High Voltage Direct Current Cable Insulation Research Articles

Electrical conduction properties of PP-based nanocomposite with high thermal conductivity for HVDC cable insulation

This paper reports on thermal and electrical conduction properties of polypropylene (PP)-based nanocomposite used for high voltage direct current cable insulation. To improve the thermal conductivity of the PP/propylene-based elastomer (PBE) blend (noted as PB), boron nitride (BN) nanosheets with high thermal conductivity are added. The pretreatment process through template method is to obtain a three-dimensional (3D) network structure composed of BN nanosheets (noted as 3DBN). Then the 3DBN is added into the PP/ PBE blend, in which the 3D networks are locally arranged and the other BN nanosheets are randomly dispersed. Thermal conductivity is estimated, while electrical conductivity is measured under electric fields of 5–40 kV mm−1 at various temperatures. Carrier trap distribution is derived from isothermal surface potential decay method. In addition, the microstructure and crystallization characteristics of the nanocomposites are measured. The results show that the thermal conductivity of the PP/PBE blend is improved obviously by the addition of the locally arranged 3DBN, and that of PB-3DBN-10 is increased by 16% compared with that of PB. Deep traps are introduced by the dispersed BN nanosheets and the electrical conduction is reduced, which becomes increasingly evident with the filler content and temperature. It is suggested that the introduction of thermal conductive fillers with a 3D network structure and well-dispersed state can synergistically enhance the thermal conductivity and insulation performances of the PP-based material.

Polymer Composites With High Thermal Conductivity for HVDC Cable Insulation: A Review

With the development of high voltage direct current (HVDC) transmission technology, the voltage level and transmission capacity of power cable need to be further improved. Joule heat originating from the current in the cable conductor brings a great challenge to extruded cable insulation because of the low thermal conductivity of polymer insulation materials. This paper reviews the harms caused by heat accumulation in cable insulation and methods for improving the thermal conductivity of polymer materials. First, the effects of heat on cable insulation are summarized. Second, the mechanism, influencing factors, and structure design of filler-based polymer composites with high thermal conductivity are introduced. Subsequently, investigations on cable insulation materials with enhanced thermal conductivity are reviewed. At last, some problems with the design of thermal conductive networks in semi-crystalline polymer materials are discussed, and our suggestions for future work are proposed.

Optimal Content Selection of Voltage Stabilizers Using TOPSIS Method to Achieve Enhanced Material Properties for HVDC Cable Insulation Applications

Three different contents (1, 3, and 5 wt%) of 4-n-propylbenzoic acid as the voltage stabilizer are added to cross-linked polyethylene (XLPE), whereas electrical, mechanical, and thermal properties of XLPE/stabilizer blends are measured. The obtained results show that the voltage stabilizer addition facilitates a reduction in accumulated space charges and conduction current and improves the life exponent and thermal conductivity of the XLPE but negatively affects the degree of cross-linking. The optimal content of the voltage stabilizer to attain the balanced overall properties is selected by the technique for order of preference by similarity to ideal solution (TOPSIS) method. The electric field distortion rate, conduction current, and temperature sensitivity of electrical conductivity under 40 kV/mm, dc breakdown strength, thermal conductivity, gel content, elongation at break, tensile strength, crystallinity, and temperature at a maximum decomposition rate of the samples are selected as evaluation indices for the TOPSIS method, which yields the 1 wt% of 4-n-propylbenzoic acid as the optimal content. The calculated life exponent is maximum for the XLPE-1, suggesting that the high voltage direct current (HVDC) cable having XLPE insulation with the optimal content of the voltage stabilizer will have a longer service life.

Automated Space Charge Classification Inside ±500-kV HVDC Cable Insulation Using Fusion of Superpixel and Deep Features for Remote Condition Assessment

The accumulated space charges cause electrical field distortion, which is fatal to the safe and reliable operation of polymeric high-voltage direct current (HVDC) cables. Hence, this paper aims to detect and classify the space charges to ensure reliability and a longer operating life of HVDC cables. To achieve this, experiments were carried out on cross-linked polyethylene (XLPE) insulation samples and space charge distributions were recorded under altering electric fields (10–50 kV/mm) and at different temperatures (30–70 °C). Subsequently, super-pixel color features were extracted from the space charge images using the simple linear iterative clustering (SLIC) algorithm. In addition, deep features were extracted using the AlexNet convolutional neural network (CNN) model. The fusion of the handcrafted and deep features was fed to three benchmark machine-learning classifiers for the recognition of different space charge accumulation categories. The method delivered high recognition performance in spite of altering electric fields and varying temperatures. As a result, the proposed framework can detect space charges in HVDC cable insulation in real time.

Synergistic improved electrical resistivity-temperature characteristics and DC breakdown strength in insulating XLPE composites by incorporating positive temperature coefficient particles

Weakening the electrical resistivity-temperature dependence of cross-linked polyethene (XLPE) is an effective way to enhance the electric field uniformity of high-voltage direct current (HVDC) cables. Therefore, an attempt is made to suppress the negative temperature coefficient (NTC) of electrical resistivity by incorporating BaTiO3-based ceramic fillers with positive temperature coefficient (PTC) electrical resistivity into XLPE. Morphology characterization indicated that PTC particles were well dispersed in the XLPE matrix. The measurements of electrical resistivity and DC breakdown strength are carried out from 30 °C to 90 °C. The electrical resistivity and DC breakdown strength of the sample at high temperatures were significantly improved with the introduction of PTC particles, which is attributed to the deep traps near the Curie temperature region. The high doping content has a better suppression of the NTC effect. The activation energy (0.63 eV) and NTC strength (1.74) of the sample with the addition of 10 wt% of PTC are considerably reduced compared with XLPE. The content of PTC particles was positively correlated to the enhancement of electrical resistivity and DC breakdown strength. The optimum content for DC breakdown strength is about 5 wt%. The low electrical resistivity-temperature dependence of insulation performance shows a potential to obtain temperature-stable HVDC cable insulation.

Phase Field Modeling on By-Product Migration in Crosslinking Polymers for HVDC Cable Insulation Applications

Cross-linking by-products has been considered as one of the crucial factors for dielectric properties of cross-linked polyethylene, which plays an important role in the insulation performance of high voltage direct current (HVDC) cables. It migrates across cable insulation incorporating space charge effect, temperature variation of conductivity, etc., which makes it a long-standing puzzle of manipulating the electric field distribution for HVDC cable insulation, especially with the increasing voltage level. Nevertheless, there still lacks a theoretical model describing the migration of the by-products, especially for cable insulation with sizeable dimensions. In this article, a phase field model is established to simulate the migration of acetophenone (i.e., one of the by-products) through calculating the free energy landscape considering the competition between the diffusion driven by concentration gradient and the uphill diffusion caused by by-product aggregation. The results show that the time-dependence migration during degassing leads to an uneven distribution of acetophenone across the cable insulation, which is in good coincidence with the measured results for a full-size cable. Accordingly, the distortion of the electric field in HVDC cable insulation due to the radial distribution variation of acetophenone has been estimated regarding the relationship between by-product content and conductivity, and a method of manipulating the distribution of acetophenone is proposed to optimize the distribution of the electric field in cable insulation. Our work provides a numerical approach for the simulation of by-product migration in crosslinked polymers for insulation applications.

PP/PP-HI/silica nanocomposites for HVDC cable insulation: Are silica clusters beneficial for space charge accumulation?

New potential High Voltage Direct Current (HVDC) cable insulation materials based on nanocomposites are developed in this study. The nanocomposites are produced by blending of polypropylene (PP), propylene-ethylene copolymer (PP–HI) and a modified fumed silica (A-silica) in a concentration of 1 and 2 wt %. The A-silica is successfully modified with (3-aminopropyl)triethoxysilane (APTES) via a solvent-free method, as proven by infrared spectroscopy, thermogravimetry and transmission electron microscope mapping.A-silica in the polymer matrix acts as a nucleating agent resulting in an increase of the crystallization temperature of the polymers and a smaller crystal size. Moreover, the silica addition modified the crystals morphology of the unfilled PP/PP-HI blend. The composite containing A-silica with 2 wt% contains bigger-size silica clusters than the composite filled with 1 wt%. The composite with the higher A-silica concentration shows lower space charge accumulation and a lower charge current value. Besides, much deeper traps and lower trap density are observed in the composite with 2 wt% A-silica addition compared to the one with a lower concentration. Surprisingly, the presence of silica clusters with dimensions of more than 200 nm exhibit a positive effect on reducing the space charge accumulation. However, the real cause of this improvement might be due to change of the electron distribution stemming from the amine-amine hydrogen bond formation, or the change of the chain mobility due to the presence of occluded polymer macromolecules constrained inside the high structure silica clusters. Both phenomena may lead to a higher energetic barrier of charge de-trapping, thus increasing the depth of the charge traps.

Effect of Acetylated SEBS/PP for Potential HVDC Cable Insulation.

Blending thermoplastic elastomers into polypropylene (PP) can make it have great potential for high-voltage direct current (HVDC) cable insulation by improving its toughness. However, when a large amount of thermoplastic elastomer is blended, the electrical strength of PP will be decreased consequently, which cannot meet the electrical requirements of HVDC cables. To solve this problem, in this paper, the inherent structure of thermoplastic elastomer SEBS was used to construct acetophenone structural units on its benzene ring through Friedel–Crafts acylation, making it a voltage stabilizer that can enhance the electrical strength of the polymer. The DC electrical insulation properties and mechanical properties of acetylated SEBS (Ac-SEBS)/PP were investigated in this paper. The results showed that by doping 30% Ac-SEBS into PP, the acetophenone structural unit on Ac-SEBS remarkably increased the DC breakdown field strength of SEBS/PP by absorbing high-energy electrons. When the degree of acetylation reached 4.6%, the DC breakdown field strength of Ac-SEBS/ PP increased by 22.4% and was a little higher than that of PP. Ac-SEBS, with high electron affinity, is also able to reduce carrier mobility through electron capture, resulting in lower conductivity currents in SEBS/PP and suppressing space charge accumulation to a certain extent, which enhances the insulation properties. Besides, the highly flexible Ac-SEBS can maintain the toughening effect of SEBS, resulting in a remarkable increase in the tensile strength and elongation at the break of PP. Therefore, Ac-SEBS/PP blends possess excellent insulation properties and mechanical properties simultaneously, which are promising as insulation materials for HVDC cables.

Effects of Trapping Characteristics on Space Charge and Electric Field Distributions in HVDC Cable under Electrothermal Stress

Space charge behavior has a strong impact on the long-term operation reliability of high voltage–direct current (HVDC) cables. This study intended to reveal the effect of trap density and depth on the space charge and electric field evolution behavior in HVDC cable insulation under different load currents and voltages by combined numerical bipolar charge transport (BCT) and thermal field simulation. The results show that when the load current is 1800 A (normal value), the temperature difference between the inside and the outside of the insulation is 20 °C, space charge accumulation and electric field distortion become more serious with the increase in the trap depth (Et) from 0.80 to 1.20 eV for the trap densities (Nt) of 10 × 1019 and 80 × 1019 m−3, and become more serious with the increase in Nt from 10 × 1019 to 1000 × 1019 m−3 for Et = 0.94 eV. Simultaneously decreasing trap depth and trap density (such as Et = 0.80 eV, Nt = 10 × 1019 m−3) or increasing trap depth and trap density (such as Et = 1.20 eV, Nt = 1000 × 1019 m−3), space charge accumulation can be effectively suppressed along with capacitive electric field distribution for different load currents (1800 A, 2100 A and 2600 A) and voltages (320 kV and 592 kV). Furthermore, we can draw the conclusion that increasing bulk conduction current by simultaneously decreasing the trap depth and density or decreasing injection current from conductor by regulating the interface electric field via simultaneously increasing the trap depth and density can both effectively suppress space charge accumulations in HVDC cables. Thus, space charge and electric field can be readily regulated by the trap characteristics.

Dielectric Performance of Silica-Filled Nanocomposites Based on Miscible (PP/PP-HI) and Immiscible (PP/EOC) Polymer Blends

This study compares different polymer-nanofiller blends concerning their suitability for application as insulating thermoplastic composites for High Voltage Direct Current (HVDC) cable application. Two polymer blends, PP/EOC (polypropylene/ethylene-octene copolymer) and PP/PP-HI (polypropylene/ propylene - ethylene copolymer) and their nanocomposites filled with 2 wt.% of fumed silica modified with 3-aminopropyltriethoxysilane were studied. Morphology, thermal stability, crystallization behavior dynamic relaxation, conductivity, charge trap distribution and space charge behavior were studied respectively. The results showed that the comprehensive performance of the PP/PP-HI composite is better than the one of the PP/EOC composite due to better polymer miscibility and flexibility, as well as lower charging current density and space charge accumulation. Nanosilica addition improves the thermal stability and dielectric properties of both polymer blends. The filler acts as nucleating agent increasing the crystallization temperature, but decreasing the degree of crystallinity. Dynamic mechanical analysis results revealed three polymer relaxation transitions: PP glass transition ( $\beta$ ), weak crystal reorientation ( $\alpha 1$ ) and melting ( $\alpha 2$ ). The nanosilica introduced deep traps in the polymer blends and suppressed space charge accumulation, but slightly increased the conductivity. A hypothesis for the correlation of charge trap distribution and polymer chain transition peaks is developed: In unfilled PP/EOC and PP/PP-HI matrices, charges are mostly located at the crystalline-amorphous interface, whereas in the filled PP/EOC/silica and PP/PP-HI /silica composites, charges are mostly located at the nanosilica-polymer interface. Overall, the PP/PP-HI (55/45) nanocomposite with 2 wt.% modified silica and 0.3 wt.% of antioxidants making it $a$ promising material for PP based HVDC cable insulation application with $a$ reduced space charge accumulation and good mechanical properties.

Compatibility dependent space charge accumulation behavior of polypropylene/elastomer blend for HVDC cable insulation

Polypropylene (PP)/elastomer blend is a promising candidate for high voltage direct current (HVDC) cable insulation. Space charge accumulation in HVDC cable is an important factor for safe and stable operation of HVDC transmission system. In this paper, propylene based elastomer (PBE) and ethylene-octene copolymer elastomer (EOC) were added into PP to prepare the blend sample. Space charge accumulation behavior was estimated by the pulsed electro-acoustic (PEA) method, and the influence of compatibility between the PP and the elastomer on the charge accumulation behavior was investigated. Polarized optical microscope (POM), differential scanning calorimetry (DSC), X-ray diffraction (XRD) and dynamic mechanical analysis (DMA) measurements were performed to assist the understanding of the compatibility dependent space charge accumulation behavior. The obtained results showed that the amount of space charge in the PP/PBE blend was lower than that in the PP/EOC blend. As compared with the PP/EOC blend, higher crystallinity with integrated crystal morphology and β-crystal form were observed in the PP/PBE blend for better compatibility between PP and PBE. It is suggested that deeper trap centers are formed in the PP/PBE blend than in the PP/EOC blend due to the compatibility, by which the charge injection is restricted.

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).

Effect of multi-dimensional zinc oxide on electrical properties of polypropylene nanocomposites for HVDC cables

Polypropylene (PP) is one of the most important high voltage direct current (HVDC) cable insulation materials, which has been widely used in recent years. However, the accumulation of space charge is the most notable problem to limit the practical application of PP. In this paper, the PP nanocomposites filled with polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and multi-dimensional zinc oxide composite fillers (co-ZnO) prepared by melt blending method showed excellent properties due to the good compatibility between the co-ZnO filler and PP/SEBS blends. The dielectric permittivity of the composites can be up to 3.1 with 1 wt% doping of co-ZnO. Compared with PP/SEBS, the introduction of co-ZnO filler can largely suppress the space charge accumulation as well as the electric field distortion, and improve the breakdown strength by 14%. It is promising to make the PP/SEBS/co-ZnO composites as one better candidate for HVDC cable insulation materials.

Temperature dependent space charge and breakdown strength of PP/ULDPE/graphene nanocomposites for HVDC extruded cable insulation

The space charge behavior, conductivity and breakdown strength are critical issues in the design of insulation for high voltage direct current (HVDC) extruded cables, which is greatly affected by the operating temperature at different load conditions. The space charge, DC conductivity and breakdown characteristics of polypropylene (PP) based insulation at various temperatures are investigated in this paper. The space charge, DC conductivity and the breakdown strength of pure PP, PP/ultralow density polyethylene (ULDPE) blends and its graphene nanocomposites were examined at 30, 60 and 90 °C. The results show that, at elevated temperature, the homo-charge injection in PP from both electrodes is enhanced. Significant charge accumulates in PP/ULDPE blends in the vicinity of the anode at 90 °C. A small amount (0.01 wt%) of graphene nanoplatelets in PP/ULDPE blends inhibit the injection of homo-charge at elevated temperatures. No dramatic change in the breakdown strength with increasing temperature is observed. The DC conductivity increases with increasing temperature, and the conductivity significantly increases from 60 to 90 °C compared to from 30 to 60 °C. The PP/ULDPE blends with 0.01 wt% graphene nanoplatelets exhibit good properties for potential HVDC cable insulation.

Effect of multi-dimensional zinc oxide on electrical properties of polypropylene nanocomposites for HVDC cables

Polypropylene (PP) is one of the most important high voltage direct current (HVDC) cable insulation materials, which has been widely used in recent years. However, the accumulation of space charge is the most notable problem to limit the practical application of PP. In this paper, the PP nanocomposites filled with polystyrene-b-poly(ethylene-co-butylene)-b-polystyrene (SEBS) and multi-dimensional zinc oxide composite fillers (co-ZnO) prepared by melt blending method showed excellent properties due to the good compatibility between the co-ZnO filler and PP/SEBS blends. The dielectric permittivity of the composites can be up to 3.1 with 1 wt% doping of co-ZnO. Compared with PP/SEBS, the introduction of co-ZnO filler can largely suppress the space charge accumulation as well as the electric field distortion, and improve the breakdown strength by 14%. It is promising to make the PP/SEBS/co-ZnO composites as one better candidate for HVDC cable insulation materials.

Polyethylene blends with/without graphene for potential recyclable HVDC cable insulation

This paper presents investigations on a usage of graphene as a voltage stabilizer for polyethylene blends in the potential recyclable high voltage direct current (HVDC) cable insulation. Thermal and electrical properties of the blends with/without graphene were evaluated by thermo-gravimetric analysis (TGA), differential scanning calorimetry (DSC), electrical treeing tests, DC conductivity and space charge measurements. Trap energy distributions in the blends were estimated using the measured space charge decay data. The obtained results show that additive-like amounts of graphene can improve the thermal stability of the polyethylene blends. The influence of the graphene on the tree inception voltage (TIV) of the polyethylene blends is not significant, whereas the low density polyethylene/high density polyethylene (LDPE/HDPE) with/without graphene demonstrates the highest TIV comparing to the LDPE, LDPE/polypropylene, and their corresponding graphene added materials. It is found that the DC conductivity of the polyethylene blends with/without graphene increases consistently with the increase of the temperature. The conductivity of the LDPE/HDPE decreases apparently with the inclusion of graphene. It has been shown that space charge accumulations can be suppressed by the addition of graphene in the polyethylene blends. It is postulated that the high trap level and trap energy density of the polyethylene blends with graphene are closely connected to the suppression of the space charge accumulation in the blends.

Enhanced High-Temperature DC Dielectric Performance of Crosslinked Polyethylene with a Polystyrene Pinning Structure.

In this paper, we propose a method on improving direct current (DC) dielectric performance by designing a polystyrene (PS) pinning crosslinked polyethylene (XLPE) for the application of insulation materials on high voltage direct current (HVDC) extruded cable. Electrical experimental results show that the addition of PS (1–5 phr, parts per hundreds of resin) can significantly reduce DC conductivity and increase DC breakdown strength of XLPE in the test temperature range of 30–90 °C. Microstructure investigation shows PS distributed as particles could participate in the formation of a crosslinking network with the help of a crosslinking agent, thus forming a polymer pinning structure at the interface between XLPE and PS. It is believed that such a special design strengthens the structure of XLPE, which leads to the improved DC dielectric performance at elevated temperatures. Our findings may contribute a new solution for developing HVDC cable insulation materials.

Recyclable insulation material for HVDC cables in Global Energy Interconnection

With the development of modern power systems, especially that of the global energy internet, high-voltage, direct current (HVDC) cable power transmission will play an important role in the future. The key problem of HVDC cable power transmission is the need for novel cable insulation materials that have high performance, recyclability, and higher working temperature to replace traditional crosslinked polyethylene. This paper investigates the thermal and electrical properties of polypropylene (PP)/Al2O3 nanocomposites as a potential recyclable HVDC cable insulation material. The developed nanocomposites exhibit excellent thermal and electrical properties with the introduction of Al2O3 nanoparticles. Particularly, the space charge accumulation is greatly suppressed.

Polypropylene-based ternary nanocomposites for recyclable high-voltage direct-current cable insulation

Polymeric high-voltage direct-current (HVDC) cables are the core equipment in energy internet, which enable the long-distance large-capacity electric power transmission, large-scale utilization of renewable electric power and flexible interconnection of large power grids. Performance of the HVDC cables are determined by the properties of their insulation material. However, the traditional polymeric HVDC cable insulation material, crosslinking polyethylene (XLPE) is limited to relatively low working temperature which restricts the power capacity of HVDC cables. XLPE with thermosetting characteristics also presents a major barrier to material recycling and environmental pollution reduction. Here we report the ternary nanocomposites of polypropylene (PP), thermoplastic polyolefin (TPO) and MgO nanoparticles as an efficient way to recyclable HVDC cable insulation material that simultaneously possess excellent thermal, mechanical and electrical properties, especially at high temperatures. At an optimal composition, the ternary nanocomposites integrate the complementary properties of the multi-components to raise the mechanical flexibility at room temperature and greatly improve the electrical properties at high temperatures (including suppressed space charge accumulation and increased breakdown strength and volume resistivity) while retaining high melting temperature comparable to PP of about 160 °C. This work may pave a way for synergistic optimization of the overall properties of insulation materials and enabling the successful development of recyclable insulation material for large-capacity HVDC cable application.

Polypropylene based thermoplastic polymers for potential recyclable HVDC cable insulation applications

Thermoplastic materials are highly desirable for power cable insulation application because of their recyclability and ease of processing. They are particularly suitable for high voltage direct current (HVDC) cable insulation because of the absence of byproducts during cable production, which may result in the reduction of undesirable space charge accumulation and the degassing cost. Polypropylene based polymers may have the potential for recyclable thermoplastic cable insulation application because of their high melting temperatures and adjustable mechanical and electrical properties. This work aimed at evaluating the potential of isotactic polypropylenes, propylene-ethylene block copolymers and random copolymers for thermoplastic cable insulation application. We investigated the melting and crystallization behavior, crystal structure, supramolecular structure, mechanical properties, space charge distribution and DC breakdown strength of six different polymers (two isotactic polypropylenes, two block copolymers and two random copolymers). A comprehensive analysis of these parameters indicates that the propylene-ethylene binary random copolymers are more potentially suitable for thermoplastic cable insulation application.