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A power-supply system was developed for Ohmic heating (OH) to double × 1018 the amount of change magnetic flux in the primary central solenoid (CS) on the QUEST spherical tokamak. Two power supplies are connected with stacks of insulated-gate bipolar transistors, and sequentially operated to generate positive and negative CS currents. This bipolar power-supply system is controlled via a field-programmable gate array, which guarantees the safety of the entire system operation. The new OH system, assisted by electron cyclotron heating, enables the stable generation of plasma currents exceeding 100 kA. Moreover, the achieved electron density over the wide range in the major radial direction exceeds the cut-off density for one of the high-power microwave sources in QUEST. This strategy yields target plasmas for future experiments with the electron Bernstein wave.

In this paper, the authors propose a method of improvement of power quality by static synchronous compensators (STATCOMs) for low voltage system (single-phase STATCOMs) added the function of harmonic suppression. In the proposal method, the STATCOMs control the voltage of the distribution system within the proper range by compensating the reactive power. Additionally, they are operated as the filtering function and suppress harmonics of the voltage of the distribution system. The filtering gain is determined by PI controlling the error between the voltage total harmonic distortion of the point voltage at the installation of STATCOMs and the target value. In order to verify the validity of the proposal method, the numerical calculations are carried out by using an analytical model of distribution system which interconnected photovoltaic generation systems. Simulation results show the effect of improvement of power quality on both sides of voltage deviation and harmonics.

As technology has improved in recent years, it has become possible to create new valuable functions by combining various devices and sensors in a network. This concept is referred to as the Internet of Things (IoT), and predictive maintenance is a new valuable function associated with the IoT. In large-scale experimental facilities with many researchers, it is not desirable that experiments cannot be performed due to sudden failure of equipment. For this reason, it is important to predict the failure in advance based on the measurement results of sensors and to perform repairs in a planned manner. On the Q-shu University experiment with steady-state spherical tokamak (QUEST) large experimental device, it is necessary to drive a large current of 50 kA, and the diagnosis of its power line deterioration is well performed as predictive maintenance through the evaluation of its contact resistances of several micro ohms order on the network. In addition, as an example of the IoT, mechanisms to assist safe operation, such as a sound alarm system and an entrance management system, are built by sharing experimental information between devices via the network., Computer Science; Electrical Engineering; Safety engineering; Internet of Things; System Diagnostics; Electric Power Transmission; System Fault Detection; Computer-Aided Engineering; Predictive maintenance; IoT; safety operation; FPGA; QUEST

In tokamaks, the temperature of the plasma-facing wall is an important parameter for achieving particle balance and therefore steady-state operation. QUEST, which is a middle-sized spherical tokamak, has hot walls that act as plasma-facing walls. They can be actively heated with sheath heaters and actively cooled with water. To control the wall temperature, heating and cooling systems have been developed. These systems adjust the power of the sheath heaters and the motor valves of the cooling system, respectively. The two systems communicate via Ethernet through UDP and control the hot-wall temperature cooperatively. The plasma control system (PCS) in QUEST has also been modified, especially with respect to gas fueling, in order to enable long-duration plasma sustainment. A feedback controller has been installed in the PCS, together with a mass flow controller, allowing Hα emission from the plasma which is used as a reference signal, to be well controlled. Plasma density calculations using a field-programmable gate array are proposed for the feedback control system.

Recently, non-linear loads have diversified and increased. It results in deterioration of power quality on a distribution system, since the equipment, which includes the non-linear loads, generates harmonics. The harmonics cause not only deterioration of power quality but also further various problems. Nowadays, the state of harmonics is becoming evident by the recent researches. However, since the harmonic analysis by using a detailed model of distribution system is too difficult, the analysis is carried out by a simplified model. In this paper, the authors construct an analytical distribution system model in consideration of the non-linear loads with the harmonic sources based on the practical data. Moreover, the influence analyses of harmonics are carried out by using this model. In addition, the authors estimate the harmonic sources in the simplified distribution system.

The plasma control system (PCS) of QUEST is developed according to the progress of QUEST project. Since one of the critical goals of the project is to achieve the steady-state operation with high temperature vacuum vessel wall, the PCS is also required to have the capability to control the plasma for a long period. For the increase of the loads to processing power of the PCS, the PCS is decentralized with the use of reflective memories (RFMs). The PCS controls the plasma edge position with the real-time identification of plasma current and its position. This identification is done with not only flux loops but also hall sensors. The gas fueling method by piezo valve with monitoring the Hα signal filtered by a digital low-pass filter are proposed and suitable for the steady-state operation on QUEST. The present status and prospect of the PCS are presented with recent topics.

In the present RF-driven divertor plasma of QUEST, it has been observed that orbit-driven current flows in the open magnetic surfaces outside of the closed magnetic surfaces. To observe this phenomenon and the characteristics of the orbit-driven current, current density profiles have been calculated on two different equilibrium conditions. We calculated current density profiles from particle guiding center orbits both for the fundamental and the second harmonic resonances for the 8.2 GHz electron cyclotron current drive. From this calculation, hollow current density profiles have been obtained with significant characteristics on both conditions. Only positive current distribution has been observed in the open magnetic surfaces outside of the closed magnetic surfaces.

In order to evaluate the magnetic surface current density distributions, the electron guiding center orbits were calculated with the equilibrium fitting plasma equilibrium analysis. The current density distributions in electron cyclotron-heated plasma were estimated from the orbit analyses in the equilibrium plasma shaping for the fundamental and the second-harmonic resonant electrons separately. The current density distribution profiles on the equatorial plane were obtained for the electrons with initially positive and negative velocities parallel to the magnetic field direction as well as the trapped electrons. The surface averaged current density profiles of the closed flux surfaces were also evaluated. A significant amount of the positive current density distributions appeared outside the last closed flux surface (LCFS), while all the negative current density distributions were evaluated inside the LCFS. The trapped electrons being the second-harmonic electron cyclotron resonant contributed the negative current inside the LCFS. The positive current density distributions outside the LCFS were significant for the driven current density for both the fundamental and second-harmonic resonances.

SUMMARY In recent years, the deterioration in the power quality of a distribution system such as the aggravation of voltage imbalance and current imbalance or the increase of harmonics has been apprehended. It is thought to be causally related to the diversification of load by progress of power electronics technology and the increase of the number of interconnection of distributed generators for home use with the reduction of the effect on the environment. The purpose of this study is development of power conditioner systems (PCSs) with a function of improvement of power quality in the distribution system. In this paper, the authors propose a control method of restraint of harmonics and improvement of voltage imbalance in the distribution system utilizing the PCSs of photovoltaic systems. Moreover, it includes a determination method of the interconnection power factor and the optimal gain of filtering operation of PCSs so as to minimize the voltage imbalance factor and so as to minimize the maximum value of voltage total harmonic distortion in the distribution system by real-time feedback control with measured data from the information technology switches of distribution line. The numerical calculation is carried out in order to verify the validity of the proposal method.

In recent years, digital grid, which is composed of multiple small grids connected asynchronously by Digital Grid Routers (DGR), has been proposed as a new electrical power grid. The DGR consists of multi-functional inverters connected by a common DC bus. This study proposes a disturbance-rejecting method for the output current of the multi-functional inverter in the DGR by using hysteresis current control. The proposed method can suppress not only the harmonic elements of the utility but also the resonance made by suddenly changes of the reference current. The simulation results show good performances of the proposed control method.

In recent years, the debasement of power quality of a distribution system such as the increase of power system harmonics has been apprehended. It is thought to be causally related to the diversification of loads by progress of power electronics technology and the increase of the number of interconnection of distributed generators for home use with the reduction of the effect on the environment. The harmonics cause not only deterioration of power quality but also further various problems. In this paper, the authors propose an estimation method of harmonic sources by separating the harmonic current flowing through the distribution lines. In addition, the authors carry out the numerical calculation and experiment in order to verify the validity of the proposed method.

SUMMARY In recent years, the number of renewable energy sources (RESs) such as photovoltaic generation systems and wind power generation systems connected to the grid has been increasing as a way of reducing negative effects on the environment. The outputs of these RESs vary rapidly because of the influence of the weather and the conditions of the location. Therefore, there are concerns that the point voltages in a distribution system may vary drastically and that the voltages may deviate from the appropriate voltage range as a result of the influence of the RES connected to the distribution system or to the diversification of loads. Furthermore, there are concerns about adverse effects on electric power quality, such as voltage imbalances and harmonics. In this paper, we propose a cooperative voltage control method for a distribution system using system voltage control equipment in order to reduce the capacity of the static synchronous compensator. Numerical calculations were performed in order to verify the validity of the proposed method.

In this study, the authors aim to develop the interconnection inverter called “Proton” used for the “Digital Grid” which has been proposed as a new electric power grid. The authors propose an islanding detection function and fault ride thorough (FRT) function of interconnection inverter. In the proposed method, the output current is controlled to be proportional to the voltage of the grid. During the momentary voltage drop, the output current of the inverter is decreased, and when the grid recovers, the output current is increased to the rated value automatically. The proposed method satisfies FRT requirement without any complicated detection and system stop process. The numerical calculation is carried out in order to verify the validity of the proposed method.

A drift error correction technique with machine vision and a real-time equilibrium calculation code have been developed on the QUEST (Q-shu university experiment with the steady-state spherical tokamak) for steady-state operation. The drift error caused by the long time-integration of magnetic raw signals has to be removed. With a captured image of the plasma’s cross section, the plasma’s position is identified by use of image filters. The measured magnetic flux values are corrected to the calculated flux values estimated by using this plasma position. The correction with the captured image work as expected in the preliminary result using a flashlight instead of a plasma.

A 28~GHz system with a high-power gyrotron tube has been used for the QUEST spherical tokamak to form an over-dense plasma for electron Bernstein wave heating (EBWH) and current drive with an 8.2~GHz-wave. Non-inductive high-density plasma ramp-up experiments with dual-frequency (dual$-f$) electron cyclotron (EC) [ 8.2~GHz and 28~GHz ] waves were conducted. A spontaneous density jump (SDJ) to an over-dense state was observed as a bifurcation phenomenon in the dual$-f$ wave experiment. The over-dense plasma on the 8.2~GHz-wave was non-inductively ramped up to 25~kA, and was maintained for 0.4~s under stable plasma equilibrium after two such jumps in one shot. Heating to mildly energetic electrons and bulk electrons was observed even in the over-dense region. The electrostatic EBWH effect on the mildly energetic electrons in the over-dense region is assessed following a dispersion analysis of the 8.2 GHz-wave. The bulk electron heating effect observed is explained as heat exchange from mildly energetic electrons heated by the electrostatic EBWH wave. Remarkably, a high hard-X-ray-radiation temperature ~ 500~keV was also observed in tangential viewing for current-carrying electrons in the over-dense core region. Synergetic heating from the overlap of different 28~GHz EC harmonic resonances as well as higher harmonic heating is discussed for maintaining the highly energetic electrons in the over-dense core region. In addition, the SDJ process and mechanism are considered based on the discussion of the electron heating effects with the 8.2 GHz-wave.

Global particle balance in non-inductive long-duration plasma on QUEST has been investigated. Approximately 70% of the fuel hydrogen (H) was retained in the wall and then was almost exhausted just after the discharge. The global recycling ratio (Rg), defined as the ratio of the evacuated H2 flux to that injected, was found to gradually increase during discharges and subsequently rose rapidly. To study the growth of Rg, the thermal desorption spectra after deuterium implantation in a specimen exposed to QUEST plasma was analyzed with a model which includes reflection, diffusion, solution, recombination, trapping, and plasma-induced desorption in the re-deposition layer. The model reconstructs the growth of Rg during a long-duration plasma and indicates solution plays a dominant role in the growth.

A plasma control system to sustain divertor configurations is developed on QUEST (Q-shu university experiment with steady-state spherical tokamak). Magnetic fluxes are numerically integrated at 100 kHz using FPGA (Field-Programmable Gate Array) modules and transferred to a main calculation loop at 4 kHz. With these signals, plasma shapes are identified in real time at 2 kHz under the assumption that the plasma current can be represented as one filament current. This calculation is done in another calculation loop in parallel by taking advantage of a multi-core processor of the plasma control system. The inside and outside plasma edge positions are controlled to their target positions using PID (proportional-integral-derivative) control loops. Whereas the outside edge position can not be controlled by the outer PF coil current, the inside edge position can be controlled by the inner PF coil current.

In this study, we made experiments by the simulated PV system for understanding the cause of disconnection when instantaneous voltage sag occurs. Moreover, it is understood that the PCS with the voltage phase jump detection method which is one of the main causes of the disconnection. We modeled it in the simulation. When fault occurs, even though the residual voltage is more than 20 % and the voltage sag time is within 1.0 sec, the PCS disconnected by the voltage phase jump detection was confirmed in the simulation results. If a large number of PV systems are interconnected in the power system, we verified through the simulation that the interaction caused by the disconnection which generated by the voltage phase jump detection. We proposed threshold limit value of islanding detection. The validity of proposed method was verified through the simulation in this study.

QUEST has a divertor configuration with a high and a negative n-index, and the problem of plasma vertical position instability control in QUEST is still under extensive study for achieving high efficiency plasma. The instability we considered is that the toroidal plasma moves either up or down in the vacuum chamber until it meets the vessel wall and is extinguished. The actively controlled coils (HCU and HCL) outside the vacuum vessel are serially connected in feedback with a measurement of the plasma vertical position to provide stabilizing control. In this work, a robust controller is employed by using the loop synthesis method, and provides robust stability over a wide range of n-index. Moreover, the gain of the robust controller is lower than that of a typical proportional derivative (PD) controller in the operational frequency range; it indicates that the robust controller needs less power consumption than the PD controller does.

A CW phased-array antenna system for electron cyclotron/Bernstein wave heating and current drive (ECWH/CD, EBWH/CD) experiments was developed in the QUEST. The antenna was designed to excite an elliptically polarized pure O-mode wave in oblique injection for the O-X-B mode conversion scenario, and its good performance was confirmed at a high power level. Long pulse discharges with a plasma current of 10 kA and 15 kA were non-inductively attained for 37 s and 20 s, respectively, with only radio frequency (RF) power. Divertor configurationswere also obtained in the RF-sustained plasmas. A new operational window for sustained plasma current was observed in the high-density plasma with a higher RF incident power. Two new heating and current drive systems with an 8.56GHz klystron and a 28GHz gyrotron are being prepared to conduct CW EBWH/CD experiments in the high-density plasma. © 2012 The Japan Society of Plasma Science and Nuclear Fusion Research.

In this paper, we proposed the mathematical model of campus by measurement data and theoretical approach. We measure dynamic characteristic and static characteristic of load in a system. Power capacitors in the university are assumed to be electric power customer's load. The load model is presumed by combining them and the linear model calculated theoretically. The line voltage and the line current are measured, and we calculate the line load capacity in distribution system of campus. The load fluctuation of distribution system during a day is observed. The load will be modelized by evaluating a lot of data. We can forecast the demand of electric power by few information that is the load model and the measurement data of receiving end.

In recent years, the debasement of the power quality of a distribution system such as deterioration of voltage imbalance and current imbalance or the increase of harmonics has been apprehended with diversification of the load by progress of power electronics technology and with increase of the amount of connection of a distributed generators for home use with the reduction of the effect on the environment. In this paper, the authors propose the restraint method of harmonics and improvement of voltage imbalance in distribution system utilizing the power conditioner systems of photovoltaic systems and verify the validity of the proposal method by the numerical calculation using an analytical model of distribution system.

Recently, the number of system interconnection of the renewable energy sources (RES) such as the photovoltaic generation (PV) and wind power generation is increasing drastically, and there is in danger of changing the voltages in a distribution system by the precipitous output variation of RESs. In this study, the authors propose one voltage control method of the distribution system by the power factor control of plural PV systems in consideration of cooperation with the load ratio control transformer (LRT) of laggard control response installed beforehand in the distribution system. In the proposed method, the slow voltage variation is controlled by LRT, and the steep voltage variation uncontrollable by LRT is controlled by plural PV systems, as a result, all the node voltages are controllable within the proper limits. In order to verify the validity of the proposed method, the numerical calculations are carried out by using an analytical model of distribution system which interconnected PV systems.

The amount of distributed generation (DG) such as photovoltaic systems and wind power generator systems installed in distribution systems has been increasing because of reduction of the effects on the environment. However, harmonic problems in the distribution system are a concern in the context of increased connection of DGs through inverters and the spread of power electronics equipment. This paper proposes a method for the restraint of voltage total harmonic distortion (THD) in a whole distribution network by active filter (AF) operation of multiple power conditioner systems (PCSs). It also proposes a method for determination of the optimal gain of AF operation so as to minimize the maximum value of voltage THD in the distribution network by real-time feedback control with measured data from information technology (IT) switches. In order to verify the validity of the proposed method, numerical calculations are carried out using an analytical model of a distribution network of interconnected DGs with PCSs. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(3): 19–29, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22354

Particle retention and recycling in plasma fusion devices are generally associated with the diffusion of atomic hydrogen into the materials. The resulted permeation of atomic hydrogen is known as plasma driven permeation (PDP). This permeation may also be significant, even in the walls, which are not directly exposed to the plasma. Under similar conditions, the permeation flux (Γperm) of hydrogen through a 30 μm thick Ni membrane heated at 412–575 K has been measured in the spherical tokamak QUEST. Γperm is being measured during the scans of different operating parameters like RF power (PRF), chamber pressure (Pchamber), discharge widths (τdis) and vertical magnetic field (BZ). Simultaneously edge plasma density and spectral intensities of atomic (Balmer) lines and molecular (Fulcher) bands have been compared with the permeation measurements. A linear relationship has been established between the time integrated Γperm i.e. permeation fluence (Qperm) and the time integrated Hα intensity i.e. Hα fluence (Qα). Qperm also shows a strong relationship with the edge plasma density and various spectral fluences. The obtained results are discussed for exploring the applicability of the permeation probes in measuring the atomic flux near the first walls.

In recent years, the number of connection of distributed generators (DGs) such as the photovoltaic generation (PV) and wind power generation is increasing, and there is in danger of changing the voltages in a distribution system by the precipitous output variation of DGs. In this study, the authors propose one voltage control method of a distribution system by the static var compensator (SVC) in consideration of cooperation with the load ratio control transformer (LRT) of laggard control response installed beforehand in the distribution system. The proposed method may be able to make the rated capacity of SVC small and may be able to reduce the aggravation of power factor of the distribution system, by setting up the dead band of voltage control appropriately. And the authors propose one determination method of the necessary minimum rated capacity and optimum control parameters of SVC in view of the cost of equipment and high-speed controllability. In order to verify the validity of the proposed method, the numerical calculations are carried out by using a distribution system model.

Recently, the total number of distributed generators (DGs) such as photovoltaic generation system and wind turbine generation system connected to an actual distribution network has increased drastically. The distribution network connected to many distributed generators must be operated keeping reliability of power supply, power quality, and loss minimization. In order to accomplish active distribution network operation to take advantage of many connections of DGs, a new coordinated operation of distribution system with many connections of DGs is necessary. In this paper, the authors propose a coordinated operation of distribution network system connected to many DGs by using newly proposed sectionalizing switch control, sending voltage control, and computation of available DG connection capability. In order to check the validity of the proposed coordinated operation of distribution system, numerical simulations using the proposed coordinated distribution system operation are carried out in a practical distribution network model. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 167(3): 46–57, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20655

Recently, the harmonic troubles in a distribution network are worried in the background of the increase of the connection of distributed generation (DG) and the spread of the power electronics equipments. As one of the strategies, control the harmonic voltage by installing an active filter (AF) has been researched. In this paper, the authors propose a computation method to determine the optimal allocations, gains and installation number of AFs so as to minimize the maximum value of voltage total harmonic distortion (THD) for a distribution network with DGs. The developed method is based on particle swarm optimization (PSO) which is one of the nonlinear optimization methods. Especially, in this paper, the case where the harmonic voltage or the harmonic current in a distribution network is assumed by connecting many DGs through the inverters, and the authors propose a determination method of the optimal allocation and gain of AF that has the harmonic restrictive effect in the whole distribution network. Moreover, the authors propose also about a determination method of the necessary minimum installation number of AFs, by taking into consideration also about the case where the target value of harmonic suppression cannot be reached, by one set only of AF. In order to verify the validity and effectiveness of the proposed method, the numerical simulations are carried out by using an analytical model of distribution network with DGs.