Time-domain Simulation Method Research Articles

Design of Novel Metal Nanostructures for Broadband Solar Energy Conversion

Solar power holds great potential as an alternative energy source, but current photovoltaic cells have much room for improvement in cost and efficiency. Our objective was to develop metal nanostructures whose surface plasmon resonance (SPR) spectra closely match the solar spectrum to enhance light absorption and scattering. We employed the finite-difference time-domain simulation method to evaluate the effect of varying key parameters. A novel nanostructure with SPR absorption matching a region of the solar spectrum (300 to 1500 nm) that contains 90% of solar energy was successfully designed. This structure consists of a large gold-silica core-shell structure with smaller gold nanoparticles and nanorods on its surface. Such complex nanostructures are promising for broad and tunable absorption spectra. In addition, we investigated the SPR of silver nanoparticle arrays, which can achieve scattering close to the solar spectrum. We demonstrated an improvement in efficiency of over 30% with optimal nanoparticle radius and periods of 75 nm and 325 nm, respectively. In combination, our studies enable high-efficiency, tunable, and cost-effective enhancement of both light absorption and scattering, which has potential applications in solar energy conversion as well as biomedical imaging.

CRONE Toolbox for system identification using fractional differentiation models

This paper presents the latest developments for the continuous-time system identification toolbox with fractional models (or fractional order systems): the CRONE toolbox. This toolbox is to be run with Matlab which includes time-domain identification algorithms for estimating continuous-time models directly from sampled data. The originality of the implemented algorithms is that they allow either fixing fractional differentiation orders or estimating them along with transfer function coefficients. One of the main issues when dealing with fractional models is their time-domain simulation. Three different time-domain simulation methods can be used independently from system identification methods. Output Error (OE), state variable filters (SVF) and (optimal) instrumental variables (IV) methods for ARX and OE models are provided to the end-user. The object oriented programming of the toolbox allows overloading standard script names. As a consequence, an end-user familiar with standardMatlab operators and scripts can use straightforwardly the CRONE toolbox.

Finite difference time domain simulation of optical properties of annular cavity arrays

Optical properties of two-dimensional periodic annular cavity arrays in hexagonal packing are investigated by finite difference time domain simulation method in this paper. According to simulated reflectance/transmission spectra, electric field distribution and charge distribution, we confirm that multiple cylindrical surface plasmon resonances, which result in reflectance dips, can be excited in annular cavities by linearly polarized light. Mechanism of the cylindrical surface plasmons is investigated. A coaxial waveguide mode TE11 is excited in the annular cavities and a Fabry-Perot resonance is fulfilled along the depth direction of the annular cavities at the resonance wavelengths. While the number of reflectance dips and wavelengths of these dips in reflectance spectra are dependent on the geometric sizes of the annular cavities, the periodicity and polarization of incident light do not affect their reflectance spectra dramatically. Incident light beams with resonant wavelengths are localized in annular cavities with large electric field increasing and dissipate gradually due to metal loss. Reflectance dips can be tuned from 350 to 2000 nm by adjusting geometric size parameters of the annular cavities, such as outer and inner radii of the annular gaps, gap sizes and metal film thickness values. Reflectance dips shift toward longer wavelength with increasing inner and outer radii of the annular gaps, metal film thickness and with reducing the gap distance. In addition, infiltrate liquids in the annular gaps will result in a shift of the resonance wavelengths, which makes the annular cavities good refractive index sensors. A refractive index sensitivity up to 1850 nm/RIU is demonstrated. The refractive index sensitivities of annular cavities can also be tuned by their geometric sizes. Annular cavities with large electric field enhancement and tunable cylindrical surface plasmons can be used as surface enhanced Raman spectra substrates, refractive index sensors, nano-lasers and optical trappers.

Focusing surface plasmon polaritons in archimedes' spiral nanostructure

Surface plasmon polaritons (SPPs) are a hybrid mode of a light field and metallic collective electrons oscillated resonantly and excited at the metal/dielectric interface. Recently extensive research has been carried out due to its technological potential in nano-optics. The SPPs coupling, focusing, waveguiding and resonance enhancement are hot spots in this field. In particular, to find a simple method that can focus SPPs into a highly confined spot with the size beyond the diffraction limit is still a big challenge. In this work, we have fabricated the Archimedes' spiral structures with different structural parameters on an Au film by using focused ion beam etching technique. Through changing the chiralities of the incident circularly polarized light and the spiral structure, we have studied theoretically and experimentally the focusing properties of the Archimedes spiral structures with different parameters. We find that besides the chiralities of the incident light and the spiral structure, the pitch of screw of the spiral structure and the wavelength of the excited light also affect the surface plasmon field. The resulting surface plasmon fields inside the structure are the zero-order, first-order, and high-order evanescent Bessel beams. By using a phase analysis and a finite-difference time-domain simulation method, we calculate the electric field and phase distribution in different spiral structures. A near-field vortex mode with different spin-dependent topological charges can be obtained in the structures. Furthermore, the results of the scanning near-field optical microscopy measurements verify the theory and simulation results. The method of using an Archimedes' spiral structure to focus SPPs provides a new route to manipulate the SPPs optical field in nanoscale. Based on theoretical calculation and FDTD simulation in this work, we have studied the physical process of the optical field manipulation in spiral structures. The significant and innovated points of this work are: a) We have developed the phase theory, and analyzed the field manipulation process of spiral structures with different parameters and chiralities at different circular polarization and wavelengths. b) A more effective and convenient way is used for SPPs focusing in linearly polarized light and circularly polarized light. c) A near-field vortex surface mode with different spin-dependent topological charges is obtained for the structure. This work can be considered to have applications in SPPs tweezers, highly integrated photonic devices.

A plasmonic metal grating wavelength splitter

A plasmonic metal grating wavelength splitter is theoretically investigated and experimentally demonstrated. Based on the periodical waveguide theories, the negative real part of the propagation constant of surface plasmon polaritons (SPPs) () in metal grating is derived in a wavelength range which is determined by the grating parameters. The transmission prohibition at the negative is utilized to realize the wavelength splitting by the metal grating with different grating periods and fill factors on the left and right half. The metal grating plasmonic splitter is simulated by the finite difference time domain simulation method, the characteristics of which are consistent well with theoretical predictions. The plasmonic wavelength splitter is fabricated by electron beam lithography and the ion beam etching process. The SPPs excited by an incident wavelength of 532 and 650 nm are experimentally split and observed under an optical microscope using a charge-coupled device camera.

Study of Sub-Synchronous Interaction in Doubly-Fed Wind Power Systems

When doubly fed wind power generation system transmits power to power grid through the series compensation, once the external disturbances is close to the natural sonant frequency, there is a trouble of synchronous oscillation between the rotor side and the series capacitor. The paper analyzes the condition of resonance and its mechanism. According the actual argument provide by north china wind power plant, we use PSCAD to build a 1.25MW doubly fed wind power generation model to analyze the synchronous oscillation. We adopt time-domain simulation method to analyze that as the change of series compensation degree the system subsynchronous resonance frequency changes as well as the influence to doubly fed fan current and power. The simulation result is basically the same as the fault wave record monitoring data result of actual site when wind power plant take off the network .Consequently to verify the validity of the simulation platform. To verify the improvement of the converter control strategy to build simulation platform.

Power System Transient Stability Margin Estimation using Artificial Neural Networks

This paper presents a methodology for estimating the normalized transient stability margin by using the multilayered perceptron (MLP) neural network. The complex relationship between the input variables and output variables is established by using the neural networks. The nonlinear mapping relation between the normalized transient stability margin and the operating conditions of the power system is established by using the MLP neural network. To obtain the training set of the neural network the potential energy boundary surface (PEBS) method along with time domain simulation method is used. The proposed method is applied on IEEE 9 bus system and the results shows that the proposed method provides fast and accurate tool to assess online transient stability.

Study of Sub-Synchronous Control Interaction (SSCI) Based on the Test Signal Method

Large-scale wind turbine generators with power electronic converters that operate near series compensated transmission lines are susceptible to un-damped sub-synchronous oscillations. This sub-synchronous oscillation is called Sub-synchronous Control Interaction (SSCI). Based on PSCAD / EMTDC simulation platform the establishment of Double Fed Induction Generator (DFIG) equivalent inverter controlled sources and network model, and verify the correctness of the equivalent model. Analysis the impact of series compensation and rotor-side converter PI parameters characteristic of the SSCI based on test signal method and time-domain simulation method. The results show that increasing the degree of series compensation , inner gain constant increases and inner integral time decreases will help increase SSCI, outer ring of PI parameters has little effect on the SSCI .

On the symmetry and topology of plasmonic eigenmodes in heptamer and hexamer nanocavities

Plasmonics is expected to play a key role in nanotechnology, leading to intriguing routes in many engineering and biological applications. Recently, it has been realized that toroidal resonances could be an alternative to electric and magnetic resonances, which have governed the innovation of plasmonic applications so far. In a previous contribution, we proved the existence of toroidal moments in an oligomeric void-plasmonic structure [1]. In this article, we investigate the role of topology and symmetry in decomposing the various dipolar, quadrupolar, and toroidal moments, using energy-filtering transmission electron microscopy supported by three-dimensional finite-difference time-domain method simulations. The consequences of changing the topology on the toroidal character are discussed by comparing results obtained from nanoholes forming heptamer and hexamer nanocavity systems that were drilled into a thin silver film.

Near-Field Broadband Light Transparency via Tunneling and Cavity Effects

This work presents a broadband optical transparency structure consisting of a metal film perforated by an array of strip cavities. We theoretically demonstrate the metal film with side-coupled grooves system in model structure by the coupled model theory. The transparent light with a great bandwidth is obtained as a result of the tunneling and the resonance effect of the cavities by employing the finite-difference time-domain method simulation. By changing the parameter of the grooves in the structure, the transparency property shows a great adjustment in both wavelength and bandwidth. These structures have potential applications for transparent conductor devices.

A simplified method for time domain simulation of cross-flow vortex-induced vibrations

A new method for time domain simulation of cross-flow vortex-induced vibrations of slender circular cylindrical structures is developed. A model for the synchronization between the lift force and structure motion is derived from already established data for the cross-flow excitation coefficient. The proposed model is tested by numerical simulations, and the results are compared to experimental observations. When a sinusoidal cross-flow motion is given as input to the algorithm, the generated force time series are generally in good agreement with experimental measurements of cross-flow force in phase with cylinder velocity and acceleration. The model is also utilized in combination with time integration of the equation of motion to simulate the cross-flow vibration of a rigid cylinder. The resulting amplitude and frequency of motion as functions of reduced velocity are compared to published experimental results. In combination with the finite element method, the model is used to simulate cross-flow vibrations of a flexible cylinder in shear flow. Comparison with experiments shows that the model is capable of reproducing important quantities such as frequency, mode and amplitude, although some discrepancies are seen. This must be expected due to the complexity of the problem and the simple form of the present method.

Investigation of the Effects of Various Types of Wind Turbine Generators on Power-System Stability

It has become more and more demanding to investigate the impacts of wind farms on power system operation as ever-increasing penetration levels of wind power have the potential to bring about a series of dynamic stability problems for power systems. This paper undertakes such an investigation through investigating the small signal and transient stabilities of power systems that are separately integrated with three types of wind turbine generators (WTGs), namely the squirrel cage induction generator (SCIG), the doubly fed induction generator (DFIG), and the permanent magnet generator (PMG). To examine the effects of these WTGs on a power system with regard to its stability under different operating conditions, a selected synchronous generator (SG) of the well-known Western Electricity Coordinating Council (WECC three-unit nine-bus system and an eight-unit 24-bus system is replaced in turn by each type of WTG with the same capacity. The performances of the power system in response to the disturbances are then systematically compared. Specifically, the following comparisons are undertaken: (1) performances of the power system before and after the integration of the WTGs; and (2) performances of the power system and the associated consequences when the SCIG, DFIG, or PMG are separately connected to the system. These stability case studies utilize both eigenvalue analysis and dynamic time-domain simulation methods.

Fast and Accurate Transient Analysis of Buried Wires and its Applications

An improved finite difference time-domain (FDTD) method based on the telegraph equations of transmission lines is proposed. The original time-domain simulation method that resembles the well-established FDTD scheme based on the Maxwell's equations has been used in this paper to tackle the modeling problems of buried and overhead transmission lines. In this paper, a group of time domain responses of the ground impedance or ground admittance can be extracted analytically by means of a correlation between the ground impedance and the ground admittance. Thanks to the analytical technique, the accuracy of this time-domain solution is improved significantly. As a result, the application of this time-domain algorithm can be greatly expanded beyond the modeling of common transmission lines. By combining with the shunt and series connection difference equations, the method can be used to analyze an entire buried grounding network. Because the correlation between the ground impedance and the ground admittance is related to the propagation constant of electromagnetic waves, the analytical technique makes the propagation velocity unchanged during the iterative calculation. Therefore, the improved FDTD method allows the grounding network to be analyzed more accurately and efficiently.

Enhancement of antireflection property of silicon using nanostructured surface combined with a polymer deposition

Silicon (Si) nanostructures that exhibit a significantly low reflectance in ultraviolet (UV) and visible light wavelength regions are fabricated using a hydrogen etching process. The fabricated Si nanostructures have aperiodic subwavelength structures with pyramid-like morphologies. The detailed morphologies of the nanostructures can be controlled by changing the etching condition. The nanostructured Si exhibited much more reduced reflectance than a flat Si surface: an average reflectance of the nanostructured Si was approximately 6.8% in visible light region and a slight high reflectance of approximately 17% in UV region. The reflectance was further reduced in both UV and visible light region through the deposition of a poly(dimethylsiloxane) layer with a rough surface on the Si nanostructure: the reflectance can be decreased down to 2.5%. The enhancement of the antireflection properties was analyzed with a finite difference time domain simulation method.

Novel periodic dielectric ring waveguide and design of compact dual wavelength demultiplexer

A novel one dimensional periodic dielectric ring waveguide (PDRWG) is proposed and its dispersion properties are studied in details. Based on the proposed waveguide structure, a compact dual wavelength demultiplexer is designed. By a finite-difference time-domain (FDTD) simulation method, the performance of the designed device is calculated and analyzed. The dual wavelength demultiplexer can split the communication wavelengths of 1.31 and 1.55 mm into different waveguides and output from different ports with relative high transmittances of about 98% and 92%, respectively. The PDRWG may find applications in multi-wavelength demultiplexers or other novel integrated photonic devices.

Construction and analysis of an automatic multirate time domain simulation method for large power systems

A multirate method for time domain simulation of large power systems is presented in this paper. Analysis of the proposed multirate method shows how the coupling between fast and slow components affects the numerical stability of the method. Numerical results for two power system examples are presented. Comparison with the corresponding standard integration method shows that substantial gain in computational work and CPU time can be obtained.

Beneficial roles of Al back reflectors in optical absorption of Si nanowire array solar cells

We investigate the influence of Al back reflectors on the optical absorption spectra of Si nanowire (NW) arrays by using the finite-difference time-domain simulation method. A flat Al layer enhances the absorption in the NW array due to not only the reflection-induced optical path length enlargement but also reflection of light between NWs and localized surface plasmon induced optical field confinement. An Al underlayer with a grating structure allows grating-coupled surface plasmon polariton excitation and raise the optical absorption in the Si NWs. Interplay among all these factors on the optical absorption and expected solar cell performance of the NW arrays is discussed.

Air-gap embedding GaN template for enhanced emission from light-emitting diodes

Selective growth by metal-organic chemical vapor deposition (MOCVD), and electrochemical etching of a heavily Si-doped GaN (n+-GaN) interlayer were employed to obtain air-gaps embedded in a u-GaN layer. As confirmed by Raman spectroscopy, the introduction of an n+-GaN, which was later etched to obtain air-gaps, also enhanced the strain-compliance of GaN epilayer on sapphire substrate. An enhanced electroluminescence emission was observed from the light-emitting diodes (LEDs) fabricated on the air-gap embedding template. Using theoretical LED simulation, it was discerned that the increase in optical emission from the LED was caused predominantly by the redirection of photons at GaN/air-gap interface. Finite-difference time domain (FDTD) simulation method was employed to understand the mechanism of optical emission enhancement and its spatial variation over the LED surface.

Simulation and study on mitigation measures of frequent subsynchronous oscillation with low amplitude at multi-power plants

Subsynchronous oscillation (SSO) with low amplitude that exceeds cumulative fatigue threshold of the generator shaft frequently could significantly reduce the shaft’s service life, which is a new SSO problem that emerges in recent years. According to the real recording oscillograph, the basic reason for frequently over-threshold SSO with low amplitude at multi-power plants was analyzed based on Hulunbuir League system. The sensitivities of the electrical damping to the main electrical parameters in the contributing loop of subsynchronous torsional interaction were calculated. Based on the sensitivities, a simulation method was presented, which was used to excite the same oscillation as the actual case by exerting disturbance on the firing angle. The limitation of wide-band and narrow-band supplementary subsynchronous damping controller (SSDC) for mitigating this kind of SSO was analyzed based on the electromagnetic transient simulation model of Hulunbuir League system. The difference of supplementary excitation damping controller (SEDC) and parallel-form FACTS connected to the generator terminal was compared from the aspects of response time and the ability of damping torque supplying. The analysis indicates that their response time is similar but FACTS has stronger ability of damping torque supplying than SEDC. Time-domain simulation method was used to compare the mitigation effects of SEDC, static var compensator (SVC) and static synchronous compensator (STATCOM). Considering the mitigation effect, the floor space limit of the power plant and so on, STATCOM was considered as the best mitigation measure. A control strategy of cascaded STATCOM for engineering application was presented and the capacity for SSO mitigation as well as output characteristics was analyzed. The analysis indicates that STATCOM using the proposed control strategy has better mitigation effect and output characteristics with smaller capacity.

Online Application and Fast Solving Method for Critical Clearing Time of Three-Phase Short Circuit in Power System

For transient stability analysis, the direct method runs fast but with poor accuracy due to the over-simplification of component models; the time-domain simulation method is accurate but time-consuming and thus not suitable for online application. In this paper, results of the Potential Energy Boundary Surface (PEBS) method applied on the simulation examples and actual system calculation are analyzed, and the range of error is discussed. Via complementarity of direct method and time-domain simulation method, a new hybrid method is proposed: in the calculation of the critical clearing time (CCT), the result of the direct method is used as the initial value and then the time-domain simulation method is employed for further search, which can meet the requirements of online application in terms of accuracy and speed. Based on the power system online dynamic security assessment and early warning system (the PDSA), the fast solving method of three-phase short-circuit CCT is applied on the East China 500 kV power grid, and a detailed analysis of the result is presented. The engineering practice testifies the enhancement of CCT calculation in speed and accuracy.