Woo-Ju Shin

Validity Analysis on the Positioning of Superconducting Fault Current Limiter in Neighboring AC and DC Microgrid

In a smart grid, various kinds of distributed generation (DG) sources could be connected into the main power grid in order to enhance the reliability of the power system. The combination of ac and dc distribution grid are also considered for the efficient connection of renewable power resources. In this case, one of the critical problems due to these integrations is the excessive increase in the fault current because of the presence of DG within the smart grid. In order to protect the smart grid from increasing fault current, a superconducting fault current limiter (SFCL) could be applied, which has negligible power loss and capability to limit initial fault currents effectively. This paper presents feasibility analysis results of the positioning of the SFCL and its effects on reducing fault current in a smart grid having ac and dc microgrid. The detailed power system was implemented with a microgrid having wind farm and low voltage dc grid connected with a photovoltaic farm. Transient analyses were performed for the worst case faults with the different SFCL arrangements. The strategic location of SFCL in the power grid, which could limit fault currents and has no negative effect on the DG sources, was found to be the connection point of integration of the each DG sources in the ac and dc microgrid.

Breakdown Characteristics of Liquefied

High voltage cryogenic insulation issues need to be addressed in order to promote the commercialization of high temperature superconducting (HTS) equipment. One of the critical components for superconducting devices is the bushing whose role is to safely supply high current to the device. Due to a steep temperature gradient, commercial bushings which have been insulated with SF6 gas could not be directly applied to cryogenic equipment due to liquefaction of SF6 in the cryogenic environment; therefore, alternative suitable structure and insulation methods should be developed. As a fundamental step in the development of the optimum bushings for HTS devices, the breakdown characteristics of liquid nitrogen mixed with liquefied insulating gases such as N2, SF6 and CF4 have been investigated. In particular, we noted the insulation characteristics of CF4 gas whose liquefication temperature is much higher than SF6 gas. Thus, in order to investigate the possibility of substituting CF4 gas for SF6 gas for the bushings of HTS electrical equipment, impulse tests, AC withstanding voltage tests, and partial discharge (PD) tests have been performed. As a result of these tests, it was shown that mixtures of liquefied insulating gases have a much higher breakdown voltage compared to pure liquid nitrogen. Especially in a cryogenic environment, the usage of SF6 gas should be evaluated due to freezing effects. On the other hand, CF4 gas has shown excellent insulation properties even in a cryogenic environment and could be utilized as an insulation gas for high voltage bushings of HTS electrical equipment.

{\rm SF}_{6}

Since the discovery of high-temperature superconductor (HTS), liquid nitrogen(LN2) has not been only utilized as a coolant of superconducting electric equipment but also as an insulation material in cryogenic environment due to its dielectric performance. It also has a lot of advantages over other cryogenic liquid such as less expense and harmless substance, thus it has been widely used in the development of superconducting devices. Up to now, a lot of research works dealing with the breakdown characteristics of LN2 for distribution-class superconducting devices have been presented worldwide but, few research works about breakdown characteristics of liquid nitrogen in extra high voltage class have been reported due to the limitation of cryogenic test facilities in extra high voltage (EHV) class. In order to study the cryogenic EHV insulation technologies, we have built the cryogenic dielectric test facilities including a fiber reinforced plastic (FRP) big cryostat with cryogenic bushing, a 400 kV AC overvoltage and a 1.6 MV lightning impulse test systems. Using these facilities, we focused on the breakdown characteristics of liquid nitrogen in EHV level which is rather different comparing to the distribution level. With real scale big cryostat, AC overvoltage test and impulse tests have been performed. From the test results, the breakdown characteristics of liquid nitrogen in EHV were suggested. And these test results could be used as basic insulation design data to develop transmission-class superconducting electric equipment.

and

One of the critical matters yet to be solved for commercial applications of extra high voltage superconducting devices is the optimum design and development of high voltage cryogenic bushings which could withstand both severe insulation requirements and a steep temperature gradient due to the cryogenic environment. Neither conventional extra high voltage bushings insulated by SF6 gas nor composite materials are directly applicable to cryogenic bushings due to an extremely low temperature environment. In order to obtain suitable dielectric performance of bushings in the cryogenic environment, we focused on an alternative insulation gas instead of SF6 such as CF4, which shows excellent dielectric performance under extremely low temperatures, and also on the optimum design of cryogenic bushings, which have a longer creepage distance compared to conventional bushings. In this paper, design factors of cryogenic bushings were discussed, and test results of 60 and 100 kV extra high voltage prototype bushings were discussed in detail. Consequently, it was possible to obtain satisfactory results to verify the insulation level of newly designed extra high voltage cryogenic prototype bushings for superconducting electric power applications.

{\rm CF}_{4}

Due to ac loss in superconducting materials, high cryogenic costs are inevitable when superconducting devices are operated in ac power networks. Thus, dc electric power networks would be regarded as a better choice for the operation of superconducting devices. In order to develop superconducting devices for a dc network, the dc insulation characteristics, which are much different from the ac insulation characteristics, should be clarified. In this paper, in order to investigate the dc insulation characteristics of polypropylene laminated paper (PPLP), which is generally used for dc superconducting cable, a dc breakdown test and a dc electric field analysis were performed. For the dc breakdown test, specimens with three layers of PPLP with one butt gap were fabricated. In order to reveal the breakdown characteristic of PPLP, a dc electric fields calculation in the media at the moment of breakdown was performed considering capacitive and resistive field distributions. Consequently, the capacitive electric field and resistive electric field distributions were determined using dc field analysis techniques and it was found that the butt gap edge is affected enough by the high field strength to cause the breakdown. Furthermore, it was deduced that the butt gap edge acted as a triple-junction point which causes the breakdown.

Gases in Liquid Nitrogen for High Voltage Bushings in a Cryogenic Environment

Cryogenic dielectric insulation skills play a significant role in the development of superconducting electric equipment for transmission and distribution electric network. Nowadays, newly developed nano-composites have shown enhanced electrical, thermal, and mechanical properties of polymer insulation materials. And the application of nano-composites in high voltage power systems could be implemented in the near future. Among the various nano-composites, epoxy nano-composites have been paid much attention as a new insulating material for high voltage insulation. In this paper, we presented experimental results of epoxy nanocomposites in liquid nitrogen and determined the possible applications of nano-composites as insulating material for superconducting equipment. In order to determine their dielectric breakdown properties in liquid nitrogen, various kinds of epoxy based nano-composites have been made by mixing SiO2, Al2O3, TiO2 fillers, respectively. AC withstand voltage test and partial discharge (PD) inception voltage tests have been performed to verify the insulation breakdown characteristics of nano-composites in cryogenic environment. Consequently, it was deduced that the breakdown strength of epoxy nano-composites have been improved compared to epoxy with micro-fillers in cryogenic environment. And epoxy nano-composites immersed in liquid nitrogen showed a similar breakdown voltage characteristic when compared with that of insulating oil. In addition, the effects of nano-composites have been varied according to the concentration of nano-fillers and their content of fillers.

Breakdown characteristics of liquid nitrogen for transmission-class superconducting electric equipment

Recently, various high temperature superconducting (HTS) power applications have been developed and prepared for field tests and commercial applications. Comparing to conventional power applications, it could offer several advantages such as reduced size and weight, high efficiency, decreased losses, no oil, nonflammable and decrease of CO2 emissions. Besides overload operation is possible with no loss of lifetime. For HTS power applications in low temperature and high voltage environments, partial discharge (PD) measurements in cryogenic dielectric materials of HTS power applications are very important because PD was regarded as primary source for ageing and breakdown of cryogenic materials. One of the diagnostic methods for safety of the power components, the detection of PD taking place inside the apparatus has been widely investigated. The first method, phase resolved partial discharge (PRPD) Analysis was developed in the early 1970s taking the phase information of the applied AC voltage into account. We also proposed a pattern analysis method named chaotic analysis of PD(CAPD) for PDs occurred in liquid nitrogen, considering three normalized parameters obtained from the values between two consecutive PD pulses: amplitude difference (Pt), occurring time difference (Tt) and correlation between Tt and Pt. This pattern analysis method can identify the type of defects by means of PD pattern classification without employing the phase information of the applied voltage signal. For the experimental investigation, three artificial defects have been fabricated considering possible defects formed during the manufacturing process of HTS power applications: turn to turn insulation, floating particle and protrusion. And PD signals originated from these artificial defects are measured and analyzed by means of CAPD. Throughout this work, it seems that the correlation between the consecutive PD pulses, depending on the nature PDs, could be clarified by CAPD.

Experimental Design and Test of 100 kV Extra High Voltage Prototype Bushing With

High temperature superconducting devices utilize liquid nitrogen (LN2) both for cryogenic liquids and insulating media. Regarding the insulating performance of LN2, it shows excellent characteristics comparable to insulating oil which is widely used for conventional electric equipment. But, detailed complex insulating structure in LN2 composed of cryogenic liquids, solid insulating material and conductor material has not been fully considered yet. In order to commercialize superconducting devices, insulating characteristics of LN2 should be analysed and confirmed to the level of insulating oil. In this work, we focused on the breakdown characteristics of LN2 especially for different electrode materials. Aluminum, Stainless steel, and Copper electrode systems were fabricated and tested in LN2 to verify uniform and non-uniform characteristics of LN2 according to the electrode materials. AC withstand voltage test and lightning impulse test has been performed. From the test results, it was revealed that the breakdown voltages were significantly changed according to the electrode materials. Especially for Stainless steel, it shows prominent enhancement of breakdown voltage compared to copper and aluminum electrode in LN2. Consequently, it was deduced that optimum choice of conductor materials in LN2 is another critical factor to determine complex insulating performance of superconducting devices.

{\rm CF}_{4}

Korea Electric Power Corporation has developed a 154-kV superconducting fault current limiter (SFCL). This report is a part of the design process of the SFCL, particularly for fixation of posts supporting the superconducting element on the cryostat wall side. For supporting the superconducting element, the use of a post insulator is inevitable; however, the post insulator and cryostat with liquid nitrogen (L-N2) during operation of the SFCL form three junction points where electric field is intensified. In this study, we aim to design the metal shield in order to relax the electric field intensity at triple points (TPs) through numerical analysis of electric field distribution. For the electric field distribution analysis, a commercial software based on the finite-element method was employed. Each design for the metal shield was checked whether it makes the electric field intensity at the TP sufficiently lower than dielectric strength in L-N2 for 750-kV input and whether there is any electrically weak point on the metal. The designs of the metal shields were improved through four critical steps where thermal contraction, manufacture tolerance, and insulation distance in L-N2 were considered. It was experimentally verified that there was no electric breakdown in L-N2 between the metal shield and the fiber-reinforced plastic post insulator for the lightning impulse test and the ac breakdown voltage test according to the IEC 60137 standard.

as Insulation Gas for Superconducting Equipment

One of the most important insulating components for developing a 154 kV superconducting fault current limiter (SFCL) is post insulator, which supports 154-kV SFCL modules in a cryostat. In order to design an optimum post insulator, the top priority is to determine the creepage distance of post insulator. In our previous research, surface flashover voltages along smooth glass fiber reinforced polymer (GFRP) rods immersed in liquid nitrogen (LN2) were experimentally investigated. To reduce the cryostat volume, ribbed post insulators with increased creepage path were then considered. However, due to difficulties with producing ribbed GFRP post insulators, ribbed specimens were made of polytetrafluoroethylene instead. Four kinds of test specimens were prepared to determine the position and the number of ribs attached to the surface. From the experimental results, it was found that the ribbed insulator has shown better insulation performance in LN2 and number of ribs could affect the surface flashover voltage of post insulator. Consequently, ribbed model for 154-kV SFCL post insulator was designed and successfully tested. Finally, real 154-kV SFCL system with newly designed ribbed post insulator was installed and Basic Impulse Level (BIL) test and ac required withstand voltage were successfully carried out.