Jong-Geon Lee

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.

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.

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

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.

Insulation Design of a Stop Joint Box of 80-kV DC HTS Cables Based on DC Electric Field Analysis

Among the accessories of the dc high-temperature superconducting (HTS) power cable, a stop joint box (SJB) is an essential component for the long-distance transmission cable connections and the separation of the cooling section. The main insulating material of the SJB is a polypropylene laminated paper (PPLP), which is already generally used for dc HTS power cable. In order to separate the cooling section, an epoxy spacer is located at the center of the SJB. Due to the weakness of surface breakdown characteristics compared with that of punctual breakdown and different nature of dc electric field distribution determined by the ratio of conductivities of dielectric materials, the epoxy spacer configuration of the SJB should be carefully designed based on dc electric field analysis. In this paper, the optimum configuration of the epoxy spacer of the SJB was investigated based on dc electric field analysis. On the basis of the initial SJB model, which is already designed for the joint box of the conventional ac oil-filled power cable, a variety of improved SJB models with different epoxy spacer configurations were considered. In dc electric field simulations, the main objective was to reduce the tangential dc electric field intensity at the interface between PPLP and epoxy spacer. To find the optimum epoxy spacer configuration, a parametric sweep technique was applied, and an appropriate SJB model could be identified. However, its tangential dc electric field intensity remained still higher than that of the design criteria for the dc electric field from the experimental results. Therefore, as a novel method, a thin Kraft layer between the PPLP and the epoxy spacer was added to reduce electric field intensification. The simulation result of the newly designed insulation structure revealed the possibility of lowering the tangential electric field. Furthermore, different electrical conductivity values were applied to the additional layer instead of the original Kraft value in order to meet the critical electric field intensity. Finally, 1E-11 S/m, among those values, showed the best simulation result, which was lower than the design criteria for the dc electric field. To verify the effect of the additional layer for dc electric field mitigation, miniature SJB specimens were fabricated, and dc breakdown tests were performed using a step-by-step test method. Based on the experimental results, SJB specimens with an additional layer showed the highest breakdown strength compared with that of SJB specimens without an additional layer. Thus, the optimum configuration of SJB determined by dc electric field analysis was approved by experimental works. Consequently, these insulation design skills based on dc electric field analysis could be usefully adopted in developing the SJB of 80-kV dc HTS power cables.

Effect of humidity and electrode roughness on the AC and impulse breakdown characteristics of dry-air

Dry-air is recognized as an attractive insulating medium for substituting SF6 gas in high voltage devices. Recently, Japanese manufacture announced that 72.5 kV has been successfully developed using dry-air as a main insulating medium. However, only a few fundamental research works of dry-air have been reported compared to those of SF6. In this paper, two kinds of experiments were conducted to clarify the breakdown characteristics of dry-air. First, the effect of humidity on the breakdown characteristics of dry-air was deeply investigated. And secondly, the effect of electrode roughness on the breakdown voltage of dry-air has been investigated. In order to make artificial specific humidity of dry-air, the experiment chamber has been controlled in vacuum condition under 0.2 Torr and some water was injected before filling the dry-air. Dry-air used for the experiments was composed of about 21% Oxygen and 79% nitrogen and its humidity was controlled of -85°C dewpoint temperature. Water could be successfully vaporized in dry-air, and then AC withstand test and lightning impulse breakdown test were performed using sphere to plane electrode airgap configurations. Next, similar breakdown tests were performed considering polished condition, and artificially made surface roughness with 16 μm and 25 μm on the high voltage electrode respectively. From the test results, it was shown that impulse breakdown voltages were slightly decreased with increasing electrode roughness. However, AC withstand voltages did not show any remarkable difference regarding surface roughness, and it was revealed that the breakdown voltage of dry-air was not affected by different humidity condition ranging from 300 PPM to 1,500 PPM humidity in dry-air. AC withstand voltage and negative impulse voltage in this humidity range did not show noticeable difference.

Electromagnetic field analysis of low voltage DC circuit breaker for the enhancement of arc driving force

The aim of this paper is to perform fundamental simulation for design of 1.5 kV DC circuit breaker of high speed. The simulation was focused on methods for improvement of the arc driving force. Considering that design for arc runner is a main component which affects the arc driving force, two main design parameters of arc runner: slope of arc runner and electromagnetic force of the blow out coil, were simulated with variable shape, and then research was conducted on exploration of optimum arc runner shape with calculation of Lorentz force about each shape. The electromagnetic field analysis based on Finite Element Method (FEM) was performed by utilizing COMSOL Multiphysics. For analyses, a related model was designed for a practical 1.5 kV DC circuit breaker. In addition, transformative works of model was performed to conduct effective analysis on Lorentz force in the arc runner. As a result, arc is more effectively transferred and focused in driving part using core pin and electrode magnetic field which is formed as applying to Blow out coil. Furthermore, it was identified that the more the slope of arc runner increases, the more increase the density of Lorentz force and acceleration of arc at the same time. Additional researches and developments for advancement in driving force is required.

A comparative study on electrical and thermal stress distribution across fundamental components of conventional and superconducting hybrid type HVDC circuit breakers

Super grids are considered to be the technology of the future that will allow transmission and trade of huge volumes of electricity over long distances. HVDC is the preferred choice for developing super grids as HVAC transmission of bulk power over long distances is less efficient. However, the key obstacle in developing HVDC super grids is the absence of HVDC circuit breakers (DCCB) which can quickly detect and isolate the fault. The huge electrical and thermal stresses subjected to DCCB during DC current interruption are one of the primary challenges in developing DCCB. Especially, IGBT valves in hybrid DCCB are the most expensive and sensitive part and their cost drastically increase with their maximum rating. This research paper presents a comparative study on the electrical and thermal stress distribution across IGBT valves of conventional hybrid DCCB (CDCCB) and Superconducting hybrid DCCB (SDCCB) during fault conditions. Realistic CDCCB and SDCCB models were developed by considering the characteristic values of commercially available IGBT module. To create the electrical and thermal stress profiles, CDCCB and SDCCB were placed in HVDC test bed model and transient simulations were performed. The voltage distribution, current distribution and energy dissipation profiles across IGBT valves were developed for CDCCB and SDCCB. Comparative analysis of the profiles showed that the SDCCB components were subjected to relatively lower electrical and thermal stresses.

Fault current characteristics of multi-terminal HVDC system

Owing to the type and structure of HVDC system, fault current characteristics can be varied. Until the present, most of studies have been dealt with fault current characteristic of HVDC system which focused on point-to-point connection. However, there were few announcements about the fault current characteristic of MTDC system. In this works, as a first step to determine the characteristics of circuit breaker for MTDC network, simulation works of fault current characteristics in MTDC system was investigated. Two types of MTDC system model were designed by Matlab/Simulink; Tree-like topology and Meshed topology. Pole to pole fault which is the severe fault in HVDC system was imposed and its characteristics was analyzed and compared. From the simulation result, the characteristics of fault current in MTDC system were analyzed and basic requirements for HVDC circuit breaker were suggested.