Experimental study of a transformer with superconducting elements for fault current limitation and energy redistribution

Numerous proposed and developed superconducting fault current limiters and self-limiting transformers limit successfully fault currents but do not provide uninterrupted supplying of consumers. A design investigated in the work combines the functions of a conventional transformer with the functions of fast energy redistribution and fault protection. The device constitutes a transformer containing an additional high-temperature superconducting (HTS) coil short-circuited by a thin film HTS switching element. Fault current limitation and redistribution of the power flow to a standby line are achieved as a result of a fast transition of the superconducting switching element from the superconducting into the normal state. Transient and steady-state characteristics were experimentally investigated. A mathematical model of the device operation was proposed, and the calculated results were found to be in good agreement with the experimental data. The application field and basic requirements to such devices were discussed and it was shown that the proposed device meets these requirements.Comment: 15 pages incl. 4 figures. Submitted to "Cryogenics

Entanglement of dipolar coupling spins

Entanglement of dipole-dipole interacting spins 1/2 is usually investigated when the energy of interaction with an external magnetic field (the Zeeman energy) is greater than the energy of dipole interactions by three orders. Under this condition only a non-equilibrium state of the spin system, realized by pulse radiofrequence irradiations, results in entanglement. The present paper deals with the opposite case: the dipolar interaction energy is the order of magnitude or even larger than the Zeeman one. It was shown that entanglement appears under the thermodynamic equilibrium conditions and the concurrence reaches the maximum when the external field is directed perpendicular to the vector connecting the nuclei. For this direction of the field and a system of two spins with the Hamiltonian accounting the realistic dipole-dipole interactions in low external magnetic field, the exact analytical expression for concurrence was also obtained. The condition of the entanglement appearance and the dependence of concurrence on the external magnetic field, temperature, and dipolar coupling constant were studied.Comment: 9 pages, 5 figure

Low temperature MQ NMR dynamics in dipolar ordered state

We investigate analytically and numerically the Multiple Quantum (MQ) NMR dynamics in dipolar ordered spin systems of nuclear spins 1/2 at a low temperatures. We suggest two different methods of MQ NMR. One of them is based on the measurement of the dipolar energy. The other method uses an additional resonance ({\pi}/4)_{y}-pulse after the preparation period of the standard MQ NMR experiment in solids and allows one to measure the Zeeman energy. The both considered methods are sensitive to the contribution of remote spins in the interaction and to the spin system structure. The QS method is sensitive to the spin number in the molecule while the PS method gives very similar time dependencies of the intensities of MQ coherences for different spin numbers. It is shown that the use of the dipolar ordered initial state has the advantage of exciting the highest order MQ coherences in clusters of 4m identical spins, where m=1,2,3..., that is impossible to do with the standard MQ method. MQ NMR methods based on the dipolar ordered initial states at low temperatures complement the standard NMR spectroscopy for better studying structures and dynamic processes in solids.Comment: 12 pages, 4 figures will be published in Solid State Nuclear Magnetic Resonance (2013

Comparison of a Self-Limiting Transformer and a Transformer Type FCL with HTS Elements

A superconducting fault current limiter of the transformer type (inductive FCL) based on magnetic coupling between a superconducting element and a protected circuit has been investigated by many authors for various parameters and performances of a superconducting element. Another design of the device preventing high short-circuit currents is a self-limiting transformer combining the functions of a usual power transformer with the functions of a current limiter. In the presented work we compare the parameters, operation and application of these two devices. The operation of the devices is investigated experimentally on small models fabricated using the same superconducting element. The parameters of the full-scale devices are evaluated. It is shown that the requirements to superconducting elements are practically the same for both devices.Comment: submitted to Applied Superconductivity Conference 200

Thermal state of superconducting films on wide substrates

The stationary thermal state and propagation of a normal zone in a long superconducting film on a wide substrate are analyzed analytically. Expressions describing voltage-current characteristics and temperature-current dependence of the film are derived for the flux creep, flux flow and normal regions. It is shown how the flux creep influences the conditions of the thermal stability. In particular, it is found that the bistability of the thermal state can appear in this regime. Under the boiling crisis, the temperature-current dependence for a film differs markedly from that for a wire and is characterized by a smooth temperature increase with the current. A "mixed" regime is analyzed where the flux flow and normal states exist simultaneously with the boundary between them parallel to the film axis. Expressions for the propagation velocity of a normal zone along a narrow film are obtained which show that this velocity in films is sufficiently higher than in wires.Comment: 17 pages, 10 figures, submitted to "Cryogenics