• Coupled systems mechanics
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• v.7 no.6
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• pp.649-668
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• 2018

In this paper, we present a numerical model for fluid-structure interaction between structure built of porous media and acoustic fluid, which provides both pore pressure inside porous media and hydrodynamic pressures and hydrodynamic forces exerted on the upstream face of the structure in an unified manner and simplifies fluid-structure interaction problems. The first original feature of the proposed model concerns the structure built of saturated porous medium whose response is obtained with coupled discrete beam lattice model, which is based on Voronoi cell representation with cohesive links as linear elastic Timoshenko beam finite elements. The motion of the pore fluid is governed by Darcy's law, and the coupling between the solid phase and the pore fluid is introduced in the model through Biot's porous media theory. The pore pressure field is discretized with CST (Constant Strain Triangle) finite elements, which coincide with Delaunay triangles. By exploiting Hammer quadrature rule for numerical integration on CST elements, and duality property between Voronoi diagram and Delaunay triangulation, the numerical implementation of the coupling results with an additional pore pressure degree of freedom placed at each node of a Timoshenko beam finite element. The second original point of the model concerns the motion of the outside fluid which is modeled with mixed displacement/pressure based formulation. The chosen finite element representations of the structure response and the outside fluid motion ensures for the structure and fluid finite elements to be connected directly at the common nodes at the fluid-structure interface, because they share both the displacement and the pressure degrees of freedom. Numerical simulations presented in this paper show an excellent agreement between the numerically obtained results and the analytical solutions.

• The Transactions of the Korean Institute of Power Electronics
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• v.26 no.2
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• pp.75-81
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• 2021

This study analyzes the resonance characteristics caused by the mutual interference between LCL filters and the grid impedance under the parallel operation of the grid-connected inverter using the LCL filter. These characteristics are verified through simulation and experiment. Two inverters are used to connect to the grid in parallel, and the system parameters, including the LCL filter, are set to the same conditions. In the case of inverters running in parallel at the point of common coupling, the presence of grid impedance causes mutual interference between the LCL filters of each inverter, and the deviation of the filter resonance frequency is analyzed to understand the parallel inverter. The correlation between the number of devices and the size of grid impedance is simulated by PSIM and verified by MATLAB. By connecting the real-time digital simulator Typhoon HILS to the DSP 28377 control board, the mutual interference characteristics are tested under the condition of two inverters running in parallel. The experimental and analysis results are the same, indicating the validity of the analysis.

• Journal of Electrical Engineering and Technology
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• v.11 no.2
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• pp.320-328
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• 2016

The introduction of renewable energy sources into the AC grid can change and weaken the strength of the grid, which will in turn affect the stability and robustness of the doubly-fed induction generator (DFIG) wind farm. When integrated with weak grids, the DFIG wind turbine with vector power control often suffers from poor performance and robustness, while the DFIG wind turbine with synchronized control provides better stability. This paper investigates the critical short circuit ratios of DFIG wind turbine with vector power control and synchronized control, to analyze the stability boundary of the DFIG wind turbine. Frequency domain methods based on sensitivity and complementary sensitivity of transfer matrix are used to investigate the stability boundary conditions. The critical capacity of DFIG wind farm with conventional vector power control at a certain point of common coupling (PCC) is obtained and is further increased by employing synchronized control properly. The stability boundary is validated by electromagnetic transient simulation of an offshore wind farm connected to a real regional grid.

• Journal of Electrical Engineering and Technology
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• v.3 no.2
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• pp.152-161
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• 2008

This paper deals with a new wave shaping technique for cost effective harmonic mitigation in ac-dc converter feeding Vector Controlled Induction Motor Drives(VCIMD's) for improving power quality at the point of common coupling(PCC). The proposed harmonic mitigator consists of a polygon connected autotransformer based twelve-pulse ac-dc converter and a small rating passive shunt filter tuned for $11^{th}$ harmonic frequency. This ac-dc converter eliminates the most dominant $5^{th},\;7^{th},\;and\;11^{th}$ harmonics and imposes the reduction of other higher order harmonics from the ac main current, thereby improving the power quality at ac mains. The design of autotransformer is carried out for the proposed ac-dc converter to make it suitable for retrofit applications, where presently a 6-pulse ac-dc converter is used. The effect of load variation on VCIMD is also studied to demonstrate the effectiveness of the proposed ac-dc converter in a wide operating range of the drive. Experimental results obtained on the developed laboratory prototype of the proposed harmonic mitigator are used to validate the model and design of the ac-dc converter.

• The Transactions of the Korean Institute of Power Electronics
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• v.27 no.5
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• pp.410-416
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• 2022

A grid-connected inverter with critical loads uses mode transfer control to supply stable voltage to the load. An islanding detection method should also be used to quickly detect the grid fault and disconnect the inverter from the grid. However using the existing islanding detection method to detect islanding is difficult due to the small fluctuation of the voltage and frequency of the point of common coupling. This study proposes a reactive power P&O islanding detection method by using the positive feedback technique. The proposed method always injects a small variation of reactive power. When a grid fault occurs, the injected reactive power accelerates the reactive power injection reference. As a result, the reactive power reference value and the sensed reactive power become mismatched, and islanding is detected. Reducing the amount of real-time injected reactive power results in high efficiency and power factor. The simulation and experimental results of a 3 kW single-phase inverter are provided to verify the proposed islanding detection method.

• Journal of Power Electronics
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• v.16 no.4
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• pp.1551-1564
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• 2016

The high penetration level of inverter-based distributed generation (DG) power plants is challenging the low-voltage ride-through requirements, especially under unbalanced voltage sags. Recently, a flexible injection of both positive- (PS) and negative-sequence (NS) reactive currents has been suggested for the next generation of grid codes. This can enhance the ancillary services for voltage support at the point of common coupling (PCC). In light of this, considering distant grid faults that occur in a mainly inductive grid, this paper proposes a complete voltage support control scheme for the interface inverters of medium or high-rated DG power plants. The first contribution is the development of a reactive current reference generator combining PS and NS, with a feature to increase the PS voltage and simultaneously decrease the NS voltage, to mitigate voltage imbalance. The second contribution is the design of a voltage support control loop with two flexible PCC voltage set points, which can ensure continuous operation within the limits required in grid codes. In addition, a current saturation strategy is also considered for deep voltage sags to avoid overcurrent protection. Finally, simulation and experimental results are presented to validate the effectiveness of the proposed control scheme.

• Journal of Power Electronics
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• v.18 no.4
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• pp.1245-1254
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• 2018

The stability of a grid-connected system (GCS) has become a critical issue with the increasing utilization of renewable energy sources. Under grid faults, however, a grid-connected inverter cannot work efficiently by using only the traditional droop control. In addition, the unbalance factor of voltage/current at the common coupling point (PCC) may increase significantly. To ensure the stable operation of a GCS under grid faults, the capability to compensate for grid imbalance should be integrated. To solve the aforementioned problem, an improved voltage-type grid-connected control strategy is proposed in this study. A negative sequence conductance compensation loop based on a positive sequence power droop control is added to maintain PCC voltage balance and reduce grid current imbalance, thereby meeting PCC power quality requirements. Moreover, a stable analysis is presented based on the small signal model. Simulation and experimental results verify the aforementioned expectations, and consequently, the effectiveness of the proposed control scheme.

• Journal of Electrical Engineering and Technology
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• v.13 no.2
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• pp.711-721
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• 2018

This paper presents a single-phase asymmetric half-bridge cascaded multilevel inverter based series active power filter (SAPF) for harmonic voltage compensation. The effect of level number on performance of the proposed SAPF is examined in terms of total harmonic distortion (THD) and system efficiency. Besides, the relationship between the level number and the number of switching device are compared with the other multilevel inverter topologies used in APF applications. The paper is also aimed to demonstrate the capability of the SAPF for compensating harmonic voltages alone, without using a passive power filter (PPF). To obtain the required output voltage, a new switching algorithm is developed. The proposed SAPF with levels of 7, 15 and 31 is used in both simulation and experimental studies and the harmonic voltages of the load connected to the point of common coupling (PCC) is compensated under two different loading conditions. Furthermore, very high system efficiency values such as 98.74% and 96.84% are measured in the experimental studies and all THD values are brought into compliance with the IEEE-519 Standard. As a result, by increasing the level number of the inverter, lower THD values can be obtained even under high harmonic distortion levels while system efficiency almost remains the same.

• Proceedings of the Korean Institute of IIIuminating and Electrical Installation Engineers Conference
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• 2003.11a
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• pp.283-288
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• 2003

This paper presents the result of survey of case study for Harmonics in electrical Installations of buildings. Recently, many power electronic equipments(power converter, computers, air conditioners electronic ballasts for fluorescent lamps and so on) are used in office buildings, and harmonic current from them influence the other equipments in a distribution line. Notably, voltage distortion or voltage harmonics may approach or exceed is allowable level in power distribution system. Individual electric power consumers and end-users and responsible for reducing current harmonics while companies or utilities are responsible for reducing voltage harmonics at the point of common coupling in distribution system. As for harmonics, which one of the electric power qualities, it becomes important to obtain harmonic voltage/current distribution of the power system precisely because the use of power electronic apparatus in increasing. However, there are some difficulties on evaluating the measured data in comparison with the simulated result. The primary cause was indentitied with the resonance of harmonics form many sources. To suppress harmonics in electrical installations of buildings, one of many methods suggest that resonance frequencies are controlled by modulating the capacities of high-voltage customer's capacitors.

• Journal of Power Electronics
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• v.18 no.6
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• pp.1855-1865
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• 2018

This paper presents a control strategy for voltage-controlled multi-function distributed generation (DG) combined with an active power filter (APF) and a reactive power compensator. The control strategy is based on droop control. As a result of local nonlinear loads, the voltages of the point of common coupling (PCC) and the currents injecting into the grid by the DG are distorted. The power quality of the PCC voltage can be enhanced by using PCC harmonic compensation. In addition, with the PCC harmonic compensation, the DG offers a low-impedance path for harmonic currents. Therefore, the DG absorbs most of the harmonic currents generated by local loads, and the total harmonic distortion (THD) of the grid connected current is dramatically reduced. Furthermore, by regulating the reactive power of the DG, the magnitude of the PCC voltage can be maintained at its nominal value. The performance of the DG with the proposed control strategy is analyzed by bode diagrams. Finally, simulation and experimental results verify the proposed control strategy.