Bang-Wook Lee

Hybrid Superconducting Fault Current Limiter of the First Half Cycle Non-Limiting Type

Resistive superconducting fault current limiters (SFCLs) are assumed to be a prospective solution for commercialization. But in spite of excellent current limiting performances realized by resistive SFCLs, commercialization and installation of SFCLs have been delayed due to difficulties in overcoming several technical problems such as a coordination with conventional relays. Resistive SFCLs may have difficulty in controlling an initiation of current limiting operation that sometimes is very important for coordination with relays, because the superconductor in the SFCL will quench immediately after the fault current reaches its critical current. In order to solve these problems, a novel hybrid SFCL of the first half cycle non-limiting type was developed. This paper presents the characteristics of this hybrid SFCL, its configuration and test results.

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.

A Novel Model of HVDC Hybrid-Type Superconducting Circuit Breaker and Its Performance Analysis for Limiting and Breaking DC Fault Currents

The key obstacle in integrating high-voltage direct current (HVDC) point-to-point networks into meshed multiterminal HVDC networks (MTDC) is the absence of dc circuit breakers (DCCBs), which can timely and reliably isolate the faulty HVDC network from the MTDC. In this paper, a novel hybrid-type superconducting DCCB model (SDCCB) is proposed. The SDCCB has a superconducting fault current limiter (SFCL) located in the main line, to limit the fault current until the final trip signal to the SDCCB is given. After the trip signal, insulated-gate bipolar transistor (IGBT) switches located in the main line will commutate the fault current into a parallel line, where dc current is forced to zero by combination of IGBTs and surge arresters. DC fault current behavior in MTDC and fundamental requirements of DCCB for MTDC were described, followed by an explanation of the working principles of the SDCCB. To prove the viability of the SDCCB, a simulation analysis demonstrating SDCCB current interruption performance was done for changing the intensity of dc fault current. It was observed that the passive current limiting by SFCL caused significant reduction in fault current interruption stress for SDCCB. Furthermore, fundamental design requirements for SFCL, including the effect of SFCL quenching impedance on SFCL voltage rating and energy dissipation capacity, were investigated. Finally, advantages and limitations of the SDCCB were highlighted.

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.

Improvement of Recovery Characteristics of a Flux-Lock Type SFCL Using a Superconductor's Trigger

To achieve the effective performance of the flux-lock type superconducting fault current limiter (SFCL), the countermeasures to improve the recovery characteristics of the flux-lock type SFCL are required. In this paper, as one of the solutions, the method using a superconductors trigger when the quench in the high-TC superconducting (HTSC) element occurred was suggested. To verify the suggested method in case of the fault occurrence, the control circuit to detect the quench of HTSC element and the current limiting resistor (CLR) to limit the fault current were designed and fabricated. With the fabricated control circuit and the CLR, the fault current limiting and recovery characteristics of the flux-lock type SFCL using the suggested method were tested. Through the analysis for the experimental results, the recovery characteristics of the flux-lock type SFCL could be confirmed to be improved than the SFCL not using the superconductors trigger.

Impact of SFCL on the Four Types of HVDC Circuit Breakers by Simulation

Recently, studies on HVDC circuit breaker (CB) prototypes have shown successful test results. Nevertheless, effective and reliable solutions regarding massive fault energy during dc fault interruption have not yet been commercialized, and dc current breaking topologies on methods of achieving artificial zero should be somewhat modified. As an alternative, one feasible solution is to combine fault current limiting technologies with dc breaking topologies. In this paper, we studied the application of resistive superconducting fault current limiters (SFCLs) on various types of HVDC CB in order to estimate the effects of combining fault current limiters and conventional dc breakers. For the simulation works, four types of dc breaker topologies were modeled, including a mechanical CB using black-box arc model, a passive resonance CB (PRCB), an inverse current injection CB, and a hybrid HVDC CB. In addition, a resistive SFCL was simulated and added to the dc breakers to verify its interruption characteristic and distributed energy across HVDC CB. From the simulation results, we found that the maximum fault current, interruption time, and dissipated energy stress on the HVDC CB could be decreased by applying SFCL. In addition, it was observed that, among four types of HVDC CB, PRCB with SFCL exhibited the best observable enhancement.

Comparison Between PD Inception Voltage and BD Voltage of PPLP in

With the rapid development of world economics, demand for electricity in metropolitan areas has increased dramatically. The high-temperature superconductivity (HTS) cable is one of the most promising technologies for solving the bottleneck of the transmission system. However, power system planners have not yet considered the HTS cable as a mature technology due to its weak insulation characteristics with respect to those of conventional power cables. The electrical insulation design is one of the major issues for HTS cable. It is evident that the insulation in the dc cable should be investigated further at this time. For the insulation design of the HTS cable, it is necessary to investigate the breakdown (BD) characteristics and the partial discharge (PD) inception voltage of the liquid nitrogen (LN2)/polypropylene laminated paper (PPLP) composite insulation system. In this paper, BD voltage and PD inception voltage of PPLP immersed in LN2 was compared under ac and dc stresses. Various kinds of PPLP specimens considering the different layers and the butt gaps were prepared. The PD inception voltage was measured in a shield room according to IEC 60270. Consequently, it was investigated that characteristics of the PD inception voltage and the BD voltage was rather different when ac and dc voltages were respectively applied in LN2, and these results could be used for the insulation design of ac and dc HTS cables.

\hbox{LN}_{2}

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.

Considering HTS Cable Insulation

A compact cryogenic cooling system is developed for small HTS magnets to be continuously refrigerated in subcooled liquid nitrogen at temperatures below 77 K by a cryocooler. This thermal design is particularly useful when electrical insulation and compactness are significant at the same time. HTS elements are immersed in a cylindrical liquid-nitrogen vessel, and a copper band is brazed around the exterior sidewall of the cylinder at vertical location just under the liquid level in order to achieve peripherally uniform cooling with a GM cryocooler. The vapor space above liquid nitrogen in the vessel provides a fully open room for current leads and mechanical supports to the magnets. Heat is removed from the HTS magnets by natural convection of subcooled liquid nitrogen to the cold sidewall. The key components are designed based on heat transfer analysis. A prototype is fabricated, and it is successfully demonstrated that the thermal load is effectively removed and temperature is spatially uniform.

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

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.