Electrical Insulation Performance Research Articles

Epoxy insulator with surface graded-permittivity by magnetron sputtering for gas-insulated line

To reduce the electric field concentration induced flashover in AC gas insulated line (GIL), a novel concept of surface functionally graded material (SFGM) is proposed in this paper, and the continuously graded high-A layer was fabricated by depositing BaTiO 3 on the insulator surface. The iterative method was introduced to optimize the thickness distribution of the BaTiO 3 layer. After four iterations, the layer thickness distribution almost reaches its optimal solution, and the maximum electric field strength is reduced by approximately 70% compared with that of the conventional insulator. To fabricate the optimized SFGM insulator, a rotatable baffle with the designed gap was placed above the insulator during the sputtering process. For comparison, a uniformly sputtered insulator with the 4μm BaTiO 3 layer and a discretely graded insulator with four gradients were prepared. Both electric field simulations and experiments were performed to evaluate the electrical performance of different insulators. The results show that, compared with the conventional insulator, the sputtered insulators, especially the optimized SFGM insulator, can significantly improve the uniformity of the electric field distribution along the insulator surface under a 50 Hz voltage. Accordingly, the discharge inception voltage and flashover voltage of the insulator both increased significantly. Under negative standard lightning impulse voltage (1.2/50 μs), the sputtered insulators show insulation performance comparable to that of the conventional insulator.

Electrical insulation performance of cross‐linked polyethylene/MgO nanocomposite material for ±320 kV high‐voltage direct‐current cables

AbstractPolymeric nanocomposite insulations are receiving the widespread attention of the electrical cable industry. This paper presents the electrical insulation performance of as‐received commercial 320 kV high‐voltage direct‐current (HVDC) cross‐linked polyethylene (XLPE)/MgO nanocomposite material with reference to the pure XLPE. The results of this commercial‐grade electrical insulation material are not reported hitherto. The scanning electron microscopy confirms the well‐dispersed nanofiller inside the polymer matrix. The DC electrical insulation performance is investigated by DC breakdown strength, space charge, DC electrical conductivity, and surface potential decay measurements. The test samples are subjected to the thermal aging at 135°C for 30 days. The un‐aged nanocomposite exhibits 20% higher DC breakdown strength, negligible hetero space charge accumulation, and the lower DC conductivity by one order than the un‐aged pure XLPE. Moreover, thermally aged nanocomposite shows more restraint to the deterioration of its properties. After the thermal aging, the DC breakdown strength decreased by 38% and 20% in the pure XLPE and its nanocomposite, respectively. Thermally aged nanocomposite shows negligible hetero charges and an increase in the DC conductivity by one order. However, the pure XLPE shows higher hetero charge accumulation and the increased DC conductivity by one order. In un‐aged XLPE nanocomposite higher crystallinity, higher deep trap density, and the lower carbonyl index are found to be the primary attributes of its improved performance. It is postulated that these significantly unchanged attributes can be the reason for the better anti‐thermal aging properties of the XLPE nanocomposite.

Dual‐direction high thermal conductivity polymer composites with outstanding electrical insulation and electromagnetic shielding performance

AbstractThe rapid dissipation of accumulated heat and efficient electromagnetic interference (EMI) shielding attract considerable attention due to the gradual integration and miniaturization of electronic devices. In an attempt to simultaneously overcome these issues, dual‐direction high thermal conductivity (TC) materials with outstanding electrical insulation and EMI shielding performance are urgently demanded. Herein, a tailor‐made sandwich network structure with insulated outer layer and EMI shielding middle layer is constructed by compression molding. The sandwich network structure endows the composite (#AB0.7/AM0.7/AB0.7) with high TC values of 3.051 and 3.365 Wm−1 K−1 at through‐plane and in‐plane directions, respectively, excellent electrical insulation (6.61 × 1013 Ω cm, 3.25 kV/mm) and superior EMI shielding performance (>27.68 dB from 8.2 to 12.4 GHz). All these results demonstrate that the prepared composite with tailor‐made sandwich network structure is a promising candidate as ideal thermal management material for electronic devices.

Studying AC Flashover Performance of Suspension Insulators Under Natural Cold Fog and Wet Deposition Conditions

The electrical performance of insulators under comprehensive conditions including low air pressure, pollution and icing, is an important basis for the selection of external insulation. However, the problem of cold wet deposition has not attracted extensive and considerable interest in research. Based on flashover tests of suspension insulators in field test station, the AC flashover performance of insulators was studied. The results show that the duration of existence of the arc throughout the composite insulator surface in case of condensation was significantly lower than that in case of freezing fog flashover because of a change of the sheds surface roughness, which leads the arc melt the frozen fog droplets on the sheds surface and extends its flashover time. Compared with the flashover characteristics of the hydrophilic insulators in frost fog, the composite insulators exhibited clear arc extinguishing and arc starting phenomena. Moreover, the wind velocity has affected the flashover performance of suspension insulators in cold fog. Studying flashover characteristics of suspension insulators can contribute to the design of the external insulation of transmission lines and substations under natural cold fog and wet deposition regions.

AC Flashover Voltage Model for Polluted Suspension Insulators and an Experimental Investigation in Salt Fog

In recent years, the electrical performance of polluted suspension insulators operating in coastal foggy regions has attracted wide attention because of the occurrence of flashover. Surface wetting by salt fog accretion increases the surface conductivity and degrades the flashover voltage of insulators. Normally, the effect of conductivity under fog condition and pre-contamination on the flashover of polluted insulators are analyzed separately. Therefore, this does not reveal the flashover characteristic in salt fog, which requires further research. This paper describes salt fog experiments with an AC test voltage conducted using suspension insulators in a 110-kV power system. The electrical strength performance of insulators with salt fog treatments is analyzed, and the effects of salt deposit density (SDD) and the fog water conductivity (FWC) on the flashover of three types of suspension insulators are studied. The concept of additional SDD is proposed to quantify analyze the effects of salt fog. The results show that there is less dependence on FWC at higher pollution levels than at lower pollution levels. That is, the effect of FWC on 50% AC flashover voltage ($U_{50}$ ) can be neglected for SDD >0.15 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , whereas FWC has a strong effect on $U_{50}$ for SDD <; 0.15 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . FWC has a greater influence on $U_{50}$ of ceramic insulators than on that of silicone rubber insulator. A model for calculating the critical flashover voltage of insulators is proposed by considering the combined effect of FWC and pre-polluting SDD.

Influence of Crystallization Effect During Icing Water Phase Transition on the Flashover Characteristics of Ice-Covered Insulators

The crystallization effect during the phase transition of icing water has a significant impact on the electrical performance of an ice-covered insulator. However, the spatial distribution of conductive ions in the ice layer remains poorly understood. In this study, the crystallization effect was confirmed through a condensation test of the potassium permanganate solution. The influence of freezing water conductivity, ice thickness and equivalent salt deposit density on the spatial distribution of conductive ions was determined by a triangular ice sample, wherein the effect on the flashover characteristics of ice-covered insulator was investigated. Results show that MnO4− migrates to the unfrozen area during freezing, thus accumulating on the droplet top. The conductivity of melted water on the exterior surface of an ice sample is considerably higher than that of icing water. The former can reach a maximum of 5.7 times the latter when the freezing water conductivity method is adopted. This value is 9.7 for a polluted ice sample. A highly conductive water film forms on the insulator surface because of the crystallization effect in the melting period, thereby improving the leakage current and reducing the residual ice resistance, which results in the decrease of flashover voltage.

The Shed Hole Influence on the Electrical Performance of Composite Insulator

In recent years, shed holes appeared on the surface of in-service composite insulators have been observed in transmission lines in the southern and middle regions of China. Their presence can shorten the insulator creepage distance and therefore may impair the composite insulator’s electrical performance. In this paper, extensive experiments were carried out to study the influence of shed hole (shed hole diameter, shed hole arrangement, shed hole percentage) on the electrical performance including pollution flashover voltage, pollution-rain flashover voltage, dry flashover voltage, wet flashover voltage, and lightning impulse voltage. The experimental results showed that the presence of shed hole had insignificant impact on the dry flashover voltage, wet flashover voltage, and lightning impulse when the shed diameter was 1.0 mm. However, the shed hole influence on the pollution flashover and pollution-rain flashover performance was appreciable. When the shed hole diameter is 1.0 mm, both the pollution flashover voltage and the pollution-rain flashover voltage are higher with alternating arrangement than that with straight-line arrangement. With the same shed hole diameter and pollution degree, the electrical performance of pollution-rain flashover is better than the pollution flashover. It is also found that the diameter of shed hole is increased by 3% to 60% after the pollution flashover test.

Rejuvenation of Retired Power Cables by Heat Treatment: Experimental Simulation in Lab

In this work, heat treatment was performed on three retired 110 kV AC cables with service years of 0, 15 and 30, and the effects on the thermal and electrical performance of the cable insulation were investigated. First, each cable with a length approximately of 5 m was prepared and cut into five equal segments. Four segments of each cable were annealed at a temperature of 90, 95, 100, or 105 °C by building a small circuit to simulate cable operation. Then, a section at the middle of each segment was cut out, and the insulation layer was peeled away. The peeled-off layers from the inner, middle and outer positions were selected as the test samples. Subsequently, Fourier transform infrared spectrometry (FTIR), and differential scanning calorimetry (DSC) were performed, and the DC conduction current and dielectric breakdown strength were measured. The best thermal properties of the highest melting point, crystallinity, and smallest melting range were found when the cables were annealed at 100, 105, and 105 °C for the inner, middle, and outer positions, respectively. The highest dielectric breakdown strength and lowest electrical conductivity were found at temperatures of 95 or 100 °C for the inner and middle positions, respectively, and at 105 °C for the outer position. The FTIR results showed that thermal annealing for hundreds of hours did not adversely affect the molecular chains. It is found that this type of heat treatment could be a feasible method to rejuvenate retired cables, and a conservative temperature of 95 °C is regarded as the optimum annealing temperature.

이종 필러 추가에 따른 에폭시 마이크로 복합체의 절연성능 연구

In this paper, essential properties that affect electrical insulation performance of epoxy micro composites were studied. In particular, the effect of adding co-filled micro sized fillers to epoxy composites were explored. The fillers considered in this study include 40 [um] BN, 22 [um] SiO₂, 5 [um] Al₂O₃ and 5 [um] TiO₂. For each composite, dielectric constant, dielectric loss, tracking resistance and breakdown field intensity were tested. When BN and SiO₂ particles were added, the dielectric constant and the dielectric loss increased. By co-filling TiO₂ and Al₂O₂ particles to the BN/Epoxy composite, a change in dielectric properties were observed. Different performance among epoxy composites were observed depending on characteristics of the co-filled filler. Experiment results demonstrated the effectiveness of epoxy composites with co-filled micro particles. Results of this research could contribute to optimized design of epoxy composites for improved insulation performance.

Novel transparent Er3+ doped perovskite glasses fabricated by containerless solidification

Perovskite glasses with the compositions of La0.85-xErxBi0.15Al0.5Ga0.5O3 (x = 0.025, 0.05, 0.1, 0.125, 0.15) and remarkable electrical insulation performance were synthesized by containerless solidification. The x = 0.15 amorphous material had the highest Vicker's hardness (7.34 GPa). The transmittance reached as high as 86% in the infrared (IR) region. The absorption peaks in the UV–vis–NIR region of transmittance spectra could be attributed to the transitions of Er3+ ions from the ground state 4I15/2 to excited states. All transparent samples emitted intense green emissions when excited at 379 nm, corresponding to the 2H11/2 and 4S3/2 to 4I15/2 transitions of Er3+ ions. The average decay life-time decreased from 25.3 μs to 6.1 μs with increasing Er3+ ion content, revealing the interaction within Er–Er clusters.

An efficient synergistic system for simultaneously enhancing the fire retardancy, moisture resistance and electrical insulation performance of unsaturated polyester resins

Unsaturated polyester resins (UPR) are usually used in the field of electronic appliances, but their inherent flammability and hygroscopicity severely limit their wide application. In this research, dimelamine pyrophosphate (DMPY) was synthesized and constituted a synergistic flame retardant system (FRs) with aluminum diethylphosphinate (ADP). The FRs was applied to flame retarded UPR thermosets and the fire retardancy, moisture resistance, mechanical properties and flame retarded mechanisms of UPR/FRs were comprehensively investigated. When as low as 15 wt% FRs was incorporated, UPR/FRs achieved UL-94 V-0 rating during vertical burning tests and the limiting oxygen index value reached 27.9%. FRs stimulated UPR matrix to form uniform, stiff and dense char layer on materials surface during combustion process. Besides, the phosphine and phosphine oxygen radicals and inert gases generated from FRs exerted the inhibition and dilution effect in gas phase. Thus, the heat release and effective heat combustion of UPR/FRs were conspicuously suppressed. Moreover, UPR/FRs retained satisfactory flame retardancy and mechanical performance after water resistance tests. The moisture resistance of UPR/FRs was evidently enhanced compared with UPR and it led to the maintenance of electrical insulation properties of water treated UPR/FRs. Overall, the superior fire safety, moisture resistance and electrical insulation performance promised the flame retardant UPR thermosets crucial application values in the key areas.

Thermal response properties and surface insulation failure mechanism of epoxy resin under arc ablation

Arc ablation during the operation of HVDC circuit breakers can cause irreversible damage to the epoxy resin (EP) insulation pull rod. This is one of the critical factors that affect the reliability and lifetime of a HVDC circuit breaker. In this work, we built an arc ablation experimental platform and studied the thermal response properties of EP insulation under arc ablation. The surface charge evolution characteristics and the dielectric properties were analyzed to reveal the underlying surface insulation failure mechanism of EP after arc ablation. The results indicate that arc ablation can cause severe carbonization to EP insulation. With increasing arc duration, the EP gradually transforms into carbide and pore products. Arc ablation can decrease the molecular moment orientation polarizability and the relative dielectric constant of EP insulation, and thereby increasing the current density formed by Schottky injection. Therefore, when the external electric field is applied, the charge on EP insulation surface migrates from high voltage electrode to the ablating area, causing the regional accumulation of surface charge and the formation of local failure area. This can decrease the surface electrical performance of EP insulation and lead to insulation failure finally.

Effect of nano filler concentration on leakage current and partial discharge properties of zepoxy nano composites

Electrical insulation materials play a key role in stable and uninterrupted operation of electrical power system. Researchers working in the field of electrical insulation have turned their focus to nano composite insulation materials for better performance and reliability. A typical nano composite insulation material is a combination of nano particles and base materials. Since last 25 years silicon rubber and epoxy are two widely used electrical insulation materials for outdoor and indoor applications respectively. To further enhance the electrical insulation performance of these materials addition of nano particles has proven to be of practical value. This study presents an investigation on zepoxy (a military grade epoxy) incorporated with three different nano particles at varying percentages. The main aim was to find the nature of relationship between leakage current (LC) and partial discharge (PD). In addition to that optimum composition which results in lowest values of PD and LC was also investigated for each of three nano composites. The results of this study help to understand the interdependence between LC and PD for indoor electrical insulation based on zepoxy. LC and PD play a vital role in condition assessment of any insulation material.

Effect of Er interlayer on microstructure, composition, electrical and mechanical properties of erbium oxide coating on steel

The properties of a coating depend on its integrity and the interface properties between the coating and the substrate. Erbium oxide coatings with and without pre-sputtered Er interlayer were reactive sputtered and annealed at 700 °C and 900 °C to investigate the microstructure evolution in a high-temperature environment and the effect of Er interlayer on the composition, topography, microstructure, mechanical and electrical insulation properties of the coatings. X-ray diffraction (XRD) and Raman spectroscopy (Raman) showed that the presence of the Er interlayer effectively reduced the coating stress and promoted the growth of thermally stable cubic Er2O3, which may responsible for the improvement of the mechanical property of Er2O3/Er coating, and 700 °C annealed Er2O3/Er sample has the highest elastic modulus and hardness. Meanwhile, impurity was detected by XRD, Raman and energy dispersive spectroscopy (EDX) in the annealed samples, and their compositions, locations, and formation mechanism were analyzed. The presence of Er interlayer can hinder the formation of low-resistance spinel Fe–containing oxide nanocrystals, therefore, the electrical insulation performance of the composite coating did not deteriorate significantly after annealing, and still meets the requirements of ITER insulating coating.

Investigation of high temperature electrical insulation property of MgO ceramic films and the influence of annealing process

MgO ceramic films were deposited as insulating films for thin film sensors by electron beam evaporation on Ni-based substrate at 25 °C, 200 °C and 300 °C (denoted as MgO-25, MgO-200, and MgO-300). The structures and morphologies of the MgO ceramic films were investigated by X-ray diffraction and scanning electron microscopy, which indicated that MgO films exhibited (111) and (222) preferential orientation growth and a more compact microstructure with increasing substrate temperature. Furthermore, the insulation resistance of MgO films was enhanced at higher substrate temperature, the MgO-300 ceramic film increased by approximately one order of magnitude compared to MgO-25 ceramic film. Notably, the insulation resistance of the MgO-300 ceramic film was 1.5 MΩ when the test temperature reached 1000 °C. Additionally, the influence of annealing on the electrical insulation properties of MgO-300 was evaluated via a leakage current test at 25–1000 °C. The results showed that the stability of the electrical insulation performance was improved mainly due to the growth of grains and repair of defects. Moreover, the conductivity was below 1.2 × 10−6 S/m and the calculated insulation resistance was greater than 0.55 MΩ at 1000 °C after the fourth annealing, which displays that the relative error of the apparent resistance is less than 0.03% by the shunt circuit law. Therefore, their excellent electrical insulation performance demonstrated that MgO films have the potential to be employed as an insulating layer for thin film sensors at high temperature.

Laser surface texturing of Ti-6Al-4V by nanosecond laser: Surface characterization, Ti-oxide layer analysis and its electrical insulation performance.

The development of smart biomedical implants with intrinsic communication system between sensors and actuators, or between implant and patient, remains a challenge for scientific community. Titanium and its alloys, especially Ti6Al4V, are the most used materials in the implant's fabrication. The present work is concerned with implant internal communication printing process and presents a detailed study on titanium alloy (Ti6Al4V) surface texturing and their characterization (morphological and chemical) produced by a Nd: YAG laser, aiming to create localized electrical insulation zones, necessary to accommodate electrical communication systems. The characterization of the textured surface was carried out by scanning electron microscopy and the heat affected zone was analyzed by X-ray diffraction. The results showed the formation of α-Ti, Ti6O and Ti2O phases on the textured surface. Through simulations in TINA software, the outer oxide layer formed during laser texturing had efficiency above 90% as insulator when an electrical current was applied.

Modification mechanism of low-density polyethylene insulation by hydrophilic and hydrophobic porous SiO2 nanoparticles

To study the modification effect and mechanism of porous nanoparticles on electrical insulation performance, SiO2/low-density polyethylene (LDPE) nanocomposites with different filler concentrations are prepared by filling two kinds of porous SiO2 nanoparticles. The microstructure and dispersion of SiO2 in LDPE are characterized by scanning electron microscope [ the effect of nanofilling on the crystallinity of LDPE is analyzed by Differential Scanning Calorimetry (DSC)]. The mechanism of insulation modification of nanocomposites is studied by space charge and direct current conductance analysis of charge transport and Weibull distribution of breakdown field strength. By filling nanoscale SiO2 particles, the crystallinity of the LDPE matrix can be changed effectively. It is indicated that the hydrophobic SiO2 nanofillers with higher dispersity in LDPE matrix are more effective than hydrophilic SiO2 nanofillers to improve the crystallinity of LDPE matrix. The LDPE crystallinity of hydrophobic 1 wt% SiO2/LDPE nanocomposite approaches the highest value through heterogeneous nucleation. Compared with pure polyethylene, the conductivity of nanoporous SiO2 filled polyethylene composites is reduced obviously and the lowest conductivity is obtained for the nanocomposites filled with hydrophobic SiO2 nanoparticles in 1 wt% filling rate. The space-charge distribution and electric breakdown field strength further demonstrate that the filling of 1 wt% hydrophobic SiO2 nanoparticles can efficiently inhibit carrier injection and increase the breakdown field strength. The heterogeneous nucleation of porous SiO2 nanofillers increases the crystallinity of LDPE, and thus decreases the carrier mobility and increases the probability of charge charge trapping. The dispersion of nanofillers in composite is improved significantly by the symmetrical group –CH3 on the surface of hydrophobic SiO2 nanoparticles, which effectively suppresses the space-charge accumulation and evidently increases the electric breakdown field strength. It is proved that the special nanoscale interface in nanodielectrics can substantially improve the dielectric properties of polyethylene.

Dielectric, impedance and piezoelectric properties of (K0.5Nd0.5)TiO3-doped 0.67BiFeO3-0.33BaTiO3 ceramics

A novel (0.67-x)BiFeO3-0.33BaTiO3-x(K0.5Nd0.5)TiO3 (KNT100x, x = 0.0, 0.02, 0.04, 0.06, 0.08 mol%) ceramics were fabricated and their microstructure and electrical properties were studied. All samples displayed a pseudo-cubic symmetry, and adding of KNT had little effect on grain size. The dielectric analysis displayed the dispersion increases with the addition of KNT compositions, showing strong relaxor properties. Besides, high dielectric constant (ε’) of 23000 and dielectric peak temperature (Tm) of 390 °C remain at 1 kHz in the x = 0.02 sample while the dielectric loss (tanδ) dropped below 0.5 in the range of 30–400 °C, showing excellent electrical insulation performance. In addition, doping of KNT had obvious influence on the strain, and a large strain (Smax) of 0.26% was obtained at x = 0.02 due to the increase of electrical insulation. More importantly, the strain at 50 kV cm−1 enhanced significantly with temperature increasing, reaching a maximum strain of 0.75% with a small hysteresis coefficient of 30% at 110 °C. Particularly, KNT02 exhibited excellent fatigue resistance within 105 fatigue cycles. Presumably these results are attributed to the coexistence of ferroelectric and non-ergodic relaxor domains and the thermally activated domain wall motion.

Thermally Conductive and Electrical Insulation BNNS/CNF Aerogel Nano-Paper.

Adding heat conducting particles to a polymer matrix to prepare thermally conductive and electrical insulation materials is an effective approach to address the safety issues arising from the accumulation of heat in the working process of electronic devices. In this work, thermally conductive and electrical insulation nano-paper, consisting of Boron Nitride nano-sheet (BNNS) and cellulose nanofiber (CNF), was prepared using an aerogel 3D skeleton template method. For comparison, BNNS/CNF nano-paper was also produced using a simple blending method. At a BNNS loading of 50 wt%, the thermal conductivity of BNNS/CNF aerogel nano-paper and blended nano-paper at 70 °C are 2.4 W/mK and 1.2 W/mK respectively, revealing an increase of 94.4%. Under similar conditions, the volume resistivity of BNNS/CNF aerogel nano-paper and blended nano-paper are 4.0 × 1014 and 4.2 × 1014 Ω·cm respectively. In view of its excellent thermal conductivity and electrical insulation performance, therefore, BNNS/CNF aerogel nano-paper holds great potential for electronic-related applications.

HVDC electrical insulation performance based on charge activity in oil-pressboard composite insulation structures

For more reliable HVDC power transmission systems, it is necessary to enhance HVDC electrical insulation performance in power equipment such as an oil-immersed AC/DC converter transformer. In this paper, the electric field distributions under AC, DC steady-state (DC-SS) and DC polarity reversal (DC-PR) conditions are discussed in oil-pressboard (PB) composite insulation structures. Firstly, based on discussion about the charge accumulation and charge dynamics at oil/PB interfaces, the contribution ratio of deposited charges under DC-SS to the following emerging electric stresses at DC-PR conditions are quantitatively clarified. Secondly, we found that the DC electric field distributions could significantly be controlled by PB arrangement in oil. In particular, both PB configurations and the conductivity ratios of oil and PB would play critical roles to determine electric field distribution, under DC-SS conditions. Furthermore, using an actual AC/DC converter transformer structure, electrical insulation performances under various testing conditions such as AC, DC-SS and DC-PR conditions were comparatively investigated. It was concluded that the most critical testing condition against breakdown strength could be the DC-PR conditions under the higher conductivity ratio of oil and PB.