One of the key components for dc HTS cable for long-distance line is a stop joint box (SJB). Up to now, the SJB has not been commercialized yet due to some difficulties of dc electrical insulation matters in LN 2 . Basically, the structure of SJB would be similar to the SJB of oil-filled cable because they adopt liquid insulation and sectionalized cooling separations. The insulation structure of conventional SJB based on ac electric field was no more suitable for dc; hence, there is a need to design suitable insulation structure, which can be applied for dc. In this work, in order to find a solution to reduce dc electric field concentration on the polypropylene laminated paper (PPLP) above epoxy spacer, double layer structure composed of PPLP and Kraft were suggested. To verify the effectiveness of this structure for relieving dc electric field, simulation and experiments were performed. From analytical works, the Kraft layer between PPLP and epoxy has shown noticeable electric field mitigation. Furthermore, surface breakdown tests considering PPLP/Kraft combined layers were conducted to verify the insulation properties of double layer structure. Finally, it was deduced that Kraft layer was effective to enhance surface breakdown voltage. Surface breakdown characteristics of PPLP and Kraft were relevant to the surface roughness of insulating materials.Index Terms-DC electric field analysis, epoxy spacer, HTS, Kraft, power cable, PPLP, stop joint box (SJB), surface breakdown.
1051-8223
For the insulation design of dc high-temperature superconducting (HTS) equipment, dc electric field analysis should be performed. As the dc electric field distribution is mainly determined by the relative electrical conductivities of the various insulating materials used, the conductivities of these materials should be precisely measured. In particular, in cryogenic environment, the measurement of electrical conductivity could not be easily conducted due to the difficulty of measuring extremely low leakage current. In this paper, investigation on the measurement of volume and surface electrical conductivity of various cryogenic insulants, including polypropylene laminated paper (PPLP), Kraft, and glass fiber reinforced plastic (GFRP), was carried out. For the measurement of volume electrical conductivity in LN2, the infiltration of LN 2 into the specimen should be considered in order to avoid inaccurate measuring data. Thus, in order to increase the reliability for volume electrical conductivity, the PPLP specimen deposited by copper was adopted, and the comparison between ordinary PPLP and PPLP with copper deposition was made. As a result, it was suggested that the copper deposition could be a valid method to prevent the infiltration of LN 2 when the electrical conductivity of thin paper was measured. Consequently, volume and surface electrical conductivity values of PPLP, Kraft, and GFRP have been measured and summarized.Index Terms-Copper deposition, DC electric field analysis, electrical conductivity, GFRP, Kraft, PPLP.
1051-8223
The influx of a switching impulse during DC steady-state operations causes severe electrical stress on the insulation of HVDC cables. Thus, the insulation should be designed to withstand a superimposed switching impulse. All major manufacturers of DC cables perform superimposed switching impulse breakdown tests for prequalification. However, an experimental approach to study space charge dynamics in dielectrics under a switching impulse superposed on DC voltage has not been reported yet. This is because, unlike the DC stress, it is not possible to study the charge dynamics experimentally under complex stresses, such as switching impulse superposition. Hence, in order to predict and investigate the breakdown characteristics, it is necessary to obtain accurate electric field distribution considering space charge dynamics using a numerical approach. Therefore, in this paper, a numerical study on the switching impulse superposition was carried out. The space charge dynamics and its distribution within the dielectric under DC stress were compared with those under a superimposed switching impulse using a bipolar charge transport (BCT) model. In addition, we estimated the effect of a superimposed switching impulse on a DC electric field distribution. It was concluded that the temperature conditions of dielectrics have a significant influence on electric field and space charge dynamics.
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