In this paper a possible mechanism of current in medium is presented. Comparison between this current and the magnetization current was made. Expression for this current was derived. This work is helpful to understand...In this paper a possible mechanism of current in medium is presented. Comparison between this current and the magnetization current was made. Expression for this current was derived. This work is helpful to understanding the interaction between medium and electromagnetic wave.展开更多
Numerical and analytical investigations of the thermosolutal instability in a viscoelastic Rivlin-Ericksen fluid are carried out in the presence of a uniform vertical magnetic field to include the Hall current with a ...Numerical and analytical investigations of the thermosolutal instability in a viscoelastic Rivlin-Ericksen fluid are carried out in the presence of a uniform vertical magnetic field to include the Hall current with a uniform angular velocity in a porous medium. For stationary convection, the stable solute gradient parameter and the rota- tion have stabilizing effects on the system, whereas the magnetic field and the medium permeability have stabilizing or destabilizing effects on the system under certain condi- tions. The Hall current in the presence of rotation has stabilizing effects for sufficiently large Taylor numbers, whereas in the absence of rotation, the Hall current always has destabilizing effects. These effects have also been shown graphically. The viscoelastic effects disappear for stationary convection. The stable solute parameter, the rotation, the medium permeability, the magnetic field parameter, the Hall current, and the vis- coelasticity introduce oscillatory modes into the system, which are non-existent in their absence. The sufficient conditions for the non-existence of overstability are also obtained.展开更多
In this paper, an investigation of the effects of some physical parameters and Hall current on magneto hydrodynamics (MHD) fluid flow with heat flux over a porous medium was carefully examined, taking into considerati...In this paper, an investigation of the effects of some physical parameters and Hall current on magneto hydrodynamics (MHD) fluid flow with heat flux over a porous medium was carefully examined, taking into consideration Hall effects where the temperature and concentration are assumed to be oscillating with time. Furthermore, perturbation method is used in solving the governing equations. The profiles of velocity, temperature and concentration are presented graphically, going into the problem the primary and secondary velocity are presented and compute for some physical parameters such as mass Grashof number (<em>Gc</em>), Schmidt number <em>Sc</em>, Prandtl number (<em>Pr</em>) viscoelastic parameter (<em>K</em><sub>1</sub>) and hall current parameter (<em>m</em>). Results indicated that primary velocity increases with increase in values of <em>Gc</em> on one hand and on the other hand it decreases with increase in the values of <em>Pr</em>, <em>K</em><sub>1</sub> and <em>m</em>. Secondary velocity demonstrated opposite trend.展开更多
In contrast to the conventional direct current railway electrification system(DC-RES),the medium voltage direct current(MVDC)-RES is considered promising for long-distance high-speed corridors.In the MVDC-RES,traction...In contrast to the conventional direct current railway electrification system(DC-RES),the medium voltage direct current(MVDC)-RES is considered promising for long-distance high-speed corridors.In the MVDC-RES,traction substations(TSSs)are placed much farther and train loads are much heavier than in the conventional DC-RES.Hence,the MVDC-RES brings a drastic change in catenary voltage,TSS spacing,and train loading,which affects rail potential and stray current.In this connection,this work performs some significant quantitative analysis of rail potential and stray current in the MVDC-RES environment.An MVDC simulation model is proposed and different grounding schemes are analyzed for a single-train and two TSSs scenario as well as for a multi-train multi-TSS scenario.According to the simulation and analysis,the maximum values of rail potential and stray current at MVDC-RES distances and the maximum safe distance between adjacent TSSs are determined.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
In order to improve the quality of cultivated land in Liaoning Province in the light of local conditions, medium- and low-yielding farmland in Liaoning Province are divided into various zones according to characterist...In order to improve the quality of cultivated land in Liaoning Province in the light of local conditions, medium- and low-yielding farmland in Liaoning Province are divided into various zones according to characteristics of the terrain, and the practical and feasible improvement measures are put forward based on the current situation and main existing problems in the medium- and low-yielding farmland in different areas,展开更多
Traditional small current grounding system has many advantages. Pilot operation shows that optimized one has even better operation characteristics. It has proven to be a geenrallly properand relatively perfect neutyal...Traditional small current grounding system has many advantages. Pilot operation shows that optimized one has even better operation characteristics. It has proven to be a geenrallly properand relatively perfect neutyal grounding method in urban MV network.展开更多
The increase in the installed capacity of wind energy conversion systems(WECS) has triggered the devel-opment of more demanding grid codes and additional requirements on performance.In order to meet these require-ment...The increase in the installed capacity of wind energy conversion systems(WECS) has triggered the devel-opment of more demanding grid codes and additional requirements on performance.In order to meet these require-ments the industry trend has shifted to full-scale power converter interfaces in modern multi-megawatt WECS.As con-sequence,a wide variety of new power converter topologies and WECS configurations have been introduced in recent years.Among them,current source converter(CSC) based configurations have attracted attention due to a series of advantages like:simple structure,grid friendly waveforms,controllable power factor,and reliable grid short-circuit pro-tection.This paper presents the latest developments in CSC interfaces for WECS and related technologies such as modulation methods,control schemes and grid code compatibility.展开更多
为解决船舶中压直流(medium voltage direct current,MVDC)电力系统直流电流开断困难,以及发生短路时故障电流上升率高且峰值大的问题,提出一种基于耦合电抗器的阻容限流型固态直流断路器拓扑。以晶闸管(silicon controlled rectifier,S...为解决船舶中压直流(medium voltage direct current,MVDC)电力系统直流电流开断困难,以及发生短路时故障电流上升率高且峰值大的问题,提出一种基于耦合电抗器的阻容限流型固态直流断路器拓扑。以晶闸管(silicon controlled rectifier,SCR)作为主开断器件,通过耦合电抗器来辅助晶闸管开断,并在直流系统发生故障时,通过换流过程将阻容限流元件接入,有效限制故障电流上升率和峰值,减少故障开断所需时间。基于所提拓扑设计了6 kV/4.2 kA的直流断路器模型,在PSCAD/EMTDC中进行仿真,并与现有拓扑进行对比分析。仿真结果表明:所设计断路器可针对直流系统不同的运行状态,按照不同的控制策略顺利完成对直流电流的开断,并且在开断速度、限流能力和金属氧化物避雷器(metal oxide arrester,MOA)耗能方面均具有一定优势。展开更多
文摘In this paper a possible mechanism of current in medium is presented. Comparison between this current and the magnetization current was made. Expression for this current was derived. This work is helpful to understanding the interaction between medium and electromagnetic wave.
文摘Numerical and analytical investigations of the thermosolutal instability in a viscoelastic Rivlin-Ericksen fluid are carried out in the presence of a uniform vertical magnetic field to include the Hall current with a uniform angular velocity in a porous medium. For stationary convection, the stable solute gradient parameter and the rota- tion have stabilizing effects on the system, whereas the magnetic field and the medium permeability have stabilizing or destabilizing effects on the system under certain condi- tions. The Hall current in the presence of rotation has stabilizing effects for sufficiently large Taylor numbers, whereas in the absence of rotation, the Hall current always has destabilizing effects. These effects have also been shown graphically. The viscoelastic effects disappear for stationary convection. The stable solute parameter, the rotation, the medium permeability, the magnetic field parameter, the Hall current, and the vis- coelasticity introduce oscillatory modes into the system, which are non-existent in their absence. The sufficient conditions for the non-existence of overstability are also obtained.
文摘In this paper, an investigation of the effects of some physical parameters and Hall current on magneto hydrodynamics (MHD) fluid flow with heat flux over a porous medium was carefully examined, taking into consideration Hall effects where the temperature and concentration are assumed to be oscillating with time. Furthermore, perturbation method is used in solving the governing equations. The profiles of velocity, temperature and concentration are presented graphically, going into the problem the primary and secondary velocity are presented and compute for some physical parameters such as mass Grashof number (<em>Gc</em>), Schmidt number <em>Sc</em>, Prandtl number (<em>Pr</em>) viscoelastic parameter (<em>K</em><sub>1</sub>) and hall current parameter (<em>m</em>). Results indicated that primary velocity increases with increase in values of <em>Gc</em> on one hand and on the other hand it decreases with increase in the values of <em>Pr</em>, <em>K</em><sub>1</sub> and <em>m</em>. Secondary velocity demonstrated opposite trend.
文摘In contrast to the conventional direct current railway electrification system(DC-RES),the medium voltage direct current(MVDC)-RES is considered promising for long-distance high-speed corridors.In the MVDC-RES,traction substations(TSSs)are placed much farther and train loads are much heavier than in the conventional DC-RES.Hence,the MVDC-RES brings a drastic change in catenary voltage,TSS spacing,and train loading,which affects rail potential and stray current.In this connection,this work performs some significant quantitative analysis of rail potential and stray current in the MVDC-RES environment.An MVDC simulation model is proposed and different grounding schemes are analyzed for a single-train and two TSSs scenario as well as for a multi-train multi-TSS scenario.According to the simulation and analysis,the maximum values of rail potential and stray current at MVDC-RES distances and the maximum safe distance between adjacent TSSs are determined.
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.
文摘In order to improve the quality of cultivated land in Liaoning Province in the light of local conditions, medium- and low-yielding farmland in Liaoning Province are divided into various zones according to characteristics of the terrain, and the practical and feasible improvement measures are put forward based on the current situation and main existing problems in the medium- and low-yielding farmland in different areas,
文摘Traditional small current grounding system has many advantages. Pilot operation shows that optimized one has even better operation characteristics. It has proven to be a geenrallly properand relatively perfect neutyal grounding method in urban MV network.
文摘The increase in the installed capacity of wind energy conversion systems(WECS) has triggered the devel-opment of more demanding grid codes and additional requirements on performance.In order to meet these require-ments the industry trend has shifted to full-scale power converter interfaces in modern multi-megawatt WECS.As con-sequence,a wide variety of new power converter topologies and WECS configurations have been introduced in recent years.Among them,current source converter(CSC) based configurations have attracted attention due to a series of advantages like:simple structure,grid friendly waveforms,controllable power factor,and reliable grid short-circuit pro-tection.This paper presents the latest developments in CSC interfaces for WECS and related technologies such as modulation methods,control schemes and grid code compatibility.
文摘为解决船舶中压直流(medium voltage direct current,MVDC)电力系统直流电流开断困难,以及发生短路时故障电流上升率高且峰值大的问题,提出一种基于耦合电抗器的阻容限流型固态直流断路器拓扑。以晶闸管(silicon controlled rectifier,SCR)作为主开断器件,通过耦合电抗器来辅助晶闸管开断,并在直流系统发生故障时,通过换流过程将阻容限流元件接入,有效限制故障电流上升率和峰值,减少故障开断所需时间。基于所提拓扑设计了6 kV/4.2 kA的直流断路器模型,在PSCAD/EMTDC中进行仿真,并与现有拓扑进行对比分析。仿真结果表明:所设计断路器可针对直流系统不同的运行状态,按照不同的控制策略顺利完成对直流电流的开断,并且在开断速度、限流能力和金属氧化物避雷器(metal oxide arrester,MOA)耗能方面均具有一定优势。
