다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

To design reliable high voltage cables, clean materials with superior insulating properties capable of operating at high electric field levels at elevated temperatures are required. This study aims at the electrical characterization of a byproduct-free crosslinked copolymer blend, which is seen as a promising alternative to conventional peroxide crosslinked polyethylene currently used for high voltage direct current cable insulation. The characterization entails direct current (DC) conductivity, dielectric response and surface potential decay measurements at different temperatures and electric field levels. In order to quantify the insulating performance of the new material, the electrical properties of the copolymer blend are compared with those of two reference materials; i.e., low-density polyethylene (LDPE) and peroxide crosslinked polyethylene (XLPE). It is found that, for electric fields of 10–50 kV/mm and temperatures varying from 30 °C to 70 °C, the DC conductivity of the copolymer blend is in the range of 10−17–10−13 S/m, which is close to the conductivity of crosslinked polyethylene. Furthermore, the loss tangent of the copolymer blend is about three to four times lower than that of crosslinked polyethylene and its magnitude is on the level of 0.01 at 50 °C and 0.12 at 70 °C (measured at 0.1 mHz and 6.66 kV/mm). The apparent conductivity and trap density distributions deduced from surface potential decay measurements also confirmed that the new material has electrical properties at least as good as currently used insulation materials based on XLPE (not byproduct-free). Thus, the proposed byproduct-free crosslinked copolymer blend has a high potential as a prospective insulation medium for extruded high voltage DC cables.

/pdf/electrical-characterization-of-a-new-crosslinked-copolymer-r3igmodhte.pdf

Electrical Characterization of a New Crosslinked Copolymer Blend for DC Cable Insulation

This paper focuses on the multi-dimensional performance analysis including the oxidation performance, crystal morphology, thermal properties, mechanical properties and dielectric behavior of the thermally aged extra high voltage XLPE submarine cable insulation. Results show that the antioxidation and thermal oxidative aging of XLPE insulation are coexisting and competing during the thermal degradation process. In the early stage of thermal degradation, the antioxidation of XLPE insulation is dominant resulting in the decline of the spacing between spherulites. In the later stage of thermal degradation, thermally oxidative aging of XLPE insulation is dominant, which leads to the damage of the crystalline morphology and the rapid generation of aging products. It is indicated that the mechanical strength of the XLPE insulation is mainly determined by its internal crystalline morphology, while the dielectric properties of the XLPE insulation are governed by the crystalline morphology, aging products and micro interface. With the thermal aging time extended, the breakdown strength of XLPE insulation first increases and then decreases, which is determined by its amorphous region and aging products.

Multi-dimensional Analysis and Correlation Mechanism of Thermal Degradation Characteristics of XLPE Insulation for Extra High Voltage Submarine Cable

Effect of degassing treatment durations on physico-chemical and electrical properties of 500 kV extra HVDC XLPE cable insulation

This paper presents the influence of thermal ageing on the physico-chemical and electrical properties of 500 kV extra high voltage direct current (EHVDC) XLPE cable insulation. EHVDC XLPE cable sections are placed in a thermal oven and thermally aged at 106 °C (melting peak temperature) for 250, 500 and 1000 h. After ageing, the inner, middle and outer layers of the cable insulation are characterized by Fourier transform infrared (FTIR) spectroscopy, oxidation induction time, X-ray diffraction (XRD), space charge, DC conductivity and DC breakdown measurements. The physicochemical analysis results show that the cross-linking by-products in the cable insulation are difficult to be completely removed during the ageing process, whereas the thermal ageing cannot completely consume the antioxidants in the materials. The space charge measurement results show that the hetero charges are accumulated inside the unaged middle layer samples. However, the space charge accumulation and electrical conductivity of the samples are significantly reduced after 1000 h ageing. Moreover, the DC breakdown strength of the inner and middle layer samples is continually increased with the increase of the ageing time. It is elucidated that the thermal ageing leads to the re-crystallization and re-degassing of samples, improving the space charge behavior and the charge carrier trapping phenomenon. It is proposed that the temperature of 106 °C could be used as a suitable degassing temperature for the EHVDC XLPE cable production.

Effect of Thermal Ageing on Physico-Chemical and Electrical Properties of EHVDC XLPE Cable Insulation

This paper reports the effect of thermal ageing on the space charge characteristics, DC conductivities and trap energy distributions of 320 kV HVDC XLPE. Specimens were prepared by a heating press method in the laboratory. The specimens were placed in a thermal oven and thermally aged at 130 °C up to 30 days. Space charges were measured by a pulsed electro-acoustic setup. The results showed that the reference samples had limited hetero charge accumulations near both electrodes in the samples. The homo-charge accumulation was appeared with an increase of the aged time. The space charge accumulation in the samples was transferred from hetero-charge accumulation to homo-charge accumulation with an increase of the ageing time. The electric field distribution in the sample gradually increased with the appearance of the homo-charge accumulation, whereas the homo charges evidently distorted the electric field in the samples with the increasing ageing time. The DC conductivities of the aged samples consistently increased with the increase of the measured electric field. The obtained results of apparent mobility of the charge carriers, the activation energy and the trap energy density illustrated that thermal ageing could facilitate the homo-charge accumulation in the sample. The dominant effect of thermal ageing on the sample was elucidated by using various analytical techniques. It was found that the thermal ageing would affect the material properties and facilitate the increase of the trap energy density and the space charge accumulations in the samples.

Effect of thermal ageing on charge dynamics and material properties of 320 kV HVDC XLPE

Graphene as a novel additive in low density polyethylene (LDPE), high density polyethylene–low density polyethylene blend (10%HDPE–LDPE) and polypropylene–low density polyethylene blend (10%PP–LDPE), is investigated for promising eco-friendly insulating materials in high voltage cables. The composites with graphene amounts of 0, 0.002 wt%, 0.02 wt%, and 0.2 wt% are prepared by the solution mixing method. With attempt to disclose the structure–property relationships, the composites are extensively characterized by physical techniques including Fourier transform infrared spectroscopy, X-ray diffraction, atomic force microscopy, transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis and differential scanning calorimetry, and electrical measurements including electrical treeing, direct current conductivity and space charge distributions. Results show that graphene can significantly enhances the thermal stability of LDPE and 10%PP–LDPE, slightly decrease the degree of crystallinity, change the size of the crystal diameters, and improve the phase distribution of the polymer blends. Consequently, graphene can prolong the electrical tree initiation time and block the extension of electrical tree. Besides, it can modulate the trap distributions, which will further change the conductivity and ability to suppress space charge accumulations. This excellent performance of graphene could be attributed to its good exfoliation, uniform dispersion and outstretched morphology in the polymer matrix, as well as its strong ability to capture electrons.

Graphene Enhanced Electrical Properties of Polyethylene Blends for High-Voltage Insulation

In this paper, a thermo-electric field coupling model was established to perform simulations on a typical three-core AC 10 kV XLPE cable in bipolar DC operation by the finite element analysis software ANSYS. The obtained results showed that when the conductor temperature of the cable was 70 oC and the cable was laid in the environmental temperature of 40 °C, the estimated cable ampacity could reach up to 440 A. The maximum of electric field in the cable insulation was settled below 3 kV/mm to minimize the effect of space charge for the cable in DC operation. It was found that the cable could run at the bipolar DC ± 10 kV with enough safety margin. Moreover, the maximum transmission power of the cable running at the bipolar DC ± 10 kV mode was 1.37 times of the power in the AC operation. It is promising for upgrading the present AC grid into future DC grid.

Numerical analysis of thermo-electric field for 10 kV AC XLPE cable in DC operation

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.

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

This paper reports a new application of graphene as voltage stabilizer for polyethylene blends of the future environmental-friendly cable insulation materials. Electrical treeing in needle-plane electrode test objects of low-density polyethylene (LDPE) and its high-density polyethylene (HDPE), polypropylene (PP) blends with/without graphene were recorded by a real-time microscope observation system and a synchronized partial discharge (PD) detection system. The obtained results showed that the electrical tree initiation time of the LDPE decreased continually with the addition of 10 wt% HDPE and PP, respectively. It has been shown that the addition of 0.2 wt% graphene improves resistance of the tree inception of the LDPE and its blends. The LDPE/HDPE blended specimen with filled graphene had the longest tree initiation time (TIT) within all the tested samples, which was one time of the TIT of pure LDPE objects, and over 150% extension of the TIT of the unfilled LDPE/HDPE samples. It was also found that the graphene filled LDPE blends usually showed higher discharge magnitude than the pure LDPE material. It was postulated that the addition of grapheme would facilitate the PD in the tree channels of the samples with voltage stabilizer, but the influence of graphene voltage stabilizer on the microstructure and morphology of the blends could also enhance the tree inception resistance in the materials.

Electrical treeing of polyethylene blends with/without voltage stabilizer

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