This paper discusses a 6.6‐kV transformerless STATCOM intended for installation on industrial and utility distribution systems in the near future. In addition, this paper provides experimental results obtained from a laboratory model rate at 200 V and 10 kVA. The authors propose such a control method as to superimpose a sixth harmonic zero‐sequence component on each of three‐phase voltage references. This helps to stabilize the voltage of the inner midpoint in the DC link. As a result, the laboratory model installs two bidirectional buck‐boost choppers on the DC link for the purpose of stabilizing the voltages of two outer midpoints. Experimental results obtained from the laboratory model verify the validity of the system design, giving promise of the viability of the 6.6‐kV transformerless STATCOM. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 168(2): 60–68, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20733
This paper describes the design and performance of a 6-kW, full-bridge, bidirectional isolated dc-dc converter using a 20-kHz transformer for a 53.2-V, 2-kWh lithium-ion (Li-ion) battery energy storage system. The dc voltage at the high-voltage side is controlled from 305 to 355 V, as the battery voltage at the low-voltage side (LVS) varies from 50 to 59 V. The maximal efficiency of the dc-dc converter is measured to be 96.0% during battery charging, and 96.9% during battery discharging. Moreover, this paper analyzes the effect of unavoidable dc-bias currents on the magnetic-flux saturation of the transformer. Finally, it provides the dc-dc converter loss breakdown with more focus on the LVS converter.
This paper presents a front-to-front (FTF) system based on modular multilevel cascade converters (MMCC). It is called an FTF system because the ac sides of the MMCCs are connected together making a front-to-front configuration via a medium-frequency transformer for voltage matching and galvanic isolation. The system configuration is applicable to dc power collections. Moreover, it is suitable as a power converter for multi-terminal dc power networks since it can handle dc faults inherently without using costly dc circuit breakers. Simulated results using a "PSCAD/EMTDC" software package verify the operating principles and control method of the FTF system.
This paper discusses and compares phase-shifted pulsewidth modulation (PS-PWM) and level-shifted PWM (LS-PWM) in a modular multilevel single-delta bridge-cells (SDBC) inverter for utility-scale grid-tied photovoltaic (PV) applications. A unique characteristic of this inverter application is the capability to operate even under imbalanced power distribution among its bridge cells. An imbalanced energy distribution may degrade harmonic performance and limit the inverter operating range. The focus of this research lies in analyzing the following two fundamental properties; waveform quality and energy balancing capability when either PS-PWM or LS-PWM is applied. This paper highlights the benefits of LS-PWM over PS-PWM in this particular application, and demonstrates that the SDBC inverter is suitable for applications with imbalanced power distribution. Experimental results obtained from a three-phase 10-kW downscaled SDBC inverter with six bridge cells per cluster show good agreement with those from theoretical analysis.
A parallel technique for current-controlled pulse-width-modulated (PWM) inverters is described. Two voltage source inverters, the output terminals of which are connected in parallel through current balancers, are used as a main circuit. In this scheme, excellent characteristics both in steady states and in transient states are obtained, keeping the average values of the cross current and zero sequence current at zero level. This current control scheme is applicable to large-capacity GTO inverters because good performance is attained even if the switching frequency is only a few hundred hertz, as is shown in the experimental results.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
This paper presents a three-phase transformerless cascade pulsewidth-modulation (PWM) static synchronous compensator (STATCOM) intended for installation on industrial and utility power distribution systems. It proposes a control algorithm that devotes itself not only to meeting the demand of reactive power but also to voltage balancing of multiple galvanically isolated and floating dc capacitors. The control algorithm based on a phase-shifted carrier modulation strategy is prominent in having no restriction on the cascade number. Experimental waveforms verify that a 200-V 10-kVA cascade PWM STATCOM with star configuration has the capability of inductive to capacitive (or capacitive to inductive) operation at the rated reactive power of 10 kVA within 20 ms while keeping the nine dc mean voltages controlled and balanced even during the transient state.
The combined system of a series active filter and a shunt passive filter that is proposed by the authors has the ability to eliminate a major disadvantage of shunt passive filters, namely, harmonic amplification. The series active filter needs a much smaller kVA rating than a conventional shunt active filter, and as a result, the combined system has good filtering characteristics and high efficiency. This paper presents an optimum design of the shunt passive filter that makes possible a great reduction in the required kVA rating of the series active filter. It can minimize the peak voltage across the series active filter and reduce the required kVA rating of the filter to 60 percent. A computer simulation geared to practical applications of large three‐phase. twelve‐pulse thyristor rectifiers is used to compare the compensation characteristics of the optimized system with those of a combined system that uses a conventional shunt passive filter. Experimental results obtained with a laboratory model of the filter corroborate the design approach.
This paper describes position-sensorless control of an interior permanent magnet synchronous (IPM) motor, which is characterized by real-time position estimation based on magnetic saliency. The real-time estimation algorithm detects motor current harmonics and determines the inductance matrix, including rotor position information. An experimental system consisting of an IPM motor and a voltage-source pulsewidth modulation inverter has been implemented and tested to confirm the effectiveness and versatility of the approach. Some experimental results show that the experimental system has the function of electrically locking the loaded motor, along with a position response of 20 rad/s and a settling time of 300 ms.
This paper provides an intensive discussion on a modular multilevel cascade conversion system based on double-star bridge cells (DSBCs), which is referred to simply as a DSBC conversion system. This conversion system is defined as a power conversion system consisting of multiple DSBC converters and medium-frequency isolation transformers. A voltage fluctuation appearing across the floating dc capacitor of each bridge cell is investigated. Theoretical and numerical considerations regarding both circulating currents and the medium frequency lead to a reduction in the peak-to-peak voltage fluctuation. The system is therefore applicable to a wide variety of motor drives with small capacitors. Simulated waveforms verify principles of operation, which are obtained from the 6-kV, 5-MW system consisting of a single DSBC converter for grid connection, two DSBC converters for three-phase star-connected RL loads, and two 560-Hz transformers for galvanic isolation among the grid and the loads.
No abstract is provided for this article.
The authors present the analysis of the neutral-point potential variation of the neutral-point-clamped voltage source PWM (pulse-width-modulation) inverter (NPC-VSI) for AC motor drives and static VAr compensators (SVC). The potential variation is analyzed with the focus on the current flowing out of or into the neutral point of the DC link. The theoretical minimum capacity of the DC link capacitors is discussed for its application to both a vector-controlled induction motor system of 2.2 kW and a SVC system of 10 MVA, 6.6 kV, 60 Hz. It is shown that the proposed control scheme makes it possible to suppress the neutral-point potential variation within a few percent, so that the total capacity of the DC link capacitors in the NPC-VSI is almost the same as that in the conventional voltage source inverter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
This paper discusses automatic gain adjustment in a fully-digital-controlled shunt active filter. This is the first step in cooperative control of the multiple active filters based on voltage detection for harmonic damping throughout power distribution systems. In general, the active filter should be equipped with an optimal control gain corresponding to the characteristic impedance of a distribution line. However, it is difficult to know circuit parameters of a real distribution line having various shunt capacitors and loads. Therefore, a main purpose of the gain adjustment is to make the active filter damp out harmonic propagation without considering the circuit parameters. In addition, the gain adjustment can reduce the compensating currents and losses in the active filter, and moreover it can avoid over-damping performance. Experiment results obtained from a 200-V, 20-kW laboratory system verify the effectiveness of the active filter equipped with automatic gain adjustment.
This paper describes techniques that Japanese power electronics engineers can use to improve the quality of their technical papers written in English and the accompanying technical presentations. These techniques are based on the experience that the author has gained by participating in international conferences. Such techniques are important to discuss because the significant differences exist in writing and presentation styles between Japanese and American engineers. The author believes that such style differences are caused by differences between the Japanese and American cultures. The objective of this paper is to help Japanese engineers understand these cultural differences and improve their technical communication skills in English. The paper also includes some practical tips on writing and presentation techniques for improving the quality of their technical communications in English.
Since their basic compensation principles were proposed around 1970, active filters have been studied by many researchers and engineers aiming to put them into practical applications. Shunt active filters for harmonic compensation with or without reactive power compensation, flicker compensation or voltage regulation have been put on a commercial base in Japan, and their rating or capacity has ranged from 50 kVA to 60 MVA at present. In near future, the term of active filters will cover a much wider sense than that of active filters in the 1970s did. The function of active filters will be expanded from voltage flicker compensation or voltage regulation into power quality improvement for power distribution systems as the capacity of active filters becomes larger. This paper describes present states of the active filters based on state-of-the-art power electronics technology, and their future prospects toward the 21st century, including the personal view and expectation of the author.
This paper presents a control scheme and comprehensive analysis for a unified power flow controller (UPFC) on the basis of theory, computer simulation and experiment. This developed theoretical analysis reveals that a conventional power feedback control scheme makes the UPFC induce power fluctuation in transient states. The conventional control scheme cannot attenuate the power fluctuation, and so the time constant of damping is independent of active and reactive power feedback gains integrated in its control circuit. This paper proposes an advanced control scheme which has the function of successfully damping out the power fluctuation. A UPFC rated at 10 kVA is designed and constructed, which is a combination of a series device consisting of three single-phase pulsewidth modulation (PWM) converters and a shunt device consisting of a three-phase diode rectifier. Although the dynamics of the shunt device are not included, it is possible to confirm and demonstrate the performance of the series device. Experimental results agree well with both analytical and simulated results and show viability and effectiveness of the proposed control scheme.
This paper describes one of Japanese smart grid related activities, "Demonstration Projects for Next Generation Optimum Control of Power Transmission and Distribution Network" which deals with issues associated with mass penetration of photovoltaic power generation systems. Research subjects covered by this project and some results obtained so far are presented.
