• The Transactions of the Korean Institute of Power Electronics
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• v.21 no.6
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• pp.465-472
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• 2016

A nonlinear load causes harmonic distortion and hampers the performance of other loads or distributed generation (DG) sources connected to the point of common coupling (PCC). This study proposes a new control strategy to reduce harmonic components, such as fifth and seventh harmonics, at the PCC under nonlinear loads in islanded microgrids, which comprise more than two DG sources. The proposed control scheme enables the DG source to share the power commanded by the control center as well as to compensate for the voltage harmonics at the PCC. The reference current is estimated from the voltage harmonics and injecting power; thus, the control scheme is implemented without any additional hardware devices. The simulation and experimental results are presented to verify the effectiveness of the proposed control method.

• Journal of Electrical Engineering and Technology
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• v.5 no.4
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• pp.614-622
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• 2010

This paper presents a control method, which reduces the pulsating torque and DC voltage problems of a doubly fed induction generator (DFIG)-based wind turbine system. To reduce the torque and power ripple, a current control scheme consisting of a proportional integral (PI) controller is presented in a positive synchronously rotating reference frame, which is capable of providing precise current control for a rotor-side converter with separated positive and negative components. The power theory can reduce the oscillation of the DC-link voltage in the grid-side converter. In this paper, the generator model is examined, and simulation results are obtained with a 3 kW DFIG-based wind turbine system to verify the proposed control strategy.

• Proceedings of the KIPE Conference
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• 2017.07a
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• pp.268-269
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• 2017

This paper presents an improved current control strategy for a three-phase grid-connected inverter under distorted grid conditions. Distorted grid condition is undesirable due to negative effects such as power losses and heating problem in electrical equipments. To enhance the power quality of distributed generation systems under such a condition, a modulated finite control set - model predictive control (MFCS-MPC) scheme will be proposed, in which the optimal switching signals of inverter are chosen by online basis using the principle of current error minimization. In addition, the moving average filter (MAF) is used to improve the phase-lock loop in order to obtain the harmonic-free reference currents on the stationary frame. The usefulness of the proposed MFCS-MPC method is proved by the comparative simulation results under different operating conditions.

• Journal of Power Electronics
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• v.16 no.1
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• pp.297-309
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• 2016

Grid-connected inverters (GCIs) with an LCL output filter have the ability of attenuating high-frequency (HF) switching ripples. However, by using only grid-current control, the system is prone to resonances if it is not properly damped, and the current distortion is amplified significantly under highly distorted grid conditions. This paper proposes a synchronous reference frame equivalent proportional-integral (SRF-EPI) controller in the αβ stationary frame using the parallel virtual resistance-based active damping (PVR-AD) strategy for grid-interfaced distributed generation (DG) systems to suppress LCL resonance. Although both a proportional-resonant (PR) controller in the αβ stationary frame and a PI controller in the dq synchronous frame achieve zero steady-state error, the amplitude- and phase-frequency characteristics differ greatly from each other except for the reference tracking at the fundamental frequency. Therefore, an accurate SRF-EPI controller in the αβ stationary frame is established to achieve precise tracking accuracy. Moreover, the robustness, the harmonic rejection capability, and the influence of the control delay are investigated by the Nyquist stability criterion when the PVR-based AD method is adopted. Furthermore, grid voltage feed-forward and multiple PR controllers are integrated into the current loop to mitigate the current distortion introduced by the grid background distortion. In addition, the parameters design guidelines are presented to show the effectiveness of the proposed strategy. Finally, simulation and experimental results are provided to validate the feasibility of the proposed control approach.

• The Transactions of the Korean Institute of Electrical Engineers
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• v.40 no.6
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• pp.567-573
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• 1991

Full direct digital control of induction motor driver is implemented with a minimal hardware structure. This paper deals with the presentation of a low-cost single-chip microprocessor-based control system for three-phase PWM generation and vector control that control speed of the induction motor using the field-oriented control method. Rotor flux is estimated using the indirect sensing method based on the rotor circuit equation in the synchronously rotation reference frame, and slip angle and rotor position are calculated from rotor angular velocity and stator current. Through simulation and experiment, it is shown that the proposed scheme gives good static and dynamic performance to the induction motor drive.

• Current Applied Physics
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• v.18 no.11
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• pp.1441-1446
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• 2018

We demonstrate an acousto-optic mode converter based on a tapered optical fiber to efficiently generate orbital angular momentum states of light. In our scheme an acoustic wave is deployed to the waist of tapered optical fiber where two degenerate $HE_{21}$ modes leading to +1 and -1 orbital angular momentum eigen-modes are resonantly excited. The excitation of $TM_{01}$ and $TE_{01}$ modes is suppressed by enlarging the intermodal index difference between near-degenerate spatial modes. Numerical calculation for optimization of the taper diameter is provided. The experimental characterization of generated states is performed by analyzing the output far-field pattern and the spatial interference fringes with a uniform reference beam.