| 1. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Charge decay on enamel wire surface
- 2014
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Ingår i: Proceedings of International Conference on High Voltage Engineering and Application 2014 (ICHVE 2014), ISBN 978-1-4799-6613-4. - 9781479966134 ; , s. 1-4, paper A-4-3-
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Konferensbidrag (refereegranskat)abstract
- Partial discharge resistant enamel insulation filled with chromium oxide (Cr2O3) has been developed recently. In the paper, dynamics of surface charges on samples of the material is analyzed and is compared with that on conventional enamel. The experiments were conducted on test objects prepared as windings of enameled wires. Surface charging was implemented using dc corona from a needle electrode which was placed against the grounded winding. After charging, the potential induced by deposited charges was measured by non-contact technique. The surface charge density pattern was reconstructed utilizing Φ-matrix method. The decay of charges accumulated on insulation surfaces was recorded. It was found that the decay process was considerably faster for the filled enamel than for the conventional one, which may be one of the factors contributing to the improved partial discharge resistance of the former.
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| 2. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Charge transport in a double-layered enamel insulation during surface potential decay
- 2018
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 25:2, s. 702-712
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Tidskriftsartikel (refereegranskat)abstract
- This work presents a numerical model for studying charge dynamics during decay of pre-deposited surface charges on an enamel insulation comprising two layers of different materials. It is shown that the considered surface charge decay is dominated by charge injection from insulation surface into the bulk and the drift of injected carriers along the direction of induced electric field. These features allow for estimating charge carriers' mobilities in the studied insulation. The developed model incorporates experimental outcomes as well as the proposed effect of an internal interface between insulating layers on hindering charge transport through the bulk. The transient process of charge accumulation at the internal interface is exhibited and correlated with the well-known Maxwell-Wagner-Sillars polarization model. Contribution of charge injection at each insulation surface into the bulk to the measured potential decay is evaluated and compared.
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| 3. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Charge Transport in LDPE Nanocomposites Part I—Experimental Approach
- 2016
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Ingår i: Polymers. - : MDPI AG. - 2073-4360. ; 8:3, s. 87:1-87:19
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Tidskriftsartikel (refereegranskat)abstract
- This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.
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| 4. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Charge Transport in LDPE Nanocomposites Part II—Computational Approach
- 2016
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Ingår i: Polymers. - : MDPI AG. - 2073-4360. ; 8:4, s. 103:1-103:16
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Tidskriftsartikel (refereegranskat)abstract
- A bipolar charge transport model is employed to investigate the remarkable reduction in dc conductivity of low-density polyethylene (LDPE) based material filled with uncoated nanofillers (reported in the first part of this work). The effect of temperature on charge transport is considered and the model outcomes are compared with measured conduction currents. The simulations reveal that the contribution of charge carrier recombination to the total transport process becomes more significant at elevated temperatures. Among the effects caused by the presence of nanoparticles, a reduced charge injection at electrodes has been found as the most essential one. Possible mechanisms for charge injection at different temperatures are therefore discussed.
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| 5. |
- Hoang, Tuan Anh, 1985
(författare)
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Charge transport in polymer-based insulating materials for high voltage applications: effect of single- and multi-layered structures
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polymer composites have been widely employed as electrical insulating materials for high voltage components and devices such as power cables, gas-insulated systems, rotating machines, line and post insulators, etc. Such materials are usually made by introducing inorganic oxides (SiO2, TiO2, Al2O3, MgO) into polymeric matrices e.g. polyethylene (PE), epoxy, silicone rubber. Reported experimental evidences indicate improvements which can be achieved in dielectric strength, partial discharge resistance, and life time of the polymer composites as compared to their base materials. However, the reasons behind the advancements in materials’ electrical performance have not been clearly elucidated in many cases. In this context, the present thesis concentrates on analyzing the influence of filler particles on transport of charge carriers in two polymer-based insulations namely nanofilled PE and microfilled enamel, which are novel insulating materials for high voltage direct current (HVDC) cables and electric motors, respectively. The contribution of internal interfaces between insulating layers existing in multi-layered structures to their electrical conduction is also considered. As for PE nanocomposites and its unfilled counterpart, specimens of thin films were prepared, whereas multi-layered structures were made by pressing them together at high temperature. Charging currents measured at various temperatures indicated a remarkable reduction in DC conductivity of the nanofilled dielectrics as compared to PE, making the former preferable insulating materials for HVDC cables, especially for those working under enhanced electric stresses. The observed effect was associated with the decreased charge mobility and increased trap energy in the nanocomposites as compared to the corresponding properties of unfilled PE. By comparing the measured currents obtained on single-layered and multi-layered structures, the impact of insulation–insulation interface on electrical conduction was revealed and analyzed. Further, the field dependencies were established for the transient currents and the quasi-steady state currents that were utilized for examining the dominant conduction mechanisms in the studied materials. A computer model was employed for studying the generation and transport of charge carriers in the insulations at various temperatures. The simulations demonstrated that apart from the reasons mentioned above, the weakening charge transport process in the nanocomposites is in great extent caused by the suppression of charge injection at the electrode–insulation interfaces. On the other hand, the insulation coatings of enameled wires are inherently multi-layered structures consisting of at least two layers of different materials. This study focuses on partial discharge resistant enamel insulation that was created by introducing chromium oxide (Cr2O3) particles of micrometer size into the base polymer. Experiments were conducted mainly on multi-layered enamel coatings, while some tests were also performed on single-layered flat samples and filler powder. Thus, contributions of each phase (filler and polymer matrix) to the electrical conduction and dielectric response of the filled enamel were highlighted. Charge transport in the multi-layered enamel coatings was studied in details by analyzing the surface potential decay characteristics. Accordingly, potential decay mechanisms due to the combination of charge injection, polarization, and intrinsic conduction were revealed, each of them dominating within a certain time interval. Based on that, a computer model of charge transport in multi-layered insulating structures was developed to reproduce the measured surface potentials. In this model, the consideration of the barrier effect of the internal interface is of utmost importance.
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| 6. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Charging and Discharge Currents in Low-Density Polyethylene and its Nanocomposite
- 2020
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Ingår i: Energies. - : MDPI AG. - 1996-1073 .- 1996-1073. ; 13:6, s. 1518-1535
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Tidskriftsartikel (refereegranskat)abstract
- Charging and discharge currents measured in low-density polyethylene (LDPE) and LDPE/Al2O3 nanocomposite are analyzed. The experiments were conducted at temperatures of 40–80 °C utilizing a consecutive charging–discharging procedure, with the charging step at electric fields varying between 20 and 60 kV/mm. A quasi-steady state of the charging currents was earlier observed for the nanofilled specimens and it was attributed to the enhanced trapping process at polymer–nanofiller interfaces. An anomalous behavior of the discharge currents was found at elevated temperatures for both the studied materials and its occurrence at lower temperatures in the nanofilled LDPE was due to the presence of deeply trapped charges at polymer–nanofiller interfaces. The field dependence of the quasi-steady charging currents is examined by testing for different conduction mechanisms. It is shown that the space-charge-limited process is dominant and the average trap site separation is estimated at less than 2 nm for the pristine LDPE and it is at about 5–7 nm for the LDPE/Al2O3 nanocomposite. Also, location of the trapping sites in the band gap structure of the nanofilled material is altered, which substantially weakens electrical transport as compared to the unfilled counterpart.
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| 7. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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DC electrical conductivity of LDPE-based nanocomposites
- 2015
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Ingår i: 9th International Conference on Insulated Power Cables, Jicable’15, 21 – 25 June 2015, Versailles – France (2015). ; , s. paper B6.5-
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Konferensbidrag (refereegranskat)abstract
- This work presents studies of the effect of nanofillers on dc electrical conductivity of polymeric materials, potentially applicable in high voltage direct current (HVDC) cable insulation. Samples of low-density polyethylene (LDPE) filled with nanoparticles of aluminium oxide (Al2O3) and magnesium oxide (MgO) up to 3 wt% were prepared, on which charging currents were measured at 40 and 60 °C and electric stress of 32 kV/mm. Experimental results for nanocomposites were compared with those of reference LDPE. It was found that the addition of nanofillers led to a significant decrease in electrical volume conductivity and the decline depended strongly on filler type, its content and surface treatment. Further, the influence of specimen thickness on the conductivity was examined for verifying the operating conduction mechanisms. An indication was found that the space charge limited current (SCLC) mechanism dominated the conduction process in the reference material, while this effect ceased for the nanocomposites.
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| 8. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Electrical Characterization of a New Enamel Insulation
- 2014
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 21:3, s. 1291-1301
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Tidskriftsartikel (refereegranskat)abstract
- Investigations of dielectric properties of a newly developed enamel wire insulation created by adding chromium (III) oxide (Cr2O3) filler to polyamide-imide enamel base are presented. Results of measurements of electrical conductivity and complex permittivity at various temperatures as well as surface potential decay are discussed and compared with corresponding properties of standard enamel insulation. Contributions of different polarization relaxation processes in both enamels are examined based on the obtained master curves of dielectric response. In addition, the properties of chromium oxide filler are characterized separately and utilized further for analyzing its impact on the performance of enamel wire insulation by means of computer simulations. The experimental and simulated results demonstrate that the introduction of chromium oxide yields changes in the electrical properties that allow for mitigating the voltage stress in a wound insulation system. Furthermore, a correlation between the obtained results and the earlier described improved resistance to partial discharge activity of the chromium oxide filled enamel is discussed.
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| 9. |
- Hoang, Tuan Anh, 1985
(författare)
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Electrical Characterization of Partial Discharge Resistant Enamel Insulation
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Adjustable speed drives for rotating machines have become increasingly popular as they provide possibilities of smooth and accurate process control as well as for energy savings. In such systems, due to the fact that the voltage applied at terminals of motor windings is no longer purely sinusoidal but characterized by a high content of harmonics, the appearing electrical and thermal stresses yield premature failures of the winding insulation, mainly because of an increased partial discharge (PD) activity. To tackle the problem effectively, PD-resistant enamels have been developed, by introducing various inorganic fillers into the base polymer of the wire insulation, that exhibit an increased resistance to PD activity. The effect seems, in most of the cases, to be strongly dependent on the dielectric properties of the filler used and the degree of interactions with the host materials.In this respect, a recently developed enamel insulation, in which the top layer of the multilayer coating is filled with micro-particles of chromium (III) oxide (Cr2O3), has demonstrated exceptional properties. To elucidate the effect of material properties on the PD behavior, the presented in this thesis study concentrates on electrical characterizations of the new enamel insulation by measuring and analyzing its electric conductivity and complex dielectric permittivity and comparing these parameters with of the properties of enamel without the filler. In addition, the decay of surface potential induced by deposition on the enamel wire surfaces electric charges from a corona source is examined. The performed analyses show that the addition of Cr2O3 filler results in a highly conductive and dispersive material, as compared to the base enamel. As a consequence, the top coating layer of the enamel wire insulation activates suppression of PD activity, while the insulating properties of the bulk material are kept unchanged. The presence of chromium oxide results in enhanced interfacial polarization and low frequency dispersion (hopping conduction) of the enamel. The material conductivity is increased because of enhancement of charge carrier mobility as well as a shift of the maximum of trap density distributions towards lower energy levels.. The mechanism of the surface potential decay is analyzed by means of a model in which the combination of charge injection, dipolar polarization and intrinsic conduction appear to be the most important contributors to the observed surface charge dynamics.
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| 10. |
- Hoang, Tuan Anh, 1985, et al.
(författare)
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Mechanisms of Surface Potential Decay on Enamel Wire Coatings
- 2015
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - : Institute of Electrical and Electronics Engineers (IEEE). - 1558-4135 .- 1070-9878. ; 22:6, s. 3470-3480
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Tidskriftsartikel (refereegranskat)abstract
- Results of measurements of surface potential decay on coating layer of enameled wires are presented. In experiments, surfaces of winding samples were charged by dc corona of both polarities and decay of surface potential induced by charges deposited on the materials was recorded utilizing non-contact technique. The power-law type of time dependences of surface potential decay rates was obtained that indicated a dominant role of charge injection and/or dipolar polarization in the decay process. Furthermore, several decay mechanisms were employed for examining the experimental data, which revealed that potential decay was initially governed by the injection of charges due to electron transfer between the deposited charges and the material. Later on, the contribution of polarization to decay process was significant, while intrinsic conduction became the prevailing decay mechanism only at the latest stage as polarization was stabilized.
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