Frequency Scanning Method Research Articles

A simple mechanical model of turbulence

This work examines the control and stabilization problems of vibrations in a hierarchical chain of oscillators with hysteresis couplings. Hysteresis coupling is formalized within the Bouc — Wen phenomenological model. The mass, stiffness, and damping properties of the oscillators are set to follow a specific scaling rule and decrease exponentially along the chain, thus forming a hierarchy. The model is verified using Kolmogorov’s hypotheses. To do this, energy spectra are constructed under hysteresis in coupling and without it at different amplitudes of the external excitation. As a result of computational experiments, it is shown that for a chain with hysteresis couplings at a high amplitude of excitation, the energy spectrum curve sufficiently corresponds to Kolmogorov’s hypotheses. The amplitude-frequency characteristics of the system are calculated under hysteresis in coupling using the frequency scanning method. In numerical experiments, frequency ranges of external excitation are identified, which correspond to the chaotic behavior of oscillators and their synchronization.

Towards Time-Resolved Multi-Property Measurements By Single-Frequency FRS

The filtered Rayleigh scattering technique (FRS), extended by the method of frequency scanning, has historically been limited to time-averaged multi-property flow measurements. In our recently published work, we present a concept that potentially enables the combined measurement of time-resolved pressure, temperature and three-component (3C) velocity fields. It is based on the observation of the region of interest from six perspectives and a single excitation frequency. This work summarizes and expands on a follow-up publication that experimentally verifies this concept on an aspirated circular duct flow. For this purpose, the results obtained from single-frequency data processing are compared with reference pressures, temperatures and corresponding LDA velocity measurements. Overall, a very good agreement is found for all operating points with accuracies of 3.4% in pressure, 1.3% in temperature and ±2 m/s in axial velocity. Concerning precision, a newly developed multistage evaluation procedure enables values for pressure, temperature and velocity as low as 3 hPa, 2.2 K and 1.7 m/s. In a second flow configuration, an axial swirler is introduced into the duct. The resulting secondary flow structure and deformation of the axial velocity field caused by swirler geometry and support are very well captured with the single-frequency analysis. A closing discussion on the implementation challenges of a single-frequency multi-property FRS instrument with pulsed laser radiation reveals significant obstacles to overcome. Due the considerable optimization potential identified, chances are high that true time-resolved multi-property measurements by FRS will become a reality.

Stability Enhancement Method of Standalone Modular Multilevel Converters Based on Impedance Reshaping

Modular multilevel converters (MMCs) are susceptible to subsynchronous oscillations (SSOs) caused by impedance interactions in the power line. Current research into the stability of MMCs focuses mainly on voltage feed-forward control, while the effect of current feed-forward control is neglected. This paper proposes a current feed-forward compensation method based on impedance reshaping for standalone MMCs. Initially, an impedance model was developed to identify the stability risks caused by the interaction between the MMC and power line impedance. The proposed method feeds the current compensation signal into the modulation circuit, thereby improving the control signal and suppressing the impedance interaction between the MMC and the power line. The analysis of the harmonic state space (HSS) method verifies that the proposed approach effectively reduces the negative damping region in the frequency band where the SSO is located. Additionally, the impedance frequency scanning method confirms the accuracy of impedance modeling. Using the MATLAB/Simulink platform and StarSim HIL hardware-in-the-loop experimental platform, the SSO phenomenon of the MMC is simulated, and the results show that the proposed method can effectively suppress harmonic currents during SSO, which verifies the accuracy of the stability analysis and the feasibility of the proposed method.

Determination of dielectric properties of plaster blocks for sealing masonry using non-destructive frequency scanning methods

With the increasing use of traditional and new models of wireless communication systems, the study and determination of the electrical characteristics of materials used in civil construction is an important topic to establish an understanding of how the radio frequency signal behaves inside built environments. This study presents an extensive process of characterization of electrical parameters of plaster blocks used in the construction of walls. Different from the literature where a prior estimation of the data occurs to enable sampling parameters to be obtained, this work proposes an innovative way of obtaining them entirely based on the analysis of the material through frequency measurements, which results in a greater level of precision of the data results. Analyses are made in the frequency range from 0.7 to 5.2 GHz, which has been used for several wireless communication standards. To carry out the electrical characterization of dielectric materials, a non-invasive methodology is proposed based on an innovative combination of the Nicolson–Ross–Weir Method (NRW) and the Ray Tracing Method. Through the proposed methodology and an extensive campaign of measurements using frequency scanning equipment, calculated and experimental data of Shielding Effectiveness (SE), complex relative electrical permittivity, loss tangent, attenuation coefficient and conductivity were obtained for a wide range of frequency, considering different samples of plaster blocks. The obtained results are compared to those available in the related literature, confirming the accuracy of the proposed analysis.

A novel visual representation method for multi-dimensional sound scene analysis in source localization problem

A visual representation method, DoAgram, for multi-dimensional sound scene analysis is suggested. The visual representation of the sound source localization result gives intuitive information about the estimated one to the end user. Also, image-based deep learning is popularly used in the acoustic field nowadays, so such a visualized one can be used for data augmentation. To analyze the spatial sound scene for the moving source, the method displays the estimated azimuth angle and elevation angle of the source, and its corresponding time stamp and frequencies as RGB color channels and metadata by mapping the spatial coordinate to color space. Even though the suggested method is human-interpretable, decoding is needed for the quantitative analysis. Therefore, the time and frequency scanning method, and a histogram to estimate the DoA of the source are proposed. An experiment is conducted in an anechoic chamber to localize two quadcopter drones that have a mean angular velocity of 8°/s ± 9°/s (95 % CI) and 25°/s ± 31°/s (95 % CI), respectively, and the spatial sound scene analysis is implemented using the proposed methods. The test result shows that the trajectories with respect to the time of each source are well separated. Also, an additional test is conducted using an open-access audio dataset for machine learning. The cumulative source mapping method is adopted for the spatial sound scene analysis, and the decoded result shows that the DoAgram is feasible to adopt for machine learning applications.

Investigation of the Feasibility of the Dynamic Equivalent Model of Large Photovoltaic Power Plants in a Harmonic Resonance Study

In recent years, there have been several harmonic resonance accidents around the world that involve renewable energy power plants. The frequency scanning method is the most widely used technique in engineering practice to evaluate the severity of resonance due to its simple operation and clear physical meaning. However, when establishing electromagnetic transient simulation models and conducting frequency scans, a single generation unit or a few renewable generation units are usually used to represent the original power plant for the purpose of reducing model complexity and improving the simulation efficiency. Such a practice has been found to be effective in dynamic studies around the fundamental frequency. However, its feasibility in harmonic resonance studies has not yet been fully investigated. Because of this research gap, a feasibility study is conducted in this paper by using a real-life photovoltaic power plant. The detailed harmonic model of the plant is first established using the harmonic linearization method, and the equivalent harmonic model is then developed using the power loss conservation method. The feasibility of the equivalent model was investigated in detail, and the impact of the different impedance models on the resonance analysis was analyzed. The results indicate that the conventional dynamic equivalent model can effectively reflect the harmonic resonance characteristics of photovoltaic power plants. Furthermore, a more simplified model that ignores the inductance of the collector line is recommended in this paper to further reduce the modeling complexity.

A new sequence domain EMT-level multi-input multi-output frequency scanning method for inverter based resources

A new sequence domain EMT-level multi-input multi-output frequency scanning method for inverter based resources

A new stationary frame multi-input multi-output EMT-level frequency scanning method for inverter based resources

Impedance-based stability analysis (IBSA) is an effective method to identify subsynchronous interaction (SSI) problem between the inverter-based resources (IBRs) and series compensated or weakly tied AC grids. The electromagnetic transient (EMT) level positive sequence and dq-frame frequency scanning methods (p-scan and dq-scan, respectively) are used to obtain the sequence single-input single-output (SISO) and dq multi-input multi-output (MIMO) impedance of IBRs, respectively. The dq MIMO impedance usage in IBSA provides more accurate results compared to the sequence SISO impedance. This paper proposes an EMT-level αβ-frame frequency scanning method (αβ-scan) to obtain the αβ MIMO impedance and its usage in IBSA. The αβ-scan requires significantly less time compared to dq-scan for the convergence of MIMO IBR impedance representation and offers similar accuracy with dq-scan in IBSA. The accuracy of the proposed αβ MIMO IBSA is validated by comparing with dq MIMO IBSA and EMT simulations on series capacitor SSI cases with different type IBRs (Doubly-fed induction generator (DFIG)-based wind park (WP) and full-scale converter (FSC)-based WP). This paper also uses a multivariable structure function(MSF)-based method for the first time in IBSA of SSI to achieve Bode plot-based analysis for providing better visualized presentation of resonance frequency and stability margins compared to generalized Nyquist criterion.

A Gray-Box Stability Analysis Method of Grid-Connected Inverter Considering Synchronization Dynamics

The Grid-Connected Inverter (GCI) can be considered a gray box when circuit and controller parameters are missing due to intellectual property rights or parameter variations caused by aging, which poses an impediment to assessing the stability of the system. This paper presents a gray-box stability analysis method based on impedance identification of GCI considering the synchronization dynamics. The impedance frequency responses of GCI are measured by the frequency scanning method on the dq-frame. Meanwhile, the influence of synchronization dynamics and background harmonics is theoretically investigated. A vector fitting (VF) algorithm, co-designed with impedance identification, is then applied to generate polynomial transfer functions. Based on the obtained transfer functions, the stability of the GCI can be judged by the distance relationship between the prohibited area boundary and the center of the gershgorin-circle through the distance formula. Finally, the experiments of both RT-LAB and experimental prototypes are conducted to verify the feasibility of the proposed method.

Harmonic stability analysis of single‐phase converters in electric multiple units based on an impedance model with harmonic linearization

AbstractLow‐frequency oscillation, harmonic oscillation and harmonic instability issues in railway vehicle‐grid systems pose challenges to the system stability and safe operation. Previous studies constructed the linear time‐invariant impedance model of vehicle‐grid system, which ignored the internal frequency coupling and loss of model accuracy. To fill this gap, the impedance modelling method based on the harmonic linearisation method of the vehicle‐grid system is proposed. The derived model provides a clear physical insight into the harmonic coupling relations between ac and dc variables and improves the model accuracy. Then, a systematic analysis of harmonic stability in the railway vehicle‐grid system is analysed based on the proposed model. The electrical and control parameters on system stability at the frequencies above and below the fundamental frequency are summarised. The mechanisms of these oscillation issues in railway vehicle‐grid systems are then discussed. Finally, the model accuracy and stability analysis results are verified by the frequency scan method and hardware‐in‐the‐loop results.

Passivity Enhancement Strategy of Grid-Connected Inverter System Based on the Adaptive Active Damper

The grid-connected inverter system may be unstable when connected to a weak grid, and installing an active damper as a virtual resistor at the PCC is convenient for stabilizing the system. This article proposes a passivity enhancement strategy for the grid-connected inverter system via the adaptive active damper. Furthermore, the admittances of the grid-connected inverter system with the active damper are derived, and the real parts of the admittances of the whole system have been calculated. And the admittance measurement results obtained by the frequency scanning method approximately match the theoretical results. Additionally, the virtual resistor’s stability region is discussed, the bandwidth of the phase-locked loop is recommended, ensuring the stabilizing effect of the active damper under the weak grid. And the online resonance frequency detection algorithm guarantees the passivity enhancement of the active damper when grid impedance changes. Finally, controller hardware-in-the-loop tests validate the theoretical analysis of passivity enhancement based on the active damper.

Impact of Pseudo-Random Modulation on Measured Conducted EMI

This paper demonstrates that using conventional frequency scanning methods for the evaluation of conducted electromagnetic interference generated by DC/DC converters can give misleading results when pseudo-random modulation is used as part of the converter control scheme. The paper therefore proposes that the resolution bandwidth of the super heterodyne EMI test receiver used for measurements and the dwell time should be adjusted to match the control parameters of the random modulation scheme. The use of different values for resolution bandwidth and dwell time is demonstrated and measurements for random and fixed modulation schemes are compared.

Fast Peak Searching Method for Brillouin Gain Spectrum Using Positive-Slope Inflection Point

A Fast peak searching method for Brillouin gain spectrum (BGS) using positive-slope inflection point is proposed. In this method, the positive-slope inflection point of Brillouin gain spectrum is used to terminate the frequency scanning (FS) progress adaptively and determine the Brillouin frequency shift (BFS). The number of frequency scanning points and data acquisition time is effectively reduced. The basic measurement principle, algorithm, and performance evaluation are presented. Based on experimental results over 9.91km-long optical fiber, the measurement time by conventional FS method is 50.13s, whereas it is only 15.62s by fast peak searching method. Under the same hardware, RMSE of measured BFS is 0.67MHz, which is nearly the same as the results obtained by conventional FS method. When the signal-to-noise ratio at the end of optical fiber (<i>SNR</i>_E) is within a reasonable range (i.e., <i>SNR</i>_E &gt; 2.4dB), the proposed method can effectively save processing time. The measurement speed of fast peak searching method is increased by about three times than that of conventional FS method, nearly without the loss of measurement accuracy.

Turnkey generation of Kerr soliton microcombs on thin-film lithium niobate on insulator microresonators powered by the photorefractive effect

Generation of Kerr soliton microcombs on microresonators enables power-efficient, phase-coherent, and broadband frequency teeth generation, which has revolutionized a wide range of scientific areas such as astronomy, metrology, spectroscopy, communications, etc. However, compared with a conventional frequency scanning method that requires a complex start-up and feedback control, turnkey generation of soliton microcombs remains challenging and a more promising solution is desired. Here, we propose for the first time and numerically demonstrate that turnkey generation of soliton microcombs can be achieved on thin-film lithium niobate on insulator (LNOI) microresonators for polarization along the ordinary axis of lithium niobate (LN) for which the photorefractive (PR) effect dominates. The PR effect shows power-dependent refractive index change, which is strong and opposite to that of the Kerr effect and thermal effect, thus enables the self-routing and converge of the total pump-resonator detuning into the existence region of soliton. Our results show that initiated with a certain amount of initial pump-resonator detuning on either blue- or red-detuned side, generation of soliton microcombs can self-start, self-route, and finally get stable without any artificial frequency scanning. Moreover, we show that deterministic and turnkey generation of single soliton microcombs can be achieved by leveraging a phase-modulated pump laser. Thanks to the inherent electro-optic effect of LNOI, a lab-on-a-chip device with monolithically integrated high-speed phase modulators and high-Q microresonators is feasible.

Through-Plane and In-Plane Thermal Diffusivity Determination of Graphene Nanoplatelets by Photothermal Beam Deflection Spectrometry.

In this work, in-plane and through-plane thermal diffusivities and conductivities of a freestanding sheet of graphene nanoplatelets are determined using photothermal beam deflection spectrometry. Two experimental methods were employed in order to observe the effect of load pressures on the thermal diffusivity and conductivity of the materials. The in-plane thermal diffusivity was determined by the use of a slope method supported by a new theoretical model, whereas the through-plane thermal diffusivity was determined by a frequency scan method in which the obtained data were processed with a specifically developed least-squares data processing algorithm. On the basis of the determined values, the in-plane and through-plane thermal conductivities and their dependences on the values of thermal diffusivity were found. The results show a significant difference in the character of thermal parameter dependence between the two methods. In the case of the in-plane configuration of the experimental setup, the thermal conductivity decreases with the increase in thermal diffusivity, whereas with the through-plane variant, the thermal conductivity increases with an increase in thermal diffusivity for the whole range of the loading pressure used. This behavior is due to the dependence of heat propagation on changes introduced in the graphene nano-platelets structure by compression.

Harmonic Resonance Identification and Mitigation in Power System Using Modal Analysis

Due to a rising share of power electronic devices in power networks and the consequent rise in harmonic distortion, impedance resonances are an important issue. Nowadays, the frequency scan method is used for resonance phenomena identification and analysis. The main disadvantage of the method is its inability to decouple different resonance phenomena. This means that the method is also unable to provide sufficient information about the effects that the parameters of network elements have on different resonance phenomena. Furthermore, it was also noted that despite the fact that the harmonic resonance mode analysis is well described in the literature, there is a lack of systematic approach to the analysis procedure. Thus the main objective of this paper is to address this disadvantage and to propose a systematic approach to harmonic resonance analysis and mitigation, utilizing modal analysis. In the first part of the paper, dominant network nodes in terms of resonance amplification of harmonics are determined. This is done by analysis of the eigenvalues of the network admittance matrix. Using the eigenvalue analysis results, key parameters of network elements involved in a specific resonance are determined next. This is performed by calculating the critical mode (i.e., the mode that experiences resonance) sensitivity coefficients with respect to network parameters. In the second part of the paper, the procedure for modal resonance frequency shift is presented. The shift is performed by changing the value of a selected parameter so that the modal resonance frequency matches the desired resonance frequency value. The parameter value is calculated with the Newton–Rhapson method. Presented analysis considers both parallel and series resonances. The effectiveness of the proposed method is demonstrated on an actual industrial-network model.

Dynamic equivalent modeling for power converter based on LSTM neural network in wide operating range

The internal parameters and topology of the power converter are unknown in some practical cases. Existing modeling methods based on impedance frequency scanning method can only guarantee that the dynamic modeling is effective at a single working point. To make the established dynamic model effective in a wide range, an equivalent modeling method for power converter based LSTM Neural Network is presented. At first, the equivalence of black-box modeling problem and deep loop neural network is studied. Then, dynamic modeling method black-box power converter on wide operating range by using LSTM neural network is proposed. Finally, the simulation results under large disturbance and multi-operating points show that the proposed method is effective under wide operation range.

Detection of and Countermeasure Against Thermal Covert Channel in Many-Core Systems

The thermal covert channels (TCCs) in many-core systems can cause detrimental data breaches. In this article, we present a three-step scheme to detect and fight against such TCC attacks. Specifically, in the detection step, each core calculates the spectrum of its own CPU workload traces that are collected over a few fixed time intervals, and then it applies a frequency scanning method to detect if there exists any TCC attack. In the next positioning step, the logical cores running the transmitter threads are located. In the last step, the physical CPU cores suspiciously engaging in a TCC attack have to undertake dynamic voltage frequency scaling (DVFS) such that any possible TCC trace will be essentially wiped out. Our experiments have confirmed that on average 97&#x0025; of the TCC attacks can be detected, and with the proposed defense, the packet error rate (PER) of a TCC attack can soar to more than 70&#x0025;, literally shutting down the attack in practical terms. The performance penalty caused by the inclusion of the proposed DVFS countermeasures is found to be only 3&#x0025; for an <inline-formula> <tex-math notation="LaTeX">$8\times 8$ </tex-math></inline-formula> many-core system.

A Robust Circuit and Controller Parameters’ Identification Method of Grid-Connected Voltage-Source Converters Using Vector Fitting Algorithm

This article presents a vector fitting (VF) algorithm-based robust circuit and controller parameters’ identification method for grid-connected voltage-source converters (VSCs). The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$dq$ </tex-math></inline-formula> -domain impedance frequency responses (IFRs) of the VSCs are first measured using the frequency scanning method, based on which the corresponding measured phasor-domain IFRs are calculated. Then, polynomial transfer functions are generated by applying the VF algorithm on the measured phasor-domain IFRs, from which the circuit and controller parameters, i.e., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$LCL$ </tex-math></inline-formula> filter parameters, digital sampling time, current controller parameters, and phase-locked loop parameters, are identified. The influence of measurement noise on parameters’ identification accuracy and corresponding countermeasure to mitigate the adverse influence are also theoretically investigated. The proposed method is able to identify the circuit and controller parameters when detailed parameters are missing due to industrial secrecy or parameters variation caused by operating condition change, temperature fluctuation, or aging. The effectiveness of the proposed circuit and controller parameters’ identification method is validated by theoretical demonstration, OPAL-RT-based real-time simulation, and experimental validation.

Development of Inplantable Surface Acoustic Wave Sensor for High Voltage Cable Core Temperature Monitoring in Intermediate Joint

In this paper, a kind of implantble passive wireless surface acoustic wave (SAW) sensor and its reader are developed to measure the operation temperature of high voltage crossed-linked cable core. A small type SAW temperature sensor is embedded into cable intermediate joint. A radio frequecny transceiver of the reader based on discrete devices is optimally designed to achieve high signal-to-noise ratio and receiving sensitivity. The characteristic of resonant frequency and temperature of SAW sensor is determined by frequency scan method. Through AC withstand voltage and temperature rise test of 110kV cable, it proves that the prototype can accurately measure the operation temperature of 110kV cable core in intermediate joint under high operation temperature and strong electromagnetic environment.