The use of superconducting power cables is one of the promising ways for handling large volumes of electric power efficiently in the future. It is necessary to study the behavior of electrical insulation at cryogenic temperature to evaluate the long-term reliability of superconducting cables. At cryogenic temperature, there are many practical studies on electrical insulation characteristics of fluids such as liquid nitrogen or liquid helium and composite insulating systems, but few studies on solids enabling a high-voltage design. This paper describes the electrical breakdown and space charge of ethylene-propylene rubber (EPR) samples with fillers for the DC electrical insulation system at cryogenic temperatures. The results show that the breakdown strength at cryogenic temperature is much higher than that at room temperature. This may be attributed to less carrier injection from the electrode.
Heat and mass transport phenomena in supercritical air near the maxcondentherm point has been investigated using a laser holography interferometer. In a previous study, it was confirmed that a large temperature (density) gradient, which could not be observed in supercritical nitrogen, was formed in supercritical air. We also obtained an experimental result, which suggested the existence of piston effect. At that time, heat was added from a planar heater located at the bottom of an experimental cell. In this mode, the heat and mass transport phenomena always accompany natural convection. To prevent the effect of natural convection, heat was added from the top of the cell in the present study. Here, we show the experimental results and discuss the heat and mass transport phenomena observed in supercritical air.
In order to investigate fatigue strength and fracture mechanisms of Cu-Nb composite multistrand wires, fatigue tests at room temperature and fractographic observations were carried out using Cu-20, -30, and -40wt%Nb composite wires fabricated by in-situ processing. Fatigue strength in the high cycle region increases significantly as the content of Nb increases, as in the case of U.T.S., while the strength in the low cycle fatigue region is the lowest in Cu-30wt%Nb wire, as in the case of 0.2% proof stress. Fractographic observations revealed that slip plane decohesion facets and inclusions within the Cu wire sheath were possible sites for the crack initiation. Evidence on the roles of the Nb filaments and Cu-Nb filaments acting as barriers against crack propagation was obtained by observations of the longitudinal section of fatigued specimens.
Quench detection is an essential technique to protect superconducting coils from excessive ohm-generated heating after quenching. We previously proposed an electric quench detection method based on detecting active power, which is less susceptive to electromagnetic noise. However, this method is vulnerable to high voltage because it requires a central voltage tap to detect the active power (or cancel the reactive power). In other words, a lead-wire from the tap may short circuit as the result of high voltage. In this paper, we present an electric quench detection method without a central voltage tap. In this method, an inductive voltage detection coil is used instead of a central voltage tap. The inductive voltage measured by the inductive voltage detection coil and that of the superconducting coil are in phase, therefore the inductive voltage (or reactive power) of the superconducting coil can be canceled by offsetting the inductive voltages. Like the previous method, this method can also detect quenching regardless of noise superposition. Through the quench detection tests for a Bi-2223/Ag HTS coil, we show the feasibility of the proposed method for detecting the quench of a large AC superconducting coil.