Jaehong Kim

Mode Adaptive Droop Control With Virtual Output Impedances for an Inverter-Based Flexible AC Microgrid

A decentralized power control method in a single-phase flexible ac microgrid is proposed in this paper. Droop control is widely considered to be a good choice for managing the power flows between microgrid converters in a decentralized manner. In this work, to enhance the power loop dynamics, droop control combined with a derivative controller is used in islanded mode. In grid-connected mode, to strictly control the power factor in the point of common coupling (PCC), a droop method combined with an integral controller is adopted. Small-signal analysis of the proposed control is shown both in islanded and grid-connected mode. The proposed control scheme does not need any mode switching action. Thus, it is relatively simple in control for full mode of operation. Smooth transitions between the operation modes and the effectiveness of the proposed control scheme are evaluated through simulation and experimental results.

Optimization and characterization of UV-curable adhesives for optical communications by response surface methodology

Novel organic UV-curable material (UCM) containing fluorine and methoxy-silane has been specifically developed for the fabrication of optical-communications systems devices. Response surface methodology (RSM) was used to evaluate the effect of UCM and colloidal silica content in UV-curable adhesive formulations on the refractive index, curing, adhesion strength (dry and wet) and coefficient of thermal expansion (CTE). A central composite design for two variables at five levels was chosen as the experimental design. UCM for all six responses was highly significant and colloidal silica was significant for conversion and adhesion strength (dry and wet). UCM and colloidal silica had significant interactive effects on adhesion strength and CTE. Based on the fitted model, in order to get the optimal conditions for the application of optical adhesives in high-performance optical components, UCM should be set over 20 wt% and colloidal silica be set below 10 wt%.

A Current Distortion Compensation Scheme for Four-Switch Inverters

The four-switch inverter, having a lower number of insulated gate bipolar transistors(IGBTs), has been studied for the possibility of reducing the inverter cost. But it has a limited performance in the low-frequency region, because the balance among the phase currents collapses due to the fluctuation of the center tap voltage of the DC-link capacitors. This problem could be solved if the DC-link capacitance is infinitely large, but it is a costly solution. In this paper, this problem is looked at from the perspective of source impedance and the voltage variation caused by the current flow through the capacitors. The source impedance of the center tap is large compared with other normal IGBT arms. This causes an asymmetry among the three voltage sources, resulting in phase current distortion and unbalance. Second, the capacitor voltage change caused by current flow is another source of current distortion and unbalance. The voltage errors are derived, and based on them, a compensation method is proposed. Effectiveness of the proposed method is demonstrated by the simulation and experimental results.

Inverter-Based Local AC Bus Voltage Control Utilizing Two DOF Control

Small- and medium-sized distributed generation (DG) systems in a microgrid normally have inverters, and the inverter outputs are connected to the grid through an LCL filter. In this paper, we are interested in designing a DG inverter that behaves like a uninterruptible power supply (UPS) for its dedicated loads, while enabling power flow control with the microgrid. If the grid power is lost, then the DG system can be transferred smoothly to the island (stand-alone) operation. This islanding capability provides a higher reliability of the local ac bus than the conventional power system. In order to have such a UPS functionality, the inverter is desired to have a high control bandwidth, which is required for regulating the ac bus voltage at the presence of nonlinearities and unpredictable behaviors of the loads. As for a fast controller, a two-degree-of-freedom (DOF) controller is applied to the LC filter plant with the inverse dynamic model. The voltage command levels are selected considering the amount of power share with the grid. The effectiveness of the proposed control scheme is shown by experimental results.

Commutation Control for the Low-Commutation Torque Ripple in the Position Sensorless Drive of the Low-Voltage Brushless DC Motor

This paper discusses a commutation control method aimed at reducing the commutation torque ripple in sensorless drive of brushless direct current motors. These motors are generally used for low-cost applications because of relatively high efficiency and low manufacturing cost. On the other hand, they show a high torque ripple characteristic caused by nonideal commutation currents. This limits their application area, especially for low-voltage applications. In order to minimize torque ripple for the entire speed range, a comprehensive analysis of commutation torque ripple was made according to three commutation control methods whereupon an optimal current vector trajectory for low torque ripple was devised. The effectiveness of the proposed method was verified through simulation and experimental results.

Asymmetric Duty Control of a Dual-Half-Bridge DC/DC Converter for Single-Phase Distributed Generators

A dual-half-bridge (DHB) converter is integrated with a half-bridge 60-Hz inverter as a converter/inverter system for a small distributed generator. This topology provides an isolation between the power source and the load with a 100-kHz transformer, and therefore, the system volume is small. On the other hand, it reduces the number of switching devices greatly. However, the half-bridge inverter causes severe capacitor-voltage fluctuations. The unbalanced voltage problem can be solved by controlling the converter switching: In addition to the conventional phase-shift control method, an asymmetric charging method is used that charges the upper and lower secondary capacitors differently. This means that the voltage imbalance is corrected by adjusting the switching time of the secondary switches. Furthermore, a decoupling control algorithm can be derived from this approach. The usefulness of this method is then validated by simulation and experimental results.

Determination of key variables affecting surface properties of UV curable coatings using experimental design

Abstract The influence of prepolymer (urethane acrylate or polyesteracrylate) type, triacrylate (trimethyolpropane triactylate or ethoxylated trimethylolpropane triacrylate) type and the concentration of silicone acrylate on the surface properties of UV cured films was studied. The effect of these variables on the pencil hardness and gloss was determined by using a full factorial experimental design. The results showed that the prepolymer type and the concentration of silicone acrylate were the most significant factors affecting surface hardness and gloss of UV cured films, respectively. Specifically, the coating formulation with polyester prepolymer resulted in improved surface hardness of the UV cured film, while increasing the concentration of the silicone acrylate significantly decreased the gloss. In addition, the predictive mathematical models obtained through analysis of variance provide a reasonable approximation of actual experimental measurements.

Asymmetric duty control of the dual-active-bridge DC/DC converter for single-phase distributed generators

The dual-active-bridge (DAB) and a half-bridge inverter are integrated back-to-back for a converter/inverter system. The power source is isolated by a high frequency transformer, and a 60Hz inverter is directly connected to the secondary side of the converter via DC link capacitors. Thus, the proposed converter-inverter topology greatly reduces the size and the number of parts count, while providing isolation. But, the inverter causes the DC link voltage unbalance, leading to coupled dynamics between converter and inverter. In this work, a voltage balancing algorithm and a decoupling control algorithm are derived from the average model of the DAB converter. Its usefulness is validated by experimental results.

Two arm inverter with three level boost converter for three phase induction motors

The three-level boost converter is installed at the DC link side of the two arm inverter. The three level boost converter is capable of charging upper or lower DC link capacitors selectively. Utilizing this selective charging capability, a DC link center tap voltage balancing control method is proposed. Further, the boost converter enables to achieve unity power factor. Supporting experimental results were made with a small DC link capacitance.

On-line compensation of dead-time and inverter nonlinearity with disturbance voltage observer

This paper discusses and analyses a simple on-line compensation scheme for dead-time and inverter nonlinearity in the pulse width modulated (PWM) voltage source inverter (VSI). Dead-time effect and voltage drop in switching devices cause nonlinearity between reference and output voltage. In a conventional three-phase six-switch inverter, this nonideal condition adds extraneous harmonics that badly disturb voltage characteristics. In its turn, voltage disturbance causes distortion of the current waveform and degrades performance. In this paper, an on-line dead-time compensation method based on inverse dynamics control is proposed, and it is much simpler than conventional full/reduced order observation methods adopted in dead-time compensation. Disturbance voltages are observed on-line with no additional circuitry or off-line measurements. The observed disturbance voltages are fed back to the voltage reference for compensation. Stability problem of the proposed observer arisen from inverter delay and parameter mismatch was analysed. The proposed method is applied to a surface-mounted permanent-magnet synchronous motor (SPMSM) drive. The effectiveness of the proposed scheme is validated by the experimental results.