Showing papers on "Frequency response published in 2021"

Tunable Terahertz Dielectric Resonator Antenna

Abstract: An annular dielectric resonator (DR) antenna (DRA) is implemented for THz applications. A silicon made DR is loaded with graphene disk for obtaining the tunability in the frequency response. The physical parameters of silicon annular DR can be set to obtain the resonance at any frequency in the lower THz band and can be tuned by changing the chemical potential of graphene nano-disk placed at the top of the DR. The response of antenna is preserved after changing the chemical potential of graphene. The higher order hybrid electromagnetic mode is excited in the antenna structure. The proposed research work provides a way to implement the antenna for THz frequency with high gain around 3.8 dBi and radiation efficiency in the range 72 − 75%.

Rapid Power Compensation-Based Frequency Response Strategy for Low-Inertia Power Systems

Abstract: To reduce the frequency deviation and the rate of change of frequency (RoCoF) in a low-inertia power system, some converters are required to provide the frequency response (FR) power normally associated with the frequency deviation and/or the RoCoF, by droop/inertia/PD control. In this article, a rapid power compensation (RPC)-based FR strategy is developed to optimize the ability to compensate grid imbalance power, by fully exploiting the converter idle capacity. To this end, first, mathematical proof demonstrated the improved performance of the RPC strategy in terms of frequency deviation suppression versus droop control, and in terms of RoCoF suppression versus inertia control, with identical converter capacity limit. Moreover, it is proven that the RPC strategy can achieve consistent FR performance with respect to the optimal PD control, i.e., it can maximize the suppression of frequency deviation and RoCoF simultaneously, yet avoiding the limitations due to unknown grid parameters. Finally, by analyzing the operation modes and identifying the pertinent switching logic, the detailed implementation of the proposed RPC strategy is developed. Its superb FR performance is verified by the experiment results in a two-converter low-inertia system, and simulation results in an IEEE four-machine two-area system.

Nonlinear secondary resonance of FG porous silicon nanobeams under periodic hard excitations based on surface elasticity theory

Abstract: To impart desirable material properties, functionally graded (FG) porous silicon has been produced in which the porosity changes gradually across the material volume. The prime objective of this work is to predict the influence of the surface free energy on the nonlinear secondary resonance of FG porous silicon nanobeams under external hard excitations. On the basis of the closed-cell Gaussian-random field scheme, the mechanical properties of the FG porous material are achieved corresponding to the uniform and three different FG patterns of porosity dispersion. The Gurtin–Murdoch theory of elasticity is implemented into the classical beam theory to construct a surface elastic beam model. Thereafter, with the aid of the method of multiple time-scales together with the Galerkin technique, the size-dependent nonlinear differential equations of motion are solved corresponding to various immovable boundary conditions and porosity dispersion patterns. The frequency response and amplitude response associated with the both subharmonic and superharmonic hard excitations are obtained including multiple vibration modes and interactions between them. It is found that for the subharmonic excitation, the nanobeam is excited within a specific range of the excitation amplitude, and this range shifts to higher excitation amplitude by incorporating the surface free energy effects. For the superharmonic excitation, by taking surface stress effect into account, the excitation amplitude associated with the peak of the vibration amplitude enhances. Moreover, in the subharmonic case, it is demonstrated that by increasing the porosity coefficient, the value of the excitation frequency at the joint point of the two branches of the frequency-response curve reduces. In the superharmonic case, it is revealed that an increment in the value of porosity coefficient leads to decrease the peak of the oscillation amplitude and the associated excitation frequency.

Hybrid Frequency-Selective Rasorber With Low-Frequency Diffusion and High-Frequency Absorption

Abstract: In this article, a novel frequency-selective rasorber (FSR) is proposed, which provides a high-efficiency transmission band and a broadband radar cross section (RCS) reduction by hybridizing low-frequency diffusion and high-frequency absorption. The FSR comprises three layers. A broadband second-order bandpass frequency-selective surface is designed as the bottom layer, which has a reflection–transmission–reflection frequency response, and acts as an equivalent ground plane for the upper layers outside the transmission band. A circuit analog absorber and a coding metasurface are designed as the middle and top layers, respectively. They use absorption and diffusion to reduce the RCS at high- and low-frequency bands. In the coding metasurface, miniaturized and absorptive techniques are utilized to suppress the harmonic resonances. The whole FSR unit cell exhibits a cross-polarized reflection–transmission–absorption frequency response. A co-polarized transmission magnitude larger than −1.5 dB from 7.4 to 12.1 GHz and a co-polarized reflection magnitude less than −10 dB from 3.1 to 20 GHz are observed in the full-wave simulation under the normal incidence. An FSR sample with $20\times20$ unit cells is designed and measured for verification. The measurement agrees well with the simulation, and it shows a significant monostatic RCS reduction from 3.3 to 20 GHz in comparison with a metallic plane of the same size.

Locating Inter-Turn Faults in Transformer Windings Using Isometric Feature Mapping of Frequency Response Traces

Abstract: Power transformers usually confront various mechanical and electromagnetic stresses during an operation that may lead to defects in their windings. The short circuit in the windings is one of those severe defects. Early detection of short-circuits is necessary as extra heating in the shorted location can lead to progressive damage in windings insulation. Frequency response analysis (FRA) is a well-known method to diagnose short-circuits in transformers. Despite the accuracy of FRA, the interpretation of the obtained frequency response traces (FRTs) is still an intricate task. Due to the unknown impact of faults on FRTs, extracting efficient features from such traces is necessary for the interpretation of transformer's frequency response. In this article, an isometric feature mapping (Isomap) is used as a nonlinear dimensionality reduction technique to locate interturn faults in transformer windings due to its capability of capturing the nonlinear phenomena in FRT of power transformers. It is revealed that, after constructing the isometric mapping for a transformer, there is no need for any expertise to detect fault location even in nondirect (high impedance) short-circuits. In other words, it can be the first step for the automated interpretation of FRA of power transformers.

Frequency Regulation in Isolated Microgrids Through Optimal Droop Gain and Voltage Control

Abstract: This article presents an adaptive active power droop controller and voltage setpoint control in isolated microgrids for optimal frequency response and stability after disturbances. The control scheme involves an optimal and model predictive control approach that continuously adjusts the active power droop gains and the voltage setpoints of Distributed Energy Resources (DERs) to maintain the frequency of the system within acceptable limits and enhance the primary frequency response of the system, while taking into account the active power sensitivity of the microgrid loads to the system’s operating voltage. The proposed control scheme is tested, validated, and compared with previously proposed techniques using time-domain simulations for a test system based on a CIGRE medium voltage benchmark microgrid under different realistic conditions, demonstrating the advantages of the proposed approach.

An Adaptive PV Frequency Control Strategy Based on Real-Time Inertia Estimation

Abstract: The declining cost of solar Photovoltaics (PV) generation is driving its worldwide deployment. As conventional generation with large rotating masses is being replaced by renewable energy such as PV, the power system’s inertia will be affected. As a result, the system’s frequency may vary more dramatically in the case of a disturbance, and the frequency nadir may be low enough to trigger protection relays such as under-frequency load shedding. The existing frequency-watt function mandated in power inverters cannot provide grid frequency support in a loss-of-generation event, as PV plants usually do not have power reserves. In this article, a novel adaptive PV frequency control strategy is proposed to reserve the minimum power required for grid frequency support. A machine learning model is trained to predict system frequency response under varying system conditions, and an adaptive allocation of PV headroom reserves is made based on the machine learning model as well as real-time system conditions including inertia. Case studies show the proposed control method meets the frequency nadir requirements using minimal power reserves compared to a fixed headroom control approach.

An Improved Synchronization Stability Method of Virtual Synchronous Generators Based on Frequency Feedforward on Reactive Power Control Loop

Abstract: The synchronization stability of the virtual synchronous generator (VSG) under grid fault is an important issue for maintaining stable operation in the power system. Existing work has pointed out a low-pass filter (LPF) with a sufficiently low cutoff frequency in the reactive power control loop (RPCL) can improve the transient stability. Yet, the underlying mechanism was unknown. Moreover, as a key index of VSG and precondition of synchronization stability, the frequency response is rarely studied. In this article, based on the linearized model for qualitative analysis, combined with the nonlinear model for quantitative analysis, the underlying mechanism of improving synchronization stability using an LPF in the RPCL is revealed. Furthermore, to avoid increasing the system order and solve the conflict between transient stability and frequency response, an improved synchronization stability method is proposed by feedforwarding the frequency difference between the VSG and grid to the RPCL. The frequency response is also acquired based on the combined linearized and nonlinear model, which shows that the frequency feedforward method can further enhance the frequency stability. How to design the coefficient of the frequency feedforward path with different inertia requirements is also presented. Finally, this method is verified by experimental results.

On the chaotic behavior of graphene-reinforced annular systems under harmonic excitation

Abstract: In this study, a mathematical derivation is made to develop a nonlinear dynamic model for the nonlinear frequency and chaotic responses of the graphene nanoplatelets (GPLs)-reinforced composite (GPLRC) annular plate subject to an external harmonic load. Using Hamilton’s principle and the von Karman nonlinear theory, the nonlinear governing equation is derived. For developing an accurate solution approach, generalized differential quadrature method (GDQM) and perturbation approach (PA) are finally employed. Various geometrically parameters are taken into account to investigate the chaotic motion of the annular plate subject to a harmonic excitation. The fundamental and golden results of this paper could be that the chaotic motion and nonlinear frequency of the annular plate are hardly dependent on the value of the length to thickness ratio (lGPL/wGPL) of the GPLs. Moreover, utilizing GPLs in the shapes close to square (lGPL/wGPL = 1) presents higher frequency of the annular plate. Also, increase in lGPL/tGPL indicates that using GPLs with lower thickness relative to its length provides better frequency response

Grid-Supporting Battery Energy Storage Systems in Islanded Microgrids: A Data-Driven Control Approach

Abstract: Islanded microgrids have low real and reactive power generation capacity and low inertia. This makes them susceptible to large frequency and voltage deviations, which deteriorate power quality and can cause frequency or voltage collapse. Grid-supporting battery energy storage systems are a possible solution as they are able to respond quickly to changes of their real and reactive power set-points. In this paper, a data-driven grid-supporting control system for battery energy storage systems, which requires no changes to the inverters inner real and reactive power control loops compared with a conventional grid-supporting inverter, is proposed. Tuning the data-driven controller does not require a dynamic model of the microgrid. Instead, the frequency response of the microgrid is identified and used directly to optimally tune the controller for $H_\infty$ performance and robustness criteria. The performance of the data-driven controller is verified through real-time software-in-the-loop electromagnetic-transient simulation, where it is compared with an inverse-droop controller and is shown to significantly reduce voltage and frequency deviations.

Approximating Trajectory Constraints With Machine Learning – Microgrid Islanding With Frequency Constraints

Abstract: In this paper, we introduce a deep learning aided constraint encoding method to tackle the frequency-constraint microgrid scheduling problem. The nonlinear function between system operating condition and frequency nadir is approximated by using a neural network, which admits an exact mixed-integer formulation (MIP). This formulation is then integrated with the scheduling problem to encode the frequency constraint. With the stronger representation power of the neural network, the resulting commands can ensure adequate frequency response in a realistic setting in addition to islanding success. The proposed method is validated on a modified 33-node system. Successful islanding with a secure response is simulated under the scheduled commands using a detailed three-phase model in Simulink. The advantages of our model are particularly remarkable when the inertia emulation functions from wind turbine generators are considered.

Polarization Insensitive and Transparent Frequency Selective Surface for Dual Band GSM Shielding

Abstract: This article proposes a single-layer, dual-band, optically transparent frequency-selective surface (FSS) for GSM shielding. The design evolves from a fractal cross dipole to achieve dual-band response, polarization insensitivity, and wide angle of incidence stability with a miniaturized size. A two-axis symmetric structure helps to provide a stable frequency response for incident waves with different polarization angles up to 60°. Capacitive loading further reduces the unit size and extends the maximum stable incident angle. The working mechanism of the design has been explained through an equivalent lumped element circuit model (ECM), which provides a generic approach to retune or optimize the design for another frequency band. Both the full-wave EM and circuit simulations are in good agreement. A prototype has been realized by screen printing a custom silver nanowire (Ag NW)-based transparent ink on a flexible polymer substrate. A decent reflection performance has been achieved from 0.71 to 1.25 GHz and from 1.73 to 2.16 GHz in measurements, which are consistent with the simulations. Optical measurements reveal a transparency of 81.6%. Printed FSS is completely flexible, and the performance does not deteriorate after bent or rolled up conditions. Field testing on a glass box demonstrates decent shielding from the incoming GSM signals. The highly transparent and flexible nature of the FSS structure makes it suitable for mounting on glass windows of cars or home environments for EM shielding purposes.

Design of Digital OTAs With Operation Down to 0.3 V and nW Power for Direct Harvesting

Abstract: In this paper, passive-less fully-digital operational transconductance amplifiers (DIGOTA) for energy- and area-constrained systems are modeled and analyzed from a design viewpoint. The digital behavior of DIGOTAs is modeled as an equivalent small-signal differential-mode circuit with zero bias current, and a common-mode feedback loop operating as a self-oscillating threshold sampler. Such continuous-time equivalent circuits are used to derive an explicit model of the main performance parameters that are generally adopted to characterize OTAs. This provides an insight into circuit operation and allows to derive practical guidelines to achieve a given design target. Among the others, an explicit model is derived for the DC gain, the frequency response, the gain-bandwidth product, the input-referred noise, and the input offset voltage. The models are validated via direct comparison with multi-die measurement results in CMOS 180 nm. From an application viewpoint, the voltage (power) reduction down to 0.25 V (sub-nW) uniquely enable direct harvesting (e.g., with solar cells), suppressing any intermediate DC-DC conversion stage. This further enhances the area efficiency advantage of DIGOTA stemming from its fully-digital nature, making it well suited for cost-sensitive and purely-harvested systems.

Performance analysis of a quasi-zero stiffness vibration isolation system with scissor-like structures

Abstract: To isolate low-frequency vibration, a novel single-degree-of-freedom vibration isolation system with quasi-zero stiffness (QZS) and nonlinear damping using geometric nonlinearity is proposed in this study. One of the remarkable features of this system is the use of scissor-like structures (SLSs) to achieve the nonlinear stiffness and damping. The length difference between the connecting rods in SLS is considered. First, both the stiffness and damping characteristics are derived and analyzed in detail. Then, the frequency response and force transmissibility are obtained using the harmonic balance method. Finally, the effects of structural parameters on the isolation performance are investigated. Theoretical results show that the proposed QZS vibration system can not only isolate low-frequency vibration but also suppress the high-amplitude vibration in the resonant region. Besides, increasing nonlinear damping has little influence on the isolation performance in high frequencies. The proposed QZS vibration system can outperform a classical counterpart.

Coordinated Control Strategy of Virtual Synchronous Generator Based on Adaptive Moment of Inertia and Virtual Impedance

Abstract: The application of virtual synchronous generators in the power system eases the pressure on the grid caused by penetration of lots of power electronic devices, but worsens the dynamic frequency stability of the grid Meanwhile, in order to solve the problem of power coupling in low-voltage micro-grid systems, most measures introduced virtual impedance technology to adjust the equivalent output impedance of the system Therefore, most studies have adopted the adaptive control strategy of the moment of inertia, to improve the dynamic frequency adjustment of the grid However, it will cause the frequency response speed reduction problem In order to deal with the contradiction between the moment of inertia and frequency response speed, the voltage phasor relationship of the micro-source - grid system under signal disturbance in the introduction of virtual impedance is analyzed Then an adaptive virtual impedance control strategy is proposed to accelerate the active power adjustment process of the virtual synchronous generator system, which helps to complete the first frequency modulation and improve the inertia adjustment ability Finally, combining these two control strategies, a coordinated adaptive moment of inertia and virtual impedance control strategy is proposed From the perspective of the moment of inertia and virtual impedance, the proposed method fully exploits the control advantages of the virtual synchronous generator system and achieves the purpose of adjusting the inertia of the virtual synchronous generator system while considering the acceleration of the frequency response speed The comprehensive simulation results verify the feasibility and effectiveness of the proposed method

Encoding Frequency Constraints in Preventive Unit Commitment Using Deep Learning with Region-of-Interest Active Sampling

Abstract: With the increasing penetration of renewable energy, frequency response and its security are of significant concerns for reliable power system operations. Frequency-constrained unit commitment (FCUC) is proposed to address this challenge. Despite existing efforts in modeling frequency characteristics in unit commitment (UC), current strategies can only handle oversimplified low-order frequency response models and do not consider wide-range operating conditions. This paper presents a generic data-driven framework for FCUC under high renewable penetration. Deep neural networks (DNNs) are trained to predict the frequency response using real data or high-fidelity simulation data. Next, the DNN is reformulated as a set of mixed-integer linear constraints to be incorporated into the ordinary UC formulation. In the data generation phase, all possible power injections are considered, and a region-of-interest active sampling is proposed to include power injection samples with frequency nadirs closer to the UFLC threshold, which enhances the accuracy of frequency constraints in FCUC. The proposed FCUC is investigated on the IEEE 39-bus system. Then, a full-order dynamic model simulation using PSS/E verifies the effectiveness of FCUC in frequency-secure generator commitments.

A Two-Level Simulation-Assisted Sequential Distribution System Restoration Model With Frequency Dynamics Constraints

Abstract: This paper proposes a service restoration model for unbalanced distribution systems and inverter-dominated microgrids (MGs), in which frequency dynamics constraints are developed to optimize the amount of load restoration and guarantee the dynamic performance of system frequency response during the restoration process. After extreme events, the damaged distribution systems can be sectionalized into several isolated MGs to restore critical loads and tripped non-black start distributed generations (DGs) by black start DGs. However, the high penetration of inverter-based DGs reduces the system inertia, which results in low-inertia issues and large frequency fluctuation during the restoration process. To address this challenge, we propose a two-level simulation-assisted sequential service restoration model, which includes a mixed integer linear programming (MILP)-based optimization model and a transient simulation model. The proposed MILP model explicitly incorporates the frequency response into constraints, by interfacing with transient simulation of inverter-dominated MGs. Numerical results on a modified IEEE 123-bus system have validated that the frequency dynamic performance of the proposed service restoration model are indeed improved.

The frequency-response behaviour of flexible piezoelectric devices for detecting the magnitude and loading rate of stimuli

Abstract: Piezoelectric sensors with good flexibility and high sensitivity have attracted extensive interest in wearable electronics. Here, we report a novel frequency-response behaviour of piezoelectric voltage of a sensor that is based on a piezoelectric enhanced composite film of P(VDF-TrFE) and BaTiO3. The piezoelectric voltage enhances with the increase of frequency and becomes stable beyond the critical frequency. Such a demonstrated sensing characteristic of the piezoelectric sensor depends on the inner resistance of the voltmeter, which is determined by whether the loading of stimuli can be completed within the period of piezoelectric voltage measurement in the testing circuit. By utilizing the frequency-response behaviour in different frequency ranges, the flexible piezoelectric device exhibits excellent capabilities to quantitatively detect the magnitude and loading rate of stimuli. As a proof-of-concept demonstration, a flexible pressure sensor is successfully integrated with a bionic bee to monitor the flight status (i.e., strain rate and strain) of the vibrating wings. This work demonstrates that flexible piezoelectric sensors have great prospects for application in the field of bionic flying robots, thus paving the way forward for the development of smart self-sensing flexible electronics.

Ultra-Wideband Rectenna Using Complementary Resonant Structure for Microwave Power Transmission and Energy Harvesting

Abstract: An ultra-wideband (UWB) rectenna (fractional bandwidth >100%) using a novel wideband complementary matching stub is proposed for microwave power transmission and energy harvesting A simple resonant structure, ie, LC series–parallel resonant circuit, is embedded to the L-shaped complementary matching stub Due to the unique frequency response of the LC resonant circuit, the proposed matching stub can exhibit “open” and “short” circuits as a function of frequency, thereby acting as a complementary matching circuit covering a relatively wide frequency range Having utilized the proposed matching stub, the nonlinear input impedance of the rectifier can be tuned to conjugately match the antenna impedance throughout the frequency band of interest Simulated and measured results show that the proposed rectenna has good matching performance ( $S_{11} dB) and high RF-dc conversion efficiency (>50%) over a relatively wide frequency range from 09 to 3 GHz (for GSM, Wi-Fi, and WLAN bands) The maximum conversion efficiency of 734% is realized at 3-dBm input power It is evident that the proposed resonant structure-based matching scheme is a promising and effective solution to facilitate the UWB rectenna design with stably high efficiency over a very wide frequency band