in a low-density polyethylene (LDPE) matrix in different ratios. First, the NPs were silanized with different aminosilanes, (3-aminopropyl)triethoxysilane, and 3-aminopropyl(diethoxy)methylsilane (APDMS). Subsequently, a-BiB, an initiator for SI-ATRP, was attached to the amino groups, showing higher immobilization ratios for APDMS and confirming that fewer self-condensation reactions between silanes took place. In a third step SI-ATRP of LMA at different times was performed to render PLMA-grafted NPs (NP-PLMAs), showing good control of the polymerization. Reactions were conducted for 20 to 60 min, obtaining a range of molecular weights between 23?000 and 83?000 g/mol, as confirmed by size-exclusion chromoatography of the cleaved grafts. Nanocomposites of NP-PLMAs at low loadings in LDPE were prepared by extrusion. At low loadings, 0.5 wt % of inorganic content, the second yield point, storage, and loss moduli increased significantly, suggesting an improved interphase as an effect of the PLMA grafts. These observations were also confirmed by an increase in transparency of the nanocomposite films. At higher loadings, 1 wt % of inorganics, the increasing amount of PLMA gave rise to the formation of small aggregates, which may explain the loss of mechanical properties. Finally, dielectric measurements were performed, showing a decrease in tan d values for LDPE-NP-PLMAs, as compared to the nanocomposites containing unmodified NP, thus indicating an improved interphase between the NPs and LDPE.

  • 26.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Janus, Patrick
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    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 Aging2015In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 22, no 2, p. 1071-1079, article id 7076808Article in journal (Refereed)
    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.

  • 27.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Surface charge dynamics studied by the temporal evolution of the corona charging current2015In: Journal of Electrostatics, ISSN 0304-3886, E-ISSN 1873-5738, Vol. 76, p. 222-230Article in journal (Refereed)
  • 28.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Surface discharge analysis at trapezoidal testing voltage waveforms2015In: 2015 IEEE Eindhoven PowerTech, PowerTech 2015, IEEE conference proceedings, 2015Conference paper (Refereed)
    Abstract [en]

    Surface discharge (SD), as one important partial discharge (PD) source, were measured in the test cell representing the surface discharge, with polycarbonate plate as the insulating material covered completely on the bottom ground electrode. 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. The results show that 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.

  • 29.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Tenbohlen, Stefan
    Effect of Partial Discharges on Thermal Breakdown of Oil Impregnated Paper2014In: Conference Proceedings of ISEIM 2014, IEEE conference proceedings, 2014, p. -199Conference paper (Refereed)
    Abstract [en]

    In this paper the effect of partial discharges on the thermal breakdown voltage of oil-impregnated paper is investigated. A disc shaped cavity in a stack of paper sheets formed a PD source. The paper sample was exposed to partial discharges for 18 hours. Dielectric spectroscopy measurements were performed on the samples before and after exposure to PD activity. The dielectric spectroscopy results showed a big change in the real and imaginary part of the permittivity. These results were used to calculate activation energy and heat losses inside the insulation. In order to investigate the possibility of thermal breakdown in the system, the oil-impregnated paper system was simulated by using the FEM software Comsol Multiphysics. The heat produced by PD pulses and dielectric losses was considered in the simulation. The results show that for the oil-impregnated paper insulation, the PD activity can reduce the thermal breakdown voltage by a factor up to 4 times.

  • 30.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Letter to the editors2014In: IEEE Electrical Insulation Magazine, ISSN 0883-7554, E-ISSN 1558-4402, Vol. 30, no 4, p. 7-7Article in journal (Refereed)
  • 31.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Niasar, Mohamad Ghaffarian
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Partial Discharge Analysis in a Metal-dielectric Air Gap on Machine Insulation at Arbitrary Testing Voltage2014In: Proceedings of the International Symposium on Electrical Insulating Materials, 2014, p. 216-220Conference paper (Refereed)
    Abstract [en]

    Partial Discharge (PD) tests were performed in a metal-dielectric air gap between a spherical electrode and the surface of machine insulation, which consists of mica, epoxy resin and glass-fiber. The PD activities were investigated at three types of applied voltages: triangular, square and trapezoidal waveform. In the trapezoidal waveform, the duration of the linearly rising part of the applied voltage was varied. The effect of the time derivative of the applied voltage dU/dt on the PD activities was studied. The PD activities were studied at a power frequency (50 Hz) and at a lower frequency (10 Hz) of the applied voltage. The results show that the time derivative of the applied voltage has a significant effect on the PD behavior. In each waveform, most of the PD pulses occur during the linearly rising period, but never occur during the linearly falling period. At lower frequency, compared with the positive pulses, the number of negative ones decreases dramatically with the increasing rising time, and PD activities even disappear at the triangular waveform.

  • 32.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response of Aged Transformer Bushings Utilizing Power System Transients2013In: 2013 4th IEEE/PES Innovative Smart Grid Technologies Europe, ISGT Europe 2013, IEEE , 2013, p. 6695398-Conference paper (Refereed)
    Abstract [en]

    The reliability of power transformer bushings is of great importance. Failures may lead to severe consequences. Frequent diagnostics by off-line methods is not feasible as the interruption is costly. Therefore it is of interest to have the possibility to perform on-line diagnostics. Transformer bushings can become aged during their operation periods. Several phenomena such as partial discharges, thermal effects and humidity may change the strength of the insulation in power transformer bushings. There are several methods which provide diagnostics of such components, but some of them can only be done offline in maintenance periods and some online methods would lack useful information about the insulation characteristics. In this paper, a new method is investigated, which uses natural transients such as switching and lightning impulses as stimuli to get the on-line dielectric response of the insulation system. Also, laboratory investigations are reported on three in-service aged 132 kV power-transformer bushings. The dielectric response measured under excitation by transient voltage is compared with results from low-voltage dielectric spectroscopy.

  • 33.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Nikjoo, Roya
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of Dielectric Material on Decay of Surface Charge Deposited by Corona Discharge2013In: Proceedings of the 18th International Symposium on High Voltage Engineering (ISH), 2013, p. 753-758Conference paper (Refereed)
    Abstract [en]

    This work investigates the effect of different dielectric barriers on coronadischarge in the needle-plane geometry, where a layer of dielectric material was placedover the surface of the plane electrode. Different dielectric materials used werepolytetrafluoroethylene (PTFE), polyethylene (PE), polycarbonate (PC), polyvinylchloride(PVC), epoxy and pressboard. The discharges were generated by the application ofperiodic negative step voltage pulses. The results show that the evolution of coronapulses over time depends on the dielectric properties of the barriers. Unlike pressboardwith a higher conductivity, the dielectric materials with a lower conductivity such as PTFEand PE would have more surface charges deposited on its surface during the firstcharging period of 100 ms, and the charges would not disappear during the relaxationperiod of 10 s, therefore, the discharge activities can be reduced significantly after thefirst charging period.

  • 34.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wang, Xiaolei
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Insulation Condition Diagnostics of Oil-impregnated Paper by Utilizing Power System Transients2013Conference paper (Refereed)
    Abstract [en]

    Transients of varied spectral content, such as lightning and switching impulses, are inevitable phenomena that happen in power systems. These natural transients can be used as the stimuli for online dielectric spectroscopy for monitoring power components. The wide range of frequency in those impulses makes it possible to evaluate the dielectric response of their insulation. This paper presents a new technique for online diagnostic of oil-impregnated paper by utilizing those natural transients. Paper with different moisture levels was used to investigate the effectiveness of the proposed technique.The presence of water in the impregnated paper can be seen within those range of frequencies existed in high voltage transients. However, the validity of the results depends on the bandwidth of the applied impulse. Change in frequency response due to different moisture content in oil impregnated paper can be detected by this method. The results have been verified with results from low-voltage frequency-domain dielectric spectroscopy (FDS).

  • 35.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Letter to the Editors on Articles on PD Detection2013In: IEEE Electrical Insulation Magazine, ISSN 0883-7554, E-ISSN 1558-4402, Vol. 29, no 5, p. 6-7Article in journal (Refereed)
  • 36.
    Wang, Xiaolei
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Lumped-circuit modeling of surface charge decay in a needle-plane geometry2013In: Proceedings of the 23rd Nordic Insulation Symposium (Nord-IS 13), 2013, p. 183-186Conference paper (Refereed)
  • 37.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Ghaffarian Niasar, Mohamad
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric response measurement of power transformer bushing by utilizing high voltage transients2012In: 2012 Annual Report Conference on Electrical Insulation and Dielectric Phenomena (CEIDP), IEEE , 2012, p. 503-506Conference paper (Refereed)
    Abstract [en]

    Insulation has a significant role in most of power components failures. Therefore, proper diagnostics of insulation condition which can be done non-destructively and online in the field is of interest. In this paper, a new insulation diagnostic technique is proposed which utilizes natural transients in power system for online dielectric response measurement. In the proposed technique, lightning and switching impulses with wide range of frequency are used as stimuli for dielectric spectroscopy. Oil impregnated papers and condenser bushing are the objects under investigation and dielectric response of them is derived at those transients. Dielectric responses obtained by this method are verified by the responses measured by insulation diagnostic systems IDAX and LCR meter.

  • 38.
    Setréus, Johan
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hilber, Patrik
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Arnborg, Stefan
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Identifying Critical Components for Transmission System Reliability2012In: IEEE Transactions on Power Systems, ISSN 0885-8950, E-ISSN 1558-0679, Vol. 27, no 4, p. 2106-2115Article in journal (Refereed)
    Abstract [en]

    This paper presents a method to quantify and rank transmission system components by their importance for system reliability under different load scenarios. Each component is ranked by three separate importance indexes based on its expected outage rate and impact on: 1) system security margin; 2) load supply; and 3) generation units. By studying these three interests individually, a more complete view of the risks to system reliability can be assessed. The method is demonstrated on a detailed power system model (7000 components) of a significant part of the Great Britain transmission system at 400 and 275 kV. The results show how sensitive the component indexes are to the load scenario. The method provides an input for decision-making when planning maintenance and new investment and can be used as a complement to deterministic criteria.

  • 39.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric response and partial discharge measurements on stator insulation at varied low frequency2010Doctoral thesis, monograph (Other academic)
    Abstract [en]

    This is a study of potential improvements of diagnostic methods used on high-voltage generators and motors.  It considers offline electrical measurements on the main insulation of stator windings, where a sinusoidal voltage is applied between the winding and the stator-core, and the total current through the insulation (dielectric spectroscopy, DS) and the rapid current-pulses arising from discharges (partial discharge, PD) are measured.

    The proposed methods differ from existing practice in industrial DS and PD measurements in that the applied voltage is varied in amplitude and in frequency, harmonics of the voltage and current are measured, and the DS and PD measurements are made simultaneously, with comparison of results.  Based on literature, models and measurements, the problems and advantages of these methods are assessed in this work.

    Harmonics provide a way of separating linear and nonlinear sources of current, and reveal the waveform of the current.  Measurement of total PD charge by DS methods provides complementary information to the conventional PD measurement; the difference in results between these types of measurement is shown by literature and experimental results to be large.  Simultaneous measurement allows direct comparison of the relation between the DS and PD results, and saves time compared to separate measurements.  The varied frequency, down to the millihertz range, provides additional information about the insulation.  Much of the potential for DS methods on machine insulation is spoiled by the end-winding stress grading.  Models and measurements of the currents in this grading are presented, with discussion of how much effect the disturbance has and how well it can be predicted by modelling.

  • 40.
    Taylor, Nathaniel
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Stator End-winding Currents in Frequency-domain Dielectric Response Measurements2010In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 17, no 5, p. 1489-1498Article in journal (Refereed)
    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.

  • 41.
    Taylor, Nathaniel
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Differences between PD charges measured by Partial Discharge and Dielectric Spectroscopy systems2009In: Proceedings of the 21st Nordic Insulation Symposium (NordIS), 2009, p. 69-73Conference paper (Refereed)
    Abstract [en]

    In some rather idealised cases, similar values of Partial Discharge (PD) apparent charge may be obtained using a PD pulse measurement system or a capacitance and loss measurement system such as Dielectric Spectroscopy (DS). In many practical diagnostic situations, however, the results are widely different, for several possible reasons. The relative virtues of the two types of measurement are discussed here, on the grounds of useful diagnostic information. Some work on simultaneous measurement by these two methods is presented, and measurements from simple test objects are shown as examples of the factors limiting capture of the full PD charge by pulse measurement.

  • 42.
    Taylor, Nathaniel
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Partial discharge measurement at varied low frequency, and some results from thermally aged stator insulation2007In: Proceedings of the 20th Nordic Insulation Symposium (NordIS), 2007, p. 185-188Conference paper (Refereed)
    Abstract [en]

    Measurement of partial discharge (PD) at varied low frequency has the potential for greater information about an insulation system than is obtained with single-frequency measurements, and the advantage of low current-requirements of the driving source.  With frequency becoming another controlled variable, there is a heightened need to consider how to present the data.  From the desire to compare and interpret results of measurements at different frequencies there arises the need for appreciation of the effect of recent excitations on the result of a PD measurement.  Results of low-frequency PD measurements on three stator coils are presented here: a comparison of two shows strong variation in frequency dependence, and measurements on the other show the variation of a PD measurement due to varied recent history of excitation of the test-object.  Some indices for comparison of data at different frequencies and amplitudes are considered.

  • 43.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES).
    Diagnostics of stator insulatin by dielectric response and variable frequency partial discharge measurements: a study of varied low frequencies in stator insulation, with particular attention to end-winding stress-grading2006Licentiate thesis, monograph (Other scientific)
    Abstract [en]

    Stator insulation is a critical to the reliability of electrical generators and motors. It is common industrial practice to use electrical measurement of partial discharges (PD) and some form of current-voltage measurement as part of the condition assessment of stator insulation at maintenance times. Extension of these methods by the recently investigated methods of high-voltage dielectric spectroscopy (HV-DS) and variable frequency phase- resolved partial discharge analysis (VF-PRPDA) may offer r usefully increased information about the condition of a stators insulation while requiring less power from the test voltage source than with conventional power-frequency measurements.

    HV-DS and VF-PRPDA have independent variables of the amplitude and frequency of a sinusoidal voltage applied to the insulation system. The dependent variables are the smooth currents of HV-DS and the discharge pulse charges of VF-PRPDA; these may be analysed in many ways, typically as complex capacitance, the harmonic spectrum of currents when there is PD activity or other non-linearities in the insulation system, and various measures of PD pulse distribution.

    The methods provide complementary information and have a common need of a variable frequency high voltage driving source. This makes the simultaneous use of these methods a matter of interest, as further information can be gained without extra time and with a total equipment size and cost smaller than that of both separate systems.

    In this thesis, results are presented from several directions of work relevant to the application of the low frequency diagnostic methods, HV-DS and VF-PRPDA, to machine insulation.

    The contribution to measured frequency domain dielectric response from the current into the non-linear stress grading of stator end-windings has been studied from physical and numerical models. As well as the effecects on the dielectric response (complex capacitance), the harmonic spectrum of the current into the grading, and the distribution and waveform of the potential along the grading is shown from the numerical models. The frequency and amplitude dependent response of the stress-grading is of importance due to its significacant contribution to the measured dielectric response of a whole stator and to the harmonic currents due to other non-linear phenomena such as partial discharge currents.

    Short dielectric response measurements have been made on a complete hydro-generator before and after a period of maintenance, giving a better idea of the practical limitations that time constraints and a large test-object put on possible amplitudes and frequencies for driving the test object.

    Two new epoxy-mica stator coils have been studied with both DS and VF-PRPDA before and after accelerated thermal aging, as a preliminary step for seeing what changes can be detected electrically and for studying how the PD and DS methods differer in their measurement of PD.

  • 44.
    Taylor, Nathaniel
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    The dielectric response of stator end-winding stress-grading2005Conference paper (Refereed)
  • 45.
    Taylor, Nathaniel
    et al.
    KTH, Superseded Departments, Electrical Systems.
    Edin, Hans
    KTH, Superseded Departments, Electrical Systems.
    Utilisation of voltage and frequency dependence of stress-grading materials in dielectric diagnostics2004In: 2004 ANNUAL REPORT CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA, NEW YORK: IEEE , 2004, p. 178-181Conference paper (Refereed)
    Abstract [en]

    Diagnostics of electrical insulation is increasingly performed with the frequency and amplitude of the applied voltage varied over wide ranges. Cable terminations and high-voltage machine end-windings have stress grading systems whose response alters measured capacitance and loss components even in measurements at power frequency. Non-linear stress grading materials also contribute harmonic currents to the measured response. Excitation at frequencies other than power frequency changes the grading's potential-distribution, affecting the measured capacitance and possibly inciting partial discharges that contribute to measured loss and harmonic currents. Understanding of the effects of such stress grading systems is important for analysis and possible compensation of dielectric measurement data.

  • 46.
    Ghaffarian Niasar, Mohamad
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Kiiza, Respicius Clemence
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Frequency Response of Oil-impregnated paper: the Effect of Partial Discharges Compared to other InfluencesManuscript (preprint) (Other academic)
  • 47.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Dielectric Response Measurement with Impulse Stimulus on AC: Measurement Considerations, and Laboratory Testing on a BushingManuscript (preprint) (Other academic)
  • 48.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Effect of Superimposed Impulses on AC Partial Discharge Characteristics of Oil-Impregnated PaperManuscript (preprint) (Other academic)
  • 49.
    Laurell Lyne, Åsa
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Transport Science, Highway and Railway Engineering.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Jäverberg, Nadejda
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Birgisson, Björn
    KTH, School of Architecture and the Built Environment (ABE), Transport Science, Highway and Railway Engineering.
    Low Frequency Dielectric Spectroscopy of Bitumen Binders.Manuscript (preprint) (Other academic)
  • 50.
    Nikjoo, Roya
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Hollertz, Rebecca
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wåhlander, Martin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Taylor, Nathaniel
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Edin, Hans
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Surface Discharge Characteristics of Oil-impregnated Paper with SiO2 and ZnO nanoparticles under AC with Superimposed ImpulseManuscript (preprint) (Other academic)
1 - 50 of 50
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