Cable Insulation Materials Research Articles

Electro-thermal breakdown measurement of 500 kV cross-linked polyethylene submarine cable insulation material and its lifetime model analysis

The first 500 kV AC cross-linked polyethylene (XLPE) cable in the world has been installed in China in 2019. The insulation performance of insulation material under the electro-thermal stress is the basis for its service life. At here, the AC breakdown voltage values and the breakdown time of the 500 kV AC XLPE submarine cable insulation material under the electro-thermal step stress was analyzed. The lifetime prediction accuracy of a new proposed model was compared to the classic FALLOU, SIMONI/RAMU and CRINE lifetime model. Results show that the AC breakdown strength and lifetime of the XLPE samples increase slowly at low temperatures (40–65°C) and decrease rapidly at high temperatures (65–85°C). The lifetime prediction error of the classic FALLOU, SIMONI/RAMU and CRINE models present that their fit goodness cannot meet the accuracy requirement. The new lifetime model using stepwise regression method could optimize the combination of variables, which considers the coupling effect of electrical and thermal stress. The evaluation error of the new proposed model is better than the FALLOU, SIMONI/RAMU and CRINE model, which is suitable for analyzing the lifetime of the AC 500 kV XLPE submarine cable insulation material.

Temperature Stability of DC Conductivity and Breakdown of Crosslinked LLDPE

In this paper, a way to improve the temperature stability of DC conductivity and breakdown characteristics of polyethylene (PE) is proposed using a crosslinked linear low density polyethylene (LLDPE). The ratio of DC conductivity, as well as breakdown strength values under 90 to 30°C is used to determine the temperature stability of DC properties. The closer it is to 1, the better the performance of temperature stability. The results show that when using crosslinked-LLDPE material, the ratio of DC conductivity under 90 to 30°C is 10.5, while it is 461.6 for crosslinked-LDPE samples with the same DCP content. And the ratio of DC breakdown strength is 0.97 for crosslinked-LLDPE, while it is 0.32 for crosslinked-LDPE. The gel permeation chromatography (GPC) method, hot-set test, differential scanning calorimetry (DSC) analysis and scanning electron microscopy (SEM) images show that the fine arrangement and smaller molecular weight between crosslinks of molecular and morphology structure that LLDPE owns contribute to the improvement in comparison with LDPE. The findings may contribute a solution to the development of HVDC power cable insulation materials.

Examining impact of vapor-induced crosslinking duration on dynamic mechanical and static mechanical characteristics of silane-water crosslinked polyethylene compound

Silane-crosslinked polyethylene (Si-XLPE) compounds are widely used cable insulation materials, cured by water as the crosslinking inducer. Herein, an attempt was made to evaluate the alterations of dynamic mechanical and static mechanical properties of Si-XLPE by increasing the water vapor-triggered crosslinking duration. The criterion for determining the progress of crosslinking was based on an industrially-used method (hot set test), and results indicated 16 h of vapor exposure as an appropriate duration for approaching maximum attainable crosslinking extent. Progress in crosslinking during this period was confirmed by FTIR spectroscopy. DMA was employed to assess the variations of dynamic mechanical behavior. Extending the crosslinking duration affected the storage modulus noticeably, and decreased it due to adverse effect of crosslinking on crystallinity. DSC confirmed the diminishment of crystallinity owing to the extended crosslinking time. Furthermore, prolonged vapor exposure period impacted β and γ transitions due to the influence of formed crosslinks on the mobility of chains. Tensile testing was carried out to ascertain the static mechanical characteristics of crosslinked samples. Findings demonstrated that tensile strength at yield increased below 5%, and tensile stress at break grew around 17%. Besides, elongation and Young's modulus experienced nonmonotonic changes due to the lengthened crosslinking period.

Modification of Polyethylene-Based Cable Insulation Material

Modification of Polyethylene-Based Cable Insulation Material

Material development approach matching to an injection moulding rejuvenation for cable insulation failure

This paper aims to construct a material developing approach matching to an injection moulding rejuvenation technique for cable insulation failure. The work starts from a chemical composition testing to the cable original insulation material. The applied composition should be determined by considerations of property requirements, including insulation, mechanical behaviour and environmental adaptability. In addition, a stable and standard manufacture technology should be groped to assure having a stable repair quality. Finally, proper material should be verified by testing for meeting normal the cable insulation requirements. By the above approach, typical material development is performed and the results by a code-based insulation property testing reveal the present approach has successfully been applied to develop a proper material matching to injection moulding rejuvenation technique for cable insulation failure.

Investigation of Electrical Tree Growth of XLPE Nano Composites using Time–Frequency Map and Clustering Analysis of PD Signals

Failure of underground cables due to electrical treeing phenomena, i.e. formation of electrical discharges in the imperfections of cable insulation, is a major problem faced by electrical utilities. Many research works on nanocomposites are being carried out to improve the electrical treeing resistance of the XLPE cable insulation material. Technological advancements in communication and data analytic systems have shown the way for implementing online continuous partial discharge (PD) monitoring systems for high voltage apparatus. Hence collection of PD database of XLPE nanocomposites in the laboratory during entire electrical tree growth process is important for implementing efficient condition monitoring systems and relatively little work has been published in this area. In this work, PD characteristics of XLPE nanocomposites with 1, 3, 5 and 10 wt% silica were investigated. Electrical tree growth and corresponding phase resolved PD (PRPD) pattern were analysed with respect to time. Cluster analysis of equivalent time–frequency mapping of PD signals was carried out with respect to tree growth time period. Statistical analysis was performed for the entire set of PD data. Results show that cluster analysis of T–F map of PD data is useful in estimating early failure of insulating material due to treeing. Addition of silica nano fillers in the range of 3–5 wt% concentration significantly improves the PD resistance and breakdown time of XLPE material.

Flexible Fabric-Based IDC Sensors for Conformal Curved Surface Applications

Interdigital capacitor (IDC) sensors are attractive in-situ sensors that can be used to measure permittivity variations in materials. Such sensors could be powerful enablers for cable insulation aging detection. Since insulation aging is correlated with a change in its material permittivity an IDC can measure that change via its measured capacitance. A new thin, flexible fabric-based IDC sensor is introduced for application on conformal curved surfaces such as an electrical cable surface. A multielectrode IDC and its backplane were fabricated using conductive copper polyester taffeta fabrics that were adhesively bonded to nonconductive polyester fabric. The sensor because of its soft flexible structure eliminates the problem of sensor conductor damage which is common for printed circuit board (PCB) type sensors when conformed to the surface of a cable. It also significantly reduces the airgap between the electrodes and the surface that they must contact in order to measure the capacitance. Airgap reduction and consistency are important in order to accurately predict permittivity variations in cable insulation material due to aging. Both PCB-based sensors and the new fabric-based sensors were built and tested on unaged cables and cables that underwent accelerated aging. Results demonstrate the proposed advantages of the novel fabric-based IDC sensor.

Block polypropylene/styrene-ethylene-butylene-styrene tri-block copolymer blends for recyclable HVDC cable insulation

Polypropylene (PP) is regarded as the most promising candidate for eco-friendly high-voltage direct current (HVDC) cable insulation material due to its excellent thermal and electrical properties. In this paper, a block polypropylene (BPP) was selected as the matrix and blended with styrene-ethylene-butylene-styrene tri-block copolymer (SEBS) by melt blending. The thermal behavior, dynamic mechanical behavior, crystallization behavior, and mechanical and electrical properties of the BPP/SEBS blends were investigated. It was found that SEBS could efficiently improve the flexibility while maintaining the excellent heat resistance of PPB. Furthermore, adding an appropriate amount of SEBS had a positive effect on suppressing the space charge and increasing the breakdown field strength. For the BPP/SEBS blend containing 20 wt% SEBS, a desirable combination of thermal, mechanical, and electrical properties was achieved. This work may help pave the way for developing eco-friendly PP insulating material in HVDC cable applications.

Investigation into the toxicity of combustion products for CR/EPR cables based on aging period

The toxicity of combustion product gas of CR/EPR cables, which are widely used in nuclear power plants, has been investigated according to the aging period. The cable specimens were prepared by thermal acceleration aging method, from non-aged to 40-year-old cables at spans of 10 years. The cable sheath and insulation materials were tested separately. The test was conducted in accordance with the Naval Engineering Standard 713 standard code, which measures the amount of 13 exhaust toxic gases when 1 gram of the test specimen is burnt. The toxicity index and the amount of emission gases were varied irregularly with respect to the aging period, and the reason for this result could not be clearly explained due to the lack of information on cable material. However, the reason could be qualitatively estimated to have two conflicting effects: (1) Reduced flame retardant performance as the devolatilization process proceeded over time, and (2) the enhanced stabilization of double bonds between carbon compounds and flame retardant elements, such as chlorine or bromine, as the cable is aged. The toxicity of the accelerated aging period of 20 years was much higher than that of non-aged cables, which signifies increased probability of fire risks of aged cables from the viewpoint of toxicity. Furthermore, when comparing the adjusted toxicity index calculated without CO, CO2, and NOx emission, it can be clearly determined that the average toxicity index of aged cables is higher than that of non-aged cables. Thus, in conclusion, human hazardousness of a cable fire increases from the viewpoint of toxicity of emission gases as it deteriorates, so safety regulations or maintenance guides for old nuclear power plants should be reinforced considering this toxicity variation. The results obtained from this study can be used to improve the reliability of operator manual actions for safety-shutdown and fire-suppression activities and to provide basic information for studies on fire suppression.

Thermal properties and space charge behavior of thermally aged polypropylene/elastomer blends nanocomposite

Polypropylene (PP)-based composites including PP/thermoplastic polyolefin (TPO) elastomer blends and PP/TPO-based nanocomposite are rather promising insulation materials for HVDC cables. Aging is an inevitable issue in practical operation and needs to be investigated thoroughly. In this paper, PP/TPO blends and PP/TPO/ZnO nanocomposite were prepared and thermally aged at 130 °C for various periods (28 days at longest). The micro-morphology, thermal properties and high-temperature space charge behavior were investigated. The results show that PP and TPO still exhibited excellent compatibility without phase separation after thermally aging. The crystallinity was not influenced after thermally aging, but a new endothermic peak at about 140 °C appeared in the melting process of thermally aged composites. Space charges accumulated in the two thermally aged composites, but dissipated slowly in PP/TPO, while dissipated rapidly first and then reduced in PP/TPO/ZnO nanocomposite. The calculated apparent charge mobility and trap level distribution based on space charge dissipation show that deep traps increased in aged PP/TPO, but decreased in aged PP/TPO/ZnO nanocomposite, which may be correlated with the chemical variation of thermally aged polymer matrix. The work may give a reference on the aging research of PP-based composite material for recyclable HVDC cable insulation.

Research on Mechanical, Physicochemical and Electrical Properties of XLPE-Insulated Cables under Electrical-Thermal Aging

The stabilization of cross-linked polyethylene (XLPE) cables is crucial to the safe working of power systems. In order to investigate the aging characteristics of cable insulation in the process of operation, 10 kV XLPE cables were electrically-thermally aged at an AC voltage of 26.1 kV and three temperatures: 103°C, 114°C, and 135°C. Cable samples at five aging stages were cut into slices and tested to determine their mechanical, physicochemical properties and dielectric strength. The changes in these properties were analyzed in terms of aging time. The mechanical strength and oxidation induction time have the same changing trend of decrease, because the chemical bonds of the cable insulation material are fractured when the XLPE cable is aged. The AC space charge presented a trend of gradual accumulation with aging time. The breakdown field strength after AC voltage application decreases monotonically with aging time, which could be a characteristic index to evaluate the degradation degree of the cable insulation. This research contributes to the comprehension of degradation mechanism and discovery of aging assessment criteria, which is of great significance in assessments of insulation properties and fault detection on power cables.

Preparation and electrical properties of 4-acetoxystyrene grafted polypropylene for HVDC cable insulation

Polypropylene (PP) is considered to be a potential eco-friendly cable insulation material. However, space charge accumulation in PP under high electric stress limits its application for HVDC cable. In this paper, PP was modified with 4-acetoxystyrene (AOS) by melt grafting to improve electric properties. The molecular structure of the polypropylene-graft-4-acetoxystyrene (PP-g-AOS) was characterized by means of Fourier Transform Infrared Spectroscopy (FT-IR). The effects of temperature, screw speed, time, initiator content and monomer content on grafting degree of PP-g-AOS were investigated. Furthermore, the influence of grafting degree of PP-g-AOS on space charge suppression, volume resistivity and dc breakdown field strength was investigated. The FT-IR result showed that AOS was successfully grafted onto PP. It was found that under the proper grafting conditions, the obtained PP-g-AOS exhibited a maximum grafting degree of 1.31%. Moreover, PP-g-AOS with grafting degree of 1.14% can suppress space charge efficiently, and increase both volume resistivity and dc breakdown strength. This provides a new idea for the study of polypropylene HVDC cable insulation.

Recyclable polypropylene-based insulation materials for HVDC cables: Progress and perspective

Polypropylene (PP)-based recyclable materials have attracted tremendous interest for HVDC cable insulation application due to their superior electrical properties, e.g., high thermal stability and superior recyclability. Compared with crosslinked polyethylene (XLPE), PP-based materials exhibit the advantages of not only higher working temperature but also facile and efficient cable manufacturing with reduced cost, which are highly desired in HVDC cable manufacturing. Considering their promising advantages, PP-based materials have received significant attention from both academia and industry in the field of HVDC cable insulation. In order to adopt PP as cable insulation material, the mechanical flexibility of PP should be improved. However, the regulation of the mechanical properties inevitably influences the electrical properties of PP. So extensive researches have been conducted on the regulations of the mechanical and electrical properties of PP. This review summarizes the research progress in recyclable polypropylene-based materials for HVDC cable insulation application. Particular attention is placed on the electrical property regulations and material structure-property relationship. The challenges that remain to be addressed and the opportunities for future studies in PP-based recyclable HVDC cable insulation materials are also presented.

Comparison of Effects of Ethylene-Based and Propylene-Based Copolymer on Tailoring the Properties of Polypropylene

Recently, polypropylene (PP) is considered as the potential recyclable HVDC cable insulation material. Ethylene-based copolymer (EBC) and propylene-based copolymer (PBC) were introduced into PP to tailor the properties of PP and make it more capable for HVDC cable insulation application. This paper focused on the effects of two additions on those propertied such as microstructure, thermal properties, electrical properties and space charge accumulation of PP. The results indicated that PBC was uniformly dispersed in the matrix of PP without adhesion, and its particle size did not increase with the increase of its content in the matrix. With the increase of PBC, the glass transition peak of polypropylene decreased slowly, indicating a better performance on flexibility at low temperature. With the increase of content of copolymer, the DC volume resistivity of the composite material decreased gradually, but samples with PBC had higher resistivity than samples with EBC. In the term of space charge, the addition of PBC could help suppressing the accumulation of space charge better. To sum up, PP/PBC composite material could be an option for recyclable DC cable insulation material.

Polycyclic Compounds Affecting Electrical Tree Growth in Polypropylene Under Ambient Temperature

Polypropylene (PP) has great potential to be used as recyclable high-voltage direct current (HVDC) cable insulation material. Electrical tree is the key factor that leads to cable insulation failure and limits the increase of HVDC transmission voltage level. In this study, three different polycyclic compounds, namely 4,4'-bis(dimethylamino)phenylazo, 2-hydroxy-2-phenylacetophenone and 4-phenylbenzophenone, are added into PP matrix with the addition content of 0.1 wt%. The electrical treeing experiments are conducted under impulse superimposed DC voltage at 30, 70 and 90 °C. Then the polycyclic compound with the best effect on inhibiting electrical tree is selected, and its filling ratio is further changed from 0.1 wt% to 0.3 wt% to get the optimal inhibition effect. It is found that the addition of 2-hydroxy-2-phenylacetophenone can inhibit the electrical tree degradation. However, the addition of 4-phenylbenzophenone and 4,4'-bis(dimethylamino)benzene will increase the electrical tree length and accumulated damage. As the content of 2-hydroxy-2-phenylacetophenone increases, the electrical tree length and accumulated damage decrease firstly, and then increase. The effect of polycyclic compounds on the electrical tree degradation is affected by the relative polarity of impulse superimposed DC voltage and temperature. The experimental results reveal that adding 0.1 wt% 2-hydroxy-2-phenylacetophenone polycyclic compound to PP can effectively inhibit the electrical tree degradation, which has great potential for application in recycle HVDC cables.

Trap Level Characteristics and DC Breakdown Performance of Isotactic/Syndiotactic/Atactic Polypropylene Blend Insulation

Polypropylene (PP) material as a DC cable insulation material has great prospects as a recyclable alternative to cross-linked polyethylene. This paper aims at investigate the effect of PP blends crystallinity morphology and trap energy level distribution on DC breakdown strength. Five different ratios iPP/sPP/aPP blends samples are prepared by melt-blending method. Polarizing optical microscope (POM) experiments are conducted to characterize the crystallinity morphology of five iPP/sPP/aPP blends and isothermal surface potential decay (ISPD) methods are employed to measure the trap levels distribution in composite dielectric. The DC breakdown strength is tested by ball-plate electrodes equipment, and the result of DC breakdown experiment indicates that sPP30 exhibits the highest breakdown field strength, which is 29.2% higher than sPP0. The addition of sPP can act as a nucleus to help the crystallization behavior of iPP, and the distribution of spherulites in sPP30 is the most uniform and dense among the five iPP/sPP/aPP blends samples, which can improve the trap characteristics of polymers. The ISPD results show that the decay rate of the sPP30 sample is the slowest at 50 °C, 70 °C and 90 °C, indicating that sPP30 is more conducive to suppressing the dissipation process of carriers during the depolarization process. Moreover, sPP30 has higher deep trap density and lower shallow trap density and exhibits superior breakdown performance. It is deduced that the addition of sPP can effectively regulate the crystallinity morphology of iPP, and then effectively improve the trap energy level distribution of iPP/sPP/aPP blends, and it shows the improvement of DC breakdown performance macroscopically.

Material progress toward recyclable insulation of power cables part 2: Polypropylene-based thermoplastic materials

Progress in the development of polyethylene (PE) based thermoplastic materials for power cable insulation has been reviewed in the first article on this subject [1]. New catalysis technique, copolymerization, blending, nanotechnology and additives have been used to tailor the electrical property of PE in the past few decades. However, these methods have brought about limited improvement in the thermomechanical property of PE. For instance, crosslinking reactions can improve the thermomechanical property of PE so that the cable operation temperature can be increased from 70°C to 90°C of low density polyethylene (LDPE) cables (Table 1); however, as discussed in the first article, crosslinking brings in undesirable effects. Apart from the ultra-low recyclability, the crosslinking and degassing also lead to potential safety concerns during cable installation and operation, in addition to long duration and high energy consumption needed for cable production. Since the beginning of the 21st century, polypropylene (PP) based thermoplastic insulation has attracted tremendous interests because of its high temperature stability and excellent recyclability. Compared with XLPE, PP possesses advantages of not only increased operating temperature higher than 110°C but also facile manufacturing processing with reduced cost, increased production efficiency and unlimited cable length, which are highly desirable in the power cable industry. As a promising future solution, PP based thermoplastic cable insulation starts to receive significant interest and attention from academia and industry [2]. However, conventional PP cannot be used directly as power cable insulation because of its high hardness, high crystallinity and low flexibility. In recent years, PP has been modified as thermoplastic cable insulation. This second article discusses the structure-property relationship of PP and reviews the progress on the development of PP based thermoplastic materials for power cable insulation.

Design and Model of Series-Connected High-Voltage DC Multipliers

The generation of high dc voltages is necessary for feeding many technical applications and testing the insulation material of high-voltage dc transmission lines and ac power cables with long lengths. The article proposes a modular multiplier high-voltage dc generator with integrated Cockcroft-Walton (CW) circuits. In this new topology, CW circuits are connected in series to produce a high dc output voltage with low regulation factor (<; 2%) and a negligible ripple factor without the need for costly filters to produce a nearly constant output voltage. The modules are fed by low input ac voltage sources that have the same magnitude and frequency. Numerical simulations using MATLAB/Simulink confirm the viability of the proposed generator. The generator is finally validated by experimental tests on a kV-size prototype.

Further discussion on the reaction behaviour of triallyl isocyanurate in the UV radiation cross-linking process of polyethylene: a theoretical study.

Further theoretical investigation on the reaction behaviour of triallyl isocyanurate (TAIC) in the UV radiation cross-linking process of polyethylene (PE) is accomplished by density functional theory for high voltage cable insulation materials. The reaction potential energy information of the 13 reaction channels at B3LYP/6–311 + G(d,p) level are identified. These have been explored that the TAIC take part in the reaction behaviour on ground state during UV radiation cross-linking process and TAIC intra-molecular isomerization reaction itself. In addition, the results show that the effect of multiplication and acceleration for the cross-linking reaction of trimethylopropane trimethacrylate (TMPTMA) would be better than that of TAIC. It has further clarified the reasons why UV radiation cross-linking reaction of PE had been initiated by benzophenone (Bp), and the TAIC or TMPTMA needed to take part. The results obtained in the present study could directly guide both the optimization of UV radiation cross-linking PE process and the development of the insulation material of high-voltage cable in real application.

Temperature and trap distribution dependence of electrical tree growth characteristics in polypropylene/elastomer blends for recyclable cable insulation

Polypropylene (PP)/elastomer blends as a promising cable insulation material have to face the challenge of electrical treeing which possibly leads to unexpected insulation failure. In this work, two types of elastomers, i.e. polyolefin elastomer and propylene-based elastomer, have been blended into isotactic PP. Effects of the elastomer content and type on electrical treeing behaviours have been investigated under different ambient temperatures. Isothermal surface potential decay measurement has been performed to explore the influence of trap distribution on the treeing behaviour, scanning electron microscope and differential scanning calorimetry measurements were performed to characterise the structure of blends. It was found that with the increase of elastomer content, the trap energy level was reduced and the shallow trap density was increased, which resulted in the enhancement of the tree growth rate and the decrease in the breakdown time. With the temperature changing from 20 to 100°C, the growth rate tended to be increased except that an anomalous reduction in the rate occurred at 80°C. It is suggested that the compatibility between PP matrix and elastomer plays an important role in determining the morphology of blends, which in turn influences the trap distribution and the temperature dependent treeing behaviours.