Koji Orikawa

A Single-Phase Current-Source PV Inverter With Power Decoupling Capability Using an Active Buffer

This paper proposes a new circuit configuration and a control scheme for a single-phase current source inverter with a power decoupling circuit which is called as the active buffer. The proposed inverter achieves low-DC-input voltage ripple and also provides sinusoidal current that can achieve unity power factor, without large passive components in DC bus such as smoothing inductors and electrolytic capacitors, which are conventionally required in order to decouple the power pulsation caused by single-phase power source. In this paper, the fundamental operations of the proposed inverter are demonstrated experimentally. From the experimental results, the input voltage ripple is 8.87% and the output current THD is 4.24%. In addition, the output power factor over of 99% and a maximum efficiency of 94.9% are obtained. Finally, it is confirmed that the maximum power density of the conventional circuit and the proposed circuit are 2.75 kW/L at the switching frequency 70 kHz and 4.86 kW/L at the switching frequency 80 kHz, respectively.

Reduction of a boost inductance using a switched capacitor DC-DC converter

The loss analysis of a switched capacitor converter (SCC) with a boost reactor is discussed in this paper. The proposed SCC can deliver an output voltage independent from the circuit structure comparing to other voltage control methods. The main feature of this circuit is that most of the energy for the boost up function is transferred from a flying capacitor which results the inductance of the input reactor can be reduced. The reactor size of the proposed SCC is smaller than that of the conventional boost converters. A 1000 W 3-level SCC prototype has been built and tested. The efficiency of 97.8% is achieved at 1000 W when the boost up ratio is three times. Moreover, the loss analysis shows the main loss of the proposed SCC is the diode conduction loss.

Experimental verification of an EMC filter used for PWM inverter with wide band-gap devices

This paper discusses a volume of an EMC filter and a cooling system that are used for a PWM inverter with wide band-gap devices. At first, the volume of reactor that is used for EMC filter such as common mode coke coils and differential mode choke coils are estimated by theoretically. Then, the relationship between the carrier frequency of the PWM inverter and the total volume of filter reactors are clarified by simulation. Moreover, the relationship between the carrier frequency and the volume of a cooling system is calculated based on experimental results. As a result, the total volume of the inverter system that contains filter reactors and cooling system will be reduced by 54% at the carrier frequency of 300 kHz by using a two stage filter compared to case of the carrier frequency of 150 kHz by using one stage filter. In addition, an induction motor is driven by a prototype of GaN-FET inverter system with a 300-kHz carrier frequency. As a result, the conduction noise is suppressed below the limit of CISPR. Therefore, the proposed design method for EMC filters is valid in the experiment. Furthermore, the power loss of EMC filter is less than 2% compared with the total loss of the GaN-FET inverter system.

Isolation system with wireless power transfer for multiple gate driver supplies of a medium voltage inverter

In this paper, a multiple wireless power transfer system for multiple gate driver supplies of a medium voltage inverter is developed. The proposed isolation system achieves a galvanic isolation with an air-gap of 50 mm using a wireless power transfer with magnetic resonance coupling. It easily respects the standard of galvanic isolation, which is established by International electrotechnical commission (IEC). Moreover, the power is supplied from one transmitting board to six gate drivers without a solid magnetic core. In this paper, the isolation system is developed and tested. It is clarified that the isolation system transmits power of not less than 300 mW to each gate drivers beyond an air-gap. However, sum of the output power of the each receiving board are limited up to approximately 3.5 W because of a voltage drop in the equivalent series resistances of the transmission coils.

Modular Multilevel Converter for wind power generation system connected to micro-grid

This paper discusses a AC-DC converter which is constructed by a Modular Multilevel Converter (MMC) for a wind power generation system connected to Micro-grid. The proposed system which is constructed the MMC with an H-bridge cell achieves to convert from a generator voltage of 3.3 kV into DC bus voltage of 340 V. Moreover, as a fundamental evaluation, the experimental result by miniature model of 700 W confirms that the proposed system achieves the step-down operation from input voltage of 200 V into DC voltage of 65 V. Finally, the proposed system maintains the capacitor voltage of each cell to the voltage command. Furthermore, the maximum voltage error between the voltage command of the cell capacitor and the measured voltage is 10% or less.

Improvement of output voltage with SVM in three-phase AC to DC isolated matrix converter

This paper discusses and evaluates the performance of a 50-kVA three-phase AC to DC matrix converter. Comparing to the conventional converter, which consists of a PWM rectifier and an inverter, the matrix converter does not require a large reactor at the three-phase input side, and require no DC smoothing capacitor at the DC link part. A space vector modulation based on the virtual AC-DC-AC conversion with four-step commutation patterns is proposed for the converter. From the experimental results, the converter can obtain 2.49% THD on the input current and achieves approximately 91.4% efficiency at 40kW.

Circuit-Parameter-Independent Nonlinearity Compensation for Boost Converter Operated in Discontinuous Current Mode

This paper proposes a current control method for discontinuous current mode (DCM) in order to achieve the same control performance as continuous current mode (CCM) in a boost converter. By utilizing the duty ratio at the previous calculation period to compensate for a DCM nonlinearity, the controller, which is designed for CCM, can also be used in DCM. In the frequency analysis, the cutoff frequency of the proposed DCM current control agrees exactly to the design value, which is 2 kHz, whereas the cutoff frequency of the conventional DCM current control results in high error of 47.5%. In the current step response experiment with a 360-W prototype and the switching frequency of 20 kHz, the experimental DCM current response almost agrees with the conventional CCM current response, which are 380-μs rise time for both CCM and DCM, 9% and 8% overshoot for CCM and DCM, respectively. Furthermore, the computation time of the proposed DCM current control is shorter 35% than the conventional DCM current control.

DC to single-phase AC Voltage Source Inverter with power decoupling circuit based on flying capacitor topology for PV system

A novel power decoupling circuit using a flying capacitor topology is proposed in this paper. The inverters, which are connected to a single-phase grid, have single-phase power fluctuation at the twice the grid frequency. Thus, bulky electrolytic capacitor is generally used as DC link capacitor. In the proposed circuit, the power fluctuation is compensated by the flying capacitor in the flying capacitor DC-DC converter. The proposed circuit does not need an additional magnetic component in comparison with the conventional system, which has a boost chopper and an inverter. The proposed converter is experimentally tested with a 1-kW prototype. A voltage ripple at twice the frequency on the inverter DC voltage is suppressed from 35.1% to 4.6%. Moreover, the maximum efficiency of 94.5% with an output power of 1.0 kW is achieved. Finally, the design method of the boost-up inductor for proposed circuit is estimated by experiment. As a result, the error of the ripple current between design and measurement value is 4.7%.

Minimum flying capacitor for N-level capacitor DC/DC boost converter

This paper proposes a multilevel flying capacitor dc/dc boost converter (FCBC) with a small capacitance of the flying capacitor for the reduction of converter size. In this paper, the design methods for the boost inductor and the flying capacitor in the n-level FCBC are principally and experimentally confirmed. Furthermore, the boost inductor is designed based on maximum product of voltage-time, regardless the number of level. Meanwhile, the minimum capacitance of the flying capacitor is designed based on the maximum switching device voltage stress, regardless of the output voltage ripple influences. Moreover, the achieved maximum efficiency of the designed three-level FCBC is 98.5% at the output power of 1 kW. Finally, the effectiveness of small capacitances of the flying capacitor in the three-level and five-level FCBCs is analyzed. The characteristics of the distorted voltage across the input voltage source and the boost inductor, and the distorted current to the output side (output capacitor and load) are investigated with several capacitances of the flying capacitors in the three-level and five-level FCBCs. As a result, it is experimentally confirmed that the distorted voltage is drastically reduced by increasing the number of level. Besides, the distorted current is almost the same although the number of level is increased. Therefore, based on the experimental results, it is shown that small capacitance of the flying capacitor can be used in the n-level FCBC without compromising the stability of converter operation.

Minimization of interconnected inductor for single-phase inverter with high-performance disturbance observer

When a inductance of a single-phase grid-connected inverter is minimized, the effect of the disturbance to the current control such as the dead-time error voltage increases. This paper proposes a disturbance compensation method by using a high-performance disturbance observer. In particular, in the control system, the current controller is implemented by the digital signal processor, whereas the high-performance disturbance observer is implemented by the field-programmable gate array in order to suppress the disturbance at high switching frequency. This results in the improvement of the current control performance. With the experimental prototype, in case of the minimized inductance (%Z = 0.5%), the total harmonic distortion (THD) of the output current is improved from 11.1% to 2.9% at rated load by applying the proposed disturbance compensation method. Furthermore, with the proposed disturbance compensation method, it is confirmed that THD of the output current is improved by over 68% at all load range.