电网电压跌落下DFIG电磁暂态过程及转子侧控制策略研究

以双馈风力发电机(DFIG)作为对象,对转子侧变换器的定子磁链定向矢量控制是基于假设电网电压稳定。而矢量控制下对电网电压波动时控制性能较差。对DFIG在电网电压跌落时的电磁暂态过程进行分析,仿真得出定子电流的波形与频谱分析,在控制策略中引入定子等效励磁电流,从而引入两个补偿量,重新构建控制框图并进行对比仿真分析显示,加入了补偿量的控制策略与矢量控制方法相比,转子侧的故障电流得到了抑制,提高了DFIG在电网电压跌落时的运行能力。     

电网短路时交流励磁风电机组网侧变换器控制策略

电网短路故障时交流励磁用双脉宽调制（PWM）变换器应提供足够的励磁电压实现交流励磁发电机的不间断运行，要求双PWM变换器直流链电压在故障时波动较小。分析并提出一种电网短路故障时交流励磁风电机组电网侧变换器的控制策略，该方案在电压跌落时仅利用电流内环控制电网侧变换器，并于电压正常时采用带前馈的双闭环电压控制策略控制电网侧变换器。通过仿真验证了所提出的方案在电网短路故障发生和切除时稳定控制直流链电压的有效性，为故障过程发电机不脱网励磁控制奠定了基础，同时该方案也能有效保护直流侧电容及提高系统的稳定性。     

永磁直驱风电系统低电压运行特性的分析

通过构建永磁直驱风电系统的仿真模型，实现网侧变换器输出有功和无功功率的解耦控制，增加卸荷负载以提高其应对电压跌落等故障的穿越能力，对风电系统运行在单位功率因数、超前和滞后功率因数情况下的跌落特性进行了仿真分析，讨论了电压跌落期间风电系统对电网的无功支持。仿真结果表明，直驱式永磁同步电机风电系统具有较强的低电压穿越能力，可以安全运行在不同功率因数下，同时能在电网电压故障期间对系统提供一定的无功支持。     

全功率变流器永磁直驱风电系统低电压穿越特性研究

随着风电机组安装容量的不断上升，风电系统在电网故障情况下的运行变得尤为重要，电网导则要求风电机组在电网电压瞬间跌落一定范围内不脱网运行。针对使用背靠背全功率变流器的永磁直驱风电系统，提出一种在电网电压瞬间跌落情况下不脱网运行的方法。电网发生电压瞬间跌落时，网侧变流器运行在静止无功补偿（STATCOM）模式，依据电网电压跌落的深度决定发出无功电流的大小，通过快速提供无功电流来稳定电网电压，实现直驱型风电系统的低电压穿越功能。仿真和实验结果表明电网电压故障时使直驱风电系统运行在STATCOM模式可以有效提高低电压穿越能力。     

无差拍控制提高变速异步风力发电系统的故障承受能力

变速风力发电机对于电网扰动、故障以及电网电压跌落非常敏感，当上述情况发生时风力发电机将从电网切除，直到电网恢复正常才能重新投入运行。如何在电网发生扰动时保持风力发电机的正常运行是一个有挑战性的课题。未来的电网规范甚至要求在电网发生故障时风力机组仍能保持运行，这就要求必须安装保证风电机组低电压运行的控制装置。文中针对采用全载换流器与电网进行连接的异步风力发电机组提出了相应的风电机组低电压控制器及控制策略，无须进行硬件改造即可实现电网故障情况下风电机组不退出运行。其中，风电机组电气子系统控制器基于无差拍控制策略进行设计。算例给出了300 kW的风电机组在30%电压跌落情况下的暂态过程，没有加入控制器之前风电机组将停机，加入之后则可以保持运行，验证了所提方法的正确性。     

电网故障时永磁直驱风电机组的低电压穿越控制策略

为提高永磁直驱风电机组所并电网的运行稳定性,研究电网故障下永磁直驱风电机组的运行特性以及提高其低电压穿越运行能力,文中提出一种适用于采用双脉宽调制变换器并网的永磁直驱风电机组的低电压穿越运行控制方案。通过在电网故障时限制发电机的电磁功率来限制输入至直流侧电容和电网侧变换器的功率,通过在电网故障时采用考虑发电机功率信息的网侧变换器电流闭环控制来实现直流链电压稳定控制,从而有效实现发电系统的低电压穿越运行。系统仿真结果表明,所提出的控制方案无需增加硬件保护装置,在电网对称及非对称故障下均可有效实现永磁直驱风电机组的低电压穿越运行。     

加速功率型电力系统稳定器输入变量的获取

采用阻尼分析方法，在分析加速功率型电力系统稳定器（PSS）工作原理的基础上，阐述了PSS输入状态量的含义及其作用。分析了电网运行方式或电网结构变化对PSS工作特性的影响，提出了目前加速功率型PSS中不宜采用发电机组机端电压频率信号代替发电机组转速信号的观点。     

双馈感应风力发电机网侧变换器低压运行与控制

电压型双PWM变换器用作双馈风力发电机的交流励磁电源在风力发电系统中得到广泛应用,但在电网故障时会引起电网侧变换器直流链电压波动,影响双馈感应风力发电机的不间断运行。分析了引起交流励磁发电机电网侧变换器直流链电压波动的因素,提出了在电压跌落时采用基于小波模型方法,通过引入恰当的前馈项,使流入母线电容的电流为零。与传统的前馈控制策略进行仿真比较分析,验证了改进的前馈控制策略能在负载突变和电网电压骤降的情况下,有效地控制变换器直流母线电压,提高了直流电压调节速度,减小了振荡幅值,同时也起到保护直流链电容的作用,提高了电力系统的稳定性。     

双馈异步风力发电机开关频率恒定的直接功率控制

与矢量控制相比,直接功率控制(DPC)结构简单,应用于风电系统的双馈异步发电机(DFIG)能简化变频器控制结构,提高系统动态性能。在分析DFIG暂态数学模型的基础上推导了内部状态量与控制量之间的关系,提出了分别基于转子磁链、转子电流和电磁转矩的3种DPC策略,并通过引入空间矢量调制(SVM)技术使DPC策略的开关频率保持恒定。这些策略在电网正常情况下能获得优良的静态性能,而在非正常运行状态各自表现出不同的动态特性,能适用于不同的控制目标。理论分析与仿真实验证明,电网正常情况下各定频DPC可有效实现有功、无功功率的解耦控制;电网电压波动时基于转子磁链DPC(RFDPC)可使DFIG快速进入稳定状态,缩短振荡时间;基于转子电流DPC(RCDPC)可抑制转子电流振荡,防止变频器过流;基于电磁转矩DPC(EMTDPC)可消除电磁转矩脉动,减少对机组转轴剪切应力冲击。     

不平衡电压下双馈异步风力发电系统的建模与控制

提出了不平衡电网电压条件下双馈异步发电机（DFIG）在正、反转同步速旋转坐标系中的完整数学模型，推导和分析了不平衡电网电压条件下DFIG定子输出瞬时有功、无功功率组成。在此基础上，提出了小值稳态不平衡电网电压条件下增强DFIG不间断运行能力的4种可供选择的控制方案。讨论了不同不平衡控制目标下转子正、负序电流指令值计算原则，设计了正、反转同步速旋转坐标系中DFIG的双 dq转子电流控制器的不平衡控制方案，实现了不平衡电网电压条件下转子正、负序电流的独立跟踪控制，有效地提高了小值稳态不平衡电网条件下风电机组的不间断运行能力。仿真研究验证了理论分析的正确性和所提出的双 dq转子电流不平衡控制方案的有效性。     

Call for Papers

正As the eldest and most distributed professional journal of water industry,Water Wastewater Engineering(W WE)gained very wide acceptance from the colleagues working in this field in this country since start publication in 1964.Now it is administrated by the Ministry of Construction of the People’s Republic of China and sponsored by the China Civil Engineering Society etc.     

库水波动下库岸边坡地下水浸润线解析解及误差分析

解析法是库岸边坡地下水浸润线计算中便于实际应用的方法,但该方法须基于若干假定并对潜水运动基本方程线性化后才能求解。针对各假定和线性化过程建立不同的地下水渗流数学模型,用解析法和有限元法解答上述数学模型,分析各误差大小及其规律。结果表明：库岸垂直处理带来的误差要小于方程线性化处理和不考虑非饱和渗流带来的误差,而且它们都随渗透系数变大而减小;在三峡库区库水调度情况下,假定库水位等速变化带来的误差对大多岸坡而言可以忽略;计算库岸边坡地下水浸润线时,解析法只适用于水位变化幅度相比含水层厚度较小,且几何边界规则、岩土结构简单、岩土体渗透性较好时的情况。     

Changes in Summer Irrigated Crop Area and Water Use in Southeastern Turkey from 1993 to 2002: Implications for Current and Future Water Resources

Changes in summer irrigated cropland acreage and related water use are estimated from satellite remote sensing and ancillary data in semi-arid Southeastern Turkey where traditionally dry agricultural lands are being rapidly transformed into irrigated fields with the help of water from the Euphrates-Tigris Rivers. An image classification methodology based on thresholding of Landsat NDVI images from the peak summer period reveals that the total area of summer irrigated crops has increased three-fold (from 35,000 ha to over 100,000) in the Harran Plain between 1993 and 2002. Coupled analysis of annual irrigated crop area from remote sensing and potential evapotranspiration based estimates of irrigation water requirements for cotton indicate a corresponding increase in agricultural water use from about 370 million cubic meters to over one billion cubic meters, a volume in accordance with the state estimates. These estimates have important implications for understanding the rapid changes in current agricultural withdrawals in Southeastern Turkey and form a quantitative basis for exploring the changes in future water demands in the region. For example, expansion of irrigated lands have led to a steady decrease in potential evaporation due to increased roughness and decreased humidity deficit in the Harran Plain. Assuming that the changes in future evaporation conditions will be of similar nature, water use for irrigation is expected to decrease over 40 percent in future irrigation sites. Incorporating this decrease in overall planning of the irrigation projects currently under construction should lead to improved management, and by extension, sustainability of water resources in the region.     

SPATIAL AND TEMPORAL WATER QUALITY VARIABILITY IN AQUATIC HABITATS OF A CULTIVATED FLOODPLAIN

The floodplains of lowland rivers contain diverse aquatic habitats that provide valuable ecosystem services but are perturbed when intensively cultivated. Hydrologic, water chemistry and biological (fish) conditions in five aquatic habitats along the Coldwater River, Mississippi, were measured for more than 4 years: the river, two severed meanders that functioned as backwaters, a managed wetland and an ephemeral channel draining cultivated fields. Off‐channel habitats were connected to downstream regions 0.10% to 32% of the dry season and 24% to 67% of the wet season. The median temperatures for the five monitored sites ranged from 18°C to 23°C, the median total solids concentration for all sites was 135 mg L^?1, the median total phosphorus was 0.29 mg L^?1 and the median total nitrogen was 1.56 mg L^?1. Chemical and physical water quality displayed strong seasonal differences between the wet winter/spring and the dry summer/fall periods so that temporal variation consisted of gradual seasonal trends superimposed on strong diurnal variations. All off‐channel habitats exhibited periods of hypoxia and temperatures >30°C during the dry season. Between‐site gradients of water and habitat quality were strongly coupled to water depth and runoff loading. The rehabilitation of one backwater by increasing water depth and diverting agricultural runoff was associated with improved water quality and fish species richness relative to an adjacent untreated backwater. The diversion of polluted runoff and the use of water control structures to maintain greater water depth were observed to be effective management tools, but the former reduces the water supply to habitats that tend to dry up and the latter reduces connectivity. Published in 2011 by John Wiley & Sons, Ltd.     

Site modelling methods for detecting hydrologic alteration of flood frequency and flood duration in the floodplain below the carlyle dam,lower Kaskaskia River,Illinois, USA

Reports concerning the influence of dams on river hydrology vary among researchers, interest groups and government agencies. These often contradicting statements may occur because changes in hydrology caused by dams are distinct for each dam and river watershed. The objective of this research was to use site specific techniques to determine if the 1967 installation of the Carlyle Dam, lower Kaskaskia River, Illinois, altered flood frequency and duration within the forested floodplain located below the dam. Results indicated a decrease in flood duration and frequency, and a decrease in annual flood frequency variation at a site 6.4 km below the dam. Pre‐dam versus post‐dam differences in flood frequency and duration at the site 32.2 km below the dam were related to climate rather than dam effects. Although dam impacts are a concern, this research shows that distance downstream from the dam and downstream tributary and watershed characteristics should be considered before assuming that the dam has changed hydrologic parameters for portions of rivers. This research also indicates that areas of the lower Kaskaskia River may still maintain hydrologic ecological integrity, and could be targeted for restoration and adaptive management purposes. Hydrologic modelling combined with river gage and on‐site well measurement techniques presented in this study could provide detailed flood frequency and duration information for land use, sociological and geomorphological questions in focus areas within river floodplains. Copyright © 2008 John Wiley & Sons, Ltd.     

浅谈优化Vray渲染在制作三维场景的应用研究

用Vray三维渲染引擎来渲染三维场景,其质量和速度是由一些影响因素和参数设置决定的,不同的参数和因素,可以得出不同的三维渲染效果.因此,分析、优化三维渲染质量和速度的影响因素,正确、合理的选择参数对提高Vray最后渲染阶段的质量和速度非常重要.     

固结、渗流和沉积控制方程研究

本文从两相流体动力学的守恒方程出发，考虑固相、液相的相对运动，推导了可以考虑场的不均匀性和流体压缩性的统一形式的控制方程，并对方程所基于的物理概念和相互联系进行了必要的分析。通过对方程适当形式的表达，可导出用于大、小变形固结、渗流和沉积的简化方程，显示了在不同领域的三个理论存在一定的统一性。方程推导过程并未对本构关系加以限制，适用范围较广。依据实际问题的要求，作了不同形式的简化。     

四川省及重庆市中型水电站效益分析

该文分析了四川省及重庆市几座中型水电站的调查材料表明：四川省及重庆市中型水电站不仅在川渝主网和地方电网中起着很大的作用，并具有巨大的社会效益，同时还有综合利用效益，说明修建中型水电站的优越性．     

掺MgO混凝土的宏细观试验研究

利用MgO膨胀剂的延迟微膨胀效应能显著提高大体积混凝土的力学性能,产生的预压应力还能有效提高大体积混凝土抵抗温度开裂的能力,但目前对其微观机理研究不多。采用宏观与微观相结合的方式,研究了不同活性指数、不同掺量的MgO混凝土的力学性能,利用SEM/EDS微观分析手段揭示了MgO对混凝土性能影响的作用机理。试验结果表明：MgO取代部分水泥掺入混凝土中,混凝土的力学性能降低;掺量为4%~5%时,活性指数为100s的MgO混凝土的膨胀率大于活性指数为50 s的MgO混凝土;MgO混凝土的膨胀特性与Ca,Si,Mg,Al等元素的分布情况有关,掺入的MgO将改变界面区Ca,Si,Mg元素的富集特性,Ca元素和Mg元素更易于在界面区富集,导致混凝土宏观力学性能的降低。     

Statistical tools for thermal regime characterization at segment river scale: Case study of the Ste‐Marguerite River

Suitable thermal fish habitats are constrained by both maximum and minimum temperature tolerances. A multivariate and geostatistical approach was developed to estimate stream thermal characteristics at the river segment scale. Data from 22 temperature‐monitoring stations during summer 2007 were used to estimate monthly maximum temperature as well as thermal characteristics such as the number of events, the cumulative degree–days and the associated duration over specific temperature thresholds of 19 and 21°C. The probability of exceeding these temperature thresholds has also been interpolated. The methodology relies on the construction of a multivariate space using physiographic and hydrological characteristics of gauging stations as inputs in a canonical correlation analysis (CCA). A geostatistical interpolation technique, ordinary kriging, was subsequently used to perform interpolation in the physiographical space constructed using CCA. Results from this study were obtained for thermal characteristics estimated into two different interpolation spaces: (1) a 7 metrics space, and (2) an 8 metrics space. Cross‐validation technique has been performed and satisfactory results were obtained. Kriging thermal characteristics (magnitude and duration) into the 7 metric space for a 19°C threshold exceedance leads to best results with Relative Root Mean Square Error (RRMSE) ranging between 9.66 and 15.08%. The study shows that kriging in a multivariate space is a promising tool for water resources managers, especially in cases where risk mapping for lethal or sub‐lethal temperature thresholds may be required for a specific fish species. Copyright © 2010 John Wiley & Sons, Ltd.