Kuniomi Oguchi

A multilevel-voltage source rectifier with a three-phase diode bridge circuit as a main power circuit

A novel system obtaining very low distorted waveforms of input AC currents of three-phase rectifier circuits is proposed. This rectifier system consists of a three-phase diode bridge circuit as a main power circuit, AC reactors, and an additional circuit which serves as an active filter with low switching frequencies and a forced commutation circuit for the diodes. The authors describe the operation principle, analysis of the waveforms of input voltage and current, multiple phase-shifted systems, and experimental results. The proposed system gives excellent input current waveforms and has many advantages over conventional systems.<<ETX>>

A novel six-phase inverter system with sixty-step output voltages for high-power motor drives

A novel six-phase voltage-fed inverter system is proposed. The system consists of two main three-phase inverters, coupling reactors, and an additional single-phase inverter. The ripple voltage injected into the DC link by the additional inverter results in 60-step load voltages with the help of the coupling reactors. The main inverters operate at the same frequency as the output and the additional inverter operates at a frequency six times higher. The advantages and disadvantages of the system are quantitatively discussed.

Small-scale hydropower

A novel harmonics-reducing system of cascaded-induction generators has been proposed, and its effectiveness has been confirmed by the simulation results. Comparing the characteristics of the cascaded generators with those of the single generator, we see that the power efficiency decreases by 1-3%, and the input power factor decreases by around 20%, but the power factor can be improved by controlling the reactive power of the power-recovery inverter. Since the proposed system has the advantages of a low-cost system, a brushless structure, and adjustable-speed characteristics, we anticipate that its use will be practical for small hydroelectric power plants.

Coupling rectifier systems with harmonic cancelling reactors

This paper shows that double-diode rectifiers can generate 12-step voltages by using well-designed line-side reactors, although with relatively large self-inductances. The usage of three-level rectifiers can alleviate this disadvantage. The double three-level rectifier system proposed features the following advantages over conventional transformer-coupled systems: small in size, light in weight, and more economical; control capabilities of output DC voltage in a limited range; low current and voltage ratings required for additional circuits; and highly reliable operation due to 12-step operation capability without additional circuits.

Voltage-phase shifting effect of three-phase harmonic canceling reactors and their applications to three-level diode rectifiers

For replacing a conventional transformer-coupled system, a novel double rectifier system with 24-step input voltages is proposed for mid-power applications. The system consists of double three-level voltage-type diode rectifiers coupled with three harmonic canceling reactors with a total power capacity of 13.4-18.2% of the DC output power. Additional switches of the unit rectifiers act as forced commutation circuits and the reactors absorb lower harmonics. The proposed system has the advantages over conventional transformer coupled systems of: economical design; small size; light weight; high-quality input currents; and highly reliable operation.

An Improved Three-Phase Multistepped-Voltage Inverter Combined with a Single-Phase Inverter Through Switching Devices

A new multistepped-voltage waveform inverter with a total harmonic distortion (THD) of 6.25 percent is described. The proposed system consists of a three-phase inverter whose conduction intervgl is smaller than 180 °, a triple-frequency single-phase inverter, and power semiconductor switches that link the two inverters. The advantages of the proposed system over conventional stepped-voltage inverters are that it requires only a small number of switching devices, no three-phase output transformers, and an output filter of reduced size. The relation between THD and the number of steps of an output voltage waveform is discussed. Comparison with other stepped voltage inverters and experimental results using power transistors are also presented.

Multilevel current-source and voltage-source converter systems coupled with harmonic canceling reactors

Novel current-source and voltage-source power converter systems with 36-step input waveforms are proposed. Each of the systems consist of two main power converters coupled with harmonic canceling AC-side reactors and an additional converter coupled with DC-side interphase transformers. The additional converter injects into the DC links ripple currents or voltages having a frequency six times that of the line frequencies. The proposed systems are suitable for high power applications because of their low switching frequencies and the low distortion of their waveforms. The system configurations and operations are presented for comparison of the two types. Experimental results show close agreement with theoretical prediction.

Three-phase multilevel voltage source converters with low switching frequencies and less-distorted input voltages

A novel, modified neutral-point-clamped (NPC) converter is proposed. It generates quasi-18-step input voltages and gives nearly sinusoidal input currents at a switching frequency of triple the power line frequency. Theoretical and experimental work has shown that the proposed converter is suitable for large-capacity power converters for electric utility applications, because it makes the most of the circuit characteristics of high-voltage converters, i.e., the series connection of switching devices, and it produces fairly good input voltage and current waveforms at low switching frequencies. The authors present the operating principle, the analysis and synthesis of the input voltage waveforms in a single and a double converter system, and experimental results.<<ETX>>

A low-cost system of variable-speed cascaded induction generators for small-scale hydroelectricity

A novel variable-speed cascaded induction generator system is proposed for small-scale hydroelectric power plants, in which the second induction generator has double three-phase stator windings (or six-phase windings). The proposed low-cost system can provide a brushless, robust structure and less distorted currents without using an expensive PWM rectifier on the machine side and an expensive adjustable-blade water turbine. The simulated results showed that the system has sufficient capabilities for practical use.

A novel control method for single-phase slow switching multilevel rectifiers

A novel control method for single-phase multilevel rectifiers is proposed to meet harmonic current limiting standards with minimum input filter inductance. Since the proposed rectifier consists of a main power circuit of a diode bridge and two auxiliary semiconductor switches in parallel with the main circuit, it provides several features; high efficiencies, high input power factor, and low EMI. The simulation and the experimental results show that the proposed control method reduces the input filter inductance minimum.