Showing papers by "Hong-Hee Lee published in 2016"

Development of a Three-to-Five-Phase Indirect Matrix Converter With Carrier-Based PWM Based on Space-Vector Modulation Analysis

Abstract: This paper proposes an indirect matrix converter (IMC) topology to supply a five-phase load directly from three-phase ac power sources without bulky electrolytic capacitors and explains its operating principle. Using the emerging topology, which combines the five-leg inverter into the rectifier stage of the conventional IMC, the three-to-five-phase IMC topology reduces the number of power switches and results in a simpler commutation compared with the conventional three-to-five-phase direct matrix converter. In addition, a simple approach to realize the carrier-based pulsewidth modulation (CBPWM) method for a three-to-five-phase IMC is presented, and the proposed CBPWM method is analyzed based on the space-vector approach. As a result, the IMC with the proposed CBPWM method can be more easily implemented compared with IMCs with the space-vector PWM method. A three-to-five-phase IMC control platform is realized by coordinating a TMS320F28335 DSP (Texas Instruments) with an EPM7128SLC84-15 field-programmable gate array (Altera). The simulation and experimental results are provided to verify the effectiveness of the CBPWM strategy.

A Modulation Scheme for Matrix Converters With Perfect Zero Common-Mode Voltage

Abstract: This paper presents a novel space vector modulation (SVM) method combined with a modified four-step commutation to achieve a perfect zero common-mode voltage (CMV) for matrix converters (MCs). The new SVM method was developed by using only rotating input voltage vectors, which results in the CMV for MCs being zero. However, it causes some noise in practical applications that drive MCs with traditional four-step commutation. In order to solve this noise problem, a modified four-step commutation is also proposed to perfectly eliminate the CMV for MCs without any noise. In the modified four-step commutation, a delay time is designed to deal with misjudgment of input voltages. The proposed SVM method and four-step commutation are easily implemented via software and achieve good performance in input/output current waveforms. Prototype experiments were carried out to evaluate the performance of the proposed method.

A Novel Dual-Battery Energy Storage System for Wind Power Applications

Abstract: In this paper, a novel dual-battery energy storage system (DBESS) is proposed to firmly dispatch the intermittent wind power onto the grid with a lower system operation cost. Thanks to the DBESS, a wind farm can commit to integrate constant power in each dispatching time interval. In the proposed DBESS, the battery energy storage system (BESS) that takes the charged role is active when the dispatch power is lower than the wind power, and another is enabled if the dispatch power is higher than the wind power. These roles of the BESSs switch when one BESS is either fully charged or deeply discharged. To extend the BESS lifetime, the dispatch power is optimized to ensure both BESSs are operating under full charge–discharge cycles, so the system's operating cost is reduced significantly by using the proposed DBESS. In addition, short-term power dispatch control is considered to deal with wind power forecasting errors. In order to evaluate the proposed optimized system, we performed several numerical studies using a 3-MW wind turbine generator with real wind power data. Moreover, a wind-battery power test bed is implemented in the laboratory to validate the feasibility of the proposed DBESS.

A DSVM Method for Matrix Converters to Suppress Common-mode Voltage With Reduced Switching Losses

Abstract: This paper proposes a direct space-vector modulation (DSVM) method that can reduce the common-mode voltage (CMV) as well as the switching losses for matrix converters in a high voltage transfer ratio. Even though the previous DSVM improvements can reduce the CMV peak value to 42%, compared to the conventional DSVM method, they incur high switching losses. The DSVM method proposed in this paper provides the same CMV peak value as previous methods with lower switching losses. The switching losses are theoretically analyzed through the accumulated switched voltage in a switching period and compared to those of previous methods. Simulation and experimental results are given to verify the input/output waveforms, CMV reduction, and lower switching losses of the proposed DSVM method.

Embedded quasi-Z-source inverters based on active switched-capacitor structure

Abstract: A novel topology with applying the concept of the embedded technique to an active switched-capacitor qZSI topology named an active switched-capacitor embedded quasi-Z source inverter (ASC-EqZSI) topology is proposed. By inserting one diode and switch to the classic qZSI and embedding a single dc voltage source at the impedance network, the proposed topology has high boost factor, and performs a reduced voltage stress on the switching devices of PWM inverter. The performances of the proposed topology are validated through simulation and experimental results.

Effective power dispatch capability decision method for a wind-battery hybrid power system

Abstract: With integration of a battery energy storage system (BESS) into the wind power system, the wind power variation can be mitigated to dispatch a constant power to the grid. To effectively supply the constant power dispatch to the grid with a limited BESS, the power dispatch capability should be defined primarily at beginning of each dispatching interval to cooperate with the transmission system operator. This study introduces a method to determine the power dispatch capability of a wind-battery hybrid power system with the minimal battery capacity. The power dispatch capability is decided under the condition that the battery power is maintained below its rating and that the state of charge of the battery is guaranteed within a safe range. For economic BESS operation, the battery capacity, including the power and energy ratings, is also determined cost-effectively to obtain the constant power dispatch. Based on the long-term wind power profile, the authors develop the determination process to minimise the battery capacity which ensures that the BESS sufficiently mitigates wind power variation to dispatch the constant power into grid under all wind conditions. To evaluate the proposed determination method, a case study is carried out with a 3-MW wind turbine generator and real wind speed data measured on Jeju Island in Korea.

An adaptive power distributed control method to ensure proportional load power sharing in DC microgrid considering equivalent line impedances

Abstract: This paper proposed a distributed control method for dc microgrid to ensure the proportional load sharing by taking into account the different line impedance. In the proposed method, the operation point of each DG is effectively defined based on the power rating and the instantaneous power of the DG to achieve the proportional load power sharing. A low bandwidth communication is used to transmit the data required to determine the power reference for all DGs. In order to balance the power per unit requirement, the output voltage of each DG is controlled by a power controller to adjust the desired operating point. Therefore, all DGs can operate at the balanced operating point on the droop curve to ensure the proportional load power sharing. This paper also considers the load shedding to prevent the dc microgrid from operating under overload condition. The effectiveness of the proposed method is verified by simulation and experiment which are carried out with 2.8kW prototype dc microgrid.

Microgrid communication network with combined technology

Abstract: Smart microgrid concept deserves communication assisted protection, control and management. In this paper, information within microgrid for monitoring, control and protection purpose is carried out and mapped according to power system automation standard IEC61850. Multi-agent technology is adopted to define the characteristics of different intelligent devices. Furthermore, wired and wireless technology based integrated, robust and reliable communication network is proposed for real time monitoring and control of microgrid. Performance analysis of the network is done by modelling microgrid nodes and traffic according to IEC61850 with Optimized Network Engineering Tool (OPNET). Simulation results are provided to claim the performance of the proposed network for reliable operation of microgrid in both islanded and grid connected modes.

Adaptive virtual impedance control scheme to eliminate reactive power sharing errors in islanded microgrid

Abstract: This paper proposes an enhanced distributed generation (DG) unit with an adaptive virtual impedance control approach in order to solve the inaccurate reactive power sharing problem. The proposed method can adaptively regulate DG units with the aid of the equivalent impedance, and the mismatching problem of the feeder impedance is compensated by sharing the reactive power accurately. The proposed control strategy can be implemented directly without any pre-knowledge of the feeder impedances. Simulations are performed to validate the effectiveness of the proposed control approach.

Carrier-based PWM strategy for post-fault reconfigured 3-level NPC inverter under imbalanced dc-link voltages

Abstract: This paper presents an investigation on carrier-based PWM method of post-fault reconfigured topology for the three-level neutral-point-clamp (3-level NPC) inverter. The operation of 3-level NPC under device failure conditions is analyzed and the proposed fault-tolerant PWM strategy is able to continuously the operation of the inverter. The proposed carrier-based method is simple without complexity calculation, the duty cycles are defined based on the comparison between the modulation signals and high frequency carrier signal. Furthermore, the balanced output currents are obtained under the imbalanced dc-link voltage condition because the modulation signals are determined based on the feedback dc-link voltage. The proposed carrier-based PWM is verified by numerical simulation and it is validated by hardware experimentation on a laboratory prototype.

A modulation strategy to eliminate CMV for matrix converters with input power factor compensation

Abstract: A space vector modulation (SVM) method to eliminate common-mode voltage (CMV) for matrix converters (MCs) can effectively solve many problems in electrical systems. However, the input power factor (IPF) of the MC is significantly degraded under light-load conditions due to the input filter's characteristics. In this paper, a new switching strategy is proposed to compensate the IPF for the zero CMV-SVM method in MCs. The proposed strategy is based on the study of rotating-vector behavior and rearrangement of duty cycles in order to achieve the zero CMV, unity or the maximum achievable value of IPF, as well as a good performance of MCs. Simulation results are provided to validate the correctness of the proposed strategy.

Three-vector modulation scheme to improve output performance for five-leg indirect matrix converter fed open-end load

Abstract: This paper proposes a three-vector modulation (TVM) scheme for a five-leg indirect matrix converter (IMC) fed a three-phase open-end load (OEL) to control both rectifier and inverter stages with zero common-mode voltage (CMV) and low output current distortion. In order to reduce the CMVs at the load terminals and to eliminate the CMV across the load phases, three nearest active vectors are selected to generate the reference output voltage. Similarly, thanks to the TVM scheme for the rectifier stage, the average value of the dc-link voltage is kept at constant, which leads to minimize the output current distortion. Additionally, the interest features of the conventional OEL systems such as higher voltage transfer ratio and better output voltage quality with three-level voltage waveform are also inherited in the cost effective topology of a five-leg IMC fed OEL. Simulation and experimental results are provided to verify the performance of the proposed TVM modulation scheme.

A single-phase unified power quality conditioner with an enhanced repetitive controller

Abstract: This paper proposes a robust and flexible control strategy for a single-phase unified power quality conditioner (S-UPQC) to solve the power quality problem due to the distorted input supply voltage and the nonlinear load. In order to enhance the performance of the series and shunt active power filters (APFs) in S-UPQC, an advanced current and voltage control schemes are introduced based on the modified repetitive controller (RCs) connected with a proportional-integral (PI) controller. Unlike the conventional control scheme for the series and shunt APFs, the proposed scheme is implemented without any harmonic extractor. Furthermore, the dynamic performance is improved significantly due to the reduced delay time of the RC, and the computation time is also reduced. The effectiveness of the proposed control schemes for the S-UPQC is validated through simulation.

An Islanding Microgrid Power Sharing Approach Using Adaptive Virtual Impedance control scheme

Abstract: 315 Abstract –This paper proposes an enhanced distributed generation (DG) unit with an adaptive virtual impedance control approachin order to address the inaccurate reactive power sharing problems. The proposed method can adaptively regulate the DG unit thanks to the equivalent impedance, andthe effect of the mismatch in feeder impedance is compensatedto share the reactive power accurately.The proposed control strategy can be implemented directly without any requirement of pre-knowledge of the feeder impedances. Simulations are performed to validate the effectiveness of the proposed control approach.

A Simplified Predictive Current Control for Matrix Converters with Reduced Common-mode Voltage

Abstract: Predictive current control (PCC) is an effective method to control matrix converters (MCs) due to its advantages such as simplicity, fast dynamic response, and flexibility to control different variables. However, the high amount of calculations for the PCC is an obstacle for its real application. To overcome this problem, this paper proposes a simplified PCC method for MCs by utilizing the sector distribution. In addition, the proposed method reduces the peak value of common-mode voltage (CMV) to 42% by using a mediumvalued phase voltage to generate the zero vectors. Simulation results are shown to validate the effectiveness of the proposed PCC method.

Enhanced Reactive Power Sharing and Voltage Restoration in Islanded Microgrid

Abstract: Parallel distributed generators (DGs) in the islanded micro-grid are normally controlled with the aid of the droop control scheme. However, the traditional droop control methods which use the P-ω and Q-E curve to share power between DGs are 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 restoring. In order to achieve the control objective, a secondary control is implemented with both central controller and local controller by using the low bandwidth communications. The effectiveness of the proposed control scheme is analyzed through the simulation. Keywords— islanded microgrid, reactive power control, secondary control.

A Model Predictive Current Control Method for Voltage Source Inverters to Reduce Common-Mode Voltage with Improved Load Current Performance

Abstract: This paper presents a new model predictive current control (MPCC) method in order to reduce common-mode voltage (CMV) for a three-phase voltage source inveter (VSI). By utilizing twenty-one vectors instead of eight vectors used in the conventional MPCC method,the proposed MPCC method can not only reduce the CMV but also improve load current performance. Simulation results are given to verify the effectiveness of the proposed MPCC method.

Flexible Source Current Reference Generation for Predictive Current Control of Matrix Converter under Unbalanced Input Voltages

Abstract: This paper presents a new predictive current control (PCC) method to achieve the coordinate control of power and current of the matrix converter under unbalanced input voltages. In order to control the power fluctuation in the input side, the flexible source current reference is generated based on the positive-negative sequence components of the input voltage. The optimal switching state to adjust source and load currents is selected by minimization 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.