| 1. |
- Kumara, Sarath, 1979, et al.
(författare)
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DC Flashover Characteristics of a Polymeric Insulator in Presence of Surface Charges
- 2012
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 19:3, s. 1084-1090
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Tidskriftsartikel (refereegranskat)abstract
- Effect of surface charges on dc flashover characteristics of a composite polymeric insulator is studied by means of experiments and theoretical calculations. The considered insulator consisted of a glass fiber reinforced epoxy core covered with a layer of silicone rubber and terminated by metallic electrodes with rounded smooth edges. In the experiments, the insulator surface was charged by external corona while keeping the electrodes grounded and different charging levels were realized by varying its intensity. A series of disruptive discharge tests were carried out on the charged insulator under negative dc voltages. It was revealed that negative deposited surface charges led to an enhancement of the flashover performance whereas positive ones reduced the flashover voltage level. A theoretical model has been developed and utilized for analyzing the experimental results. In the model, surface charge density profiles deduced from measured surface potential distributions were used as boundary conditions for calculations of electric fields. The measured and calculated flashover voltages were found to be in agreement indicating that the observed variations in the flashover characteristics could be attributed to the modifications of the electric field produced by the surface charges.
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| 2. |
- Kumara, Sarath, 1979, et al.
(författare)
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Surface Charges on Cylindrical Polymeric Insulators
- 2012
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 19:3, s. 1076-1083
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Tidskriftsartikel (refereegranskat)abstract
- Distributions of charges are investigated on surfaces of polymeric cylindrical insulators that emerge due to dc corona discharges in surrounding air and by stressing with dc voltage. The studied model insulator consisted of a glass fiber reinforced epoxy core covered with a sheath of silicone rubber, terminated by metallic electrodes with rounded smooth edges. Surface potential distributions were measured 1 min after completing the charging process by means of a Kelvin electrostatic probe connected to an electrostatic voltmeter. The measured surface potential profiles were utilized to calculate corresponding charge density distributions. The obtained results demonstrated significant differences in the surface charge patterns for different charging conditions, which in turn may affect flashover withstand of the insulator.
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| 3. |
- Alam, Shahid, 1986, et al.
(författare)
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Contribution of Gas Neutralization to the Potential Decay on Silicon Rubber Surfaces at Different Ambient Pressures
- 2014
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Ingår i: ICHVE 2014 - 2014 International Conference on High Voltage Engineering and Application, Poznan, Poland, 8-11 September 2014. - 9781479966134 ; , s. 1 -4
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Konferensbidrag (refereegranskat)abstract
- Surface charging and charge decay on insulating polymers are inherent phenomena in HVDC insulation systems and knowledge about them is essential for proper insulation design, testing and co-ordination. In the present study, surface potential decay on silicone rubber samples is analyzed at different pressures of ambient air. For both polarities of pre-deposited surface charges, the intensity of gas neutralization and its contribution to the total charge/potential decay are examined by means of experimental measurements. The results of the study indicate that gas neutralization is efficient at high magnitudes of surface potentials and for materials with relatively low conductivity. Reduction of air pressure weakens the intensity of the background ionization in gas and leads to diminishing amount of free ions. Under these conditions, the contribution of gas neutralization to the total charge decay is reduced and the decay process is determined solely by properties of solid material (bulk and surface electric conductivities)
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| 4. |
- Alam, Shahid, 1986, et al.
(författare)
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Effect of Interfaces on Surface Potential Decay on Double Layered HTV Silicone Rubber Samples
- 2016
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Ingår i: Proceedings of International Conference on Dielectrics. - 9781509028030 ; 1, s. 309-312
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Konferensbidrag (refereegranskat)abstract
- Interfaces between materials constituting composite HVDC insulation systems are typically associated with abrupt changes of the materials’ dielectric properties, which cause accumulation of interfacial charges. These may affect various processes in the insulation system, including dynamics of charge transport and electric field distribution as well as its flashover performance. In the present study, the influence of macroscopic interfaces between layers made of high temperature vulcanized silicone rubbers on surface potential decay characteristics is analyzed. It is shown by comparing results of measurements on single and double layered samples that the rate of the potential decay may be significantly changed (increased or reduced) depending on how the layered structure is designed. The performed analyses show that the observed features cannot be explained by means of classical Maxwell-Wagner model and introducing additional charges at the investigated interface is necessary to successfully model the experimental data. The presented discussion concentrates on defining possible sources of such interfacial charges.
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| 5. |
- Alam, Shahid, 1986, et al.
(författare)
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Field-dependent electric conductivities of silicone rubbers deduced from measured currents and surface potential decay characteristics
- 2019
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Ingår i: International Journal of Polymer Analysis and Characterization. - 1563-5341 .- 1023-666X. ; 24:1, s. 54-62
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of DC electric conductivities of four types of silicone rubber-based polymers for use in high voltage insulation systems of power components are reported. The field dependences of the conductivities obtained by two different techniques, namely by utilizing steady-state currents through materials’ samples placed between metallic electrodes and by employing surface potential decay characteristics in an open circuit configuration, are compared and discussed. It is shown that the surface potential decay technique allows for a wider range of electric field strength and reduces the time span of the measurements.
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| 6. |
- Alam, Shahid, 1986, et al.
(författare)
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Potential Decay on Silicone Rubber Surfaces Affected by Bulk and Surface Conductivities
- 2015
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Ingår i: IEEE Transactions on Dielectrics and Electrical Insulation. - 1558-4135 .- 1070-9878. ; 22:2, s. 970-978
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Tidskriftsartikel (refereegranskat)abstract
- A decay of electric surface potential on a pre-charged gas-solid interface involves several charge transport processes and each of them may become dominant under certain environmental conditions (temperature, pressure, humidity). In the present paper, surface potential decay on flat samples of different kinds of silicone rubber used in HVDC applications is analyzed at reduced pressures of ambient air that allows for minimizing the involvement of the gas phase, i.e. surface charge neutralization by gas ions. Effects imposed solely by bulk and surface conduction in the solid material are studied experimentally and by means of computer simulations. The results allow for evaluating threshold values of volume and surface electric conductivities at which these mechanisms become most essential. Field dependent bulk conductivities are deduced from the surface potential decay characteristics obtained for the studied materials.
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| 7. |
- Alam, Shahid, 1986
(författare)
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Surface Charge Dynamics on Polymeric Insulating Materials for High Voltage Applications
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To meet increasing demands in electric energy, it is essential to enhance production of electricity from renewable energy sources (solar, wind and hydro). Such generation sites, however, are usually separated from consumption sites by long distances. An efficient transportation of energy requires implementations of high voltage direct current (HVDC) transmission systems, which operate today at rated voltages up to ±800kV. To provide electric insulation for such voltage levels, polymeric insulators are preferable due to a number of advantages over traditionally used ones made of glass or porcelain. The use of polymers, however, leads to surface charging and charge dynamics on insulating elements, which are inherent phenomena in HVDC insulation systems. Thus, knowledge about these processes is essential for proper insulation design, testing and co-ordination. Therefore, the conducted research aimed at providing information about fundamental mechanisms of electric charge transport in HVDC insulation and focused on analyzing roles of gas phase and properties of solid materials on surface charge dynamics. The study was conducted utilizing flat samples of several types of HTV silicon rubber and cross-linked polyethylene, which are widely used in different HVDC applications. The electrical conductivities and dielectric permittivities of the materials were measured in time and frequency domain, respectively. To study variations of surface charges, the samples were exposed to corona generated in air from nearby sharp electrode that yielded accumulation of electric charges on gas-solid interfaces. Surface potentials induced by the deposited charges were measured at different instants after charging that allowed for obtaining surface potential decay characteristics for the studied materials. The measurements were conducted for both polarities of pre-deposited surface charges at different pressures of ambient air that provided a possibility to control the intensity of neutralization of the deposited surface charges by free counter ions present in air and to evaluate relative contribution of this process to the charge/potential decay. It was found that a reduction of air pressure weakened the intensity of the background ionization in gas and led to diminishing amount of free ions. Under these conditions, the contribution of gas neutralization to the total charge decay was reduced and decay mechanisms were determined solely by the properties of solid materials. Effects imposed by bulk and surface conduction in the solid material on surface charge dynamics were studied by means of experimental measurements and computer simulations. The obtained results allowed for evaluating threshold values of the volume and surface conductivities at which these transport mechanisms become essential. It is demonstrated that bulk conduction becomes dominant mechanism of surface potential decay if volume conductivity of the material is above ~10-16 S/m. The results of the modeling agree well with the measured characteristics if materials’ field-dependent conductivities are taken into account. The performed parametric studies also demonstrate that surface conduction may influence the potential decay if the corresponding conductivity exceeded ~10-17 S.
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| 8. |
- Alam, Shahid, 1986, et al.
(författare)
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Surface Potential Decay on Silicon Rubber Samples at Reduced Gas Pressure
- 2013
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Ingår i: Proceedings of the 23rd NORDIC INSULATION SYMPOSIUM, June 9-12, 2013 Trondheim, Norway. - 9788232102747 ; , s. 19-22
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Konferensbidrag (refereegranskat)abstract
- Accumulation of interfacial charges is an inherent feature of HVDC insulation based on solid and gaseous media. The collected surface charges can alter the geometrical electric field leading to undesirable phenomena such as partial discharges and even unexpected flashovers. In the present paper, surface potential decay on silicone rubber samples is analyzed at reduced pressures of ambient air that allows for elimination of surface charge neutralization by gas ions. Thus, influences imposed by bulk and surface conduction in the solid material are studied by means of computer simulations and experimental measurements. The results allow for identifying levels of bulk and surface conductivities above which the corresponding charge decay mechanism becomes dominant. It is shown that with a negligible space charge effect and significant surface leakage, there exists a notable spread of charge along gas-solid interface yielding visible crossover phenomenon in charge decay characteristics. It is also demonstrated that the effect of space charge in the material bulk on surface potential decay can only be significant within layers of material finer than ~100 µm.
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| 9. |
- Alam, Shahid, 1986
(författare)
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Surface potential dynamics on insulating polymers for HVDC applications
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The use of high voltage direct current (HVDC) technology in power transmission systems is continuously expanding. Nowadays, HVDC transmissions operate at voltages up to 800 kV and higher levels are being developed. To secure continuous and reliable transportation of electric energy in such systems, materials used for electrical insulation should satisfy stringent requirements related to their performance under high electrical stresses. These concern, in particular, charge accumulation and its dynamics on surfaces of insulating elements which affect distributions of electric fields and may even influence flashover performance. Thus, the conducted study aimed at increasing understanding of surface charge dynamics on insulating polymers that is essential for proper design, testing and co-ordination of HVDC insulation. The work was performed utilizing flat samples (thicknesses ~2 mm and ~300 µm) of several types of high temperature vulcanized silicon rubbers. The materials were first characterized by measuring their electrical conductivities and complex dielectric permittivities. A non-contact technique, based on application of Kelvin type electrostatic probe, was thereafter used to measure surface potentials and their decay characteristics on single- and double-layered samples of these materials. The samples were located on a grounded metallic base and their open surface was pre-charged by means of a corona source in air under atmospheric pressure. The dynamic behavior of surface potential was afterwards investigated at various air pressures (1 bar, 600 mbar and 300 mbar) and temperatures (from room temperature to 70 oC), which allowed for minimizing the influence of gas phase on the decay of the deposited charges and for examining solely the effect of solid material properties. Furthermore, a computer model describing the surface potential dynamics has been developed and utilized for analyzing the results of the experiments. The performed study has demonstrated that deposition of charges generated by corona on the open material surface induce potential distribution decaying with time but continuously preserving its initial space distribution. The decay is found to be slower at reduced gas pressures. It also depends on material conductivity, being faster on more conductive materials as well as at increased temperatures, well responding to the thermal activation of conduction processes. These facts indicate that bulk conduction is the dominant mechanism of surface potential decay under conditions of the present study, which could also be confirmed by the computer simulations. It was in addition observed that the decay on double-layered structures could be faster as compared to that on single-layered ones, if a more conductive material was used for the base layer, which remained in contact with the grounded metallic electrode. A model of interfacial polarization was employed to analyze this effect. The analyses of surface potential decay also allowed for independently determining bulk conductivity of the investigated materials and its variation with electric field strength, yielding results comparable with those obtained by means of the conventional method. The determination of material conductivity based on surface potential decay provides a number of advantages, in particular, a reduced measuring time and a wider range of the analyzed electric field strength.
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| 10. |
- Alam, Shahid, 1986, et al.
(författare)
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Temperature and Field Induced Variations of Electric Conductivities of HTV Silicone Rubbers Derived from Measured Currents and Surface Potential Decay Characteristics
- 2020
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Ingår i: Energies. - : MDPI AG. - 1996-1073 .- 1996-1073. ; 13:11, s. 2982-2992
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Tidskriftsartikel (refereegranskat)abstract
- The temperature and field dependencies of electric conductivities of two types of silicone rubber-based polymers intended for use in high voltage direct current applications are presented and discussed. The conductivities obtained with the standard method by measuring a current through the material sample placed between metallic electrodes in response to the applied voltage are compared with those deduced from the measured potential decay on pre-charged material surface in an open circuit configuration. The measurements were conducted in the range of the applied electric field strength (0.5–5) kV/mm and temperatures ranging from 22 °C to 70 °C. It is shown that the values of the conductivities obtained by the two methods are in agreement and their temperature dependences obey Arrhenius law yielding similar activation energies.
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