Low-frequency Section Research Articles

Fast Low Frequency Fault Location and Section Identification Scheme for VSC-Based Multi-Terminal HVDC Systems

Penetration of DC networks is rapidly increasing in modern power systems. As transient behavior of DC grids is different from that of AC ones, AC grid protection schemes might not be applicable to protect DC networks. Exact fault location is vital for protection of DC networks to reduce repair cost and achieve fast power recovery. This paper proposes a novel topology independent multi-terminal DC scheme to identify fault section and location for DC transmission systems. Voltage and current signals measured at the terminals are analyzed by the least squares error (LSE) algorithm. Then, the proposed scheme uses low frequency components of the signals to locate DC faults offline. Unlike some other methods, it does not require sophisticated sensors and hardware to capture very high frequency content of the signals. Hence, the proposed scheme can be implemented in practice without additional measuring infrastructure. The performance of the proposed scheme is evaluated under various fault conditions for different topologies of DC networks in both overhead lines and cables. Numerous simulations carried out demonstrate that the proposed scheme is quite accurate and provides excellent performance under different fault resistances, sections, and locations.

An extended nonlinear Schrödinger equation for water waves with linear shear flow, wind, and dissipation

Based on potential flow theory, governing equations are developed for surface gravity waves affected by wind, dissipation, and a linear shear flow (LSF). The LSF is composed of a uniform flow and a shear flow with constant vorticity. Using the multiple-scale analysis method, a two-dimensional nonlinear Schrödinger equation (NLSE) describing the evolution of freak waves in water of finite depth is derived by solving the governing equations. The modulational instability (MI) of the NLSE is analyzed, and it is shown that uniform up-flow and positive vorticity require a lower angular frequency to sustain the MI than uniform down-flow and negative vorticity. Moreover, the low-frequency section requires stronger wind velocity to sustain the MI than the high-frequency section. In other words, young waves are more sensitive to the MI than old waves. In addition, the free surface elevation of freak waves as a function of time is examined for different uniform flows, vorticities, and wind forcing, and the results are compared with a measured freak-wave time series from the North Sea. It is found that the theory agrees with the observations. Furthermore, the LSF affects the height and steepness of freak waves, while wind forcing affects their symmetry. Hence, the MI, wave–current interactions, and wind–wave interactions may be responsible for generating freak waves in realistic ocean scenarios.

An Adaptive Kalman Filter Bank for ECG Denoising.

Model-based Bayesian frameworks proved their effectiveness in the field of ECG processing. However, their performances rely heavily on the pre-defined models extracted from ECG signals. Furthermore, their performances decrease substantially when ECG signals do not comply with their models- a situation generally occurs in the case of arrhythmia-. In this paper, we propose a novel Bayesian framework based on Kalman filter, which does not need a predefined model and can adapt itself to different ECG morphologies. Compared with the previous Bayesian techniques, the proposed method requires much less preprocessing and it only needs to know the location of R-peaks to start ECG processing. Our method uses a filter bank comprised of two adaptive Kalman filters, one for denoising QRS complex (high frequency section) and another one for denoising P and T waves (low frequency section). The parameters of these filters are estimated and iteratively updated using expectation maximization (EM) algorithm. In order to deal with nonstationary noises such as muscle artifact (MA) noise, we used Bryson and Henrikson's technique for the prediction and update steps inside the Kalman filter bank. We evaluated the performance of the proposed method on different ECG databases containing signals having morphological changes and abnormalities such as atrial premature complex (APC), premature ventricular contractions (PVC), Ventricular Tachyarrhythmia (VT) and sudden cardiac death (SCD). The proposed algorithm was compared with several popular ECG denoising methods such as wavelet transform (WD) and empirical mode decomposition (EMD). The comparison results showed that the proposed method performs well in the presence of various ECG morphologies in both stationary and non-stationary environments especially at low input SNRs.

Parameters Identification and Application of Equivalent Circuit at Low Frequency of Oil-Paper Insulation in Transformer

The low frequency section of frequency domain spectroscopy (FDS) of oil-paper insu- lation system can effectively reflect the ageing state and moisture of the oil-immersed paper. Meanwhile, steady-state insulation resistance is the one of indexes for the condition of transformer. In actual measurement, these two both are time-consuming and poor accessibility, thereby restraining their field applications. Aim at this problem, this paper analyses the polarization process of the oil-paper insulation in transformer, and adopts a low frequency equivalent circuit model for the oil-paper insulation to characterize the polarization process at low frequency. The simulated annealing particle swarm optimization (SAPSO) algorithm is proposed to identify the parameters in equivalent circuit, which are affected by change of insulation state. Compared with the experimental results, the errors of the steady-state insulation resistance and the tan $\delta $ below 10−3 Hz, which are calculated by the identified parameters, are less than 2.5%. This method can avoid the influence from external environment and reduce the measurement time. The time and frequency domain parameters calculated can provide reference for the ageing assessment of oil-immersed paper.

<italic>N</italic>-Tuple Flying Capacitor Multicell Converter—A Generalized Modular Hybrid Topology

This paper presents a generalized topology for multilevel converters based on flying capacitor multicell structure. The proposed topology consists of two modular high-frequency (HF) and low-frequency (LF) sections or cells. The HF cells consist of flying capacitors and semiconductor switches operating at carrier frequency. The LF cells consist of dc sources and semiconductor switches operating at line frequency and its multiples. The proposed topology can be implemented with arbitrary number HF and LF cells. An important advantage of the proposed topology is its reduced components when compared to other multilevel converter topologies in the same family. Another interesting feature is the relatively low blocking voltage requirement for HF switches, making it an attractive choice for high-power applications. A modulating algorithm is devised to generate the gating pulses for the HF and LF cells using phase-shifted carrier-based pulsewidth modulation. As a particular case, the operation of a 25-level topology is explained by the help of illustrating waveforms. A comprehensive comparative study is presented to illustrate the advantages of the proposed topology in terms of the number of components and other performance criteria. Various simulations are carried out to verify the operation of the proposed topology at different operating conditions. The theoretical analysis is experimentally verified using a scaled-down laboratory setup.

Accelerating frequency domain dielectric spectroscopy measurements on insulation of transformers through system identification

Frequency domain dielectric spectroscopy (FDS) is a non-destructive method widely used to evaluate the state of dielectric materials. Traditional FDS measurement is conducted in frequency sweep mode between 1 kHz and 1 mHz. However, this method requires long measurement time when sweeping in low frequencies (1 mHz–1 Hz), thereby limiting its field applications. Hammerstein model is selected as the mathematical model for insulation systems according to known property of dielectric and established based on system identification theory. An equivalent mathematical model fitting >97% with the insulation system is obtained. The equivalent model can be used to calculate dielectric response parameters and construct an FDS. The proposed method requires 10 min to obtain dielectric response parameters under any frequency excitation in the low-frequency section. The measurement time decreases by up to 90% compared with that of traditional FDS. Comparison experiments are conducted between system identification method and traditional FDS measurement applied to different insulation systems, such as insulation pressboards, testing transformers, and field power transformers. Results show that frequency responses are consistent between the equivalent model and insulation system, ensuring the accuracy of FDS curve obtained by the proposed method.

Distributed Source Coding of Hyperspectral Images Based on Three-Dimensional Wavelet

To reduce the possibility of poor efficiency and weak anti-error capability while encoding and transmitting hyperspectral images, we present a distributed source coding scheme for hyperspectral images based on three-dimensional (3D) set partitioning in hierarchical trees (SPIHT). First, the 3D wavelet transform is performed on the hyperspectral image. Thereafter, the low frequency section is regarded as the Key frame and the high frequency section as the Wyner–Ziv frame to enable independent SPIHT coding through different transmission channels. The Wyner–Ziv encoder uses Turbo channel coding to create high frequency information that reflects the details of the image with better anti-error capacity, while the low frequency information shows the main energy of the image. In this study, we used SPIHT coding to acquire a bit stream with quality scalability. Results show that the proposed scheme is more efficient during coding, while at the same time providing improved anti-error capability and quality scalability of the bit stream.

High-Precision Spectral Decomposition Method Based on VMD/CWT/FWEO for Hydrocarbon Detection in Tight Sandstone Gas Reservoirs

Seismic time-frequency analysis methods can be used for hydrocarbon detection because of the phenomena of energy and abnormal attenuation of frequency when the seismic waves travel across reservoirs. A high-resolution method based on variational mode decomposition (VMD), continuous-wavelet transform (CWT) and frequency-weighted energy operator (FWEO) is proposed for hydrocarbon detection in tight sandstone gas reservoirs. VMD can decompose seismic signals into a set of intrinsic mode functions (IMF) in the frequency domain. In order to avoid meaningful frequency loss, the CWT method is used to obtain the time-frequency spectra of the selected IMFs. The energy separation algorithm based on FWEO can improve the resolution of time-frequency spectra and highlight abnormal energy, which is applied to track the instantaneous energy in the time-frequency spectra. The difference between the high-frequency section and low-frequency section acquired by applying the proposed method is utilized to detect hydrocarbons. Applications using the model and field data further demonstrate that the proposed method can effectively detect hydrocarbons in tight sandstone reservoirs, with good anti-noise performance. The newly-proposed method can be used as an analysis tool to detect hydrocarbons.

The impact of frequency and register on cognate facilitation: Comparing Romanian and Vietnamese speakers on the Vocabulary Levels Test

The Cognate Facilitation Hypothesis proposes that L2 learners register phonetic/orthographic similarities between L1 and L2 lexical items, ascribing L1 meanings to L2 counterparts. This study's purpose was to examine how cognate facilitation varies in degree between frequency and registers (academic versus general). Data were collected from L1 speakers of Romanian or Vietnamese via Schmitt's (2000) Vocabulary Levels Test (VLT), at the 10,000 and Academic levels together with a newly designed VLT comprising extremely low-frequency words. The Romanian group outperformed the Vietnamese one on the cognate-rich Academic level only on account of cognates, but the effect size was moderate and due to dispersion. On the 10,000 level, the Romanian group outperformed the Vietnamese one and once again cognate facilitation was evident only in the Romanian group. However, this time the effect size was large. The same was true in our low-frequency section. Our findings underline the special advantage that cognates bestow on L2 learners in low-frequency bands. For testing, we recommend that performance on cognates and non-cognates be considered separately rather than holistically. The findings also highlight the probability that, in highly specialized disciplines, speakers' language backgrounds would impact the ease or difficulty of acquiring discipline-specific terminology.

Study on precursory characteristics of granite failure based on infrasonic energy

To study signal characteristics of precursory infrasound of granite failure, coal-rock stress loading device and infrasound acquisition device were adopted to perform infrasound test in uniaxial loading process for granite samples. Wavelet filtering, infrasonic energy methods were used to process and analyze the test results. The results show that the infrasonic waves caused by the failure of rock samples are mainly in the middle and low frequency. Moreover, in the loading process, the ratio of relative energy of infrasound in different frequency sections and the total relative energy of the whole low frequency section both exhibit significant regularity of stage change, which is in consistent with the compaction stage, elastic stage, plastic stage of rock deformation. Additionally, the precursory characteristics of infrasound before rock failures are prominent and easy to be recognized. Meanwhile, infrasonic wave has the advantage of low attenuation in propagation process, which grants the superiority of infrasound prediction technology. A new method of rock failure prediction based on infrasound is formed, which provides an important reference for rock damage prediction in coal mining process.

Algorithm for automatic image dodging of unmanned aerial vehicle images using two-dimensional radiometric spatial attributes

Unmanned aerial vehicle (UAV) remote sensing technology has come into wide use in recent years. The poor stability of the UAV platform, however, produces more inconsistencies in hue and illumination among UAV images than other more stable platforms. Image dodging is a process used to reduce these inconsistencies caused by different imaging conditions. We propose an algorithm for automatic image dodging of UAV images using two-dimensional radiometric spatial attributes. We use object-level image smoothing to smooth foreground objects in images and acquire an overall reference background image by relative radiometric correction. We apply the Contourlet transform to separate high- and low-frequency sections for every single image, and replace the low-frequency section with the low-frequency section extracted from the corresponding region in the overall reference background image. We apply the inverse Contourlet transform to reconstruct the final dodged images. In this process, a single image must be split into reasonable block sizes with overlaps due to large pixel size. Experimental mosaic results show that our proposed method reduces the uneven distribution of hue and illumination. Moreover, it effectively eliminates dark-bright interstrip effects caused by shadows and vignetting in UAV images while maximally protecting image texture information.

Speckle transfer function for partial correction adaptive optics image reconstruction.

The residual aberrations of adaptive optics (AO) images can be removed by the speckle reconstruction method. To achieve accurate photometry, the knowledge of the speckle transfer function (STF) is necessary. In this Letter, a new method is proposed to evaluate the STF for speckle image reconstruction based on the partial correction AO. The STF is divided into two sections: the low-frequency section and the high-frequency section, and two different generalized Fried parameters for the partial wavefront compensation are used to calculate the STF values of those two different sections. This method is tested with the different seeing conditions and the various correction modes. The results show that the combined STFs have satisfactory agreement with the simulated AO STFs and give more accurate results than the STFs obtained from the traditional spectral ratio method. Further discussions show that the method is more reliable when the AO is operated in low-order correction mode because the approximation of the STF and the two generalized Fried parameters have better accuracy when the pupil diameter is much larger than those generalized Fried parameters.

Algorithm for relative radiometric consistency process of remote sensing images based on object-oriented smoothing and contourlet transforms

Traditional relative radiometric normalization methods generally depend on global statistical linear parameters, do not consider two-dimensional radiometric distribution, and do not eliminate foreground objects in an image. Thus, we present a method for relative radiometric consistency process based on object-oriented smoothing and contourlet transforms. Object-level smoothing is applied to both the reference image and the image to be normalized so as to reduce the influence of foreground objects on background radiation extraction. Then, high-frequency and low-frequency sections of an image are separated by contourlet transforms to preserve high-frequency texture information of the image to be normalized, with low-pass filtering applied to the low-frequency sections to gather the background radiation difference. Finally, contourlet reverse transforms are used to reconstruct the radiometrically normalized images. Test results show that the proposed method is effective for radiometric normalization of images with both large-scale and small-scale radiometric characteristics. The proposed method can not only normalize linear and nonlinear radiation differences at the same time, but also maximally preserve image texture information. It can improve the visual effects of normalized images and increase change detection accuracy.

Study on Power Characteristics of Hybrid Wind, Photovoltaic Generation and Energy Storage

In this paper, power characteristics of hybrid wind, photovoltaic generation and energy storage device is studied. Two-stage Smooth Energy Storage Control Strategy is designed in order to effectively inhabit both high frequency and low frequency section of wind and photovoltaic power. A topology of demonstration project of hybrid wind power, photovoltaic, energy storage and power transmission in north China is considered for simulation. Power characteristic of different hybrid generation mode is studied, and the simulation results show reliability and effectiveness of the control strategy. The results studied have the important significance for grid planning and operation.

The Application of Fault Diagnosis Based on EMD and Real Modulation Zoom Method

It is first to make Empirical Mode Decomposition (EMD) and achieve Hilbert-Hung transform (HHT) of envelope spectrum after Hilbert transformation, which can resolve the cross fault existing in Hilbert transformation is utilized merely. However, it can’t improve the resolution of envelope spectrum, which doesn’t adapt the characteristic of high resonant frequency of rolling bearing and low fault frequency. The paper utilizing the principle of real modulation shift frequency moves the EMD decomposition components of high frequency section to low frequency section and makes analysis of envelope spectrum after Hilbert transformation, which resolves the problem of low frequency resolution of pure HHT.

Development of the Two-Stage Planar Fem for the Terahertz Band on the Basis of the ELMI Accelerator

Project of the two-stage FEM-generator based on the ELMI accelerator with two parallel sheet electron beams formed in a single cathode unit is under development currently. In this scheme the first beam is used in the low-frequency section of the FEM for generating of high-power 75 GHz radiation. This radiation than goes through the coupling waveguides to the second channel and operates as a pumping wave which scatter on the second beam into the terahertz band. Our paper is devoted to modeling of various stimulated scattering regimes in this high-frequency section. Regime of the selfamplified spontaneous emission (SASE) is considered as well as a generator with resonator for the scattered radiation formed with advanced Bragg structures, based on coupling of traveling and quasi-critical waves

A.C Impedance Spectrum and Microstructure of Reactive Powder Cementitious Materials with Low w/b

In order to understand microstructure characteristics and properties of reactive powder cementitious materials with low w/b, the A.C impedance test on reactive powder cementitious materials is carried out. The correlation between the impedance parameters, microstructure and properties of reactive powder cementitious materials is also analyzed. Results indicate that the impedance-frequency curve both for reactive powder mortar and ordinary cement mortar consists of vertical part in range of low frequency section and horizontal part within high frequency section. But the angle between vertical part and horizontal part of curve for reactive powder mortar is larger compared with ordinary specimen. There is a good linear relationship between the real part of impedance and angular frequency ω-1/2 for mortar when frequency is from 10Hz to 100Hz. And the slope φ of line increases with the additions of reactive powder and the lowering w/b. The mechanical strength and microstructure of reactive powder cementitious materials with low w/b can be expressed well by the A.C impedance characteristic parameters.

Analytical Solution to Excited Waveform of Bias Control on Electro-Hydraulic Vibration Exciter

For precisely controlling the bias position of an electro-hydraulic vibration exciter, a scheme of a parallel mechanism of a two-dimensional valve (2D valve) and a servo valve is proposed. In the low frequency section, the mathematical model of the electro-hydraulic vibration exciter is simplified reasonably. A vibration central position is first analytically derived by assuming that 2D valve connected with parallel valve is equivalent to a single slide valve with neutral positive opening and the time-average flow rate through them is identical. And then the analytic solutions to excited waveforms superimposed on the bias position are further obtained. Finally, the experimental system is built to verify the theoretical analysis. The results reveal that this approximate analytical solution could describe excited waveform of bias control on electro-hydraulic vibration exciter. When the opening area of 2D valve is a constant, the bias position follows a linear relation with the throttling areas of the parallel valve which is no more than the maximum position. The excited waveform is close to the sinusoidal waveform. At the same opening area of the parallel valve, the bias position is reduced as the area coefficient of 2D valve increases. The proposed scheme not only ensures the frequency and the amplitude to be controlled independently but also the bias position to be adjusted precisely.

Analysis on the Excited Waveforms of an Electro-Hydraulic Vibration Exciter Using a Two-Dimensional Valve

The frequency bandwidth of the conventional electro-hydraulic vibration exciter is highly dependent on the servo valve's response speed, which is always limited to a rather narrow range. A novel scheme of electro-hydraulic vibration exciter using a two-dimensional valve (2D valve) to control a hydraulic actuator is proposed in order to extend the frequency range. In this 2D valve, the rotary and linear motions of a spool are controlled independently to produce the reciprocating motion of the piston of the cylinder. The frequency and the amplitude of the excited waveform are separately controlled by the rotary motion and the sliding motion of the spool respectively. The frequency is related to the rotary speed of the spool and the coupling pattern between the grooves on the spool and windows on the sleeve. This configuration could easily extend the frequency of the hydraulic vibration exciter by increasing the rotary speed of the spool and the number of the grooves on the spool area. Based on the work principle of the electro-hydraulic vibration system, the mathematical models are first established considering a nonlinear relation between pressure and flow. Then the analytic solution to excited waveforms is solved using the subsection integration method. And the expression equations of correlated parameters are given. Finally, real excited waveforms are obtained to validate the theoretical analysis result. The results reveal that the load of electro-hydraulic vibration exciter using a 2D valve is mainly the elastic force in low frequency section, so the ascent and descent slopes of excited waveform demonstrated somewhat asymmetrically in respect that the direction of the elastic load is changed. The extent of asymmetry depends on the 2D valve's axial opening. When it exceeds a critical value, the excited waveform tends to the saturated state because of pressure saturation. The exciter waveform is also influenced by hydraulic resonance as the input frequency approaches the hydraulic resonant frequency.

Joint algorithm for permutation problem in frequency-domain blind speech source separation

A new method for solving the permutation problem in the frequency-domain Blind Source Separation(BSS) was presented.This new method divided the whole frequency-domain into three sections and applied different permutation algorithms to different sections.The neighboring frequency correlation and harmonic frequency correlation coefficient of signal amplitudes were used together for the low-frequency section,the direction of arrival estimation for speech sources for the mid-frequency section,and the combination of correlation coefficient comparison and direction estimation for the high-frequency section,respectively.Experimental results show that the new method provides a more robust and precise solution to the permutation problem than the algorithm only with interfrequency correlation coefficient of signal amplitudes or only with direction of arrival estimation for sources.