基于XGBoost算法的电力系统暂态稳定评估

针对暂态稳定评估问题的特点,在改进极限梯度提升(XGBoost)算法的基础上进行暂态稳定评估。根据电网物理特点,定义能够反映电力系统稳态运行状态的特征集;研究XGBoost算法用于暂态稳定评估的过程:针对暂态稳定预测中2类错误严重程度不同的特点,定义包含注意力系数的对数损失函数,使得模型对不稳定样本的误预测情况减少;使用Logistic函数将模型输出概率化,用于衡量XGBoost模型输出的可靠程度,预防部分误预测;给出针对任意系统随机产生样本集的方法。IEEE 39节点系统仿真结果表明,XGBoost算法在准确率上均高于其他几类常用机器学习算法,优化后的损失函数降低了不稳定样本错误分类的可能性,使该算法的召回率较优于其他方法,且概率化输出的形式有助于评估模型输出的可靠程度,降低了误预测的概率。     

含大型光伏电站的动态安全域

从域的角度研究大型光伏电站对电力系统暂态稳定性的影响,用等容量的光伏电站替换常规发电机,计算含大型光伏电站的动态安全域(LPV-DSR)。通过对典型电力系统的大量仿真发现LPV-DSR边界仍可以用超平面拟合。通过与接入光伏电站前系统的DSR进行比较,得出3条经验规律:LVP-DSR边界与接入光伏电站前系统的DSR边界近似平行;从利于系统暂态稳定性的角度看,光伏电站换流器的最佳控制策略为定有功功率控制和定交流电压控制;同一种控制策略下,DSR范围随着接入系统光伏容量的增大而单调变化。     

雷击下金属氧化物避雷器ATP仿真模型分析

在雷电冲击下,对金属氧化物避雷器的动态特性和仿真计算中等效模型的建立进行了分析。讨论了采用分段线性法、单指数和多指数函数拟和法、线性和非线性混合拟和法分析金属氧化物避雷器伏安特性的优缺点。指出金属氧化物避雷器在电磁暂态计算程序(ATP)中宜采用Pinceti模型,在金属氧化物电阻片中电阻非线性的计算宜采用多指数函数拟和法。     

Analysis of PVC Pipe-Wall Viscoelasticity during Water Hammer

This research work focuses on the analysis of hydraulic transients in polyvinyl chloride (PVC) pipes, which are characterized by a viscoelastic rheological behavior. Transient pressure data were collected in a pipe rig consisting of a set of PVC pipes. The creep function of the PVC pipes was determined by using an inverse transient model based on collected transient pressure data and compared with that obtained by carrying out mechanical tensile tests of PVC pipe specimens. The numerical results obtained from the transient solver have shown that the attenuation, dispersion, and shape of transient pressures were well described. The incorporation of the viscoelastic mechanical behavior in the hydraulic transient model has provided an excellent fitting between numerical results and observed data. Calibrated creep function based on inverse analysis fit the one determined by mechanical tests well, which emphasized the importance of pipe-wall viscoelasticity in hydraulic transients in PVC pipes.     

Functional dopant profiling of optical coherent transient materials

A new novel method that functionally maps the distribution of dopants in a photon echo material is proposed that relies on imposing a set of linear orthogonal gradient magnetic fields for a controlled hyperfine splitting of energy levels to create characteristic quantum beats when illuminated with a laser pulse with sufficient bandwidth to interrogate these levels. In this approach, a spectroscopic finger print of the dopant sites due to concentration and field susceptibilities in the sample is achieved through a Fourier decomposition of the radiative relaxation decay in an approach analogous to nuclear magnetic resonance (NMR) spectroscopy due to the imposition of a controlled spatial–spectral encoding scheme. An example of such an interrogative approach uses a three pulse stimulated sequence necessary to probe a gradient resolved voxel. This three pulse approach can be combined with the conventional confocal imaging technique to provide information about the underlying chemistry of dopant distribution along each imaging plane which is useful in guiding the design and manufacturing process of optical crystals. In combination with gradient induced quantum beats, the entire inhomogeneous bandwidth can be interrogated. The proposed approach would scan this entire bandwidth at much faster rate enabling characterization of a large number of crystals than is currently possible through mechanical scanning with a confocal microscopy based spectroscopic technique as well as providing functional dopant profiling which is not currently possible with conventional approaches.     

Inrush Current Parameters Associated with Back-to-Back Switching of Capacitors in a Bank

Back-to-back switching of 50 kVAr capacitors, in a capacitor bank of 200 kVAr is investigated experimentally. The switching inrush currents in a three-phase, 200 kVAr, 415 V Automatic Power Factor Correction panel at power frequency are captured in the laboratory. The characterizing features of inrush current are reported and analyzed. The commercially available two makes of switches (contactors) are used to compare the inrush currents with and without their pre-insertion resistors (PIR). The inrush current scenario of the experimental setup is modeled in EMTDC/PSCAD to ascertain the results. The ratio I_peak/I_N for both the contactors without PIR exceeded the limit mentioned in IEC 60831-1 Ed 3.0 : 2014.     

Effects of shutter transients in molecular beam epitaxy

We have studied the effects of shutter transients (STs) in molecular beam epitaxy (MBE). Two series of samples were grown by MBE and evaluated by X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements. The effects of STs were evaluated by growth rate (GR) analysis using a combination of growth time (GT) and thickness evaluated by XRD and XRR measurements. We revealed two opposite effects of STs: (1) overshoot of GR and (2) increase in GR with GT and subsequent saturation. Each effect was consistent with the previous studies; however, the previous studies showed no relationships between these two effects. By considering closing time of the shutter, the two opposite effects were well understood.     

Reconstruction of missing data in multidimensional time series by fuzzy similarity

The present work addresses the problem of missing data in multidimensional time series such as those collected during operational transients in industrial plants. We propose a novel method for missing data reconstruction based on three main steps: (1) computing a fuzzy similarity measure between a segment of the time series containing the missing data and segments of reference time series; (2) assigning a weight to each reference segment; (3) reconstructing the missing values as a weighted average of the reference segments. The performance of the proposed method is compared with that of an Auto Associative Kernel Regression (AAKR) method on an artificial case study and a real industrial application regarding shut-down transients of a Nuclear Power Plant (NPP) turbine.     

心力衰竭时大鼠心肌细胞肌浆网钙回摄的变化

目的：探讨心力衰竭时心肌细胞肌浆网钙回摄异常的机制。方法：结扎大鼠冠状动脉左前降支制备慢性心衰模型。4周后采用酶解法分离假手术组和心衰组大鼠心室肌细胞,利用激光扫描共聚焦显微镜检测心肌细胞肌浆网钙含量。采用western?blot技术检测心肌细胞肌浆网钙调蛋白SERCA2a、PLN、PP1的表达水平。结果：心衰组的大鼠心肌细胞荧光强度小于假手术组,并且心衰组大鼠心肌细胞咖啡因诱导的钙瞬变也低于假手术组;心衰组大鼠心SERCA2a表达水平降低,PP1的水平升高,PLN总体水平不变,但磷酸化的PLN水平降低。结论：钙泵表达量下降和钙泵活性降低是心力衰竭时心肌细胞肌浆网钙回摄减少的主要原因。     

Influence of Distributed Generation on Distribution Networks During Faults

Distributed generation is playing an increasingly important and positive role in decreasing network losses, reducing electricity prices, maintaining environmental and economical benefits, reducing the investment in generation capacity, and delaying the upgrade of the transmission system. Therefore, researchers have become interested with distributed generation and its connection with the main power network. But before connecting these resources with the main grid on the low-voltage level, the worst-case scenario that could occur must be analyzed first. This article introduces a procedure to study the effect of the distributed generation units on distribution systems during short-circuit periods. This effect is measured in terms of fault current, transient time, voltage total harmonic distortion, voltage sag, and voltage swell. The main objective of this study is to study the influence of distributed generation on distribution systems so the maximum distributed generation penetration that can be adopted in a distribution system without degrading the system performance is determined. A general purpose alternative transients program version of the electromagnetic transient program is used to accomplish the above goals. The proposed method is tested on IEEE 13-node test feeder distribution system to demonstrate its performance.