Frequency Response Approach Research Articles

Validation of the structural response of a 70kW VAWT

This paper describes a project for predicting and measuring the structural response of a 70 kW vertical axis wind turbine (VAWT), while offering insights for optimizing efficiency and ensuring longevity. We employ the traditional frequency response approach and model the turbine behavior using its stationary modal shapes. Results for three key sensor locations – lower central mast bending, upper blade flapwise bending, and central horizontal arm out-of-plane bending – reveal that the codes accurately predict the critical resonance and capture the dominant harmonic amplitudes. In addition, it is shown how a statistical treatment of the steady state solution can lead to predictions of the fatigue cycle distribution enabling accurate life predictions. The program demonstrates the potential to significantly improve the design and operation of VAWT using simple, steady state, simulation codes for capturing VAWT aerodynamics. The model demonstrates sufficient accuracy while offering computational efficiency compared to more complex models.

Design of Lead, Lag and Lead-Lag Compensators Based on Frequency Response Approach

In the control systems, the compensators are widelyused to improve system dynamics, characteristics of the openloop system and the stability. The degree of convergence of theoutput waveform for any compensator depends upon theproper selection and tuning of the compensator. The dynamicstructure and the control properties of lead, lag and lead-lagcompensators are presented in this work. A simple methodbased on the frequency domain is adopted to design the threedifferent compensators. The parameters which have effect oneach compensator performance were studied and explainedhow to choose them to achieve the optimal performance. Thecompensator algorithms are implemented with MATLABsimulation in order to control the DC motor motion. The resultsand the comparative analysis confirm the effectiveness of theproposed three compensators and improve the systemresponse.

Sensing the ocean electric fields via a self-supported CNT sponge

A self-supported CNT sponge is composed of uniformly twisted CNTs with high aspect ratio that enables the large specific surface area and good carrier conduction to go beyond the conventional carbon-based materials, shedding light on its high electrochemical activity in salt water. Herein, we demonstrate the comparable performance of the CNT sponge as the electrode pair to the most commonly used Ag/AgCl for sensing underwater electric fields. The CNT sponge electrodes exhibit a high electrochemical reactivity in salt water with a low noise of ∼10 nV/rt(Hz)@100 Hz within a wide range of the electric field frequency (e.g., 10–105 Hz), while the frequency response approaches a constant magnitude across the same range of frequency. The performance of the CNT sponge electrodes in ocean electric field sensing is further verified by measuring the electrical signal response curve at various characteristic frequencies within a small water container, and also under a simulated source within the water pool. The high chemical stability and low costs of carbon-based CNT sponges pave the way to their potential applications.

Design, implementation and performance evaluation of multi-function boost converter

This paper introduces a single-phase multi-function converter (MFC) circuit that can perform the power electronics functions of a DC–DC, a DC–AC, an AC–AC, and an AC–DC boost converter. The power circuit of the proposed converter contains only four bi-directional switches, which is sufficient side by side with a switched logic-based control system to execute converter functions. The proposed converter can implement the boosting converter function, which is a drawback of the conventional single-phase matrix converter, which is limited to performing the buck converter functions. The mathematical analysis of each converter function is presented. A state space averaging technique is employed to obtain the time-invariant state space model for a better understanding of the converter characteristics. The frequency response approach is used to demonstrate the effect of circuit parameters on converter stability using the time-invariant state-space model. Moreover, the performance analysis of the system is checked through the building of a simulation model of the proposed converter via MATLAB/Simulink and MATLAB/Simscape toolbox integration. Additionally, the actual performance of the proposed converter is verified through a laboratory prototype experimental study. The experimental results confirm those obtained by computer simulation, clearly demonstrating the successful operation of the proposed circuit as a multi-function converter with acceptable levels of total harmonic distortion (THD).

Fast Frequency Support Through LED Street Lighting in Small Non-Synchronous Power Systems

Small-sized non-synchronous islands, which are inherently characterized by low system inertia, will face in the next years the challenge of integrating larger and larger amount of renewable energy sources, leading to further reduction of inertia. Several means to support inertia and frequency transients have been proposed in the literature, including energy storage systems, distributed generation, and demand response. LED street lighting systems are widespread control resources, which in a smart grid scenario, can be regulated by means of local or remote control signals. This article investigates the possibility to regulate the power consumption of LED street lighting to provide frequency support in a system characterized by reduced total inertia. Tests are carried out integrating the simulation of the non-synchronous power system of an Italian small island with the response of an actual LED street lamp, in a Power Hardware-in-the-Loop testing environment. Both synthetic inertia and fast frequency response approaches are tested and discussed.

A data-driven nonlinear frequency response approach based on the Loewner framework: preliminary analysis

We propose a hybrid method based on the combination of computed-aided nonlinear frequency response analysis with the Loewner framework, for the characterization of nonlinear dynamical processes with application in electrochemistry. The method is data-driven, i.e., requiring only samples of the first two generalized transfer functions of the underlying system, given as values of the sampled nonlinear frequency response. Then, the established system fitting and complexity reduction approach (in the frequency domain), known as the Loewner framework, is used to extract the system's invariant quantities. In this analysis, we have used a nonlinear electrical circuit model as a test case, for which the new method provides good results.

Lift enhancement by a flapped trailing edge at low Reynolds number: A frequency response approach

The capability of an oscillating trailing edge flap to enhance the lift generated by a rigid NACA-0012 airfoil at 2.1×104 Reynolds number is investigated. Experiments are performed at two angles of attack, namely α0=0∘ that represents a fully attached flow and α0=10∘ that represents a near-stall flow condition. The presented data and analysis cover sinusoidal oscillations of the flap at five reduced frequencies between k=0.02 and 0.12, and three oscillating amplitudes δA=5∘, 8° and 10° relative to the airfoil’s angle of attack. Enhancement to the mean lift, lift-to-drag ratio, and unsteady lift dynamics are discussed. The validity of quasi-steady formulation of Leishman’s model in predicting the lift is quantitatively assessed in light of the measurements. The unsteady lift dynamics generated by flap oscillations are modeled by constructing frequency response functions with the quasi-steady lift as the input and unsteady lift as the output. The experimentally determined gain variations are compared with analytically-derived gain functions. The effects of rate and amplitude of flap oscillations on lift cycle variations are examined using aerodynamic hysteresis response.

Event-Driven Fast Frequency Response for Energy Storage System Considering State of Charge

The increasing penetration of renewable energy sources reduces the equivalent inertia and spinning reserve. Power systems are becoming more and more vulnerable when subjected to active power deficits. With the trait of quick response and flexible ramp, the energy storage system (ESS) offers a promising capability to counter frequency decline after large disturbances. This paper proposes an event-driven fast frequency response (EFFR) approach for the ESS to sustain frequency stability. A centralized EFFR control framework is established, in which the wide area information is collected, and the disturbance power is broadcasted to ESSs under contingencies. Moreover, a fuzzy logic controller (FLC) is employed to determine the power set-point of the ESS based on the disturbance power and state of charge (SOC). A case study is presented to validate the feasibility and effectiveness of the proposed approach.

Design of a PID-Lead Compensator for a Twin Rotor Aerodynamic System (TRAS)

This paper deals with a Twin Rotor Aerodynamic System (TRAS). It is a Multi-Input Multi-Output (MIMO) system with high crosscoupling between its two channels. It proposes a hybrid design procedure that combines frequency response and root locus approaches. The proposed controller is designated as PID-Lead Compensator (PIDLC); the PID controller was designed in previous work using frequency response design specifications, while the lead compensator is proposed in this paper and is designed using the root locus method. A general explicit formula for angle computations in any of the four quadrants is also given. The lead compensator is designed by shifting the dominant closed-loop poles slightly to the left in the s-plane. This has the effect of enhancing the relative stability of the closed-loop system by eliminating the oscillation in its transient part but at the expense of greater rise time. However, for some applications, long rise time may be an allowable price to get rid of undesired oscillation. To demonstrate the proposed hybrid controller's performance numerically, a new performance index, designated by Integral Reciprocal Time Absolute Error (IRTAE), is defined as a figure to measure the oscillation of the response in its transient part. The proposed controller enhances this performance index by 0.6771%. Although the relative enhancement of the performance index is small, it contributes to eliminating the oscillation of the response in its transient part. Simulation results are performed on the MATLAB/Simulink environment.

Design of a PID-Lead Compensator for a Twin Rotor Aerodynamic System (TRAS)

This paper deals with a Twin Rotor Aerodynamic System (TRAS). It is a Multi-Input Multi-Output (MIMO) system with high crosscoupling between its two channels. It proposes a hybrid design procedure that combines frequency response and root locus approaches. The proposed controller is designated as PID-Lead Compensator (PIDLC); the PID controller was designed in previous work using frequency response design specifications, while the lead compensator is proposed in this paper and is designed using the root locus method. A general explicit formula for angle computations in any of the four quadrants is also given. The lead compensator is designed by shifting the dominant closed-loop poles slightly to the left in the s-plane. This has the effect of enhancing the relative stability of the closed-loop system by eliminating the oscillation in its transient part but at the expense of greater rise time. However, for some applications, long rise time may be an allowable price to get rid of undesired oscillation. To demonstrate the proposed hybrid controller's performance numerically, a new performance index, designated by Integral Reciprocal Time Absolute Error (IRTAE), is defined as a figure to measure the oscillation of the response in its transient part. The proposed controller enhances this performance index by 0.6771%. Although the relative enhancement of the performance index is small, it contributes to eliminating the oscillation of the response in its transient part. Simulation results are performed on the MATLAB/Simulink environment.

Diagnosis of Inter-Turn Shorts of Loaded Transformer Under Various Load Currents and Power Factors; Impulse Voltage-Based Frequency Response Approach

Inter-turn short circuits within the transformer winding are diagnosable through the impulse voltage based frequency response analysis (IFRA). If the diagnosis is made Online, interruption to the power network can be avoided. However, the transformer’s load may influence the frequency response based diagnostic process, and this aspect has not yet been investigated. Motivated by this research gap, experimental investigations were carried out on a three-phase 5 kVA, 440 V/440 V Star/Delta transformer and a three-phase 315 kVA, 11 kV/433 V, Dyn11 distribution transformer, when they were supplying power to loads of different magnitudes and power factors. The influence of the load current and its power factor on the IFRA was observed under the transformer’s healthy condition and with inter-turn short circuits in one of the windings. Investigations under these different loaded conditions revealed that a signature FRA plot developed at a particular load condition cannot be directly used for assessing the condition of the transformer at a different load. Both the magnitude and the power factor of the load are found to influence the frequency response. A careful interpretation of the Frequency response of the transformer, based on its load, was found effective in detecting and locating the inter-turn shorts.

Distribution Feeder-Scale Fast Frequency Response via Optimal Coordination of Net-Load Resources—Part I: Solution Design

This article is the first of a two-part series that develops and experimentally demonstrates a first-of-its-kind hierarchical control solution for optimally dispatching thousands of deferrable loads and distributed energy resources (DERs) across a distribution feeder to provide fast frequency response (FFR) within 500 ms to the bulk power system. This approach rapidly coordinates resources online after a frequency event occurs, allowing fast-changing, behind-the-meter (BTM) resources to be incorporated and aggregate FFR power set points to be achieved more quickly and accurately than existing approaches. We also present a solution for determining the optimal amount of headroom to operate solar inverters with to minimize opportunity cost while ensuring the FFR response viability of a building with the inverter and deferrable loads. In Part I, we develop practical algorithms for fast, cost-based optimal dispatch at multiple aggregation scales (single building, multiple buildings, and full distribution feeder), establish their optimality, and demonstrate via simulation that they are faster than state-of-the-art, coordinated frequency response approaches. In Part II, the entire platform is implemented and experimentally verified using a unique power hardware-in-the-loop demonstration, including more than 100 powered loads and DERs connected to a real-world distribution network model and over 10,000 net-load resources dispatched.

Influence of the Load on the Impulse Frequency Response Approach Based Diagnosis of Transformer’s Inter-Turn Short-Circuit

Frequency Response Analysis (FRA) is an Off-line condition monitoring technique for the transformers and is successful, in diagnosing mechanical faults and inter-turn short-circuits. Only limited literature is available for On-line FRA works, and the effect of the transformer’s load on the fault diagnosing capability of FRA has not yet been analysed. Motivated by this research gap, an experimental investigation was done to analyze the effect of load on IFRA based inter-turn short-circuit diagnosis. A single-phase 1 kVA, 240V/240 V transformer supplying R and R-L loads and a three-phase 5 kVA, 440 V/440 V transformer supplying an induction motor at various load levels were tested. Inter-turn short-circuits were emulated in their windings and, the effectiveness of IFRA in diagnosing the same was investigated. Comparisons were made between the frequency response of the loaded transformers in their healthier conditions and emulated fault conditions, based on the transfer function plots and the statistical parameters. Investigations reveal that the loads alter the useful spectrum of IFRA. The frequency response of the healthier transformer itself varies based on the transformer’s load and necessitates a careful focus on the sensitive frequency spectra for effective inter-turn short-circuit diagnosis in loaded transformers.

A novel approach based on simulation of tunable MEMS diaphragm for extrinsic Fabry–Perot sensors

A new tunable structure with a Three Leaf Clover (TLC) geometry has been proposed for use in diaphragm-based acoustic pressure sensors. The sensitivity and frequency response of this new structure, which will be an alternative to conventional circular diaphragms, was accomplished using Finite Element Method (FEM) and numerical analysis techniques. As a result of the analysis, sensitivity and frequency response approaches were obtained for the tunable-TLC structure. These approaches provide researchers to design diaphragms more convenient. Conventional circular diaphragms can be easily converted to TLC diaphragms by means of micro-electromechanical systems (MEMS) thus can be tuned up to 5 times for sensitivity and up to 1/3 times the fundamental frequency without altering the thickness and radius of the diaphragm. Obtained approach expressions were compared with FEM results for practical design purposes in a wide range of tunable parameters, and the average errors were below 2.5% for the sensitivity and below 0.5% for the fundamental frequency response, respectively.

Measurement and modeling of lift enhancement on plunging airfoils: A frequency response approach

A frequency response approach is taken to model unsteady aerodynamics resulting from plunging oscillations of an airfoil. The oscillations range covers high angles of attack (up to 65°) and reduced frequencies (up to 0.95) with particular attention to the range of flow nonlinearities over which lift enhancement is observed. Experiments were performed in a wind tunnel on an airfoil section at a Reynolds number of 80,000. Airfoil acceleration and lift force data were used to determine the frequency response functions between the measured unsteady lift and the quasi-steady lift. The results show enhanced lift amplitude over the stall regime at a reduced frequency of 0.7. The flow dynamics associated with this observation were identified from water tunnel experiments. Particular attention is paid to the flow separation and consequent formation and convection of the leading edge vortex. An optimization-based system identification approach is performed to represent the unsteady lift by a fourth-order dynamical system. This representation is suitable for sensitivity, dynamics and control analyses especially for applications where unsteady aerodynamics can be exploited for the purpose of lift enhancement.

Development of a high speed and precision wire clamp with both position and force regulations

This paper presents the mechanism and robust control of a monolithic wire clamp to achieve fast and precision operations for strong and robust micro device packaging. The wire clamp is piezoelectrically actuated and a two-stage flexure-based amplification was designed to obtain large and parallel jaw displacements. The grasping forces of the wire clamp were evaluated based on finite element analysis (FEA), and the force measurement was presented. The wire clamp was manufactured using wire EDM technique and the position and force transfer functions were obtained based on the frequency response approach. The position/force switching control strategy was employed to regulate the motion position and grasping force, and the position/force switching controller composed of a PID controller for position control and a sliding model controller (SMC) for force control was designed. Experimental tests were carried out to investigate the performance of wire clamp with the position/force switching controller during the grasping and releasing operations. The results show that the wire clamp exhibits good performance and demonstrate that high speed and precision grasping operations can be realized through the developed wire clamp and the control strategy. A novel monolithic wire clamp mechanism with a two-stage amplification was presented.The wire clamp has a large amplification factor and parallel jaw motion can be realized.Both the position and force of the wire clamp were regulated.Fast and precision grasping and releasing operations have been realized through the wire clamp.

An Improved Control Strategy for a Four-Leg Grid-Forming Power Converter under Unbalanced Load Conditions

This paper proposes an improved multiloop control strategy for a three-phase four-leg voltage source inverter (VSI) operating with highly unbalanced loads in an autonomous distribution network. The main objective is to balance the output voltages of the four-leg inverter under unbalanced load conditions. The proposed control strategy consists of a proportional-integral (PI) voltage controller and a proportional current loop in each phase. The voltage controller and the current control loop are, respectively, used to regulate the instantaneous output voltage and generate the pulse width modulation (PWM) voltage command with zero steady-state tracking error and fast transient response. A voltage decoupling feedforward path is also used to enhance the system robustness. Since the outer voltage loop operates in the synchronous reference frame, tuning and stability analysis of the PI controller is far from being straightforward. In order to cope with this challenge, the stationary reference frame equivalent of the voltage controller in the rotating frame is derived. Subsequently, a systematic design based on a frequency response approach is provided. Simulation results are also carried out using the DIgSILENT PowerFactory software to verify the effectiveness of the suggested control strategy.

Detrending moving average algorithm: Frequency response and scaling performances.

The Detrending Moving Average (DMA) algorithm has been widely used in its several variants for characterizing long-range correlations of random signals and sets (one-dimensional sequences or high-dimensional arrays) over either time or space. In this paper, mainly based on analytical arguments, the scaling performances of the centered DMA, including higher-order ones, are investigated by means of a continuous time approximation and a frequency response approach. Our results are also confirmed by numerical tests. The study is carried out for higher-order DMA operating with moving average polynomials of different degree. In particular, detrending power degree, frequency response, asymptotic scaling, upper limit of the detectable scaling exponent, and finite scale range behavior will be discussed.

An AutomaticP‐Phase Arrival‐Time Picker

Presented is a new approach for picking P ‐phase arrival time in single‐component acceleration or broadband velocity records without requiring detection interval or threshold settings. The algorithm P PHASE P ICKER transforms the signal into a response domain of a single‐degree‐of‐freedom (SDOF) oscillator with viscous damping and then tracks the rate of change of dissipated damping energy to pick P ‐wave phases. The SDOF oscillator has a short natural period and a correspondingly high resonant frequency, which is higher than most frequencies in a seismic wave. It also has a high damping ratio (60% of critical). At this damping level, the frequency response approaches the Butterworth maximally flat magnitude filter, and phase angles are preserved. The relative input energy imparted to the oscillator by the input signal is converted to elastic strain energy and then dissipated by the damping element as damping energy. The damping energy yields a smooth envelope over time; it is zero in the beginning of the signal, zero or near zero before the P ‐phase arrival, and builds up rapidly with the P wave. Because the damping energy function changes considerably at the onset of the P wave, it is used as a metric to track and pick the P ‐phase arrival time. The P PHASE P ICKER detects P ‐phase onset using the histogram method. Its performance is compared with picking techniques using short‐term‐average to long‐term‐average ratio, and a picking method that finds the first P ‐phase arrival time using the Akaike information criterion. A large set of records with various intensities and signal‐to‐noise ratios is used for testing the P PHASE P ICKER , and it is demonstrated that P PHASE P ICKER is able to more accurately pick the onset of genuine signals against the background noise and to correctly distinguish between whether the first arrival is a P wave (emergent or impulsive) or whether the signal is from a faulty sensor. Online Material: MATLAB script for P ‐phase arrival time picking.

Power System Stabilizers Design for Multimachine Power Systems Using Local Measurements

In this paper, a technique for designing fixed parameter decentralized power system stabilizers (PSSs) for interconnected power systems is proposed. In the proposed method, local information available at each machine in the multimachine environment, is used to tune parameters of PSS. Conventional design techniques such as P-Vr frequency response approaches, and the method of residues are based on complete system information, wherein phase angles of residues are consistent with the P-Vr phase response and can be used with confidence for design purposes. It is shown that magnitude and phase information of the proposed GEP TF agrees closely with that of P-Vr TF and yields a robust stabilizer. Nonlinear simulation and eigenvalue analysis show the efficacy of the proposed stabilizer to damp out the interarea and local modes of oscillations effectively over a wide range of operating conditions. Superiority of the proposed approach over the conventional approaches is demonstrated with simulation studies on two widely used multimachine systems.