Publications

Direct connection of the synchrotron magnet power supply to the utilities causes the effect of pulsed reactive and active power in the AC line. Conventionally, Static Var Control system compensates the reactive power generated by the thyristor converter to reduce the flicker in the power line. However, it is necessary to control not only a reactive power but also an active power for the future large scale synchrotron magnet power supply in order to reduce the dissipation power and to realize the stabilization in the AC line. An adjustable-speed rotary condenser is capable of not only reactive power control but also active power control since it utilize a flywheel effect of the rotor. Research and development on these problems are now under going using a model system of 7.5 kW rotary condenser with flywheel (GD/sup 2/=3kg-m/sup 2/). Control and characteristic of an adjustable-speed rotary condenser and the experiment results will be presented.

The progress of microelectronics and microprocessors has advanced digitization of control circuits so that sensors have been expected to have digital output. Current sensors are basic in electrical drives, and the control accuracy depends on their performance. However, a current sensor using Hall-device has a non-linear error of 1%. In order to solve this problem, a Hall-effect current sensor, or a so-called closed loop current sensor, has been developed, which is combined with a current transformer. The current of a secondary winding is compensated by the Hall-device so that the magnetic field in the toroidal core is kept at zero. In this case, the non-linear error is eliminated.In this paper, a digital output current sensor of 12 bits is developed, which is a combination of a closed loop current sensor with a voltage comparator and a counter. As this circuit includes a nonlinear component such as the voltage comparator, the limit cycle caused by non-linearity is analyzed. It is described that the oscillation can be suppressed by linearization using a flush type A/D converter of 4 bits.

No abstract is provided for this article.

This paper proposes a hybrid active filter for damping of harmonic resonance in industrial power systems. The hybrid active filter consists of a small-rated active filter and a 5th-turned passive filter. The active filter is controlled to behave as a variable resistor by adjusting a feedback gain which changes from a negative to positive value. The negative resistance cancels a positive resistor inherent in the passive filter, so that the hybrid active filter acts as an ideal passive filter with infinite quality factor. This results in a significant improvement of harmonic damping accompanied by harmonic mitigation, compared with the passive filter used alone. Moreover, the active filter acts as a positive resistor to prevent an excessive harmonic current from flowing into the passive filter. Experimental results obtained from a 20-kW laboratory model verify the viability and effectiveness of the hybrid active filter proposed in this paper.

This paper achieves a feasibility study of a 6.6kV, 1MW loop controller that consists of a transformerless back-to-back configuration using two 5-level diode-clamped converters. However, the loop controller requires reducing the zero-sequence current circulating between the two distribution lines below than 0.2 A in rms, in order to avoid malfunction of line-to-ground fault protection relays. Moreover, all the dc voltages across four capacitors in the dc link have to be controlled equally. This paper presents a solution to these problems. Two common-mode chokes are installed at the ac side of each converter to suppress high-frequency zero-sequence currents, while feedback control is applied to eliminate low-frequency zero-sequence currents. Two bidirectional buck-boost dc-dc converters are employed to keep the four capacitor voltages equal. Simulation results verify viability and effectiveness of the loop controller, along with the developed theoretical analysis.

This paper deals with an inverter system integrating a small-rated passive EMI filter with a three-phase voltage-source PWM inverter. The purpose of the EMI filter is to eliminate both common-mode and normal-mode voltages from the output voltage of the inverter. The motivation of this research is based on the well-known fact that the higher the carrier or switching frequency, the smaller and the more effective the EMI filter. An experimental system consisting of a 5-kVA inverter, a 3.7-kW induction motor, and a specially-designed passive EMI filter was constructed to verify the viability and effectiveness of the EMI filter. As a result, it is shown experimentally that both three-phase line-to-line and line-to-neutral output voltages look purely sinusoidal as if the inverter system were an ideal variable-voltage, variable-frequency power supply when viewed from the motor terminals. This results in completely solving serious issues related to common-mode and normal-mode voltages produced by the inverter.

This paper presents a modular push-pull PWM converter (MPC) for a battery energy storage system, which is intended for grid connections to medium- or high-voltage power systems. The converter per phase consists of a center-tapped transformer and two arms based on a cascade connection of multiple bidirectional PWM chopper cells with floating dc capacitors. This paper proposes a new control method for a three-phase MPC, which makes it possible to regulate six average voltages of the floating dc capacitors in each arm independently without any interference. This attractive feature eliminates a need of arm-balancing control, leading to a simpler and more reliable system. The validity of the operating performance and control method developed in this paper is confirmed by experiment using a three-phase, 200-V, 5-kW downscaled system.

This paper proposes a hybrid active filter for damping of harmonic resonance in industrial power systems. The hybrid filter consists of a small-rated active filter and a 5th tuned passive filter. The active filter is characterized by detecting the 5th harmonic current flowing into the passive filter. It is controlled in such a way as to behave as a negative or positive resistor by adjusting a feedback gain from a negative to positive value, and vice versa. The negative resistor presented by the active filter cancels a positive resistor inherent in the passive filter, so that the hybrid filter acts as an ideal passive filter with infinite quality factor. This significantly improves damping the harmonic resonance, compared with the passive filter used alone. Moreover, the active filter acts as a positive resistor to prevent an excessive harmonic current from flowing into the passive filter. Experimental results obtained from a 20 kW laboratory model verify the viability and effectiveness of the hybrid active filter proposed in this paper.

It has previously been confirmed that the guinea pig has aggregations of 10-20 lymphoid follicles at the junction of the nasal cavity and the nasopharyngeal duct. The vascular architecture of this nasal-associated lymphoid tissue (NALT) was studied by the corrosion cast/scanning electron microscope method. The NALT was supplied by branches of the inferior nasal artery. These afferent arterial branches gave off arterioles to the follicles and the interfollicular regions, where the arterioles ramified into capillaries. Some of these arterioles reached the subepithelial region to form a single-layer dense capillary network. The subepithelial capillaries gathered into short collecting venules, which in turn drained into high endothelial venules (HEV) in the interfollicular region. The HEV, which also receives tributaries from the follicular and interfollicular capillary plexuses, descended in the interfollicular regions and finally flowed into the efferent veins at the bottom of the NALT. Indentations impressed by high endothelial cells (HEC) were prominent on the surface of the HEV casts, and their frequency was larger in the upper course or segments than in the lower. This suggests that the incidence of HEC in the upper segments is higher than in the lower segments, and these findings are consistent with the hypothesis that some substances which are taken up into the subepithelial capillaries and transported to the venules induce differentiation and maintain of HEVs.

This paper describes a 6.6-kV battery energy storage system based on a multilevel cascade PWM (pulse-width-modulation) converter with star configuration. It discusses design concepts with and without a line-frequency transformer for grid connection. The control system consists of SOC(state-of-charge)-balancing control and fault-tolerant control. The former is indispensable for effective utilization of battery energy while the latter is required for maintaining continuous operation during a converter-cell or battery-unit fault. A 200-V, 10-kW, 3.6-kWh laboratory system combining a three-phase cascade PWM converter with nine NiMH (nickel-metal-hydride) battery units is designed, constructed, and tested to verify the validity and effectiveness of the proposed control system.

This paper focuses on the common dc-link voltage between a three-phase diode rectifier and a modular multilevel cascade inverter based on double-star chopper cells (MMCI-DSCC) for a medium-voltage motor drive. This motor drive can be operated even when no capacitor exists on the dc link. However, a nonnegligible, but predictable, amount of switching-ripple voltage occurs on the dc link. This paper achieves modeling and analysis of the switching-ripple voltage, thus making it possible to design a small-sized dc passive filter consisting of series connection of a film capacitor and a damping resistor. A 400-V, 15-kW downscaled system is used to confirm the effectiveness of the analysis and the dc filter. Experimental results show that the switching-ripple voltage can be attenuated satisfactorily by the dc filter, and that the power loss dissipated in the damping resistor is negligible, compared to the rated power of 15 kW.

No abstract is provided for this article.

This paper presents a theoretical analysis and control of a three-phase modular multilevel cascade converter based on double-star chopper-cells (MMCC-DSCC). The analysis and control are characterized by applying the so-called “αβ0” and “dq0” coordinate transformations to the converter. Existing analysis and control methods were unable to use any coordinate transformation because the three-phase converter was considered as three sets of single-phase converters that were analyzed and controlled independently. As a result, it was difficult to avoid mutual interferences among the three-phase ac currents, circulating currents inside the converter, and dc current. Power-flow analysis using the two coordinate transformations succeeds in decoupling these currents, as well as in deriving lucid relationships between the power flow and the circulating currents. Moreover, the power-flow analysis makes it possible to design a decoupled digital controller for balancing and regulating the dc mean voltages of all the floating dc capacitors. The validity of the design is confirmed by computer simulation using a software package of PSCAD/EMTDC.

No abstract is provided for this article.

This paper deals with the high-frequency leakage current caused by a voltage-source PWM inverter. A nonnegligible amount of leakage current would flow through stray capacitors between stator windings and a motor frame due to a, large step change of the common-mode voltage produced by the PWM inverter. An equivalent circuit for the leakage current is proposed, which is expressed by a series resonant circuit. The analysis applying the equivalent circuit gives us such a conclusion that the connection of a common-mode choke in series between the inverter and motor terminal is not effective to reduce the rms and average value of the leakage current, but effective to reduce the peak value. Furthermore, this paper shows that the modeling and analysis are valid in designing a previously proposed suppression method of the leakage current.