Makoto Hamabe

Research activities of DC superconducting power transmission line in Chubu University

A experiment of a DC Superconducting power transmission line using HTS conductor was started in Chubu University, Japan in autumn 2006. The first cooling down began in October 2006. The coolant is liquid nitrogen, and the cooling system used a cryogenic cooler and cold pump. The operation temperature is the range of 72 K - 80 K. The power cable has a total length of 20 m, and composed of thirty-nine Bi-2223 HTS tape conductors with critical current approximately100 A. The power cable achieved a 2.2 kA. The insulation voltage of the cable is 20 kV. In order to reduce the heat leakage and to avoid the current imbalance in the HTS tapes, we installed the Pelteir Current Lead (PCL) for each of the nineteen HTS tapes of the cable, with remaining twenty HTS tapes connected individually by the usual copper leads (CCL). Depending on the visual observation, and the measurement of temperatures of the current leads, the heat leakage of the PCL is lower than that of CCL. We installed a current transformer for each individual HTS tape conductor circuit, and measured the current of each HTS tape conductor. We measured the critical current of each HTS tape after the cable was installed into the cryostat, and degradation was not observed. Since the variation of the current in each tape is less than 10%, we eliminated the problem of current imbalance. Computational Fluid Dynamics is used to estimate the pressure drop, showing that the straight-tube cryostat has the advantage against the bellows- and corrugated-tube cryostats to reduce the pressure drop of the circulation of coolant. We proposed further to use the siphon for circulating the coolant in order to reduce the circulation losses and costs. We proposed that the voltage of the system be kept below 30 kV in order to use low cost power inverters. This choice can increase the storage energy of the power transmission line itself if we do not use a co-axial cable system because of it allows use of large current. And the magnetic energy of power grid is estimated to 4.5MJ/km for ±30 kA.

Recent Progress of Experiment on DC Superconducting Power Transmission Line in Chubu University

A test stand of a DC superconducting power transmission cable was finished to construct in October 2006 in Chubu University, Japan, and three cooling cycles were carried out to measure the properties of the cable. Critical current of HTS tapes in the cable was measured at every cooling cycle and shows the similar temperature dependence; conclusively, the HTS tapes suffered no damage after cooling and heating process between the each cycle. Peltier current leads were partly installed in the test stand, and it was measured that the temperatures of the feedthrough near to the liquid nitrogen were decreased in spite of a current feeding.

Status of a 200-Meter DC Superconducting Power Transmission Cable After Cooling Cycles

We constructed a facility of a 200-m HTS power transmission test cable (CASER-II) in 2010. Generally, an HTS cable contracts about 0.3% when it is cooled from room temperature to liquid nitrogen (LN2) temperature. The contraction of the 200-m HTS cable corresponds to 0.6 m. In order to realize the HTS power transmission system, it is an essential issue to absorb the mechanical stress of the HTS cable during the cycles of cooling-down and heating-up. The CASER-II uses smooth pipes as the cryogenic pipe for the cable line to reduce the pressure drop of the liquid nitrogen flow, whereas the other HTS cables use corrugated pipes to absorb the mechanical stress. The CASER-II employed (1) the movable terminals at the cable end, and (2) the extendable bellows inserted in the cryogenic pipe, to absorb the contraction of 0.6 m in cooling cycles. Even at the 4th cooling-down test, no damage was observed in the CASER-II. Use of the smooth cryogenic pipe enabled low pressure drop with low LN2 flow rate, and negative pressure drop appeared at less than 5 L/sec of the LN2 flow rate. This negative pressure drop was caused by the LN2 flow assisted by siphon effect due to the difference of LN2 density along the cryogenic pipe line with elevation of 2.6 m.

Double Peltier Current Lead for Heat Leak Reduction at the Terminals for Superconducting Direct Current Applications

For superconducting direct current applications, heat leak reduction at the terminal is a key issue for high performance systems and especially for small ones such as the distribution in internet data centers. We propose a double Peltier current lead (PCL), where the suitable combination of two Peltier modules can enhance the performance of PCL. Using the model parameters of actual thermoelectric materials, we estimated the heat leak on PCL using a thermal balance equation. At the double PCL, the large temperature difference on the current lead can be split to two thermoelectric materials and then the performance of PCL can be enhanced. As each of the thermoelectric materials has better working temperature range, an optimized combination of shape factors can be used for the high performance current lead at the terminals for superconducting application systems.

Cryogenic System for DC Superconducting Power Transmission Line

Due to the recent developments of the electric power devices and the HTS power cables, DC superconducting power transmission system is considered to be practical. As one of the R & D issues of the DC superconducting power transmission system, we measured the radiation heat of the thermally-isolated stainless steel pipes with different surfaces. As the result, one sheet of aluminum foil pasted on the inner pipe surface reduced the heat radiation to 1.98 m/W between LN2 and the room temperature of 300 K.

Critical Current and Its Magnetic Field Effect Measurement of HTS Tapes Forming DC Superconducting Cable

We constructed a test stand of 20 m-class DC superconducting power transmission cable in Chubu University, in 2006. The cable consisted of thirty-nine Bi-2223 HTS tapes in two layers; nineteen tapes were used in the first layer and twenty tapes were used in the second layer. One of the features of the cable is that each HTS tape in the first layer is electrically isolated. Therefore, we can apply the current to the individual HTS tapes in the first layer. We measured critical current of the one HTS tape in the first layer when current was applied to the other HTS tapes, and compared the result with the magnetic field calculation due to the applied current. The critical current increased by 2 ~ 4% when the current was applied to all other eighteen HTS tapes in the same layer. From the magnetic field calculation, we concluded that the self magnetic field of the measured HTS tape was eliminated at the tape edge by the applied magnetic field from the neighbor HTS tapes, and then the critical current increased.

RADIATION HEAT MEASUREMENT ON THERMALLY-ISOLATED DOUBLE-PIPE FOR DC SUPERCONDUCTING POWER TRANSMISSION

Multilayer insulator (MLI) is a strong tool for use as a radiation heat shield, though the use of MLI has disadvantages in construction and evacuation for a long superconducting power cable. We have proposed the “MLI-free” radiation heat shielding for DC superconducting power cable and have measured the radiation heat transfer for thermally-isolated double-pipes with different surfaces. Here, Zn coating, MLI, and Al-foil sheet were tested. Consequently, from the radiation heat of 9.7 W/m for bare stainless-steel pipe, Zn-coated stainless-steel surface reduced to 2.6 W/m, whereas the use of MLI reduced to 0.2 W/m. It is expected that the simultaneous use of Zn coating and MLI can reduce the number of total MLI sheets to reduce the evacuation time.

Thermoelectric Characteristics of Si/Ge Superlattice Thin Films at Temperatures Less Than 300 K

We have studied the thermoelectric characteristics of Si/GeAu superlattice thin films at temperatures ranging from 290 K to 75 K and compared them to those of the SiGeAu alloy thin film. In the annealed Si/GeAu superlattice, the electrical resistivity was lower than that of the unannealed Si/GeAu superlattice at all temperatures. The annealed Si/GeAu superlattice showed a high thermoelectric power of 105 µV/K at 290 K. At temperatures less than 200 K, however, the polarity of the thermoelectric power of the unannealed Si/GeAu superlattice switched from positive to negative and a large negative thermoelectric power of -4.6 mV/K was attained at 80 K. On the other hand, the characteristics of all samples showed no magnetic field effect at all temperatures. To explain the causes of the extremely small variations in the characteristics by the magnetic field effects, we calculated the transport coefficients for the SiGeAu alloy using the two-band parabolic model, and compared them to those of experimentally measured values. When a large amount of acceptor concentration from Au doping and the very low carrier mobility were assumed, similar transport coefficients to the measured ones resulted.

Residual Magnetic Field Measurement of BSCCO and YBCO Tapes by a Hall Probe

A serious damage can happen to the power cable manufactured from the high-temperature superconducting (HTS) tapes due to overcurrent conditions or because of technical errors during the cable assembling. To avoid the cable damage in any urgent case, a quick interruption of the transport current is necessary. Comprehensive understanding of the current cutoff time of the HTS tape is required to restart the operation of the power transmission line. In this paper, we applied 100-A transport current to BSCCO and YBCO tapes. After interrupting the current, we measured the magnetic field profiles of each HTS tape. We compare the magnetic field profiles for the fast and slow cuts of the current. The cutoff time is 0.15 ms for the fast cut and 100 s for the slow one. The current density profiles are obtained by solving the inverse problem. Our results show that the magnetic field profiles are quite different from each experimental condition and HTS tape, which are related to the hysteresis of HTS tape characteristics.

Cooling cycle test of DC superconducting power transmission cable

We constructed a test stand of a 20 m DC superconducting power transmission cable in Chubu University in 2006. The cable uses thirty-nine Bi-2223 tapes. Four cycles of a cooling and current-feeding test have been carried out after the construction of the test stand. The cable suffered rapid temperature decrease of 20 K/hour at the daytime during the cooling process by cold nitrogen gas and liquid to the liquid nitrogen temperature. Nineteen HTS tapes in the cable are electrically isolated from each other, and the superconducting characteristics of these HTS tapes can be measured separately. At every cooling cycle, critical current of these isolated HTS tapes were measured. Consequently, no reduction of the critical current characteristics was observed through the 4 cooling cycles. We considered that this endurance of the cable against the cooling cycle is due to unfixed cable end structure of the cable, and that the cable was free from the mechanical stress in spite of the 6 cm shrinkage of the cable length under the low temperature.