154 kV급 초전도 한류기의 핵심 절연부품 설계 및 성능 평가

Abstract

The electric power demand is expected to increase continuously because modern industry aims for acceleration and high efficiency which bring on economic development and living welfare. As a result, the total amount of electric power production and transmission increases which causes the growth of fault current. In South Korea, network power system is adopted in order to promote reliability and stability of electric power supply. When fault occurs at network power system, however, the impedance of system decreases and its aftermath, fault current extremely augments. Beside, the electric power equipment is increasing due to increment of electric power requirement which leads to explosive rise of fault current. As a result, the fault current gradually exceeds the interrupting capacity of substations. In order to solve such problem, one of the newly developed superconducting power apparatus, the superconducting fault current limiter (SFCL), has been paid much attention as an emerging solution.

Unlike conventional fault current limiter, SFCL does not need to be replaced even though the fault current increases. Moreover, SFCL has almost zero impedance so that the voltage drop does not occurs which eliminates the losses of the power system. Despite such advantages of SFCL, there are many problems to be solved in order to develop superconducting apparatuses. One of the unsolved problem is insulation design on cryogenic environment. The research on insulation design for SFCL is greatly lacking and some previous studies performed at advanced companies are disclosed to the public.

In this paper, the characteristics of insulation material for cryogenic environment was analyzed, and suitable design method for insulation components of 154 kV SFCL was established. In order to evaluate the insulation characteristics of liquid and gas nitrogen, which are known as optimum coolant for high-temperature SFCL, the experimental study on the electrical breakdown field strength was performed. From the result, standard electric field intensity for each coolant was evaluated to determine the clearance for cryogenic environment. And then, the design of electrical bushing and post insulator for SFCL application was progressed by means of the evaluated insulation characteristics.

The electrical bushing is generally divided as solid type and gas filled type. In order to determine an optimum type for cryogenic application, prototype bushings were developed. In addition, the comparative study was progressed utilizing the result of lightning impulse test and AC withstand voltage test. In case of post insulator, It should endure the weight of SFCL modules and secure the voltage withstand capability. In order to satisfy the above-mentioned requirements, the design of prototype model and its experimental studies were progressed by means of the lightning impulse test and AC withstand voltage test. From the result, the suitable design method for post insulator for 154 kV SFCL application was deduced. Until now, there is no significant standard for SFCL design. Therefore, test procedure was progressed utilizing IEC 60060 “”, IEC 60076 “” and IEC 60137 “”, which deal with the test of conventional high voltage power apparatus.

Consequences in this paper is summarized as follows.

First, the critical insulation design components for 154 kV SFCL was evaluated from the investigations on SFCL development and cryogenic insulation works. The main insulating component was deduced as follows:

· The insulation characteristics of liquid nitrogen and gas nitrogen, which are the coolant for SFCL. · Electrical bushing that is applicable to cryogenic environment. · Post insulator which can endure the weight of SFCL modules and secure the insulation performance between SFCL modules and cryostat.

Second, in order to verify the dielectric performance of liquid nitrogen and gas nitrogen, which were used for the insulation materials of SFCL, dielectric breakdown tests were performed. In addition, dielectric breakdown tests of GFRP, which has excellent mechanical and electrical characteristics in cryogenic environment, were also carried out. These experimental works had been conducted considering a variety of electrode structures and gap distances. From the experimental test results, electric field criterion was determined and this criterion is a main design factor for insulation design for SFCL.

Third, real scale bushings considering various structures including different composite solid and gas materials were made. These bushings were used for experimental tests to assure applicability of high voltage cryogenic environment. As a result, Prototype 154 kV bushing for cryogenic environment was successfully manufactured. In addition, dielectric breakdown tests of liquid nitrogen above 100 kV level could be successfully performed. In this research, insulation design technology for manufacture of 154 kV SFCL was developed. Manufactured 154 kV SFCL has been installed at KEPCO Go-Chang test center and electrical insulation performance test and evaluation of this was lastly passed. Moreover, insulation design technology, which is most important core factor, is also essential for developing the cryogenic electric power apparatuses such as superconducting cable, transformer and power storage system. Therefore, insulation design technology and experimental data obtained by this research will be widely used for development of cryogenic electric power apparatuses.