Controversy exists on the magnitude and variability of farm nutrient balances and quality of arable land in sub-Saharan Africa with Kenya among those affected negatively. This study investigates quality of arable land...Controversy exists on the magnitude and variability of farm nutrient balances and quality of arable land in sub-Saharan Africa with Kenya among those affected negatively. This study investigates quality of arable land by fitting multivariate multilevel model to farm nutrient balance data collected from five agro-climatic zones of Kenya (arable lands). Objectives of the study were to investigate the magnitude and variability of Nitrogen, Phosphorus and Potassium (NPK) farm nutrient balances in arable lands of Kenya, study effects of agro-climatic zones on nutrient balances and to determine effects of household resource endowments on NPK nutrient balances. The study concludes that agro-climatic zones differ with respect to farm nutrient balances;that livestock resource endowments and hired labour have positive effects on the magnitude and direction of farm nutrient balances;and that household ownership of large capital resources do not guarantee a positive effect on farm nutrient balances. The study recommends integration of sound livestock practices and application of agro-climatic zone differentiated interventions in future strategies for addressing farm nutrient balances and arable land quality, and the use of large sample sizes and relevant factors/covariates in future analysis to shed additional insights on farm nutrient balances and on how arable land quality can be improved.展开更多
针对一种新型并联全桥子模块构成的模块化多电平换流器（paralleled full bridge sub-module MMC,PFB-MMC）拓扑,提出一种适用于任意新型双端口MMC的电磁暂态通用等效建模方法。传统单端口子模块MMC的等效算法要求全部子模块流过相同的...针对一种新型并联全桥子模块构成的模块化多电平换流器（paralleled full bridge sub-module MMC,PFB-MMC）拓扑,提出一种适用于任意新型双端口MMC的电磁暂态通用等效建模方法。传统单端口子模块MMC的等效算法要求全部子模块流过相同的桥臂电流,这对双端口MMC不再适用。针对这一问题,适用于双端口MMC的迭代求解算法被提出。MMC桥臂中全部子模块的内部节点和模块间的互联节点均被迭代消去,整个桥臂在电磁暂态解算器中被等效成仅含4个外部节点,尽管在这一过程中求解的步骤有所增加,但依然能够完整保留各桥臂全部的内部信息。对降阶导纳矩阵进行求解后,即可完成之前被等效消去的节点信息例如电容电压、连接节点电流的更新。因此,该文提出的建模方法,能够在精确仿真系统内、外部动态特性的同时,大幅提高其电磁暂态仿真速度。在PSCAD/EMTDC环境中验证所提出模型的精度和加速比。展开更多
RTDS(real time digital simulation)上小步长仿真模块对开关元件的数量有严格的限制,而模块化多电平换流器(modular multilevel converter,MMC)的每个子模块都有2个电力电子开关元件IGBT。随着系统容量的不断增加,如何在有限的硬件资...RTDS(real time digital simulation)上小步长仿真模块对开关元件的数量有严格的限制,而模块化多电平换流器(modular multilevel converter,MMC)的每个子模块都有2个电力电子开关元件IGBT。随着系统容量的不断增加,如何在有限的硬件资源中实现多电平系统的仿真成为实时仿真系统的瓶颈。对如何在RTDS大步长的环境中建立正确的IGBT元件等效模型进行了分析,根据IGBT在MMC子模块(sub-module,SM)中的工作原理,采用开关函数建立IGBT的等效数学模型。该模型能够有效减小仿真占用的资源,扩大仿真容量,在大步长环境下能够正确表征IGBT的运行特性。采用Matlab验证了所建立的IGBT模型的可行性和正确性。展开更多
针对超大规模模块化多电平换流器高压直流输电(modular multilevel converter based HVDC,MMC-HVDC)在诸如PSCAD/EMTDC、MATLAB等电磁暂态仿真环境中仿真速度极慢问题,基于节点电压分析法,提出超大规模MMC电磁暂态仿真提速模型。所提出...针对超大规模模块化多电平换流器高压直流输电(modular multilevel converter based HVDC,MMC-HVDC)在诸如PSCAD/EMTDC、MATLAB等电磁暂态仿真环境中仿真速度极慢问题,基于节点电压分析法,提出超大规模MMC电磁暂态仿真提速模型。所提出模型本质为将MMC在计算过程中的超大规模矩阵分解为相应数目的小矩阵,在与原有矩阵同解以保证仿真精度的前提下,仿真速度显著提升。所提出的模型具有极强的通用性,且可以进行开关器件级别的故障研究。在PSCAD/EMTDC中搭建的不同电平数MMC验证了提速模型的仿真精度及提速效果。展开更多
文摘Controversy exists on the magnitude and variability of farm nutrient balances and quality of arable land in sub-Saharan Africa with Kenya among those affected negatively. This study investigates quality of arable land by fitting multivariate multilevel model to farm nutrient balance data collected from five agro-climatic zones of Kenya (arable lands). Objectives of the study were to investigate the magnitude and variability of Nitrogen, Phosphorus and Potassium (NPK) farm nutrient balances in arable lands of Kenya, study effects of agro-climatic zones on nutrient balances and to determine effects of household resource endowments on NPK nutrient balances. The study concludes that agro-climatic zones differ with respect to farm nutrient balances;that livestock resource endowments and hired labour have positive effects on the magnitude and direction of farm nutrient balances;and that household ownership of large capital resources do not guarantee a positive effect on farm nutrient balances. The study recommends integration of sound livestock practices and application of agro-climatic zone differentiated interventions in future strategies for addressing farm nutrient balances and arable land quality, and the use of large sample sizes and relevant factors/covariates in future analysis to shed additional insights on farm nutrient balances and on how arable land quality can be improved.
文摘针对一种新型并联全桥子模块构成的模块化多电平换流器（paralleled full bridge sub-module MMC,PFB-MMC）拓扑,提出一种适用于任意新型双端口MMC的电磁暂态通用等效建模方法。传统单端口子模块MMC的等效算法要求全部子模块流过相同的桥臂电流,这对双端口MMC不再适用。针对这一问题,适用于双端口MMC的迭代求解算法被提出。MMC桥臂中全部子模块的内部节点和模块间的互联节点均被迭代消去,整个桥臂在电磁暂态解算器中被等效成仅含4个外部节点,尽管在这一过程中求解的步骤有所增加,但依然能够完整保留各桥臂全部的内部信息。对降阶导纳矩阵进行求解后,即可完成之前被等效消去的节点信息例如电容电压、连接节点电流的更新。因此,该文提出的建模方法,能够在精确仿真系统内、外部动态特性的同时,大幅提高其电磁暂态仿真速度。在PSCAD/EMTDC环境中验证所提出模型的精度和加速比。
文摘RTDS(real time digital simulation)上小步长仿真模块对开关元件的数量有严格的限制,而模块化多电平换流器(modular multilevel converter,MMC)的每个子模块都有2个电力电子开关元件IGBT。随着系统容量的不断增加,如何在有限的硬件资源中实现多电平系统的仿真成为实时仿真系统的瓶颈。对如何在RTDS大步长的环境中建立正确的IGBT元件等效模型进行了分析,根据IGBT在MMC子模块(sub-module,SM)中的工作原理,采用开关函数建立IGBT的等效数学模型。该模型能够有效减小仿真占用的资源,扩大仿真容量,在大步长环境下能够正确表征IGBT的运行特性。采用Matlab验证了所建立的IGBT模型的可行性和正确性。
文摘针对超大规模模块化多电平换流器高压直流输电(modular multilevel converter based HVDC,MMC-HVDC)在诸如PSCAD/EMTDC、MATLAB等电磁暂态仿真环境中仿真速度极慢问题,基于节点电压分析法,提出超大规模MMC电磁暂态仿真提速模型。所提出模型本质为将MMC在计算过程中的超大规模矩阵分解为相应数目的小矩阵,在与原有矩阵同解以保证仿真精度的前提下,仿真速度显著提升。所提出的模型具有极强的通用性,且可以进行开关器件级别的故障研究。在PSCAD/EMTDC中搭建的不同电平数MMC验证了提速模型的仿真精度及提速效果。
