佐藤 之彦

This paper proposes a quasi-MPPT (maximum power point tracking) control for integrating multiple energy harvesters of different characteristics. Energy harvesting (EH) technology is a promising technology and is expected to be widely used in various fields. However, the amount of the harvested power is inherently limited and strongly depends on the weather and other conditions. Therefore, multiple energy harvesters are often integrated to increase total harvested energy. In the case of integrating multiple energy harvesters of different characteristics, the control tends to be complicated due to the different characteristics of the energy harvesters. The proposed quasi-MPPT control enables the multiple energy harvesters to work near their MPP voltages by referring to the command voltages derived from preliminary identification of the energy harvesters and measurement of the open-circuit voltages. It achieves harvesting of approximately maximum powers of the multiple energy harvesters by a simple control strategy using a power management circuit. The effectiveness is validated by using an experimental system that consists of photovoltaic (PV) and thermoelectric (TE) energy harvesters.

The voltage drop in GaN HEMTs during reverse conduction is larger than that of Si-MOSFETs, causing an increase in dead-Time loss and a rise in the bootstrap capacitor voltage when a bootstrap circuit is applied. We propose connecting a low-side device in parallel with a Schottky barrier diode as a solution. This allows the bootstrap capacitor to be charged at the appropriate voltage because the voltage drop is lower. This also reduces dead-Time losses but increases the losses due to capacitance across the connected diode terminals. These losses increase with frequency and are not negligible in ultrahigh frequency switching operations such as 1MHz. This study examines the effect of using anti-parallel diodes for GaN devices.

Carrier frequency of N-level flying capacitor converter (FCMLC) is equivalently (N-1) times higher than that of the 2-level converter with the same carrier frequency. In consequence, its control bandwidth is extended. However, updating the command value in that equivalent carrier period causes capacitor voltages to deviate from predetermined voltages. It is caused by mismatches between charging and discharging times of flying capacitors. Imbalanced flying capacitor voltages cause problems such as increase of harmonics in the output current and applied voltages to the devices. In this study, we derive the discrete-time state-space equation of FCMLC considering flying capacitor voltages and analyze stabilities of flying capacitors. Depending on the analysis, a compensation method for capacitor voltages is proposed, and the effectiveness of it is verified by the experiments.

In motion control systems, detailed characteristics of current control systems are usually not considered. However, as the performance of the overall system advances, higher-performance current control systems are required. This paper proposes a novel PI current control method for realizing deadbeat control characteristics. Conventional deadbeat control is complicated because it is necessary to calculate the pulse width for each sampling cycle. On the other hand, proposed method achieves deadbeat characteristics easier than conventional method by simply determining the PI gains. It can also be applied to multi-level inverters. By simulations and experiments, we confirm that the deadbeat characteristic is obtained. The robustness of the system against disturbances and modeling errors is also confirmed.

This paper proposes a control method for a bidirectional isolated three-phase AC/DC Dual Active Bridge (DAB) converter based on matrix converter (MC) that realizes a wide output voltage range. This method achieves the reduction of excessive high frequency link current and soft switching with online calculation by changing two modulation methods according to the input and output voltage ratio. The proposed method realizes higher efficiency in expanded output voltage range with reduced online calculation burden. The feasibility and validity of the proposed method are verified by experimental demonstration.

In this paper, we study a control method that maintains stability regardless of the number of connected power converters in islanded microgrids. It is known that variations in the number of the power converters connected to the islanded microgrids may lead to system instability, since the change of the number of the connected power converters is equivalently considered as variations of the grid impedance. Therefore, we study a control method for the individual power converter that is robust against the grid impedance variations and clarify the stability range, since it is possible to achieve robust stability of the whole microgrid system by configuring it only with the power converters that are robust against the variations of the grid impedance. We present stability analysis, and simulation and experimental verifications of the proposed robust stabilization method.

A power flow controller (PFC) that realizes flexible power flow control in DC power networks is investigated herein. In previous studies, an additional node called a compensation node was introduced in PFC. This compensation node is necessary for regulating the link voltage that decreases owing to circuit loss. However, as the compensation node is not directly related to the power flow control, adding the compensation node may increase the cost. In this paper, we propose a link voltage control method for PFC without the compensation node. Adding the current command for the link voltage control to the current command for the power flow control makes it possible to realize flexible power flow control and constant link voltage control without the compensation node. The effectiveness of the proposed method is verified experimentally.

This paper proposes a sensorless link voltage control method for bidirectional power flow controller for next-generation DC power network. In our previous works, we studied methods to keep the link voltage at a constant value. The methods were implemented based on feedback of the measured link voltage values. It therefore needs additional costs for measuring equipment of the link voltage. In this paper, we propose a link voltage control method that does not use the measured link voltage value, and enables to keep the link voltage certainly higher than the node voltages. The method well simplifies the implementation and reduces the costs. The effectiveness of the proposed method is verified by experimental results.

Filters are inevitable for grid-connected inverters to attenuate the current harmonics caused by the pulse width modulation which is usually used in power conversion systems. High-order filters have attracted much attention because they attenuate the current harmonics effectively. Nevertheless, the high-order filters have some resonances which cause instability of the system. In addition, the resonance frequencies shift to high as the inductors and capacitors are smaller. It implies that the resonance frequencies may be beyond the Nyquist frequency in downsizing the filter. This complicates the stability and performance analyses of the system. This paper investigates rigorous input-output stability and analyses a robust performance based on the sampled-data control theory regardless of whether the resonance frequencies are beyond the Nyquist frequency or not. Our analysis contributes to downsizing the filter synthesis while the stability and the robustness are guaranteed even if the resonance frequencies are beyond the Nyquist frequency. The effectiveness of the proposed method is verified through simulations and experiments.

A high-frequency link three-phase ac/dc converter based on matrix converter is widely studied because of its small size of passive components, and some modulation methods have been proposed. However, these methods have not achieved zero-voltage-switching (ZVS) and sinusoidal line currents under bidirectional operation with PQ control. This article presents a new modulation method to realize ZVS and sinusoidal line currents under bidirectional operation by accurate control parameters (duty cycle and phase shift) for high-frequency link bidirectional three-phase ac/dc dual-active-bridge (DAB) converter. The converter can realize ZVS by phase-shift-modulation similarly to the dc/dc DAB converter. The line currents have nonlinear characteristics to phase shift and duty cycle. Therefore, the proposed method uses a nonlinear mathematical model to determine the accurate duty cycle and phase shift in order to realize sinusoidal line currents. The accurate duty cycle and phase shift are determined by real-time numerical calculation. Experimental results employing a 1-kW laboratory prototype verify the capability to control active and reactive power with sinusoidal line current. It is confirmed that the proposed method can realize the sinusoidal line current with total harmonic distortion of less than 4% under the bidirectional operating condition at the rated power.

In motion control systems, detailed characteristics of power converters are usually not considered. Practically, voltage ripple, harmonics, and electromagnetic interferences (EMI) generated in widely used 2-level inverters become concerns to realize a high-performance control. Compared with 2-level inverters, multi-level (ML) inverters essentially reduce these problems. Furthermore, the equivalent carrier frequency of N-Ievel ML inverters is expected to be N-l times higher than that of the 2-level inverters in case that a carrier phase-shifted modulation is utilized. This paper focuses on the equivalent carrier frequency and studies the performance and stability of current control systems by using ML inverters.

Many techniques have been proposed as power supply methods for high-side gate drivers. However, most techniques that do not depend on the topology of the main circuit have drawbacks related to the size and cost. This paper proposes a novel floating power supply based on capacitive isolation. The proposed circuit is designed to operate independently of the main circuit, and it is cheaper and smaller than conventional circuits. The proposed method is applied to a diode-clamped multilevel topology and is validated by experiments.

This paper studies a resonance suppression control based on virtual resistance concept for parallel inverters in islanded microgrids. At the inverter systems with LC output filters, resonances often occur due to various reasons. To suppress the resonances, additional resistances are often used to dampen the system (passive damping). However, additional resistance inevitably causes additional losses. On the other hand, many active damping approaches have also been studied. In this paper, we study one of the active damping methods, the resonance suppression control based on virtual resistance concept. In particular, the resonance suppression control implemented at the current-controlled inverter and that implemented at the voltage-controlled inverter in islanded microgrids are comparatively studied. The effectiveness is evaluated by analyses of frequency characteristics, simulation studies, and experimental results in the case of two parallel inverters in islanded microgrids.

This paper proposes a novel commutation method that improves efficiency of bidirectional isolated AC/DC Dual-Active-Bridge (DAB) converter based on matrix converter (MC). Although some modulation methods have been proposed, characteristics of the converter in low power region have not been discussed. In this region, efficiency of converter decreases because of commutation fault, which increases stress to devices and leads to shorten lifetime of the converter. In this paper, an advanced commutation method to reduce the commutation failure is proposed. The proposed method combines current commutation and voltage commutation implemented by a simple algorithm based on the waveform of modulation signal without information of the current direction. In addition, seamless and safe operation is inherently assured owing to no requirement of change in switching operation mode. The validity of the proposed method is verified by experiment employing a laboratory prototype.

The paper confirms that removing antiparallel silicon carbide (SiC) Schottky barrier diodes (SBDs) from SiC-based inverters offers positive effects without critical impact on inverter loss and electromagnetic interference (EMI) issues, moreover, the removal of SBDs reduces the inverter losses in many cases and noise emissions. This conclusion leads to the possibility to improve the power densities by removing SBDs. However, the removal of SBDs may cause some disadvantages such as an increase of the reverse conduction loss and influence of the body diode recovery phenomenon. Therefore, a comprehensive investigation of these advantages and disadvantages is necessary. In this paper, design criteria are proposed to clarify the conditions in which SiC-based inverters without SBDs have advantages over those with SBDs from the viewpoint of losses. On the other hand, to achieve the removal of SBDs, it is also necessary to confirm that removing SBDs does not cause severe EMI issues. The paper confirms that switching noises are reduced by the removal of SBDs; this is due to the larger damping effect of the SiC MOSFETs without SBDs than that of SiC MOSFETs with SBDs. The validity of the theoretical analyses and design criteria is confirmed with comprehensive experimental results.

In recent years, AC and DC microgrids have been extensively studied. By constructing microgrids, it is possible to promote installing renewable energy resources. Most of components (renewable energy sources, storage devices, load, and so on) are connected to the microgrids through power converters. Therefore, due to spread of microgrids, the importance and the use of power converters have been increasing. As a practical solution to build microgrids, our group has proposed a concept of "plug-and-play" converter. The concept means that the plug-and-play converter is flexibly applied to various components with a unified circuit topology in three-phase AC, single-phase AC, and DC microgrids. As one of the essential functions for development of the plug-and-play converter, this paper realizes universal grid interconnection control method. The proposed control method enables to transmit and receive desired active power in three-phase AC, single-phase AC, and DC microgrids, without changing the control system and circuit topology. Simulation studies verify the validity of the universal grid interconnection control method.

This paper presents a high power density silicon carbide (SiC)-based inverter, with a two-level voltage-source structure having forced air cooling, which provides a high volumetric power density of 70 kW/liter or 50 kW/kg in gravimetric terms. In order to achieve a power density greater than that of conventional inverters, the losses must be reduced or the cooling performance must be improved. Small light-weight SiC MOSFET power modules with directly soldered foil fins having good thermal conductivities, are developed in this study. The antiparallel SiC Schottky barrier diodes (SBDs) are removed from the modules to improve the power density. Gate drivers are developed to reduce the switching losses and switching time. A prototype of the proposed high power density inverter, which includes the developed power modules and proposed gate driver, is fabricated. The volume and weight of the prototype inverter are approximately 0.5 liter and 660 g, respectively. Experimental results confirm that the prototype inverter can operate continuously with an output power up to 35 kW. Therefore, the power density of the prototype inverter is approximately 70 kW/liter or 50 kW/kg. The efficiency of the prototype inverter is found to be more than 98%. Hence, the measures undertaken in this study have been verified to improve the power density of the inverters. The proposed high power density inverters can be applied in future aircraft and other electric vehicles.

This paper describes about a two-stage power conversion system with high frequency isolated AC/DC converter to interface the AC distribution network with the dc load. This system converts the three phase AC power input from the grid to DC power output for supplying the DC load. The proposed system is composed of three-phase to single-phase matrix converter, high frequency isolated transformer and full-bridge AC/DC converter. The matrix converter directly converts the 50Hz frequency AC input to a high frequency AC output without a dc-link. The output of the matrix converter is then processed by the full-bridge converter via a high frequency isolation transformer to interface with DC network. The system performance is demonstrated by both simulation and experimental work.

This paper presents a new modulation method for a bidirectional isolated three-phase AC/DC Dual-Active-Bridge (DAB) converter to realize higher efficiency in wide output voltage range. The proposed modulation method realize the bidirectional power flow control with Zero Voltage Switching (ZVS) in all switching devices and low conduction losses by using 5 level PWM and PSM (Phase Shift Modulation). Experimental results employing a 10kW prototype converter using SiC-MOSFETs verifies the effectiveness of the proposed method.

This paper presents a high-speed, low loss, and low noise gate driver for silicon-carbide (SiC) MOSFETs. We propose a gate boost circuit to reduce the switching loss and delay time without increasing the switching noise. The proposed gate driver enables converter-level efficiency improvements or power density enhancements. SiC MOSFETs have attracted significant interest as the next generation power devices. In general, the switching performance of power devices exhibits a trade-off between switching loss and noise. SiC-MOSFETs are expected to switch faster than Silicon IGBTs; however, faster switching might cause switching noise problems such as unwanted electromagnetic interferences (EMI). In this paper, we propose a gate driver topology that improves the switching performance of SiC-MOSFETs, and confirm the reduction in switching loss and delay time through experimental results.

To realize flexible power flow control for DC power networks, we have proposed a bidirectional power flow controller (BPFC) based on a bidirectional buck-boost converter. It is possible to control the power flow by intentionally introducing additional voltage difference between two terminals. However, we have not investigated the efficiency of BPFC for power flow control in detail so far (in previous papers). Because many power converters are expected to be installed in future DC power networks, efficiency investigation is vital. In this paper, we first reveal the energy efficiency of BPFC and study ways to improve the efficiency. Then, we propose a novel structure of the BPFC, which drastically improves its efficiency. We call the proposed structure floating bidirectional power flow controller (F-BPFC). The efficiency and characteristics of the F-BPFC are experimentally compared with those of the BPFC and its superiority is clarified in this paper.

In AC motor drive, the harmonic components in the output voltage of inverters generate additional iron losses in magnetic cores and additional copper losses in windings of the motor. Particularly in the case of high speed motors, the ripple of the current flowing through the motor increases, because the inductance is designed to be small. Therefore, the additional losses caused by the ripple of the current are often serious problems in the high speed motors.In this research, as a solution to these problems, multilevel inverters are applied to high speed motor drive systems. Multilevel converters can generate multilevel output voltage using internal voltage sources. Thus, they can reduce harmonics and EMI.In this paper, the loss reduction effect in the high speed motors by the multilevel inverters is confirmed by simulation and experiment.

SiC-MOSFETs are expected as promising power switching devices from the view point of high-speed switching and lower on-state resistance. In the multilevel inverters, the number of switching devices in the main current path increases. Thus, lower on-state resistance of the SiC-MOSFETs is very useful to reduce conduction loss of the inverter. In addition, to reduce required capacitance of the flying capacitors, higher switching frequency operation is indispensable. From this point of view, the higher switching frequency in high power applications realized by the SiC-MOSFETs is very attractive. In this way, the SiC-MOSFETs can dramatically enhance the advantages of the flying capacitor multilevel inverters. In this paper, as a basis of design consideration for flying capacitor multilevel inverters using SiC-MOSFETs, various experimental investigations employing four different prototypes are carried out. The effect of the number of the output levels and thermal management are demonstrated. In addition, key issues in extension to higher voltage applications are investigated.

This paper studies a novel control approach to multi-terminal power flow controller (MTPFC) for next-generation DC power network. We have proposed and validated bidirectional power flow controller (BPFC) and MTPFC as promising power flow controllers for next-generation DC power networks. In BPFC, total power imbalance possibly causes voltage fluctuation of the inter-stage link capacitor. The fluctuation affects performance of the power flow control. Then, in MTPFC, a compensation node that keeps the inter-stage link capacitor voltage at a predetermined constant value, plays an important role for precise operation of MTPFC. However, the introduction of the compensation node needs additional costs. In this paper, we propose a novel control approach that properly controls power flows even in case that there are fluctuations of the voltage of the inter-stage link capacitor. The control approach is realized by using measured values of the voltages for the control system. The proposed approach is validated by experimental results.

This paper presents a new modulation method for high-frequency link bidirectional three-phase AC/DC converter based on matrix converter. Since the matrix converter does not have electrolytic capacitor, it achieves high power density and long lifetime system. The modulation method performs bidirectional active and reactive power control with sinusoidal line current by using optimal duty cycle and phase shift. The optimal duty cycle and phase shift are determined by numerical calculation based on the system mathematical model. Experimental results employing a 1kW laboratory prototype verify the capability to control active and reactive power with sinusoidal line current.

This paper proposes a new method of eliminating DC bias flux of high frequency transformer in a bidirectional isolated AC/DC converter. The converter treated in this paper has a matrix converter and a full-bridge converter in primary and secondary sides of a high-frequency isolation transformer, respectively. Exciting current of the transformer pulsates when the matrix converter and full-bridge inverter are controlled for eliminating dc bias flux at the same time. Therefore, this paper proposes a decoupling control method considering physical model of the transformer and shows its effectiveness by simulation.

This paper deals with offshore wind system with DC collector grid, in which a high-voltage (HV) DCDC converter is utilized between the DC collector gird and HVDC transmission line. This study is focused on the circuit configuration of the HV DC-DC converter. Conventionally, a modular multilevel converter (MMC) is used for AC-DC conversion in HVDC systems. However, adopting the MMC topology for the HV DC-DC converter cannot achieve high efficiency because of hard switching. This paper proposes a novel configuration of a 1-GW +/- 25-kV/+/- 350-kV dual-active bridge DC-DC converter for offshore wind system, which comprises two series-connected IGBTs to reduce the transformer weight and increase the efficiency. The proposed configuration can achieve the efficiency of 98.5%.

We studied vibration power generation using piezoelectric elements in order to determine effective rectification methods to increase harvested energy from vibration power generation systems. First, we experimentally clarified the output characteristics of two kinds of passive rectification methods for vibration power generation systems. Then, an appropriate rectification method for various kinds of loads was studied. In order to increase the harvested power from an integrated vibration power generation system with multiple piezoelectric elements, we considered an appropriate circuit structure for connecting multiple piezoelectric elements. The effectiveness was verified experimentally.

Grid-connected voltage source converters are widely used in various applications. In these Grid-connected converters, volume of the passive components such as AC side inductors and harmonic filters becomes sometimes a practical problem. As a solution to reduce the volume of the passive components in the grid-connected converters, flying capacitor multilevel converters are expected. The multilevel converters can generate multilevel voltage waveforms in the AC side with smaller instantaneous deviation from its reference. Therefore, the multilevel converter can miniaturize the AC side inductors and harmonic filters. In this paper, to obtain guidelines to reduce the volume of the passive components, the minimum required capacitance of the flying capacitors and required inductance of the AC side inductors are investigated.

DC microgrid has been drawing much attention in recent years. In DC microgrid, introduction of renewable energies to demand sides possibly causes power imbalance and reverse power flows on feeders. These problems cause increase of distribution losses and voltage rises in feeders. To overcome these problems, a power flow control method among multiple DC feeders (nodes) is required. We have studied bidirectional power flow controller (BPFC) based on a bidirectional buck-boost converter. It realizes power flow control by intentionally generating voltage difference between two terminals. Furthermore, BPFC has been extended to multi-terminal power flow controller (MTPFC) that realizes power flow control among multiple DC nodes. However, output voltages of BPFC and MTPFC are excessively higher than minimally required voltages for realizing the power flow control. That leads to unnecessary increase of conversion capacity of the circuit, and therefore seriously deteriorates the efficiency. Then, we studied a novel circuit structure, floating bidirectional power flow controller (F-BPFC). F-BPFC outputs the voltage just equal to the minimally required voltage for realizing the power flow control. Therefore, it is able to be implemented with low conversion capacity, and significantly improves the efficiency. In this paper, to realize high efficiency power flow control among multiple DC nodes, we extend F-BPFC to multi-terminal cases. We call it, floating multi-terminal power flow controller (F-MTPFC). We study the power flow control by using F-MTPFC with introduction of a novel control method. The effectiveness, feasibility, and efficiency of F-MTPFC are validated by experimental studies.

We studied vibration power generation using piezoelectric elements in order to determine effective rectification methods to increase harvested energy from vibration power generation systems. First, we experimentally clarified the output characteristics of two kinds of passive rectification methods for vibration power generation systems. Then, an appropriate rectification method for various kinds of loads was studied. In order to increase the harvested power from an integrated vibration power generation system with multiple piezoelectric elements, we considered an appropriate circuit structure for connecting multiple piezoelectric elements. The effectiveness was verified experimentally.

This study investigates power flow control methods for next-generation DC power networks. It has been anticipated that numerous distributed generators and energy storage devices will be introduced in next-generation power systems. Therefore, arbitrary and flexible power flow control methods for such power systems are required. In this work, we study a power flow control method that combines node voltage control and additional voltage insertion control on links (link voltage control) for controlling next-generation DC power networks. The effectiveness of the proposed power flow control method is demonstrated by experimental results. It is shown that the proposed power flow control method can be expected to be one of the promising approaches for the realization of next-generation power systems.

Multilevel inverters can reduce harmonics and EMI. Therefore, miniaturization of output filter is realized compared with general 2-level inverters. Especially, flying-capacitor multilevel inverters have an advantage over other multilevel circuit topologies in terms of downsizing. In the flying capacitor topologies, multilevel voltage waveform is generated by the capacitors (called flying capacitors) which maintain their predetermined voltage. In this paper, flying capacitor inverters used in grid-connected applications are investigated. Power density of the inverters is evaluated so that it can be used as a guideline to realize downsizing and loss reduction of the inverters. Firstly, the loss of the power devices, flying capacitors, and interconnection inductors that are major loss components of the inverters are calculated theoretically. Furthermore, the volume of heatsinks for power devices, flying capacitors and interconnection inductors are calculated theoretically. Based on these theoretical results, power density of the inverters is calculated. As a result, it is clarified that the multilevel inverter can reduce the total volume of flying capacitors and interconnection inductors compared with the conventional 2-level inverters. In addition, by increasing the PWM frequency, advantages of the flying capacitor multilevel inverter are maximized. In this study, the maximum power density of the main circuit portion is 4.97W/cm(3) in 2-level inverter, 25.11W/cm(3) in multilevel (9-level) inverter. Thus, it is demonstrated that the multilevel inverter is advantageous to realize high power density inverters.

Multilevel converters can generate the multilevel output voltage using internal voltage sources. Thus, they can reduce harmonics and EMI. From the practical point of view, implementation of the source of multilevel voltage without increase in the volume of the converters is strongly desired. In this context, the flying capacitor multilevel converters are most promising candidates for the practical multilevel converters.AC motor drive is one of typical applications of inverters. In this case, the additional losses caused by the harmonics in the inverter outputs often become practical problems. As a solution to these problems, the multilevel inverters are expected to reduce the harmonic losses. As the number of output levels in the multilevel inverters increases, the harmonics of the output current further decrease. However, the losses in the inverter may increase due to the number of the circuit components in the paths of the main currents will increase.In this paper, as a guideline to minimize the total losses of the multilevel inverters and the motors, we investigate the loss components in the flying capacitor multilevel inverters and induction motors for several different number of the output levels under various operating conditions.

This paper presents fast, low loss, and low noise gate driver for Silicon-Carbide (SiC) MOSFETs. We proposed gate boost circuit to reduce switching losses and switching delay time without increasing switching noise. The proposed gate driver makes it possible to improve converters efficiency or enhance power density of converters. SiC power devices have attracted huge interest as next generation power devices. Normally, switching performances of power devices have tradeoff between switching losses and switching noise. SiC-MOSFETs are expected to be able to faster than Silicon IGBTs, but faster switching might cause switching noise problem such as electromagnetic interferences (EMI). We proposed gate boost circuit to improve switching performances of SiC-MOSFETs, and also confirmed that the proposed gate driver reduce switching losses and delay time with experimental results.

This paper studies efficiency improvement of DC power network by using multi-terminal power flow controller (MTPFC). We have proposed MTPFC for flexible and precise power flow control in DC power networks. The circuit topology and the control method have been investigated and it has been clarified that MTPFC is effective for power flow control in DC power networks. In this paper, we address efficiency improvement by reducing the loss in distribution line of power network by using MTPFC. A case study is conducted and the effectiveness for efficiency improvement is discussed.

Multilevel inverters have very attractive features, such as lower harmonics in the output, lower EMI, and reduction of the required voltage rating of power semiconductor devices. Among them, lower harmonics in the output can reduce the volume of the output harmonic filter and additional losses caused by the harmonics. Therefore, multilevel inverters are expected to realize higher power density and higher efficiency. In this paper, as a basis of the quantitative investigation of these features, the harmonics in the PWM output voltage of multilevel inverters are analyzed theoretically. As an application of the theoretical results, the usefulness of the theoretical results is verified by the prediction of the harmonic contents of the load current.

DC power networks with introduction of distributed generation (DG) and energy storage (ES) systems have been drawing much attention in recent years. To control power flows in DC power networks, voltage of each terminal is usually an only controllable variable. Therefore, implementation of control method is generally simple, but it lacks flexibility particularly when the structure of the power network is complicated. To overcome the problem, a more flexible power flow control method that includes a power flow controller implemented on the distributed line has been proposed. The controller makes additional voltage differences on the distribution line and realizes bidirectional and flexible power flow control. In this paper, we consider DC power networks with complicated connections of DG and ES systems. For implementing simple and intuitive power flow control, we propose a multi terminal power flow control method that consists of a multi terminal power flow controller (MTPFC) and a compensation node for MTPFC. The control method makes the DC power network simpler and enables to realize flexible and robust power flow control. The effectiveness and the feasibility of the proposed control method are validated by experimental studies.

Multilevel converters are expected as a fundamental solution to harmonics and EMI problems caused by power converters. In general, the multilevel converters generate the output voltage using voltages of internal capacitors or floating voltage sources. From the practical point of view, implementation of the source of multilevel voltage without increase in the volume of the converter is strongly desired. In this context, flying capacitor multilevel converters are the most promising candidates for the practical multilevel converters. In this paper, the minimum required capacitance of the flying capacitors is investigated for the case of grid-connected inverters in which the reduction of the harmonics and EMI is strongly required. The minimum required capacitance is determined considering the allowable range of voltage variation in the flying capacitors to ensure proper operation both in the steady state and transient conditions. For the determination of the minimum capacitance, theoretical expression of the voltage ripple in the flying capacitors is derived. The validity of the theoretical expression is confirmed experimentally. In addition, the transient operation under severe disturbances such as a sudden change in power flow and a short-time power line fault is confirmed experimentally.

In motion control systems, detailed characteristics of power converters and motors are usually not considered. Practically, voltage ripple, harmonics, and electromagnetic interferences (EMI) generated in widely used 2-level converters become concerns to realize a high-performance control. Those possibly degrade the control performance especially in sensitive applications such as the case of small motors, high-speed and high-precision systems, and haptic systems. In this paper, performance improvement of motion control systems by applying multi-level converters is studied experimentally in a force control system of a DC motor. Some experimental results clarify that force control gain and cutoff frequency of DOB (disturbance observer) are able to be larger in the case of multi-level converters compared with the case of 2-level converters. It is confirmed that the multi-level converter improves the force control performance compared with the case of 2-level converter even when an identical control system is used.

Multilevel converters have a number of capacitors as sources of multilevel voltages. The volume of the capacitors should be minimized to realize a high-power-density converter. In this paper, the selection criteria of the capacitors in a flying capacitor converter are investigated. In this type of converter, the required capacitance, consequently, volume of the capacitors are decreased when the PWMcarrier frequency is higher. However, there is a limitation on the reduction in the volume due to the temperature increase caused by the power losses in the capacitors when the volume becomes small during high-PWM-frequency operation. On the basis of an investigation including the temperature increase, it is clarified that a capacitor with a high capacitance density is not necessarily the best for the flying capacitor depending on the operating conditions.

400-V direct current (400-V dc) distribution systems are promising candidate of highly efficient and reliable distribution systems for data centers. To realize the 400-V dc distribution systems, development of methods for high-speed overcurrent protection is one of the important issues. In this paper, as a solution to this problem, a semiconductor dc circuit breaker using SiC static induction transistors (SiC-SIT's) is investigated. The SiC-SIT's has extremely low on-state resistance and very large safe operating area (SOA). These properties are attractive in the application of the circuit breakers for 400-V dc distribution systems. A novel control method of the gate voltage waveform to reduce the transient overvoltage and resonance during the interruption process is proposed. The effectiveness of the proposed method is confirmed by actual operation tests employing an experimental prototype of the dc distribution system. All of the result is confirmed by the fabricated SiC-SIT circuit breaker prototype.

Multilevel converters can essentially reduce harmonics even when their switching frequency is low. Among the various topologies of the multilevel converters, flying capacitor converters are considered to be promising converters for realizing high power density. However, the main circuit of the flying capacitor converters has many capacitors. Therefore, in this study, the volume of the capacitors in the flying capacitor converters is determined by taking into consideration the allowable ripple voltage and temperature rise in the capacitor. (c) 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 186(4): 81-91, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22361

Multi-level inverters have very attractive features such as lower harmonics in the output, lower EMI, and reduction of the required voltage rating of power semiconductor devices. Among them, lower harmonics in the output can reduce the volume of the output harmonic filter and additional losses caused by the harmonics. Therefore, multi-level inverters are expected to realize higher power density and higher efficiency. In this paper, as a basis of the quantitative investigation of these features, the harmonics in the PWM output voltage of multi-level inverters are analyzed theoretically. As an application of the theoretical results, the usefulness of the theoretical results is verified by the prediction of the harmonic contents of the load current. © 2014 The Institute of Electrical Engineers of Japan.

To realize massive integration of distributed renewable energy resources in future power system, the development of efficient measures for their integration is required. Using DC grid is more efficient and more compatible for the integration of renewable energy resources and energy storage devices. In the next-generation DC power network, many kinds of nodes which consist of generators, loads, energy storage devices, and so on and links (distribution lines) are connected. Thus, the grid structure is complex, and flexible power flow controls are essential. In a DC distribution network, the only controllable parameter is voltage of nodes, thus it is difficult to control the power flow of each link independently. In this paper, we study a power flow control on the links named link voltage control based on a bidirectional buck-boost converter implemented on the link. In addition to the voltage difference between the nodes, the link voltage controller generates additional voltage difference on the link intentionally. Thus, it is possible to control power flow of a specific link independently without affecting power flow of other links. In this way, flexible controls of power flows in a multi-terminal DC power network become possible. The effectiveness of the power flow control is demonstrated by experiments. The high controllability of the link voltage controller will contribute to the realization of future DC power grid.

Multi-level converters can essentially reduce the harmonics and Electro Magnetic Interference (EMI). The number of output levels should be higher drastically to realize extremely high quality output and low EMI. However, as the number of levels increases, the number of circuit components also increases and implementation will be difficult significantly. Thus, flexible building platform of the multi-level converters suitable for the higher number of the output levels should be developed. As a solution to this problem, a concept of Multi-Level converter Building Modules (MLBM) to realize the multi-level converter with a higher number of output levels systematically has been proposed in ECCE 2013. A prototype of the MLBM has been constructed as a 5-level flying capacitor converter with extendable configuration. However, in the previous version of the MLBMs (MLBM Type-I) without any heat sinks, there is a restriction to increase the rated power of the converter when plural MLBMs are combined. In this paper, the improved thermal management of the MLBM is investigated to realize higher power capacity as well as higher number of levels. The module configuration to improve the heat dissipation of devices by using aluminum-based printed circuit board (MLBM Type-II) is developed. The proposed configuration enables to obtain the higher power density in the multi-level converter with higher number of output levels.

In the field of motion control systems, voltage ripple, harmonics, and electromagnetic interferences (EMI) in an output of converters negatively affect the control performance. However, so far, there has been practically few works that explicitly consider the power converters in motion control systems. Currently, general 2-level converters are widely used. However, it is concerned that the 2-level converter becomes one of the obstacles to realize high-performance control because the output voltage has high harmonics and EMI. Those possibly degrade the position or force control performance. As the solution to this problem, linear amplifiers may be useful because they outputs continuous voltage without pulsed waveform. However, low efficiency of the linear amplifiers becomes critical issue as the power converter. In this paper, performance improvement of the motion control systems by applying multilevel converters is investigated as a solution to realize both high control performance and high efficiency at a higher level. From the results of some experiments, it is seen that current waveform is distorted in the case of using the 2-level converter due to the output voltage ripple of the converter. In addition, the operation of disturbance observer (DOB) causes current ripple with lower frequency. On the other hand, it is confirmed that 9-level converter reduces the ripples of the current effectively. Moreover, EMI which is major concern in high-performance control system such as extremely small motor applications for high-precision control, can be improved by using the multi-level converter even without any filters.