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

In the last few decades, there has been a trend involving the use of nanoscale fillers in a variety of applications. Significant improvements have been achieved in the areas of their preparation and further applications (e.g., in industry, agriculture, and medicine). One of these promising materials is magnesium oxide (MgO), the unique properties of which make it a suitable candidate for use in a wide range of applications. Generally, MgO is a white, hygroscopic solid mineral, and its lattice consists of Mg2+ ions and O2− ions. Nanostructured MgO can be prepared through different chemical (bottom-up approach) or physical (top-down approach) routes. The required resultant properties (e.g., bandgap, crystallite size, and shape) can be achieved depending on the reaction conditions, basic starting materials, or their concentrations. In addition to its unique material properties, MgO is also potentially of interest due to its nontoxicity and environmental friendliness, which allow it to be widely used in medicine and biotechnological applications.

Synthesis, Properties and Selected Technical Applications of Magnesium Oxide Nanoparticles: A Review

Enhancement of Service Life and Electrical Insulation Properties of Polymeric Cables With the Optimum Content of Aromatic Voltage Stabilizer

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

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

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

This paper reports on the effects of four voltage stabilizers, m-aminophenylboric acid, 2-methoxy-5-pyridineboric acid, m-aminobenzoic acid and 4-dimethylaminobenzoic acid, on the insulation properties of polyethylene blends. 1 wt% voltage stabilizers were added into low density polyethylene (LDPE) blend containing 10 wt% high density polyethylene (HDPE) by a solution method. Physicochemical and electrical properties of the blends were analyzed by infrared spectroscopy, thermo-gravimetric analysis, differential scanning calorimetry, electric tree initiation tests, space charge and surface potential decay measurements. The results showed that the addition of the voltage stabilizers had a little effect on the thermal stability and melting properties of the LDPE/HDPE blend. When the voltage stabilizers were included into the blends, the tree initiation voltage (TIV) increased, in particular the blend with m-aminobenzoic acid showed the highest increase of 41% compared to the reference LDPE/HDPE. It was found that space charge accumulations of the blends were apparently suppressed by adding voltage stabilizers. Moreover, the blend with the m-aminobenzoic acid had the least space charge accumulation. The surface potential decay rate of the blends was significantly increased after adding voltage stabilizers. Quantum chemical calculations illustrated that the HOMO-LUMO energy gap was closely related to the TIV of the blends, whereas the m-aminobenzoic acid has the lowest energy gap level and the best effect of restraining the electrical tree initiation. The trap level calculation showed that the trap depth in the blends decreased with the addition of the voltage stabilizers, which was beneficial to suppress the accumulation of space charge in the materials.

Enhancement of insulating properties of polyethylene blends by delocalization type voltage stabilizers

With rapid development of renewable energies and dc loads in distribution grids, converting ac distribution grids into dc operation has become a promising method to utmost utilize the present ac distribution grid in large cities. This paper reports a numerical investigation of thermo-electric fields of a three-core 10-kV ac cross-linked polyethylene (XLPE) cable and cable joint in the dc operation. DC conduction current for the XLPE and silicon rubber (SiR) was evaluated at different temperatures and electric fields. The thermo-electric field simulation was performed by COMSOL Multiphysics according to the measured data. The obtained result showed that the threshold of XLPE at 65 °C is 3.1 MV/m, whereas the threshold of SiR at 60 °C is 2.2 MV/m. It was found that the maximum electric field of the cable operated in dc voltage up to 12.5 kV was below the threshold of 3.1 MV/m. The maximum electric field in the cable joint appeared at the high voltage screen tube, whereas the maximum electric field at the high voltage screen tube was less than the threshold of SiR at a temperature difference of 25 °C. Moreover, the maximum transmission power of the three-core 10-kV AC XLPE cables running at the ±10-kV bipolar dc operation was 1.36 times of the power in the ac operation. It was elucidated that the three-core ac 10-kV XLPE cable could be operated reliably at ±10 kV in bipolar dc topology with sufficient safety margin. It is beneficial to upgrade the ac distribution grid to the dc distribution grid.

/pdf/numerical-analysis-of-thermo-electric-field-for-ac-xlpe-5fwem9b6ad.pdf

Numerical Analysis of Thermo-Electric Field for AC XLPE Cable in DC Operation Based on Conduction Current Measurement

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

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