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Numerical simulations have revealed various large current phenomena with improvement in the calculation speed of a computer. For example, detailed modeling was conducted in numerical simulation of arc discharge for arc welding or circuit breaker. Especially, in application of arc plasma to material processing such as arc welding, understanding and control of thermal and dynamic interactions between plasma and material are indispensable for improving quality and efficiency of the process. In this paper, recent studies on numerical simulation of arc welding are introduced.

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Gas tungsten arc (GTA) welding is a high-quality joining technology. However, the TIG welding cannot melt the welding pool deeply. Therefore, the weld defect sometimes occurs at TIG welding because of this poor heat transfer to the anode and ability of melting deeply. In addition, the welding speed becomes slow, and the productivity is decreased. The welding parameter should be controlled in order to improve the welding productivity and prevent the weld defect. The pulsed arc welding has been developed because it can control the heat transfer with the current waveform. Some researchers have elucidated that the pulse frequency of a current is to be 100 to 500Hz is effective for improvement of welding speed. In this frequency band, the arc and the heat transfer to anode become stable. Therefore, the material can be melted easily because the heat transfer is concentrated. However, the shape of welding pool is decided by the temperature distribution near cathode, the current density distribution at the arc and welding pool and driving force in the welding pool. Especially, the Lorentz force (current density times magnetic flux density) is very important factor in order to become deep welding pool. In the case of pulsed arc welding, these phenomena are derived from current waveform, especially the current peak, base, and transition time. In this paper, contribution for the heat transfer and heat flux to anode affected by rise current transition time in pulsed arc is elucidated. As a result, when the current transition time decreases, the heat transfer becomes small, and the heat flux is concentrated to the center. Therefore, the current transition time plays important role for making welding pool.

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In reducing the operating energy thermal puffer type gas circuit breaker is very advantageous. However a detail understanding of the basic fluid-plasma interaction phenomena associated with the gas blast characteristics is very much necessary for its efficient design. Therefore the main aim of this research is to study the basic fluid-plasma interaction phenomena based on a High-Order LES (Large Eddy Simulation) turbulence model. The model configuration used in this regard is a converging-diverging type nozzle installed with an anode and a cathode inside to generate thermal arc plasma in the nozzle flow field. Here, the basic physics associated with the reduction of arc diameter due to interaction of supersonic cooler gas with the thermally expanded hot core gas and related heat transfer through turbulent mixing is discussed. The results revealed that as more and more denser cooler gas entrain to the low density hotter core gas the arc region starts growing axially downstream the region of reduced arc diameter. However near the shock location in the nozzle an abrupt change in temperature distribution is observed.

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This study aims to clarify energy balance and heat transfer in short arc lamp by constructing numerical simulation model of the short arc lamp. In this paper, influence of design of the lamp on radiation produced by the lamp is discussed by employing numerical simulation model developed by modifying numerical simulation model of free burning arc. From evaluation of energy balance in the short arc lamp, it was found that 72.5% of the input power is consumed by the radiation. Furthermore, the shape of the anode and the bulb strongly affect amount of the radiation. Amount of radiation is maximized at diameter of the bottom surface of the anode of 7.5mm and increases with decreases in diameter of the bulb, because mainly of increase in pressure in the bulb.

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The home applications of distributed generations such as photovoltaic and gas engine cogeneration system are growing and many of distributed generations are connected to a grid by a DC/AC inverter. The DC/AC inverter is controlled by a PLL (Phase Locked Loop) in order to be synchronized with power system frequency. However, the control scheme of stand-alone operation uses the voltage control, and the PLL is unnecessary. Therefore, it is hard to realize uninterrupted change between stand-alone and grid-connected operation. In this paper, we propose VSG (Virtual Synchronous Generator) control scheme in order to use the same control scheme in the stand-alone and grid-connected operation. We carried out some experiments to demonstrate the control characteristics.

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Renewable Energy Sources (RES) and Smart Grid are being focused on recently in Japan to reduce greenhouse gas emissions and energy consumption. In particular, a large amount of Photovoltaic generations (PV) will be introduced into Japanese distribution system in the near future. However, output of PV depends on weather conditions and climate condition may change rapidly or randomly. As a result, voltage in distribution system with many PVs will fluctuate and could not be manage stable in future. In contrast, a battery is effective device to control voltage and will increase in power system as an Electric vehicle and a household battery. Smart Grid is likely to have an ability to control these batteries to manage voltage effectively in the future. In this paper, authors proposed a control method of SVR and batteries of consumers distributed in power system for voltage regulation. An advantage of proposed method is that it is possible to control voltage using small batteries in the network and a benefit of users from battery is not hindered. The effectiveness of proposed method is confirmed by numerical simulation.

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Practical application of the VSC (Voltage Source Converter) HVDC system is progressing recently due to its superior control performance over the conventional line-commutated converter type HVDC system. Introduction of the VSC HVDC will continue to spread further by the appearance of MMC (Modular Multilevel Converter) topology.
At present, AC circuit breaker is tripped to clear fault when a DC fault occurs at a VSC HVDC system. Tripping AC circuit breaker delays recovery from the fault. The application of the full bridge cell MMC is considered as one of the solutions to clear the fault without tripping AC circuit breaker. However, any specific control scheme using the full bridge cell MMC has not been proposed. We propose a novel control scheme of the VSC HVDC system which can control DC fault current and recovery quickly from the fault.
We verified performance of the control scheme through transient simulation. The VSC HVDC system can maintain stable operation without overcurrent and overvoltage on semiconductor devices during entire AC and DC fault period. It was confirmed that HVDC system with the control scheme can restore power transmission quickly after both AC and DC system faults.

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Harmonics have existed in power systems for many years. Nowadays, power systems have experienced an increase in level of harmonic currents and voltages on their electrical delivery systems due to the rapid growth of power electronic devices in industry, the presence of products based on electronic component in enterprise, institutions, shops, residence and transport systems etc. The harmonic effect on power system equipment such as capacitor failure or transformer and neutral conductor overheating, has become one of the major power quality problems to be solved in the near future. To decrease harmonic distortion, the method such as passive filtering and active filtering are used to mitigate local harmonic currents and prevent them from influencing the main supply system. The reliable design of a passive filter requires a correct knowledge of the system harmonic impedance and its variations throughout the day to avoid creating a resonance condition, which could destabilize a power system. Active filters also require a good knowledge of the system harmonic impedance to ensure stable controller operation and also can be used in the generation of the filter reference currents. A technique that can estimate the system harmonic impedance from a Point of Common Coupling (PCC) could prove extremely useful information in many areas of power quality analysis. In this paper, we present the estimation of system harmonic impedance from PCC using complex Independent Component Analysis (ICA). The numerical simulations show that the proposed method can estimate the system harmonic impedance even in the condition that the load harmonic impedance is changing in mixing process.

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The electromagnetic phenomena intrinsically spread over the infinite region. Thus, the efficient handling of open boundary is one of the main issues in the electromagnetic field computations. This paper deals with the orthogonalized infinite edge element method which efficiently performs precise analysis of the infinite region. In this method, there are several parameters to achieve its high accuracy. As one of the parameters, we focus on the reference point and investigate the effect of its position setting on the accuracy. Furthermore, we also evaluate the accuracy of the calculated magnetic field at distance region. By applying boundary element method (BEM) as post-processing, it is found that the high computational accuracy in the region can be effectively achieved.

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Severe Transient Recovery Voltage (TRV) after the current interruption may appear when a fault occurs in the immediate vicinity of a power transformer without any appreciable capacitance between the transformer and the circuit breaker. These faults are called Transformer Limited Fault (TLF) that may cause higher Rate-of-Rise of TRV (RRRV) than the standard values specified for terminal fault test duties T10 and T30 of IEC 62271-100. TRVs for TLF conditions with large capacity and high voltage shell type power transformer were measured by the capacitor current injection method. The impedance frequency responses were also measured by the Frequency Response Analysis (FRA) and then TRVs were reproduced with the simplified transformer model consisting of a series connection of parallel circuit of inductance, capacitance and resistance whose parameters were evaluated by the FRA measurements. The reproduced TRV showed good agreement with the measured TRV even though deformation from a sinusoidal shape wave was observed due to superposition of higher frequency components on the TRV.

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Lightning surge analysis is very important for rational lightning protection design of distribution line. In order to establish a calculation method of a power distribution line flashover ratio due to lightning, it is important to estimate flashover behavior of a distribution insulator accurately. Insulator voltage waveform, which appears across an insulator on distribution line by a lightning stroke, tends to very short wave tail. However, flashover phenomena of the distribution insulator with the short wave tail lightning impulse voltage waveform and its modeling methods are not clarified.
This paper proposes a distribution insulator flashover model for lightning surge analysis. At first, we clarify the flashover characteristics of a distribution insulator with a short wave tail lightning impulse voltage waveform. Then, we propose the flashover model based on the integration method. The proposed model consists of simple two equations, and can be realized very easily on transient analysis programs. A comparison of results calculated by the proposed model with the experimental results shows good agreements.

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Recently, metaheuristics becomes practical technique for various optimization problems. Metaheuristics is the technique of obtaining an approximate solution efficiently for optimization problems. Genetic Algorithm (GA), Simulated Annealing (SA), Particle Swarm Optimization (PSO), etc. are typical metaheuristics. PSO is an effective method among them in terms of the processing time and the accuracy in solution.
The processing time of PSO depends on the number of the particle and the generation. The terminating condition in the conventional PSO is the preset maximum number of generations. Thus, even though the particles have already converged to the optimal solution, the search continues up to the preset one. This paper proposes a novel scheme of terminating in PSO by using the coefficient of variation (CV) which achieves the shortening of the processing time. The proposed method judges the convergence of the swarm by using CV, and terminates the search. In this paper, the validity is shown by the benchmark problems and the proposed method is applied to the optimization problem for the electric power leveling systems in the rolling mills.

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TOKYO SKYTREE^® has recently been constructed in the eastern area of central Tokyo, located in the Kanto Plain, and is the tallest free-standing broadcasting tower in the world (634m). To observe the current waveshapes of lightning striking TOKYO SKYTREE^®, the authors have installed Rogowski coils on the tower at a height of 497m. In this letter, the authors report on the lightning current waveshapes observed at TOKYO SKYTREE^®.

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