HTS POWER TRANSMISSION CABLE DEVELOPMENT IN JAPAN

Of the major players in Japan that are developing HTS power transmission cables -- Sumitomo, Furukawa, and to a lesser extent, Fujikura and Mitsubishi (Yuhya, Hosotani, and Hiraoka 1995) -- Sumitomo Cable working with TEPCO has demonstrated the best performance in both fundamental materials development and cable construction. The configuration of its 7-meter cable prototype is schematically illustrated in Fig. 3.2, and the corresponding dimensions are shown in Table 3.1 (Shibata et al. 1995). Note that in this design the HTS tapes are used not only for transmission of the primary current, but also for shielding the external pipe from the magnetic fields generated by the tapes transmitting the power. This design increases the needed quantity of costly HTS conductor, but the lower electrical losses place less strain on the cryogenic systems, which reduces cooling costs. Several characteristics of this cable are as follows:

• 3-phase, 66 kV / 1 kVA_rms (114 MVA)
• 7 m length, 130 mm diameter
• magnetic shielding layer, PLPP insulated
• 3-phase continuous current test ( 1 kA_rms, 7 hours)

Fig. 3.2. Schematic of 7-meter HTS cable prototype (Sumitomo).

Table 3.1
Sumitomo/TEPCO Cable Prototype

Table 3.2 gives characteristics of the Sumitomo HTS conductor used in assembling the 50-meter cabled conductor that is shown in Fig. 3.3.

Table 3.2
Sumitomo/TEPCO HTS Conductor

Fig. 3.3. Fifty-meter-long cabled conductor coil.

The Sumitomo HTS tapes have a high cross-sectional aspect ratio. The self-field critical current densities are not as high as those cited as being "economic" in the Joule study; however, they are presently the best in the world and represent the state of the art for long-length HTS conductors. At 10^-12 ohm * m and 10^-13 ohm * m criteria, the I_cs of the conductor are 2,900 A and 2,200 A, respectively. It should be noted that it was the requirement of the utility sponsor, TEPCO, that the critical current density for its application (fixed diameter retrofit) be 100,000 A/cm². Lower critical current densities could only be tolerated if the attendant ac losses were reduced substantially. At the time of this WTEC study, ac losses were ten times higher than acceptable. Recent measurements on each of the four individual layers constituting this cable (see Fig. 3.4 for test layout) confirm that the ac loss is described by a self-field loss of a single cylindrical bulk superconductor based on the Bean model (Saga et al. 1996).

Fig. 3.4. View of power cable test layout.

Based on Sumitomo's cabled conductor characteristics, only four tape layers are needed to carry nearly 3,000 amps (dc critical current) at 77 K. With a tape width of 4.1 mm and a cable diameter of 23 mm, one can estimate that just over 60 tapes have been used to wind this cabled conductor, which means that the average I_c of one tape is approaching 50 A at 77 K! This is nearly ten times the I_c of the Furukawa cabled conductor described below and in Table 3.3.

• 1-phase, 66 kV / 1.4 kA_rms (38 MVA)
• 5 m, 124 mm diameter
• 66 kV - class terminations
• load test (1.4 kA_rms, 15 minutes)

In the Furukawa case, the J_c and the overall tape cross-section are nearly half of the Sumitomo values; consequently, one would expect an I_c at least four times smaller. Since ten tape layers were wound at a larger diameter than in the Sumitomo cable to produce a much smaller dc critical current at 77 K, the I_c is much smaller and is estimated to be about 5 A.

Table 3.3
Furukawa/TEPCO 50 m Conductor

Although there is not much published detail about the cable prototype proposed and assembled by Fujikura, it is unique in that it has HTS tapes lying axially along the cable length instead of being wound with a pitch (Kakimoto et al. 1995; Kume et al. 1995).