V. Meerovich

Superconducting FCL: design and application

Design, parameters, and application areas of a superconducting fault current limiter (FCL) are analyzed on the basis of the requirements of power systems. The comparison of resistive and inductive designs is carried out. An example of the effective application of FCLs in distribution substations is considered and the gain from the FCL installation is discussed. It is shown that an FCL not only limits a fault current but also increases the dynamic stability of the synchronous operation of electric machines. The calculation procedure of the parameters of an inductive FCL for a specific application case is described.

Performance of an inductive fault current limiter employing BSCCO superconducting cylinders

Inductive fault current limiters operating at high levels of short-circuit currents are plagued by appearance of overheated thermal domains in active superconducting elements. Excessive growth of thermal domains may lead to a fatal mechanical destruction of the superconducting element during a fault event. It has been determined that employment of superconductors with gradual dissipation onset controlled by flux relaxation processes can efficiently prevent local overheating. Operation of such elements, fabricated by melt cast technique, has been investigated experimentally in a small-scale open-core model of an inductive fault current limiter. The results of the experiments demonstrate the feasibility of application of superconducting cylinders having properties dominated by flux relaxation processes in inductive current limiters. The most important parameter of a superconducting element designated to operate in such devices is the rate of flux relaxation and its dependence on ac current amplitude. It has been found that ac losses associated with flux relaxation in the investigated cylinders allow for a reliable limiter operation at the nominal current level. Projection of the parameters of the investigated small-scale model to the full-scale device has been performed using the concept of physical modeling. The obtained results indicate that it is possible to build a full-scale device based on flux creep dissipation mechanisms for distribution networks.

High-Tc superconducting inductive current limiter for 1 kV/25A performance

The results of investigations of an inductive current limiting device prototype based on superconducting to normal state transition in 0.2 m o/d. BSCCO rings are discussed. Thermal processes in the ring were found to have an important influence on transient response characteristics of the limiter. In a marked difference to small scale devices, the quenching process in a medium scale current limiter is accompanied by an intense heating of the HTSC ring. Because of high losses and large thermal inertia, the superconducting ring remains in the normal state during entire limitation time and at least a few AC cycles pass before the superconducting state in the rings is resumed after a fault occurrence.<<ETX>>

Experimental investigation of current-limiting device model based on high-Tc superconductors

Abstract The model of an inductive current-limiting device consists of a copper coil and a high-T c superconducting ring which are placed on a ferrite core and are coupled magnetically. The rings were prepared by extrusion of the YBaCuO submicron precursor powder and organic binder, followed by sintering in oxygen atmosphere. The principle of the devices operation is based on a rapid rise of the devices inductance at the transition of the ring from the superconducting to the normal state. The results demonstrate that the device can reduce both the transient and the steady-state fault current significantly. The influence of thermal processes in the ring on the mode of device operation in the circuit is discussed.

Thermal regimes of HTS cylinders operating in devices for fault current limitation

We reveal obstacles related to the application of HTS cylinders in current limiting devices based on the superconducting–normal state transition. It is shown that, at the critical current density achieved presently in bulk materials, and especially in BSCCO-2212, the required thickness of the cylinder wall in a full-scale inductive device is several centimetres. A simple mathematical model of the operation of an inductive fault current limiter (FCL) is used to show that such cylinders cannot be cooled in an admissible time after a fault clearing and, hence, the inductive FCLs and current-limiting transformers employing BSCCO cylinders do not return to the normal operation in the time required. For the recovery even with a non-current pause in the circuit, cylinders are needed with a critical current density an order of magnitude higher than the existing one.

Three phase inductive HTS fault current limiter for the protection of a 12 kVA synchronous generator

The concept of high temperature superconducting (HTSC) mini power plant model is presented. An HTSC fault current limiter (FCL) for the protection of a generator unit was designed. The simulation and test results of a one phase overload case are shown. Duration tests to reveal the dependency of the limited current and the FCL voltage on the activation number were performed. A new representation of the sudden short circuit current and voltage are proposed.

Dynamics of entanglement in a one-dimensional Ising chain

The evolution of entanglement in a one-dimensional Ising chain is numerically studied under various initial conditions. We analyze two problems concerning the dynamics of entanglement: (i) generation of the entanglement from the pseudopure separable state and (ii) transportation of the entanglement from one end of the chain to the other. The model investigated is a one-dimensional Ising spin-1/2 chain with nearest-neighbor interactions placed in an external magnetic field and irradiated by a weak resonant transverse field. The possibility of selective initialization of partially entangled states is considered. It was shown that, in spite of the use of a model with direct interactions between the nearest neighbors, entanglement between remote spins is generated.

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.

Analytical approach to AC loss calculation in high-Tc superconductors

Abstract Using a linear spline approximation for the E – J characteristic of a superconductor and representing the solution in form of series, analytical expressions for AC losses have been obtained. The expression explains experimentally observed frequency and magnetic field dependencies of AC losses. Cases of complete and incomplete magnetic field penetration have been distinguished. AC losses per cycle decrease with increasing frequency in the case of incomplete penetration, the case relevant to thick slabs and low amplitude magnetic fields, while in thin slabs and large magnetic fields they increase with increasing frequency, the case of complete penetration. The analysis of the analytical solutions obtained has given a simple criterion for the applicability of the critical state model (CSM) to the calculations. This criterion involves characteristics of both superconductor and applied magnetic field. The physical meaning of the criterion in terms of ratios between the characteristic decay times of magnetic field energy in a superconductor and the period of applied magnetic field has been established. It has been shown that the analytical solutions can be applied for various forms of E – J characteristics by means of defining effective critical current density and flux flow resistivity.

Nuclear polarization and entanglement in spin systems

We investigated relationships between entanglement measures and the order parameter (nuclear polarization) in nuclear spin systems controlled by the nuclear magnetic resonance (NMR) technique. Since spin polarization can be easily manipulated by the NMR technique, experimentalists are presented with an opportunity to study the dynamic properties of entanglement, i.e., the creation and evolution of entangled states. Our approach may constitute the basis for researching the relations between the entanglement measures and measurable parameters of order in other quantum systems.