Effect of short-term versus long-term grassland management and seasonal variation in organic and conventional dairy farming on the composition of bulk tank milk

Bulk tank milk from 28 dairy farms was sampled every second month for 2 yr to assess the effects of grassland management, production system and season on milk fatty acid (FA) composition, concentrations of fat-soluble vitamins, Se, and milk sensory quality. Grassland management varied in terms of time since establishment. Short-term grassland management (SG) was defined as establishment or reseeding every fourth year or more often, and long-term grassland management (LG) was defined as less frequent establishment or reseeding. Fourteen organic (ORG) dairy farms with either short-term or long-term grassland management were paired with 14 conventional (CON) farms with respect to grassland management. Within ORG farms, SG farms differed from LG farms in herbage botanical composition, but not in concentrate FA concentrations, dry matter intake, or milk yield. Within CON farms, herbage composition, concentrate FA concentrations, dry matter intake, and milk yield showed no or insignificant variations. The ORG farms differed from CON farms in herbage botanical composition, concentrate FA concentrations, concentrate intake, and milk yield. Compared with ORG-LG farms, ORG-SG farms produced milk fat with higher proportions of C10:0 and C12:0 associated with higher herbage proportions of legumes (Fabaceae) and lower proportions of other dicotyledon families. Compared with milk from CON farms, milk fat from ORG farms had higher proportions of most saturated FA and all n-3 FA, but lower proportions of C18:0 and C18:1 cis-9 associated with higher forage proportion and differences in concentrations of FA in concentrates. Compared with the outdoor-feeding periods, the indoor feeding periods yielded milk fat with higher proportions of most short-chain and medium-chain FA and lower proportions of most C18-FA associated with grazing and higher forage proportions. Milk concentrations of α-tocopherol and β-carotene were lower during the grazing periods. Inclusion of fishmeal in organic concentrates may explain higher Se concentrations in organically produced milk. Milk sensory quality was not affected in this study. In conclusion, grassland management had minor effects on milk composition, and differences between ORG farms and CON farms may be explained by differences in concentrate intake and concentrate FA concentrations. Milk produced on ORG farms versus CON farms and milk produced during the outdoor versus indoor feeding periods had potential health benefits due to FA composition. In contrast, the higher milk-fat proportions of saturated FA in milk from ORG farms may be perceived as negative for human health.     

The economic value of organic dairy farms in Vermont and Minnesota

This study quantifies the overall economic values of organic dairy farms in Vermont and Minnesota and estimates the economic impacts of organic dairy farm sales relative to an equivalent level of sales from conventional dairy farms in those states. This question is of interest because the development of the organic dairy sector has allowed some farms that likely would not have remained in the conventional dairy business to continue being economically viable as organic dairy farms. Thus, these sales provide an economic impact in regions when this milk is exported to nonproducing regions. Organic and conventional dairy farm financial data in Vermont and Minnesota were collected and assembled to develop dairy farm production functions by region and dairy type. These production functions were then used in state-level input-output models to calculate economic impacts. The opportunity costs of organic dairy farm production were also estimated by comparing the relative statewide economic impacts of organic and conventional dairy farms if both experience a hypothetical 5-million-dollar increase in sales. Between 2008 and 2010, Vermont’s 180 organic dairy farms annually contributed $76.3 million in output (the value of an industry’s production within the state), 808 jobs, $34.1 million in gross state product, and $26.3 million in labor income to Vermont’s economy. Between 2009 and 2011, Minnesota’s 114 organic dairy farms annually contributed $77.7 million in output, 552 jobs, $32.1 million in gross state product, and $21 million in labor income to Minnesota’s economy. In Vermont, organic dairy farm sales revenue would result in greater state-wide impacts of 3% in output, 39% in labor income, 33% in gross state product, and 46% in employment relative to the impacts from an equivalent level of sales revenue to conventional dairy farms. In Minnesota, these economic impacts are 4, 9, 11, and 12% greater, respectively, for organic dairy farms relative to conventional dairy farms. This study concludes that organic dairy farms may contribute more to the local economy than average and similar-size conventional dairy farms in the Northeast and Upper Midwest and that organic dairy farm milk production supports economic development in rural communities.     

应用于风电场的SVC与SVG性能比较

应用于风电场的动态无功补偿装置主要有TCR型静止无功补偿装置(SVC)与静止无功发生器(SVG)两大类。基于山西省电力公司关于风电场动态无功补偿装置的测试项目,从工作原理、性能指标以及实用特性三方面对SVC与SVG进行综合分析,对比得出结论:二者均能满足当前国标对动态无功补偿装置提出的要求,且各具利弊,SVG的发展前景更为乐观。     

计及风电不确定性的暂态稳定概率分析

研究风电不确定性对电力系统暂态稳定性的影响。采用威布尔分布模拟风速的概率分布,建立适用于含大规模风电的电力系统暂态稳定分析的双馈型风电机的动态模型。以IEEE 10机39节点系统为算例,在MATLAB和电力系统分析软件DAS-TSAT平台上,运用蒙特卡洛方法,计算得到电力系统暂态稳定性各项指标的概率分布及其随风速变化规律。结果显示,大规模风电接入电网后,在一定的风速范围内,系统暂态稳定性能随着风速的增大而增强。     

故障情况下海上风电机群领导者-跟随者分布式控制策略研究

针对风电机组单机故障和风电场网络通讯故障,以分布式控制取代集中控制,设计2种故障情况下的领导者-跟随者分布式控制策略,减小故障对系统的不利影响。首先,基于Hamilton能量理论,将单机系统模型扩展为风电机群系统模型;然后针对机组故障情况,提出领导者-跟随者分布式控制策略,使得风电机群在领导者或跟随者单机故障下能够保持稳定运行;继而考虑通信故障情况,设计弱连通拓扑下的领导者-跟随者控制策略,以使系统稳定且有功功率输出稳定。最后,通过仿真验证,在机组故障停机和网络通信故障下,领导者-跟随者控制策略能够增强系统的稳定性与可靠性。     

大型风电场等值建模与仿真分析

在研究大型风电场动态特性时,对风电场进行等值建模是一个重要课题。根据风机位置和尾流效应的影响对风电场进行区域划分,将风力发电机组合并简化,在PSCAD/EMTDC仿真平台上建立风电场整体简化模型,对不同的等值模型进行对比分析,给出了各种模型的优缺点和各自的实用性,并提出在动态仿真中风电场简化模型应用的建议。     

风电场接入容量分析与计算

从环境保护和可再生能源利用的角度考虑,应尽量扩大风力发电的规模,但风电本身的特点使得它的并网运行对电网的电能质量以及安全稳定运行构成一定的威胁。风力发电的原动力是自然风,因此风电场的选址主要受风资源分布的限制;风力发电的原动力是不可控的,它是否处于发电状态以及出力的大小都取决于风速状况,风速的不稳定性和易变性决定了风电场的出力也具有波动性和易变性的特点。因此,计算电网中允许接入的风电场最大装机容量是风电发展亟待解决的问题。本文对风电场接入的无功电压等方面的特性进行分析,并对风电场的接入容量进行分析、计算。     

京津唐电网第一个百万千瓦风电基地并网运行分析

京津唐电网第1个百万千瓦风电基地总容量为1300MW,由12个风电场组成,并网线路总长度为605km,到并网点的最长送电距离达189km,计划于2010年前后全部并网运行。根据电网实际运行参数,针对风电场并网后电网的稳态运行和故障情况进行全面的仿真计算分析。对并网线路产生的充电功率、容性及感性无功补偿装置的配置、风电机组有功出力变化引起并网线路潮流波动、系统故障对风电机组的影响及电压的控制措施、风机提供的短路电流等分别进行了研究,提出满足电网安全运行的具体建议。     

基于实时通信的风电场接地故障保护方案

风电系统的保护必须考虑电网和风电机组产生的故障,其中接地故障使相电压升高到线电压和产生瞬间过压,能降低绝缘并对设备造成损害.文中主要讨论用IEC 61850中的通用面向对象变电站事件(GOOSE)报文解决馈线接地故障保护问题,从分析一种实际的风电场和风电机组保护模型以及接地保护启动功能出发,介绍了接地故障中风电智能电子设备(IED)信息通信模型和各IED之间的交互模型,并用PSCAD软件对风电接地故障进行了仿真.设计了保护方案中的GOOSE跳闸报文的实现,通过对GOOSE报文进行模型实验和分析各IED之间交互的时间序列,可证明用信息化手段能很好地解决接地故障,避免风电机组受到损害.     

基于钒电池储能系统的风电场并网功率控制

随着风力发电并网容量的增加,风电场功率波动对电网的影响越来越大.为提高风电场并网运行的稳定性,在其出口处增加新型环保钒氧化还原液流电池(VRB)储能系统,以有效调节并网功率.根据VRB的等效数学模型,分析了VRB荷电状态与端电压之间的变化特点,采用一级双向DC/AC变换器作为VRB储能系统的功率调节器,设计了相应的充放电控制与能量管理策略,并对具有VRB储能单元的风电场并网系统进行了建模和仿真.仿真结果表明,在风速波动的情况下,采用VRB储能系统能够快速、有效地平滑风电场输出的有功功率波动,并可为电网提供一定的无功支持,有效地改善了风电场的并网运行性能.