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Because of the small square dimension, a soldered-stacked-square (3S) high-temperature superconducting (HTS) wire with 1 mm width is considered as a candidate for cabling large current conductors through twist process. For studying the twist structure, in this work, we prepared several twisted 3S wire samples fabricated from the same piece of the 3S wire with 10 m length. For the 3S wire, the uniformity of critical current was evaluated. As well, we investigated basic electrical and mechanical characteristics for the twisted 3S wire. The main results show that the critical current distribution for the 3S wire is relatively uniform at the average value of 90 A; the allowable minimum twist pitch of the twisted 3S wire is about 60 mm; the critical current of the twisted 3S wire shows isotropy characteristics; through the test for frequency dependence of self-field loss, we can know the eddy-current loss in the twisted 3S wire can be ignored, and the measured losses are identical to the calculated one from the thin strip equation; the minimum bending diameter and allowable tensile force for the twisted 3S wire are approximately 80 mm and 400 N, respectively.

In a high-voltage dc (HVDC) transmission system, a high-temperature superconducting (HTS) dc cable is a feasible candidate due to its high power density. However, the dc in the grid includes ac ripple generated from switching devices and the environment, which can cause ac losses. In this paper, two pentagonal conductors consisting of second-generation (2G) wires are prepared, and their ac losses are investigated under the application of current with ac and dc components. Moreover, a numerical analysis is performed for both conductors to compare the measured losses. The results show that the measured ac losses correspond well with the numerical ones calculated by our models. The dc offset current does not affect the ac losses when the total current is smaller than the critical current, and the ac losses depend on the amplitude of the ac component. All measured losses are verified by evaluating the dependence on the arrangement of the voltage leads, the contact position, and the frequency.

We have operated the 22.9-kV high temperature superconductor cable system installed at Icheon Substation of Korea Electric Power Corporation (KEPCO), which consists of a conductor and a shield using YBCO-NiW wires. We prepared the same 5-m cable sample as the Icheon Substation in order to understand its ac loss characteristic. Especially, the contact resistance between copper current leads and YBCO wires was minimized through the increased soldering surface between them. Besides, four voltage leads are attached to both the conductor and the shield with an equal angle of 90°. Under ac transport current, which was supplied to the conductor and shield in opposite direction, losses of the 5-m sample were tested for various contact positions of voltage leads, frequencies, and transport periods. The result shows that the measured ac losses for various contact positions in the conductor and shield are the same. However, their loss exponents are very different. More details will be presented in this work.

We have operated the two 22.9 kV/50 MVA HTS cable systems installed at Gochang Test Yard and Icheon Substation of Korea Electric Power Corporation (KEPCO), respectively; they have a double layer conductor and a single layer shield, consisting of BSCCO and YBCO-NiW wires. In general, the ac loss of a multilayer cylindrical conductor like the KEPCO HTS cables is evaluated from the conductor's current and the loss voltage of a lead soldered onto its outermost layer. To experimentally investigate what the voltage lead on the outermost layer picks up in this study, we prepared a double layer BSCCO conductor sample, and attached the four voltage leads of the outer, inner, contactless, and total leads. Then its ac loss for these leads was examined for different layer current distribution, and finally compared with a numerically calculated one. The results show that the outer-lead, which is often used in the double layer conductors, takes only the outer layer's loss voltage like the contactless-lead. Therefore, for ac loss evaluation of our double layer sample, a new voltage lead, total-lead, is presented besides the conventionally used outer-lead. The total-lead best fits with the numerical calculations, regardless of layer current distribution. However, the outer-lead is not suitable, especially for nonuniform distribution, although it still now is valid only for uniform distribution.