Jae-Sang Hwang

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.

Comparative Evaluation Between DC and AC Breakdown Characteristic of Dielectric Insulating Materials in Liquid Nitrogen

Due to the existence of AC loss in superconducting materials when an alternating voltage is applied, high cryogenic costs are inevitable to operate superconducting devices in AC networks. Therefore applications of superconducting devices in DC electric power networks could be regarded as the optimum choice for superconducting devices, because superconductors show exactly zero resistance to a DC source. Recently, DC superconducting devices such as DC cables have received noticeable attention as DC power transmission lines. In order to develop DC superconducting devices, the DC insulation characteristics in cryogenic liquids should be clarified. However, up to now, limited research has been reported in this field. In this paper, to clarify the different breakdown characteristics of DC and AC applications, various kinds of cryogenic dielectric sheets including Kraft, Kapton (polyimide) and Nomex (polyamide) papers have been prepared. Furthermore, a penetrating breakdown test for three kinds of sheets and turn-to-turn breakdown tests have been performed in liquid nitrogen (LN2). Consequently, it was found that the prepared sheets have shown 1.7-2.7 times higher dielectric strength than those of AC. Moreover, the Nomex and Kraft sheets have shown a remarkable increase in their dielectric strength in liquid nitrogen compared to air. However, the dielectric strength of the Kapton sheet did not show a remarkable increase in liquid nitrogen. From the turn-to-turn breakdown test, it was proved that the dielectric strength has been linearly increased according to the wrapping number of the sheets and that the DC breakdown voltage was 1.1-2.5 times higher than AC.

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

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.

Experimental and Analytical Study on DC Breakdown Characteristics of Butt Gap Condition in

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.

\hbox{LN}_{2}/\hbox{PPLP}

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.

Composite System

High-voltage direct current (HVDC) technology is considered to have some important advantages over traditional high-voltage alternating current, such as higher overall efficiency and smaller power losses for long-distance transmission. In addition, applications of superconducting cables in dc electric power networks may realize real zero impedance, and the economic and technical advantages could be maximized. Therefore, many research institutes have tried to develop advanced superconducting cables for HVDC grids with higher reliability, by considering insulation diagnosis in order to avoid unexpected failures. As one of the plausible diagnostic methods for power cables applied to the ac grid, the detection of partial discharges (PDs) taking place inside the apparatus has been widely investigated. With regard to the related PD pattern analysis, a phase resolved PD analysis (PRPDA), which was first developed in the early 1970s, accounts for the phase information of the applied ac voltage. In 2001, we also proposed a method for pattern recognition, i.e., chaotic analysis of PD (CAPD), that considers three normalized parameters obtained from the values between two consecutive PD pulses: amplitude difference (Pt), occurring time difference (Tt), and applied voltage difference (Vt). However, none of the proposed methods of pattern analysis can be employed for PD under dc stress. Therefore, in this paper, we propose a modified CAPD for the related pattern recognition of possible defects inside a joint box and termination of an HVDC superconducting cable. PDs are produced from four artificial defects and are then detected by a self-designed and fabricated sensor, for which the analysis was performed based on our newly modified CAPD.

Evaluation of Uniform and Non-uniform Breakdown Characteristics of Liquid Nitrogen With Different Electrode Materials

For the insulation design of dc high-temperature superconducting (HTS) equipment, dc electric field analysis should be performed. As the dc electric field distribution is mainly determined by the relative electrical conductivities of the various insulating materials used, the conductivities of these materials should be precisely measured. In particular, in cryogenic environment, the measurement of electrical conductivity could not be easily conducted due to the difficulty of measuring extremely low leakage current. In this paper, investigation on the measurement of volume and surface electrical conductivity of various cryogenic insulants, including polypropylene laminated paper (PPLP), Kraft, and glass fiber reinforced plastic (GFRP), was carried out. For the measurement of volume electrical conductivity in LN2, the infiltration of LN2 into the specimen should be considered in order to avoid inaccurate measuring data. Thus, in order to increase the reliability for volume electrical conductivity, the PPLP specimen deposited by copper was adopted, and the comparison between ordinary PPLP and PPLP with copper deposition was made. As a result, it was suggested that the copper deposition could be a valid method to prevent the infiltration of LN2 when the electrical conductivity of thin paper was measured. Consequently, volume and surface electrical conductivity values of PPLP, Kraft, and GFRP have been measured and summarized.

Identification of Insulation Defects Based on Chaotic Analysis of Partial Discharge in HVDC Superconducting Cable

As polypropylene laminated paper (PPLP) is usually wound around the high-voltage conductor of a dc high-temperature superconducting power cable several times, it is needed to determine whether electrical conductivity of PPLP could be increased according to the number of layers. In thin films without layers, electrical conductivity increases according to the thickness of materials. For this reason, the effect of the number of PPLP layers on electrical conductivity needs to be verified because the dc electric field distribution is highly influenced by electrical conductivity. In this paper, to determine the effect of PPLP layers on electrical conductivity, we measured the conductivity of specimens with single, double, and quadruple layers of PPLP in liquid nitrogen (LN2), and compared the results according to layer number. In addition, we measured the electrical conductivity of specimens with single, double, and quadruple layers of Kraft to verify if there was a layer effect or not. The electrical conductivity of the PPLP and Kraft specimens did not change with an increase in the number of layers. Therefore, we concluded that the electrical conductivity of single-layered PPLP and Kraft specimens can be used in dc electric field simulations.

A Study on the Measurement of Volume and Surface Electrical Conductivity of Cryogenic Insulants for DC HTS Equipment

One of the key components for dc HTS cable for long-distance line is a stop joint box (SJB). Up to now, the SJB has not been commercialized yet due to some difficulties of dc electrical insulation matters in LN2. Basically, the structure of SJB would be similar to the SJB of oil-filled cable because they adopt liquid insulation and sectionalized cooling separations. The insulation structure of conventional SJB based on ac electric field was no more suitable for dc; hence, there is a need to design suitable insulation structure, which can be applied for dc. In this work, in order to find a solution to reduce dc electric field concentration on the polypropylene laminated paper (PPLP) above epoxy spacer, double layer structure composed of PPLP and Kraft were suggested. To verify the effectiveness of this structure for relieving dc electric field, simulation and experiments were performed. From analytical works, the Kraft layer between PPLP and epoxy has shown noticeable electric field mitigation. Furthermore, surface breakdown tests considering PPLP/Kraft combined layers were conducted to verify the insulation properties of double layer structure. Finally, it was deduced that Kraft layer was effective to enhance surface breakdown voltage. Surface breakdown characteristics of PPLP and Kraft were relevant to the surface roughness of insulating materials.

Comparison of the Electrical Conductivity of Polypropylene Laminated Paper (PPLP) and Kraft in LN 2 According to the Number of Layers

Conventional protection devices installed for the protection of excessive fault current in the electric power system have a certain response time delay resulting in power system to pass initial peaks of fault current. Superconducting Fault Current Limiter (SFCL) is a novel technology which has negligible power loss and capability to quench initial fault currents instantly and it also could be utilized to cope with excessive fault current problems in upcoming microgrids. This paper presents feasibility analysis results of positioning of the SFCL and its effects on reducing fault current in microgrids. The detailed power system having AC and low voltage DC microgrids consist of integrated wind and photovoltaic farms integrated with a traditional power system is modeled. Transient analyses for the microgrids were performed for the worst case faults with SFCL installed at key locations of the microgrids. It has been observed that SFCL should not be installed directly at the substation or the branch network feeders. This placement of SFCL results in abnormal fault current contribution from the DG sources. The strategic location of SFCL in power grid which limits all fault currents and has no negative effect on the DG sources is found to be in front of the point of integration of the DG sources with the microgrid.