Papers by Keyword: Space Charge

Abstract: In this paper, trap levels around the glass transition temperature (T_g) of polymers have been characterized using Thermally Stimulated Current (TSC) technique. Deconvolution on α-peaks of the T_g for PE (-104 °C), plasticized PVC (-35 °C), PMMA (90 °C) and PET (96 °C) were carried out based on the first-order kinetic theory for non-Debye relaxation. Using temperature, T from TSC experimental data, we have successfully separated the α-peaks of the thermoplastic polymers. It is found that the complex curve of α-peaks can composed of four (4) to eight (8) sub peaks. Dominant sub peaks were identified at T_max = -105 °C, -34 °C, 89 °C and 92 °C for PE, pPVC, PMMA and PET, respectively. These peaks show activation energy, E_a of shallow and deep trap centers ranged from 0.3 eV to 4.6 Ev where they represent the depolarization of localized dipoles and space charges relaxations in the polymers.

Abstract: Moisture has a detrimental effect on oil-paper insulation life by lowering electrical breakdown strength. How does the space charge behavior of the oil-paper insulation system consisted by oil gap and oil immersed insulation pressboard with different moisture content The space charge characteristics of the mixed insulation consisted of oil gap and pressboard with three moisture content were investigated using the PEA measurement system. The space charge accumulation behaviors at the interface between oil gap and pressboard were analyzed. The distortion of the electrical field in the oil gap and pressboard was also analyzed. Results show that there are many charges injected into the oil and the pressboard. The charges accumulated at the interface between oil gap and pressboard. The charges accumulated in the mixed insulation system become less with the moisture content increased. The difference of the electric field strength in the oil gap and pressboard become smaller with the moisture content increased.

Abstract: ±1000kV DC wall bushing is a complex insulation system working in different conditions, such as DC, AC and polarity reversal. No matter what kind of conditions, the space charge effects on the electric field distribution of the bushing are remarkable. A finite element model for ±1000kV DC wall bushing is established based on space charge theory, to analyze the electric field distribution in different conditions, especially when polarity reversal occurs. The research results provide useful references for the electric field analysis and insulation optimization of UHV (ultra high voltage) DC wall bushings.

Abstract: The photo-stimulating discharge (PSD) and ultraviolet-visible spectrometry were used to characterize the polyethylene (PE) and nano-hybrid PE trap before and after ultraviolet aging. In PE with the increasing of ultraviolet aging time, shallow traps reduce gradually and deep traps increase gradually, but for the nano-hybrid PE, shallow traps increase gradually and deep traps reduce gradually. Experiments show that: in the process of aging PE produces oxidation products such as carbonyl, hydroxyl etc. The incorporation of impurities can inhibit the injection of space charges, so as to achieve the purpose of improving electric field distortion in the body of the PE.

Abstract: MgO/XLPE composites, composed of micro-and nanosized MgO material having different particle size as inorganic filler and cross linked polyethylene (XLPE), were prepared and investigated to the space charge dynamics of these composites for high voltage DC insulation. Added MgO material, having 50nm to 1μm in size, were synthesized from the calcination of Mg (OH)₂ nanoplatelets which was controlled by potassium content in Mg (OH)₂, varying the dilution volume ratio of distilled water/ Mg (OH)₂ suspension. MgO/XLPE composites were obtained by thoroughly mixing LDPE and dicumyl peroxide (DCP) as the cross-linking agent and silane modified MgO before kneading to enhance the dispersion degree of the composites. The space charge distributions of the composites were observed by controlling the particle size and content of a dispersed MgO in XLPE matrix. To prevent the breakdown under high DC stress, it was found that the reduction of space charge of MgO/XLPE composite depends upon the smaller size of MgO particle and the its content above 0.5 phr (per hundred part of resin, wt%). MgO material and MgO/ XLPE composites were characterized with TEM, XRD, BET, ICP, and the space charge properties of the composites were measured by a pulsed electro-acoustic method (PEA).

Abstract: Arc breaking at the function of large current is one of the bottlenecks for the development of vacuum breaker to higher voltage level. Simulation on arc is one of the most effective methods in this field. Before-arc process model of vacuum circuit breaker (VCB) whose breaking current is 20kA current and clearance is 10mm is developed here based on Boltzmann equation. Collisions between charged particles and metal vapor and electrodes are considered in the simulation. The physical nature of the sheathe and arc columns formation, the space charge densities and their effects on electric field strength before the steady state of arc plasma are simulated here.

Abstract: Based on space charge theory, electric field and potential distribution of ±1000kV DC wall bushing under DC and AC work condition is analyzed in this paper. The position and amount of space charge can be deduced from space charge theory and surface charge theory. By analyzing electric field and potential distribution under DC and AC voltage work condition with and without space charge, valuable conclusions are drawn, which provides valuable reference for insulation design.

Abstract: The influence of humidity content on the electrical and dielectric properties of a composite made from recycled TetraPak® containers and granulated cork was studied. The material components have been dried before preparation and after the composite was conditioned by keeping the samples in a dry environment (desiccator) or in an oven at high temperature (70°C in air). The differences observed in electrical properties (investigated by isothermal charge and discharge current measurement) and dielectric properties (measured using dielectric relaxation spectroscopy) show that the thermal treatment at high temperature is more efficient on removing water and slows down the re-absorption rate.

Abstract: The isothermal charging current and the isothermal discharging current in low mobility materials are analyzed either in terms of polarization mechanisms or in terms of charge injection/extraction at the metal-dielectric interface and the conduction current through the dielectric material. We propose to measure the open-circuit isothermal charging and discharging currents just to overpass the difficulties related to the analysis of the conduction mechanisms in dielectric materials. We demonstrate that besides a polarization current there is a current related to charge injection or extraction at the metal-dielectric interface and a reverse current related to the charge trapped into the shallow superficial or near superficial states of the dielectric and which can move at the interface in the opposite way that occurring during injection. Two important parameters can be determined (i) the highest value of the relaxation time for the polarization mechanisms which are involved into the transient current and (ii) the height of the potential barrier W0 at the metal-dielectric interface. The experimental data demonstrate that there is no threshold field for electron injection/extraction at a metal-dielectric interface.

Abstract: For the characterization of the new materials and for a better understanding of the connection between structure and properties it is necessary to use more and more sensible methods to study molecular movement at nanometric scale. This paper presents the experimental basis for a new electrical method to study the fine molecular movements at nanometric scale in dielectric materials. The method will be applied for polar and non-polar materials characterization. Traditionally, the electrical methods used to study the molecular movements are based on the movements of the dipoles that are parts of the molecules. We have proposed recently a combined protocol to analyze charge injection/extraction, transport, trapping and detrapping in low mobility materials. The experimental results demonstrate that the method can be used to obtain a complex thermogram which contains information about all molecular movements, even at nanoscopic level. Actually during the charging process we are decorating the structure with space charge and during the subsequent heating we are observing an apparent peak and the genuine peaks that are related to charge de-trapping determined by the molecular movement. The method is very sensitive, very selective and allows to determinate the parameters for local and collective molecular movements, including the temperature dependence of the activation energy and the relaxation time.