A harmonic analyzed parameterization of tide-induced mixing for ocean models

The tide-induced mixing plays an important role in the regulation of ocean circulation.Numerical simulation of continental shelf circulation is found to exhibit an unreasonable vertical thermohaline structure without consideration of tide effects.In this study,we establish a harmonic analyzed parameterization of tide-induced（HAT） mixing,by which means to derive time-depended function of mixing coefficient based on harmonic analysis of the vertical mixing coefficient.By employing HAT mixing parameterization scheme,a series of numerical experiments are conducted for the Yellow Sea.Numerical results show that an ocean circulation model with the HAT mixing involved is capable of reproducing the reasonable thermohaline structure of the Yellow Sea Cold Water Mass,similar to structures produced by explicit tidal forcing on the open boundary.The advantage of the HAT method is its faster computation time,compared with models that directly resolve explicit tidal motion.The HAT parameterization for the tide-induced mixing has potential to improve both the accuracy and efficiency of ocean circulation and climate models.

Establishment of Numerical Wave Flume Based on the Second-Order Wave-Maker Theory

With growing computational power, the first-order wave-maker theory has become well established and is widely used for numerical wave flumes. However, existing numerical models based on the first-order wave-maker theory lose accuracy as nonlinear effects become prominent. Because spurious harmonic waves and primary waves have different propagation velocities, waves simulated by using the first-order wave-maker theory have an unstable wave profile. In this paper, a numerical wave flume with a piston-type wave-maker based on the second-order wave-maker theory has been established. Dynamic mesh technique was developed. The boundary treatment for irregular wave simulation was specially dealt with. Comparisons of the free-surface elevations using the first-order and second-order wave-maker theory prove that second-order wave-maker theory can generate stable wave profiles in both the spatial and time domains. Harmonic analysis and spectral analysis were used to prove the superiority of the second-order wave-maker theory from other two aspects. To simulate irregular waves, the numerical flume was improved to solve the problem of the water depth variation due to low-frequency motion of the wave board. In summary, the new numerical flume using the second-order wave-maker theory can guarantee the accuracy of waves by adding an extra motion of the wave board. The boundary treatment method can provide a reference for the improvement of nonlinear numerical flume.

Surface second harmonic generation of chiral molecules using three-coupled-oscillator model

Based on the three-coupled-oscillator molecular model we proposed, the relation between the second-order susceptibilities of a chiral film and the molecular hyperpolarizabilities is given. The effect of microscopic parameters on the second-order susceptibilities is simulated numerically and the difference between the efficiencies of s-polarized secondharmonic fields induced by the left- and the right-handed circularly-polarized fundamental beams is discussed. The theoretical basis for studying second-order nonlinear optical properties of the chiral molecular media with a tripod-like structure is provided in this paper.

Enhancement of second-order harmonic generation in polarized BaTiO_3 thin films embedded with Ag nanoparticles

RECENTLY, much attention has been paid to optical properties of ferroelectric thin films. Be-

MMC器件损耗分布与电容电压纹波综合优化方法

逆变工况下,半桥子模块下部绝缘栅双极晶体管(insulated gate bipolar transistor,IGBT)(简称T_(2)管)损耗占比较大,减小其损耗有利于提升设备运行可靠性。同时,抑制电容电压纹波有利于减小电容需求及提高功率密度。然而,现有优化控制策略未关注损耗分布优化及电容电压纹波间的矛盾,难以兼顾设备运行可靠性及功率密度。为此,文中提出兼顾减小T_(2)管损耗及抑制电容电压纹波的综合优化方法。首先,通过分析电荷量对器件损耗及电容电压纹波的影响路径,阐述减小T_(2)管的通态损耗与抑制电容电压纹波间的内在矛盾。接着,通过引入罚函数,建立计及T_(2)管损耗及电容电压纹波的综合目标函数。然后,以主动旁路策略为例,通过分析二倍频环流与三次谐波电压注入对T_(2)管损耗和电容电压纹波的影响规律,提出基于二倍频环流及三次谐波电压注入的综合优化方法。最后,在MATLAB/Simulink及PLECS中搭建仿真模型进行验证。仿真结果表明:该综合优化方法兼顾了T_(2)管损耗与电容电压纹波优化,一定程度上增加了设备可靠性与设备的功率密度。

计及交直流谐波传递的MMC-BTB系统阻抗特性分析及环流配合策略

基于模块化多电平换流器的背靠背系统(back-to-back system based on modular multilevel converter,MMC-BTB)中换流器间控制交互密切,导致振荡事故频发。现有研究中忽略谐波传递的单端模型无法准确刻画MMC-BTB阻抗特性,也较少从振荡抑制层面考虑两换流器间的协同配合。为此,该文考虑MMC-BTB中换流器间谐波传递与衰减机理,依据多种环流控制方式的阻抗等价性提出一种基于环流协调控制的振荡抑制策略。首先,基于等效电路定性分析谐波的流通路径,并定量推导MMC-BTB的双端模型;然后,沿谐波路径对两侧环流控制的交互机理开展分析,证明其对改善MMC-BTB阻抗特性的关键作用,并提出环流控制的协调配合与参数优化策略;最终,通过Matlab/Simulink仿真验证理论分析与所提策略的正确性。

Tide simulation in a global eddy-resolving ocean model

The tide plays a pivotal role in the ocean,affecting the global ocean circulation and supplying the bulk of the energy for the global meridional overturning circulation.To further investigate internal tides and their impacts on circulation,it is imperative to incorporate tidal forcing into the eddy-resolving global ocean circulation model.In this study,we successfully incorporated explicit tides(eight major constituents)into a global eddy-resolving general ocean circulation model and evaluated its tidal simulation ability.We obtained harmonic constants by analyzing sea surface height through tidal harmonic analysis and compared them with the analysis data Topex Poseidon Cross-Overs v9(TPXO9),the open ocean tide dataset from 102 open-ocean tide observations,and tide gauge stations from World Ocean Circulation Experiment.The results demonstrated that the State Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics/Institute of Atmospheric Physics(LASG/IAP)Climate System Ocean Model 3.0(LICOM3.0)effectively simulated tides,with errors predominantly occurring in nearshore regions.The tidal amplitude simulated in LICOM3.0 was greater than that of TPXO9,and these high-amplitude areas exhibited greater errors.The amplitude error of the M_(2) constituent was larger,while the phase error of the K_(1) constituent was more significant.Furthermore,we further compared our results with those from other models.

温阳通络调营卫法论治系统性硬化症

探讨温阳通络调营卫法在系统性硬化症(SSc)中的辨治及临床应用。SSc属于中医学“皮痹”范畴,主要病因病机为素体阳虚,营卫失和,风、寒、湿等邪气乘机侵袭,致气血凝滞、经络不通,肌肤失养,继则累及全身。此病临床症状复杂多变,多表现在皮肤病变及雷诺现象,亦可出现关节、消化道及脏器病变。阳虚与营卫不和皆为SSc的内因之一,痰浊瘀血阻滞、经络不通为其病理基础,临床应温阳通络调和营卫并治。

一种关于MMC谐波抑制的新型无源控制方法

针对模块化多电平变换器(modular multilevel converter,MMC)中由于桥臂共模电压的波动分量而产生的桥臂环流谐波抑制问题,在无源控制的基础上,提出一种结合改进重复控制的无源比例积分重复控制(passivity-based control-proportional integral-repeat control,PPRC)方法。首先,对MMC主电路内部电流和桥臂环流进行数学建模;其次,理论分析基于欧拉拉格朗日的无源控制和改进的重复控制,设计总体控制框图;最后,在动态条件下将所提PPRC方法与无源控制方法、无源比例积分控制方法进行仿真实验对比。3种控制方法的环流、桥臂电流和输出电流的对比分析结果表明,所提PPRC方法对桥臂环流中的谐波成分有着更好的控制性能。

模块化多电平换流器子模块电容电压波动与内部环流分析

为了分析模块化多电平换流器(modular multilevel converter,MMC)子模块电容电压波动和内部环流的基本成分,以开关函数为出发点,对电容电压波动和环流进行了数学分析。分析结果表明:基频波动和2倍频波动是电容电压波动的基本成分,并得出其解析表达式;内部环流仅包含偶次谐波成分,且以2次谐波环流为主,并给出其幅值的解析表达式。针对环流对MMC运行的影响,分析了MMC交流侧相电压中的谐波成分,结果表明环流导致MMC输出相电压中的奇次谐波含量增大,尤以3次谐波最为明显,且给出了3次谐波幅值的计算表达式。仿真结果表明上述理论分析是准确的,为子模块电容值和桥臂电感的选取提供了理论基础。

逆变器并联系统中谐波环流抑制的研究

逆变器中,电压瞬时值反馈调节对死区等因素造成的输出电压波形畸变有着很好的校正作用,但在实验中发现,对于大功率逆变器,由于滤波电感及并机电抗都很小,使得死区等因素可引起很大的谐波环流,而电压瞬时值反馈控制对这种谐波环流的抑制能力是非常有限的。该文为研究谐波环流的产生机理及瞬时值反馈控制对谐波环流的抑制作用,建立了基于谐波扰动的逆变器模型。研究表明,瞬时值反馈控制对谐波环流的抑制能力与其波形校正能力存在相互关系。通过对电压单环,电压电流双环控制的谐波环流抑制能力的比较,发现波形控制效果较好的瞬时值反馈控制,对谐波环流也有较好的抑制作用。仿真与实验验证了上述结论。

组串式LC型光伏逆变器并网谐波环流及其抑制策略分析

组串式LC滤波型三电平并网逆变器并联入网结构广泛用于光伏发电系统中,其非隔离特性可导致逆变器之间差模谐波环流问题。为分析谐波环流的影响因素和具体形态,首先在理想情况下建立了差模谐波环流模型,得出谐波环流与功率器件开关暂态的关系,以及环流谐波峰峰值与各逆变器不同步的载波交错角度和电网矢量角度的关系,并考虑非理想情况下寄生参数对谐波环流的影响。在此基础上,分别分析3种典型的环流抑制方案对模型中输出环流的影响,得出各方法的优缺点和适用范围。

变流器并列运行系统中非特征谐波环流分析及其抑制方法

冀北电网中,先后出现了变流器的非特征次谐波环流问题。某500 k V变电站变流器并联运行时发生了一次故障,以该故障为例,通过分析变流器故障录波数据,发现多台变流器并列运行容易发生非特征谐波环流。通过分析环流与变流器拓扑结构之间的关系,得出级联H桥多电平变流器更容易发生非特征谐波环流;然后,通过电磁暂态仿真复现了故障现象,仿真结果验证了所得结论的正确性。最后,提出了解决非特征次谐波环流的措施。

基于谐波线性化的模块化多电平换流器阻抗建模

模块化多电平换流器(MMC)的小信号阻抗建模是分析含MMC的电力系统谐振及稳定性的关键。MMC是一个多频率、非线性、周期性时变的系统,具有复杂的内部动态特性,传统的小信号建模方法无法直接用于MMC。采用谐波线性化法对MMC的交流侧小信号阻抗进行建模,计及谐波环流和电容电压稳态纹波的影响,并揭示了环流控制对MMC交流侧阻抗的作用机理。利用MATLAB/Simulink搭建三相MMC详细时域仿真模型,仿真测量结果验证了解析模型的正确性。

模块化多电平变换器的环流谐波抑制策略

在考虑子模块电压扰动的基础上,提出模块化多电平变换器(MMC)环流的等效控制模型,并在传统比例-积分环流控制器中加入谐振环节来有效抑制环路中的电压谐波扰动项,对环流中的低次谐波起到较好的衰减作用。为了获得更优的抑制效果,将重复控制引入到环流控制中,对扰动项中的各阶次都进行了很好的抑制,相比于多谐振控制的指定阶次抑制方法,重复控制策略形式更加简单、抑制效果更好。最后,对提出的多谐振抑制策略和重复控制策略进行了实验验证。

并联逆变系统谐波环流的抑制方法及功率计算

为了抑制并联逆变系统的谐波环流,提高系统的电压输出质量,分析了谐波环流产生的原因,利用下垂特性将谐波功率环引入系统控制,并基于此提出多反馈并联控制策略及其参数设计方法,并采用更加实时的谐波环流功率计算方法,实现基波及主要次谐波的功率均分,有效抑制谐波环流。实验结果证明,基于新的谐波功率计算的多反馈并联控制策略能更好地抑制谐波环流。提高并联逆变系统的电压输出质量。

并联逆变器馈电PMSM调速系统谐波和环流控制

针对大功率场合下并联逆变器馈电永磁同步电机(PMSM)调速系统中存在的环流问题和电机电流谐波性能差的问题,提出一种并联逆变器馈电永磁同步电机控制策略来协调控制不同并联逆变器模块实现电机总电流波形优化,并有效地抑制并联逆变器模块间环流。分析并联逆变器馈电PMSM调速系统的电路结构及并联逆变器环流的等效电路模型,提出针对并联逆变器PMSM调速系统的移相空间矢量脉宽调制(SVPWM)技术和随机SVPWM技术,并通过在SVPWM中动态分配零矢量作用时间来抑制各逆变器模块间的环流。仿真和实验结果均验证了所提出的控制策略的有效性,采用移相SVPWM和随机SVPWM策略可改善电机电流的谐波性能,采用环流抑制环节后可以有效地消除电路中的环流。

多极隐极发电机励磁绕组匝间短路时的定子分支环流谐波特性

为研究多极隐极发电机励磁绕组匝间短路故障机理,揭示多极隐极发电机励磁绕组匝间短路特性,详细推导了多极隐极发电机空载运行时励磁绕组正常与故障状态下,励磁磁动势、定子空载电动势及故障后定子绕组并联支路间环流的表达式。对比分析了故障前、后定子相绕组感应电动势以及定子分支环流,揭示了转子极对数p与故障后定子感应电动势及定子分支环流谐波次数之间的关系;并对励磁绕组短路位置相同、短路程度不同以及短路程度相同、短路位置不同时的谐波环流特性进行了分析,进而得到了定子分支环流与励磁绕组短路位置及短路程度间的关系。以一台核电半速隐极发电机为例,对励磁绕组同位置、不同程度以及同程度、不同位置的短路过程进行了仿真,仿真结果与解析结果一致,验证了理论推导的正确性,为多极隐极发电机励磁绕组匝间短路故障在线定位提供了理论基础。

变压器电流差动保护改进方法

针对一种适用于Y,d或D,y接线变压器的电流差动保护改进方法进行了研究,该方法采用修正的差动电流来补偿励磁电流和剩磁的影响,制动电流与传统比率式差动保护相同。变压器铁心饱和前,采用一、二次侧相电流差作为差动电流;饱和后,则计算磁化电流来修正差动电流。该保护不受谐波和剩磁影响,不需要附加制动或闭锁技术。通过EMTP仿真将该保护与传统谐波制动原理的差动保护进行了比较,结果表明,励磁涌流和过励磁时,该保护能够保证不误动;内部故障时则不会像传统谐波制动原理的差动保护受制动信号影响而延迟动作。动模实验进一步验证了该保护的可行性和优越性。

EAST纵场电源均流技术

EAST纵场电源由两组双反星形整流电路直接并联而成,要求单组电源的环流和两组电源之间的环流必须满足规定。从纵场电源的数学模型着手,通过谐波分析得到产生环流的原因,对于单组电源的环流,通过改变电路的阻抗特性来抑制,而对于两组电源之间的环流,则通过控制的方法进行动态调节。仿真和实验结果证明,该方案能在保证纵场电源稳定的前提下,取得良好的均流效果。