Studies of superconducting or cryoresistive power transformer are summarized. Since 1960, feasibility studies of superconducting transformer have been made by Mcfee, Wilkinson Jones, Lorch, Harrowell and the experimental models of superconducting transformers have also been constructed and tested by Mcfee and Jones. Though Mcfee thought that a superconducting transformer could compete with a usual transformer at high power region more than several ten MVA, others reported that it would not be economic until a superconductor having the extremely low AC loss and the high critical current density under high magnetic field were developed. In addition to the development of such a new superconductor, they pointed out that the improved winding construction having the low leakage magnetic field, superinsulation vessel for winding, and protection of the superconductor against fault conditions should be studied. Feasibility studies of cryoresistive transformers have been made by Wilkinson. He has estimated economic terms power that could be saved if the conductor in several hundred MVA cryoresistive transformers were pure aluminium maintained at 20°K by liquid hydrogen or beryllium maintained at 77°K by liquid nitrogen. Compared the possible saving in conductor loss with the drive power used by refrigerators and with the cost of refrigeration plant, it was found that in cryoresistive transformers, there was no promise of net saving over conventional costs that would warrant the constructional and operational complexities entailed in deep refrigeration. The experimental models of cryoresistive transformers have been made by Lair Liquid Ltd. and Hitachi Ltd.. In the former, a capacity of which was 253kVA, pure aluminium at 20°K have been used as the conductor. In the latter, a capacity of which is 1, 200kVA, copper have been used as the conductor and maintained at 130°K by liquid fluorocarbon cooled by the heat of the evaporation of liquid natural gas, which have been used more and more as fuel of power plants. This type of the cooling systems would save the cooling cost of the cryoresistive transformer.
Characteristics of temperature control are quite important to insure the reliable operation of the superconducting power transmission cable in practice. Three typical control schemes are considered for the cable, i.e. proportional (P), proportional integral (P-I), and proportional integral and differential (P-I-D) type feed back control. Theoretical and numerical analysis show that the first scheme leaves permanent deviation, the second scheme gives smooth convergence, and the third scheme shows fast response for the foundamental oscillation mode, but there remains high frequency oscillations in this type of control. Therefore we concluded that the second scheme is most suitable control system for the cable, and may be applicable for back up protection of it. This paper discusses these three control schemes in detail.
The resistance-temperature (R-T) characteristics of 1/8W Matsushita carbon resistors of grade ERC-18 SG (military grade) with the nominal values of 48, 82, 100, 220, and 330Ω have been measured in the region 4.2K-25mK and their usefulness as thermometers in this regionis confirmed. As to the 82Ω resistor, the measurement was extended to temperatures below 10mK. The temperature dependence of the resistance was found to be linear on the log-log plot over a wide range below 50mK. The sensitivity remains finite even at 6mK, but below 10mK rapid temperature measurements were prevented by a considerable increase in the thermal relaxation time. Measurements of the characteristics of some numbers of 100Ω resistors from each of two different lots showed that resistors from the same lot separate into two groups with different characteristics. This becomes appreciable with decreasing temperature below 4.2K, so that it is difficult to presume the behaviour of Matsushita resistors below 4.2K from the characteristics at higher temperatures, in contrast to the case of e.g. Speer resistors. The empirical formulae representing the R-T characteristics are discussed.