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The effect on space charge accumulation of the introduction of nano-filler into polyethylene is usually studied using thin 100 ?m samples in electric fields reaching 100 kV/mm. However, the insulation thickness in an actual HVDC cable is usually some centimeters. Thus there is a great thickness difference between the experiment sample and the insulation application. Here we investigate the thickness dependence of space charge accumulation in unfilled and LDPE doped with nano-SiO2 over a range of temperatures in a field of 50 kV/mm. It is found that the space charge in LDPE/SiO2 nanocomposites exhibits marked thickness dependence whereas in the unfilled LDPE it shows very little thickness dependence. A simulation based on the bipolar injection model and Takada?s trapping potential model has been carried out in order to explain the differences. It is found that when the simulation included the effects of charge injection, extraction, trapping, recombination, mobility and diffusion, it is able to explain the heterocharge formation in the unfilled LDPE, and the thickness dependence in the two kinds of materials. [ABSTRACT FROM AUTHOR]

The article presents a study which examines the effect of temperature gradient on the design and extent of space charge accumulation of high voltage direct current (HVDC) cables. Measurements of space-charge accumulation were conducted on HVDC cable models applied with different temperature gradients across the cable insulation and are compared with other charge-supply mechanisms. Results of the study show the significant role played by nonuniform conductivity due to temperature gradient in space charge generation in HVDC cable models.

The article analyzes the interface and space charge accumulation in polymeric high-voltage direct-current (HVDC) cables. It is observed that in the DC steady state the field distribution of space charge is determined only by the conductivity mismatch instead of the permittivity mismatch. It is pointed out that the optimization of the design of the cable and accessories should consider a complex system. This is because the nonhomogeneities in permittivity and conductivity can result into an interface charge that alters the insulation field distribution. Meanwhile, the increase in HV cable design fields shows that microscopic effects have to be taken into account when the electric field surpasses the threshold for space charge accumulation.

The article discusses the results of space charge measurements related to charge injection from the semicon/insulation interface and accumulation in the insulation bulk that considers the different combinations of insulation/semicon. These results came from the study titled "Benefits of HVDC Links in the European Power Electrical System and Improved HVDC Technology." The study investigates the effect of interfaces between electrode/semicon/insulation or different insulating materials on space charge injection and accumulation in insulation. Furthermore, the study aims to provide principles and guidelines for an appropriate choice of semicon and insulation among materials to be used for HVDC cables applications.

The PEA technique is used to measure the distribution of space charge in an epoxy resin after polarisation for one week at an applied field of 7.14 kV/mm over a range of temperatures. The decay of the space charge is followed for times up to 114 h after removal of the voltage and analysed in terms of a number of alternative decay mechanisms. It is shown that the rate-determining stage of the decay mechanism is that of a thermally activated process that has been associated with charge de-trapping. At times greater than 102 s the de-trapping process behaves as though the space charge field does not exist and the retention time of the space charge depends only upon the depth of the deepest occupied traps and the temperature. [ABSTRACT FROM AUTHOR]

This paper has the aim of providing a view of a lively debated topic which has broad impact on the design of electrical apparatus and new insulating materials, that is, the interaction between space charge and aging processes of polymeric insulation. Aging models developed in recent decades that consider explicitly or implicitly the contribution of space charge to insulation degradation, under both dc and ac voltage, are dealt with, with the intention to point out their range of validity. Some conventional phenomenological models that have been used for much more than two decades without referring to space charges can be exploited to account for electrical field and activation energy modification due to space charge. These, together with models conceived considering space charges as the driving force for aging, are especially examined. In addition, recent models that disregard the action of space charge as an ageing factor, but consider space charge as the consequence of degradation processes are also discussed. [ABSTRACT FROM AUTHOR]

A model for electrical tree initiation in epoxy resins is presented in which the process is driven by the generation of new charge traps as a result of energy transferred to the polymer via charge recombination processes. The electroluminescence intensity expected from the model is computed and shown to be in agreement with the experimental data. In particular it is shown how the initial emission due to recombination can change to an emission arising from impact excitation in a natural way when the trap density reaches a level sufficient for the high electric field of the tree initiation region to connect shallow traps to form conducting filamentary paths in the form of percolation clusters. This result allows the electroluminescence behavior to be correlated with the observed onset of filamentary damage. The model is also shown to be able to explain the decrease in emission intensity observed at the time of transition between the two mechanisms for luminescence. [ABSTRACT FROM AUTHOR]

This paper presents a discussion of the concept of threshold for trapped space charge accumulation in solid dielectrics submitted to a dc field. The starting point is the fact that it is often possible to define a critical field separating an ohmic type of conduction from a non-linear regime in the current-voltage characteristic of solid dielectrics. In the space charge limited conduction theory, this critical field corresponds to the onset of space charge accumulation. However, other conduction processes, such as hopping conduction for example, can also explain non-linearity in the current-voltage characteristic, which does not involve space charge. It is proposed to check for the existence of a critical field for space charge accumulation using complementary techniques, i.e., space charge detection and electroluminescence techniques. Polyethylene, polyester and polycarbonate were investigated as being representative of three different families of polymers. It is shown that similar values of thresholds are found for a given material using the three above-mentioned techniques, lending support to the physical explanation of a threshold for trapped space charge accumulation. [ABSTRACT FROM AUTHOR]

The life expressions of models of insulation aging are functions of temperature and field as well as material parameters. A methodology is presented that allows these models to be applied to a cable geometry in which there is a radial variation of both field and temperature. In this way material parameters can be extracted from cable data. The methodology is illustrated using one such model and the parameters deduced from cable failure distributions are compared with those obtained for thin films. This comparison allows conclusions to be drawn about how the aging process affects specimens of the same material with different volumes. [ABSTRACT FROM AUTHOR]

Part I. Investigates the possibility that the local strain is an elemental property of the polymer morphology. Relation of the elemental strain to the bulk modulus through the elemental volume of the affected chain section and the number density of contributing centers; Estimates for the smallest magnitude of the local field that is able to initiate aging; Correlation of the estimates with the maximum number of electronic charges that such a void may trap.

Examines the formation of space charge packets in crosslinked polyethylene (XLPE) tapes from un-aged cable insulation using the pulsed electroacoustic technique. Description of a sequence of positive charge packets observed to transit the sample; Calculation of the internal field showing field requirement for initiation; Discussion of the results in terms of the theories of charge packet formation.

Part II. Presents the derivation of a modified life model, considering the possibility of the local strain as an elemental property of the polymer morphology. Connection between the life model and space discharge densities as revealed by the experiments; Correlation with analytical measurements; Application of the connection to electrical aging and space charge data for crosslinked polyethylene materials.

Reports on the results of the research program carried out to quantify the reliability of joint moldings for direct current (dc) operation in submarine telecommunications cables. Microtome sections; Joint moldings of same insulation materials for microtome sections; Moldings of different design and insulation materials; Test procedure for system's reliability; Quality assurance requirements; Conclusions.