Woo-Seok Kim

Design of HTS Magnets for a 600 kJ SMES

Development of a 600 kJ Superconducting Magnetic Energy Storage (SMES) system is in progress by Korean Electric Research Institute (KERI). High-temperature superconducting (HTS) wires are going to be used for the winding of the system. The design of the HTS windings for the system is presented in this paper. We considered BSCCO-2223 wire for the HTS windings. The operating temperature of the winding was decided to be 20 K which will be accomplished by conduction cooling method using cryo-coolers. Auto-Tuning Niching Genetic Algorithm was adopted for an optimization method of the HTS magnets in the SMES system. The objective function of the optimal process was minimizing the total amount of the HTS wire. We also estimated the AC loss which can be generated in the discharge period. These HTS windings are going to be applied to the SMES system whose purpose is the stabilization of the power grid

Operation and performance analyses of 350 and 700 MHz low-/high-temperature superconductor nuclear magnetic resonance magnets: A march toward operating frequencies above 1 GHz

Since 2000, a three-phase program with a final goal to complete a 1 GHz high-resolution low-/high-temperature superconductor (LTS/HTS) nuclear magnetic resonance (NMR) magnet has been conducted at the Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology (MIT). In a LTS/HTS magnet assembly, a HTS insert is placed in the cold bore of a LTS background magnet. To date, two LTS/HTS magnets have been designed, constructed, and tested: a 350 MHz (LH350) in phase 1 and a 700 MHz (LH700) in phase 2. The program’s target has recently been upgraded from the original goal of 1 GHz to a new goal of 1.3 GHz. In this paper, we present extensive performance analyses of the two LTS/HTS NMR magnets. Spatial homogeneity and temporal stability of LH350 and LH700, examined with harmonic analysis, and four key issues that became evident in the operation of these two magnets are discussed: (1) field constant reduction, (2) “large” residual Z1 gradient and its temporal decay, (3) large one-periodic tesseral fi...

Normal Zone Propagation in 2-Dimensional YBCO Winding Pack Models

This paper presents the results of a study of 2-dimensional normal zone propagation (NZP) in winding pack models comprised of 12 straight YBCO tapes stacked with wet, i.e., epoxied, insulating spacers. In each winding pack, the YBCO tapes were connected electrically in series in such a way that a transport current through the YBCO tapes zigzagged from one end of the winding pack to the other end of the winding pack. An insulated heater strip placed at the middle of the stack was energized to trigger a quench in YBCO tapes with a combination of heater taps which permits the length of the heated section to be varied for 1- or 2-dimensional NZP. Voltage taps of various lengths were placed within the winding packs to measure the transverse and longitudinal NZP speeds. Measurements with wet-spacers in which a large contact thermal resistance at each YBCO tape and spacer interface is chiefly responsible for very slow transverse NZP velocities, effectively eliminate the interface thermal resistance, resulting in faster transverse NZP speeds. Numerical simulation that neglects interface thermal resistance agrees quite well with experiment.

An LTS/HTS NMR Magnet Operated in the Range 600–700 MHz

As part of our 3-phase program to develop a high resolution 1 GHz LTS/HTS NMR magnet, a system composed of a 600 MHz LTS background NMR magnet and an HTS insert operating in the range 75 to 100 MHz has been built and tested. Operated at 4.2 K, immersed in a bath of liquid helium, the Phase 2 LTS/HTS magnet, the LTS magnet in persistent mode and the HTS insert powered by a stable supply, generated an NMR frequency of 692.2 MHz (16.26 T): the LTS magnet, with 234.4 A, at 588.7 MHz (13.83 T); and the HTS insert, with 115.95 A, at 103.5 MHz (2.43 T). This paper presents results of the magnet operation during test geared to the design of room temperature shim coils and ferromagnetic tiles. Magnetization effects of the HTS insert on field distribution are also reported.

Characteristic tests of a 1 MVA single phase HTS transformer with concentrically arranged windings

A 1 MV A single phase high temperature superconducting (HTS) transformer was manufactured and tested. The rated voltages of primary and secondary of the HTS transformer are 22.9 kV and 6.6 kV respectively. BSCCO-2223 HTS tape was used for HTS windings of 1 MV A HTS transformer. In order to reduce AC loss generated in the HTS winding, the type of concentric arrangement winding was adopted to a 1 MV A HTS transformer. Single HTS tape for primary windings and 4 parallel HTS tapes for secondary windings were used considering the each rated current of the HTS transformer. A core of HTS transformer was fabricated as a shell type core made of laminated silicon steel plate. And a GFRP cryostat with a room temperature bore was also manufactured. The characteristic tests of 1 MV A HTS transformer were performed such as no load test, short circuit test and several insulation tests at 65 K using sub-cooled liquid nitrogen. From the results of tests, the validity of design of HTS transformer was ascertained.

Development of a superconducting linear synchronous motor

High Tc superconducting (HTS) wires were applied to primary coils of a small size permanent magnet linear synchronous motor. Three-phase racetrack shaped test coils were fabricated and tested. The HTS coils were cooled down to 33 K by using a GM-cryocooler in order to overcome the decrease of the critical current density due to high intensity of magnetic field in the coil. A vacuum chamber and thermal shield for reduction of the radiation and the convection heat penetrations were fabricated. The parameters of the linear motor were calculated by using finite element method, and verified by experiment. It is possible to achieve large thrust force compared with that of a conventional machine because large current can flow in HTS primary coil.

Normal Zone Propagation in YBCO Winding Pack Models

This paper presents the results of a study, experimental and simulation, of normal zone propagation (NZP) in winding pack models comprised of several lengths of YBCO tapes, each sandwiched by insulating spacers. A heater strip extending over the entire length of, and placed at the midpoint of, each winding pack model was energized over the entire length to trigger a quench in the middle YBCO tape. In each winding pack model, the YBCO tapes were connected electrically in series in such a way that a transport current through the YBCO tapes zigzagged from the YBCO tape at one side of the winding pack model to that at the other side of the winding pack model. The transverse speed of normal zone propagations through the different insulating spacers were measured by experiments and estimated by simulations.

Stress Analysis of HTS Magnet for a 600 kJ SMES

Auto tuning niching genetic algorithm was used to design optimal HTS magnets for the 600 kJ class SMES system under several design constraint conditions. Constraint conditions were operation loss of magnet (less than 2 W), inductance of magnet (less than 24 H), the number of double pancake coils (about 10 DPCs), the number of turns of DPC (less than 300 turns), outer diameter of DPC (close to 800 mm) and total length of HTS wire in a DPC (less than 500 m). As a result of optimum design, we obtained design parameters for the 600 kJ SMES magnet according to two operating currents, 360 A and 370 A. However, even though the HTS magnet was designed optimally in respect to the electromagnetics, consideration of mechanical integrity due to the stress by Lorentz force must not be neglected for the stable operation of the SMES system. Therefore, we developed a program, through the finite element method (FEM), for stress analysis due to Lorentz force in operation of the SMES system. In this paper, the stresses (radial and hoop stress) imposed on the designed HTS magnets were calculated by the program, and the results of stress analysis were discussed.

Design of a 1 MVA high T/sub c/ superconducting transformer

A 1 MVA transformer using BSCCO-2223 high T/sub c/ superconducting (HTS) tapes was designed. The rated voltages of each sides of the transformer for primary and secondary are 22.9 kV and 6.6 kV respectively. Double pancake HTS windings, which have advantages of insulations and distribution of high voltage, were adopted. Four HTS tapes were wound in parallel for the windings of low voltage side. Each winding was composed of several double pancake windings and four parallel conductors of secondary winding were transposed in order to distribute the currents equally in each conductor. The core of the transformer was designed as a shell type core made of laminated silicon steel plate and the core is separated from the windings by a cryostat with a room temperature bore. Configuration of the cryostat made of nonmagnetic and nonconducting material and a liquid nitrogen sub-cooling system were used in order to maintain the coolants temperature of 65 K.

Layout Synthesis and Loop Parameter Optimization of a Low-Jitter All-Digital Pixel Clock Generator

This paper presents a dual-loop ADPLL suitable for video pixel clock generation. The dual-loop architecture where a fast subloop works as a DCO inside a main loop is chosen since it can efficiently suppress the ring oscillator phase noise. In order to satisfy a wide tuning range and fine resolution requirements in the pixel clock PLL, the TDC and DCO with two-step and bottom-up control are employed. An s-domain model is utilized to tune the loop parameters so that the dual-loop PLL has the optimum jitter performance. The measurement results match well with the proposed s-domain model and clearly show the effectiveness of the dual loop in suppressing the phase noise of the DCO. We propose a new cell-based layout technique to avoid performance degradation during automatic placement and routing, which is compatible with the conventional digital design environment. A chip that occupies 0.032 mm2 has been fabricated in the 28-nm CMOS technology. The synthesized DCO shows 1.7-LSB DNL which is close to a full-custom layout. The rms integrated jitter is only 15 ps at 250 MHz, even though PLL operates at the extremely low input frequency of 100 kHz. Power consumption is 3.1 mW at 250 MHz with a 1.0-V supply.