可再生能源大规模并网导致电力系统转动惯量降低,在扰动事件下的频率稳定问题突出。时域仿真存在计算量大、运算耗时长等缺陷,难以满足复杂多变运行方式和海量预想事故下的频率指标快速评估需求。为了实现功率扰动事件下系统惯性中心多...可再生能源大规模并网导致电力系统转动惯量降低,在扰动事件下的频率稳定问题突出。时域仿真存在计算量大、运算耗时长等缺陷,难以满足复杂多变运行方式和海量预想事故下的频率指标快速评估需求。为了实现功率扰动事件下系统惯性中心多维频率指标(极值频率、最大频率变化率、准稳态频率)的快速评估,该文将深度学习引入到频率稳定研究中,提出一种基于改进堆栈降噪自动编码器(improved stacked denoising autoencoders,ISDAE)的智能化评估方法。首先,利用随机森林算法筛选出重要特征变量作为输入数据,实现输入数据降维;然后,将多个降噪自动编码器堆叠,构建深度学习网络结构;采用"预训练-参数微调"方法训练网络参数,引入Dropout技术提高算法泛化能力、防止过拟合,基于均方根反向传播(root mean square back propagation,RMSprop)优化方法对网络参数进行微调,减小陷入局部最优的概率;最后,根据离线训练得到的ISDAE网络结构实现扰动事件后系统惯性中心的多维频率指标在线评估。在修改后的IEEE RTS-79系统进行测试,与时域仿真、浅层神经网络以及未改进的SDAE方法所得结果进行比较,验证所提方法的快速性、准确性以及良好的泛化能力。展开更多
<strong>Purpose: </strong><span style="font-family:;" "=""><span style="font-family:Verdana;">The purpose of this study was to assess the impact of free caesa...<strong>Purpose: </strong><span style="font-family:;" "=""><span style="font-family:Verdana;">The purpose of this study was to assess the impact of free caesarean section on maternal-fetal prognosis. </span><b><span style="font-family:Verdana;">Materials and Methods:</span></b><span style="font-family:Verdana;"> This was a descriptive, cross-sectional, comparative analytical study of one year of non-free caesarean section and three years of free with retrospective data collection. Our study took place from January 1, 2004 to December 31, 2004 and from January 1, 2007 to December 31, 2009. </span><b><span style="font-family:Verdana;">Results:</span></b><span style="font-family:Verdana;"> Caesarean section completion rates are 1.28%, 1.30%, 1.53%, 1.32% respectively in 2004, 2007, 2008, 2009. The average age was 27 years with extremes of 15 to 40 years. The peak of evacuations was recorded in 2007 with a rate of 89.4% and the peak of direct admissions in 2009 with a rate of 14.9%. An adequate means of transport was used in 77.20%. In 22.80% of cases an inadequate means was used. The indications of caesarean sections were dominated by preeclampsi</span></span><span style="font-family:Verdana;">a</span><span style="font-family:;" "=""><span style="font-family:Verdana;"> 211/1045 caesarean section followed by acute fetal suffering 179/1045;86/1045;placenta previa 64/1045;presentations of the shoulder </span><i><span style="font-family:Verdana;">i.e.</span></i><span style="font-family:Verdana;"> 53/1045;pre-rupture syndrome 44/1045;retro-placental hematoma 27/1045. In terms of maternal prognosis: we noted maternal deaths with a rate of 2.4%</span></span><span style="font-family:Verdana;">, </span><span style="font-family:Verdana;">2</span><span style="font-family:Verdana;">.</span><span style="font-family:Verdana;">4%</span><span style="font-family:Verdana;">, </span><span style="font-family:Verdana;">4</span><span style="font-family:Verdana;">.</span><span style="font-family:Verdana;">3%</span><span style="font-family展开更多
This study explores the irregularity and complexity of strong earthquake ground motions from the perspective of fractal geometry, and constructs a relation with the frequency content of the ground motions. The box-cou...This study explores the irregularity and complexity of strong earthquake ground motions from the perspective of fractal geometry, and constructs a relation with the frequency content of the ground motions. The box-counting fractal dimensions and five representative period parameters of near-fault ground motions from the Chi-Chi and Northridge earthquakes are calculated and compared. Numerical results indicate that the acceleration and velocity time histories of ground motions present the statistical fractal property, and the dominant pulses of near-fault ground motions have a significant influence on their box dimensions and periods. Further, the average box dimension of near-fault impulsive ground motions is smaller, and their irregular degree of wave forms is lower. Moreover, the box dimensions of ground motions reflect their frequency properties to a large extent, and can be regarded as an alternative indicator to represent their frequency content. Finally, the box dimension D of the acceleration histories shows a considerably negative correlation with the mean period T. Meanwhile, the box dimension of the velocity histories Dye is negatively correlated with the characteristic period T and improved characteristic period Tgi.展开更多
文摘可再生能源大规模并网导致电力系统转动惯量降低,在扰动事件下的频率稳定问题突出。时域仿真存在计算量大、运算耗时长等缺陷,难以满足复杂多变运行方式和海量预想事故下的频率指标快速评估需求。为了实现功率扰动事件下系统惯性中心多维频率指标(极值频率、最大频率变化率、准稳态频率)的快速评估,该文将深度学习引入到频率稳定研究中,提出一种基于改进堆栈降噪自动编码器(improved stacked denoising autoencoders,ISDAE)的智能化评估方法。首先,利用随机森林算法筛选出重要特征变量作为输入数据,实现输入数据降维;然后,将多个降噪自动编码器堆叠,构建深度学习网络结构;采用"预训练-参数微调"方法训练网络参数,引入Dropout技术提高算法泛化能力、防止过拟合,基于均方根反向传播(root mean square back propagation,RMSprop)优化方法对网络参数进行微调,减小陷入局部最优的概率;最后,根据离线训练得到的ISDAE网络结构实现扰动事件后系统惯性中心的多维频率指标在线评估。在修改后的IEEE RTS-79系统进行测试,与时域仿真、浅层神经网络以及未改进的SDAE方法所得结果进行比较,验证所提方法的快速性、准确性以及良好的泛化能力。
文摘<strong>Purpose: </strong><span style="font-family:;" "=""><span style="font-family:Verdana;">The purpose of this study was to assess the impact of free caesarean section on maternal-fetal prognosis. </span><b><span style="font-family:Verdana;">Materials and Methods:</span></b><span style="font-family:Verdana;"> This was a descriptive, cross-sectional, comparative analytical study of one year of non-free caesarean section and three years of free with retrospective data collection. Our study took place from January 1, 2004 to December 31, 2004 and from January 1, 2007 to December 31, 2009. </span><b><span style="font-family:Verdana;">Results:</span></b><span style="font-family:Verdana;"> Caesarean section completion rates are 1.28%, 1.30%, 1.53%, 1.32% respectively in 2004, 2007, 2008, 2009. The average age was 27 years with extremes of 15 to 40 years. The peak of evacuations was recorded in 2007 with a rate of 89.4% and the peak of direct admissions in 2009 with a rate of 14.9%. An adequate means of transport was used in 77.20%. In 22.80% of cases an inadequate means was used. The indications of caesarean sections were dominated by preeclampsi</span></span><span style="font-family:Verdana;">a</span><span style="font-family:;" "=""><span style="font-family:Verdana;"> 211/1045 caesarean section followed by acute fetal suffering 179/1045;86/1045;placenta previa 64/1045;presentations of the shoulder </span><i><span style="font-family:Verdana;">i.e.</span></i><span style="font-family:Verdana;"> 53/1045;pre-rupture syndrome 44/1045;retro-placental hematoma 27/1045. In terms of maternal prognosis: we noted maternal deaths with a rate of 2.4%</span></span><span style="font-family:Verdana;">, </span><span style="font-family:Verdana;">2</span><span style="font-family:Verdana;">.</span><span style="font-family:Verdana;">4%</span><span style="font-family:Verdana;">, </span><span style="font-family:Verdana;">4</span><span style="font-family:Verdana;">.</span><span style="font-family:Verdana;">3%</span><span style="font-family
基金National Natural Science Foundation of China under Grant Nos.50978047 and 11332004National Basic Research Program of China under Grant No.2010CB832703
文摘This study explores the irregularity and complexity of strong earthquake ground motions from the perspective of fractal geometry, and constructs a relation with the frequency content of the ground motions. The box-counting fractal dimensions and five representative period parameters of near-fault ground motions from the Chi-Chi and Northridge earthquakes are calculated and compared. Numerical results indicate that the acceleration and velocity time histories of ground motions present the statistical fractal property, and the dominant pulses of near-fault ground motions have a significant influence on their box dimensions and periods. Further, the average box dimension of near-fault impulsive ground motions is smaller, and their irregular degree of wave forms is lower. Moreover, the box dimensions of ground motions reflect their frequency properties to a large extent, and can be regarded as an alternative indicator to represent their frequency content. Finally, the box dimension D of the acceleration histories shows a considerably negative correlation with the mean period T. Meanwhile, the box dimension of the velocity histories Dye is negatively correlated with the characteristic period T and improved characteristic period Tgi.
