The present invention relates to a termination for connecting superconducting and high temperature superconducting (HTS) cables operating at sub-ambient temperatures to cables operating at ambient temperature.
It is known that superconductors are metals, alloys, or oxides thereof, and in general are compounds having practically zero resistivity below a transition temperature, i.e. the critical temperature. A superconducting cable must be operated below its critical temperature, therefore it is cooled during use by, for example, cryogenic cooling fluids. Metal and alloy superconductors have critical temperatures below 20° K while metal oxide (ceramic) superconductors have higher critical temperatures on the order of 80° K thus distinguishing them from the former materials and separating them into a class known as high temperature superconductors that are used to make HTS cables. Because of the brittleness of high temperature superconductors, the cable making material is presently manufactured in the form of tapes known as HTS tapes.
Because of their negligible resistance, superconducting power cables lose only about one-half percent of power during transmission, compared to a 5 to 8 percent loss of traditional power cables and deliver about three to five times more power through the same area than traditional power cables. As the rapid growth of urban areas increases demand for electricity, the ability of HTS cables to transmit more power while using equivalent amounts of space as traditional cables are increasingly important.
To be useful, a superconducting cable must have terminations such that the cold superconductor may be connected to a conventional resistive conductor in an ambient temperature environment. The two primary functions carried out by a superconducting cable termination are providing transition from the cryogenic superconducting environment to ambient conditions and transitioning the large radial voltage gradient in the cable to the much lower gradient tolerable after termination.
Generally, an HTS cable has a coaxial configuration comprised of an energized inner superconductor (phase or line), at least one layer of electrical insulating material, and an outer layer of superconductor placed at zero potential (neutral, ground, or shield). Multiple layers of energized superconductor and electrical insulation may be present in some cables to transmit three phase power. An HTS cable is generally made by winding HTS tapes over a hollow tube known as a former. The former provides mechanical support for the HTS tapes and electrical insulation as well as a path for cryogenic fluid circulation from one end of the cable to the other for cable cooling. The coolant, in some HTS cable designs, permeates the cable structure and thereby becomes an important part of the electrical insulation. In this function, the coolant must also be kept at a pressure where bubbles do not form during operation and the coolant pressure may then be above ambient pressure. HTS cable is housed in a conduit with thermal insulation to keep the cable at the desired temperature and having sufficient strength to accommodate the pressure of the cooling fluid and protect the cable from harm. The conduit also provides an additional path for cryogenic fluid circulation from one end of the cable to the other for cable cooling. Terminations are located on each end of the HTS cable to affect the transition from the superconducting cable, generally cooled by pressurized cryogenic fluid such as liquid nitrogen, to external bushings at ambient temperature.
Various types of terminations have been used in the prior art, but these terminations are complex, subject to stress and susceptible to failure.
A common prior art design has two sets of bushings, a cold bushing and a warm bushing, at two separate boundaries. At the first boundary the cold bushing separates the HTS cables cooled by cold, pressurized liquid nitrogen from another region that is warmer and either is in a vacuum or is filled with an insulating gas such as nitrogen or SF6. At the second boundary the warm bushing separates the vacuum or insulating gas region from ambient conditions (i.e. 295° K and one atmosphere). The cold bushing in such designs is a highly stressed component and prone to failure. The bushing experiences significant thermal/mechanical stresses during cooldown of the cable and must be designed for cable current (several kA) and, for the inner conductor, has to have sufficient solid insulation for the rated voltage (˜10-100 kV). The bushing must also have sufficient electrical insulation to withstand the rated voltage.
In one known embodiment, described by C. Bogner in “Transmission of Electrical Energy by Superconducting Cables”, pages 5145-16 in S. Foner and B. B. Schwartz ed., Superconducting Machines and Devices, NATO Advanced Study Institute, Entreves, Italy, 1973, Plenum Press (1974) a terminal for a single-phase superconducting cable comprises a vacuum container inside which a casing filled with low-temperature liquid helium is disposed.
U.S. Pat. No. 6,049,036 discloses a terminal for connecting a multiphase superconducting cable to room temperature electrical equipment. The terminal includes a casing with cooling fluid, inside which three cable superconductors are connected with a resistive conductor the end of which is connected to the room temperature equipment phases at the outside of the casing. The design features internally cross connections between the three shield conductors at the cold end eliminating the need for the shield conductors to ambient conditions, although an external connection is provided to establish ground potential. In this design, the internal portion of the resistive conductor ends are filled by gaseous coolant that forms an interface with the liquid coolant somewhere along the resistive conductor and this interface is held in place by gravity, thus vertical orientation is required in this invention. Further, this invention has a high voltage insulator that forms a vacuum boundary that extends from room temperature to coolant temperature.
U.S. Pat. No. 4,485,266 discloses a termination for connecting a single coaxial superconducting power transmission line to an ambient bushing that operates in the horizontal position. The invention has a completely sealed horizontal conduit that connects the cold superconducting cable to a room temperature sulfurhexafluoride insulated bushing. The sealed conduit is a very complex structure that provides electrical insulation between phase and shield as they warm and transition to normal conductors, each of which has its own independently cooled heat exchanger that controls the temperature gradient along the conductor.
U.S. Pat. No. 3,902,000 discloses a termination for connecting a single coaxial superconducting cable to an ambient temperature bushing. The patent provides for a low temperature stress cone to expand the dimensions of the insulation prior to encountering the vertical temperature gradient region. This is done because the coolant, helium, has poor dielectric properties in the warm gaseous state. Gaseous coolant is vented from the top of the termination to provide cooling for the temperature transition zone. The inner conductor is connected to a conventional bushing having conventional dielectric fluid at the warm end.
Prior art terminations utilized either vertical configuration or a very complicated horizontal section with independent cooling circuits to control temperature gradients in the transition zone between the superconducting and normal conducting cables. The present invention considerably simplifies the design of terminations for HTS cables by using a unique and innovative technique employing the thermal gradient along the termination's copper conductors to eliminate the requirement for vertical orientation or independent cooling circuits. This produces an HTS cable that is more reliable due to the inherent simplicity of the termination design.