基于修正开关函数的LCC-HVDC整流站直流侧阻抗建模

电网换相换流器高压直流输电凭借输送容量大、距离远等优势在国内得到广泛应用。换流站作为交直流系统连接枢纽,其精确建模具有十分重要的意义。该文提出一种将换流变压器等值漏感折算至直流侧后的开关函数修正模型,该模型从根本上计及换流站的换相过程,能够准确计算出稳态直流电压;同时基于该修正模型,建立了LCC-HVDC整流站的小信号直流侧低频阻抗模型。最后,在PSCAD/EMTDC中利用Cigre Benchmark模型,对提出的开关函数修正模型及建立的LCC整流站阻抗模型进行仿真验证,证明模型的正确性。

基于模块化多电平矩阵式换流器的分频输电系统低频侧阻抗建模及稳定性判别

针对风电机组经分频输电并网系统低频侧稳定性判别的问题,该文采用阻抗分析方法,结合基于阻抗矩阵k-范数的稳定判据,实现了并网系统低频侧稳定性的快速判别。首先,基于模块化多电平矩阵式换流器(modular multilevel matrix converter,M3C)的数学模型,针对M3C低频侧接风电机组并网这一典型场景设计了相应的无源控制器和对应的滤波电路,并对风电机组的模型进行了简化。其次,推导了M3C低频侧和风电机组的dq轴阻抗模型并利用扫频法验证了该文阻抗建模的正确性。然后,针对该系统低频侧稳定性设计了基于系统阻抗矩阵范数的快速判稳方法。最后,利用MATLAB对风电机组经分频输电并网系统低频侧的稳定性进行了判别并基于PSCAD仿真验证了该方法的有效性。

应用灰色系统理论预测沥青涂层在土壤模拟液中的寿命

目的基于灰色系统理论预测模型建立涂层腐蚀寿命的预测模型,利用该模型计算不同厚度的无溶剂环氧煤焦沥青涂层在滨海氯盐土模拟液中的涂层寿命。方法采用电化学方法,测定不同厚度无溶剂环氧煤焦沥青涂层涂覆的碳钢在滨海氯盐土模拟液中的交流阻抗值。基于灰色系统理论GM(1,1)模型将滨海氯盐土模拟液中测试的无溶剂环氧煤焦沥青涂料的低频阻抗模值和浸渍时间关系,建立涂层腐蚀寿命的预测模型。结果三种不同厚度无溶剂环氧煤焦沥涂层试样在侵蚀性溶液中浸泡240 d时,低频阻抗模值仍保持在8.3×10~8Ω·cm^2以上,应用预测公式计算的滨海氯盐土模拟液中三种无溶剂环氧煤焦沥涂层低频阻抗模值的实测值与预测值的相对误差小于6.0%,C<0.35,P=1,该模型的预测精度好。涂层厚度为200μm的无溶剂环氧煤焦沥青层预测寿命t=781 d,涂层从浸泡初期阶段进入中期阶段。随着涂层厚度的增加,涂层寿命愈长,当涂层厚度为600μm时,其预测寿命达到2926 d。结论所建涂层腐蚀寿命的预测模型表现出较好的拟合精度和预测可靠度。

托卡马克等离子体环向转动对测地声模的影响

采用回旋动理学方程推导得到了环向转动托卡马克等离子体中测地声模的色散关系,分析了环向转动对测地声模、低频模和声波分支的频率以及无碰撞阻尼率的影响。结果表明,测地声模的频率会随着环向转动马赫数而逐渐增大,而其无碰撞阻尼率则会随着环向转动马赫数而迅速减小。此外,低频模和声波分支的频率以及无碰撞阻尼率都会随着环向转动马赫数而逐渐减小,其中环向转动对声波分支的频率以及无碰撞阻尼率的影响非常小,基本上可以忽略。

EAST反磁剪切q_(min)≈2条件下磁流体力学不稳定性及内部输运垒物理实验结果简述

托卡马克装置内,建立和维持内部输运垒结构是提高等离子体约束的重要保障.本文简单概述了EAST反磁剪切q_(min)≈2实验条件下建立和维持内部输运垒的关联物理过程:“离轴锯齿”和双撕裂模不稳定性;快离子激发的阿尔芬波不稳定性;热粒子激发的低频模不稳定性等.首先,“离轴锯齿”是判断实验条件q_(min)≤2的重要依据.文中详细介绍了“离轴锯齿”的激发条件、分类方式和先兆模结构等基本特征,其崩塌事件由m/n=2/1双撕裂模磁重联诱发产生.其次,在“离轴锯齿”振荡期间,快离子很容易激发比压阿尔芬本征模和反剪切阿尔芬本征模.这两类阿尔芬本征模的环向模数为1≤n≤5,径向位置为环向区域1.98 m≤R≤2.07m(磁轴R0≈1.9 m,归一化小半径0.2≤ρ≤0.45).简述阿尔芬波的激发条件和3种不同物理量(热压力梯度、快离子分布函数和环向流速剪切)等之间的关系.第三,在“离轴锯齿”振荡期间,热压力梯度可以诱发低频模不稳定性.利用一般鱼骨模色散关系很容易得出EAST上低频模的基本特征:1)离子抗磁漂移频率大小;2)阿尔芬波极化方向;3)反应型动理学气球模特征.低频模不稳定性的激发不依赖快离子,主要发生在高的压力梯度区α∝(1+τ)(1+ηi),τ=Te/T_(i),ηi=Lni/LT_(i),也即是足够高的τ和ηi.最后,“离轴锯齿”不稳定性的抑制和内部输运垒结构的建立.EAST内q_(min)≈2条件下内部输运垒建立过程中包括3个重要过程:1)切向(NBI1L)注入比垂直方向(NBI1R)注入的中性束更容易缓解“离轴锯齿”的爆发周期;2)存在“离轴锯齿”的情况高效缓解微观不稳定性,且此位形更有利于内部输运垒结构的建立;3)内部输运垒建立过程中伴随阿尔芬波(1≤n≤5)不稳定性的激发,内部输运垒维持期间存在热离子温度梯度激发的中尺度微观不稳定性(5≤n≤10)等.因此,理解和掌握“离轴锯齿”实验条件的建立和抑制、阿尔芬波的激发和快离子的再分布、热压力梯度相关不稳定性等物理过程,对于内部输运垒形成机制的理解具有重要的借鉴意义.

光老化丙烯酸聚氨酯涂层防护性能的电化学表征

采用荧光紫外辐照UVB+冷凝的人工加速试验对丙烯酸聚氨酯清漆进行老化试验,用电化学阻抗谱(EIS)评价不同暴露时间涂层的屏障性。结果表明:随着暴露时间增加,一定频率下的低频阻抗模值|Z|下降,表现为时间的变化函数;傅立叶变换红外光谱(FTIR)分析表明,紫外暴露下发生高分子断链涂层降解,而在老化12天后|Z|值的升高可能与分子的重构变化有关;因而,EIS谱的|Z|值可作为一种较为有效的表征参数来反映UV老化后有机涂层的防护性能。

Low-frequency data analysis and expansion

The use of low-frequency seismic data improves the seismic resolution, and the imaging and inversion quality. Furthermore, low-frequency data are applied in hydrocarbon exploration; thus, we need to better use low-frequency data. In seismic wavelets, the loss of low-frequency data decreases the main lobe amplitude and increases the first side lobe amplitude and results in the periodic shocking attenuation of the secondary side lobe. The loss of low frequencies likely produces pseudo-events and the false appearance of higher resolution. We use models to examine the removal of low-frequency data in seismic data processing. The results suggest that the removal of low frequencies create distortions, especially for steep structures and thin layers. We also perform low-frequency expansion using compressed sensing and sparse constraints and develop the corresponding module. Finally, we apply the proposed method to real common image point gathers with good results.

等离子体部分填充的波纹慢波结构中电磁波色散特性

对有限厚环形等离子体层填充的波纹慢波结构中电磁波色散特性进行了分析。研究结果表明,对于ω>ωp的高频TM0n模,模式色散曲线随等离子体密度的增加而上移;ω≤ωp的低频等离子体模为一系列呈周期变化的密度谱,部分模式能与电子注发生较强的同步互作用。等离子体模的时间增长率随等离子体密度的增加快速增加,而TM0n模则快速减小或缓慢增加,因此在较高的等离子体密度下,高频模式可能被抑制。

大气环境中霉菌对氟碳彩涂板老化行为的影响

目的通过对氟碳彩涂板在海南文昌和云南西双版纳大气环境中曝晒后的失效行为进行对比研究,探明霉菌因素对氟碳彩涂板老化行为的影响。方法运用扫描电子显微镜(SEM)、电化学阻抗谱(EIS)以及傅里叶变换红外光谱(FT-IR)等分析手段,对不同周期涂层试样的表面形貌和防护性能进行了监测和分析,探究海南文昌和云南西双版纳两种不同环境室外自然曝晒下,氟碳涂层的老化失效和行为。结果SEM照片表明,随着曝晒时间的增加,两个地区涂层试样均发生不同程度的老化,且西双版纳涂层试样表面出现明显的霉菌菌落附着。EIS结果表明,与文昌试样相比,西双版纳暴晒的涂层低频阻抗模值下降较快。FT-IR结果表明,西双版纳地区氟碳涂层老化的主要原因是涂层中的丙烯酸树脂在光照和真菌等微生物共同作用下发生分子链断裂。相比之下,在光照时间、辐射强度均高于西双版纳的情况下,文昌大气暴晒后的试样表现出较好的耐候性能。结论霉菌引起的微生物降解加速了涂层的光老化进程,使得西双版纳暴晒试样老化程度较大。

DNN-Based Speech Enhancement Using Soft Audible Noise Masking for Wind Noise Reduction

This paper presents a deep neural network(DNN)-based speech enhancement algorithm based on the soft audible noise masking for the single-channel wind noise reduction. To reduce the low-frequency residual noise, the psychoacoustic model is adopted to calculate the masking threshold from the estimated clean speech spectrum. The gain for noise suppression is obtained based on soft audible noise masking by comparing the estimated wind noise spectrum with the masking threshold. To deal with the abruptly time-varying noisy signals, two separate DNN models are utilized to estimate the spectra of clean speech and wind noise components. Experimental results on the subjective and objective quality tests show that the proposed algorithm achieves the better performance compared with the conventional DNN-based wind noise reduction method.

Observation of the abyssal western boundary current in the Philippine Sea

Mooring observations were conducted from July 16, 2011 to March 30, 2012 east of Mindanao, Philippines （127°2.8＇E, 8°0.3＇N） to observe the abyssal current at about 5 600 m deep and 500 m above the ocean bottom. Several features were revealed： 1） the observed abyssal current was highly variable with standard deviations of 57.3 mrn/s and 34.0 ram/s, larger than the mean values of-31.9 and 16.6 mm/s for the zonal and meridional components, respectively; 2） low-frequency current longer than 6 days exhibited strong seasonal variation, flowing southeastward （mean flow direction of 119.0° clockwise from north） before about October 1, 2011 and northwestward （mean flow direction of 60.5° counter-clockwise from north） thereafter; 3） the high-frequency flow bands were dominated by tidal currents O1, K1, M2, and S2, and near-inertial currents, whose frequencies were higher than the local inertial frequency. The two diurnal tidal constituents were much stronger than the two semidiumal ones. This study provides for the first time an observational insight into the abyssal western boundary current east of Mindanao based on long-term observations at one site. It is meaningful for further research into the deep and abyssal circulation over the whole Philippine Sea and the 3D structure of the westem boundary current system in this region. More observational and high-resolution model studies are needed to examine the spatial structure and temporal variation of the abyssal current over a much larger space and longer period, their relation to the upper-layer circulation, and the underlying dynamics.

Reducing-resistance mechanism of vibratory excavation of hydraulic excavator

Based on the working principle of vibratory excavation of hydraulic excavator,the expression of digging resistance changed with time under sine wave inspiritment was deduced;a comparison analysis was given after calculating the forces status of rock and soil under static load and vibratory load respectively by using MATLAB;and then RFPA-2D(rock failure process analysis code)was used to make comparison of simulation experiment on rock and soil failure process under static load and vibratory load.The results demonstrate that,compared with the normal excavation under the same situation,the digging resistance and the energy consumption can be reduced by respectively 30%and 60%at maximum,and that the working efficiency can be increased by 45%at maximum owing to vibratory excavation.

Defects detection in typical positions of bend pipes using low-frequency ultrasonic guided wave

In order to analyze the possibility of detecting defects in bend pipe using low-frequency ultrasonic guided wave, the propagation of T(0,1) mode and L(0,2) mode through straight-curved-straight pipe sections was studied. FE(finite element) models of bend pipe without defects and those with defects were introduced to analyze energy distribution, mode transition and defect detection of ultrasonic guided wave. FE simulation results were validated by experiments of four different bend pipes with circumferential defects in different positions. It is shown that most energy of T(0,1) mode or L(0,2) mode focuses on extrados of bend but little passes through intrados of bend, and T(0,1) mode or L(0,2) mode is converted to other possible non-axisymmetric modes when propagating through the bend and the defect after bend respectively. Furthermore, L(0,2) mode is more sensitive to circumferential notch than T(0,1) mode. The results of this work are beneficial for practical testing of pipes.

Use of low-frequency signals to improve imaging quality under high-velocity basalt

Wave equation wave field numerical modeling technology is applied to the observation that deep layer imaging is difficult below a screening layer of high-velocity basalt. Three simple high-velocity basalt models are designed on the basis of basalt formation characteristics. The analysis of deep-layer reflection seismic signal energy shows that lowfrequency seismic signals are capable of both penetrating the thin high-velocity basalt layer and reducing the diffraction noise caused by the rough surfaces. The simulation experiment of a complete 2D basalt model confirms that the low-frequency signals can be used to boost the quality of deep-layer imaging under the high-velocity basalt layer and achieve good results in low-pass filter processing of actual data.

Currents and Electric Fields Induced in Anatomically Realistic Human Models by Extremely Low Frequency Electric Fields

There is increasing public concern about biological interactions with and the potential health effects of low frequency electric and magnetic fields. Recently, the ICNIRP （International Commission on Non-Ionizing Radiation Protection） has published new exposure guidelines with regard to these fields. The aim of this paper is to demonstrate the calculation of the currents and electric fields induced in the human body by external electric fields at 60 Hz, using numerical human models of anatomically-realistic human bodies, and to compare those results with the basic restrictions proposed by the new guidelines. As a result, in the case that a human is exposed to an electric field of 1 kV/m at 60 Hz the short-circuit current of 18 μA flows though the ankles. Furthermore, the electric field of 40 mV/m in the nervous tissue of the adult model is induced by exposure to external electric fields at the reference level, which is enough smaller than the basic restrictions established in the ICNIRP guidelines for occupational exposure.

Application of Empirical Mode Energy to the Analysis of Fluctuating Signals

After an aerial object enters the water, physical changes to sounds in the water caused by the accompanying bubbles are quite complex. As a result, traditional signal analyzing methods cannot identify the real physical object. In view of this situation, a novel method for analyzing the sounds caused by an aerial object’s entry into water was proposed. This method analyzes the vibrational mode of the bubbles by using empitical mode decomposition. Experimental results showed that this method can efficiently remove noise and extract the broadband pulse signal and low-frequency fluctuating signal, producing an accurate resolution of entry time and frequency. This shows the improved performance of the proposed method.

SCALABLE PERCEPTUAL AUDIO REPRESENTATION WITH AN ADAPTIVE THREE TIME-SCALE SINUSOIDAL SIGNAL MODEL

This work is concerned with the development and optimization of a signal model for scalable perceptual audio coding at low bit rates. A complementary two-part signal model consisting of Sines plus Noise (SN) is described. The paper presents essentially a fundamental enhancement to the sinusoidal modeling component. The enhancement involves an audio signal scheme based on carrying out overlap-add sinusoidal modeling at three successive time scales, large, medium, and small. The sinusoidal modeling is done in an analysis-by-synthesis overlap- add manner across the three scales by using a psychoacoustically weighted matching pursuits. The sinusoidal modeling residual at the first scale is passed to the smaller scales to allow for the modeling of various signal features at appropriate resolutions.This approach greatly helps to correct the pre-echo inherent in the sinusoidal model. This improves the perceptual audio quality upon our previous work of sinusoidal modeling while using tile same number of sinusoids. Tile most obvious application for the SN model is in scalable, high fidelity audio coding and signal modification.

Further Study of Typhoon Tracks and the Low-Frequency (30-60 Days) Wind-Field Pattern at 850 hPa

The association of typhoon tracks over the western Pacific with the low-frequency wind-field pattern of atmospheric intraseasonal (30-60 days) oscillation at 850 hPa is further studied by using observational data analyses. Comparative analyses of the composite wind fields at 850 hPa, contrasting the atmospheric intraseasonal oscillation (ISO) with the original circulation, show that the typhoon tracks are closely related to the wind pattern of the ISO but are not obviously related to the original wind fields. Case studies of two typhoons in 2006 also show that the low-frequency wind-field pattern, particularly the maximum-value line (belt) of low-frequency cyclonic vorticity at 850 hPa, is closely related to the typhoon track. Therefore, the lowfrequency circulation pattern and the maximum-value line (belt) of low- frequency cyclonic vorticity at 850hPa can be used to predict typhoon tracks over the northwestern Pacific.

拉曼光谱在石墨烯结构表征中的应用

石墨烯是sp2碳原子紧密堆积形成的二维原子晶体结构,因其独特的结构与性质引起了科学家们的广泛关注.拉曼光谱是一种快速而又简洁的表征物质结构的方法.主要综述了拉曼光谱技术在石墨烯结构表征中应用的一些最新进展.首先,在系统分析石墨烯声子色散曲线的基础上介绍了石墨烯的典型拉曼特征(G'峰、G峰和D峰),讨论了G'峰、G峰和D峰在石墨烯层数的指认和石墨烯边缘与缺陷态分析中的应用;然后,通过对石墨烯拉曼G峰和G'峰的峰位、峰型以及强度的分析,讨论了石墨烯的层间堆垛方式、掺杂、基底、温度和应力等对石墨烯的电子能带结构的影响;最后,介绍了石墨烯中的二阶和频与倍频拉曼特征以及石墨烯的低频拉曼特征(剪切和层间呼吸振动模),并讨论了其对石墨烯结构的依赖性.

Estimation of the electromechanical characteristics of power systems based on a revised stochastic subspace method and the stabilization diagram

Estimating low-frequency oscillation modes and the corresponding mode shapes based on ambient data from WAMS measurements has a promising prospect in power system analysis and control.Based on the stochastic subspace method,this paper proposes a revised stochastic subspace method by introducing reference channels,which can estimate the modes and the mode shapes simultaneously with great computational efficiency.Meanwhile,the accuracy of the estimated results is not degraded.To discriminate the real modes from the spurious ones,the stabilization diagram is introduced.A novel algorithm is designed to deal with the stabilization diagram which can detect the real modes automatically.Tests conducted on the IEEE-118 system indicate that the proposed method has good performance in terms of both computational efficiency and accuracy,and has the potential of being used on-line.