Patent GB-1331979-A (GB)
Title: Superconductor switching devices
IPC: H01L39/20
Filed: 1970-12-14
Abstract: 1331979 Superconductor devices; arcing- preventing in switches SIEMENS AG 14 Dec 1970 [13 Dec 1969] 59392/70 Headings H1K and H1N [Also in Division H2] A superconductor current limiting switch comprises a superconductor coil and a magnetic screen of superconductor material, the arrangement being such that below a predetermined current level the magnetic flux due to the coil is forced to follow an extended path by the screen but at that current level the magnetic flux penetrates the screen and takes a shorter path so that the magnetic field in the coil suddenly increases and switches the whole of the coil to the normal state. As shown, Fig. 1, two coils 1, 2 comprising strip form conductors wound round a thin plate 5 of plastics material are separated by a superconductor magnetic screen 3 having rounded ends 6 and are enclosed in a screening box 7 also of superconductor material. The coils are connected in series and for normal load currents the magnetic flux follows the paths S1 extending round the ends of the screen 3. When an overload occurs the magnetic flux increases until at a predetermined level the edge of the screen are driven normal allowing the flux to enter so that it contracts to the direct paths S2 between the adjacent ends of the coils. This suddenly increases the magnetic field applied to the coils and drives them normal so that the device provides a high resistance. The device is connected in parallel with a resistor into which the load current is diverted and limited to a value which can be interrupted by a conventional circuit breaker. In a second embodiment, Fig. 5a, a plurality of series connected coils are provided each comprising a Nb strip 31 wound round a tubular former 32, of glass fibre reinforced epoxy resin, having a longitudinal slot, the coils being arranged coaxially with a superconductor magnetic screen 36 extending through the aligned slots of their formers. The coils are separated by coaxial superconductor screens 33 having curved ends 39 and are enclosed in an outer screen 34 and a cryostat enclosure 41, 44, 46, 47 utilizing liquid nitrogen as a thermal screen. Layers of insulating material may be incorporated between the coils and the screens. The coils may also have strips of insulating material (not shown) set on edge between adjacent turns to prevent flash overs. The ends of the coil formers are provided with polepieces 38 of iron powder embedded in plastics material to direct the magnetic flux. Although only three coils are shown, in a practical embodiment a larger number are employed. The superconductor screen comprises a plate of soft superconductor material e.g. Pb or Pb-Bi alloy or may comprise a plurality of Nb strips secured together by insulating adhesive layers or by adhesive coated plastics strips and reduce eddy current effects, Fig. 6 (not shown). An overload cut out system, Fig. 4, comprises a superconductor current limiting device 21 of the above-described type, a safety switch 22, a limiting resistor 23 and a conventional circuit breaker 24 connected in a power line 25 having inductance 26 and resistance 27. The system carries a normal load current of 1KA at 220KV but when an overload occurs the superconductor device 21 switches to its normal state when the current reaches 2KA and exhibits a resistance of 2K# which diverts the current through resistor 23 which has a value of 155 ohms. The current through device 21 drops to 100A in 50 to 100 ÁS and safety switch 22 is opened 20 to 50 mS after the start of the overload to prevent the superconductor device overheating. The current flowing through resistor 23 is then interrupted by means of circuit breaker 24 100 to 150 mS after the start of the overload.