Space Charge Measurements Research Articles

Space Charge Data Analysis for Thermal Pulse Method

In this work, a direct space charge calculation method for thermal pulse method data analysis is proposed. Combining the equation of temperature in the tested film with the heat conduction equation, the Fredholm integral equation of the first kind with the direct goal of space charge distribution in the frequency domain is deduced. This ill-posed equation is solved by approximation algorithm with some numerical examples in this study. The simulation results show that the new data analysis method is able to reduce unreasonable oscillations in great extent. The distinct low sensitivity to the deliberately added noise for calculating space charge profiles is found as another advantage of the method. The related fast and effective calibration method is also suggested for space charge measurement and the subsequent calibration method for electric field profiles with or without applied field.

Direct Measurement of Accumulated Space Charge Using External Currents

In the past few decades, immense contributions have been put in by researchers to measure the accumulation of space charge in dielectric materials. At the same time, the direct measurement of the amount of space charge in materials has been completely neglected, probably due to the difficulties in measuring currents at high-voltage (HV) electrode. Although the amount can be measured indirectly with the available methodologies, the measured results suffer from accuracy issue. Also, indirect measurements involve complex, post measurement mathematical analysis such as deconvolution and filtering to get the amount of accumulated charge. Keeping the lacunas in mind and contrary to the basic approach of the present methodologies, a novel wireless system is developed which separately monitors the ultralow, volumetric, electrode currents flowing into and out of the material at both HV and ground electrodes. The obtained current data are further processed to observe the accurate temporal variation in the accumulated charge using the fundamental concept of charge conservation law. Some more parameters have also been identified which unmistakably determine the presence of the packet-like-charge phenomenon in dielectric materials such as low-density polyethylene. Furthermore, for comparing the results of the proposed electrode system, space charge measurements have been done using pulsed electroacoustic system. Based on the comparison of the results, it is concluded that the newly developed electrode system monitors the charge accumulation closely but with less effort and complexity.

PEA measurement of low‐density polyethylene using spin‐coated P(VDF‐TrFE) film and acoustic propagation analysis based on viscoelasticity

AbstractPulsed electroacoustic (PEA) space charge measurement of low‐density polyethylene using a spin‐coated poly(vinylidene fluoride‐trifluoroethylene) thin film is demonstrated. The space charge profile of a low‐density polyethylene film with a thickness of 150 μm is measured by PEA method. Compared with conventional 9 and 30‐μm piezoelectric films, the space charge profile obtained by the 3‐μm spin‐coated film shows better spatial resolution. Furthermore, acoustic attenuation of a PEA signal is also analytically studied using the Kelvin‐Voigt model for viscoelastic materials.

Review of space charge measurement by pulsed electro-acoustic technique

<p>Pulsed electro-acoustic (PEA) is well known as a non-destructive technique for testing and describing space charge profiles in the insulation. Degradation due to moisture, ageing, external flashover, partial discharge and space charge are considered as insulation defects that is an issue in high voltage insulator. Space charge may distort the internal electric field that may lead to an electrical breakdown. For instance, a method based on an acoustic phenomenon can be used to describe the space charge profiles. This paper compares several earlier researches on different specimen of PEA method and reviews their results to find the space charge distribution profiles. The scope of the review includes types of methods used. The acoustic technique is considered for this study. It has been observed that the PEA technique successfully gives the same result as theoretical that is based on the obtained profile. Linear low-density polyethylene (LLDPE) composite demonstrates about 89 % lower charge density value compared to cross-linked polyethylene (XLPE) and polypropylene (PP) composite. While the electric field value of ethylene-vinyl acetate (EVA) sample is 75 % lower than low-density polyethylene (LDPE) sample. This study found that the time interval, specimen thickness and the electric field sway the space charge profiles. </p>

Charge transport dynamics and the effects on electrical tree degradation under DC voltages in thermally aged silicone rubber

Electrical tree is one of the major issues for insulation failure in high voltage power cables. In this paper, we investigate electrical tree degradation under DC voltages in thermally aged silicone rubber (SIR). Charge transport dynamics are revealed by means of space charge measurement and simulation to give a better insight into the electrical tree initiation and evolution. The results show that a thermal oxygen reaction enhances the SIR cross-linked networking and thus gives rise to a higher tree initiation voltage in the early stage of aging. Subsequently intensified thermal degradation and cracking reaction accelerate SIR cross-linked network breakdown and generate micro-cracks. The increased deep traps and significantly decreased charge carrier mobility after thermal aging result in greater homo charge accumulating near the needle tip. The mechanism of DC electrical tree initiation in SIR is clarified based on the trap theory. It is argued that more trapped charges in the low-density region near the tip in the late thermally aged stage would decrease the tree initiation voltage, while the resultant homo charges would in turn reduce the electric field around the tip. Therefore, after thermal aging, the reduction of tree initiation voltage under DC voltage is less than that under AC voltage.

Influence of the Pulsed Voltage Connection on the Electromagnetic Distortion in Full-Size HVDC Cable PEA Measurements.

Nowadays, with the widespread use of High Voltage Direct Current (HVDC) cables in power systems, the measurements of space charges in full-size cables are becoming more relevant. One of the most common methods used for space charge measurements is the Pulsed Electro-Acoustic (PEA) method. This paper analyzes two factors that influence the electromagnetic interference on the piezoelectric signal. These factors are the connection of the injected pulsed voltage at the PEA test cell and the grounding of the PEA test cell. The influence was analyzed by means of experimental tests to compare different configurations and the electromagnetic distortion created in each one of them. It was observed that the physical location of the pulsed voltage at the electrode has a very important impact on the magnitude of the electromagnetic distortion. Moreover, it is shown that the physical connection of the grounding and the existence of a parasitic capacitance at the PEA test cell are also an important source of distortion.

Time-resolved measurement of space-charge evolution in dielectric films or slabs by means of repeatable laser-induced pressure pulses.

A new variant of the Laser-Induced Pressure-Pulse (LIPP) method for repeatable, time-resolved space-charge profile measurements is proposed and demonstrated. Automated deposition of a fresh laser-target film before each illumination leads to good repeatability of the LIPP and thus allows for the detection of time-resolved changes in the space-charge distribution over many hours. We describe and discuss the experimental setup and its features, compare the repeatability of the LIPP measurements on the same sample without and with re-preparation of the test cell, and present the time-resolved evolution of the space-charge profile in a two-layer arrangement of a silicone-grease and a silicone-elastomer film as an example. Finally, the temperature dependence of the space-charge evolution during polarization under high voltage and during depolarization in short circuit is shown. Possible uses and future developments of the new LIPP approach are also discussed.

Investigation of effects of charge injection and intrinsic ionic carriers in low-density polyethylene and cross-linked polyethylene

The effects of charge carrier injection and intrinsic ionic carriers on the conduction process in superclean low-density polyethylene and cross-linked polyethylene (LDPE and XLPE) and the associated development of space charge are investigated. The carrier mobility is measured directly by space charge measurements in a double- and triple-layer structure. It is shown that the difference between superclean LDPE and XLPE in the field-dependence of the current density results from the dominance of ions in XLPE introduced by cross-linking as compared to injected carriers in the case of superclean LDPE.

Space Charge Accumulation at Material Interfaces in HVDC Cable Insulation Part I—Experimental Study and Charge Injection Hypothesis

On-site installation of accessories on extruded polymeric high voltage cables in a common practice. The procedure requires the shaping of the physical interface between the cable insulation surface and the pre-molded accessory body. On such interfaces, rough surfaces should be avoided in order to limit space charge accumulation in the insulation, which affects the cable performance by reducing insulation life-time, creating conditions for local field enhancement, and, respectively, the formation of possible breakdown path e.g. by electrical treeing. Space charge measurements on cable insulation peelings were undertaken to assess the space charge injection and accumulation on interfaces with varying degrees of surface roughness in order to improve understanding on this subject. The results of the measurements confirm the hypothesis regarding the enhancement of charge injection from rough surfaces when electric field strength exceeds a certain level. The accumulated charge density in the material is shown to strongly depend on the field strength and temperature in both polarization and subsequent depolarization measurements. These results emphasize that a bipolar charge transport model that incorporates field and temperature dependencies of charge injection, trapping, detrapping, and recombination processes needs to be adopted to accurately describe the observed electric conduction phenomena.

Space Charge Measurement and Modelling in Cross-Linked Polyethylene

Cross-linked polyethylene, commercially known as XLPE, is widely used as an insulating material in high voltage cables. However, space charge accumulation under the DC field is one of the most challenging problems in the further development of XLPE insulated cable. Due to the potential electrical degradation ageing process triggered by the accumulated space charges, the IEEE standard 1732 was established for measuring space charge in HVDC extruded cables as the qualification tests. Previous research has revealed that space charge originates from either charge injection at the electrodes or ionization of impurities presenting inside the bulk. In the light of this, this paper aims to simulate the accumulation of space charge in XLPE under DC stress. Space charge measurements have been carried on the fresh and degassed XLPE samples. A modified bipolar charge transport model, by considering the dissociation of impurities, has been employed to simulate the space charge behavior in XLPE. Compared with the experimental observations, the simulation results can reveal appropriate features of hetero charge formation. Both the calculated charge dynamics and field variation are consistent with the experiment results. The restrictions and potential improvements of this preliminary model are also discussed for its future application of XLPE cables.

Space Charge Accumulation at Material Interfaces in HVDC Cable Insulation Part II—Simulations of Charge Transport

Extruded high voltage direct current (HVDC) cable systems contain interfaces with poorly understood microscopic properties, particularly surface roughness. Modelling the effect of roughness on conduction in cable insulation is challenging, as the available results of macroscopic measurements give little information about microscopic charge distributions at material interfaces. In this work, macroscopic charge injection from interfaces is assessed by using a bipolar charge transport model, which is validated against a series of space charge measurements on cable peelings with different degrees of surface roughness. The electric field-dependent conduction and charge trapping effects stimulated by the injection current originating from rough surfaces are assessed. It is shown that by accounting for roughness enhanced charge injection with the parameters derived in part I of the paper, reasonable agreement between computed and measured results can be achieved at medium field strengths (10–40 kV/mm).

Study on Correction Method of Signal Distortion under Temperature Gradient for Space Charge Measurement of HVDC Cables

Study on Correction Method of Signal Distortion under Temperature Gradient for Space Charge Measurement of HVDC Cables

Carrier transport in LDPE and its nanocomposites

The mobility of positive and negative charge carriers has been separately measured independently of carrier concentration in low-density polyethylene (LDPE) and its nanocomposites using space charge measurements in a bilayer system. It was found that the carrier mobility of positive charge carriers was larger than that of negative charge carriers for both LDPE and its nanocomposites and that the carrier mobility was one to two orders of magnitude less in the nanocomposite than unfilled LDPE under the same conditions. The mobility is interpreted in terms of a field-assisted hopping model whose estimated activation energies were verified by measurements at different temperatures. The form of the charge traps and carrier transport is interpreted in terms of known features and electron and hole traps and the effect of nano-particles upon these features is discussed.

Improvement of electrical and material properties of epoxy resin/ aluminum nitride nanocomposites for packaging materials

This paper presents the influence of Aluminum Nitride (AlN) nanoparticles on the electrical and material properties of epoxy resin (EP). The EP/AlN nanocomposites with different concentrations of nano-AlN fillers are prepared. The dispersion of the nano-AlN particles in the composites is analyzed by a field emission scanning electron microscope (FESEM). The electrical properties are investigated by the space charge and DC conductivity measurements, whereas the material properties are studied by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively. The results show that the homo-charge accumulation appears near both electrodes during the polarization, but there are limited negative charges left near both electrodes in the depolarization for the pure EP sample. There is no space charge accumulation in the 1 wt% and 2 wt% EP/AlN nanocomposites. The electric field distortion of the pure EP sample is 20%. Moreover, the electric field distortion initially decreases with the increase of the nano-AlN content, but it increases for the 2 wt% nano-AlN sample. Temperature has a dominant influence on the DC conductivity of the EP/AlN nanocomposites comparing to the pure EP. However, the DC conductivity of the nanocomposites becomes stable at high temperatures. It is also found that the weight loss of the samples decreases with the addition of the nano-AlN and the 1 wt% nano-AlN sample has the highest glass transition temperature. It is elucidated that the high apparent mobility and activation energy facilitate the space charge transport and suppressing the space charge accumulation. Furthermore, the nano-AlN filler can increase the trap level and trap energy density of the deep traps in the sample. The dielectric loss of the EP at high frequency is reduced with the content of 1 wt% nano-AlN. Furthermore, the addition of the nano-AlN can improve the thermal stability of the EP. The 1 wt% nano-AlN sample has the superior electrical insulation and material performance amongst the tested materials.

Investigation of the Space Charge and DC Breakdown Behavior of XLPE/α-Al2O3 Nanocomposites.

This paper describes the effects of α-Al2O3 nanosheets on the direct current voltage breakdown strength and space charge accumulation in crosslinked polyethylene/α-Al2O3 nanocomposites. The α-Al2O3 nanosheets with a uniform size and high aspect ratio were synthesized, surface-modified, and characterized. The α-Al2O3 nanosheets were uniformly distributed into a crosslinked polyethylene matrix by mechanical blending and hot-press crosslinking. Direct current breakdown testing, electrical conductivity tests, and measurements of space charge indicated that the addition of α-Al2O3 nanosheets introduced a large number of deep traps, blocked the charge injection, and decreased the charge carrier mobility, thereby significantly reducing the conductivity (from 3.25 × 10−13 S/m to 1.04 × 10−13 S/m), improving the direct current breakdown strength (from 220 to 320 kV/mm) and suppressing the space charge accumulation in the crosslinked polyethylene matrix. Besides, the results of direct current breakdown testing and electrical conductivity tests also showed that the surface modification of α-Al2O3 nanosheets effectively improved the direct current breakdown strength and reduced the conductivity of crosslinked polyethylene/α-Al2O3 nanocomposites.

Space charge dynamics of acetophenone and cumyl alcohol and their synergistic effect in LDPE

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.

Influence of Thermal Aging on Space Charge Accumulation and Dissipation of Cross-Linked Polyethylene With Different Cross-Linking Agent Contents

This article presents the influence of thermal aging on space charge accumulation and dissipation of cross-linked polyethylene (XLPE) with different cross-linking agent contents. In this article, dicumyl peroxide (DCP) and low-density polyethylene (LDPE) respectively are used as cross-linking agent and basic experimental material. Additionally, an improved PEA method is used to measure space charge densities during voltage application and removal when thermal aging time respectively is 0, 5, 15, 30 and 80 days due to higher accuracy. And experimental results indicate thermal aging can introduce negative charge traps into pure LDPE and make space charges easier to inject and accumulate in pure LDPE and XLPE, but space charge densities in XLPE increase more rapidly as thermal aging time increases, moreover, thermal aging can simultaneously increase densities of deep and shallow charge traps in pure LDPE and XLPE, etc. And we have also explained the change of space charge accumulation and dissipation of pure LDPE and XLPE after thermal aging from the perspective of micro-structure change of pure LDPE and XLPE through the quantitative analysis of average charge densities. Besides, theoretical model of the average charge decay has also been summed up, and this model can perfectly reflect the dissipation of space charges in pure LDPE and XLPE of different thermal aging time.

Enhancement of insulating properties of polyethylene blends by delocalization type voltage stabilizers

This paper reports on the effects of four voltage stabilizers, m-aminophenylboric acid, 2-methoxy-5-pyridineboric acid, m-aminobenzoic acid and 4-dimethylaminobenzoic acid, on the insulation properties of polyethylene blends. 1 wt% voltage stabilizers were added into low density polyethylene (LDPE) blend containing 10 wt% high density polyethylene (HDPE) by a solution method. Physicochemical and electrical properties of the blends were analyzed by infrared spectroscopy, thermo-gravimetric analysis, differential scanning calorimetry, electric tree initiation tests, space charge and surface potential decay measurements. The results showed that the addition of the voltage stabilizers had a little effect on the thermal stability and melting properties of the LDPE/HDPE blend. When the voltage stabilizers were included into the blends, the tree initiation voltage (TIV) increased, in particular the blend with m-aminobenzoic acid showed the highest increase of 41% compared to the reference LDPE/HDPE. It was found that space charge accumulations of the blends were apparently suppressed by adding voltage stabilizers. Moreover, the blend with the m-aminobenzoic acid had the least space charge accumulation. The surface potential decay rate of the blends was significantly increased after adding voltage stabilizers. Quantum chemical calculations illustrated that the HOMO-LUMO energy gap was closely related to the TIV of the blends, whereas the m-aminobenzoic acid has the lowest energy gap level and the best effect of restraining the electrical tree initiation. The trap level calculation showed that the trap depth in the blends decreased with the addition of the voltage stabilizers, which was beneficial to suppress the accumulation of space charge in the materials.

Investigation of charge transport in LDPE/SiO2 nanocomposite based on the simultaneous observation of charge and current behaviour

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.

A comparison of space charge behaviors in coaxial cable and film sample under temperature gradient

Space charge in DC cables affect the long-term reliability of the cable during operation. A temperature gradient (TG) develops across the cable insulation due to Joule heat in the conductor. Extensive researches on space charge in film insulations under TG have been reported. However, up to date, it is still questionable whether the measurement results of film samples can directly reflect the space charge characteristics in an actual coaxial cable. In this study, the space charge behavior in a 10 kV XLPE cable was investigated under different temperatures and electrical stresses using an improved pulse electro-acoustic (PEA) measurement system. The results show that the coaxial geometry and the TG both enhance hetero-charge accumulation near the cable outer semi-conductive layer. Space charge in film samples sliced from the same cable under the same conditions were also measured and compared to that in the cable. It is found that the space charge behaves similarly in terms of charge variation laws and distributions under different electrical stresses and TGs regardless of the type of sample structure, the two space charge measurement methods show well equivalence.