Time-domain Simulations Research Articles

A proposed methodology for mollification of subsynchronous oscillation in DFIG-based wind farm

High levels of series compensation in transmission lines, particularly those integrated with wind farms, exacerbate the risk of sub-synchronous oscillation (SSO), a phenomenon detrimental to system stability. Gate-controlled series capacitor (GCSC) is one of the used means to mitigate the SSO and enhance power transfer capability. In the previous studies, the combined proportional-integral controller with supplementary damping controller was an effective approach for controlling the GCSC and mitigating the SSO. In this paper, the ability to cascade a proportional-resonant controller with a PI controller for damping the SSO is investigated. The proposed method’s performance is evaluated through extensive time-domain simulations using the modified IEEE First benchmark model. The simulations incorporate a diverse range of wind speeds, compensation levels, as well as sub-synchronous control interactions. Furthermore, a comparative analysis with recent SSO damping techniques is conducted to assess the strategy’s performance under varying operating conditions. The comparative results overwhelmingly demonstrate the superiority and effectiveness of the proposed strategy in alleviating SSO, especially at low wind speeds and high compensation levels.

Engineering an Ultrafast Ambient NO2 Gas Sensor Using Cotton-Modified LaFeO3/MXene Composites.

This work presents a room-temperature (RT) NO2 gas sensor based on cotton-modified LaFeO3 (CLFO) combined with MXene. LaFeO3 (LFO), CLFO, and CLFO/MXene composites were synthesized via a hydrothermal method. The fabricated sensor, utilizing MXene/CLFO, exhibits a p-type behavior and fully recoverable sensing capabilities for low concentrations of NO2, achieving a higher response of 14.2 times at 5 ppm. The sensor demonstrates excellent performance with a response time of 2.7 s and a recovery time of 6.2 s, along with notable stability. The sensor's sensitivity is attributed to gas interactions on the material's surface, adsorption energy, and charge-transfer mechanisms. Techniques such as in situ FTIR (Fourier transform infrared) spectroscopy, GC-MS (gas chromatography-mass spectroscopy), and near-ambient pressure X-ray photoelectron spectroscopy were employed to verify gas interactions and their byproducts. Additionally, finite-difference time-domain simulations were used to model the electromagnetic field distribution and provide insight into the interaction between NO2 molecules and the sensor surface at the nanoscale. A prototype wireless IoT (Internet of Things)-based NO2 gas leakage detection system was also developed, showcasing the sensor's practical application. This study offers valuable insight into the development of room-temperature NO2 sensors with a low detection limit.

Distributed secondary frequency control and state of charge (SoC) balance for droop-controlled battery energy storage systems (BESSs) with a unified SoC relative variation rate

The state of charge (SoC) balance, power sharing, and frequency restoration are common control objectives of battery energy storage systems. However, the SoC balance scheme induced by the power allocation through existing droop controllers can cause the capacity parameters of battery cells to be unequal to the droop coefficient, which is the result of battery capacity degradation. Under this limitation, previous capacity-based droop controllers and the secondary controllers are no longer suitable to address the imprecise power sharing and frequency restoration caused by this problem. Therefore, a power allocation scheme based on the current SoC level ratio is designed to induce a new droop controller and ensure that the SoC simultaneously drops to 0. In order to restore frequency in a distributed manner and obtain the SoC level ratio, a distributed nominal frequency controller, SoC average estimator, and power-sharing controller are designed based on multi-agent systems in both asymptotic and finite-time manners. In the asymptotic scheme, the steady-state performance of the SoC estimator is adjustable, and power sharing and frequency restoration are zero errors. In the finite-time scheme, the influence of parameters on convergence time is well analyzed, and some conservative calculation methods are provided. Several time-domain simulation examples are designed on an improved IEEE-57 bus system to verify the distribution of asymptotic and finite-time schemes.

A novel flexible infinite element for transient acoustic simulations

This article addresses the efficient solution of exterior acoustic transient problems using the Finite Element Method (FEM) in combination with infinite elements. Infinite elements are a popular technique to enforce non-reflecting boundary conditions. The Astley–Leis formulation presents several advantages in terms of ease of implementation, and results in frequency-independent system matrices, that can be used for transient simulations of wave propagation phenomena. However, for time-domain simulations, the geometrical flexibility of Astley–Leis infinite elements is limited by time-stability requirements. In this article, we present a novel infinite element formulation, called flexible infinite element, for which the accuracy does not depend on the positioning of the virtual sources. From a software implementation perspective, the element proposed can be seen as a specialized FEM element and can be easily integrated into a high-order FEM code. The effectiveness of the flexible formulation is demonstrated with frequency and time-domain examples; for both cases, we show how the flexible infinite elements can be attached to arbitrarily-shaped convex FE boundaries. In particular, we show how the proposed technique can be used in combination with existing model order reduction strategies to run fast and accurate transient simulations.

Al@Al2O3 core-shell plasmonic design for the dilemma between high responsivity and low dark current of MoS2 photodetector

The localized surface plasmon resonance (LSPR) effect induced by metal nanoparticles (NPs) can solve the problem of low light absorption in two-dimensional (2D) materials limited by atomic scale. However, the accompanying problem is the rise in dark current due to plenty of electrons from metal NPs injecting into the 2D materials, which decreases the performance of plasmonic photodetectors. Here, we designed the structure of Al NPs coated with Al2O3 by low temperature oxidation treatment method to balance the dilemma between high photoresponse and low dark current. Raman spectrum and finite-difference time-domain simulations were used to verify that Al2O3 does not affect the LSPR effect of Al NPs. Compared to that of the pristine MoS2/Al photodetector, the MoS2/Al@Al2O3 plasmonic photodetector achieved a fourfold decrease in dark current, threefold increase in detectivity, and 1.5-fold increase in responsivity. As a result, the optimized plasmonic device achieves a high responsivity of ∼1719 A/W, an excellent detectivity of ∼6.0 × 1011 Jones, and an ultra-fast response speed of ∼15 ns. Our work reveals that constructing metal NPs covered by ultra-thin oxide layer is a feasible strategy for plasmonic photodetectors to decrease dark current and achieve high performance index.

Design of a novel state-feedback robust \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_2/H_{\\infty }$$\\end{document} sliding-mode controller for a hydraulic turbine governing system

This paper presents a novel state-feedback robust sliding-mode controller (SFRSMC) based on a mixed H2/H∞\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_2/H_{\\infty }$$\\end{document} approach to enhance the control performance of hydraulic turbine governing systems (HTGS) with complex conduit systems under external load disturbances and control signal uncertainties. A state-space model incorporating the dynamic responses of the HTGS is developed. The SFRSMC is designed using the sliding-mode equivalent control principle, with a disturbance observer to estimate and mitigate unknown disturbances. To balance robustness and optimality, mixed H2/H∞\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_2/H_{\\infty }$$\\end{document} linear matrix inequalities (LMIs) are utilized to determine the critical performance sliding matrix via an auxiliary feedback control method. The effectiveness of the proposed control scheme is validated through time-domain simulations under various operating scenarios, including different parameter variations of the HTGS.

Optimization of hierarchical textured PDMS film with wide-angle broadband anti-reflection for light trapping in solar cells

We developed a hierarchical random pyramid (HRP)-polydimethylsiloxane (PDMS) film as an antireflective layer for metal halide perovskite solar cells (PSCs). To create the HRP structure on the PDMS surface, we first patterned the Si surface using alkaline etching and Ag-assisted chemical etching (Ag-ACE) and then transferred the pattern onto the PDMS surface. The resulting PDMS exhibited hierarchical structures of inverted micro-pyramids with nano-scale surface features. The optical performance of the textured PDMS film was observed across the entire wavelength range of visible light (λ = 300–800 nm), attributed to the enhanced light-trapping effect resulting from the increased fractal dimensions (Df) of the hierarchical nanostructures. Under optimal conditions, an average reflectance of 3.13 % was observed, along with a 14 % improvement in transmittance compared to that of the reference. Finite difference time-domain simulations were used to investigate the origins of these optical enhancements, conclusively linking them to the HRP structures. The optimized structures also showed superhydrophobicity with a contact angle (θCA) of approximately 155.2°. When applied to PSCs (MAPbI3), the optimized textured PDMS film led to a 12 % increase in short-circuit current density (Jsc) from approximately 23.1 to 25.98 mA/cm2. Furthermore, the power conversion efficiency of the MAPbI3 p-i-n structured devices was enhanced by 18.21 %, representing a 13 % improvement. Our results confirm that the HRP-PDMS film enhances superhydrophobicity and light trapping, while mitigating the transmittance loss due to changes in the incident angle. This study suggests potential strategies for overcoming the limitations of light absorption in solar cell applications.

Optimizing the chiral optical response in nanostructures using plasmonic Fano resonance

This study investigates the chiral enhancement effects of plasmonic Fano resonance modes in planar metallic nanostructures. The nanostructure consists of a central Z-shaped or 卍-shaped element surrounded by six clustered gold nanorods, focusing on the coupling between these doubly rotationally symmetric structures. This coupling induces plasmonic Fano resonance, which significantly enhances the chiral response. Under normal incidence of circularly polarized light, the maximum chiral response can reach up to 41%. Finite-difference time-domain simulation and multipole expansion analysis reveal the fundamental origin of this enhanced chiral response: the selective excitation of electric dipoles and toroidal dipoles in polarization. The study demonstrates that rotationally symmetric structures and coupling effects play a crucial role in modulating the chiral response of nanostructures.

Femtosecond laser surface modification induced the tunable of nonlinear optical property of single-crystal silicon

Tunable nonlinear optical properties were realized by femtosecond laser surface modification of single-crystal silicon with periodic grating-like structures. The as-modified samples exhibit a nonlinear behavior with a transition from reverse saturation absorption (RSA) to saturation absorption (SA) with the increase of laser power due to the introduction of oxide and defect in Si. The maximum third-order nonlinear absorption coefficients can reach −1.16 × 10−4 cm/W. And the epsilon-near-zero wavelength of as-modified samples can be tuned from 191 nm to 508 nm. The finite-difference time-domain simulation results are in good agreement with the test results of the samples.

Stability Improvement of Sulbagsel Electricity System Integrated Wind Power Plant Using SVC-PSS3C Based on Improved Mayfly Algorithm

This study thoroughly investigates the enhancement of power system stability through the coordinated design of Static VAR Compensator (SVC) and Multi-Band Power System Stabilizer type 3C (MB-PSS3C). An Improved Mayfly Algorithm (IMA) is employed to optimize the placement and tuning of SVC and MB-PSS3C within the Southern Sulawesi (Sulbagsel) integrated Wind Power Plant (WPP) electricity system. Mayfly intelligence is inspired by the mating and flight behaviors of adult mayflies. In its standard form, this algorithm cannot be applied to high-dimensional, nonlinear, and complex space problems. This study proposes an Exponent Decreasing Inertia Weight (EDIW) strategy to enhance the performance of the standard Mayfly Algorithm (MA). The IMA demonstrates superior performance compared to other methods, achieving a minimum fitness function value of 80.0582 and converging rapidly by 8th iteration. For SVC, the optimization analysis involved reviewing the voltage profile at each bus and analyzing Optimal Power Flow (OPF). For MB-PSS3C, the optimization focused on evaluating system stability through damping analysis, time-domain simulations, and eigenvalue visualization. The application of IMA-based SVC improved reactive power management in transmission systems, resulting in enhanced voltage profiles, OPF, and reduced transmission losses. Specifically, transmission losses decreased by 3.12% in the normal system case study and by 2.88% in the N-1 contingency case study. Meanwhile, the IMA-based MB-PSS3C enhanced system stability by increasing system damping, reducing generator oscillation overshoot, and improving system eigenvalues. The implementation of MB-PSS3C using IMA across 14 generators achieved the highest damping ratio of 0.7375, compared to 0.7274 for MB-PSS3C with MA.

Time-domain simulation of nonlinear motions of moored floating body in irregular waves using rankine source method

This study introduces a three-dimensional time-domain Indirect Boundary Integral Equation Method (IBIEM) for simulating the motion of the floating body in both regular and irregular waves. The method systematically addresses second-order wave loads, focusing on low-frequency motions induced by low-frequency wave loads, crucial for the stability of offshore structures. Unlike traditional methods, the time-domain IBIEM simplifies the analysis by simultaneously considering all wave components of irregular waves, making the calculation of nonlinear wave loads more straightforward and intuitive. An approximate second-order equation of motion is proposed, simplifying the computational process by eliminating the need for solving boundary value problems for second-order radiation forces. The IBIEM is coupled with a quasi-static mooring model based on the elastic catenary equation, accurately capturing the influence of mooring systems on the response of the floating body. Validation against model tests and established numerical methods demonstrates the accuracy of the developed code in predicting the dynamic behavior of offshore structures. Applied to various scenarios, including a truncated cylinder, the barge, H2FPSO, and K-Semi, the results show strong agreement with experimental data, confirming the reliability and effectiveness of the method in simulating complex wave-structure interactions, making it a valuable tool for offshore structure design and analysis.

A general optimization framework for nanofabrication using shadow sphere Lithography: A case study on chiral nanohole arrays

Shadow sphere lithography (SSL) offers unparalleled advantages in fabricating complex nanostructures, yet optimizing these structures remains challenging due to vast parameter spaces. This study presents a general optimization framework for SSL-fabricated nanostructures, demonstrated through chiral metamaterials. The approach combines a custom SSL program, a novel mathematical model for eliminating redundant structures, and machine learning (ML) analysis of finite-difference time-domain (FDTD) simulations. Applied to rotated nanohole arrays (RHAs), this framework efficiently navigates a 7200-structure parameter space, identifying optimal configurations with circular dichroism (CD) and g-factor up to 3.23˚ and 0.28, respectively. Experimental validation of optimized RHAs shows good agreement with predictions, exhibiting twice the chiral response of random configurations. Notably, the framework reduces the dataset by 86%, significantly decreasing computational costs. This optimization framework enables faster, more systematic, and more efficient optimization of structures manufactured using SSL, potentially accelerating discoveries in nanophotonics, plasmonics, and chiral sensing applications.

Theoretical and experimental study on the polarization-independent flanged nanowire array infrared absorber

An infrared (IR) absorber is a crucial component for thermal detectors, requiring high absorptance over a broad wavelength range while maintaining low heat capacity for optimal performance. Most thermal detectors use a thin film IR absorber that is suspended in air, supported by a layer beneath it for mechanical stability. However, this support layer increases heat capacity without contributing to IR absorptance, thereby reducing the performance of thermal detectors. In this paper, we introduce a polarization-independent nanowire array absorber using flanged nanowires with a C-shaped cross-section. This C-shaped design provides mechanical stability, eliminating the need for a support layer. Although nanowire array is generally known to exhibit polarization characteristics, the unique structure of the proposed flanged nanowires enables them to achieve polarization-independent properties, resulting in high absorptance similar to that of film absorbers. We theoretically analyzed the polarization-independent characteristics of the flanged nanowires using an optical circuit model and optimized the flanged nanowire structure using finite-difference time-domain (FDTD) simulations. Finally, we experimentally demonstrated the polarization-independent characteristics of the flanged nanowires and confirmed their high absorptance comparable to that of film absorbers.

An Open-Source Julia Package for RMS Time-Domain Simulations of Power Systems

This paper presents RMSPowerSims.jl, an open-source Julia package for the time-domain simulation of power systems. The package is designed to be used in conjunction with PowerModels.jl, a widely used Julia package for power system optimization. RMSPowerSims.jl provides a framework for the simulation of power systems in the time domain, allowing for the study of transient stability, frequency stability, and other dynamic phenomena. The package is designed to be intuitive and flexible, allowing users to easily define custom models for network components and disturbances, while also providing a range of pre-constructed models for common power system components. RMSPowerSims.jl simplifies the process of performing RMS simulations on power system models developed using the PowerModels.jl ecosystem, and provides an easy-to-use modeling that reduces the barrier to entry for new users wishing to perform RMS simulations. The accuracy of the package is verified against DIgSILENT PowerFactory for short-circuit and load-increase disturbances, using the New England 39-bus system. The active power generation delivered by several generators in the network, and the voltage magnitudes of selected busbars are analyzed and noted to be in close agreement with those obtained using PowerFactory. The computational performance of the package is compared to that of PowerFactory and is found to be comparable for load-step simulations; however, PowerFactory is found to be considerably faster for short-circuit simulations. As computational performance is not a priority at this stage of development, this is expected, and speed optimization is planned for future work. RMSPowerSims.jl is available under an open-source license and can be downloaded from GitHub.

Time Domain Design of a Marine Target Tracking System Accounting for Environmental Disturbances

Environmental disturbances represent significant challenges to the performance and accuracy of autonomous systems, especially in marine environments, where their impact varies based on disturbance severity and the employed guidance law. This paper comprehensively investigates a marine target tracking system using time-domain simulations incorporating realistic environmental disturbances. Three guidance laws and four key performance indicators are analysed to evaluate system performance under disturbed and ideal conditions. A robust and systematic evaluation pipeline is developed and applied to a case study featuring a scaled tugboat model. This approach provides a reliable method to assess tracking accuracy and robustness in adverse conditions. The results are selected from a wide range of possibilities to show the effect of the disturbances on the selected target tracking motion control scenario with two manoeuvres and two environmental conditions. The results are measured through the selected key performance indicators, and several phases are identified for each manoeuvre to extend the analysis not only to the global KPI values but also to the partial values of defined phases. They reveal the quantitative effects of environmental disturbances, exposing different system behaviours and trends. These findings demonstrate the effectiveness of the proposed pipeline in quantifying tracking system performance, delivering useful understandings of the system under environmental disturbances. The broader implications of this study are substantial, offering enhanced predictive accuracy for the performance of the analysed systems, particularly in the context of target tracking. Furthermore, introducing numerical key performance indicators facilitates a more rigorous comparison of different system characteristics, enabling informed decisions in designing and optimising autonomous operations in challenging environments.

Numerical Estimation of Bending in Holographic Volume Gratings by Means of RCWA and Deep Learning

In this paper, we introduce a novel approach to model bending phenomena on holographic volume gratings based on Rigorous Coupled Wave Analysis (RCWA), in which the bending as a phase in the dielectric permittivity expansion is introduced, and the Shooting Method (SM) is employed to solve the resulting system of equations. Further validation of our model is conducted by comparing its predictions to those obtained from reference Finite-Difference Time-Domain (FDTD) simulations and Coupled Wave Theory (CWT, referring to Kubota’s model that includes the bending phenomenon). Furthermore, we propose a methodology for estimating the bending from the diffraction efficiency curves in transmission volume gratings based on deep learning models, with a subsequent study of their accuracy and applicability.

Analysis of SiNx Waveguide-Integrated Liquid Crystal Platform for Wideband Optical Phase Shifters and Modulators

In this study, the potential of employing SiNx (silicon nitride) waveguide platforms to enable the use of liquid-crystal-based phase shifters for on-chip optical modulators was thoroughly investigated using 3D-FDTD (3D finite-difference time-domain) simulations. The entire structure of liquid-crystal-based Mach–Zehnder interferometer (MZI) optical modulators, consisting of multi-mode interferometer splitters, different tapering sections, and liquid-crystal-based phase shifters, was systematically and holistically investigated with a view to developing a compact, wideband, and CMOS-compatible (complementary metal-oxide semiconductor) bias voltage optical modulator with competitive modulation efficiency, good fabrication tolerance, and single-mode operation using the same SiNx waveguide layer for the entire device. The trade-off between several important parameters is critically discussed in order to reach a conclusion on the possible optimized parameter sets. Contrary to previous demonstrations, this investigation focused on the potential of achieving such an optical device using the same SiNx waveguide layer for the entire device, including both the passive and active regions. Significantly, we show that it is necessary to carefully select the phase shifter length of the LC-based (liquid crystal) MZI optical modulator, as the phase shifter length required to obtain a π phase shift could be as low as a few tens of microns; therefore, a phase shifter length that is too long can contradictorily worsen the optical modulation.

Accelerated strategies for impact identification of composite laminate using sparse sensor networks and model-based virtual time reversal

Barely visible impact damages are notorious threats to the health of composite structures. Detection of the impact is critical to promise the performance of composite structures. Recently, the authors proposed an impact identification method based on sparse sensor networks and virtual time reversal, which is verified on composite laminates. However, the computational demands of virtual time reversal simulations pose a notable limitation to this approach. To improve the efficiency of the method, several acceleration measures are developed in this article. First, the re-emitted wavefield analysis is accelerated using a frequency-domain method rather than the time-domain simulation, which largely reduces the computational costs. Subsequently, the GPU parallel acceleration algorithm is configured to further improve the efficiency. The proposed acceleration strategy is experimentally validated by various impact events on a composite laminate. The results render that the proposed acceleration strategy can promise the accuracy of the results of impact identification while significantly promoting computational efficiency, in which the time consumption plummets to only 1.94 s. Moreover, factors affecting the efficiency of the proposed method are discussed. The time consumption is positively correlated with the number of sensors involved in the identification. In addition, as the number of candidate points increases, the GPU encounters difficulty in handling all complex matrix operations simultaneously, leading to a substantial rise in computation time. Furthermore, the effect of signal lengths on the efficiency is minimal and largely negligible.

In-situ fabricated hexagonal PDMS microsphere arrays for substrate-mode light extraction in blue fluorescent organic light emitting diodes

To inhibit the total reflection in the substrate-mode light loss of organic light-emitting diodes (OLEDs) and fully take advantage of the excellent light converging and out-coupling effect of the hexagonal array of microsphere, we in-situ fabricated the hexagonal array of polydimethylsiloxane (PDMS) microsphere onto the substrate of OLEDs via the porous template for substrate-mode light extraction. Firstly, regular honeycomb porous polystyrene (PS) template was prepared via a self-assembly “breath figure” process. The PS molecular weight, solvent, and humidity play an important role on the uniformity of pore distribution and pore diameter according to Young-Laplace equation. Based on the optimized porous PS template, the PDMS microsphere array was fabricated by pattern transferring, which indicates more prominent light diffraction than the flat PDMS film. Finite-difference time-domain (FDTD) simulation denotes that higher duty ratio of the PDMS microsphere array contributes to higher light out-coupling efficiency. Therefore, a PDMS microsphere array with larger duty ratio of 84.0 % was applied in the extraction of external light for the blue fluorescent OLED device. The maximum luminance, current efficiency, and power efficiency are all improved, and the external quantum efficiency (EQE) is increased by 18.81 % with spectral stability.

Multi-timescale modeling and order reduction towards stability analysis of isolated microgrids

Microgrids incorporate a significant proportion of renewable energy sources and power electronic converters in the energy conversion process, creating a sustainable and clean energy infrastructure. However, the multi-timescale dynamics of microgrids are interactively coupled under a nonlinear structure, which makes it difficult to gain insight into the instability mechanisms without a high-fidelity reduced-order model that preserves the main dynamic behaviors of the system. For the isolated AC microgrid dominated by voltage source inverters (VSI), a detailed state-space model of the system, including the inverter, network, and load, is first developed. Based on this model, the eigenvalue analysis is carried out, and a participation factor analysis tool is also utilized to identify the relevant dynamics that have a strong impact on the system's dominant mode. Furthermore, to simplify the system modeling process without losing essential dynamic interactions, a novel multi-timescale coupled reduced-order model is proposed using a transfer function-based order reduction method, which retains the open-loop gain characteristics to preserve the critical couplings between fast inner loop dynamics and slow droop control dynamics. Finally, the accuracy of the reduced-order model is verified by comparing it with the detailed model and the conventional singular perturbation reduced-order model through eigenvalue distribution and time-domain simulation analysis.