A study on the ageing of polymeric materials in the presence of space charge (original) (raw)
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Application of polymer ageing models to power cables
Annual Report Conference on Electrical Insulation and Dielectric Phenomena, 2000
Ageing models have been developed to predict the lifetime of polymeric insulation subject to electro-thermal stresses. We present here a method for applying the models to situations in which the field is not constant over the whole specimen, as for cable geometry. The method has been applied to characteristic lifetime data from AC ageing experiments on cables. The results are presented, and the effect of insulation volume upon the model parameters is discussed.
The missing link - The role of space charge in polymeric insulation lifetime
Proceedings of 2014 International Symposium on Electrical Insulating Materials, 2014
Polymeric insulation in power equipment experiences degradation and ageing under service conditions. The determination of remaining life plays a key part in the asset management. This requires an ageing indicator which can be used to monitor the status of the insulation. In this paper a simple model based on trap creation has been proposed and a concept of critical trap density can be used to describe the lifetime of the insulation. Assuming the power law relationship between the trap creation and the electric field, the empirical inverse power law can be derived. This allows one to relate the physical mechanisms and ageing processes (trap generation). Since the number of traps and energy depth of traps have direct influence on charge dynamics, using the two energy levels of trap depth model developed previously it is possible to estimate the trap density through changes in space charge dynamics. Further work is necessary to establish the direct relationship between trap density and the remaining life of the polymeric insulation.
The electro-mechanical effect from charge dynamics on polymeric insulation lifetime
AIP Advances, 2015
For polymeric material used as electrical insulation, the presence of space charges could be the consequence of material degradations that are thermally activated but increased by the application of an electric field. The dynamics of space charge, therefore, can be potentially used to characterize the material. In this direction, a new aging model in which parameters have clear physical meanings has been developed and applied to the material to extrapolate the lifetime. The kinetic equation has been established based on charge trapping and detrapping of the injected charge from the electrodes. The local electromechanical energy stored in the region surrounding the trap is able to reduce the trap-depth with a value related to the electric field. At a level where the internal electric field exceeds the detrapping field in the material, an electron can be efficiently detrapped and the released energy from detrapping process can cause a weak bond or chain scission i.e. material degradat...
A Review of Aging Models for Electrical Insulation in Power Cables
Energies
Electrical insulation is an integral part of power components. The aging of electrical insulation is an undeniable fact that limits the operational lifetime of power components. Apart from regular aging, abnormal stresses and the development of defects are real threats because of their contribution in accelerating the aging rate and thereby leading to a premature failure of the power components. Over the decades, various studies have been carried out to understand the aging behavior of electrical insulation mainly considering electrical and thermal stresses. Similarly, a number of mathematical (aging) models have been developed based on the theoretical and experimental investigations and evidences. However, a dependable formulation of the models that can provide more practical estimation of the insulation degradation profile has not been achieved yet. This paper presents a comprehensive review of the aging models considering single and multistress conditions. Further, the paper disc...
Ac Aging and Space-Charge Characteristics in Low-Density Polyethylene Polymeric Insulation
Journal of Applied Physics, 2005
In the present work efforts have been made to investigate the influence of ac aging on space-charge dynamics in low-density polyethylene ͑LDPE͒. LDPE films with 200 µm were aged under various electric stress levels at 50 Hz for various times at ambient temperature. Space-charge dynamics in the samples after aging were monitored using the pulsed electroacoustic technique. It has been revealed that the space charge under ac aging conditions is related to the level of the applied field, duration of the voltage application, as well as the electrode materials. By comparing with the results of unaged sample the results from aged sample provide a direct evidence of changing trapping characteristics after ac aging. Negative space charge is present in the bulk of the material and the total amount of charge increases with the aging time. The amount of charge increases with the applied field. Charge decay test indicates that the charges are captured in deep traps. These deep traps are believed to form during the aging and related to change caused by injected charge. By using different electrode materials such as gold, brass alloy, and polyethylene loaded with carbon black, it was found that the electrode has an important role in the formation of charge, hence subsequent changes caused by charge. The charge dynamics of the aged samples under dc bias differ from the sample without ac aging, indicating changes brought in by ac aging. Chemical analysis by Fourier transform infrared spectroscope and Raman microscope reveals no detectable chemical changes taken place in the bulk of the material after ac aging. Finally, the consequence of the accumulation of space charge under ac conditions on the lifetime of the material has been discussed. The presence of deeply trapped space charge leads to an electric stress enhancement which may shorten the lifetime of the insulation system.
Space charge analysis of electrothermally aged XLPE cable insulation
Journal of Non-Crystalline Solids, 2007
Cross-linked polyethylene (XLPE) is currently widely used as an insulating material for power cables due to its good physical properties, however when in use it undergoes an electrical ageing process. Its ability to trap electric charge can give rise to space charge accumulation in the bulk of the polymer and produce localised electric stresses that can lead to cable failure, since the electric field will be increased above the design stress in some regions favouring the initiation of degradation there. In this work the PEA (pulsed electroacoustic) method was used to compare the charge dynamics in three samples (XLPE cable peelings) aged in different ways (electrothermally in the laboratory, field aged in service and thermally aged in the laboratory). Very different transient behavior was found depending upon the ageing history. This is related to differences in the migration of chemical species in the insulation layer, which are known to act as charge traps. All materials showed heterocharge peaks when the space charge reached stability, the magnitude of which seems to be related to the severity of the ageing.
Research of Service Condition Impacts on Cable Polymer Insulation
2020
During last decades the intensive increase of using of polymer insulation materials in high voltage electric equipment such as cables, apparatus, surge arresters, transformers, and insulators takes place. With all wide well-known advantages the challenge of polymer insulation material deterioration and conditions of defects formation under influence of different service factors in full-scale are poorly studied. Water trees formation and its growing are concerned with humidity impact on the reduction of polymer insulation electric strength under non-uniform electric field. Water diffuses in polymer structure under electric field intensity and concentrates in volumes with high local heterogeneity, where water trees have a tendency to grow. Despite unanimous agreement that the combination of water and electric field in polymer insulation affects its dielectric properties, there is no clear understanding of the water-tree aging process. Besides proposed deterioration physical models or registered service failure statistics have not been useful for calculation of insulation degradation level or remaining life. In paper the results of laboratory research of polymer material dielectric losses over high frequency range are represented. The dielectric losses measured by using of non-destructive dielectric response measurement techniques showed a good correlation with polymer insulation deterioration. On the basis of obtained results the mathematical model and rapid method of cable remaining life assessment is offered. Thermal deterioration of polymer cable insulation (especially polyethylene insulation) under service condition cannot be ruled out. Within the framework of the project the investigations of thermal deterioration of polyethylene insulation with using of method of X-ray structure analysis are carried out. During thermal deterioration investigations the correlations between dielectric losses and temperature load at different ageing time were defined.
Energies
The present work deals with the study of the electrical behavior of cross-linked polyethylene (XLPE) used for HVDC cable insulation. The aim is to better understand the influences of electrical and thermal stresses on the insulating material in order to provide useful information for designing HVDC cables. This study was carried out on Rogowski samples made of XLPE insulation with semiconductive electrodes, aged for more than 3 years (1220 days) at three different temperatures (70, 80 and 90 °C) under two DC electric fields (30 and 60 kV/mm). Dielectric loss factor, volume resistivity and space charge accumulation were measured. Results are analyzed and cross-correlated, in order to propose possible ageing kinetics.