Nam-Sup Choi

Modeling and analysis of static and dynamic characteristics for buck-type three-phase PWM rectifier by circuit DQ transformation

The static and dynamic characteristics of buck-type three-phase pulse width modulation (PWM) rectifier are fully analyzed based on the DC and AC circuit models developed by the circuit DQ transformation. Various static power converter characteristics such as gain, real and reactive power, power factor and unity power factor conditions are completely analyzed. Transition characteristics are also analyzed by both exact small-signal models with full set of equations and simplified output models in explicit form. The usefulness of the models is verified through computer simulations and experiments with good agreement shown.

DC micro-grid operational analysis with detailed simulation model for distributed generation

This paper describes operational analysis results of the DC micro-grid using detailed model of distributed generation. Detailed model of wind power generation, photo-voltaic generation, fuel cells generation was implemented with the user-defined model of PSCAD/EMTDC software that is coded with C-language. The operation analysis was carried out using PSCAD/EMTDC software, in which the power circuit is implemented by built-in model and the controller is modeled by user-defined model that is also coded with C-language. Various simulation results confirm that the DC micro-grid can operate without any problem in both the grid-tied mode and the islanded mode. The operational analysis result confirms that the DC micro-grid make it feasible to provide power to the load stably. And it can be utilized to develop an actual system design and building.

Carrier Based Pulse Width Modulation for Matrix Converters

This paper presents an extension of direct duty ratio pulse width modulation (DDPWM) to various topologies of matrix converters. In DDPWM, by using the concept of average over one switching period, the duty ratio which is compared with the triangular carrier waveform can be obtained so as to synthesize the desired output voltages. Owing to the per-phase modular structure of the DDPWM method, it can be effectively applied to the single-phase, the two-phase and the three-phase four-leg matrix converters as well as the common three-phase to three-phase matrix converter. The feasibility and validity of the proposed method are verified by experimental results.

Fault-tolerant modulation strategy with direct duty-ratio PWM for matrix converter-fed motor drives

Direct duty-ratio PWM schemes for continuous fault tolerant operation of matrix converter-fed motor drives are presented. The proposed method features simple modular modulation structure based on per output phase concept, which requires no additional modification on the normal modulation schemes for fault-tolerant applications. Realizations of fault-tolerant strategy applied to different system configurations are also treated to enhance the system flexibility. The proposed method can be effectively applied to treat the motor open phase fault and converter switching device failure. Simulation results will be provided to verify the feasibility and availability.

New Bidirectional Intelligent Semiconductor Transformer for Smart Grid Application

This paper proposes a new bidirectional intelligent semiconductor transformer (BIST) for the smart distribution system and smart grid. The proposed BIST consists of high-voltage high-frequency ac/dc converter, bidirectional low-voltage dc/dc converter, and hybrid-switching dc/ac inverter. It features 1) input-to-output isolation with a high-frequency transformer; 2) bidirectional power flow; 3) small size and light weight; 4) capability of compensating voltage sag and/or swell; and 5) realization of three-phase structure based on single-phase module. The operational feasibility of proposed transformer was verified not only by computer simulation with PSCAD/EMTDC software but also by a hardware prototype with rating of 1.9 kV/127 V, 2 kVA, allowing a three-phase transformer of 3.3 kV/220 V, 6 kVA with three-phase construction.

Dynamic voltage restorer using PWM ac-ac converter

This paper presents dynamic voltage restorers (DVRs) which are employing a PWM ac-ac converter. The shunt-series and series-shunt configurations are shown along with the operating principles when using the basic PWM ac-ac converter topologies such as buck, boot and buck-boost converters. The steady state analysis is carried out for an example case, the DVR with buck ac-ac converter. Also, this paper proposes three-phase DVR structures. Simulation results show the validity and feasibility of the proposed DVR.

DFIG Wind Power System with a DDPWM Controlled Matrix Converter

This paper proposes a new doubly-fed induction generator (DFIG) system using a matrix converter controlled by direct duty ratio pulse-width modulation (DDPWM) scheme. DDPWM is a recently proposed carrier based modulation strategy for matrix converters which employs a triangular carrier and voltage references in a voltage source inverter. By using DDPWM, the matrix converter can directly and effectively generate rotor voltages following the voltage references within the closed control loop. The operation of the proposed DFIG system was verified through computer simulation and experimental works with a hardware simulator of a wind power turbine, which was built using a motor-generator set with vector drive. The simulation and experimental results confirm that a matrix converter with a DDPWM modulation scheme can be effectively applied for a DFIG wind power system.

High-Efficiency Grid-Tied Power Conditioning System for Fuel Cell Power Generation

This paper proposes a grid-tied power conditioning system for the fuel cell power generation, which consists of a 2-stage DC-DC converter and a 3-phase PWM inverter. The 2-stage DC-DC converter boosts the fuel cell stack voltage of 26–48V up to 400V, using the hard-switching boost converter and the high-frequency unregulated LLC resonant converter. A laboratory experimental set-up was built with the 1.2kW PEM fuel-cell stack to verify the feasibility of hardware implementation. The developed power conditioning system shows high efficiency of 91%, which is very positive result for the commercialization.

A carrier-based medium vector PWM strategy for three-level inverters in transformerless photovoltaic systems

This paper proposes a new carrier-based PWM strategy for three-level inverter in transformerless photovoltaic systems. The proposed modulation method completely eliminates leakage currents by employing only medium vectors and one zero vector which produce the same common mode voltage. Due to carrier-based scheme, the proposed PWM method can allow overmodulation and be intuitively understood and simply implemented with analog or digital hardware. To show the feasibility of the proposed PWM method, the simulation results are presented.

Seamless control mode transfer of a PCS for islanding mode operation

This paper describes a new method for the seamless transfer between voltage and current control modes of a PCS in a microgrid. Usually, microgrid is connected to a utility through STS. In case of short circuit fault a huge current may flow through the STS, therefore, it is necessary to provide a method to reduce the STS fault current. In this paper a novel method to reduce the fault current is proposed, and the method can help to make a smooth operating mode change between current and voltage control of a PCS. The proposed method is applied to 10 kVA power system including utility source, STS, PCS, and load. Simulation results show the usefulness of the proposed method.