低压微电网逆变器并离网平滑切换控制

分布式能源通过电力电子变换器与本地负载、储能设备等相结合构成微电网;并实现在并网、离网及并离网相互切换模式下的稳定运行。采用下垂控制实现微电网离网运行时逆变器输出电压和频率的稳定、并网运行时输出功率的恒定,并且不改变控制方式实现并离网的平滑切换控制。由于传统下垂控制在离网运行时,逆变器输出电压与频率存在一定的偏差;并且为了使逆变器输出阻抗呈阻性以减小控制参数对输出阻抗的影响,采用一种改进的下垂控制策略;并在逆变器双闭环控制中加入虚拟阻抗。仿真结果验证了改进下垂控制策略在离网、并网以及并离网相互切换运行时的有效性,以及相比于传统下垂控制策略的优越性。     

非线性准PR双模逆变器的设计

针对光伏发电系统并网与离网两种应用需求,讨论分析并网、离网两种逆变模式的原理及控制策略.提出一种非线性准比例谐振(PR)控制器用于光伏系统并网与离网状态切换过程中的逆变控制,拟通过引入的非线性环节增大系统响应初期增益,提高控制的快速性,以消除切换过程中的电压电流畸变,保障逆变效果的平滑性.Matlab/Simulink仿真结果表明:所提方法具有可行性与有效性.     

基于解耦双同步和改进V/F的离网逆变器控制研究

离网逆变器三相负载不平衡时,逆变器输出电压同时存在正、负序分量.针对传统控制策略下输出电压存在大量谐波的问题,提出一种基于解耦双同步和改进V/F的离网逆变器控制算法.研究正负序分离变换矩阵、dq坐标下产生的耦合项,使用解耦双同步法、V/F控制解耦后的正、负序电压分量;在Matlab/Simulink中搭建离网逆变器仿真模型.仿真结果表明,三相负载不平衡时,基于解耦双同步和改进V/F控制算法能有效减小离网逆变器输出电压谐波畸变率、提高电压质量、为负载提供优质电能.     

5 kW单相非隔离逆变器的研制

针对采用传统PI和比例谐振PR控制的并网逆变器对电网干扰信号的抑制能力较弱问题,提出了一种基于准PR控制策略的单相并网逆变器控制方案,并针对自行研制的一种可大规模用于家庭供电的体积小、成本低的单相非隔离5 kW逆变器进行了仿真研究和并网实验。从硬件和软件两方面阐述了逆变器的设计原理;采用Matlab软件中的S-Function函数进行了仿真分析,控制策略为准PR控制,将其与传统的PI和PR控制器进行了对比,并在仿真基础上用实验样机进行了实验验证。研究结果表明,所提出的准PR控制策略在消除稳态误差、提高网侧电流品质和抗电网干扰等方面,相较于PI和PR控制具有很大的优越性,不仅可以更好地实现无静差控制,且具有可扩展性。凭借准PR控制器的无静差特性,单相非隔离5 kW逆变器在家用供电系统中将具有广阔的应用前景。     

分布式可再生能源并网逆变器控制策略研究

为满足分布式可再生能源并网要求,提出一种模拟同步发电机特性的并网逆变器控制策略.引入惯性与阻尼特性,建立了并网逆变器控制策略的数学模型.采用近似同步发电机励磁无功调节、有功频率下垂特性调节,实现并网逆变器在孤岛运行时维持电网电压、频率稳定,在并网运行时实时跟踪有功、无功指令,提高系统的稳定性.仿真结果证明了所提出控制策略的可行性.     

基于二次调频的孤岛微网自适应旋转惯量控制策略

传统的虚拟同步发电机(VSG)控制策略中引入同步发电机(SG)的旋转惯量和阻尼系数,解决了大规模分布式能源接入微网时缺少惯性的问题,然而固定的旋转惯量无法兼顾负荷扰动下功率振荡及频率波动.针对这一问题,提出一种基于二次调频的旋转惯量自适应控制策略,增强系统惯性,实现功率和频率的实时动态调节.首先,借鉴SG的外特性建立VSG的数学模型,分析孤岛模式下不同旋转惯量对系统动态响应的影响;然后,在旋转惯量控制中引入角频率偏移量和变化率形成自适应惯量控制,减少系统在负荷扰动时的超调量和振荡时间,并分析重要参数对系统稳定性的影响;最后,通过Matlab/Simulink仿真软件,将其与传统VSG控制策略进行对比分析,验证了所提新型VSG控制策略的可行性和优越性.     

直驱式PMSG风力发电系统LVRT分段控制研究

选择PMSG风力发电系统作为研究对象,对网侧逆变器的功率暂态过程进行分析.通过功率解耦控制对故障时的逆变器进行控制策略修正,并配合撬棒电路进行耗能.提出一种基于变流器和撬棒电路分阶段控制的永磁同步发电机风力发电系统低电压穿越技术,对控制策略进行仿真分析验证.结果表明,在1200 kV直流输电系统发生电压跌落故障时,控制...     

基于虚拟同步发电机的微网逆变器控制策略

针对基于下垂控制的微网逆变器控制方式的不足,研究了一种新的微网逆变器控制策略.该控制技术通过本体算法的实现及调速器、励磁调节器的设计,模拟了同步发电机的工作特点与控制特性,使微网逆器在孤岛与并网模式下能稳定运行,具有良好的预并列控制及功率均分控制特性,提高了微网逆变器控制的可靠性和灵活性.仿真和试验结果表明:该微网系统能根据有功与无功负荷的变动对逆变器输出电压的幅值与频率进行快速调节,以保持系统电压和频率的稳定性;同时,在逆变器孤岛状态下并联运行时,能根据逆变器容量按比例进行负荷分配,以维持系统的功率平衡,满足了微网逆变电源的控制要求.     

电流前馈控制的光伏并网和APF统一控制策略

针对光伏并网和有源电力滤波器统一控制策略中,光伏阵列受到太阳辐射等外界环境的突变,导致直流母线中的电压振荡问题,提出一种基于电流前馈控制的光伏并网和APF统一控制策略.该控制策略采用将光伏电流前馈作用于参考电流合成的方法,加速逆变器参考电流的计算速度,提高了注入电网的电流的动态响应,抑制直流母线电压振荡.该系统既实现光伏并网,也能执行APF相关功能,抑制负载的谐波电流,补偿负载无功功率.在不增加成本情况下,提高了系统的抗干扰能力,具有良好的运用前景.     

基于自适应滑模和下垂控制的微源逆变器并联运行策略

针对微源逆变器并联运行过程中存在的负荷扰动和功率分担精度问题,在组合式三相四线制逆变器拓扑的基础上,提出一种新的三环控制策略.该策略包含滑模电压内环、虚拟阻抗环和功率外环.其中,电压内环通过自适应鲁棒三阶滑模控制有效提高了逆变器的抗干扰和动态性能;在分析滑模电压闭环逆变器等效输出阻抗的基础上,分析了输出阻抗和线路阻抗对功率分配的影响机理,进而针对不同功率等级逆变器设计虚拟阻抗反馈系数,使逆变器等效输出阻抗呈阻性并满足功率分配的要求;最后,在传统下垂控制外环中加入暂态下垂项,用于消除系统中的低频振荡,并提高逆变器的动态性能.仿真分析和单相实验结果均验证了上述控制策略的有效性和鲁棒性.     

Markedness and its Interpretation in Language Use

Language markedness is a common phenomenon in languages, and is reflected from hearing, vision and sense, i.e. the variation in the three aspects such as phonology, morphology and semantics. This paper focuses on the interpretation of markedness in language use following the three perspectives, i.e. pragmatic interpretation, psychological interpretation and cognitive interpretation, with an aim to define the function of markedness.     

Features of Women's language

Language is a means of verbal communication. People use language to communicate with each other. In the society, no two speakers are exactly alike in the way of speaking. Some differences are due to age, gender, statue and personality. Above all, gender is one of the obvious reasons. The writer of this paper tries to describe the features of women＇s language from these perspectives： pronunciation, intonation, diction, subjects, grammar and discourse. From the discussion of the features of women＇s language, more attention should be paid to language use in social context. What＇s more, the linguistic phenomena in a speaking community can be understood more thoroughly.     

An Analysis of Catherine＇s Character——Wuthering Heights

Wuthering Heights, Emily Bronte＇s only novel, was published in December of 1847 under the pseudonym Ellis Bell. The book did not gain immediate success, but it is now thought one of the finest novels in the English language. Catherine is the key character of this masterpiece, because everybody and everything center on her though she had a short life. We can understand this masterpiece better if we know Catherine well.     

Lower Paleozoic oil relationships within Williston Basin, Canada

The Williston Basin is a significant petroleum province, containing oil production zones that include the Middle Cambrian to Lower Ordovician, Upper Ordovician, Middle Devonian, Upper Devonian and Mississippian and within the Jurassic and Cretaceous. The oils of the Williston Basin exhibit a wide range of geochemical characteristics defined as "oil families", although the geochemical signature of the Cambrian Deadwood Formation and Lower Ordovician Winnipeg reservoired oils does not match any "oil family". Despite their close stratigraphic proximity, it is evident that the oils of the Lower Palaeozoic within the Williston Basin are distinct. This suggests the presence of a new "oil family" within the Williston Basin. Diagnostic geochemical signatures occur in the gasoline range chromatograms, within saturate fraction gas chromatograms and biomarker fingerprints. However, some of the established criteria and cross-plots that are currently used to segregate oils into distinct genetic families within the basin do not always meet with success, particularly when applied to the Lower Palaeozoic oils of the Deadwood and Winnipeg Formation.     

低渗透非均质砂岩油藏启动压力梯度研究

理论推导与室内实验相结合,建立了低渗透非均质砂岩油藏启动压力梯度确定方法。首先借助油藏流场与电场相似的原理,推导了非均质砂岩油藏启动压力梯度计算公式。其次基于稳定流实验方法,建立了非均质砂岩油藏启动压力梯度测试方法。结果表明:低渗透非均质砂岩油藏的启动压力梯度确定遵循两个等效原则。平面非均质油藏的启动压力梯度等于各级渗透率段的启动压力梯度关于长度的加权平均;纵向非均质油藏的启动压力梯度等于各渗透率层的启动压力梯度关于渗透率与渗流面积乘积的加权平均。研究成果可用于有效指导低渗透非均质砂岩油藏的合理井距确定,促进该类油藏的高效开发。     

On Tragic Consciousness In The Works By Ernest Hemingway

As an American modern novelist who were famous in the literary world, Hemingway was not a person who always followed the trend but a sharp observer. At the same time, he was a tragedy maestro, he paid great attention on existence, fate and end-result. The dramatis personae's tragedy of his works was an extreme limit by all means tragedy on the meaning of fearless challenge that failed. The beauty of tragedy was not produced on the destruction of life, but now this kind of value was in the impact activity. They performed for the reader about the tragedy on challenging for the limit and the death.     

Design and analysis of generic LBS application developing platform

Location based services is promising due to its novel working style and contents.A software platform is proposed to provide application programs of typical location based services and support new applications developing efficiently. The analysis shows that this scheme is easy implemented, low cost and adapt to all kinds of mobile nework system.     

Achieving Unusual States of Matter Under High Pressure

正The periodicity of the elements and the non-reactivity of the inner-shell electrons are two related principles of chemistry,rooted in the atomic shell structure.Within compounds,Group I elements,for example,invariably assume the+1 oxidation state,and their chemical properties differ completely from those of the p-block elements.These general rules govern our understanding of chemical structures and reactions.Using first principles calcula-     

Exchange-Correlation and Electronic Excitation Energies from Pairing Matrix Fluctuations

We have developed an adiabatic connection to formulate the ground-state exchange-correlation energy in terms of pairing matrix linear fluctuations.This formulation of the exchange-correlation energy opens a new channel for density functional approximations based on the many-body perturbation theory.We illustrate the potential of such approaches with an approximation based on the particle-particle Random Phase Approximation(pp-RPA).This re-     

Density Functional Theory for the Response of Periodic Systems to Electric Fields Based on the Vector Potential Approach

正The electronic and nuclear(structural/vibrational)response of 1D-3D nanoscale systems to electric fields gives rise to a host of optical,mechanical,spectral,etc.properties that are of high theoretical and applied interest.Due to the computational difficulty of treating such large systems it is convenient to model them as infinite and periodic(at least,in first approximation).The fundamental theoretical/computational problem in doing so is that