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SUMMARY This paper proposes a new control method for a high-frequency cycloconverter consisting of two half-bridge inverters and a series–resonant circuit. This cycloconverter acts as an ac-to-ac direct power conversion circuit without any dc stage. This circuit does not require a diode bridge rectifier, and thus, can be used to reduce forward voltage drops and power losses in the diodes. A new phase-shift control method is proposed to regulate the capacitor voltage in each half-bridge inverter and to achieve zero-voltage switching. The proposed phase-shift control is theoretically discussed and is also verified by an experimental circuit consisting of superjunction power MOSFETs. As a result, the proposed high-frequency cycloconverter exhibits a good power conversion efficiency as high as 97.7% at the rated power of 1.3 kW.

This paper proposes a new harmonic detection method for reducing voltage fluctuations applied across the dc capacitor in three-phase active power filters. The theoretical analysis in this paper derives the error in a conventional harmonic detection method and the resulting dc capacitor voltage fluctuations due to transient load changes. The proposed method inserts a new transfer function for compensating the detection error into the harmonic detection. It has been confirmed that the proposed method can suppress the dc-capacitor voltage fluctuations in experiments. As a result, the proposed method can reduce the capacitance of the dc capacitor in three-phase active power filters to 1/7.

This paper proposes a parameters extraction procedure based on the integrated evaluation of static and dynamic characteristics. The existing procedure available for various model is based on formal optimization with direct search, but it yields static characteristics with low accuracy, and its computation cost is high. The proposed procedure helps solve these problems since static characteristic evaluation is added to an objective function and the search range is narrowed. Validation with the simulation results of parameters optimization for a pin diode model demonstrates that the proposed procedure has high accuracy and low computation cost.

This paper discusses heat distribution on a workpiece in a zone-control induction heating (ZCIH) system. The system consists of two or more split working coils and multi inverter units, and adjust the coil currents to control the heat or temperature distribution on the workpiece. This paper theoretically reveals the relation between the coil currents and the heat distribution, and proposes a simple method to calculate the heat distribution using a three-dimensional resistance matrix. The validity of the developed theory is confirmed by performing both numerical computation based on the finite element method (FEM) and experiments using a 1.5-kW six-zone ZCIH laboratory setup.

This paper presents the operating performance of a three-phase solar power conditioner using a single-phase pulse-width modulation (PWM) method. The proposed modulation method turns on or off only one of the three phase legs in one PWM-carrier period. The proposed method enables to reduce the average switching frequency by a factor of three, and thus it has a switching power loss four times smaller than a conventional three-phase PWM method does. Experimental results obtained by using a 4-kW, 200-V setup are shown to verify the operating principle and performance. The experimental setup exhibits a high efficiency of 98% and an acceptable total harmonic distortion (THD) as low as 4.1%.

This paper deals with a new resonant gate-drive circuit for power MOSFETs. The proposed gate-drive circuit is characterized by a resonant inductor connected in series to the gate terminal of the MOSFET. It is possible to charge or discharge the input capacitance of the MOSFET by using the resonance between the inductor and the input capacitance. Experimental results are shown to verify the viability of the resonant gate-drive circuit. As a result, the proposed resonant gate-drive circuit reduces its power consumption to one tenth, compared with a conventional one. It was experimentally clarified that the proposed circuit makes it possible to improve efficiency of a high-frequency inverter using MOSFETs.

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 rated at 200V and 10kVA. The authors propose such a control method as to superimpose a 6th-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 bi-directional buck-boost choppers on the dc link for the porpose 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.

This paper deals with analysis and reduction of dc voltage fluctuations in a single-phase active filter caused by a sudden load change. The analysis reveals that the delay time of the harmonic detection causes dc voltage fluctuations in transient states. A new control method capable of reducing the dc voltage fluctuations is proposed based on the analysis. The dc capacitor can be reduced to the required capacitance to satisfy the ripple current in steady states. Experimental results are shown to verify the viability of the analysis and the proposed control method.

This paper deals with dynamic behavior of a 21-level (line-to-line) BTB system based on series connection of sixteen converter-cells under a single-line-to-ground (SLG) fault condition. When the SLG fault happens, an imbalance in the supply voltage brings an amount of negative-sequence current to the BTB system. The negative-sequence current should be mitigated because it would result in an increased supply current and a dc-capacitor voltage fluctuation. This paper proposes a new control method intended for reducing both negative-sequence current and dc-voltage fluctuation to acceptable levels. A laboratory system rated at 200 V and 20 kW confirms the validity of the control method. In addition, this paper addresses the issue of magnetic saturation in the 16 three-phase transformers.

This paper deals with a new control method for an atmospheric-pressure discharge plasma generator. The plasma generator consists of a voltage-source inverter, a step-up transformer, a resonant inductor and two parallel electrode plates. The high-frequency high voltage induced by series resonance between the equivalent capacitance of the plates and the resonant inductor generates glow plasma between the plates. However, a continuously-applied voltage may cause streamer or arc discharges instead of glow plasma, when the plates are under atmospheric pressure. This paper discusses an application of pulse density modulation (PDM) to the plasma generator, and proposes a new PDM method suitable for atmospheric-pressure plasma. A conventional PDM method repeats alternate run and stop to adjust the output power. The switching pattern is designed to minimize power variations. In contrast, the switching pattern of the proposed PDM method is designed to achieve a quick decrease in discharge current and electrode voltage during the inverter is stopped. As a result, the proposed method makes it possible to eliminate the streamer or arc discharges, leading to stable glow plasma. Experimental results are shown to verify the viability of the proposed PDM method.

This paper discusses control characteristics of line-voltage regulation by a shunt active filter intended for installation on a power distribution system. Since the power circuit configuration of the active filter is the same as a static synchronous compensator (STATCOM), it is possible to regulate the line voltage by means of controlling an adequate amount of a reactive power. In this case, no harmonic voltage affects a feedback controller for the line-voltage regulation, because the active filter eliminates harmonic components from the line voltage at the installation bus. The dynamic performance is also investigated by computer simulation and experimental results. This paper proposes a new compensation scheme capable of reducing transient voltage fluctuations caused by a phase angle change in the installation bus voltage.

This paper points out that dc magnetic-flux deviation may occur in a doubly-fed flywheel genarator with primary-current feedback control. The flux deviation discussed in this paper, which is different from the common flux deviation caused by a dc output voltage of a PWM converter, is induced by an ac output voltage of the PWM inverter operated at the angular frequency of the rotor. It may be impossible to detect such a dc flux from the primary current and/or voltage, because the dc flux induces no voltage on the primary windings. This paper reveals that the primary-current control induces a dc flux in the primary windingsand prevents the dc flux from decaying. A new control method for suppressing the dc flux deviation, which is based on calculating the exciting current from the primary and secondary currents, is proposed. Experimental results verify the viability of the control method proposed in this paper.

This paper deals with a series active filter for harmonic-current compensation of a large-capacity diode rectifier with a capacitive load. Flux satulation may occur in a series matching transformer for connecting the series active filter to the ac side of the diode rectifier. This paper develops an analog controller of a dc magnetizing current flowing in the matching transformer, along with a model for the controller, to avoid the flux satulation. Operating characteristics of the controller are theoretically and exprimentally discussed, taking into account both an offset voltage in the controller and an error in a current sensor. Finally, this paper discusses a relation between magnetizing characteristics of the matching transformer and the amplitude of the magnetizing current.

This paper deals with a self-commutated BTB (Back-To-Back) system for the purpose of power flow control and/or frequency change in transmission systems. Each BTB unit consists of two sets of 16 three-phase voltage-source converters, and their ac terminals are connected in series each other via 16 three-phase trans-formers. Hence, the BTB unit uses totally 192 switching devices capable of achieving gate commutation. This results in a great reduction of voltage and current harmonics without performing PWM control. Simulation results verify the validity of the proposed system configuration and control scheme not only under a normal operating condition but also under a single-line-to-ground fault condition.

This paper discusses harmonic propagation and damping in a long-distance distribution feeder having many capacitors for power factor correction, and presents harmonic damping characteristics of a shunt active filter based on voltage detection. A distributed-constant-circuit model for the feeder is derived to perform analysis with the focus on harmonic propagation and damping. As a result, it is clarified by theory and experiment that harmonic propagation throughout the feeder can be mitigated by installing the active filter, which acts as a harmonic terminator, on the end terminal of the feeder.

The purpose of this paper is to perform theoretical analysis of active power flowing into, or out of, a series device in a unified power flow controller (UPFC), and to reveal what amount of electric energy the dc link capacitor absorbs or releases through the series device in a transient state. This paper theoretically derives that the electric energy flowing into, or out of, the series device during a transient state is equal to the difference between the energy stored in the line inductance in a steady state before the start of the transient state and that in another steady state after the end of the transient state. This analysis helps us to determine the required capacity of the dc link capacitor, as well as the required response of dc voltage regulation in the shunt device. Experimental results obtained from a 10-kVA laboratory setup agree well with analytical results, as well as computer-simulated results.

In recent years, attention has been paid to the concept of FACTS (Flexible AC Transmission Systems), along with significant progress in power electronic technology. A high speed phase shifter, which is one of the most promising devices in the FACTS concept, has the potential of power flow control and/or voltage stability in power transmission systems.In this paper, theory and experiment reveal that conventional high speed phase shifters may cause power swing in a transient state as a result of coupling between instantaneous active and reactive power control loops. Then, a couple of new control schemes for a high speed phase shifter are proposed to achieve both power flow control and power swing damping. The second proposed control scheme is based on the control scheme of an already proposed series active filter. Simulated and experimental results agree well with analytical results, not only in steady states but also in transient states.

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.

A specially designed twelve-pulse thyristor rectifier of 5-8MVA is required as a low-voltage high-current dc power supply for super-conductive material tests. It is, however, too susceptible to low frequency variation of the utility voltage, or to the so-called “flicker” to generate a strong magnetic field with high stability.This paper presents a flicker and harmonic compensator based on the combined system of a small-rated series active filter and a conventional shunt passive filter. The main purpose of the compensator is to remove susceptibility to the flicker from the thyristor rectifier. The series active filter, which is connected in series with the utility, can compensate for flicker components present at the utility grid, the frequency range of which is from 1Hz to 20Hz. In addition, it helps almost all harmonic currents produced by the thyristor rectifier to flow into the shunt passive filter installed in parallel with the harmonic-producing load, i.e. the thyristor rectifier.

The purpose of corona discharge treaters using high frequency inverters is to render the affinity of polyethylene films to inks, glues and coatings. A frequency control scheme for such a high frequency inverter has been proposed to achieve the power control of a corona discharge treater. It is, however, impossible to realize zero current switching except for full-power operation. Partial discharge appears in the frequency control scheme under the condition of a low power because the applied voltage between electrodes is decreased as the output power is reduced.This paper presents a 30kHz 5kW voltage-source inverter using IGBTs for corona discharge treaters, which is characterized by the power control based on pulse density modulation (PDM). Although the voltage-source inverter operates at a constant dc voltage and a constant output frequency, the average output power can be controlled by adjusting a period ratio of the square wave pulse having a constant amplitude to zero voltage. The PDM inverter can always operate with zero current switching, so that switching losses and electromagnetic noises are reduced. The prototype system developed in this paper shows good corona discharge characteristics in a wide output power range from 0.5% to 100%, which makes it possible to realize both strong and weak surface treatment.

With remarkable progress of switching devices, the switching frequency of a voltage-source PWM inverter has been higher and higher. A high frequency PWM inverter gives great benefits in reduction of current ripples and acoustic noises. However, high frequency hard switching causes the increase of switching losses and EMI to be solved.The authors propose a zero-current-switching based three-phase PWM inverter which has small resonant circuits on the ac side. The current flowing in a switching device is a sum of the resonant current and the load current. Since the switching device is turned on and off at zero current, the switching losses and electromagnetic noises are greatly reduced. This paper describes the principle of the zero current switching operation, the design of the resonant circuits and the control scheme for the new soft switching inverter. Moreover, it shows interesting experimental results obtained by the zero-current-switching PWM inverter which drives an induction motor of 2.2kW.

Conventional passive filters have been broadly used to suppress the harmonics in power systems. However, they have the following disadvantages; (1) Source impedance affects the compensation characteristics. (2) Antiresonance between the source and the passive filter amplifies the harmonic current of the source at the specific frequency. (3) No harmonic compensation is performed at the frequency to which the passive filter is not tuned. On the other hand, active filters which have been studied and put into practical use have the ability to overcome the disadvantages mentioned above, but their initial and running costs are high, because they need voltage-or current-source PWM converters having high speed current response.This paper presents a novel harmonic compensator consisting of an active filter and a passive filter which are connected in series with each other. The passive filter removes the harmonics of the load, and the active filter improves the compensation characteristics of the passive filter. So the proposed filter system can eliminate the problems of the passive filter used alone and needs a small rated active filter, compared with a conventional active filter used alone. Control schemes and compensation characteristics are discussed, and experimental results of a laboratory model are shown.