Ju-Yeop Choi

High-frequency DC link inverter for grid-connected photovoltaic system

This paper proposes an inverter for the grid-connected photovoltaic system based on the transformerless inverter. This system consists of a high frequency inverter bridge, high frequency transformer, diode bridge rectifiers, a DC filter, a low frequency inverter bridge, and an AD filter. The high frequency inverter bridge switching at 20 kHz is used to generate bipolar PWM pulse, which is subsequently rectified by diode bridge rectifiers to result in a full-wave rectified sine wave. Finally, it is unfolded by a low frequency inverter bridge to result in a 60 Hz sine wave power output. In this paper, the control algorithm for synchronous current feedback control method and a maximum power point tracking (MPPT) method using DSP are described. The simulation and experimental results are shown to verify the validity of the proposed system.

Line current characteristics of three-phase uncontrolled rectifiers under line voltage unbalance condition

Three-phase uncontrolled rectifiers with capacitive filters are highly sensitive to line voltage unbalance, drawing significantly unbalanced line current even under slightly unbalanced voltage condition. This paper presents an analysis of this unbalance amplification effect for an ideal rectifier circuit without AC- and DC-side inductors. A novel way of modeling the voltage unbalance is established by introducing a deviation voltage superimposed on balanced three-phase line voltages, which allows simplified analysis of the rectifier operation. With proper approximations, closed-form expressions for symmetrical components of the fundamental line current and correspondingly the current unbalance factor are derived in terms of the voltage unbalance factor, filter reactance, and load current. The equivalent RMS value of the third harmonic current is also derived. The analysis clearly shows high sensitivity of the current unbalance to the voltage unbalance, and provides a basic guideline for rectifier circuit design. The validity of the analysis is confirmed by simulation.

On-line efficiency optimization control of a slip angular frequency controlled induction motor drive using neural networks

Online efficiency control optimization of induction motor drives is important from the viewpoints of energy saving and controlling a system whose characteristics are not fully unknown. The paper presents a neural network-based online efficiency control optimization for an induction motor drive which adopts slip angular frequency control. In the proposed scheme, a neurocontroller adjusts the slip angular frequency adaptively for minimum loss operation based on the measured input power. Simulation results show that considerable energy savings are achieved in comparison with the conventional method of operating under the condition of constant voltage to frequency ratio.

Boundary conditions of reactive-power-variation method and active-frequency-drift method for islanding detection of grid-connected photovoltaic inverters

Photovoltaic (PV) power conditioning system (PCS) has been increased around the world since PV is becoming widespread as a clean and gentle energy source for earth. As a result, high-density grid-connected PV PCS will be interconnected with distribution network. In this situation, islanding phenomenon becomes most important issues for the safety and stability. Many methods to prevent islanding phenomenon have been proposed and analyzed. Especially, active frequency drift positive feedback (AFDPF) method is to process output current waveform of PV PCS by chopping fraction. However, AFDPF method is not fully identified the chopping fraction considering output power quality and islanding prevention performance in IEEE Std 929-2000 islanding prevention test. In this paper the research for the minimum value of chopping fraction gain applied digital phase-locked-loop (DPLL) to AFDPF method considering output power quality and islanding prevention performance are performed by PSIM simulation and experiment in IEEE Std 929-2000 islanding prevention test.

Simulation of Active Frequency Drift Adding Zero Current Method for Islanding Detection

Islanding of grid-connected photovoltaic (PV) systems can cause a variety of problems and must be prevented. If the real and reactive powers of the load and PV systems are closely matched, islanding detection by passive methods becomes difficult. The active frequency drift method (AFD) enables islanding detection by forcing the frequency of the current in the island to drift up and down. In this paper, proposed islanding detection method which has period of zero current. Simulation results verified that method using PSIM.

Performance results of 15 kW BIPV sunshade system

15 kW grid-connected building integrated photovoltaic (BIPV) sunshade system has been installed at Korea Institute of Energy Research (KIER) and monitoring system has also been constructed for measuring and analyzing performance of BIPV sunshade system to observe the overall effect of environmental conditions on their operation characteristics by long-term field test. Before installation of BIPV system, the performance of BIPV components such as power conditioning unit (PCU) and PV module under standard test conditions, which was measured and evaluated through short-term performance test. The performance of BIPV system has been evaluated and analyzed for component perspective (PV array and PCU) and global perspective (system efficiency, capacity factor, and electrical power energy, power quality etc.) for one year monitoring periods and loss factors are reviewed.

Reactive power control strategy for inverter-based distributed generation system with a programmable limit of the voltage variation at PCC

In a grid connected distributed generator, the voltage variation problem at the point of common coupling(PCC) is investigated. This is one of the most frequent power quality issues for the grid connection of large amount of the input power into a weak grid. Through a simplified modeling of the distributed generator and the power network, the magnitude of PCC voltage variation is calculated using the equivalent circuit parameters and the output power of the distributed generator. In addition, the required amount of reactive power which can compensate the voltage variation is presented analytically using a vector diagram method. With the proposed compensation method in a power conditioning system(PCS) for a distributed generation, the PCC voltage variation can be minimized automatically even though the power fluctuation occurs due to the change of input power. The proposed method of the calculation and compensation of the PCC voltage variation is verified by computer simulation and experimental results.

Disturbance observer based sensorless speed controller for PMSM with improved robustness against load torque variation

An improved sensorless operation scheme for permanent magnet synchronous machine (PMSM) is proposed. A sliding mode observer is employed to estimate the Back-EMF, with which the rotor position is estimated. To improve the robustness against the load torque variation, the disturbance observer (DOB) scheme is adopted so that it estimates the load torque and generates a compensating signal to approximately cancel it. Moreover, the transient response of the rotor speed has been improved in the sense that the settling time and maximum overshoot are reduced. Simulations and experiments show the effectiveness of the proposed scheme.

Design of SRD-F-PLL system with consideration of low-pass filter for high dynamic performance during voltage disturbance

Usually, LPF(low pass filter) is used in the feedback loop of SRF(Synchronous Reference Frame) - PLL (Phase Locked Loop) system because the measured grid voltage contains harmonic distortions and sensor noises. In this paper, it is shown that the cut-off frequency of the LPF should be designed to suppress the harmonic ripples contained in the measured voltage. Also, a new design method of the loop gain of the PI-type controller in the SRF-PLL is proposed with the consideration of the dynamics of the LPF. As a result, a better transient response can be obtained with the proposed design method. The LPF frequency and PI controller gain are designed in coordination according to the steady state and dynamic performance requirement. This paper shows the feasibility and the usefulness of the proposed methods through the computer simulation and the lab-scale experiments

Performance monitoring and analysis of middle scale grid-connected PV system

This paper presents performance results of middle scale grid-connected Photovoltaic (PV) system for monitoring periods. The performance and losses of PV system is quantitatively evaluated and analyzed using calculation model with monitored data so that various PV system technologies are development. The objective of this paper is to develop reliable and valid evaluation method of Photovoltaic (PV) system performance so that maximum output is achieved over the system lifetime with performance improvement.