Cable Accessories Research Articles

Direct Current Electrical Performances of Cable Accessory Insulation EPDM Modified by Grafting Polar-Group Compound.

In order to improve electrical matching between ethylene-propylene-diene misch-polymere (EPDM) reinforce insulation and crosslinked polyethylene (XLPE) main insulation in direct current (DC) cable accessories, the glyceryl monooleate (GMO) organic compound composed of several polar-groups and one long carbon chain is employed for chemical graft modification on EPDM to ameliorate DC electrical performances. Charge trap characteristics are analyzed by testing thermal stimulation current (TSC) and verified by calculating first-principles electronic properties to elucidate the GMO-graft-modified charge trapping mechanism accounting for DC electric conductance and dielectric breakdown. The grafted GMO molecules introduce substantial shallow charge traps that lead to nonlinear profiles of electric conduction versus electric field and cause hopping transports of percolation conductance. Electric conductance of EPDM is significantly improved by GMO graft for electrical matching with XLPE, while a high level of dielectric breakdown strength is retained sufficiently for reinforce insulation in cable accessories. Shallow charge traps introduced by GMO graft are capable of capturing charge carriers to form homocharge layers near electrodes which can scatter the transporting charge carriers and exclude further charge injections, thus to mitigate the dielectric breakdown strength reduction caused by electric conductivity improvement. Electric field finite-element simulations demonstrate that the electric field in DC cable terminals can be evidently homogenized by using GMO-grafted EPDM as reinforce insulation.

Non‐linear electrical conductivity of ethylene‐propylene‐diene monomer‐based composite dielectrics by tuning inorganic fillers

AbstractIn this study, three kinds of inorganic fillers are chosen and filled into ethylene‐propylene‐diene monomer (EPDM) matrix, including silicon carbide, copper calcium titanate (CCTO), and vanadium dioxide (VO2). The electrical properties of the three modified composites at different temperatures have been systematically studied. The intrinsic conductance behaviours of different inorganic fillers on the direct current conductance and electric breakdown strength of the composite dielectrics are also analysed, and the possible mechanism for non‐linear conductivity is revealed. The electrical conductivity increases with the increase of fillers content, where 30 wt.% CCTO/EPDM composite dielectric has a high non‐linear coefficient of 5.71. The increase in temperature can further increase the conductivity of the composites, accompanying a lower breakdown strength. When the temperature rises to 70°C, the VO2/EPDM composite has a higher conductivity, but the electric breakdown strength degrades seriously. The simulation results show that the composites with non‐linear electrical conductivity characteristics can effectively uniform the electric field and solve the problem of electric field concentration at the root of the stress cone, which is beneficial to improving the safe operation of the cable accessories.

Performance evaluation of thermally aged ATH and BN co‐doped silicone rubber nano‐micro composites for power cable applications

AbstractPower cables are intended to operate at high temperatures over a long period which may change the performance of silicone rubber insulation used in high voltage cable accessories. Therefore, in the current study, to enhance the properties of silicone rubber, alumina try‐hydrate (ATH) and boron nitride (BN) micro and nanoparticles were added to the silicone rubber matrix. The synthesized undoped silicone rubber, silicone rubber with 40 wt% micro ATH, 7 wt% nano BN, and 10 wt% micro ATH + 5 wt% nano BN composites were thermally aged for 600 h at 180ºC. The composite properties were analyzed through electrical, thermal, and physicochemical techniques. To assess the electrical characteristics, water droplet‐initiated corona discharge studies as per IEC 60112 were conducted and leakage current, corona inception voltage, and erosion resistance were evaluated. A deep learning technique also was employed for the prediction of erosion resistance of the samples. Further, to examine the surface texture and chemical changes on the surface of the material, 3‐D microscopy and Fourier transform infrared spectroscopy (FTIR) was adopted. Finally, thermal distribution analysis was performed for all the composites by infrared thermography. The results demonstrated the suitability of nano‐micro co‐doped composites over nanocomposites and micro composites for power cable applications.

Insulating Materials for HVDC Cable Accessories: Effects on the Electric Field in Nonstationary Situations

Controlling the electric field distribution in HVDC cable accessories requires a tight knowledge of the properties, particularly the electrical conductivity of the insulating materials constituting the system.

Nonlinear Conductivity and Space Charge Characteristics of SiC/Silicone Rubber Nanocomposites.

To achieve a preferable compatibility between liquid silicone rubber (LSR) and cable main insulation in a cable accessory, we developed SiC/LSR nanocomposites with a significantly higher conductivity nonlinearity than pure LSR, whilst representing a notable improvement in space charge characteristics. Space charge distributions in polarization/depolarization processes and surface potentials of SiC/LSR composites are analyzed to elucidate the percolation conductance and charge trapping mechanisms accounting for nonlinear conductivity and space charge suppression. It is verified that SiC/LSR composites with SiC content higher than 10 wt% represent an evident nonlinearity of electric conductivity as a function of the electric field strength. Space charge accumulations can be inhibited by filling SiC nanoparticles into LSR, as illustrated in both dielectric polarization and depolarization processes. Energy level and density of shallow traps increase significantly with SiC content, which accounts for expediting carrier hopping transport and surface charge decay. Finite-element multiphysics simulations demonstrate that nonlinear conductivity acquired by 20 wt% SiC/LSR nanocomposite could efficiently homogenize an electric field distributed in high-voltage direct current (HVDC) cable joints. Nonlinear conductivities and space charge characteristics of SiC/LSR composites discussed in this paper suggest a feasible modification strategy to improve insulation performances of direct current (DC) cable accessories.

Numerical Analysis of Electric Field Characteristics and Interfacial Pressure of HVDC XLPE Cable Joint Considering Load Cycles

Recent innovations in HVDC extruded cable systems require the development of reliable and safe cable accessories. Cable accessories are made of several insulating materials and contain several interfaces. Interfaces made of different materials can cause electric field distortion and localized enhancement of the field. In addition, the internal temperature profiles of accessories differ depending on load conditions or installation environments, which may lead to an increase in or loss of interfacial pressure due to changes in the mechanical properties of materials. The loss of interfacial pressure degrades the contact state between materials. The micro voids formed due to pressure loss can cause partial discharge and tree, which in turn can lead to reduced lifespan and failure of the cable system. Therefore, it is necessary to study the electrical and mechanical characteristics of cable accessories considering various transient states. However, there is a limit to experimentally analyzing the actual structure. In this paper, electric field and mechanical stress for pre-molded cable joints were analyzed using an electrical model based on the conductivity of the material and a mechanical model based on elastic theory. Temperature fluctuations were simulated according to the sequence of the cable load cycle test, and time-varying electric fields and mechanical stresses were analyzed. From the simulation results, it was confirmed that the electric field and stress distribution in the joint continuously changed according to the heating and cooling periods. In addition, during the cooling cycle, the field strength at the interface near the conductor increased and the interface pressure decreased. In conclusion, it is important to ensure sufficient initial pressure so that the dielectric strength at the interface does not decrease even if there is a loss of interface pressure due to temperature fluctuations.

Influence of the Cable Accessories Installing Method on the Partial Discharge Activity in Medium Voltage Cables

This article proposes a method to modify the construction of a medium voltage (MV) heat shrinkable cable termination in cases of atypical damage to the shields of cross-linked polyethylene (XLPE) insulated cables. The proposed solutions include a modified method of assembling electric field control coating. An attempt was made to check the effect of such damage to the shields of MV cables with XLPE insulation on the level of occurrence of partial discharges within the cable termination. The investigations included testing the XRUHAKXS 1 × 240/25 cable type using the electric method (ME) and high frequency (HF) method with sinusoidal AC test voltage. As a result of the measurements, the values of total charges in the period and phase-resolved partial discharge (PRPD) patterns were obtained. The presented experimental results show the influence of the damage of the semiconducting coating surface on the occurrence of a defect in the cable termination without a modified method of control mantissa pinning. We suggest new methods of assembling MV cable accessories in the case of the presented coating damage in MV cable insulation.

Stress Cone Effect Analysis and Optimum Design of Extra High-Voltage Cable Joint Using Neural Networks

The major problem in high-voltage cable accessories is the electric field stress caused by the high-voltage level at the cable junction. In this paper, a neural network and simulation-based optimal design for extra high-voltage (EHV) cable accessory is proposed to ensure the optimum electrical design. The different kinds of geometries of the EHV underground cable accessory are realized in the Comsol Multiphysics software program, and the electric field analyses are conducted based on the finite element method. The design of high-voltage underground cable accessories is examined both within and without the optimal stress cone geometry. A neural network-based model is developed according to the analysis results obtained in the simulation. Thanks to this model, the geometry of the stress cone is determined. In the simulation-based analysis, the electric field stress caused by the high-voltage level at critical points is controlled with an optimum joint design using neural networks. The proposed model will contribute to EHV cable joint design at different stress levels.

Aging Mechanism of Silicone Rubber Under Thermal–Tensile Coupling Effect

Silicone rubber (SR) is widely used as insulation for cable accessories and the aging behavior significantly affects the cable operation. This study aimed to investigate the aging performance and mechanism of SR subjected to thermal effect and different mechanical tensions for up to 1000 h. We measure the crosslink density, mechanical and electrical properties to evaluate property variations, and material characterization methods, such as Fourier transform infrared spectrum and X-ray photoelectron spectroscopy (XPS), are conducted to explore the aging mechanism. Based on comprehensive analyses, competitive effects and synergistic effects between thermal aging and tensile aging are demonstrated. Thermal aging increases the crosslink density and improves the dielectric property, while tensile aging exhibits the opposite effects. Nevertheless, the effects of tensile aging on mechanical properties, especially stress relaxation, are significantly promoted by thermal aging. These complex effects can be attributed to the destruction of physical crosslink and the orientation of molecular chains induced by tensile stress.

Improvement of the DC electrical performance of silicone rubber for cable accessories through aromatic hydrocarbon voltage stabilizer

AbstractThe development of high voltage direct current (DC) cables poses new challenges to insulation. The electrical properties of silicone rubber (SiR) limit its application as cable accessories at high voltage levels. Therefore, this article attempted to add two kinds of voltage stabilizers with high electron affinity into the SiR to improve its DC electrical properties. The preferred addition percentage was initially determined by the breakdown strength. Properties such as conductivity, dielectric losses, and space charge distribution were also measured. The results showed that the DC electrical properties of organically modified SiR had improved comprehensively. There might be two reasons. First, the voltage stabilizer has a high electron affinity, which means it can trap high‐energy electrons, thus reducing the number of carriers and mobility. In addition, it has an electric field gradient effect, which can homogenize the electric field by migration.

Insulation Properties and Interface Defect Simulation of Distribution Network Cable Accessories under Moisture Condition

To fully understand the distribution network cable accessories’ moisture fault, the variation in insulation parameters of the cable main insulation of cross-linked polyethylene (XLPE) and the cable accessory reinforced insulation of silicone rubber (SIR) under different moisture conditions are studied experimentally. Furthermore, the effects of typical moisture defect forms and moisture positions on electric field distributions are studied by establishing the interface defect model. The experimental results show that the insulation properties of XLPE after water immersion are more seriously reduced than SIR. The resistivity of XLPE and SIR specimens decreases significantly with the increase in moisture time, decreasing by 90.3% and 48.3% after 720 h, respectively. The relative dielectric constants of the two kinds of specimens increase with different degrees after moisture, increasing by 5.16% and 7.83%, respectively, and the breakdown strength decreases by 18.3% and 13.1% after 720 h, respectively. The simulated results show that the moisture form and position have a great influence on electric field distribution. The electric field distortion caused by water trees’ defect is the most serious, followed by water film defects and water droplet defects. Besides, for different types of moisture defects, the position of the maximum distortion electric field is different.

Graphite nanoplatelets filled silicone composites with novel electrical and dielectric properties

In the design of medium and low voltage equipment such as cable accessories, generator, motor end windings or bushings, issues with electrical field enhancement occur at interfaces between insulators and conductors, resulting in accelerated material ageing. The purpose of this paper is to present a novel dielectric composite material which has the properties to mitigate this local amplification. It is a functional dielectric which resistivity decreases by several orders with electric field from 1014 to 109 Ωm up to 1 kV mm−1 while the dielectric constant decreases from 15 to 12 in the 10–2–106 Hz range. This novel material is made with graphite nanoplatelets. It may be used as a resistive or capacitive field grading material in electrical applications.

Nonlinear Materials Applied in HVDC Gas Insulated Equipment: From Fundamentals to Applications

The electric field distortion along the gas-solid interface on insulators in GIS or GIL greatly threatens their long-term insulation performance. In recent years, with the developments of field grading materials, their application on GIS/GIL insulators has been considered as an effective approach to suppressing the field distortion and enhancing the insulation property. Many reviews about field grading materials had been published. However, these articles rarely focus on the nonlinear materials and their unique characteristics when used in gas insulated equipment. Compared with traditional applications, such as cable accessories, generator coils, and motor stator bars, nonlinear materials in GIS/GILs are mainly used for grading the surface electric field and avoiding unwanted charge accumulations. Hence, there are two characteristics of the material that need to be paid special attention to-the surface insulation performances and the charge release functions-which are not so critical in previous applications. In this paper, first, types and principles of several commonly used nonlinear grading materials are summarized briefly. Then, the existing application cases of nonlinear materials in gas insulation equipment are sequentially introduced from the optimization ideas, material selection, manufacturing process, and using effects. Finally, the problems encountered in the application of field grading materials in practice are discussed. Suggestions for the follow-up studies are given in order to guide the application of nonlinear materials in GIS/GIL insulators.

An Exterior Optical Fibre Sensing Technology Used for Interface Pressure Measurement Between Cable and Accessory

Interface pressure between cable and accessory is the key to ensure the long-term reliable operation of cable accessories. The traditional measurement method of interface pressure is off-line measurement through a built-in piezoelectric sensor. However, this method is difficult to operate and damages the interface structure. In addition, the offline measurement cannot reflect the real-time dynamic change of interface pressure during the operation of cable accessories. Therefore, this paper proposes a new method for the on-line measurement of interface pressure between cable and accessory by means of an exterior optical fibre curvature sensor, whose design mechanism is based on the pure bending theory of optical fibre. Then the relationship model involving the exterior curvature of the cable accessory, the radial displacement and the inner interface pressure is established. Finally, interface pressures of the 8.7/10 kV cable accessory under different expansion ratios are measured by the calibrated optical fibre curvature sensor. The measurement results show that the optical fibre curvature sensor measures the exterior curvature of the accessory accurately; the average sensitivity reaches 7.679 nm/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , and the measurable curvature ranges from 0.03 to 0.05 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . The conversion results of interface pressure are basically consistent with the results measured directly by a traditional piezoelectric sensor or simulated mechanically by finite element software. Therefore, the exterior optical fibre sensing technology proposed in this paper is expected to be used for the on-line measurement of the interface pressure between cable and accessory.

Temperature Gradient Affecting Electrical Tree in Silicone Rubber under Impulse Superimposed on DC Voltage

In this paper, the electrical tree growth characteristics of silicone rubber (SiR) are studied under impulse superimposed on DC voltage and a temperature gradient (TG) field. The temperature difference between high voltage (HV) and ground (GD) electrodes varies from -90 to 90 °C resulting in a TG field in the samples. The electrical tree morphology, length and accumulated damage are discussed. Experimental results show that with the increase of absolute value of TG, the electrical tree length and accumulated damage continue to increase, and the tree morphology turns from branch-like tree to bush-branch hybrid tree. The positive TG has a more obvious acceleration effect on the tree growth than does the negative TG. Under the TG, the temperature dependent carrier transport results in space charge accumulation and electric field distortion at low temperature side, which is the main reason for the different growth characteristics of electrical tree. Besides, the accumulated damage of DC superimposed with heteropolar impulse voltage is larger than that under DC superimposed with homopolar impulse voltage. It is concluded that the temperature gradient and impulse superimposed on DC voltage promote the electrical tree growth of HVDC cable accessories insulation.

Graphene Nanoplatelets and Temperature Gradient Affecting Electrical Tree in Graphene/SiR Nanocomposites

In this paper, graphene/silicone rubber (SiR) nanocomposites with filler contents of 0.001-0.005 wt% are prepared to improve electrical tree inhibiting ability under different temperature gradients. The electrical treeing test is performed under three temperature gradients by controlling the temperature of high voltage and ground sides. Experimental results indicate that the SiR sample with 0.003 wt% graphene presents the best performance on electrical tree resistance. The tree length and accumulated damage are both reduced with the increase of graphene content from 0 to 0.003 wt%, but increased with the further increase of filler content from 0.003 to 0.005 wt%. The tree outline tends to be circular in neat SiR under negative temperature gradient, while it is triangular under positive temperature gradient. However, the effect of temperature gradient is weakened when graphene exists. Based on quantum dot models, the role of graphene quantum wells (QWs) in the regulation of charge transport and electrical tree resistance in SiR is revealed. This research provides an effective method to suppress electrical trees of SiR for cable accessories.

Autonomous Analysis of Infrared Images for Condition Diagnosis of HV Cable Accessories

Infrared thermography has been used as a key means for the identification of overheating defects in power cable accessories. At present, analysis of thermal imaging pictures relies on human visual inspections, which is time-consuming and laborious and requires engineering expertise. In order to realize intelligent, autonomous recognition of infrared images taken from electrical equipment, previous studies reported preliminary work in preprocessing of infrared images and in the extraction of key feature parameters, which were then used to train neural networks. However, the key features required manual selection, and previous reports showed no practical implementations. In this contribution, an autonomous diagnosis method, which is based on the Faster RCNN network and the Mean-Shift algorithm, is proposed. Firstly, the Faster RCNN network is trained to implement the autonomous identification and positioning of the objects to be diagnosed in the infrared images. Then, the Mean-Shift algorithm is used for image segmentation to extract the area of overheating. Next, the parameters determining the temperature of the overheating parts of cable accessories are calculated, based on which the diagnosis are then made by following the relevant cable condition assessment criteria. Case studies are carried out in the paper, and results show that the cable accessories and their overheating regions can be located and assessed at different camera angles and under various background conditions via the autonomous processing and diagnosis methods proposed in the paper.

HVDC 케이블 접속재용 실리콘고무/나노 실리콘카바이드 콤포지트의 전기적 특성

In order to develop HVDC cable accessory, space charge exists at the interface between XLPE and EPDM composite or at the interface between XLPE and SiR. The larger the difference in ratio between the dielectric constant and the electrical conductivity of the two insulators, the greater the amount of space charge appears at the interface. The more and more space charges are stored at the interface, the higher the electric field distortion occurs. Therefore, in developing HVDC cable connection materials, Silicone rubber/nano silicon carbide (SiC) contents composites related to insulation and electrical conduction were manufactured. Nano SiC, a semiconducting inorganic material, has been modified to reinforce the interface of the composite mixed with silicon rubber.In addition, in order to study electrical ±HVDC dielectric breakdown characteristics and dielectric characteristics, modified ano SiC was filled with 0, 1, 3, 5, 7, 9wt%, dispersed, and nanocomposite was developed.

Influence of Temperature Characteristics Based on Big Data on Cable Accessories under HV DC Field

Cable accessory is a weak link in cable line operation, which is prone to accidents. It is of great significance to study the influence of temperature characteristics on cable accessory and determine the temperature field distribution of cable accessory for ensuring the safety of cable accessory operation and improving the overall reliability of cable line. This paper mainly studies the influence of temperature characteristics based on big data analysis on cable accessories under hV DC field. This paper first gives an overview of big data related technologies, and then introduces the calculation method of cable loss, which provides a theoretical basis for analyzing the influence of temperature characteristics on cable accessories. Furthermore, by setting the contact resistance coefficient K at the conductor of the cable connector, the temperature field calculation model of the contact resistance of the cable connector is established, and the distribution law of the temperature characteristics of the cable connector when the contact resistance of the conductor is too large is studied. With the increase of the contact resistance, the temperature of the connector core and the temperature of the connector indicate that the temperature increases gradually.

Effects of Harmonic Component on Electrical Tree in EPDM for HVDC Cable Accessories Insulation

In this paper, the effects of harmonic superimposed DC voltage on electrical tree growth characteristics in ethylene propylene diene monomer (EPDM) for HVDC cable accessories insulation are studied. Harmonic component with a frequency from 50 to 450 Hz is superimposed on DC voltage to analyze the mechanism of harmonic amplitude, order and voltage polarity on electrical tree growth. The results show that the electrical treeing process is promoted with an increase in harmonic amplitude and order, which brings a longer tree length and more accumulated damage to the insulation. Especially when the amplitude of harmonic component exceeds that of DC voltage, the electrical treeing processes are significantly deteriorated, resulting from the large amount of energy released by the recombination of hetero charges. The growth of electrical tree has an obvious polarity effect which tends to be branch-like tree under positive voltage and bush-like tree under negative voltage. The polarity effect may be caused by different propagation processes of electron avalanche under different voltage polarities. As harmonic order increases, the main channel of positive electrical tree is broadened, while the negative tree becomes denser with a lot of bifurcation. These results indicate that harmonic component accelerates the initiation and propagation of electrical tree by affecting the charge transport behavior under the superimposed electric field.