Conventional Frequency Response Research Articles

Frequency response function of cracked Timoshenko beam measured by a distributed piezoelectric sensor

In the present report, a novel concept of frequency response function (FRF) is introduced for piezoelectric beam. First, a model of Timoshenko beam bonded with a piezoelectric layer is established and used for deriving the conventional frequency response function acknowledged as mechanical frequency response function (MERF). Then, the output charge produced in the piezoelectric layer is calculated from the MFRF and therefore obtained frequency-dependent function is called electrical frequency response function (EFRF) for the integrated beam. This concept of FRF depends only on exciting position and can be explicitly expressed through crack parameters. So that it provides a novel instrument to modal analysis and structural health monitoring of electro-mechanical systems, especially for crack detection in beams using distributed piezoelectric sensor. The sensitivity of EFRF to crack has been examined and illustrated in numerical examples for cracked Timoshenko beam.

A two-stage model updating method for the linear parts of structures with local nonlinearities

Finite element model updating provides an important supplement for finite element modelling. However, some studies have shown that if the tested structure involves local nonlinearities due to damages, material properties and large deformation et al., it is difficult to achieve an accurate modified model using conventional model updating methods that are based on the assumption of linear structures. To address this issue, a two-stage model updating method separating the effects of local nonlinearities is proposed in this paper. Firstly, the underlying linear frequency response function is obtained by using the conditioned reverse path method. Then, combined with the Sherman-Morrison-Woodbury formula and the model updating objective function established by the frequency response function similarity metric, then structural model updating and damage detection are carried out as the second stage. Three numerical examples are given to illustrate the effectiveness of the proposed method. This method can not only accurately identify the location and quantify the extent of structural damages, but also has the advantages of not based on sensitivity, not depending on the selection of frequency points, not repeatedly calling the initial model et al. The proposed method has high computational efficiency and avoids the numerical problems often encountered by conventional frequency response function-based model updating methods.

Research on source-load frequency response control strategy of distributed generation system

Abstract Distributed power generation, as an effective form of new energy power generation, has attracted extensive attention. However, due to the lack of physical inertia support, the stability of the distributed power supply system is threatened. Aiming at the problem of low inertia and weak damping in distributed systems, a source-load frequency response control strategy is proposed in this paper. Firstly, this paper introduces the structure and working mode of the distributed power generation system, and the system frequency characteristics are theoretically analyzed. Then, based on the operating characteristics of the conventional power system, the generator is simulated through the distributed power generation, the load-side converter is coupled to the grid frequency and further the source-load frequency response control strategy is achieved that taps the potential of frequency adjustment capability of the load side through frequency coupling control to provide auxiliary inertia support for the system, and this paper illustrates the source-load frequency adjustment response mechanism in detail. Compared with the conventional single frequency response control, the control strategy proposed in this paper can effectively alleviate the frequency adjustment pressure of distributed units, reduce the capacity of energy storage equipment due to frequency adjustment and improve the efficiency of system operation. Finally, through the simulation analysis of the distributed power generation system, the effectiveness of the control strategy proposed in this paper is verified.

Feasibility Analysis of EV+X Storage to Support Power System Frequency

With the fast growth of renewable energy generation, the power system faces the challenge of low inertia. Lower system inertia makes it more challenging to keep the frequency stable, and the conventional frequency response mechanism is not capable of ensuring frequency within the limit. In this paper, a new frequency response mechanism is proposed to help to improve the frequency performance, where electric vehicles (EV) are used as energy storage, and they will cooperate with existing primary frequency response (PFR) to form an EV+X storage supporting power system frequency. This approach is proposed based on rigorous mathematical derivation, where the relationship between frequency and active power is quantitively analysed. To validate the new mechanism’s feasibility, simulation models are built to simulate the frequency behaviour after a big disturbance, and a series of tests are conducted. Both technical and economic benefits are investigated, considering the difference in EV control strategies and the proportion of EV responses. The result shows that EV+X storage can be a promising solution to the frequency stability problem.

Combined heat and power operational modes for increased product flexibility in a waste incineration plant

The expected strong expansion of wind power may cause challenges for the electricity system in terms of grid stability, power balance, and increased electricity price volatility. This paper analyses how the new market conditions impact the operational pattern and revenue of a combined heat and power (CHP) plant. The work focuses on product flexibility that enables varied ratios between products; and thermal flexibility, to shift load in time given the differing timescales of heat and power demand. Product flexibility is given by five operational modes: conventional CHP, heat-only, CHP plus frequency response, condensing, and condensing plus frequency response. Optimization and process modeling are combined to study the plant dispatch in current and future electricity market scenarios and with thermal flexibility. The results indicate that load-shifting of heat generation together with condensing operation can increase revenue up to 4.5 M€ and plant utilization up to 100% for a 50 MWel waste-fired plant; but requires a thermal energy storage to meet hourly heat demand. The electricity price profile impacts both the revenue and operational patterns, with low-price periods favoring increased heat generation and frequency response delivery. High average electricity price and price volatility results in increased profitability of product and thermal flexibility.

Dynamic analysis and qft-based robust control design of a coaxial micro-helicopter

This paper presents the dynamic behavior of a coaxial micro-helicopter, under Quantitative Feedback Theory (QFT) control. The flight dynamics of autonomous air vehicles (AAVs) with rotating rings is non-linear and complex. Then, it becomes necessary to characterize these non-linearities for each flight configuration, in order to provide these autonomous air vehicles (AAVs) with autonomous flight and navigation capabilities. Then, the nonlinear model is linearized around the operating point using some assumptions. Finally, a robust QFT control law over the coaxial micro-helicopter is applied to meet some specifications. QFT (quantitative feedback theory) is a control law design method that uses frequency domain concepts to meet performance specifications while managing uncertainty. This method is based on the feedback control when the plant is uncertain or when uncertain disturbances are affecting the plant. The QFT design approach involves conventional frequency response loop shaping by manipulating the gain variable with the poles and zeros of the nominal transfer function. The design process is accomplished by using MATLAB environment software.

A Low-Cost Generator for Testing and Calibrating Current Transformers

Current transformers for ac power systems applications are usually calibrated by applying sinusoidal currents at different frequencies. The main limitation of this approach is that it does not take into account nonlinearities that may have a significant impact on harmonic current measurement. A more accurate characterization should be based on test currents that are similar to those found in normal working conditions. However, this demands a proper current generator able to generate multitone waveforms with proper accuracy, which is seldom available in laboratories. This paper proposes a simple approach to implement a current generator, based on a waveform generator, a power amplifier, and a transformer which allows boosting the output current capability. Its uneven input-output response can be compensated through linear prefiltering of the desired current waveform. When the generator is employed to reproduce a specific class of current waveforms, such as those typically found in ac distribution grids, an optimal compensation filter can be easily designed. This approach allows canceling systematic nonlinearities introduced by the transformer and the power amplifier, thus significantly increasing accuracy with respect to a conventional frequency response compensation, as confirmed by experimental results.

A unified stability analysis method for spontaneous oscillation condition s in thermoacoustic systems via measured frequency response data with application to standing- and traveling-wave engines

In this paper, a unified stability analysis method for thermoacoustic systems is proposed using the Nyquist stability criterion for MIMO (multi-input multi-output) causal stable open-loop systems. Toward this end, the conventional frequency response function in the thermoacoustic instability utilization literature, the transfer matrix, is replaced with a function for systems with a causal input–output relation between the incoming and outgoing traveling-wave pressure components, the scattering matrix, known in the thermoacoustic instability suppression literature, in order to represent whole subsystems including thermoacoustic cores and connected tubes as TITO (two-input two-output) stable open-loop systems. This makes it possible for the Nyquist stability criterion to be applied to the open-loop frequency response to analyze the closed-loop stability. The method provides a simpler procedure than the conventional one that is based on the Nyquist stability criterion for SISO (single-input single-output) systems in the thermoacoustic instability suppression literature because TITO open-loop systems can be assumed to be stable for thermoacoustic systems when traveling-wave pressure components are adopted as their input and output signals. It is shown that the spontaneous oscillation condition s for both standing- and traveling-wave thermoacoustic engines with various settings can be analyzed with a unified procedure, and the actual oscillation conditions are successfully estimated with the proposed method, indicating reasonable stability and/or instability margins in the same manner as in the Nyquist stability criterion for stable open-loop systems. That is, the feasibility of spontaneous oscillation is simply indicated by encirclement(s) of the origin by the Nyquist locus calculated from the measured frequency responses; multiple encirclements correspond to a distorted waveform in the measured pressure signal; the oscillation frequencies can be estimated by the outermost portion of the locus; and the distance between this portion and the origin corresponds to the steady-state pressure amplitude.

Diagnostic of transformer winding deformation fault types using continuous wavelet transform of pulse response

Impulse Frequency Response Analysis (IFRA) provides an alternative technique for detection of mechanical type faults in power transformers among researchers. However, the conventional frequency response curve obtained from the time domain transient response only contains the frequency domain information of signal, and the time domain information is neglected. Furthermore, the traditional approach of using the fast Fourier transform (FFT) in processing the non-stationary signal could result in lack of useful information. This paper proposes a novel winding deformation fault diagnostic technique which is modified from IFRA, the detected transient signals of pulse response are processed based on the continuous wavelet transform (CWT) algorithm, and the wavelet time-frequency diagram are plotted and analyzed. The simulation of a simple transformer high frequency model and experiment analysis show that the various types of winding deformation can be distinguished by the proposed technique. The data processing result by the wavelet transform conforms with the variation pattern of frequency response to different winding fault types. Besides, the verification result of experiment indicates that the wavelet time-frequency diagram obtained by the proposed technique shows better performance than the frequency response curves obtained by the conventional FFT on processing the transient signals.

On‐site calibration of air‐coil sensor for transient electromagnetic exploration

ABSTRACTThis paper proposes a time‐domain fitting method for on‐site calibration of the air‐coil sensor. The air‐coil sensor has been widely used in transient electromagnetic exploration. Due to limited bandwidth of the coil, the output signal is distorted, causing a phenomenon known as the transition process. To accurately measure the magnetic field from the output signals, the relationship between the coil induced electromotive force and the output voltage must be confirmed by on‐site calibration, which requires high calibration accuracy and demands simple operation, portable equipment, and adaptability to the environment. Conventional frequency response methods, however, requires a uniform magnetic field with various frequencies to obtain the frequency response curve of the air‐coil sensor. The time to acquire the signal correlates with the number of test frequencies, and the equipment used to generate a uniform magnetic field must be tailored to the shape of the air‐coil sensor under test. This paper constructs a relationship between the calibration file and the zero‐input response of the air‐coil sensor and designs an optimization algorithm to suppress the soil eddy current effect. This on‐site calibration method lifts the dependence on the uniform calibration field and reduces significantly the time required for calibration. The calibration source can be generated by cutting off the voltage source in parallel to the calibration coil, which greatly reduces the cost of the signal generator and provides a better solution for realizing the embedded self‐test devices. Experimental results show that the proposed method effectively improves the calculation accuracy of the apparent resistivity.

Frequency prediction method considering demand response aggregate characteristics and control effects

Due to an increase in intermittent renewable energy penetration, the mechanical inertia of power systems has gradually decreased, threatening system frequency stability. As one effective solution to this problem, demand response (DR) technologies, which enable large-scale residential loads to regulate system frequencies via load aggregators, have been widely used. Aggregated loads, while treated as a whole from the perspective of the system operator, present some aggregate characteristics related to the specificities of individual loads within each aggregator. To construct such a relationship, DR aggregate characteristics based on load heterogeneity in response latency and user comfort requirements are analysed in this paper. As a result, an aggregate model for available response capacity evaluation is constructed. To realize the effective utilization of DR resources, feasible control strategies based on aggregate characteristics are discussed and compared with an emphasis on frequency regulating effects. Furthermore, a two stage-based DR section related to control strategies is introduced into a conventional system frequency response model for post-disturbance frequency nadir prediction. Finally, simulations are performed to verify the validity and accuracy of the proposed method.

Time Effective Structural Frequency Response Testing with Oblique Impact

Structural frequency response testing is accurate in identifying the dynamic characteristic of a machinery structure. In practical perspective, conventional structural frequency response testing such as experimental modal analysis with technique (also known as impulse testing) has limitation especially on its long acquisition time. The high acquisition time is mainly due to the redundancy procedure where the engineer has to repeatedly perform the test in 3 directions, namely the axial-, horizontal- and verticalaxis, in order to comprehensively define the dynamic behavior of a 3D structure. This is unfavorable to numerous industries where the downtime cost is high. This study proposes to reduce the testing time by using oblique impact. Theoretically, a single oblique impact can induce significant vibration responses and vibration modes in all the 3 directions. Hence, the acquisition time with the implementation of the oblique technique can be reduced by a factor of three (i.e. for a 3D dynamic system). This study initiates an experimental investigation of testing with oblique excitation. A motordriven test rig has been used for the testing purpose. Its dynamic characteristic has been identified using the testing with the conventional normal impact and the proposed oblique impact respectively. The results show that the proposed oblique testing is able to obtain all the desired natural frequencies in all 3 directions and thus providing a feasible solution for a fast and time effective way of conducting the testing.

A method for the design of UFLS schemes of regional power system using improved frequency response model

Under frequency load shedding (UFLS) is an effective remedy against power system frequency collapses. An optimal approach for designing UFLS schemes of regional power system is presented in this paper. An improved multigenerator frequency response (IMFR) model that incorporates UFLS schemes is built to accurately and quickly evaluate system frequency after generator and load imbalance. Unlike the conventional system frequency response model, IMFR involves a spinning reserve, different types of governors, the static load model, and network influence. The representative operating and contingencies for designing UFLS schemes are selected with the use of IMFR. The optimization object function is defined for minimizing the amount of total load shed and considering UFLS design and performance criteria. Then, the optimal UFLS scheme is designed through IMFR and adaptive simulated annealing algorithm. Finally, the approach is applied to an actual regional power system in China. The effectiveness of the optimal UFLS scheme is investigated through a simulation.

Emulated Inertial Response From Wind Turbines: Gain Scheduling and Resource Coordination

With increasing wind penetration levels on power systems worldwide, conventional thermal plant, along with the ancillary services they provide are being displaced. To address this issue, modern variable speed wind turbines can offer a short-term controlled response to temporary power imbalances, by harnessing their stored rotational energy, a so called emulated, or synthetic, inertial response. However, unlike conventional frequency response services, the emulated inertial response is dependent on the wind turbine operating condition and provides a response which is distinct to that from synchronously connected plant. While much research has investigated the harnessing of such a response from wind turbines and its effect on system frequency, this paper investigates potential issues associated with the system-wide integration of the technology at scale. The ability for wind generation to provide frequency support through emulated inertia is assessed, as well as the potential for tuning such a distributed response resource to ensure timely and effective frequency recovery under different system conditions. It is shown that the frequency response capability of future systems and the operating protocols for system participants, including wind generation, will require development if emulated inertia technology is adopted at scale.

進行波圧力成分に関するシステム表現に基づく定在波型／進行波型熱音響システムに対する統一的な自励発振条件の解析

This paper presents a unied stability analysis method for thermoacoustic systems based on the Nyquist stability criterion for MIMO systems. The method consists of two procedures: (i) conventional frequency response function (transfer matrix) is transformed into the one for causal system whose input and output signals are incoming and outgoing traveling-wave pressure components; (ii) the transformed response is used to analyze the closed-loop stability by the Nyquist stability criterion for MIMO systems. It is shown that the stability analysis for both standing- and traveling-wave thermoacoustic engines can be carried out by a unied procedure. The analysis results are consistent with the actual situations of oscillation indicating reasonable stability and/or instability margins for various geometrical changes as well as the temperature ratios in engines.

Anomalous lasing of high-speed 850 nm InGaAlAs oxide-confined vertical-cavity surface-emitting lasers with a large negative gain-to-cavity wavelength detuning

The impact of a large negative quantum well gain-to-cavity etalon wavelength detuning on the static and dynamic characteristics of 850 nm InGaAlAs high-speed oxide-confined vertical-cavity surface-emitting lasers (VCSELs) was investigated. Three distinct lasing regimes were revealed in large square aperture (≥7 μm per side) devices with large detuning including: (1) an anomalous lasing via higher order Hermite–Gaussian modes at low forward bias current; (2) lasing via the lowest order Hermite–Gaussian modes at high bias current; and (3) simultaneous lasing via both types of transverse modes at intermediate bias currents. In contrast to conventional multimode VCSELs a two-resonance modulation response was observed for the case of co-lasing via multiple transverse modes with high spectral separation. The reduction in the oxide aperture area resulted in classical lasing via the lowest order modes with a conventional single-resonance frequency response.

Effect of Electrode Mixing Conditions on the Performance of Lithium-Ion Batteries Analyzed by Fast Fourier Transform Electrochemical Impedance Spectroscopy

High power and energy density lithium-ion batteries (LIBs) with high charge/discharge rate are required for electric vehicles. Although it is inevitable to clarify the appropriate electrode manufacturing methods to enhance the performance of LIBs, there have been many uncertain points. In the present study, we investigate the influence of positive electrode mixing conditions on the performance of LIBs at 30 °C. Test cells were assembled with positive electrodes of LiCoO2 mixed with acetylene black as conductive filler and PVDF binder, electrolyte solution of ethylene carbonate (EC)/dimethylcarbonate (DMC) (1:1 vol.) containing 1 M LiPF6, and negative electrodes of graphite mixed with PVDF binder.We employed fast Fourier transform electrochemical impedance spectroscopy (FFT-EIS) to analyze the internal resistances under discharge to take advantage of its short measurement time since measurable decrease in the state of charge (SOC) cannot be avoided during EIS measurements with the conventional frequency response analyzer (FRA). Cross-sectional scanning electron microscope (SEM) observations of the positive electrodes were also carried out.Pre-mixing of LiCoO2 particles and acetylene black under an appropriate mill rotation speed is effective for the performance enhancement. In both cases without the pre-mixing and with pre-mixing under too high mill rotation speed, the discharge capacities decrease.The impedance spectrum by FFT-EIS exhibits two semi-circles at 1000 mA/g. In the case without the pre-mixing, the high frequency arc becomes significantly large, while both high and low frequency arcs become significantly large in the case with pre-mixing by too high mill rotation speed.The appropriate pre-mixing possibly thus decreases the resistance of electron transfer, i.e. contact resistance, between the LiCoO2 particles and acetylene black by good dispersion increasing the contact between them. This view corresponds to the assignment that the high frequency arc is attributed to the surface insulating layer of LiCoO2 particles[1,2] contacting with acetylene black. The SEM observations support this view.On the other hand, the pre-mixing by too high mill rotation speed possibly increases the contact among LiCoO2 particles, decreasing the contact between the LiCoO2 particles and acetylene black. This also decreases the contact between the LiCoO2 particles and the electrolyte. This view agrees with the assignment that the high frequency arc is ascribed to the surface insulating layer of LiCoO2 particles described above while the low frequency arc represents the Li ion charge transfer at the interface between the LiCoO2 particles and the electrolyte[1,2]. The SEM observations support this view also in this case.

Invited Presentation: Impedance Analysis of Lithium-Ion Battery for Future

Batteries have become one of the most important devices because green vehicles, such as hybrid electric vehicles, battery electric vehicles, and even fuel cell electric vehicles, mount batteries. Also, solar power and wind power plants require batteries, in which the power leveling are demanded to stabilize electrical power systems. In order to operate the batteries in those systems, an understanding of the state of batteries, such as the state of charge and state of health of batteries, is very important for effective and safe operation of the batteries. Therefore, a non-destructive analysis of the condition is in high demand for the premonitory diagnosis of the on-board batteries of electric vehicles, and of installed batteries in load leveling systems. Electrochemical impedance spectroscopy (EIS), in which a small-amplitude sinusoidal potential perturbation is applied to a cell, is one of the most powerful tools for the non-destructive analysis of energy devices such as lithium-ion batteries (LIB) 1-4]and fuel cells 5,6. However, the capacity and the internal resistance of large-scale LIBs become higher and lower, respectively, for the application to electric vehicles and large-scale power storage systems: EIS using conventional frequency response analyzer (FRA) – potentiostat systems is not easy to measure the impedance of the LIB because of its low internal resistance. In this situation, we focused on the power controller of LIB system to create a signal for analyzing the battery health. In the present study, application of square wave potential/current for input signals of EIS was investigated firstly in simple electrochemical reaction to verify a new technique called “Square-potential/current electrochemical impedance spectroscopy (SP-EIS, SC-EIS)” which is a method for EIS without using FRA systems. And then, we applied SC-EIS to evaluate a degradation of commercial LIB with the chargedischarge cycling.Firstly, a redox reaction of Fe(Ⅱ)/(Ⅲ) was investigated as a simple electrochemical reaction. On the basis of the technique of fourier transform in ref. 7, 8, EIS was carried out using square potential input. Figure 1A shows Nyqust plots of [Fe(CN)6]4-/[Fe(CN)6]3- electrolyte using square potential input generated by potentiostat. The value of frequency response as ten times higher than that of input frequency was successfully obtained. The value was as almost same as that was obtained by the FRA system. From Fig.1B, applying this method to LIB, we successfully obtained frequency response using several square current inputs generated by a power controller.A commercially available laminated LIB with a nominal capacity of 5 Ah for an electric power assisted bicycle was also evaluated. As the results, fine Nyquist plots could be obtained by means of SC-EIS as well as that by means of the FRA system even in the case of degraded LIBs. Specifically, there was a good accordance between Nyquist plots obtained by SC-EIS on an LIB cell and those obtained by EIS using the FRA system with the error of each plot being less than 3% in the range of 1 kHz – 1 Hz. Therefore, the SC-EIS must be a method of great promise for widespread use in society.

Development of Galvanostatic Fourier Transform Electrochemical Impedance Spectroscopy

Here, we report development of the galvanostatic Fourier transform electrochemical impedance spectroscopy (FTEIS), which monitors impedance of electrochemical reactions activated by current steps. We first derive relevant relations for potential change upon application of a step current, obtain impedances theoretically from the relations by simulation, and verify them with experimental results. The validity of the galvanostatic FTEIS technique is demonstrated by measuring impedances of a semiconductive silicon wafer using the conventional frequency response analysis (FRA), the potentiostatic FTEIS, and the galvanostatic FTEIS methods, and the results are in excellent agreement with each other. This work is significant in that the galvanostatic FTEIS would allow one to record impedance changes during charge/discharge cycles of secondary batteries and fuel cells as well as electrochemically irreversible systems which may produce noise level chronoamperometric currents by potentiostatic techniques.

SETTLING TIME MINIMIZATION OF THREE-STAGE CROSSED FEEDFORWARD REVERSED NESTED-MILLER AMPLIFIER

Conventional frequency response compensation methods for three-stage amplifier are not appropriate for fast transient response viewpoint. Therefore, herein, we have utilized the minimum settling time (MST) approach to compensate the crossed feed-forward reversed nested Miller compensation (CFRNMC) amplifier which was previously compensated by the conventional frequency response methods. Compensation rules for the considered amplifier are proposed based on time-domain parameters. The three-stage amplifier is designed in a 90 nm CMOS technology with the power supply of 1 V. The amplifier drives the capacitor load of 100 pF and its closed-loop time response to a unit-step input has a settling time less than 34 ns with 1% settling error.