级联型混合直流系统受端的模块化多电平换流器高压直流系统(modular multilevel converter high voltage direct current,MMC-HVDC)和电网换相换流器高压直流系统(line commutated converter high voltage direct current,LCC-HVDC)会...级联型混合直流系统受端的模块化多电平换流器高压直流系统(modular multilevel converter high voltage direct current,MMC-HVDC)和电网换相换流器高压直流系统(line commutated converter high voltage direct current,LCC-HVDC)会因馈入点电气距离较近而存在强相互作用关系。为明确级联型混合直流馈入系统中MMC-HVDC对LCC-HVDC系统强度的影响,提出考虑幅值和相位的MMC等效电流源的简化等效方法,基于所提MMC等效原理将级联型混合直流馈入系统等效成单馈入LCC-HVDC系统,分析等效单馈入系统的参数计算方法,并提出等效单馈入短路比评估指标。最后通过该指标分析归纳了不同控制方式、不同电气距离下等效单馈入系统的系统强度变化情况,即为MMC-HVDC对LCC-HVDC系统强度的影响规律。机理分析和基于PSCDAD/EMTDC、MATLAB的仿真结果表明,所提等效方法具有有效性且所提指标能很好地反应MMC-HVDC对LCC-HVDC系统强度的影响。展开更多
Acoustic Doppler current profiler （ADCP） receives echoes from sound scatterers, then their speed is calcu- lated by the Doppler effect. In the open ocean, most of these backscatterers are from the plankton. The soun...Acoustic Doppler current profiler （ADCP） receives echoes from sound scatterers, then their speed is calcu- lated by the Doppler effect. In the open ocean, most of these backscatterers are from the plankton. The sound scatterers descend down to depth at around dawn, their mean speed is 2.9 cm/s, then they ascend up to the surface layer at around dusk with a mean speed of 2.1 cm/s, in the Luzon Strait. The descending speed is faster, which suggests that this zooplankton population may accelerate its downward migration under the action of the gravity. The vertical distribution of a mean volume backscattering strength （MVB- S） in the nighttime has two peaks, which locate near the upper and lower boundary layers of halocline, respectively. However, the backscatterers only aggregate near the surface layer in the daytime. The diel ver- tical migration （DVM） of sound scatterers has several characteristic patterns, it is stronger in summer, but weaker in winter, and the maximum peak occurs in September. The DVM occurrence is synchronous with the seawater temperature increasing at around dawn and dusk, it may affect the ocean mixing and water stratification,展开更多
Medium strength aluminium alloy （Al-Mg-Si alloy） has gathered wide acceptance in the fabrication of light weight structures requiring a high strength-to weight ratio, such as transportable bridge girders, military v...Medium strength aluminium alloy （Al-Mg-Si alloy） has gathered wide acceptance in the fabrication of light weight structures requiring a high strength-to weight ratio, such as transportable bridge girders, military vehicles, road tankers and railway transport systems. The preferred welding process for aluminium alloy is frequently TIG （tungsten inert gas） welding due to its comparatively easier applicability and better economy.In the case of single pass TIG welding of thinner section of this alloy, the pulsed current has been found beneficial due to its advantages over the conventional continuous current process. The use of pulsed current parameters has been found to improve the mechanical properties of the welds compared to those of continuous current welds of this alloy due to grain refinement occurring in the fusion zone. A mathematical model has been developed to predict pitting corrosion potential of pulsed current TIG welded AA6061 aluminium alloy.Factorial experimental design has been used to optimize the experimental conditions. Analysis of variance technique has been used to find out the significant pulsed current parameters. Regression analysis has been used to develop the model. Using the developed model pitting corrosion potential values have been estimated for different combinations of pulsed current parameters and the results are analyzed in detail.展开更多
文摘级联型混合直流系统受端的模块化多电平换流器高压直流系统(modular multilevel converter high voltage direct current,MMC-HVDC)和电网换相换流器高压直流系统(line commutated converter high voltage direct current,LCC-HVDC)会因馈入点电气距离较近而存在强相互作用关系。为明确级联型混合直流馈入系统中MMC-HVDC对LCC-HVDC系统强度的影响,提出考虑幅值和相位的MMC等效电流源的简化等效方法,基于所提MMC等效原理将级联型混合直流馈入系统等效成单馈入LCC-HVDC系统,分析等效单馈入系统的参数计算方法,并提出等效单馈入短路比评估指标。最后通过该指标分析归纳了不同控制方式、不同电气距离下等效单馈入系统的系统强度变化情况,即为MMC-HVDC对LCC-HVDC系统强度的影响规律。机理分析和基于PSCDAD/EMTDC、MATLAB的仿真结果表明,所提等效方法具有有效性且所提指标能很好地反应MMC-HVDC对LCC-HVDC系统强度的影响。
基金The National Basic Research Program of China under contract Nos 2007CB816003,2011CB403503 and 2012CB955601the National Natural Science Foundation of China under contract Nos 41176020 and 41176021the Scientific Research Fund of the Second Institute of Oceanography,State Oeanic Administration,under contract Nos JT1006 and JG1009
文摘Acoustic Doppler current profiler （ADCP） receives echoes from sound scatterers, then their speed is calcu- lated by the Doppler effect. In the open ocean, most of these backscatterers are from the plankton. The sound scatterers descend down to depth at around dawn, their mean speed is 2.9 cm/s, then they ascend up to the surface layer at around dusk with a mean speed of 2.1 cm/s, in the Luzon Strait. The descending speed is faster, which suggests that this zooplankton population may accelerate its downward migration under the action of the gravity. The vertical distribution of a mean volume backscattering strength （MVB- S） in the nighttime has two peaks, which locate near the upper and lower boundary layers of halocline, respectively. However, the backscatterers only aggregate near the surface layer in the daytime. The diel ver- tical migration （DVM） of sound scatterers has several characteristic patterns, it is stronger in summer, but weaker in winter, and the maximum peak occurs in September. The DVM occurrence is synchronous with the seawater temperature increasing at around dawn and dusk, it may affect the ocean mixing and water stratification,
文摘Medium strength aluminium alloy （Al-Mg-Si alloy） has gathered wide acceptance in the fabrication of light weight structures requiring a high strength-to weight ratio, such as transportable bridge girders, military vehicles, road tankers and railway transport systems. The preferred welding process for aluminium alloy is frequently TIG （tungsten inert gas） welding due to its comparatively easier applicability and better economy.In the case of single pass TIG welding of thinner section of this alloy, the pulsed current has been found beneficial due to its advantages over the conventional continuous current process. The use of pulsed current parameters has been found to improve the mechanical properties of the welds compared to those of continuous current welds of this alloy due to grain refinement occurring in the fusion zone. A mathematical model has been developed to predict pitting corrosion potential of pulsed current TIG welded AA6061 aluminium alloy.Factorial experimental design has been used to optimize the experimental conditions. Analysis of variance technique has been used to find out the significant pulsed current parameters. Regression analysis has been used to develop the model. Using the developed model pitting corrosion potential values have been estimated for different combinations of pulsed current parameters and the results are analyzed in detail.
