Kenichiro Sano

A Transformerless D-STATCOM Based on a Multivoltage Cascade Converter Requiring No DC Sources

This paper deals with a cascaded multilevel converter which has multiple dc voltage values (multivoltage cascade converter or hybrid multilevel converter) for a 6.6-kV transformerless distribution static synchronous compensator (D-STATCOM). A control method is proposed to realize dc voltage regulation of series-connected multiple cells in the STATCOM operation, making it possible to remove dc sources from all H-bridge cells. The simplified configuration without the dc sources makes the STATCOM small and lightweight. A downscaled STATCOM model rated at 220 V and 10 kVA is built and a series of verification tests is executed. Theoretical analysis and experimental results prove the stable operating performance of the proposed method in steady states and transient states.

Voltage-Balancing Circuit Based on a Resonant Switched-Capacitor Converter for Multilevel Inverters

This paper proposes a new voltage-balancing circuit for the split dc voltages in a diode-clamped five-level inverter. The proposed circuit is based on a resonant switched-capacitor converter (RSCC), which consists of two half-bridge inverters, a resonant inductor, and a resonant capacitor. A new phase-shift control of the RSCC is proposed to improve voltage-balancing performance. As a result, it is possible to reduce the inductor to one tenth in volume as compared to a conventional voltage-balancing circuit based on a buck-boost converter. Experimental results are shown to verify the viability of the RSCC-based voltage-balancing circuit.

A Surgeless Solid-State DC Circuit Breaker for Voltage-Source-Converter-Based HVDC Systems

This paper proposes a dc circuit breaker for voltage source converter (VSC) based high-voltage dc transmission (HVDC) systems. Technical challenges for applying dc circuit breakers are to increase blocking voltage and to suppress surge voltage at the current clearing. The proposed dc circuit breaker is a solid-state breaker which consists of a number of semiconductor devices in series. It maintains equal voltage balancing among the devices to apply it to high-voltage applications. Moreover, the surge voltage across the circuit breaker is reduced by employing a freewheeling diode. Considering a system rated at 300 MW in power and 250 kV in dc voltage, the conduction loss of the proposed circuit breaker is estimated to be 0.045% of the rated power. The value is smaller than the power loss of the VSCs. A downscaled HVDC system rated at 10 kW in power and 360 V in dc voltage was built and tested. A series of experimental results demonstrates the dc fault clearing and rapid restoration of power transmission.

Performance of a High-Efficiency Switched-Capacitor-Based Resonant Converter With Phase-Shift Control

This paper presents operating performance of a switched-capacitor-based resonant converter (SCRC) using a phase-shift control method. The proposed phase-shift control realizes zero-voltage switching operation, and thus achieves a high-conversion efficiency. A theoretical analysis shows that the SCRC can reduce its inductor volume compared with a conventional buck converter when the output voltage range is within 19%-81% of its input voltage. Experimental results verify the operating characteristics of the proposed method and show the improved conversion efficiency of more than 99%.

Design of a Photovoltaic Simulator With a Novel Reference Signal Generator and Two-Stage LC Output Filter

This paper presents a systematic design technique for a photovoltaic simulator. The proposed technique helps improve control loop bandwidth and system response. The photovoltaic equivalent circuit is used to generate the current-voltage reference curves. A novel technique is proposed and implemented with analog controllers to simplify the reference signal generator and to avoid sampling time delays in digital controllers. A two-stage LC output filter is implemented to push the resonant frequency higher and thus allowing a higher bandwidth control loop design while keeping the same switching ripple attenuation as in the conventional one-stage LC output filter. Design procedures for both control and power stage circuits are explained. Experimental results verify the steady state and transient performance of the proposed photovoltaic simulator at 2.7 kW output.

A resonant switched-capacitor converter for voltage balancing of series-connected capacitors

This paper describes a resonant switched-capacitor converter (RSCC) for voltage balancing of several capacitor or battery modules connected in series. This voltage balancing circuit transfers electric charge between the capacitor modules with a soft switching operation, resulting in a reduction of power loss and EMI. Moreover, a phase-shift control method enables to improve its voltage-regulation performance. Its current loops are analyzed in a circuit configuration which has more than three capacitor modules. Based on the analysis, a new controller is designed to improve the transient response. Experimental results confirm the validity and effectiveness of the proposed control method.

A boost conversion system consisting of multiple DC-DC converter modules for interfacing wind farms and HVDC transmission

Recent offshore wind farms have increased their capacity and distance from the shore. To transmit the bulk power to the shore, some projects employ high voltage dc (HVDC) transmissions. Such wind farm has a local ac grid in their site to collect power from each generator, and then the voltage is increased by bulky and heavy step-up transformers. In order to simplify and to reduce the weight of the offshore facilities, dc collection grids have been also considered in recent years. However, a technical issue is the development of high voltage and high power converters to boost the dc voltage for the HVDC transmissions. This paper proposes a novel dc boost conversion system for interfacing the wind farms and the HVDC transmissions. The proposed boost conversion system consists of multiple isolated dc-dc converter modules. The modules are connected in parallel in the wind farm side and in series in the HVDC transmission side in order to realize high power and high step-up ratio. A control method is also developed for maintaining the voltage of the offshore power system and keeping equal voltage sharing among the series connected modules. Computer simulations are carried out to verify the basic operation characteristics of the proposed system.

Improving dynamic performance and efficiency of a resonant switched-capacitor converter based on phase-shift control

This paper presents the output voltage regulation characteristics and the dynamic performance of an RSCC using a phase-shift control. The proposed phase-shift control realizes soft switching operation even if the output voltage was changed, and it achieves a higher conversion efficiency. A theoretical analysis shows that the RSCC can reduce its inductor volume compared with a buck converter when its output voltage range is limited within 19–81% of its input voltage. Experimental results verify the operation characteristics of the proposed method and show the improved conversion efficiency more than 99%.

Design of a photovoltaic simulator with a novel reference signal generator and two-stage LC output filter

This paper presents a systematic design technique for a photovoltaic (PV) simulator. The proposed technique helps to improve control loop bandwidth and system response. The PV equivalent circuit is used to generate the current-voltage reference curves. A novel technique is proposed and implemented with analog controllers to simplify the reference signal generator, and to avoid computation time delays in digital controllers. A two-stage LC output filter is implemented to push the resonant frequency higher, and thus, allowing a higher bandwidth control loop design while keeping the same switching ripple attenuation as in the conventional one-stage LC output filter. Design procedures for both control and power stage circuits are explained. Experimental results verify the steady state and transient performance of the proposed PV simulator at 2.7 kW output.

Reducing output current ripple of resonant switched-capacitor step-up converter with interleaving technique

This paper deals with a reduction method of output current ripples in a resonant switched-capacitor converter (RSCC) by applying an interleaving technique. Two RSCCs are connected in parallel and operated with their switching waveforms displaced by a half of the switching period. Application of a phase-shift control method to the RSCCs enables not only an equal current sharing between the RSCCs but also a great reduction of the current ripples in both input and output terminals. Theoretical analysis of the current ripples reveals that the interleaving operation can reduce the current ripple less than 45% of that in a conventional synchronous operation. Experimental results show the viability of the interleaving operation, and confirm the reduction effect in the output current ripples as well as the power loss in the RSCC.