Showing papers by "Hong-Hee Lee published in 2017"

Power Management Approach to Minimize Battery Capacity in Wind Energy Conversion Systems

Abstract: In a wind-battery hybrid power system, minimal battery capacity is a crucial requirement to achieve economic operation. In this paper, an optimal power control strategy based on a first-order low-pass filter is proposed to minimize the battery capacity by adjusting the filter smoothing time constant. We demonstrate the mathematical relationship between the filter smoothing time constant and the fluctuation mitigation requirement during one sampling time, so the optimal filter smoothing time constant can be easily computed to minimize the battery capacity. Moreover, an online short-term power control is also considered to maintain the battery state of charge within a safe range and to regulate the battery power below its rating. The proposed power management approach is simple and easy to implement. In order to verify the effective features of the proposed power management approach, a case study is carried out along with some experimental verification.

A Three-Vector Modulation Strategy for Indirect Matrix Converter Fed Open-End Load to Reduce Common-Mode Voltage With Improved Output Performance

Abstract: This paper proposes a three-vector modulation (TVM) strategy for a five-leg indirect matrix converter (IMC) fed three-phase open-end load (OEL) drive to reduce common-mode voltage (CMV) with improved output performance. By utilizing three nearest active voltage vectors to generate desired output voltage, the proposed TVM strategy can eliminate the CMV across the load phase and reduce the peak value of CMV at the load terminal to 42%. Compared to other modulation strategies for the IMC fed OEL drives, the problems associated with simultaneous switching of two legs and zero-sequence current do not occur with the TVM strategy. Also, a constant average dc-link voltage is achieved by using three active current vectors to synthesize reference input current, so that the output performance of the five-leg IMC is improved regarding output current and output voltage harmonic distortions. Simulation and experimental results are provided to verify the performance of the TVM strategy.

Topology of modified switched-capacitor Z-source inverters with improved boost capability

Abstract: This paper proposes a new topology named a modified switched-capacitor Z-source inverter (MSC-ZSI) based on the classical ZSI, in which an impedance network consists of symmetric two cells and each cell has two inductors and two capacitors, and one diode In comparison to the other topologies based on switched ZSI, the proposed topology provides higher boost capability and lower voltage stress across switching devices The improved performances of proposed topology are validated by both the simulation and experimental results

New virtual space vector modultation scheme to eliminate common-mode voltage with balanced neutral-point voltage for three-level NPC Inverters

Abstract: This paper proposes a new virtual space vector modulation (VSVM) scheme that can eliminate the common-mode voltage (CMV) for three-level neutral-point clamped (NPC) inverters with balanced neutral-point voltage. The proposed VSVM scheme is based on the virtual space vector concept, which constructs the virtual vectors from the original medium vectors to achieve zero CMV. Moreover, the neutral-point voltage is balanced because the new virtual vectors keep the average neutral-point current to be zero. Thus, the size of the dc-link capacitors can be significantly reduced. The simulation results are given to verify the effectiveness of the proposed VSVM scheme.

Resonant compensators to suppress input current harmonics for AC/DC matrix converters under unbalanced input voltages

Abstract: This paper presents a control strategy for the AC/DC matrix converter (MC) to achieve the constant output current as well as to reduce the input current harmonic distortion under unbalanced input voltages. The Proportional Integral (PI) controller is used to regulate the output current. Meanwhile, in order to suppress the harmonic distortion of input currents, two resonant compensators are employed in feedback paths of input currents in the αβ stationary frame. Finally, the proposed control strategy can achieve the constant output current and sinusoidal input currents without the phase-locked loop (PLL) and the extraction of the positive-negative voltage sequence. Simulation results are given to validate the effectiveness of the proposed control strategy.

A New SVM Method to Reduce Common-Mode Voltage of Five-leg Indirect Matrix Converter Fed Open-End Load Drives

Abstract: This paper proposes a cost-effective topology to drive a three-phase open-end load based on a five-leg indirect matrix converter (IMC) and a space vector modulation (SVM) method. By sharing an inverter leg with two load terminals, the proposed topology can reduce the number of power switches when compared to topologies based on a direct matrix converter or a six-leg IMC. The new SVM method uses only the active vectors that do not produce common-mode voltage (CMV), which results in zero CMV across the load phase and significantly reduces the peak value of the CMV at the load terminal. Furthermore, the proposed drive system can increase the voltage transfer ratio up to 1.5 and provide a superior performance in terms of an output line-to-line voltage with a three-level pulse-width modulation waveform. Simulation and experimental results are given to verify the effectiveness of the proposed topology and the new SVM method.

A regulated incremental conductance (r-INC) MPPT algorithm for photovoltaic system

Abstract: Maximum power point tracking techniques play an important role in improving the efficiency of photovoltaic generation system. Among various schemes, the incremental conductance (INC) method is mostly discussed in literature because of its superb tracking ability in irradiance and temperature changes. However, because the conventional INC algorithms adopt a simple duty-cycle updating law that is mainly based on the polarity information of the peak-power evaluation function, it is not possible to maximize the performance in transient as well as steady-state conditions. In order to mitigate above limitations, this paper proposes a novel regulated INC (r-INC) method. Just as a compensator in an automatic control system, it applies a digital filtering to the evaluation function and enhances the power tracking capability. Mathematical modeling of the new MPPT system also has been presented to aid the optimized design process. Comparative test results with a 120W boost peak power tracker verify the superiority of the proposed method over the conventional ones.

Optimal Power Control Strategy for Wind Farm with Energy Storage System

Abstract: The use of energy storage systems (ESSs) has become a feasible solution to solve the wind power intermittency issue. However, the use of ESSs increases the system cost significantly. In this paper, an optimal power flow control scheme to minimize the ESS capacity is proposed by using the zero-phase delay low-pass filter which can eliminate the phase delay between the dispatch power and the wind power. In addition, the filter time constant is optimized at the beginning of each dispatching interval to ensure the fluctuation mitigation requirement imposed by the grid code with a minimal ESS capacity. And also, a short-term power dispatch control algorithm is developed suitable for the proposed power dispatch based on the zero-phase delay low-pass filter with the predetermined ESS capacity. In order to verify the effectiveness of the proposed power management approach, case studies are carried out by using a 3-MW wind turbine with real wind speed data measured on Jeju Island.

A Predictive Current Control for Coordinate Control of Current and Power of Matrix Converter Under Unbalanced Input Voltages

Abstract: This paper presents a new predictive current control (PCC) method to achieve the coordinate control of current and power of the matrix converter under unbalanced input voltages. In order to avoid the complicated input voltage positive-negative sequence extraction, the flexible source current reference is constructed by filtering the square of input voltage vector with a notch filter. The optimal switching configuration to adjust source and load currents is selected by minimizating the cost function which is obtained from the sum of the absolute errors between the current references and their predictive values. Simulation results are given to validate the effectiveness of the proposed PCC method.

Fuzzy PID Controller for Reactive Power Accuracy and Circulating Current Suppression in Islanded Microgrid

Abstract: In this paper, an intelligent control scheme based on fuzzy proportional-integral-derivative controller (FPIDC) is proposed for distributed generations in islanded microgrid. The proposed FPIDC method is composed of a close-loop of the virtual impedance to compensate the feeder mismatch between distributed generators (DGs). Together with feedback of the inaccurate reactive power, the uncertainty of proportional-integral-derivative (PID) parameters is removed by adaptive tuning. Therefore, the power sharing performance is improved and the circulating current between DGs is mitigated. The dynamic response of the microgrid system with the proposed FPIDC is much better than that of the conventional PID controller. The comparison and analysis of the proposed control with conventional control are carried out to evaluate the superiority of the proposed method.

Rotor Flux Estimation for Low Speed Induction Motor Sensorless Drives with MRAS

Abstract: In order to realize the sensorless induction motor drives with the vector control strategy, the rotor flux and the torque current are generally used to find the rotor speed. In this paper, an improved rotor flux estimation method is proposed to enhance the performance in low and zero speed condition. For accurate rotor flux and torque current estimation, the flux reference is obtained by using the modified low pass filter (LPF) and a high pass filter (HPF) is added in order to remove errors in magnitude and phase around the filter cut-off frequency. And also, the rotor speed is detected to achieve sensorless induction motor vector control with the aid of the model reference adaptive system (MRAS) scheme. Thanks to the accurate flux estimation, the steady state performance becomes better especially in low and zero speed conditions. Simulation and experimental results verify the effectiveness of the proposed system.

An Enhanced Reactive Power Sharing and Secondary Voltage Restoration Control in Islanded Microgrid

Abstract: In the islanded micro-grid, parallel distributed generators (DGs) are normally controlled with the aid of the droop control scheme. However, the droop control method is still concerned to improve the accurate of reactive power sharing and variation of frequency and voltage at the point of common coupling (PCC). This paper proposes a control scheme to solve the limitation of microgrid in islanded operation such as reactive power sharing accuracy and PCC voltage and frequency drop by using a close-loop of the virtual impedance to compensate the feeder mismatch between DGs. Therefore, the power sharing performance is improved, and the circulating current between DGs is suppressed. In order to achieve the accurate reactive power sharing and PCC voltage restoration, a secondary control is implemented in both the microgrid central controller (MGCC) and local controller by using the low bandwidth communications. The effectiveness of the proposed control method is analyzed through the simulation.

A Nine-Switch Unified Power Quality Conditioner with Enhanced Repetitive Controller

Abstract: The 9-switch power converter can provide two sets of output terminals like the conventional back-to-back power converter. In this paper, the unified power quality conditioner (UPQC) is developed by using the 9-switch power converter with the enhanced repetitive controller (RC). The proposed 9-switch UPQC can mitigate concurrently harmonic current and voltage problems together with voltage sag/swell issues. And also, the switching loss is reduced significantly by using the discontinuous modulation strategy for the 9-switch UPQC. The characteristics of the proposed 9-switch UPQC are evaluated by simulation, and its superior performance is verified.

Adaptive Control of DC-DC Converter Based on Hybrid Fuzzy PID Controller

Abstract: DC-DC converter is widely used in industrial applications with the aid of proportional-integral-derivative (PID) controller. The PID controller is commonly used because of its simple, efficiency and small steady state error. However, DC-DC converter is mainly nonlinear system due to its inherent switching operation, which makes the PID parameters very difficult to be found. In this paper, an intelligent PID controller based on a hybrid combination with the fuzzy logic controller (FLC) is proposed to control DC-DC converter. The proposed fuzzy based PID controller (FPIDC) is a close-loop control of the nonlinear PID parameters by measuring output voltage of the DC-DC converter, and it is designed effectively to reduce computation burden. By tuning the PID parameters continuously according to the load condition, the performance of the DC-DC converter is improved much better. The comparison between the proposed and conventional PID control method are investigated under various operating conditions. Simulation results and model analysis are carried in MATLAB/SIMULINK to prove the effectiveness of the proposed FPIDC method.

A second-order generalized differentiator method for eliminating DC component in the three-phase PLL for grid-connected inverters

Abstract: This, paper proposes a method to eliminate the dc component in the measured three-phase grid voltage as the input signals of phase-locked loop (PLL). The dc component in the input signals of PLL causes a low-frequency oscillation of grid frequency. The dc component elimination can be achieved by replacing the integrator in the second-order generalized integrator (SOGI) with a differentiator, named as second-order generalized differentiator (SOGD). The good performance of a proposed SOGD is validated with experimental results with 32-bit DSP by using unbalanced three-phase signals with dc components.