1,721,017 research outputs found
Dynamics of space charge and electroluminescence modelling in polyethylene
No full textSpace charge has been recognized as an important factor contributing to the electrical failure of the cable insulation. Extensive efforts have been made to investigate space charge dynamics within polymeric insulations under electric stresses. Basic information about space charge has been recognized resorting to modern charge mapping techniques but the underlying mechanisms for charge transport, charge trapping characteristics are not yet well understood. Hence theoretical modelling and numerical simulation are employed to simulate the space charge and provide an insight into the charge distribution in dielectrics. This thesis comprises the quantitative analysis of space charge through numerical modelling and experimental investigations of charge trapping in polymeric insulation materials.A bipolar charge transport model which involves bipolar charge injection from the electrodes, charge transport with trapping and recombination in the bulk has been developed to simulate the dynamics of space charge in polyethylene. The build-up of space charge in polyethylene under dc electric fields has been modelled. The influence of parameters related to the properties of polyethylene on the formation of space charge has been recognized. Furthermore, this model is introduced to simulate the dynamics of corona charge decay in polyethylene. The formation of charge packets in polyethylene is also investigated using a numerical modelling approach.A fast pulsed electro-acoustic system along with a data processing program has been developed to investigate the behaviour of space charge in polyethylene under ac voltages. The understanding of space charge under ac stresses has also been simulated using the further developed bipolar charge transport model. Experiments and simulation have also been expanded into understanding electroluminescence, which is an indication of pre-aging of polymers under ac stress
Space charge and thickness dependent dc electrical breakdown of solid dielectrics
No full textA new model based on space charge dynamics under very high dc electric field has been proposed to explain thickness dependent dielectric breakdown. Space charge phenomenon under high electric field has been studied for several decades thanks to the development of new charge mapping techniques. Overwhelming evidences show that the charge packet can be formed in the material under high electric field. The formation and dynamics of the charge packet will result in local electric field enhancement that has a direct impact on breakdown. It has been found that the key factors leading to the formation of charge packet are negative differential mobility and low trapping coefficient. Take these factors into the space charge based model, our simulation results clearly show that the breakdown is dependent on the sample thickness. Through the simulation, it has been noticed that the electrical breakdown field reduction depends on several parameters such as the onset of critical electric field when breakdown occurs. By varying the ramp rate of dc applied voltage, simulation has also shown that the breakdown strength increases with the voltage ramp rat
Space charge in polyethylene under combined AC and DC voltages
No full textIn contrast to extensive publication on the behavior of space charge in polymeric insulation materials subject to individual dc or ac voltages, space charge in the situation of composite voltages has not yet been reported. In this paper, the dynamics of space charge in polyethylene subjected to combined ac and dc voltages have been experimentally investigated using the pulsed electroacoustic technique. Results show that combined voltages could significantly enhance the space charge accumulation. The dc component voltage determines the charge accumulation in the bulk of polyethylene while ac component influences the charge density on the electrodes. The amount of charge in the bulk depends on the ratio between ac and dc voltage components. This result is important to understand the electrical performance of high voltage equipment where insulation may experience the combined ac and dc voltages
Observation of negative differential mobility and charge packet in polyethylene
No full textCharge packet has been observed on many occasions but its physical mechanisms have never been properly understood. One of the models proposed by Lewis et al shows the presence of negative differential mobility with an electric field in semicrystalline polyethylene. In this paper we have observed the negative differential mobility using the transient space charge profile measured by the pulsed electroacoustic technique. By superimposing a short pulse voltage to a dc voltage, it is possible to obtain the velocity of holes at different applied fields. To the best of our knowledge we have for the first time observed negative differential mobility in polyethylene. This observation provides crucial evidence to support Lewis’s model and allows one to simulate charge packet and its behaviours
Space charge and DC dielectric breakdown in polymeric insulation
No full textmany years but the mechanisms that cause breakdown have not been fully understood. Charge injection always occurs at high electric fields and especially under dc conditions. The presence of space charge in solid dielectrics will result in distortion of electric field distribution. Dielectric breakdown can be initiated in the region where electric field enhancement takes place if the electric field exceeds the “threshold strength” of the material. Evidence of the presence of “threshold strength” has emerged from recent experimental work. In the present paper, a new model based on space charge dynamics under very high dc electric field has been proposed. Bipolar charge injection model has been successful in simulating charge dynamics in polymeric insulation under dc conditions. By setting threshold strength it is possible to achieve dielectric breakdown when the applied voltage rises linearly with time. Based on the time to dielectric breakdown (TTDB), it is possible to calculate the applied electric field at which breakdown occurs. Our simulation results clearly show that the dielectric breakdown under dc conditions is dependent on the sample thickness and voltage rising rate, i.e. the applied electric field at which the breakdown occurs decreases with sample thickness and increases with voltage rising rate. Both effects have been clearly observed in practical dc breakdown tests but the mechanisms never properly understood. The new model also allows one to examine charge dynamics prior to breakdown and influential factors that govern charge dynamics in the polymeric insulation, therefore, provides a strategy of how to improve dielectric performance of polymeric insulation
Measurement of mobility of positive charge carriers in polyethylene
No full textThe presence of space charge in insulation materials has been an issue with respect to their life performance in high voltage application systems. The dynamics of space charge in polymeric materials has been widely examined through a pulse electroacoustic (PEA) method over the decades. Theoretical efforts have also been made to explain the nature of these charge carriers. However their electrical properties are not fully revealed. The transport of charge carriers is directly linked with the conduction mechanism and the underlying trapping characteristics in polymeric insulation materials. In this paper, the velocity of positive charge carriers is measured by observing the transport of positive charge packets in additive-free low density polyethylene using PEA technique. The charge packets are generated using a pulse excitation method, i.e., adding a pulse voltage onto the applied bias dc voltage across the polymer film. The negative differential mobility was observed by inspecting the dependence of the velocity of positive charge packets on the applied electric field
Pseudonocardia lutea sp. nov., a novel actinobacterium isolated from soil in Chad
No full textGao, Yuhang, Piao, Chenyu, Wang, Han, Shi, Linlin, Guo, Xiaowei, Song, Jia, Xiang, Wensheng, Zhao, Junwei, Wang, Xiangjing (2018): Pseudonocardia lutea sp. nov., a novel actinobacterium isolated from soil in Chad. International Journal of Systematic and Evolutionary Microbiology 68 (6): 1992-1997, DOI: 10.1099/ijsem.0.002780, URL: http://dx.doi.org/10.1099/ijsem.0.00278
Actinomadura harenae sp. nov., a novel actinomycete isolated from sea sand in Sanya
No full textHu, Jiangmeihui, Han, Chuanyu, Yu, Bing, Zhao, Junwei, Guo, Xiaowei, Shen, Yue, Wang, Xiangjing, Xiang, Wensheng (2020): Actinomadura harenae sp. nov., a novel actinomycete isolated from sea sand in Sanya. International Journal of Systematic and Evolutionary Microbiology 70 (2): 766-772, DOI: 10.1099/ijsem.0.00381
The AC breakdown and Space Charge Characteristics of LDPE in the presence of Crosslinking Byproduct
No full textLDPE films were soaked into crosslinking byproducts which are acetophenone, ?-methylstyrene and cumyl alcohol. The samples were used to perform the breakdown test. The ac breakdown measurements were conducted at a ramp rate of 50V/s at room temperature. Weibull plot is used to analyze the ac breakdown result. Comparing the soaked and un-soaked samples, it does show a small change in the characteristic breakdown strength as the LDPE film were soaked into the sample depending on the byproducts. It suggests that the breakdown strength is modified by adding the byproducts in the LDPE film. However, as the range of breakdown strength of all samples are to be compared, these values fall in the same region which indicate no significant difference can be seen in the samples. AC Pulsed Electroacoustic (PEA) technique was used to measure space charge in the samples. The result obtained from this test is used to explain the breakdown phenomena in the crosslinking byproduct soaked LDPE and space charge characteristics
Numeric description of space charge in polyethylene under ac electric fields
No full textSpace charge in polyethylene-based insulation materials and its effect on the local electric field under a dc environment have been extensively examined over the last few decades while the behavior of space charge under ac stress has received less attention. Space charge phenomenon under ac electric fields becomes an important issue with increased operating field strength in many applications, such as next generation high voltage cables. In this paper, a bipolar charge transport model has been developed to simulate space charge in polymers under ac electric fields. Obtained simulation results show that there is a small quantity of phase-dependent bipolar charge accumulation in the vicinity of the electrodes that does not move into the bulk under ac stress. This causes a slight distortion of the local field in the bulk. However, at lower frequencies less than 1 Hz, there is increased charge accumulation and penetration. Comparison with available experimental data suggests that the model is capable of describing the underlying physics of charge behavior when a dielectric material is subjected to ac electric fields. Due to the weak charge movement in the bulk, the conduction current density is small and hence the displacement component dominates the total current density and this increases linearly with ac frequency
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