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定面积的紧凸体的超空间的拓扑结构 被引量:2
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作者 杨鎏 《南京大学学报(自然科学版)》 CAS CSCD 北大核心 2016年第3期558-565,共8页
主要研究了具有某种几何性质的紧凸体的超空间的拓扑结构.实际上证明了:欧氏平面R^2上面积为正数v_0的紧凸体全体,赋予Hausdorff度量拓扑所构成的超空间,是一个Q-流形其中Q表示赋予乘积拓扑的Hilbert方体[-1,1]~ω.
关键词 定面积 紧凸体 超空间 Q-流形
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直线与圆锥曲线围成定面积其弦上定比例点的轨迹 被引量:1
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作者 舒阳春 《大学数学》 2015年第4期120-122,共3页
通过一个大学数学竞赛题的推广,证明了一直线与圆锥曲线所围成定面积时,其弦上定比例点的轨迹与原圆锥曲线是同一类型的曲线.
关键词 圆锥曲线 定面积 轨迹
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基于定关断面积的混合双馈入直流输电系统换相失败抑制策略 被引量:11
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作者 朱佳 李银红 段献忠 《电网技术》 EI CSCD 北大核心 2018年第11期3735-3743,共9页
为充分发挥电压源型换流器高压直流输电(voltage source converter high voltage direct current,VSC-HVDC)对交流系统无功支撑的作用,抑制电网换相换流器高压直流输电(line-commutatedconverterhighvoltagedirectcurrent,LCC-HVDC... 为充分发挥电压源型换流器高压直流输电(voltage source converter high voltage direct current,VSC-HVDC)对交流系统无功支撑的作用,抑制电网换相换流器高压直流输电(line-commutatedconverterhighvoltagedirectcurrent,LCC-HVDC)换相失败的发生,提出了一种基于定关断面积的混合双馈入直流输电系统换相失败抑制策略。基于定关断面积得出交流电压补偿值,附加至VSC-HVDC逆变器外环无功控制环节,提升故障发生后VSC-HVDC对受端交流系统的无功支撑能力;增加VSC-HVDC动态功率限幅调节控制,优先保证无功功率的传输,同时尽可能减少有功功率的降幅。基于PSCAD/EMTDC进行了仿真分析,仿真结果证明了该控制策略可显著提升LCC-HVDC抵御换相失败的能力,且有效减少直流系统传输功率的降幅。 展开更多
关键词 混合双馈入直流输电系统 协调控制 换相失败 关断面积 动态功率限幅
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华东多直流馈入系统换相失败预防和自动恢复能力的优化 被引量:81
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作者 李新年 陈树勇 +3 位作者 庞广恒 雷霄 刘耀 王晶芳 《电力系统自动化》 EI CSCD 北大核心 2015年第6期134-140,共7页
根据华东电网2013年夏季大负荷方式BPA动态等值数据,在电磁暂态程序中采用了与直流工程控制保护一致的详细模型建立华东电网多直流馈入输电系统研究模型,实现了南瑞集团公司、许继集团有限公司以及ABB集团多种技术路线直流控制保护详细... 根据华东电网2013年夏季大负荷方式BPA动态等值数据,在电磁暂态程序中采用了与直流工程控制保护一致的详细模型建立华东电网多直流馈入输电系统研究模型,实现了南瑞集团公司、许继集团有限公司以及ABB集团多种技术路线直流控制保护详细模型的有效结合。对华东地区六回直流工程中采用的换相失败预测方法和低压限流参数进行对比分析,给出各工程间参数的差异。在此基础上,研究了优化换相失败预测参数、增加定关断面积法、增大额定关断角对抵御换相失败的作用。最后,通过对低压限流环节参数和瞬时电流限制的研究,提出了直流换相失败后的恢复策略,研究结果可为直流系统运行提供技术参考。 展开更多
关键词 多直流馈入系统 换相失败 关断面积 直流控制保护
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An Econometric Analysis on the Effect of Climate Change on Wheat Cropping Area in China 被引量:3
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作者 侯麟科 张同龙 +1 位作者 蔡颖萍 崔永伟 《Agricultural Science & Technology》 CAS 2012年第3期686-688,共3页
[Objective] This study aimed to explore the impact of climate change on wheat cropping by using province-specific historical data during 1996-2007. [Method] We established a panel data econometric model with lagged wh... [Objective] This study aimed to explore the impact of climate change on wheat cropping by using province-specific historical data during 1996-2007. [Method] We established a panel data econometric model with lagged wheat cropping area and province-specific fixed-effects model to control the unobserved factors. [Result] The results showed that the temperature positively affects wheat cropping area, while precipitation does not have such impact. [Conclusion] The study provided empirical evidence for analysis of the determinants of wheat cropping area in China. 展开更多
关键词 Climate change Wheat cropping area Fixed-effects model
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基于多角度预测的混合双馈入直流输电系统换相失败抑制策略 被引量:1
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作者 朱佳 左剑 李银红 《华南理工大学学报(自然科学版)》 EI CAS CSCD 北大核心 2018年第10期24-31,共8页
提出了一种基于多角度预测的混合双馈入直流输电系统换相失败抑制策略.结合定关断面积预测法和零序检测及αβ转换预测法的优势综合得出关断角参考值γref,利用关断角参考值γ_(ref)和关断角设定值γ_(set)的差值获得无功补偿量,附加至... 提出了一种基于多角度预测的混合双馈入直流输电系统换相失败抑制策略.结合定关断面积预测法和零序检测及αβ转换预测法的优势综合得出关断角参考值γref,利用关断角参考值γ_(ref)和关断角设定值γ_(set)的差值获得无功补偿量,附加至电压源型换流器高压直流输电(VSC-HVDC)的外环无功控制环节,调节其故障后发出的无功功率,提升电网换相换流器高压直流输电(LCC-HVDC)抵御换相失败的能力.该方法不依赖于对LCCHVDC关断角的实时测量,在实际工程中更易实现.采用PSCAD/EMTDC仿真软件进行了算例仿真分析,仿真结果证明了该控制策略的有效性. 展开更多
关键词 混合双馈入直流输电系统 协调控制 换相失败 关断面积预测法 零序检测法 αβ转换预测法
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Allometric equations for estimating leaf area index(LAI) of two important tropical species(Tectona grandis and Dendrocalamus strictus) 被引量:2
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作者 Dhaval Vyas Nirav mehta +1 位作者 J.Dina-karan N.S.R.Krishnayya 《Journal of Forestry Research》 SCIE CAS CSCD 2010年第2期197-200,I0006,共5页
Leaf area index(LAI) of Teak(Tectona grandis) and Bamboo(Dendrocalamus strictus) grown in Shoolpaneshwar Wildlife Sanctuary of Narmada District,Gujarat,India was obtained by destructive sampling,photo-grid metho... Leaf area index(LAI) of Teak(Tectona grandis) and Bamboo(Dendrocalamus strictus) grown in Shoolpaneshwar Wildlife Sanctuary of Narmada District,Gujarat,India was obtained by destructive sampling,photo-grid method and by litter trap method.An allometric equation(between leaf area by litter trap method and canopy spread area) was developed for the determination of LAI.Results show that LAI value calculated by the developed allometric equation was similar to that estimated by destructive sampling and photo-grid method,with Root Mean Square Error(RMSE) of 0.90 and 1.15 for Teak,and 0.38 and 0.46 for Bamboo,respectively.There was a perfect match in both the LAI values(estimated and calculated),indicating the accuracy of the developed equations for both the species.In conclusion,canopy spread is a better and sensitive parameter to estimate leaf area of trees.The developed equations can be used for estimating LAI of Teak and Bamboo in tropics. 展开更多
关键词 BAMBOO canopy spread area leaf area index specific leaf area TEAK tropical forest
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The Definition and Classification of Extensive and Persistent Extreme Cold Events in China 被引量:40
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作者 PENG Jing-Bei BUEH Cholaw 《Atmospheric and Oceanic Science Letters》 2011年第5期281-286,共6页
Using the observed daily temperatures from 756 stations in China during the period from 1951 to 2009 extensive and persistent extreme cold events (EPECEs) were defined according to the following three steps:1) a stati... Using the observed daily temperatures from 756 stations in China during the period from 1951 to 2009 extensive and persistent extreme cold events (EPECEs) were defined according to the following three steps:1) a station was defined as an extreme cold station (ECS) if the observed temperature was lower than its 10th percentile threshold;2) an extensive extreme cold event was determined to be present if the approximated area occupied by the ECSs was more than 10% of the total area of China (83rd percentile) on its starting day and the maximum area occupied by the ECSs was at least 20% of the total area of China (96th percentile);and 3) an EPECE was determined to be present if the extensive extreme cold event lasted for at least for eight days.52 EPECEs were identified in this manner,and these identification results were also verified using other reliable data.On the basis of cluster analysis,five types of EPECEs were classified according to the spatial distribution of ECSs at their most extensive time over the course of the EPECE. 展开更多
关键词 EPECE percentile cluster analysis
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Influence of surface roughness on flotation kinetics of quartz 被引量:6
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作者 M.Rahimi M.R.Aslani B.Rezai 《Journal of Central South University》 SCIE EI CAS 2012年第5期1206-1211,共6页
Surface roughness of quartz particles was determined by measuring the specific surface area of particles.The wettability characteristics of particles were determined by measuring the flotation rate using a laboratory ... Surface roughness of quartz particles was determined by measuring the specific surface area of particles.The wettability characteristics of particles were determined by measuring the flotation rate using a laboratory flotation cell.Experimental results show that the rod mill product has higher roughness than the ball mill product.For the particles with larger surface roughness,the flotation kinetics constant is also higher.Finally,empirical relationships between surface roughness(r) and the flotation kinetics constant(k) of quartz particles as k=A+Br+Cr0.5lnr+D/lnr+E/r and k=A+Br are presented,in which A,B,C,D and E are constants related to experimental conditions and mineralogical properties of mineral. 展开更多
关键词 GRINDING flotation kinetics surface roughness ball mill rod mill
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最佳重合闸时间的分析和计算 被引量:1
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作者 郭宗成 《赣南师范学院学报》 2001年第3期37-39,共3页
当输电线路发生永久性故障时 ,系统重合于永久性故障时对系统运行有利的最佳重合闸时间的计算至今没有定论 .本文应用等面积定则原理对此进行了探讨 。
关键词 面积 重合闸时间 功率角 电力系统 输电线路
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Preparation of National CRMs of Active Compounds from Natural Products
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作者 Duan Wenjuan Geng Yanling Wang Daijie Zhao Xianen Liu Jianhua Wang Xiao 《China Standardization》 2010年第3期19-25,共7页
According to the standard of GB/T1500.3-2008 and the guidelines of ISO/REMCO, the preparation of National Certified Reference Materials (CRMs) of active compounds was composed of the following two aspects: preparat... According to the standard of GB/T1500.3-2008 and the guidelines of ISO/REMCO, the preparation of National Certified Reference Materials (CRMs) of active compounds was composed of the following two aspects: preparation and certification. This paper presents an example of the preparation of biochanin A to illustrate the preparation of National CRMs of active compounds from natural products. Firstly, the target compound was extracted and purified from crude materials. Then, its chemical structure was identified by UV spectrum, IR spectrum, ESI-MS, and NMR. The purity of the compound was determined by HPLC with the area normalization method. The certified value assessed by comparing the purity values from six independent qualified laboratories and the value of uncertainty were given. Thermal analysis, residual solvent, homogeneity testing and stability testing was tested in the process. 展开更多
关键词 CRMs biochanin A natural products PREPARATION CERTIFICATION
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Application of a Step-by-Step Fingerprinting Identification Method on a Spilled Oil Accident in the Bohai Sea Area 被引量:4
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作者 SUN Peiyan GAO Zhenhui +4 位作者 CAO Lixin WANG Xinping ZHOU Qing ZHAO Yuhui LI Guangmei 《Journal of Ocean University of China》 SCIE CAS 2011年第1期35-41,共7页
In recent years,oil spill accidents occur frequently in the marine area of China.Finding out the spilled oil source is a key step in the relevant investigation.In this paper,a step-by-step fingerprinting identificatio... In recent years,oil spill accidents occur frequently in the marine area of China.Finding out the spilled oil source is a key step in the relevant investigation.In this paper,a step-by-step fingerprinting identification method was used in a spilled oil accident in the Bohai Sea in 2002.Advanced chemical fingerprinting and data interpretation techniques were used to characterize the chemical composition and determine the possible sources of two spilled oil samples.The original gas chromatography -flame ionization detec-tion (GC-FID) chromatogram of saturated hydrocarbons was compared.The gas chromatography-mass spectrometry (GC/MS) chromatograms of aromatic hydrocarbons terpane and sterane,n-alkane and poly-aromatic hydrocarbons (PAHs) were analyzed.The correlation analysis on diagnostic ratios was performed with Student’s t-test.It is found that the oil fingerprinting of the spilled oil (designated as sz1) from the polluted sand beach was identical with the suspected oil (designated as ky1) from a nearby crude oil refinery factory.They both showed the fingerprinting character of mixed oil.The oil fingerprinting of the spilled oil (designated as ms1) collected from the port was significantly different from oil ky1 and oil sz1 and was with a lubricating oil fingerprint character.The identification result not only gave support for the spilled oil investigation,but also served as an example for studying spilled oil accidents. 展开更多
关键词 step-by-step fingerprinting identification saturated hydrocarbons poly-aromatic hydrocarbons biomarkers Student'st-test
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On a Fixed Value Theorem for Directed Areas in Conic Circumscribed Polygons and Applications 被引量:2
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作者 YU De-sheng 《Chinese Quarterly Journal of Mathematics》 CSCD 2009年第4期485-490,共6页
Using polar equations for conic sections, we research conic circumscribed n-sided polygons(n ≥ 4) deeply on the basis of papers[1-3]. We obtain a general fixed value theorem for directed areas of some triangles in ... Using polar equations for conic sections, we research conic circumscribed n-sided polygons(n ≥ 4) deeply on the basis of papers[1-3]. We obtain a general fixed value theorem for directed areas of some triangles in conic circumscribed n-sided polygons and derive as many as n(n - 3) concurrent points of three lines and some other collinear, equiareal results in conic circumscribed n-sided polygons(n ≥ 4). So the results of papers[1-3] are unified. 展开更多
关键词 conic circumscribed polygon (directed) area fixed value CONCURRENT COLLINEAR
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Adsorption of Orange II Dye using Activated Carbon Produced from Iraqi Date-Palm Stones
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作者 Ismail Ibrahim Al-Khateeb Ali A. Al-Mehemdy 《Journal of Chemistry and Chemical Engineering》 2011年第8期715-719,共5页
A synthetic activated carbon was manufactured from date-palm stones using zinc chloride, cadmium chloride and sodium chloride at different temperatures and atmospheres. Specific surface area, RC and pH were measured a... A synthetic activated carbon was manufactured from date-palm stones using zinc chloride, cadmium chloride and sodium chloride at different temperatures and atmospheres. Specific surface area, RC and pH were measured and the activated carbon was used to absorb the orange II dye. Adsorption equilibrium was also studied, and it was found that the adsorption process on the synthetic activated carbon fit very well with the Langmuir isotherm model and less fit with Freundlich isotherm model. The specific surface area was highly affected by increasing temperature of combustion while the atmosphere of combustion had a little effects on surface area. The results showed that the atmosphere and temperature had no such effect on RC values. However, the RC of zinc chloride samples gave the highest values compared to that of cadmium chloride or sodium chlorides. The values of pH indicated an acid medium for cadmium chloride to alkali medium for sodium chloride, while the samples of zinc chloride showed a neutral medium. 展开更多
关键词 Activated carbon orange II dye date-palm stones: adsorption.
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Removal of elemental mercury by modified bamboo carbon 被引量:1
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作者 谭增强 牛国平 陈晓文 《Chinese Journal of Chemical Engineering》 SCIE EI CAS CSCD 2015年第11期1875-1880,共6页
The mercury removal performance of modified bamboo charcoal (BC) was investigated with a bench-scale fixedbed reactor, A simple impregnation method was used to modify the BC with ZnCI2 and FeCI3 separately. BET and ... The mercury removal performance of modified bamboo charcoal (BC) was investigated with a bench-scale fixedbed reactor, A simple impregnation method was used to modify the BC with ZnCI2 and FeCI3 separately. BET and XPS were used to determine the pore structure and surface chemistry of the sorbents. The role of Fe3 + in the removal of elemental mercury by modified sorbents was discussed. The experimental results suggest that the modified BCs have excellent adsorption potential for elemental mercury at a relatively higher temperature, 140 ℃. The BET surface area and average pore size of modified sorbents do not show noticeable priority compared to unmodified BC, XPS spectra indicate that Fe atoms mainly exist in the form of Fe3 + for the FeC1j-impregnated BC. Better performance of FeCl3-impregnated BC at different temperatures (20, 140 and 180 ℃) suggests the enhancement of non-chloride functional groups (Fe3 +). Inhibition effect of SOx and NO for Hg removal by BC samples is present in the study. 展开更多
关键词 Elemental mercuryBamboo charcoalFeCI3ZnCl2
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Development of capacitive sensor signal conditioning system for angular displacement measurement using timer IC LM555 and UFDC 被引量:2
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作者 Sumitesh Majumder T K Maiti 《Journal of Measurement Science and Instrumentation》 CAS CSCD 2019年第1期49-54,共6页
In this research paper,we have presented variable area type capacitive sensor signal conditioning system for angular displacement measurement and for this purpose we have used timer LM555 based astable multivibrator a... In this research paper,we have presented variable area type capacitive sensor signal conditioning system for angular displacement measurement and for this purpose we have used timer LM555 based astable multivibrator and universal frequency to digital converter (UFDC). Due to variation in angular displacement in the variable area type capacitor which is connected in the timer based astable circuit,capacitance changes which in turn changes the time period of the timer circuit output. The time period of the timer output waveform is linear with the capacitance and hence linear with angular displacement. The timer output is further processed with UFDC for the measurement. The experimental results show that the time period is linear with the angular displacement in the range of 0- 180° and the uncertainty we should associate it with this average time period value is the standard deviation of the mean,often called the standard error (SE),which is ± 0.023 μs. Because of the simplicity,this measurement system can be used in both electronic and industrial instrumentation. 展开更多
关键词 variable area type capacitive sensor timer circuit sensor interfacing astable multivibrator universal frequency to digital converter (UFDC)
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Exploring aramid as emerging contender for CO_2 capture
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作者 Sonia Zulfiqar Muhammad Ilyas Sarwar 《Chinese Journal of Chemical Engineering》 SCIE EI CAS CSCD 2016年第7期850-855,共6页
To prevent CO_2 accumulation in the atmosphere generated from scorching of fossil fuels, carbon capture and sequestration(CCS) technology is considered as a potential route to mitigate the emissions of CO_2 from reach... To prevent CO_2 accumulation in the atmosphere generated from scorching of fossil fuels, carbon capture and sequestration(CCS) technology is considered as a potential route to mitigate the emissions of CO_2 from reaching the atmosphere. Power generation from sources such as gas, coal and biomass can fulfill the energy demand more readily than many other sources of electricity production. Thus these sources may be retained as important alternative option in the global energy cycle. In order to curtail CO_2, porous aramid network was fabricated by the condensation of 1,3,5-benzenetricarbonyl trichloride and 1,3-phenylenediamine in 1,4-dioxane solvent. Aramid was characterized for various analyses including FTIR, XRD, TGA, BET surface area and pore size analysis, FESEM and CO_2 adsorption measurements. Excellent thermal stability was provided by strong amide linkages in the polymer backbone. Optimum CO_2 uptake of aramid was achieved to be 23.14 mg·g^(-1) at 273 K at 0.1 MPa. The basic amide groups of network structure showed greater affinity for CO_2.Excellent thermal stability of aramid makes it a promising sorbent for CO_2 capture in adverse conditions. 展开更多
关键词 Greenhouse gasCO2 capturePolymers
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Study on the propagation law of shock wave resulting from coal and gas outburst 被引量:2
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作者 WANG Kai ZHOU Ai-tao +2 位作者 ZHANG Pin LI Chuan GUO Yan-wei 《Journal of Coal Science & Engineering(China)》 2011年第2期142-146,共5页
According to the formation of shock wave resulting from coal and gas outburst, the gas flow of coal and gas outburst was transformed from an unsteady flow to a steady one based on selected appropriate reference coordi... According to the formation of shock wave resulting from coal and gas outburst, the gas flow of coal and gas outburst was transformed from an unsteady flow to a steady one based on selected appropriate reference coordinates, and the mathematical expressions were then established by applying mass conservation, momentum conservation equation, and energy conservation equation. On this basis, analyzed gas flow mitigation of variable cross-section area and the outburst intensity, and the relations between cross-section area, velocity, and density; the relations between overpressures and outburst intensity were deduced. Furthermore, shock waves resulting from coal and gas outburst and outburst intensity were measured by experimental setup, the overpressure and outburst intensity of different gas pressures were obtained, and the similar conditions of the experiment were numerically simulated. The averaged overpressure and gas flow velocity of variable cross-section under different gas pressures were numerically derived. The results show that the averaged overpressure and outburst intensity obtained from simulation are in good agreement with the experimental results. Moreover, the gas flow velocity of variable cross-sections approximates to the theoretical analysis. 展开更多
关键词 coal and gas outburst shock wave OVERPRESSURE gas flow velocity outburst intensity
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A Simple Model for Earthquake Instability
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作者 Li Ping'en Yin Youquan 《Earthquake Research in China》 2010年第2期179-189,共11页
Based on the heterogeneity of fault plane strength,the macro rupture process of a fault plane can be treated as the rupture accumulation process of local micro-elements in the fault surface.Assuming that the strength ... Based on the heterogeneity of fault plane strength,the macro rupture process of a fault plane can be treated as the rupture accumulation process of local micro-elements in the fault surface.Assuming that the strength of the local micro-elements follows the Weibull probability distribution,the macro-fault constitutive relationship of the complete load-deformation process is derived from a statistical mechanics viewpoint.Applying a one-dimensional earthquake mechanics model and using far-field displacement a as the control variable,the problem of earthquake instability is investigated by employing the stability theory.The results show that the system stiffness ratio(stiffness ratio of fault to surroun-ding rock) β is the important parameter that affects the occurrence of earthquakes.Earthquake instability occurs only when β < 1,and the sudden stress jump appears at the displacement turning point of the equilibrium path curve.The expression of three important parameters for earthquakes(fault half-dislocation distance after earthquake,earthquake stress drop and elastic energy release) is also given.When β≥1,the earthquake does not occur and the fault only slips slowly without an earthquake. 展开更多
关键词 Earthquake instability Fault rupture Weibuil probability distribution Fault constitutive relation Equilibrium path curve Stress sudden jump
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A Practical Approach Based on Equal Area Criteria for Stability Analysis of Weak Networks with Wind Generation Penetration
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作者 Rene Rossi Mohammad A.S. Masoum 《Journal of Energy and Power Engineering》 2012年第4期629-637,共9页
The performance of DFIG-based wind generation systems that interconnected to solid networks is well understood and prevalent in Europe and North America. However, the application of these renewable generating stations... The performance of DFIG-based wind generation systems that interconnected to solid networks is well understood and prevalent in Europe and North America. However, the application of these renewable generating stations to weak network has been examined in very limited occasions. Weak networks have a range of limitations from system capacities to CFCT restrictions which would need to be well understood prior to wind energy integration. Of particular interest would be how much wind generation could be integrated into a weak network prior to increasing voltage and frequency stability issues brought about by penetration issues. This paper introduces a simple and practical approach based on the equal area criteria to investigate the stability of weak networks. Simulation results that are presented to show the proposed approach is a viable preliminary assessment tool to determine system stability on weak networks with wind power penetration. 展开更多
关键词 Grid system weak system wind farm SWIS (south west interconnection system) CFCT (critical fault clearing time) DG (distributed generation) EAC (equal area criteria).
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