Space Charge Injection Research Articles

Effect of CNFs-Ni/LDPE electrode on space charge injection in LDPE insulating layer

The accumulation of space charge is considered to be an important key factor to accelerate the aging of the insulation of HVDC cables. How to reduce the accumulation of space charge in the insulation layer is an urgent problem to improve the voltage level of cables. In this paper, an approach is presented to prevent charge from the inner semiconductive layer to the insulating layer, using a modified semiconductive compound by doping magnetic carbon nanofibers. Through microwave-assisted heating, Ni was deposited on the surface of carbon nanofibers which have been pre-treated. Modified semiconductive compound electrodes was used to test the injection of charges. The results showed that charge injected into LDPE insulting layerwith modified CNFs-Ni/ LDPE electrode is less than that with LDPE semiconductive layer without CNFs-Ni electrode.

Fast Operations of Nonvolatile Ferroelectric Domain Wall Memory with Inhibited Space Charge Injection

The microampere-level domain wall currents in LiNbO3 single crystals have promising applications in nonvolatile ferroelectric domain wall random access memory and logic with high-density integration, ultrafast operation speeds, and almost unlimited switching cycles. For the memory commercialization, the improvements of the reliability and operation speed of the devices are challenging due to the high-field charge injection. The injected charge could compensate the domain-wall boundary charge that screens the domain switching field and reduces the domain wall current. In this work, two kinds of memory nanocells were fabricated on the surfaces of X-cut LiNbO3 single crystals to study the geometry-dependent charge injection. The striped memory cell due to the appearance of the size-driven reconstruction has a smaller coercive field than that of a clamped memory cell without relaxation of the lattice matching stress, which reduces low-frequency charge injection and increases the domain switching speed. At an operating voltage of 5 V, we observed a retention time of more than 1 week and an on/off current ratio of 2 × 104 for a striped-like cell, paving the route to integrate energy-efficient high-density domain wall memory in high reliability.

Effects of High Magnetic Field on Partial Discharge and Dielectric Breakdown Behavior of Silicone Rubber

In this article, space charge and partial discharge (PD) behavior and the breakdown strength of silicone rubber under ac and dc voltages are tested in high magnetic field. In the case of ac voltage, the effects of high magnetic field on promoting air gap breakdown result in the decrease in PD inception voltage. Polarization of specimens with higher speed and lower air gap breakdown voltage result in the increase in PD frequency and eventually causes the decrease in breakdown strength. In the case of dc voltage, energy level splitting due to Zeeman effects results in lower interface barrier, considering both the increase in electron affinity and the increase in density deference between donor-like and acceptor-like surface traps. The decrease in interface barrier causes more space charge injection. The electric field distortion caused by local space charge accumulation leads to the decrease in breakdown strength in high magnetic field.

Investigation of Charge Accumulation and Decay Processes in Epoxy-Based Composites Using Quantum Chemical Calculations Under the Effect of Electric Field

This work investigates the influence of aluminum nitride (AlN) fillers on the charge dynamics of epoxy resin (EP) and its hybrid composites through space charge measurements and quantum chemical calculations (QCCs). QCCs analyze the energy-level orbitals and trap parameters of the molecules under the electric field. The pure EP exhibits a homo-charge accumulation, whereas fewer charges are accumulated in the EP hybrid composites. The AlN filler addition significantly suppresses the space charge accumulation during polarization and facilitates the charge dissipation during the depolarization due to the lower trap density than the pure EP. It is found that the orbital energy level of the combined reaction of all monomers together is not a simple superposition of individual orbital energy levels of different monomers. The increased charge injection barrier height suppresses space charge injection and accumulation. The electric field significantly influences the hole traps more than the electron traps. The positive charges with low mobility are easily trapped inside hole traps compared with the negative charges with high mobility inside the electron traps. This results in positive charge accumulation during the polarization and slow decay during the depolarization. The increase of the electrostatic potential is more uniform in hybrid composites than in the pure EP, and no visible electron migration is observed. It indicates that hybrid composites remain stable under electric fields. Moreover, it is not easy for the hole to transport due to the deep trap after the charge injection, leading to limited visible space charge accumulation near the anode.

Transport Characteristics of Interfacial Charge in SiC Semiconductor-Epoxy Resin Packaging Materials.

The silicon carbide (SiC) wide bandgap (WBG) semiconductor power device has been widely applied for its excellent properties. However, the charge accumulated in the interface of SiC semiconductor-related insulation packaging may lead to serious material performance degradation and failure, threatening the reliability and operation life of power devices. In this research, interface charge accumulation characteristics of SiC–epoxy resin double-layered material were investigated, and space charge injection, transport, and accumulation mechanisms, as well as the related temperature effect for the SiC–epoxy resin under polarization and depolarization conditions, were studied by the pulsed electro-acoustic (PEA) technique. The results show that a charge peak appears between the SiC–epoxy resin interface, which shows the same polarity as the SiC side electrode. Charge injects from the SiC electrode, transports along with the SiC semiconductor bulk due to the high mobility, and is blocked by the interface barrier. In addition, under high temperature and high electrical stress conditions, obvious charge accumulation occurs inside the epoxy resin bulk, which was captured by the deep traps. The charge accumulation of the SiC-insulation packaging material can influence the operation of the power device and should attract more attention.

Effects of Pre-Crosslinking on Space Charge and Breakdown Characteristics of XLPE Cable Insulation

This study focused on the effect of pre-crosslinking on the electrical properties of XLPE insulation for HVDC cables. The XLPE samples were pre-crosslinked at 140, 150 and 160 °C. The space charge and DC breakdown characteristics of the XLPE samples with and without pre-crosslinking were investigated. The results show that the pre-crosslinked XLPE samples have an obvious space charge injection and transportation, resulting in more severe electric field distortion. Under the electric field of 100 kV/mm, the electric field distortion rates of the XLPE samples pre-crosslinked at 150 and 160 °C reached 51% and 72% respectively, which were much higher than that of the XLPE sample without pre-crosslinking (14%.) Compared with the XLPE without pre-crosslinking, the breakdown strengths of XLPE pre-crosslinked at 140, 150 and 160 °C were reduced by 11%, 6% and 7% respectively, at the temperature of 50 °C. It is concluded that the pre-crosslinking leads to imperfect crystallization and micro-defects in the amorphous region of XLPE, thereby leading to more space charge accumulation and reduced DC breakdown. The pre-crosslinking of XLPE insulation therefore should be prevented during HVDC cable manufacturing.

Effect of LiCoO 2 on the positive temperature coefficient effect (PTC) properties of semiconductive composites and its ability to inhibit space charge injection

The semiconductive shield layer is between the conductive core and the insulation layer of high voltage direct current transmission cables and plays an important role in suppressing the accumulation of space charge in the insulation layer. Since the prevalent positive temperature coefficient effect (PTC) of the semiconducting layer leads to cable ageing and failure, this paper proposes the use of lithium cobaltate to modify the semiconducting shield to suppress its PTC effect and improve its ability to inhibit space charge injection. The ionic conductor LiCoO2 was prepared by the sol–gel method. LiCoO2 particles with particle sizes ranging from tens to hundreds of nanometres were obtained by ball milling. The prepared LiCoO2 was used as a filler to modify the carbon black (CB)/ethylene vinyl acetate copolymer (EVA)/low density polyethylene (LDPE) composites. After melt blending and moulding cross-linking, LiCoO2/CB/EVA/LDPE semiconductive shielding specimens were successfully fabricated. Resistivity, Thermally Stimulated Depolarisation Currents (TSDC), and pulsed electro-acoustic measurements were conducted for the prepared specimens. The PTC effect, which is common in polymer composites, was successfully suppressed, and the PTC strength was reduced by 17%. The resistivity of the semiconductive shielding layer was significantly reduced. The ability of the semiconducting shield to inhibit space charge injection from the conductive core to the insulation layer was significantly improved. When LiCoO2 content is 0.5 wt%, the semiconductive shielding layer has the best performance, and the space charge in the insulation layer was reduced by 70% compared to the undoped semiconducting shield.

Temperature Effect on Dielectric Properties of Propylene Carbonate under Switching Overvoltage

Temperature can change the charge injection and migration characteristics in the liquid, affecting the insulation performance. First, the influence of temperature on the permittivity and Kerr constant of the liquid dielectric is studied. Also, the results prove that the permittivity and Kerr constant of propylene carbonate (PC) decrease with the increase of temperature. Moreover, to explore the influence of temperature on the insulation performance of propylene carbonate, we build a temperature-controllable breakdown test platform under voltage impulse, and the breakdown voltage of PC increases slightly with the increase in temperature. Based on the electrooptic effect, an optical platform is built to measure the electric field under the switching overvoltage. In addition, the space charge distribution is calculated, and the results indicated that the temperature in the range of 10 °C–40 °C is negatively correlated with the net space charge injection. Finally, the temperature effect on the rate of change and mobility of the space charges is explored.

Considerations on diverse physical phenomena leading to return voltage on insulators

A brief short circuit is not sufficient to relax the entire spectrum of the stress accumulated in an insulator during a long time polarization. Following neutralization, a return voltage appears. This phenomenon is used as a routine characterization tool in some areas of electrical engineering. Several experimental results are detailed here supporting the interest of this electrostatic technique as a laboratory characterization tool. For instance, the sensitivity of this technique may be applied to follow the ageing of cables for the aircraft industry, as well as to monitor interfacial polarization build-up at high temperature on polarized alumina plates. It has also been used to demonstrate a photoelectret effect on polyimide films, which is described.We analyze here the diverse physical processes that may be responsible for return voltage: slow dipolar relaxation processes, interfacial polarization, heterogeneous conductivity and injected space charge motion.

Space charge evolution and its effects on electrical tree degradation in silicone rubber at varied temperature under polarity reversal voltage

Electrical tree is one of the main reasons for cable failure. In this paper, the space charge evolution and electrical tree characteristics under polarity reversal voltage with varied temperature in silicone rubber (SIR) are carried out. Results show that as temperature rises from 30 °C to 120 °C, electrical tree initiation voltage declines by 17.8%. Meanwhile, the trees become denser, and the tree length is larger correspondingly. In order to ascertain the charge transportation and its influence on tree initiation process under polarity reversal voltage, we build a bipolar carrier transportation model with needle-plate electrode. Space charge transportation characteristics and corresponding electric field are simulated. Moreover, the evolution model of electrical tree under polarity reversal voltage is discussed. The presence of homo space charge leads electric field to become larger during the voltage reversal process. The segment relaxation of SIR and more, deeper space charge injection make tree initiate more easily at higher temperature.

Numerical investigation of electro-thermo-convection and heat transfer enhancement in a square enclosure with various electrode arrangements

The dielectric fluid was normally used to electrically shield high-voltage applications. However, recent studies on the electrohydrodynamic technique using the dielectric fluid were attentively conducted to obtain heat transfer enhancement in keeping with its electrical-insulating feature. This study numerically investigates the electro-thermo-convection in a two-dimensional square enclosure with hot and cold vertical walls. The heat transfer rate in the free convection inside the enclosure can be significantly improved by the injection of space charges and working of the Coulomb force. In this study, we tested several electrode arrangements, including alternately arranged electrode pairs on walls, under the intense charge injection condition. Consequently, the vertical arrangements of the electrode pairs demonstrate a higher heat transfer rate than the horizontal arrangements. The heat transfer coefficient of the vertical electrode pairs at a Rayleigh number of 5000 and an electric Rayleigh number of 200 is larger than three times that of the horizontal electrode pairs. Using fluid with a Prandtl number of 10, the results of the stream lines, isotherms, and space charge distributions are summarized herein for various thermal and electrical buoyant conditions.

Inception and Propagation of Electrical Trees in the Presence of Space Charge in HVAC Extruded Cables

This paper presents the space charge impact on the inception and propagation of electrical trees in cross-linked polyethylene (XLPE) insulation via simulations and experimentation. A 3D finite element analysis (FEA)-based modeling is proposed to simulate electrical trees via a needle embedded on the XLPE insulation. The proposed FEA model demonstrates the influence of the space charge magnitude and polarity on the initiation of partial discharges (PD). Then the critical parameters at the tip of an 8 µm. tree are to be examined. Experimentation is necessary to verify the simulation results acquired and study the mechanism of electrical trees at the inception and propagation stage. Therefore, in the experiment, a needle with a curvature radius of 5 µm. is embedded in the XLPE as a method of simulating defects, such as protrusions, voids, or cracks found in cable insulation. The power source supplies several high AC voltages on the 20 kV (rated voltage) cable under test for observing the inception and propagation behavior of the electrical trees. Phase-resolved partial discharge (PRPD) patterns are extracted at the inception and propagation stages of the electrical tree to determine the severity of the PDs occurring. A relationship is obtained for the PD magnitude with respect to the applied AC voltage while considering the positive and negative polarities of the PDs. This relationship then allows for the acquisition of the maximum electric field-tree length characteristics. The goal of the simulation of electrical trees is to provide the reader with a better understanding about the effect of space charge magnitude and polarity to comprehend the behavior of the treeing inception and propagation mechanisms. The injection and extraction of space charges is a complex phenomenon that requires further visualization through simulations. Therefore, the results of both the simulation and experiment are compared with the aim of establishing a clear relationship between space charge distribution and tree initiation.

Improvement of high voltage direct current material properties upon tailoring the morphology of crosslinked polyethylenes

Crosslinked linear low-density polyethylene (XLPE) is the most common polymeric cable insulation material for high-voltage applications with a high number of operating hours in high voltage alternating current (HVAC) systems. High voltage direct current (HVDC) power transmission and polymeric cable systems play a major role in the future and raise, besides numerous systemic benefits, challenges in the design of material properties. The main issue is injection and trapping of space charges in insulation materials under DC-fields. The objective of this work is to increase knowledge of the interplay between microstructure and material performance of XLPE under DC by tailoring its morphology beyond the capabilities of “common crystallization kinetics” upon constrained crystallization at certain elongations. It was found that the tailored oriented morphology influences the energetic depth of traps and a significant reduction of space charge density occurs. Moreover, the optimized oriented morphology leads to a significant reduction of field enhancement for field strengths ELaplace ≥ 20 kV mm−1 compared to unoriented XLPEs with spherulitic morphology. It is shown that this way of morphology tailoring results in a considerable, material dependent reduction of field exposure by a factor of 4, which promises a significant improvement in the electrical life time of polymeric insulation material used.

The effect of a conductive polyaniline/carbon black composite on the performance of a semiconductive layer

In this study, conductive polyaniline (PANI) ribbons were introduced to a semiconductive layer to improve the conductivity stability of the layer during thermal expansion and enhance its ability to inhibit charge injection into the insulating layer. To maximise the effect of PANI, PANI and carbon black (CB) were preformed into a uniformly dispersed composite, which was then added to the polymer matrix of the semiconductive layer. The experimental results show that the resistivity of the semiconductive layer containing the PANI/CB composite is more stable during thermal expansion than that of the semiconductive layer only doped with CB. This is attributed to the conductive CB network being enhanced by the PANI ribbons. In addition, due to the unique conductivity mechanism and high dielectric constant of PANI, the semiconductive layer has a strong ability to inhibit space charge injection into the insulating layer.

A Comparative Study of Gas-phase Fluorination and Nano-Al2O3 Doping on Space Charge Behavior and Trap Level in Epoxy Resin

In this work, the effect of gas-phase fluorination concurrent with nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> doping on space charge distribution, as well as trap level and density of epoxy resin, is investigated. Epoxy resins with different concentrations of Al2O3 nano-fillers (0, 1, 3, and 5 wt%) are prepared and fluorinated by a gas mixture of F2/N2 (20/80 v/v) for 30 min at 40 °C and 0.05 MPa. Fourier-transform infrared (FTIR) spectroscopy confirms the breakage of molecular-chain during the gas-phase fluorination process. The space charge behavior is observed by the pulsed electroacoustic (PEA) method before and after fluorination. PEA results indicate that 30-minute fluorination is useful against space charge injection into the bulk of epoxy resin/Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanocomposites. The trap density and trap energy level distribution are determined by isothermal surface potential decay (ISPD) measurement. The results indicate that fluorination introduces shallow traps on the surface, which can increase the conductivity of surface, and, thus, the trap depth is considerably reduced after fluorination. Moreover, it is found that incorporating 1 wt% nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> particles into epoxy resin can level up the energy level of deep traps. However, further increasing the nano-Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> concentration, e.g., 3 and 5 wt%, promises the overlap of interaction zones between polymer matrix and nanoparticles, reducing the trap energy level.

Experimental Study of Water Collection from Plume of an Induced-Draft Counter-Flow Cooling Tower Using Space Charge Injection

Plume collection from cooling towers can be a reliable solution to the water scarcity problem faced in many regions around the world. Meshes are one of the most proposed collectors in this regard that rely upon inertial collision for droplet capture and are inherently limited by aerodynamics. This study quantifies the effect of electrical forces on water collection from the plume of an Induced Draft Counter Flow (IDCF) Cooling Tower by introducing sets of copper tubes at the exit of the tower. The imparting of net charge to the exhaust plume by instigating space charge directs the vapor towards the inside wall of copper tube forming water droplets. This arrangement instead of a mesh or net system, creates a lesser obstruction to flow. Fabrication of fill/packing with a corrugated wave pattern using PVC plastic demonstrates satisfactory cooling performance of the tower. An optimized L/G ratio is found to exist for maximum collection efficiency of water from plume at definite entering fluid temperatures by investigating with the entering warm water temperatures at 40°C, 45°C and 50°C while the dry bulb temperature of air ranges from 23.5°C to 30.1°C. The electricity consumption for this arrangement fluctuates from 2.78 kWh/m3 to 5.13 kWh/m3 for two L/G ratios (23.5 and 28.3). Where maximum collection percentage occurs at two different entering fluid temperatures, the power expended is below the minimum used for typical desalination plants.

Hot Injection-Based Synthesized Colloidal CdSe Quantum Dots Embedded in Poly(4-vinylpyridine) (PVP) Matrix Form a Nanoscale Heterostructure for a High On-Off Ratio Memory-Switching Device.

Chalcogenide-based quantum dots are useful for the application of memory-switching devices because of the control in the trap states in the materials. The control in the trap states can be achieved using a hot-injection colloidal synthesis method that produces temperature-dependent size-variable quantum dots. In addition to this, formation of a nanoscale heterostructure with an insulating material adds to the charge-trapped switching mechanism. Here, we have shown that the colloidal monodispersed CdSe quantum dots and poly(4-vinylpyridine) (PVP) formed a nanoscale heterostructure between themselves when taken in a suitable ratio to fabricate a device. This heterostructure helps realize memory-switching in the device with a maximum on-off current ratio of 105. The switching in the device is mainly due to the trap states in the CdSe quantum dots. The conduction in the off state is due to thermal charge injection and space charge injection conduction and in the on state, due to the Ohmic conduction mechanism.

Electrical and thermal performances of epoxy-based micro–nano hybrid composites at different electric fields and temperatures

This paper investigates the electrical and thermal properties of pure epoxy resin (EP) and its micro–nano hybrid composites (20 wt% micro-AlN fillers with 1 wt% and 3 wt% nano-Al2O3 fillers; 50% micro-AlN with 3% nano-Al2O3 fillers) for power electronic packaging applications. Electrical properties such as space charge distribution, electrical conductivity and surface potential decay are measured. The thermal performance of the fabricated samples is estimated using thermal analysis devices. The hybrid composite consisting of 20 wt% micro-AlN and 1 wt% nano-Al2O3 fillers exhibits the least space charge accumulation, higher thermal conductivity and better thermal stability. However, the excessive addition adversely affects space charge and electrical conductivity properties. The micro–nano hybrid composites significantly exhibit higher electrical conductivity than pure EP. The microfiller addition from 20 wt% to 50 wt% significantly improves the thermal conductivity of the EP. The reduced space charge injection and accumulation in the hybrid micro–nano composites are attributed to the enhancement of the injection barrier and reduction of the charge carrier traps in these materials. A theoretical mechanism of the charge dynamics inside the samples under different test conditions is proposed to support the experimental results.

Influence of the semiconductive composites doped with Li4Ti5O12 on space charge injection in low-density polyethylene

Influence of the semiconductive composites doped with Li4Ti5O12 on space charge injection in low-density polyethylene

Effect of La2Li0.5Co0.5O4 in semiconductive nanocomposites on suppression of space charge injection to the insulating medium for high-voltage direct current cables

Space charge accumulation in the insulating layers of high-voltage direct current cables is a key factor in the degradation of such cables and limits their operational safety. It is necessary to weaken charge injection to the insulation. The semiconductive shielding layer sandwiched between the conductive core and the insulating layer plays a vital role in preventing carrier injecting into the insulation. In this study, a new method is proposed to suppress charge injection. A certain amount of La2Li0.5Co0.5O4 nanopowder was doped in the semiconductive screen to enhance the internal conductive network and block charge injection in the insulation. The influence of La2Li0.5Co0.5O4 in the nanocomposite on the positive temperature coefficient effect and space charge injection process were explored. The results revealed that the peak resistivity of the specimen containing 5 wt% La2Li0.5Co0.5O4 decreased by 50.5% compared to the composites without La2Li0.5Co0.5O4, and charge quantity in the insulating layer was reduced by 41.2%.