Sung-Hun Ko

Application of voltage- and current-controlled voltage source inverters for distributed generation systems

Voltage source inverters (VSI) have been widely used in uninterruptible power supplies, unified power flow controllers or unified power quality conditioners, and distributed generation systems (DGS). VSIs are inherently efficient, compact, and economical devices used to control power flow and provide quality supply. VSIs can be classified as voltage-controlled VSIs (VCVSIs) and current-controlled VSIs (CCVSIs), depending on their control mechanism. In this paper, a detailed comparison of VCVSIs and CCVSIs for DGS applications is presented. This paper examines the advantages and limitations of each control technique in a single-phase DGS, without incorporating additional hardware and/or extra complex control techniques. Discussions on the concepts, hypotheses, and computer simulations of different VSIs in the presence of different loads and conditions are presented. The experimental results confirm the validity of the analysis and simulations outlined. The paper provides design recommendations for the use of VCVSIs and CCVSIs in various applications

A Grid-Connected Photovoltaic System with Direct Coupled Power Quality Control

This paper presents a grid-connected photovoltaic (PV) system with direct coupled power quality controller (PQC), which uses an inner current control loop (polarized ramp time (PRT)) and outer feedback control loops to improve grid power quality and maximum power point tracking (MPPT) of PV arrays. To reduce the complexity, cost and number of power conversions, which results in higher efficiency, a single stage CCVSI is used. The system operation has been divided into two modes (sunny and night). In night mode, the current controlled inverter (CCVSI) operates to compensate the reactive power demanded by nonlinear or variation in loads. In sunny mode, the proposed system performs PQC to reduce harmonic current and improve power factor as well as MPPT to supply active power from the PV arrays simultaneously. It is shown that the proposed system improves the system utilization factor (SUF) to 100%, which is generally low for PV systems (20%). Mathematical modelling, computer simulations and experimental results for a 1 kVA CCVSI are presented

Current or Time Sharing Switches for High Efficiency Photovoltaic Power Systems

This paper presents a comparative analysis of the parallel operation of different switches in a switch mode power converter. In high power applications, multi-switch PWM power conditioners may be preferred despite a higher component count, due to the absence of low frequency filters, reduced switching losses and fault tolerance. The paper demonstrates how current sharing (CSH) and time sharing (TSH) lead to the reduction of switching stress in the parallel operation of switches in any converter. The solutions proposed in this study can be applied on different scales to other power conditioners for renewable energy applications. Discussions of the concepts, hypotheses and computer simulations are verified by 1 kW experimental results

Operational Characteristics of a Flux-Lock Type SFCL Integrated with Voltage-Controlled Voltage Source Inverter

In this paper, a flux-lock type superconducting fault current limiter (SFCL) integrated with a voltage-controlled voltage source inverter (VC- VSI) is proposed. The suggested equipment, which consists of a flux-lock type SFCL and a VCYSI, can perform the fault current limiting operation from the occurrence of a short-circuit, In addition, it can compensate the reactive power that the nonlinear load requires and also perform the un interruptible power supply (UPS) as well as the load voltage stabilization by controlling the amplitude and the phase of the inverters output voltage. The specification for a test model was determined and its various functions such as the fault current limiting and the power conditioning operations were presented and analyzed via computer simulation. Through the analytical results based on the computer simulation, the validity of the analysis was confirmed and its multi-operation was discussed.

Comarative Study on Current or Time Sharing Switches for High Efficiency DC/DC Converter

This paper presents a comparative analysis of the parallel operation of different switches in a DC/DC converter. In high power applications, multi-switch PWM power conditioners may be preferred despite a higher component count, due to the absence of low frequency filters, reduced switching losses and fault tolerance. The paper demonstrates how current sharing (CSH) and time sharing (TSH) lead to the reduction of switching stress in the parallel operation of switches in any converter. The solutions proposed in this study can be applied on different scales to other power conditioners for DC/DC converter systems. Discussions of the concepts, hypotheses and computer simulations are verified by 1 kW experimental results.