Chanjoo Lee

Development of a 13.2 kV/630 A (8.3 MVA) High Temperature Superconducting Fault Current Limiter

This paper deals with fabrication and development of a high temperature superconducting (HTS) fault current limiter (FCL) based on YBCO coated conductor (CC) wire for distribution systems. The capacity of the developed HTS FCL is 8.3 MVA and its rated voltage is 13.2 kV which corresponds to a three-phase power equipment voltage class of 22.9 kV. Tests of the developed prototype HTS FCL were conducted at Korea Electrotechnology Research Institute (KERI) accredited as a testing laboratory by the Korea Laboratory Accreditation Scheme (KOLAS). A short-circuit test and an AC dielectric withstand voltage test for the HTS FCL were conducted under sub-cooled liquid nitrogen (LN2 ) conditions of 3 bar and 65 K. The magnitude of an asymmetric short- circuit current without FCL reached 30 kApeak in a short-circuit test. The superconducting coil quenched instantaneously after the fault, and the magnitude of the fault current was limited to 3.6 kApeak within quarter cycle by the developed resistance of the superconducting coil. An AC dielectric withstand voltage test was performed, and the HTS FCL successfully withstood 143 kV for 1 minute. Also, it was found that there was no electrical or mechanical damage on the superconducting coil after the tests.

Stability analysis of a power system with superconducting fault current limiter installed

As a process of developing high temperature superconducting fault current limiter (SFCL), the stability of a power system in which SFCLs were installed was analyzed. For the investigation into the effect of SFCLs to a power system, we have proposed a simple model power system that had SFCL circuits. The modeling parameters of SFCL are obtained by experiment of a prototype SFCL, which is 440 V class and a shielding type model. This electric circuit was solved for transient performance by numerical methods. In case the SFCLs are installed in a power system, it can effectively protect synchronization both in a symmetrical three-phase fault and a single-phase line to ground fault by maintaining synchronism of the synchronous machines for a long time. By this analysis, we found a quantitative effect of SFCLs to a power system. Limiting fault currents means not only an improvement of circuit breaker abilities but also a protection of synchronism. So its synchronism protection property must be considered for a design of superconducting fault current limiters.

Thermal and Electrical Analysis of Coated Conductor Under AC Over-Current

In order to design a high temperature superconducting (HTS) winding for the fault current limiter (FCL), the resistance and the temperature of the winding should be calculated quantitatively under the over-current caused by fault condition. In this paper, a transient analysis is performed to estimate the resistance development and the temperature rise of coated conductor (CC) under AC over-current. A one-dimensional thermal conduction model with an electrical circuit model is developed for the solenoid coil configuration at 65 K cooling condition. All the composite materials except the buffer layer in CC are considered in the model. Two kinds of stabilizer materials (copper/stainless steel) are considered to investigate the current limitation of CC. The simulation results are compared with the experimental data of the commercial CC. The effect of Ag and solder layer on the simulation result are revealed for CC.

Development of 220 V/300 A Class Non-Inductive Winding Type Fault Current Limiter Using 2G HTS Wire

As a part of the 21st Century Frontier R&D Program in Korea being performed from 2004, a non-inductive winding type superconducting fault current limiter (SFCL) is being developed. The target of the second year in phase II of the program is to develop a 220 V/300 A class non-inductive winding type SFCL as a prototype for a 13.2 kV/630 A class, the final goal of phase II. This SFCL has three solenoid type non-inductively wound coils in series using a 2G high temperature superconducting (HTS) wire and it was tested in sub-cooled nitrogen of 65 K, 1 atm. A coil which is composed of four parallel windings in a bobbin and winding directions are opposite to have non-inductive characteristics. Three coils were connected in series and the total length of 108 m of 2G HTS wire was used. Short-circuit tests were performed at applied voltage of 220 V and the SFCL limited the fault current to a few kA extents at the tests. Recovery time of the SFCL was measured after short-circuit tests.

Manufacture and Test of Small-Scale Superconducting Fault Current Limiter by Using the Bifilar Winding of Coated Conductor

Superconducting fault current limiters (SFCLs) have been developed by many research groups. However, there is no standard for current limiting device. Recently, YBCO coated conductor (C.C.) which is named as 2nd-generation wire has been developed rapidly. YBCO C.C. has many advantages for applying to fault current limiting material. In this paper, a bifilar winding type SFCL was manufactured using YBCO C.C. The bifilar coil was wound as pancake type, and the length of C.C. tape used was 8 m. The short-circuit test of the SFCL was performed successfully rated on 30V/80A. The SFCL had a very low impedance in normal operation and limited the fault current effectively when a fault occurred. From the result, it could be confirmed that the bifilar winding type FCL using YBCO C.C. is feasible. Large-scale SFCL using C.C. should be developed in the future

Visualization Study on Boiling of Nitrogen During Quench for Fault Current Limiter Applications

For the development of a 13.2 kV/630 A bifilar winding type high-Tc superconducting fault current limiter (SFCL), a visualization study has been conducted to clarify boiling characteristics during the quench of the high-Tc SFCL. A series of experiments was carried out using a stainless steel strip on a G10 plate as a heating element to simulate the quench state of the high-Tc SFCL. A pulse of DC power input was applied to the strip in saturated and subcooled liquid nitrogen. The magnitude of the heat generation was varied from 10 W/cm2 to 170 W/cm2 and the period of the heat impulse was fixed at approximately 100 ms. Bubble behavior was observed by a high-speed camera through view ports of a cryostat. The boiling phenomena, the temperature rising of the strip and the recovery time were compared for different power densities and liquid nitrogen operating conditions. The bubble suppression was clearly observed with respect to the degree of subcooling

Determination of Maximum Permissible Temperature Rise Considering Repetitive Over-Current Characteristics of YBCO Coated Conductors

Here we suggest stable maximum temperature criteria of YBCO coated conductors (CCs) considering degradation of YBCO by repetitive exposure to excessive temperature rise. Stable maximum permissible temperature was experimentally determined as the maximum temperature at which CC does not suffer from critical current degradation or burnout. By comparing various over-current characteristics with short-circuit characteristics in regards to the pattern of generated resistance and joule heating flux, maximum permissible temperature rise of YBCO CC in superconducting fault current limiter (SFCL) can be determined as 400 K. Our research is expected to provide rudimental basis for the temperature design of various superconducting power applications including SFCL.

Test of DC reactor type fault current limiter using SMES magnet for optimal design

This paper deals with the operational characteristics of a three-phase DC reactor type fault current limiter (FCL) at a short-circuit test. This type FCL consists of transformers,. diodes, and a superconducting coil. In this paper, a superconducting coil, low-Tc SMES magnet, is made of Nb-Ti and the power system of the experimental circuit is 400 V/7 A class. This is a preliminary step to develop its faculties for applications to high voltage transmission line. As the results of the experiment, the values are referred to the limitation rate about 77% and 90% when the turns ratio of transformer was 1 and 2, respectively.

Electrical Breakdown Characteristics of Superconducting Magnet System in Sub-Cooled Liquid Nitrogen

Dielectric characteristics of gaseous helium (GHe) injected into the cooling system to make sub-cooled nitrogen condition with constant pressure is found to be extraordinarily weak in dielectric strength by H. Mitsuii (1998). In high voltage superconducting machines using the sub-cooled nitrogen cooling system, the current lead part in GHe environment could be electrical weak points rather than the superconducting magnet part in liquid nitrogen (LN2). In order to enhance dielectric characteristics of current lead part, three methods are suggested in this paper. The first one is enclosing the current leads with solid dielectric material and the second is exposing the current leads to high vacuum, and the last is topping up the magnet system with dielectric materials such as Glass Bubbles (GB) which is usually used as heat insulator. In addition, the extinction time of bubbles caused by the electrical breakdown is measured to determine the suitable pressure of sub-cooled nitrogen system. It is found that the characteristics of bubble extinction in LN2 are drastically enhanced over 1 bar.

Dielectric Tests of Superconducting Coils for Development of High Voltage Superconducting Machines

This paper deals with the dielectric breakdown characteristics of superconducting coils for development of high voltage superconducting machines such as superconducting fault current limiter (SFCL) and superconducting transformer. The electrode systems are simulated to investigate the dielectric breakdown characteristics of solenoid and pancake coil. Dielectric breakdown tests are carried out on several basic electrode systems: needle-plane, sphere-plane, rod-plane, cutting bar-plane, and rounded bar-plane electrodes. These results could be applied to design the high voltage superconducting magnet system. Dielectric tests of turn-turn and layer-layer in solenoid coil are performed according to the groove depth, pitch length, and the distance between two layers. Dielectric tests of turn-turn with various insulation materials are also performed in pancake coil. In addition, the deterioration of high temperature superconducting (HTS) tapes and coils due to the electrical breakdown are investigated experimentally. It is proven that the HTS tape and coil could be robust to the electrical breakdown by the laminated metal.