Dongsul Shin

A Novel Grid Synchronization PLL Method Based on Adaptive Low-Pass Notch Filter for Grid-Connected PCS

The amount of distributed energy resources (DERs) has increased constantly worldwide. The power ratings of DERs have become considerably high, as required by the new grid code requirement. To follow the grid code and optimize the function of grid-connected inverters based on DERs, a phase-locked loop (PLL) is essential for detecting the grid phase angle accurately when the grid voltage is polluted by harmonics and imbalance. This paper proposes a novel low-pass notch filter PLL (LPN-PLL) control strategy to synchronize with the true phase angle of the grid instead of using a conventional synchronous reference frame PLL (SRF-PLL), which requires a d-q-axis transformation of three-phase voltage and a proportional-integral controller. The proposed LPN-PLL is an upgraded version of the PLL method using the fast Fourier transform concept (FFT-PLL) which is robust to the harmonics and imbalance of the grid voltage. The proposed PLL algorithm was compared with conventional SRF-PLL and FFT-PLL and was implemented digitally using a digital signal processor TMS320F28335. A 10-kW three-phase grid-connected inverter was set, and a verification experiment was performed, showing the high performance and robustness of the proposal under low-voltage ride-through operation.

Implementation of Fault Ride-Through Techniques of Grid-Connected Inverter for Distributed Energy Resources With Adaptive Low-Pass Notch PLL

The amount of distributed energy resources (DERs) has constantly increased worldwide. As the power ratings of DERs have become considerably high, the grid code requirements are necessary to secure reliable power generation and transmission for the public electric network. In order to follow grid codes of the various countries and optimize the function of grid-connected inverters for DERs, a robust phase-locked loop (PLL) is essential for extracting the grid phase information accurately, when the grid voltage is polluted by harmonics and unbalanced. This paper proposes fault ride-through techniques based on a low-pass notch (LPN)-PLL. The LPN-PLL has not only fast and smooth transient responses to a sudden transition of the grid voltage but also has robustness to the distorted and unbalanced grid conditions. Therefore, the stable performance in the grid fault conditions is expected without the system trip. Furthermore, a universal voltage sag generator for the various grid codes with six parameters is proposed for the verification of low voltage ride-through (LVRT) performance. Experimental verifications are presented to show the LVRT performance based on the LPN-PLL with a three-phase grid-connected inverter (10 kV·A) and a prototype of the voltage sag generator (10 kV·A).

Stability Improvement of Interleaved Voltage Source Inverters Employing Coupled Inductors for Grid-Connected Applications

A merit of the interleaving reducing the switching harmonics allows for the reduction of the filter inductance. However, in the case of direct parallel voltage source inverters (VSIs), a severe circulating current occurs. To suppress the high frequency circulating current, coupled inductors (CIs) for the high impedance is inserted. However, the employed CIs induce a positive- or negative-sequence circulating current (i.e., a cross current) besides the zero-sequence circulating current already mentioned in the other papers. Due to different impedances developed by the current characteristics and the cross current in the CIs, VSIs becomes unstable when the resonant controllers are applied for the harmonic compensation such as 5, and 7th harmonics. In this paper, the cross currents and the impedances of CIs varying with the current characteristics are mathematically analyzed, and a novel current controller is proposed to solve this stability problem. The experimental results validate the proposed control scheme.

Analysis and Design of Coupled Inductors for Two-Phase Interleaved DC-DC Converters

Multiphase dc-dc converters are widely used in modern power electronics applications due to their advantages over single-phase converters. Such advantages include reduced current stress in both the switching devices and passive elements, reduced output current ripple, and so on. Although the output current ripple of a converter can be significantly reduced by virtue of the interleaving effect, the inductor current ripple cannot be reduced even with the interleaving PWM method. One way to solve this problem is to use a coupled inductor. However, care must be taken in designing the coupled inductor to maximize its performances. In this paper, a detailed analysis of a coupled inductor is conducted and the effect of a coupled inductor on current ripple reduction is investigated extensively. From this analysis, a UU core based coupled inductor structure is proposed to maximize the performance of the coupled inductor.

Parallel operation of trans-Z-source inverter

Recently, there is a strong demand for high power conversion system in power electronics area especially in photovoltaic (PV) power conditioning system (PCS), wind power generation, and fuel cell power generation system. However, there is a limit in increasing power level mainly due to the limitations in switching devices voltage and current rating. One way to solve this problem is to connect power converters in parallel. Parallel operation of inverter has many advantages such as modularity, ease of maintenance, (n+1) redundancy, high reliability, etc. This paper presents parallel operation of voltage-fed-trans-Z-source inverter. A 2.6 kW prototype inverter is built and tested to verify performances of the proposed inverter.

Coupled inductors for parallel operation of interleaved three-phase voltage source grid-connected inverters

In this paper, coupled inductors for parallel operation of interleaved three-phase voltage source grid-connected inverters are introduced. When DC-link capacitors of each inverter are shared and interleaving method is employed, more severe circulating currents which are zero-sequence components are emerged than the other cases. In order to suppress carrier frequency zero-sequence components of circulating currents, the filter inductors of each same phase are coupled. Moreover, this technique has additional advantages; 1) Reduction of current ripples of both coupled inductors and outputs of coupled inductors; 2) Reduced size and weight of total filter inductors; 3) Efficiency improvement of conventional topology. The experimental results are shown to verify the advantages of proposed method.

Hybrid photovoltaic/diesel green ship operating in standalone and grid-connected mode in South Korea - Experimental investigation

The paper presents experimental results from the operation of a proto-type green ship in Geoje island, South Korea. After ground testing with a stand-alone PV generation system, this PV system was added to conventional diesel ship. Proto-type green ship is consisted of photovoltaic (PV) generation system, diesel engine, battery energy storage, hybrid control system, stand-alone and grid-connected inverter. The aim of the green ship is not only to minimize the fuel consumption but also to support the power grid as a distributed generation (DG) in the near future.

1.5MVA grid-connected interleaved inverters using coupled inductors for wind power generation system

In this paper, grid-connected interleaved voltage source inverters for PMSG wind power generation system with coupled inductors is introduced. In parallel operation, the undesirable circulating current flows between inverters. There are some differences in circulating currents according to source configuration. It is mathematically analyzed and simulated for the verification of analysis. In case of interleaved inverters, the uncontrollable circulating current in switching frequency level flows between inverter modules regardless of source configuration such as common or separated if filter inductance which is placed between inverters is significantly low. In order to suppress high frequency circulating current, the coupled inductors are employed in each phase. A case of 1.5MVA grid-connected interleaved inverters using coupled inductors prototype for the PMSG wind power generation Back To Back converters in parallel are introduced and experimentally verified the proposed topology.

A Study on a Solar Simulator for Dye Sensitized Solar Cells

Dye-sensitized solar cells (DSSC) are emerging low-cost, simple alternatives to conventional solar cells. While there has been considerable study on improving the efficiency of DSSCs, there has not been sufficient research on a photovoltaic power conditioning system adaptable to DSSCs or on a solar simulator for DSSCs. When DSSCs are commercialized in the near future, the DSSC modules must be connected to an adaptable power conditioning system in order to manage the energy produced and provide a suitable interface to the load. In the process of developing a power conditioning system, a solar simulator with the characteristics of DSSCs is essential to show the performance of the maximum power point tracking. In this paper, a virtual DSSC is designed and simulated in PSIM. Irradiation factors, temperature and shadow effects are considered in dynamic link library block in PSIM which is linked to the external C routine. A 100 W converter is built to show the performance of a DSSC as the solar simulator controlled by a digital signal processor.

A seamless transfer algorithm based on frequency detection and feedforward control method in distributed generation system

This paper proposes a control strategy for grid interactive Power Conditioning System (PCS) which has a feedforward voltage control based on grid frequency detection. The PCS should supply the power to the critical load, regardless of the grid condition. In order to provide the power to the critical load continuously, the PCS should operate two modes. One is Grid-Connected mode and the other is Stand-Alone mode. It is important to know the grid condition to decide operation modes. This paper proposes a control method based on the grid frequency to detect the grid condition and consists of a current controller and a feedforward voltage controller. The grid frequency detection is able to reduce the grid fault detection time and a feedforward voltage controller helps voltage regulation fast. This proposed control strategy is verified by the simulation and experimental results.