Si-Dole Hwang

Performance Test of 100 m HTS Power Cable System

In order to meet the needs of economical and stable power supply and high current density cable with compact size, several countries carry out researches on high temperature superconducting (HTS) cables. Korea Electric Power Corporation (KEPCO) established the HTS cable test center and installed 22.9 kV, 1250 A, 50 MVA, 100 m class HTS cable system to investigate the performances and application possibility in real power grids. The HTS cable system went through seven cool down and warm-up cycles and also many kinds of performance tests for 3 years. Rated voltage-current tests, over voltage tests, and measurements of heat loss and AC loss with various currents have been examined. Moreover, DC critical current tests were performed for checking conductor degradation before warming up or after cooling down. Through the performance tests of the HTS cable system, the overall efficiency and loss factors are estimated and compared with conventional power cables in this paper. We conclude the present status of HTS cable system and application possibility in the real power grid.

The Results of Installation and Preliminary Test of 22.9 kV, 50 MVA, 100 m Class HTS Power Cable System at KEPCO

As high temperature superconducting (HTS) power cables have some merits over conventional cables, several demonstration projects on the HTS cable system are presently under way around the world. Korea Electric Power Corporation (KEPCO) also initiated an HTS cable project in 2002 with the Korean governments support. A three phase 100 m HTS cable system with a capacity of 50 MVA has been installed at Gochang test yard, located in Chonnbuk province, Korea. The HTS cable system is composed of a 100 m-long cable, two terminations and a cooling system. The rated current is 1,250 Arms and the rated voltage is 22.9 kV considering compatibility with the conventional power distribution system in Korea. Main purposes of this project are to verify the performance of an HTS cable system and to evaluate the potential of the HTS cable system from the viewpoint of power utilities. The real grid application of the HTS cable system requires the demonstration of system reliability, accumulated operation experiences, and it has to meet the practical needs of the utilities. In such a meaning, this project provides various challenges for KEPCO, and the feedback will be delivered to cable manufacturers. This user initiative test will facilitate the introduction of HTS cable systems into a real grid network. The installation process of the HTS cable system and some results of the preliminary test were presented in this paper.

Installation and Power Grid Demonstration of a 22.9 kV, 50 MVA, High Temperature Superconducting Cable for KEPCO

Korea Electric Power Corporation (KEPCO) has a great interest on the application of HTS cable to its network to relieve power congestion and enhance the reliability of the power system. Down through the years from 2005 to 2010, the 22.9 kV, 50 MVA HTS cable has been demonstrated by field test, showing affirmative results. With the financial support of Ministry of Knowledge and Economy (MKE), the new project to install and operate the HTS cable in electrical network of Korea was launched in 2008. An HTS cable with the 22.9 kV, 50 MVA, and 410 m length was installed in the KECPO grid in the end of 2010. After the integration of the HTS cable system, the commissioning items of the DC withstand voltage test, DC critical current test, and heat loss measurements were conducted in accordance with the Korean industrial standard, and self-defined specifications. The HTS cable was energized on the 154 kV Icheon substation of KEPCO on August 19th, 2011. The details of the project description, onsite installation, and commissioning tests are presented in this paper.

Electrical behavior of Bi-2223/Ag tapes under applied alternating over-currents

As a study for the protection of HTS power machines operated under AC conditions, the electrical behavior of Bi-2223/Ag tapes with applied alternating over-currents was investigated. Using the HTS tape of 57 A I/sub c/ and 106 K T/sub c/, we measured the V-I curves for the over-critical current region from 150 A/sub p/ to 507 A/sub p/. From the experimental results, 3 matters were investigated. Firstly, using the resistance increase data obtained from the curves, quench development was analyzed as a function of the amplitude and duration of the applied currents. Secondly, to understand these phenomena in detail, we investigated the current distribution between the superconductor and the metal sheath in Bi-2223/Ag tape, applying the equivalent equations and data of V-I curves. Finally, we confirmed the relationship between the resistance and temperature generated by the over-critical current experimentally. As a result, we could find the optimal operating condition considering the protection of the HTS tape against the alternating over-current.

Quench characteristics of HTS tapes with alternating currents above their critical currents

In this study, the dependence of quench characteristics of HTS tapes on the amplitude of current and duration time was investigated by applying alternating current (AC) that is several times over the critical currents of HTS tapes. When the applied AC/sub peak/ was up to three times of critical current, the joule heat was not cooled down by LN/sub 2/ completely. However, when the over-current reached four times of critical current, the superconducting region began to decrease gradually and the total resistance of HTS tapes increased rapidly. Finally, we analyzed the quench development using voltage-current characteristics and examined the influence of pitch angles on resistance increase.

AC Loss Measurement of a Short HTS Cable With Shield by Electrical Method

In order to electrically investigate the AC losses of the conductor and the shield in an actual HTS cable with shield, we attached two voltage-leads to the conductor and shield of a short cable which is the same as the Korea Electric Power Corporation (KEPCO) HTS cable under the field test, and examined it. The result shows that the polarities of the loss voltages measured from both voltage-leads are opposite. The ratio (1.6) between the losses for different transport periods measured from the conductor-lead is quite large. It is considered from these results that the conductor loss is the sum of the losses measured from the conductor-lead and shield-lead. And this conductor loss corresponds well with the numerical result too. Moreover, the total loss of the short cable measured from the proposed electrical method at 77 K was compared with that of the KEPCO cable by calorimetry at 66.6 K through our numerical model. The difference between the measured losses and numerical ones is almost the same. This means that the shield loss is measured from the shield-lead, and the conductor loss from both voltage-leads.

Hybrid Cooling System Installation for the KEPCO HTS Power Cable

In Korea, the KEPCO (Korea Electric Power Corporation) HTS power cable system has been successfully operated for evaluating the technical and economic feasibility as well as accumulating the operation experience since 2006. A cooling system composed of a liquid nitrogen decompression cooling unit and a closed-circulation system of subcooled liquid nitrogen at the KEPCOs Gochang power testing center. As the next step, we adjusted the cooling system for the purpose of testing a different cooling system using a cryocooler unit with cooling capacity of 4 kW at 77 K in the existing cooling system. It was designed to be used as a hybrid cooling system. In other words, it has as an each system of cryocooler unit or decompression pump unit. In this paper, the performance test results are summarized and the outline of the hybrid cooling system is introduced.

Application of Resistive Type SFCL to Protect Bi-2223/Ag Tape Against AC Over-Currents

Based on the electrical behavior of Bi-2223/Ag tape in transient state, the possibility of SFCL application was investigated as a fundamental study for the protection of HTS power apparatus. For the experiments, the Bi-2223/Ag tape having the critical current of 56 A and the resistive type SFCL fabricated using three elements of YBCO thin films were examined. When the applied current exceeds the rated current of the SFCL, the current was limited to sufficiently low level, however, higher than 40 Vrms was not allowed considering power density of SFCL. On the other hand, as a method to increase the power density, a shunt resister across the SFCL was adopted to increase the allowed power density. In this case, the results showed that the over current was limited below the safe range for HTS tape and the SFCL was safe, although 100 Vrms was applied to the circuit. In conclusion, the switching power capacity of a SFCL could be increased by using a shunt resistor, and HTS tape could be protected safely within the quench margin

Cryogenic System for 80-kV DC HTS Cable in the KEPCO Power Grid

DC HTS cable has advantages in the absence of ac loss. Therefore, the Korea Electric Power Corporation (KEPCO) has started a project of operating and manufacturing technology for applying an 80-kV 500-MW 500-m-long dc HTS cable to the commercial power grid since 2011. LS Cable Ltd. has joined this project for designing and manufacturing an HTS power cable, and KEPCO has taken on cryogenic system and real power grid operation. The cryogenic system for an 80-kV dc HTS cable is composed mainly of Stirling-type cryocoolers. The Stirling cryocooler has been examined and verified by real grid operation in Icheon substation, and we therefore became assured of reliability for the cooling HTS cable system. We present the cryogenic system design, installation, and some results of preliminary tests in this paper.

Loss Test for a 5-m YBCO Cable Sample of the 22.9-kV KEPCO System Under Grid Operation

We have operated the 22.9-kV high temperature superconductor cable system installed at Icheon Substation of Korea Electric Power Corporation (KEPCO), which consists of a conductor and a shield using YBCO-NiW wires. We prepared the same 5-m cable sample as the Icheon Substation in order to understand its ac loss characteristic. Especially, the contact resistance between copper current leads and YBCO wires was minimized through the increased soldering surface between them. Besides, four voltage leads are attached to both the conductor and the shield with an equal angle of 90°. Under ac transport current, which was supplied to the conductor and shield in opposite direction, losses of the 5-m sample were tested for various contact positions of voltage leads, frequencies, and transport periods. The result shows that the measured ac losses for various contact positions in the conductor and shield are the same. However, their loss exponents are very different. More details will be presented in this work.