Umer Amir Khan

Feasibility Analysis of the Positioning of Superconducting Fault Current Limiters for the Smart Grid Application Using Simulink and SimPowerSystem

One of the most important topics regarding the application of superconducting fault current limiters (SFCL) for upcoming smart grid is related to its possible effect on the reduction of abnormal fault current and the suitable location in the micro grids. Due to the grid connection of the micro grids with the current power grids, excessive fault current is a serious problem to be solved for successful implementation of micro grids. However, a shortage of research concerning the location of SFCL in micro grid is felt. In this work, a resistive type SFCL model was implemented by integrating Simulink and SimPowerSystem blocks in Matlab. The designed SFCL model could be easily utilized for determining an impedance level of SFCL according to the fault-current-limitation requirements of various kinds of the smart grid system. In addition, typical smart grid model including generation, transmission and distribution network with dispersed energy resource was modeled to determine the location and the performance of the SFCL. As for a dispersed energy resource, 10 MVA wind farm was considered for the simulation. Three phase faults have been simulated at different locations in smart grid and the effect of the SFCL and its location on the wind farm fault current was evaluated. Consequently, the optimum arrangement of the SFCL location in Smart Grid with renewable resources has been proposed and its remarkable performance has been suggested.

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 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}

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.

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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.

{\rm CF}_{4}

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.

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

DC microgrid has become a preferred choice over AC microgrid, due to the growth of critical DC loads and higher efficiency during power distribution. However, the transient behavior of DC microgrid is more severe than AC microgrid, which makes the correct DC Circuit Breaker (DCCB) selection very important. In this research paper, we have proposed an evaluation method to select a commercially available DCCB by using simulation results of the stresses subjected to DCCB in low voltage DC microgrid (LVDCM). A LVDCM having AC utility grid, photovoltaic (PV) and battery-storage was modeled. A DCCB model using Schwarzs Black Box arc model was applied in the LVDCM, and its parameters were determined by using parameter sweep method. Voltage and current stresses subjected to DCCBs were analyzed and compared with DCCBs capability based on its datasheet. An appropriate DCCB was selected by comparing the voltage and current stresses with DCCBs datasheet limit.

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

In the design of DC superconducting cables, the electrical insulation design is a critical factor in the cables performance and reliability. To evaluate the insulation design of DC superconducting cables, DC electric field analysis and experimental verifications should be performed. Wrapped polypropylene laminated paper (PPLP) tape has generally been used to insulate superconducting cable. During the wrapping process, butt gaps are inevitably introduced, and these are filled with liquid nitrogen (LN2) during normal operating conditions. Therefore, the insulation characteristics of the resulting combination of PPLP and liquid insulation should be carefully verified. The objective of this work was to determine the DC electric field transitions when a butt gap is present in a LN2/PPLP composite insulation system. DC electric field distribution and transition were simulated by using COMSOL Multiphysics® software. Also, to verify the characteristics of DC electric field transitions, two kinds of breakdown tests were performed: a ramp voltage breakdown test and a step voltage breakdown test. In both the experimental and analytical works, it was observed that the electric field distributions were totally different while the DC field transition. And, due to the different distributions of electric field, the breakdown characteristics of LN2/PPLP composite insulation systems could be altered.

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

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.