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Virtual inertia emulation could be regarded as an inevitable component of microgrids with renewable energy, enhancing microgrid inertia and damping properties. In applying this control technique, a phase-locked loop (PLL) is necessary to obtain the estimation of the system frequency data. However, the employment of PLL could cause larger frequency oscillation to the microgrid due to its dynamics. This issue would be exacerbated in a low-inertia microgrid driven by high renewable penetration, severely deteriorating the frequency stability. Thus, the effect of PLL with measurement delay is a critical issue in utilizing the virtual inertia control. To overcome such problem, this paper proposes a robust virtual inertia control for a low-inertia microgrid to minimize the undesirable frequency measurement effects, improving the microgrid frequency stability. The robust H∞ control design using a linear fractional transformation (LFT) technique is used to develop the virtual inertia control loop, considering the dynamics of PLL with measurement delay and the uncertainties of system inertia and damping. The efficacy of the proposed H∞ control method is compared to the conventional and optimum proportional-integral (PI)based inertia control. The results show that the H∞-based robust virtual inertia control is superior to both conventional virtual inertia control and optimum PI-based virtual inertia control against a wide range of microgrid operating conditions, disturbances, and parametric uncertainties.

Maintaining frequency stability of low inertia microgrids with high penetration of renewable energy sources (RESs) is a critical challenge. Solving this challenge, the inertia of microgrids would be enhanced by virtual inertia control-based energy storage systems. However, in such systems, the virtual inertia constant is fixed and selection of its value will significantly affect frequency stability of microgrids under different penetration levels of RESs. Higher frequency oscillations may occur due to the fixed virtual inertia constant or unsuitable selection of its value. To overcome such a problem and provide adaptive inertia control, this paper proposes a self-adaptive virtual inertia control system using fuzzy logic for ensuring stable frequency stabilization, which is required for successful microgrid operation in the presence of high RESs penetration. In this concept, the virtual inertia constant is automatically adjusted based on input signals of real power injection of RESs and system frequency deviations, avoiding unsuitable selection and delivering rapid inertia response. To verify the efficiency of the proposed control method, the contrastive simulation results are compared with the conventional method for serious load disturbances and various rates of RESs penetration. The proposed control method shows remarkable performance in transient response improvement and fast damping of oscillations, preserving robustness of operation.

This study proposes a dual-circularly polarized Reduced-Surface-Wave (RSW) antenna which has a compact and simple structure by the integration of a via-loaded annular-ring (VLAR)-RSW antenna with a branch-line coupler. The fundamental characteristics of a VLAR-RSW antenna are analyzed through simulations in detail. In general, stronger mutual coupling between RSW antennas occurs in comparison with conventional patch antennas when the element spacing is small, since the effect of the close physical distance becomes larger than the effect of surface wave suppression. It is shown that the VLAR antenna can reduce the mutual coupling than the conventional antenna by selecting the appropriate radius at each element spacing, even when the element spacing is small. A matching circuit is generally required for an annular-ring antenna because of the high input impedance when exciting the TM11 mode. In contrast, for the VLAR-RSW antenna, impedance matching is feasible without a matching circuit by adjusting the distance between the feed position and the shorting vias. Measured results show the performance of the developed dual-circularly polarized antenna is in agreement with the simulation. At 5.8GHz, the measured axial ratios of both polarizations are smaller than 0.6dB and the measured isolation between ports is about 29 dB. Moreover, it is shown that the developed antenna reduces mutual coupling than the conventional patch antenna because of the effect of suppressing surface waves.

After the demonstration of the giant tunnel magnetoresistance (TMR) ratios at room temperature (RT) using a crystalline MgO barrier based magnetic tunnel junctions (MTJs) in 2004 [1] , [2] , an application range of spintronic devices, including read-heads of hard disk drives (HDDs) and cells of magnetoresistive random access memories (MRAMs), has been significantly boosted. Use of MgO barriers enabled to obtain large electric output from MTJ structures since a large TMR ratio over 100% can be easily achieved due to the ∆ 1 band preferential coherent tunneling mechanism [3] . Fe/MgO/Fe(001) is known as one of the most basic structures exhibiting the giant TMR effect [2] . The structure is very simple, and thus many theoretical calculations have been demonstrated so far [3] , [4] . However, experimental TMR ratios (180~220% at RT and 290-370% at low temperature) are much smaller than those of the predictions like >1,000%. In order to create novel MTJ-based applications, such as high-capacity MRAMs, magnetic logics, and brain-morphic devices, further improvement in an experimental TMR ratio is needed. Especially, it should be essential to understand the origin of the TMR gap between experiments and theories. In this study, we revisited the simplest "Fe/MgO/Fe(001)" MTJ to obtain much larger TMR ratios by careful tuning of growth conditions for all the layers, combining sputtering and electron-beam evaporation, and MgO barrier interface modifications [5] .

In this study, we aim to realize SWIPT (Simultaneous Wireless Information and Power Transfer) at the same frequency using the OAM (Orbital Angular Momentum) mode. To investigate which OAM mode is desirable for WPT (Wireless Power Transmission) and how much isolation can be realized, we studied how the power transmission efficiency and the mode isolation was changed depending on the transmission distance in the simulation. We have revealed that the mode 0 is desirable for WPT and the high isolation can be realized using the OAM mode.

Renewable energy sources (RESs) are growing rapidly and highly penetrated in microgrids (MGs). As a result of the replacement of the synchronous generators with a large amount of RESs, the overall system inertia might be dramatically reduced which negatively affected the MG dynamics and performance in face of uncertainties, leading to weakening of the MG stability, which considers being a serious challenge in such grids. Therefore, in order to cope with this challenge and benefit from a maximum capacity of the RESs, robust control strategy must be applied. Hence, in this paper, a new application of robust virtual inertia control-based coefficient diagram method (CDM) controller is proposed in an islanded MG considering high-level RESs penetration for enhancement the system’s validity and robustness in face of disturbances and parametric uncertainties. The proposed controller’s proficiency has been checked and compared with H-infinite controller using MATLAB/Simulink which approved that the CDM controller achieved superior dynamic responses in terms of accurate reference frequency tracking and disturbance reduction over H-infinite in all test scenarios. Thus, the proposed controller alleviates the difficulties of H-infinite controller such as the experience and necessary abilities to design the form of the weighting functions for the system. Consequently, the frequency stability is improved and approved that the proposed CDM-based virtual inertia controller can significantly support a low-inertia islanded microgrid against RESs and load fluctuations.

Conventional coverage path planning algorithms are mainly based on the zigzag and spiral patterns or their combinations. The traversal order is limited by the linear or inside-outside manner. We propose a new set of coverage patterns induced from geometric folding operations, called the geometric folding pattern, to make coverage paths with more flexible traversal order. We study the modeling and parameterization of the geometric folding patterns. Then, a sampling operator is introduced. Based on the computational tools, we demonstrate the application of the proposed patterns in designing coverage paths. We show that the simple geometric folding patterns are flexible and controllable, which enables more choices for the coverage path planning problem.

In this paper, the influence of Fluorine incorporation into SiO 2 and Si nitride $(\text{Si}_{\mathrm{x}}\mathrm{N}_{\mathrm{y}})$ films which are widely used in the nano electronics has been investigated. In the $\text{Si}_{\mathrm{x}}\mathrm{N}_{\mathrm{y}}$ film, fluorine makes the electron trap level shallower with increasing F dosage. On the other hand, as previously reported in the literatures, an excess fluorine degrades the SiO 2 film markedly. Re-considering the control of Fluorine incorporation becomes important for highly reliable devices.