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Renewable energy sources such as wind turbine generators and photovoltaics produce fluctuating electric power. The fluctuating power can be compensated by installing an energy storage system in the vicinity of these sources. This paper describes a 6.6-kV battery energy storage system based on a cascade pulsewidth-modulation (PWM) converter with focus on a control method for state-of-charge (SOC) balancing of the battery units. A 200-V, 10-kW, 3.6-kWh (13-M J) laboratory system combining a cascade PWM converter with nine nickel metal hydride (NiMH) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed balancing control. Index Terms--Active-power control, battery energy storage system (BESS), cascade converter, nickel metal hydride (NiMH) battery, state-of-charge (SOC) balancing.

Electric double-layer capacitors (EDLCs) have attributes that feature high power density, quick charge/discharge time, long life cycle, and environmental friendliness. These attributes accord for increased appeal in employing the EDLCs as energy-storage devices in renewable energy systems, industrial applications, and hybrid electric vehicles as compared to other mature static energy-storage devices. This paper describes the construction of a 320-V, 12-F EDLC energy-storage bank connected to a bidirectional isolated dc-dc converter. Two types of EDLC bank configurations are considered with emphasis on their voltage-balancing circuits. Subsequently, this paper proposes a voltage-balancing circuit based on a center-tapped transformer, and includes its experimental verifications. It also discusses the charge-discharge and self-starting operation of the EDLC energy-storage system. During the charge-discharge operation, a low ripple current flowing in the EDLC bank is observed, leading to a theoretical analysis. The EDLC bank is also successfully charged to its rated voltage without any external dc charging circuit. Index Terms--Bidirectional isolated dc-dc converter, electric double-layer capacitor (EDLC), voltage balancing.

This paper describes a transformerless energy storage system based on a cascade multilevel pulsewidth modulation converter with star configuration. The system is intended for power leveling of renewable energy sources, as well as for improving power quality and reliability of a power distribution system. This paper pays attention to active-power control and voltage-balancing control that are indispensable for proper operation of the energy storage system. A 200-V 10-kW 8.8-kJ downscaled laboratory system is designed, constructed, and tested, replacing electric double-layer capacitors with large-capacity electrolytic capacitors. Experimental results obtained from the laboratory system verify the viability and effectiveness of the 6.6-kV energy storage system. Index Terms--Active-power control, cascade multilevel pulsewidth-modulation (PWM) converter, electric double-layer capacitor (EDLC), energy storage system, output-power leveling, renewable energy sources, voltage-balancing control.

This paper addresses a bidirectional dc-dc converter suitable for an energy storage system with an additional function of galvanic isolation. An energy storage device such as an electric double layer capacitor is directly connected to a dc side of the dc-dc converter without any chopper circuit. Nevertheless, the dc-dc converter can continue operating when the voltage across the energy storage device drops along with its discharge. Theoretical calculation and experimental measurement reveal that power loss and peak current impose limitations on a permissible dc-voltage range. This information may be useful in design of the dc-dc converter. Experimental results verify proper charging and discharging operation obtained from a 200-V, 2.6-kJ laboratory model of the energy storage system. Moreover, the dc-dc converter can charge the capacitor bank from zero to the rated voltage without any external precharging circuit. Index Terms--Bidirectional isolated dc-dc converter, electric double layer capacitors, energy storage system, loss analysis, starting procedure.

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. Index Terms--Cascade connection, multilevel converters, reactive power, STATic synchronous COMpensator (STATCOM), voltage balancing.

This paper describes a bidirectional isolated dc--dc converter considered as a core circuit of 3.3-kV/6.6-kV highpower-density power conversion systems in the next generation. The dc--dc converter is intended to use power switching devices based on silicon carbide (SiC) and/or gallium nitride, which will be available on the market in the near future. A 350-V, 10-kW and 20 kHz dc--dc converter is designed, constructed and tested. It consists of two single-phase full-bridge converters with the latest trench-gate insulated gate bipolar transistors and a 20-kHz transformer with a nano-crystalline soft-magnetic material core and litz wires. The transformer plays an essential role in achieving galvanic isolation between the two full-bridge converters. The overall efficiency from the dc-input to dc-output terminals is accurately measured to be as high as 97%, excluding gate drive and control circuit losses from the whole loss. Moreover, loss analysis is carried out to estimate effectiveness in using SiC-based power switching devices. Loss analysis clarifies that the use of SiC-based power devices may bring a significant reduction in conducting and switching losses to the dc--dc converter. As a result, the overall efficiency may reach 99% or higher. Index Terms--Bidirectional isolated dc--dc converter, mediumvoltage power conversion systems, power density, wide-band-gap semiconductors.

Due to the advance of information technologies, a large number of electronic products such as personal computers have been connected to power distribution systems in commercial buildings. Hence, voltage distortion on utility outlets and excessive neutral current on distribution lines have arisen and lead to a number of serious problems in the distribution system. Two control methods and the related compensation characteristics of a series active filter connected to the neutral conductor are presented in this paper. The distinct functions of the proposed active filter are the mitigation of the third-harmonic voltage and the neutral current in a three-phase four-wire distribution system in a building. The required power of the proposed active filter is less than 10% of that of the harmonic-producing loads. A control method of the dc capacitor voltage on the active filter circuit is also described. It is clarified through experiments that one of the two functions of the active filter can be realized selectively and the dc capacitor voltage of the active filter can be regulated to a desired value. Index Terms--Harmonic voltage distortion, neutral conductor, series active filter, third harmonic.

This paper deals with a 6.6-kV transformerless STATCOM cascading multiple single-phase H-bridge PWM converters in each phase. The AC voltage of the STATCOM is almost sinusoidal, so that it requires no harmonic filter. Each converter is equipped with a capacitor and a voltage sensor on the DC side, which are electrically isolated from each other. The STATCOM has the capability of self-starting and voltage-balancing without any external power supply or equipment. Experiments using a three-phase 200-V, 10-kVA laboratory system, along with computer simulations, are carried out to confirm the viability and effectiveness of the STATCOM. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(1): 55–64, 2010; Published online in Wiley InterScience (www.interscience.wiley. com). DOI 10.1002/eej.20822 [ABSTRACT FROM AUTHOR]

This paper proposes a series active filter for mitigation of the third-harmonic voltage in a three-phase four-wire power distribution system in a building. The active filter which consists of a single-phase inverter can suppress the harmonic voltage of the system. The active filter is characterized by acting not only as a capacitor but also as a resistor for the third-harmonic components. A Hilbert transformer is applied to the controller of the active filter in order to realize accurate third-harmonic detection on a single-phase active filter. Measurement results of harmonic distortion of source voltage in a building is also shown in this paper. It is clarified in a simulation and experiment that the active filter can suppress the third-harmonic voltage without increasing neutral conductor current. © 2004 Wiley Periodicals, Inc. Electr Eng Jpn, 150(1): 62–70, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10379 [ABSTRACT FROM AUTHOR]