Zvi Bar-Haim

Saturated Cores FCL—A New Approach

The saturated cores FCL exhibits several attractive technological advantages: inherent fail-safe and selectivity design, superconductivity is maintained during both nominal and fault states, the limiting process as well as the recovery after fault are passive and immediate, operation in limiting state is not time-limited, and the superconducting bias coil is made of wires available as commercial shelf-product. Despite these advantages, saturated cores FCL did not make it to commercial phase because of the large volume and heavy weight associated with its realization, a coupling problem between the AC and bias coils while in limiting state, and non-optimal limitation resulting from the presence of the bias field during fault. This work presents a novel, improved saturated cores FCL concept that overcomes the above difficulties and reopens the possibility for commercialization. Unique design topography reduces the cores volume and at the same time reduces the AC and DC magnetic coupling to about 2%. In addition, a control circuit, triggered by voltage drop across the FCL terminals, is added and disconnects the bias coil during a fault for increased limiting performances. All above-mentioned advantages of the saturated cores concept are maintained in this new design. First, a 4.2 kVA laboratory scale FCL has been designed built and studied proving the feasibility of the new design. Then, an up-scaled, 120 kVA model has been designed, built and tested at the testing laboratory of the Israel Electric Company. The prospective short current in the test bed was 5000 A, successfully limited to 2400 A. The 120 kVA model is a single phase FCL designed for 400 V, 300 A nominal conditions. Core losses and AC coils losses are 0.09% and 0.18%, respectively.

Calculated E-I characteristics of HTS pancakes and coils exposed to inhomogeneous magnetic fields

The upper limit of the operating current of LTS solenoids can be estimated as the coordinate of the crossing point of its load line with IC (B) line of the superconductor. For HTS coils this approach seems to underestimate the allowable operating current of the coil. A better approach is to obtain a full electric field distribution over the coil and to use it as the base for a more sophisticated coil design criteria. We developed an algorithm and a Matlab program for calculating distributions of the current density, magnetic field and electric field in HTS solenoids made of pancakes, considering the inhomogeneous current density distribution inside the anisotropic tape. I-V curves of several Bi-2223 coils are calculated and good agreeement of the calculated and measured critical currents, IC, and indexes, n, are attained. One can utilize the program in the coil design choosing his own criteria of coils critical current, e.g., 1) The average electric field 10−4 V/m over the coil, 2) The electric field 10−4 V/m at the weak point of the coil, 3) The energy dissipation in the entire coil, 4) Distribution of local energy dissipation.

Compact HTS cryogen-free magnet for magneto-optics research setup

Small working distances between the objective lenses and the inspected samples in magneto-optical (MO) setups limit the volume available for external magnetic field generation coils. Air or water-cooled copper coils are commonly used for field generation, however these coils usually make it possible to attain limited fields of the order of 10-50 mT, in continuous operating mode. HTS coils offer a unique and cost effective solution in such cases of limited space. A compact, 0.4 T, cryogen-free, HTS magnet has been designed, built and tested in a MO setup. The magnet is made of 3, 100 turns single pancakes, mounted on the second stage of a two-stage cryocooler. The HTS coil operates at about T=45 K and generates 0.4 T in a warm bore of 30 mm diameter. Minimization of metal components in the coil makes it possible to attain field ramp rates of 7.4 T/sec with a coil charging voltage of 20 V. Initial cool-down time of the magnet takes less than 5 hours. The magnet design and performances are described in detail and serve as an example for solutions that HTS technology may offer in limited space applications.

High-temperature superconducting magnet for use in saturated core FCL

A HTS magnet system used in a saturated core Fault Current Limiter (FCL) device is described. The superconducting magnet, operating in DC mode, is used in such FCL design for saturating the magnetic core and maintaining low device impedance under nominal conditions. The unique design of the FCL poses constrains on the DC HTS magnet. A model which meets all the necessary special requirements have been realized in a compact magnet design that is optimized for its electrical characteristics while minimizing its mass and volume. The coil, made of Bi-2223 tapes, has 50000 Ampere-turns required to maintain the core in a saturated state at nominal current in the limiting circuit. Unique, nonmagnetic cryostat made of Delrin was used. Cooling of the coil has been realized by two cold heads: one double-stage head that provides a cooling power of 6 W at 20 K and a single-stage head with a cooling capability of 40W at 70 K. This magnetic system has been successfully integrated and tested in a 120 kVA FCL model. The design, characteristics and tests of this magnetic system are described.