An improved model to estimate trapping parameters in polymeric materials and its application on normal and aged low-density polyethylenes (original) (raw)

Charge trapping and detrapping in polymeric materials: Trapping parameters

Journal of Applied Physics, 2011

Space charge formation in polymeric materials can cause some serious concern for design engineers as the electric field may severely be distorted, leading to part of the material being overstressed. This may result in material degradation and possibly premature failure at the worst. It is therefore important to understand charge generation, trapping, and detrapping processes in the material. Trap depths and density of trapping states in materials are important as they are potentially related to microstructure of the material. Changes in these parameters may reflect the aging taken place in the material. In the present paper, characteristics of charge trapping and detrapping in low density polyethylene (LDPE) under dc electric field have been investigated using the pulsed electroacoustic (PEA) technique. A simple trapping and detrapping model based on two trapping levels has been used to qualitatively explain the observation. Numerical simulation based on the above model has been carried out to extract parameters related to trapping characteristics in the material. It has been found that the space charge decaying during the first few hundred seconds corresponding to the fast changing part of the slope was trapped with the shallow trap depth 0.88 eV, with trap density 1.47 Â 10 20 m À3 in the sample volume measured. At the same time, the space charge that decays at longer time corresponding to the slower part of the slope was trapped with the deep trap depth 1.01 eV, with its trap density 3.54 Â 10 18 m À3. The results also indicate that trap depths and density of both shallow and deep traps may be used as aging markers as changes in the material will certainly affect trapping characteristics in terms of trap depth and density. V C 2011 American Institute of Physics.

Charge trapping and transport phenomenon in aged and unaged epoxy resin and polyethylene

International Transactions on Electrical Energy Systems, 2014

Polymeric dielectrics are prone to space charge trapping within their volume. This in turn affects many of their dielectric properties including breakdown strength and charge transport. In the current paper, we attempt to characterise charge accumulation and transport in polymeric dielectrics under the application of DC field. Epoxy (EP) resin and polyethylene (PE) are chosen as the dielectric materials to be investigated on account of their prevalence in the high voltage industry. Pulsed electro-acoustic measurements are performed on EP resin and PE specimens. Further, effect of ageing on charge trapping is studied for both EP resin and PE, by thermally ageing the materials under controlled conditions. In addition, the dielectric materials are exposed to a humid environment over long periods and the effect of prolonged exposure to moisture on charge trapping studied. From the experimental results, density of trapped charge is obtained for the different cases. Fourier transform infrared spectroscopy is carried out to identify chemical changes in dielectric materials due to ageing, with an aim to relating physicochemical changes in the material to charge trapping.

Modeling of transient space charge distribution in polymer

Journal of Electrostatics, 1997

Low density polyethylene (LDPE) is one of the principal polymers used for the insulation of dc power cables. The application of a dc electrical field in this material leads to the movement of injected charges and internal charges for the establishment of a field distorsion in the material. This process constitutes one of the principal causes of the dielectric breakdown from which is depended the life time. Consequently, the knowledge of the origin of space charge or internal field is important and necessary for the behaviour study of insulator under electrical stress. For this, numerical modeling of transient space charge distribution in low density polyethylene (LDPE) of 1 mm thick is presented in this paper in order to explain the experimental results on the time variation of space charge distribution under dc voltage stresses up to 100 kV at room temperature using the pressure wave propagation method (PWP) which has been recently set up in our Laboratory. These experiments have made in evidence the formation of homo-space charges close to the electrodes. The amount of these homocharges increases with time duration of the voltage application. The simulation is made on the basis of electrons and holes injection from the cathode and anode respectively with the hopping transport in the material bulk taking into account the bipolar carrier trapping phenomenon. The theoretical results of modeling have shown the accumulation of homo-space charges near the electrodes as it has been also found by experiments. Following our assumptions, it seems that, this accumulation of charges could be due to the injection process and their capture closest the electrodes.

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...

Morphology Effects on Space Charge Characteristics of Low Density Polyethylene

… Journal of Applied …, 2011

Low density polyethylene (LDPE) film samples with different morphology were prepared by three kinds of annealing methods which were different in cooling rates in this study. A pulsed electro-acoustic (PEA) space charge measurement system was improved to solve the surface discharge problems for small samples applied with a high voltage. Negative direct current (DC) fields from 50 to above 220 kV/mm were applied to the samples. The influences of morphologies on space charge and space charge packet characteristics were measured by the improved high voltage withstand (HVW) PEA system. Mobility and trap depth of released charges were calculated by space charge decay. It was found that there is a different probability of space charge packet initiation under applied field from À60 to À100 kV/mm. Average velocity and mobility of the space charge packets were calculated by space charge packet dynamics. It was found that the lower cooling rate samples have higher crystallinity, more homo-charge accumulation, lower mobility and deeper trap depth. The mechanism of morphological effects on space charge phenomena have been presumed to give a plausible explanation for their inherent relationships. The morphology in the metal-dielectric interface and in the bulk is convincingly suggested to be responsible for the injection and propagation processes of space charge. A model of positive space charge initiation in LDPE samples was also suggested and analyzed. The mechanism of morphological effects and the charge injection model are well fit with the injection and propagation processes of space charge. The different effects of morphology in the metal-dielectric interface and in the bulk of polymers are stressed.

Molecular modeling of electron traps in polymer insulators: Chemical defects and impurities

The Journal of Chemical Physics, 2001

The presence of space charge in the polymeric insulation of high-voltage cables is thought to be correlated with electric breakdown. However, a direct link between molecular properties, space charge formation and eventual breakdown has still to be established. It is clear that both physical ͑e.g., conformational disorder͒ and chemical defects ͑e.g., broken bonds and impurities͒ are present in insulating materials and that both may trap electrons. We have shown that by defining the defect energy in terms of the molecular electron affinity, a relationship is established between the electron trap and the molecular properties of the material. In a recent paper ͓M. Meunier and N. Quirke, J. Chem. Phys. 113, 369 ͑2000͔͒ we proposed methods that have made it possible to provide estimates of the energy, number and residence times of electrons in conformational traps in polyethylene. Typical physical trap energies are of the order of 0.15 eV and all are less than 0.3 eV. In the present paper we focus on the role of chemical defects, where we expect much deeper traps but at very low concentrations. Following the methodology used in our previous paper we have used density-functional theory to calculate trap energies for a set of chemical impurities and additives commonly found in polyethylene used for high-voltage cable insulation. In an extension of our approach we have estimated the effect of neighboring molecules on the trap energies of such defects. The resulting trap energy-trap density distribution reveals some very deep ͑Ͼ1 eV͒ traps presumably implicated in the formation of long-lived space charge in polymeric insulators and consequently to changes in the dielectric properties of the material.

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.

Mobility Dependence on Electric Field in Low Density Polyethylene (LDPE)

The surface potential decay evolutions with time after negative corona discharge of low-density polyethylene (LDPE) have been recorded for different charging levels. It was observed that there was a significant increase in the surface potential decay rate with the increase of charging levels. Electrical conduction characteristics in LDPE have been investigated as a function of electric fields strength. It is found that the mobility of LDPE increases with the increase of electric field and the conduction mechanism is governed by the hopping process of injected carriers between localized trapping centers. A deep trap separation of 3.2 nm, corresponding to 3.10 19 traps/cm 3 has been found.

Numerical Modeling of Charge Transport in Polymer Materials Under DC Continuous Electrical Stress

Transactions on Electrical and Electronic Materials, 2015

Our work is based on the development of a numerical model to develop a methodology for predicting the aging and breakdown in insulation due to the dynamics of space charge packets. The model of bipolar charge transports is proposed to simulate space charge dynamic for high DC voltage in law-density polyethylene (LDPE), taking into account the trapping and detrapping of recombination phenomena, this model has been developed and experimentally validation. Theoretical formulation of the physical problem is based on the Poisson, the continuity and the transport equations as well as on the appropriate models for injection. Numerical results provide temporal and local distributions of the electric field, the space charge density for the different kinds of charges, conduction and displacement current densities, and the external current.

Charging and Discharge Currents in Low-Density Polyethylene and its Nanocomposite

Energies, 2020

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 th...