Thermal-buble initiated breakdown characteristics of liquid helium and nitrogen at atmospheric pressure
Experiments show that electric field promotes to form a vapor locking in a gap with a hot spot and in turn the formation of locked vapor produces the serious reduction of breakdown voltage. Especially it is argued that the dielectric strength of liquid helium under serious hot spot conditions decreases below that for saturated gas.
Dielectric breakdown of cryogenic gases and liquids
Abstract Cryogenic gases and liquids exhibit interesting dielectric properties. Their resistivities are extremely high, their permittivities are low and their dielectric losses are negligible at low field strengths. The dielectric strength of the gases can be described by means of generalized similarity laws up to field levels of 107 V m−1. Breakdown of liquids is initiated within bubbles. All these properties and mechanisms are well understood in principle. The dielectric strength of very dense gases as well as of liquids is very sensitive to field distortions which are never completely avoidable in practice. Boundary effects have a considerable influence. Weak-linked breakdown mechanisms have to be assumed resulting in size and area effects. Some of the most important experimental results are discussed.
Development of a Niobium-copper-invar composite conductor for an AC superconducting power transmission cable
A superconducting ac power cable has been developed using Nb/Cu/Invar composite conductors and shields in a coaxial configuration with pressurized. He at 4.7 K as the primary dielectric. Two major concerns for this cable system were the accommodation of fault currents and the differential thermal expansion between the conductor assembly and the cryogenic envelope. We have shown that a Nb/Cu/Invar composite tube is capable of accommodating the fault currents as well as the differential thermal expansion. Studies of the effect of surface finish on the ac losses showed that with proper surface finish, the losses can be reduced to an acceptable level for power cable applications. The lowest 60 Hz losses measured at a peak surface magnetic field of 0.1 T correspond to 43 watts per 3-phase mile for a 138 kV, 3400 MVA power cable.