Zepeng Lv

Dependence of charge accumulation on sample thickness in Nano-SiO 2 doped IDPE

The effect on space charge accumulation of the introduction of nano-filler into polyethylene is usually studied using thin 100 μm 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 Takadas 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.

Study of the factors that suppress space charge accumulation in LDPE nanocomposites

Two mechanisms have been proposed to explain the suppression of space charge in polyethylene by the addition of nano-fillers, i.e. an interface change that reduces charge injection and a bulk modification that affects charge migration and recombination. The relative importance of each mechanism in Low Density Polyethylene (LDPE) nanocomposites is investigated by the measurement of space charge in samples of double-layer and triple-layer structures. The experimental results show that the nano-fillers reduce charge injection, but do not entirely eliminate the space charge. Bulk changes also play an important role in space charge suppression. The space charge near the interface between the unfilled LDPE and nanocomposites shows a surface charge at 20 °C, but at 60 °C a bipolar blocking phenomenon is observed in some circumstances. It is shown that these space charge distributions can be reproduced using a band theory approach with the assumption that the introduction of deep traps by nano-fillers raises the Fermi level of the nanocomposite towards the conduction level.

Space charge distribution in oil impregnated papers under temperature gradient

As the temperature gradient always exists in the insulation of power equipment, the influence of temperature gradient on oil-impregnated paper insulation is studied by experiments and simulation. Its found that in the both single-layer and double-layer structures, the heterocharges always appear near the lower temperature electrode, and increase with the enhancement of temperature gradient and field stress, resulting in more considerable field distortion. However, there are more heterocharges in the single-layer samples while the field distortion is more severe in double-layer samples. Numerical simulations were carried out to investigate the mechanism of temperature gradient effect. It is indicated that the space charge accumulation is mainly due to the temperature dependence of the charge injection and conduction. The behavior of interface charges can be well described by assuming an energy barrier for charge transportation at the interface.

Mechanism of space charge formation in cross linked polyethylene (XLPE) under temperature gradient

A bipolar charge transport model is used to simulate the formation of space charge in XLPE plaques placed under a temperature gradient. The model is used to assess the relative importance of charge migration and blocked extraction for three possible effects of the temperature gradient. The simulations were carried out for samples with different thicknesses and the results verified by comparison with experiment. In this way it has been shown that not only does the temperature gradient influence space charge accumulation through its introduction of a conductivity gradient, but also through differences in the injecting and extracting processes at electrodes of different temperature.

Estimating the inverse power law aging exponent for the DC aging of XLPE and its nanocomposites at different temperatures

XLPE and its nanocomposites have been subjected to a progressive step voltage until failure occurred, and the results analyzed in terms of an inverse power law ageing expression. The step-stress tests have been conducted at three temperature 20°C,40°C and 60°C, and the exponent n of the inverse power law ageing expression has been estimated for the two materials by a new method, which fully considers the contribution to the failure of the whole sequence of voltage steps, including ageing processes at the lower voltages, the final step and the process of voltage increase. It was found that the value of exponent n increases slightly with increasing temperature for both XLPE and its nano-composites. More importantly the value of n for the nano-composites was found to be substantially lower than that for XLPE at all temperatures. A complementary experiment in which the space charge distribution was measured under the same step-stress protocol as the breakdown tests has also been carried out and the results of both experiments are discussed.

Space charge formation and conductivity characteristics of PE and oil impregnated paper under a temperature gradient

Polyethylene (PE) and oil impregnated paper are widely used as insulation materials in high voltage power equipment however, at high voltages the space charge inside the insulating material can distort the electric field distribution and thus greatly influence aging and degradation. Moreover, there is usually a temperature gradient across the insulation when the equipment is in service, and this can also influence the space charge accumulation. In order to study the effect of temperature gradient systematically a PEA system suitable for measurement under a temperature gradient has been developed and appropriate corrections for the measurement error introduced by the temperature gradient implemented. Separate determinations of the space charge distributions of PE and oil-impregnated paper at different temperatures and voltages have been made. The results for both materials showed some similar characteristics. Obvious heterocharge accumulates at the low temperature side and the higher the temperature difference between the two electrodes or the higher the applied voltage, the more the heterocharge accumulates. As a result the electric field is greatly distorted and the maximum field always appears at the low temperature side. The effects of temperature gradient on the space charge distribution are discussed in detail, and the differences between the two materials are identified. The conductivity of both materials has also been investigated, and the space charge induced by the conductivity gradient produced by a temperature gradient in both materials has also been evaluated. The results show that in oil-impregnated paper the space charge induced by a conductivity gradient plays a great part in space charge formation, while in LDPE the conductivity gradient contributes much less to the space charge. These differences are discussed.

A new method of estimating the inverse power law ageing parameter of XLPE based on step-stress tests

In recent years, HVDC cables have been greatly improved, 500kV XLPE DC cable has been developed, and many XLPE DC cables of lower applied voltage have been used in HVDC systems. However, there still is not a unified standard for testing high voltage DC extruded cable systems, only a recommendation. Furthermore the recommendation only gives a lower limit of ageing parameter (n=10 in the inverse power law tL·Vn= constant), which directly determines the voltage used in the test. A more precise value of ageing parameter is needed in order to improve the testing method. The most common method of estimating the ageing parameter uses constant-stress tests at different voltages (fields) and is based on the inverse power model, but this method takes a lot of time and cannot give usable data when the applied voltage is low. An alternative method is to use step-stress tests based on the cumulative life model. This approach can give usable data quickly, but parameter evaluation is much harder than for the constant-stress tests and the approximations necessary for the data solution unavoidably brings errors. In this paper, a new method of estimating the ageing parameter of XLPE is proposed. Step-stress tests are also used but a new parameter evaluation method is presented, which avoids complicated approximations in the data solution. This new method can greatly reduce the errors and easily give the ageing parameter. A series of step-stress tests on 0.3 mm thick XLPE samples have been carried out to illustrate the method. The values of the ageing parameter, n, obtained from different methods are compared, and it is found that the new method gives a greater consistency between different test protocols, indicating a more accurate estimation of its value. However, a comparison between the new method and the constant stress accelerated life tests is still needed to confirm the accuracy of the new method.

Simulation of space charge distribution in polyethylene under a temperature gradient

It has been reported that a temperature gradient can have a major influence upon the space charge distribution in polyethylene (PE) and oil-impregnated paper however details of the mechanism by which these modifications are produced have not yet been clarified. The experimental results for PE under a temperature gradient show clear evidence of heterocharge next to the electrodes. This feature has been simulated previously using a bipolar injection model that includes blocked extraction. A direct comparison with PEA experiments however is difficult because the finite spatial resolution of the measurement causes the bulk space charge to overlap with that of the electrodes leaving the experimental values as a mixture of the two quantities that are generated separately in the simulation. Here we allow for the finite spatial resolution of the experimental technique by broadening the simulated charge density values using a Gaussian function. This allows us to compare the simulation and the measurements more effectively. The space charge distribution in PE is simulated by including a temperature dependent charge injection and charge transportation. Under these conditions it is found that the simulation results give a detailed match to the experimental results. They also show that both the temperature dependent charge injection and temperature dependent charge transportation are of importance in determining the space charge distribution in PE under a temperature gradient.

Effect of temperature gradient on space charge behavior in epoxy resin and its nanocomposites

The effect of temperature gradient on space charge behavior is necessary to investigate for HVDC insulation. In this paper, space charge distributions in neat epoxy resin (EP) and EP/SiO2 nanocomposites (NC) were measured under different DC stresses and temperature gradients. We found that different temperature conditions applied to the electrodes had a great impact on space charge distributions: Only homocharge accumulated near anode at isothermal conditions, on the contrast, at temperature gradient of 60 °C, negative charge injected from cathode (high temperature side) and accumulated in the bulk, heterocharge appeared near anode (low temperature side). Moreover, SiO2 nano-fillers added to NC could suppress the space charge accumulation significantly, and the pattern of space charge distribution in NC at temperature gradient of 60 °C also shows differently from that of EP. Numerical simulation based on the bipolar charge transport model was employed to study the experimental results. It shows that under temperature gradient, charge extraction plays an important role in heterocharge accumulation near the low temperature side. Moreover, it indicates that unlike the apparently measured conductivity, the charge mobility of NC does not increase rapidly with temperature in the range from 20 °C to 80 °C.

Mechanism of space charge accumulation in crosslinked polyethylene under temperature gradient

Space charge will accumulate within polyethylene when stressed under DC high voltage, and a temperature gradient will form throughout the insulation when a HVDC cable is in service. Previous research has revealed that temperature gradient can enhance hetero charge accumulation at the low temperature end, but the high temperature only helps a small quantity of homo charge accumulate, and the mechanism was explained for hetero charge formation due to ionization of impurities within the insulation and blocked extraction of injected carriers. However there is no direct evidence to distinguish whether the hetero charge is injected carriers or the ions. This paper intends to investigate and explain the influence of temperature gradient onto hetero charge formation. It was concluded that temperature gradient across the sample thickness could affect the space charge through its influence to the mobility of carriers, and the temperature difference between two electrodes could also influence the entire space charge distribution through its influence to the charge injection and extraction. The space charge distribution in cross-linked polyethylene (XLPE) samples with different thickness under different temperature differences and temperature gradient was investigated. The results show that both the temperature difference between the electrodes and the temperature gradient across the thickness can enhance the hetero charge accumulation.