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Space charge problems have been widely acknowledged for solid insulation such as polyethylene for high-voltage dc cable insulation. However, the effect of space charge on the reliability of high-voltage power module packaging insulation has not been systematically investigated. In this article, a simultaneous measurement of space-charge and isothermal relaxation current (IRC) is performed on silicone packaging insulation. An iterative algorithm based on the nonnegative least-square method is introduced to estimate multilevel trap energy distribution in silicone insulation. It is found that homocharges are accumulated around the electrodes, and more charges are accumulated with the increase in applied electric field and temperature. The local electrical field is distorted, and the threshold of the external applied electric field for charge accumulation decreases with increasing temperature. The trapped charges tend to be trapped in deep traps as the temperature increases. Furthermore, to investigate the charge dynamics in the complex power module package structure, a 2-D space-charge simulation model based on the combination of the bipolar charge transport model and finite-element method (FEM) is proposed with the parameters derived from the simultaneous measurement. According to the simulation results, the maximum electric field is located at the triple-junction position of the high-voltage power module package. With the accumulation of homocharges and the increase in temperature, the electric field decreases at the triple-point position but increases between the injected positive and negative charges. Space-charge behavior affects the local electric field of power module packaging insulation and should be thoroughly investigated in the future. [ABSTRACT FROM AUTHOR]

Charge transport in high-voltage direct-current (HVDC) cable joint with complicated structure has not been profoundly investigated. In this study, a two-dimensional electrothermal coupled charge transport model combining finite element method (FEM) and bipolar charge transport algorithm is proposed, and the effects of temperature gradient and stress cone tilt angle on charge transport and electric field distribution in a 320 kV HVDC cable joint is investigated by this model. In the simulation, the interfacial area between the cross-linked polyethylene (XLPE) cable insulation and the ethylene-propylene-diene monomer (EPDM) joint insulation is specially modeled with different trap parameters from XLPE and EPDM. To obtain the simulation parameters, simultaneous measurements of space charge and relaxation current are performed on XLPE/EPDM double-layer samples, XLPE single-layer samples, and EPDM single-layer samples. It is found that with the increase of temperature gradient, more positive charges are injected and migrate from the inner side of XLPE insulation to the interface, thus, the charges accumulated at the interface change from negative to positive under large temperature gradient. At the same time, the position of the maximum electric field changes from the inner side of XLPE insulation to the root area of the stress cone at the interface. With the increase of the stress cone tilt angle, both the positive charges in XLPE insulation and the negative charges near the stress cone become less, and the overall maximum electric field first decreases and then increases under 20°C temperature gradient. [ABSTRACT FROM AUTHOR]

In order to quantitatively analyze the interaction effects of crosslinking byproducts on charge characteristics in crosslinked polyethylene (XLPE), samples are prepared by soaking additive-free untreated polyethylene in three major byproducts, namely acetophenone (A), cumyl alcohol (C), and a-methylstyrene (M), and space charge measurement under relatively low electric fields of 3, 5, 7, and 10 kV/mm is carried out. The results show that the effect of C on space charge accumulation is comparable with that of A per unit byproduct content, and M exhibits the weakest effect. It is related to the different dissociation degrees of byproducts under different electric fields. An exponential fitting formula is proposed to clarify the significant synergistic and the antagonistic effects of the byproducts on charge accumulation. The synergistic effects on charge accumulation is obvious in LDPE-MC (LDPE soaked in the blend of M and C) at 3,7, and 10 kV/mm, but not at 5 kV/mm, and the LDPE-AC (LDPE soaked in the blend of A and C) shows the same rule at 5, 7, and 10 kV/mm, except for 3 kV/mm. The LDPE-AM (LDPE soaked in the blend of A and M) has antagonistic effects on charge accumulation at 5, 7, and 10 kV/mm, and shows a trend from synergistic effects to antagonistic effects at 3 kV/mm. For LDPE-ACM (LDPE soaked in the blend of A, C and M), synergistic effects are dominant at 3, 5 and 7 kV/mm, then antagonistic effects are obvious at 10 kV/mm. The different rule on space charge accumulation may be due to the discrepant charge injection under different electric fields according to the results of charge distribution with individual byproducts. [ABSTRACT FROM AUTHOR]

Simultaneous measurement at various temperatures of space charge and relaxation current is performed on cross-linked polyethylene (XLPE)/ethylene-propylene-diene monomer (EPDM) double layers with and without silicone grease. The simultaneous measurement correlates the charge distribution and current information for the same sample and avoids discrepancy, due to the separate measurement on different samples. It is found that the polarity of interfacial charges change to the opposite with the increase in temperature in most samples, which is related to both Maxwell-Wagner polarization and enhanced charge injection and migration at higher temperatures. Bipolar charges appear in the vicinity of interface in some XLPE/EPDM double layers with silicone grease. This phenomenon can be explained by Maxwell-Wagner polarization at two interfaces of XLPE/silicone grease and EPDM/silicone grease. The addition of silicone grease decreases the conductivity of XLPE/EPDM double layers. Based on the proposed three-component exponential decay model, it is found that the interfacial charge movement affects the polarity of induced relaxation current, and the addition of silicone grease decreases the interface trap depth. [ABSTRACT FROM AUTHOR]

This study investigates the effect of moisture content on space charge characteristics based on the dynamic space charge response and conductivity characteristics of oilpaper insulation under a DC electric field. Moisture contents ranging from 0.34–2.89% are considered under an electric field of up to 20 kV/mm. The results show that homocharge accumulates inside the oil-paper insulator near both electrodes, with the amount of charge gradually increasing as the moisture content rises. It may be related to the decreased charge injection barrier at the electrode-sample interface as the moisture level increases. The conductivity increases with the rise in moisture level. However, as the conductivity increases, the corresponding space charge accumulation during polarization does not decrease, but increases significantly. This indicates that space charge accumulation may be related to both charge injection and conductivity. The electric field distortion inside the oil-paper insulator increases as the moisture content rises, with the maximum electric field distortion rate going from 16.9 to 92.9% at 5 kV/mm and from 29.7 to 62.5% at 20 kV/mm. [ABSTRACT FROM AUTHOR]

Several signal processing methods are introduced to improve the signal detected under periodic stress in terms of the signal-to-noise (S/N) ratio and behaviour in an arbitrary phase. In the filtering process for the averaging and fitting methods, it is assumed that the variation of space charge can be neglected for a given detecting phase. The averaging process is associated with the phase identification under periodic stress. The fitting process is shown to further enhance the S/N ratio based on space charge behaviour at different positions. A fitting model is chosen based on the signal characteristics, such as the dynamic response of the charge signal and the voltage waveform in the excitation source. The process is based on the premise that the space charge distribution can be detected without significant mutation near the interpolation phase. The procedure proposed optimises signal processing that is associated with space charge. [ABSTRACT FROM AUTHOR]

In order to investigate the effect of nano-doping on partial discharge (PD) and electrical tree growth in cross-linked polyethylene (XLPE), samples of 0.5 wt% magnesium oxide MgO/XLPE were prepared and an AC experiment was carried out. A color-coded diagram for PD signal was developed to describe the PD activity during the whole process of treeing. The results show that the PD intensity in positive half cycle of voltage (PHC) is larger than that in negative half cycle of voltage (NHC), and the PD magnitude in MgO/XLPE is lower than that in XLPE. Moreover, the tree length and morphology in MgO/XLPE are similar to those in XLPE. Furthermore, a finite element simulation based on the bipolar charge transport model was applied to investigate the effect of space charge on PD under AC voltage. Simulation results suggest that the characteristics of PD and tree growth may be associated with the accumulated charge around the needle tip. The distribution of negative charges in NHC would be wider than that of positive charges in PHC, which would weaken the electric field in NHC and lead to the less PD in NHC. Moreover, the range of charge shield in MgO/XLPE may be larger and is equivalent to increasing the effective radius of curvature of needle tip, which would lead to smaller electric field and lower PD magnitude in MgO/XLPE. However, the addition of nano-MgO reduces the crystallinity of material based on the differential scanning calorimetry (DSC) result. Hence, although the PD magnitude in MgO/XLPE is lower, the MgO/XLPE researched in this paper does not show significant resistance to AC tree growth. [ABSTRACT FROM AUTHOR]

Acetophenone and cumyl alcohol, main crosslinking byproducts, will accelerate the loss of insulation life of the polyethylene-insulated high-voltage direct current (HVDC) cables. In this paper, the individual and mutual effects of these two byproducts on the space charge dynamics of low density polyethylene (LDPE) were studied by space charge measurement, and an interaction model was further proposed to explain their mutual effects. The space charge patterns were measured below 10 kV/mm to minimize the interference of the injected charge from the electrode. For the individual effect, the dissociation threshold of byproducts with electric field applied was investigated to determine the electric field where space charge begin to accumulate, combined with conductivity measurement as a further indicator. For their mutual effect, the interaction model was established based on the experimental results. Particularly, a synergistic effect was discovered in the coexistence system. [ABSTRACT FROM AUTHOR]

The charge transport characteristics of LDPE/SiO 2 nanocomposites were investigated by simultaneous measurement of space charge and relaxation current. The filling concentration of hydrophilic nano-SiO 2 ranged from 0.0 to 5.0 wt%. The effects of filling concentration on the relationship between charge accumulation and the quasi-steady current were analysed during the polarisation process. The trap energy distribution was calculated according to depolarisation current. For simplicity, a mean energy level is proposed through the overall averaging process in the detection period. Moreover, the total trap density within the measured energy level range was obtained by the integral of the energy distribution. The results show that the mean energy level and total trap density are non-monotonic with concentration. Based on the calculation results, the characteristics of charge accumulation and quasi-steady current were explained from the perspective of the trap energy distribution. Finally, a synchronous simulation of the space charge and relaxation current was carried out based on the above trap distribution parameters for pure LDPE and LDPE/SiO 2 nanocomposite with a concentration of 1.0 wt%. The simulation phenomena of charge accumulation characteristics and current decay trend were in close accordance with the experimental results, verifying the effectiveness of the analysis. [ABSTRACT FROM AUTHOR]

This paper gives a detailed introduction of a simultaneous measurement system that can obtain the space charge distribution and the thermally stimulated current (TSC) of the same sample over the temperature range 213–373 K. This measurement system adopts a 4-electrode structure and a time sharing strategy to avoid the mutual interference between the space charge measurement and the current measurement. In addition, a space charge recovery method is proposed to obtain the real space charge distribution from the original data under varying temperature situation. Furthermore, a simultaneous measurement is performed on a cross-linked polyethylene (XLPE) sample. The experimental results show that the positive charges accumulate and dominate in the bulk during polarization and the cooling stages. During the thermal stimulation stage, the accumulated charges decay after ∼330 K, and the thermally stimulated current increases significantly after ∼330 K. The analysis results imply that the release of trapped charges may be the main cause of the thermally stimulated current, and most of the charges are trapped in traps more than 1 eV. [ABSTRACT FROM AUTHOR]

In this study, a pulsed electroacoustic (PEA) system, which can measure the space charge distribution of polymer films from low to high temperatures, is introduced. The space charge behavior of direct current (DC) stressed low density polyethylene (LDPE) film is investigated at temperatures from −40 to 70 °C. The space charge results show that homocharges accumulate in the vicinity of electrodes at low temperatures (−40 to 0 °C), whereas heterocharges that appear during short circuit at the ambient temperature (30 °C), and negative charges gradually dominate the distribution with the increase of temperature. Moreover, the accumulated space charge density at −20 °C reaches the maximum within the investigated temperature range, which may be related to the traps formed by the polyethylene molecular chain branches. This finding coincides with the dynamic mechanical analysis (DMA) result and the thermally stimulated current (TSC) of LDPE reported by other researchers. Besides, the apparent mobilities of carriers at various temperatures calculated based on space charge decaying data indicates that the mobility decreases first and then increases with the increase of temperature from −40 to 70 °C. The variation of the apparent mobility can be explained by the hopping model. In addition, with the help of atomic force microscope (AFM) with Kelvin probe, the contact mode of aluminum/ LDPE/semiconductor was found to be the main reason that resulted in the slower decay of injected holes around the anode, as compared to the injected electrons around the cathode. [ABSTRACT FROM AUTHOR]

To estimate arbitrary electron and hole trap energy distribution of cross-linked polyethylene (XLPE), a modified method based on the combination of relaxation current and space charge measurement is proposed. This method is on the premise that homocharges are injected at the shallow depth of dielectrics and then detrap and flow away from electrodes. During depolarization, space charge and relaxation current are measured simultaneously by a specialized pulsed electro-acoustic (PEA) equipment. Furthermore, the decrease rates of positive and negative space charge are separately calculated and the ratio between them are employed to differentiate the contribution of electrons and holes to relaxation current. Thus, the relaxation current can be divided into two components to calculate energy distribution of electron and hole traps. Besides, simulation based on a modified bipolar charge transport model with calculated energy distribution were performed to simulate the dynamics of space charge. The experiment results of XLPE show that more hole traps than electron traps exist at energy levels above 0.86 eV. The energy distribution corroborates the decay pattern of space charge. In addition, the simulation results show the same qualitative characters as experiment results. Therefore, this method could be employed to estimate arbitrary trap energy distribution and the simulation may help give an insight of trap properties as well as space charge dynamics. [ABSTRACT FROM AUTHOR]

In a high voltage direct current (HVDC) transmission system, the devices like HVDC cable and its accessory, HVDC bush play important roles in power transmission and power transformation. The local electric field could be greatly enhanced due to the space charge accumulation in the insulation, which significantly influences the electrical performance of the electric devices. In this paper, an algorithm for solving the charge coupled systems of solid dielectrics is presented. The algorithm is based upon a finite element method (FEM) solving the Poisson equation and a finite volume method (FVM) discretizing the transportation equation on a cell-centered tessellation. Both FEM and FVM share a set of unstructured mesh. The study focuses on the capability of different upwind schemes on capture of charge density step. The numerical experiment results show that in contrast to conventional methods that constrain charge densities to piecewise constants within a cell, a higher-order approximation of charge density is obtained by reconstructing the charge densities gradient in the control volumes. And the oscillation could be avoided when a limiter function is adopted to make sure that no new extrema created between the central cell and the neighbor cells. The influences of time step, total calculation time, and the mesh structured or not are also investigated. The results show that the algorithm could provide enough accuracy when suitable time step and time duration are adopted with unstructured mesh. [ABSTRACT FROM PUBLISHER]

The simplest space charge measurement system is introduced for the direct space charge observation under arbitrary periodic stresses. An essential component of the system is the frequency allocation strategy for periodic stress fa and detection source fp. This strategy is based on the modified automatic equal phase shift (AEPS) principle, in which the relationship between fa and fp can be an integral multiple and a non-integral multiple. The phase detection in the range of 0??360? can be completed during Na cycles of periodic stress with Np detected excitation to realize phase resolution ?' = 2?/Np. Two different allocation strategies, namely, Na = 1 for the integral multiple between fa and fp and Na > 1 for the non-integral multiple between fa and fp, can be used to realize a high-phase resolution with consideration of the available frequency range for the detected source. Phase monitoring can be realized with the discrete voltage Va(i) of periodic stress at the detection phase and with the peak voltage Vg(i) of the raw signal representing the space charge density of the ground electrode. Phase delay is also introduced between the monitoring signal and real phase to avoid phase errors, especially at a high frequency and electric field. The detection phase of the space charge can be accurately identified through the monitoring signal via the data fitting or the Hilbert transform using the base wave component acquired with the Fourier transform. However, the data processing is complex for both methods. Thus, a simple and fast identification method is presented on the basis of the characteristic sequence of the AEPS principle. Finally, the feasibility and accuracy of the proposed system are successfully verified through the direct detection of the space charge within low-density polyethylene under several practical periodic stresses, namely, AC wave at 0.01 Hz, square wave at 1 kHz, and harmonic wave at 10 kHz. [ABSTRACT FROM AUTHOR]

The electrical properties of nanocomposites are closely related to their nanostructures. In order to investigate the nanostructures and space charge characteristics of magnesium oxide (MgO)/low-density polyethylene (LDPE), samples of LDPE with nano-MgO from 0.1 to 2.0 wt% are prepared, as well as pure LDPE. The nanostructures of nanocomposites are investigated by the technology of synchrotron radiation small-angle x-ray scattering (SAXS), and the space charge distributions in nanocomposites and LDPE are measured by the pulsed electro-acoustic (PEA) method. Results of the synchrotron radiation SAXS experiments reveal that there is obvious interphase between the nano-MgO particle and the LDPE matrix, the thickness of which increases along with the increase in nano-MgO concentration. The agglomeration and mass fractal exist in all nanocomposites, and become more obvious at higher concentration. Results of space charge measurements indicate that the apparent trap-controlled mobilities and the apparent trap depths are respectively larger and smaller than those in LDPE during the initial phase of depolarization when the concentrations are 0.1 and 0.5 wt%, and are respectively smaller and larger than those in LDPE when the concentrations are 1.0 and 2.0 wt%. The space charge amount decreases after the addition of nano-MgO. This is mainly due to the introduction of shallow traps which promote the charge recombination in 0.1 and 0.5 wt% MgO/LDPE nanocomposites, and the introduction of deep traps which restrict the charge transportation in 1.0 and 2.0 wt% MgO/LDPE nanocomposites. The space charge characteristics of MgO/LDPE nanocomposites are explained combined with the nanostructures obtained from the synchrotron radiation SAXS experiments. [ABSTRACT FROM AUTHOR]

In a high voltage direct current (HVDC) cross-linked polyethylene (XLPE) cable system, the temperature of the insulation varies with the load condition, which significantly influences the electrical performance of the XLPE. In this study, space charge detrapping and periodic grounded DC tree in XLPE are investigated and analyzed at various electric fields in a broad temperature range of 20–80 °C. Results reveal that with the increase in temperature, the apparent trap-controlled mobility of space charge becomes larger while its decay rate becomes smaller in the initial phases of depolarization. These findings indicate that charge detrapping becomes easier at higher temperature. The electrical tree initiation ratios, lengths, and widths all increase as temperature increases, except for the negative tree width under 16 kV at 80 °C. An increment in length and width of positive trees is observed at 80 °C. Electrical trees become thicker at higher temperature, and branches into the direction perpendicular to the applied electric field are observed at temperatures above 60 °C. Furthermore, the negative trees are easier to initiate and are thicker in shape than positive trees, and sometimes have an advantage over positive trees in length and width in the temperature range of 20–60 °C. [ABSTRACT FROM AUTHOR]

In this paper, trap distribution variation in low density polyethylene (LDPE) after filled with 1 wt% nanosilica is investigated through the charge decay after corona charging with a needle-plane electrodes system. The LDPE and LDPE/SiO2 nanocomposite were firstly polarized under needle-plane electrodes system. The electrons and holes are injected through the variation of polarization of the needle. Then the surface space charge behavior and relaxation current were measured with a simultaneous measuring equipment, which overcomes the poor correlation of separate measurement. Lastly, the apparent mobility and trap distribution are evaluated based on the decay of surface space charge and relaxation current decay. It is found that the observed surface space charge and current are associated with trap distribution from 0.855 eV to 1.000 eV during decay process from 10 s to 3600 s. In LDPE/SiO2 nanocomposite, the estimated trap density of electrons and holes is more than that of LDPE in a relatively low energy range from 0.855 eV to 0.925 eV. These traps with low energy levels as transport media for carriers may be the main reason for the higher mobility of carriers in LDPE/SiO2 than LDPE, which is testified by surface space charge decay. It is also worth noting that according to the relaxation current decay, the estimated density of the relatively deep hole traps (>0.990 eV) is more than that of electron traps in LDPE, which is not observed in LDPE/SiO2. This is in accord with other scholar's molecular simulation result that most hole traps exist in intra-chain areas and electron traps exist in inter-chain areas, reflecting the different transportation characteristics of electrons and holes. [ABSTRACT FROM AUTHOR]

An automatic equal phase shift principle (AEPS) is proposed to simplify space charge detection under periodic stress with the pulsed electroacoustic method. For pulse frequency, fp is a non-integral multiple with periodic stress frequency fa; the detecting phase can be automatically shifted with a certain value after each cycle of periodic stress. The pulse position will be at the same phase of periodic stress after several cycles. The minimum amounts of periodic stress cycles and pulse generated during this process are defined as Na (>1) and Np, respectively. They can be uniquely confirmed by the characteristic equation Na/fa = Np/fp. Moreover, the positions of pulse generated during this process are equally distributed in the periodic stress from 0o to 360° with a characteristic sequence of pulse. The phase interval, i.e. phase resolution Δ', is 2π divided by Np. Consequently, the phase resolution can be easily enhanced through an increase in the pulse amount Np by selection of fp, according to the characteristic equation. Finally, space charge behaviours in low-density polyethylene under 50 Hz AC stress were measured with three different pulse frequencies fp, i.e. 25.5, 1010 and 1001 Hz, to validate the proposed AEPS principle. Results show that the space charge distribution can be successfully acquired with a phase resolution from 7.07o to 0.36o, even if the pulse frequency is lower than the AC frequency at 25.5 Hz. This result indicates that AEPS can overcome the frequency limitation of the pulse generator, and a high phase resolution can be realised. [ABSTRACT FROM PUBLISHER]

In recent years, the multilayered polyimide films have been widely used in flexible Print- Circuit Board in order to reduce the thickness of the combination of the flexible Print- Circuit Board. The existence of space charge as a main reason for dielectric breakdown, has a great effect on breakdown characteristic. So it is important to study the space charge characteristics of the multilayered polyimide films. Fluorination can change the chemical component in surface layer of polymers and therefore can influence the charge injection from electrodes and affect the electrical properties of the surface layer. This paper presents a study aimed at clarifying the effect of fluorination on space charge behavior in multilayered polyimide films. Samples were fluorinated in a laboratory vessel at 328 K (55 °C) using a F2/N2 mixture with 20% F2 by volume and 0.05 MPa (500 mbar) for respectively 15, 30, 45 and 60 minutes. The surface chemical compositions of the films were characterized by attenuated total reflection infrared analysis. The space charge distributions in multilayered polyimide films were measured by using the pulsed electroacoustic (PEA) method. Obtained results showed that the space charge distribution of multilayered film stacking with five fluorinated samples was quite different from that of multilayered film stacking with five untreated samples. It is also suggested that the fluorination can significantly suppress the injection of space charge. [ABSTRACT FROM AUTHOR]

This paper presented the measurement results of space charge distribution and high field conduction in cross-linked polyethylene (XLPE) plate sample at various electric fields for a broad temperature range from 30 to 90 °C. The temperature effect on charge trapping and transport mechanism is discussed based on estimation of threshold characteristic and apparent mobility. It is shown that the threshold field for space charge accumulation apparently decreased with the temperature, probably associated with acceleration of charge injection and ionic dissociation. The apparent mobility of charges showed an exponential dependence on temperature, which indicates enhancement of charge detrapping and transfer rate at higher temperature. The field dependence of the space charge decreases with the temperature, results in more space charge accumulate at room temperature than at high temperature at certain high field. This behavior indicates that the space charge formation rate increases slower than the charge detrapping rate as the temperature increases. Besides, negative charge is always dominant in the sample at the measuring field and temperature range. This is probably due to stronger electron injection at XLPE-aluminum interface or more negative ion formed by dissociation of chemical species. [ABSTRACT FROM AUTHOR]