近年来,我国电气化铁路发生多起因牵引网网压低频振荡导致的机车保护装置误动作事故,对铁路安全运行造成严重的威胁.现有抑制策略多从机车侧开展,通过对机车控制器参数的修订实现抑制效果,但对于铁路行业的长远发展缺乏灵活性.针对车网...近年来,我国电气化铁路发生多起因牵引网网压低频振荡导致的机车保护装置误动作事故,对铁路安全运行造成严重的威胁.现有抑制策略多从机车侧开展,通过对机车控制器参数的修订实现抑制效果,但对于铁路行业的长远发展缺乏灵活性.针对车网系统低频振荡问题,提出从牵引网侧外接超导磁储能(Superconducting Magnetic Energy Storage,SMES)的动态抑制方案.首先,分析SMES抑制车网系统低频振荡的原理,根据车网系统低频振荡的走势对其进行分类,并依据振荡类型设定SMES接入准则;其次,建立车网系统等效阻抗模型,结合阻抗比判据和伯德图分析车网系统稳定性,对SMES接入车网系统前后系统稳定性对比分析,验证所提方案的有效性,并与传统抑制方法进行对比,验证所提方案的灵活性;最后,对单车及多车升弓并网情况下的车网系统进行仿真分析.结果表明:SMES装置接入车网系统后可快速有效地抑制车网系统低频振荡.展开更多
The re-establishment of the Hong Kong manufacturin g industry as a local pillar of economy is vital in the restructuring of the e conomy in Hong Kong. The SMEs should play a key role in such a process. It requi res ma...The re-establishment of the Hong Kong manufacturin g industry as a local pillar of economy is vital in the restructuring of the e conomy in Hong Kong. The SMEs should play a key role in such a process. It requi res major changes to take place in the operation of the enterprises in order to enhance their competitiveness and therefore the chance of success in the restruc turing process. It is suggested that the strategy of going for high value-added products and related services together with the practice of "focused factory" s hould be adopted by the SMEs in the industry. Effective change management by the top managers and owners in the SMEs is an underpinning element for the success. Basing upon the characteristics of the SMEs in the industry, a model of change is proposed. The roles that should be played by the top management and owners in the industry are suggested and discussed. It is believe that the proposal and s uggestions made would help the industry to effectively manage the changes that a re necessary for the industry to shape the competitiveness of the industry in th e world arena in future.展开更多
柔性直流配电系统中定功率控制的换流器具有恒功率负载特性,会降低系统阻尼,对系统的稳定性产生不利影响。针对该问题,在直流配电系统中加入超导磁储能SMES(superconducting magnetic energy storage)装置来提高系统的稳定性。推导了柔...柔性直流配电系统中定功率控制的换流器具有恒功率负载特性,会降低系统阻尼,对系统的稳定性产生不利影响。针对该问题,在直流配电系统中加入超导磁储能SMES(superconducting magnetic energy storage)装置来提高系统的稳定性。推导了柔性直流配电系统的反馈控制模型,采用频域分析法研究了换流器恒功率负载特性对系统稳定性的影响,并结合数学模型和频域分析,指出SMES装置能够为电网提供正阻尼,增大了系统开环传递函数在剪切频率处的相位裕度,从而改善了系统稳定性。为防止超导磁体两端电压过高,SMES装置与直流配电网连接的DC/DC换流器需具备一定的电压调节性能,因此研究了采用模块化多电平DC/DC换流器DC-MMC(modular multilevel DC/DC converter)的SMES装置,通过调节子模块个数灵活设置换流器电压变比,在实现换流器能量双向流动的同时控制超导磁体两端电压,以保护储能装置。最后通过时域仿真波形验证了采用DC-MMC的SMES装置在提高柔性直流配电系统稳定性方面的可行性和有效性。展开更多
An integrated modelling approach is developed to simulate ecological economics processes of mountainous watershed of Heihe river basin, northwest China. The objectives are 1) a hydrologic unit model HLM_HMU was develo...An integrated modelling approach is developed to simulate ecological economics processes of mountainous watershed of Heihe river basin, northwest China. The objectives are 1) a hydrologic unit model HLM_HMU was developed;2) the impacts of LUCC on the hydrological processes based on different scenarios were simulated respectively;and 3) the Payment for Ecosystem Services (PES) curve of prohibition grazing is given. The hydrological results show that the observed and simulated data have a good fit, in which the Nash–Sutcliffe efficiency, balance error, and explained variance are 0.69, -0.13, and 0.62, respectively;the model can simulate the majority of peaks well;the source code of Spatial Modelling Environment (SME) needs to be deeply understood to improve the algorithms of interpolation and conflux;the PES result shows that prohibition grazing will occur completely when the price of PES reached to 17.42 yuan, and the increased amount of ecosystem services is 2.13 × 108 m3.The main purpose of this paper is to build a better understanding of developing a meaningful integrated model in the study area to solve its ecological and economic problems.展开更多
Software engineering＇s lifecycle models havc proven to be very important for traditional software development. However, can these models be applied to the development of Web-based applications as well？ In recent yea...Software engineering＇s lifecycle models havc proven to be very important for traditional software development. However, can these models be applied to the development of Web-based applications as well？ In recent years, Web-based applications have become more and more complicated and a lot of efforts have been placed on introducing new technologies such as J2EE, PhP, and .NET, etc., which have been universally accepted as the development technologies for Web-based applications. However, there is no universally accepted process model for the development of Web-based applications. Moreover, shaping the process model for small medium-sized enterprises （SMEs）, which have limited resources, has been relatively neglected. Based on our previous work, this paper presents an expanded lifecycle process model for the development of Web-based applications in SMEs. It consists of three sets of processes, i.e., requirement processes, development processes, and evolution processes. Particularly, the post-delivery evolution processes are important to SMEs to develop and maintain quality web applications with limited resources and time.展开更多
文摘近年来,我国电气化铁路发生多起因牵引网网压低频振荡导致的机车保护装置误动作事故,对铁路安全运行造成严重的威胁.现有抑制策略多从机车侧开展,通过对机车控制器参数的修订实现抑制效果,但对于铁路行业的长远发展缺乏灵活性.针对车网系统低频振荡问题,提出从牵引网侧外接超导磁储能(Superconducting Magnetic Energy Storage,SMES)的动态抑制方案.首先,分析SMES抑制车网系统低频振荡的原理,根据车网系统低频振荡的走势对其进行分类,并依据振荡类型设定SMES接入准则;其次,建立车网系统等效阻抗模型,结合阻抗比判据和伯德图分析车网系统稳定性,对SMES接入车网系统前后系统稳定性对比分析,验证所提方案的有效性,并与传统抑制方法进行对比,验证所提方案的灵活性;最后,对单车及多车升弓并网情况下的车网系统进行仿真分析.结果表明:SMES装置接入车网系统后可快速有效地抑制车网系统低频振荡.
文摘The re-establishment of the Hong Kong manufacturin g industry as a local pillar of economy is vital in the restructuring of the e conomy in Hong Kong. The SMEs should play a key role in such a process. It requi res major changes to take place in the operation of the enterprises in order to enhance their competitiveness and therefore the chance of success in the restruc turing process. It is suggested that the strategy of going for high value-added products and related services together with the practice of "focused factory" s hould be adopted by the SMEs in the industry. Effective change management by the top managers and owners in the SMEs is an underpinning element for the success. Basing upon the characteristics of the SMEs in the industry, a model of change is proposed. The roles that should be played by the top management and owners in the industry are suggested and discussed. It is believe that the proposal and s uggestions made would help the industry to effectively manage the changes that a re necessary for the industry to shape the competitiveness of the industry in th e world arena in future.
文摘柔性直流配电系统中定功率控制的换流器具有恒功率负载特性,会降低系统阻尼,对系统的稳定性产生不利影响。针对该问题,在直流配电系统中加入超导磁储能SMES(superconducting magnetic energy storage)装置来提高系统的稳定性。推导了柔性直流配电系统的反馈控制模型,采用频域分析法研究了换流器恒功率负载特性对系统稳定性的影响,并结合数学模型和频域分析,指出SMES装置能够为电网提供正阻尼,增大了系统开环传递函数在剪切频率处的相位裕度,从而改善了系统稳定性。为防止超导磁体两端电压过高,SMES装置与直流配电网连接的DC/DC换流器需具备一定的电压调节性能,因此研究了采用模块化多电平DC/DC换流器DC-MMC(modular multilevel DC/DC converter)的SMES装置,通过调节子模块个数灵活设置换流器电压变比,在实现换流器能量双向流动的同时控制超导磁体两端电压,以保护储能装置。最后通过时域仿真波形验证了采用DC-MMC的SMES装置在提高柔性直流配电系统稳定性方面的可行性和有效性。
文摘An integrated modelling approach is developed to simulate ecological economics processes of mountainous watershed of Heihe river basin, northwest China. The objectives are 1) a hydrologic unit model HLM_HMU was developed;2) the impacts of LUCC on the hydrological processes based on different scenarios were simulated respectively;and 3) the Payment for Ecosystem Services (PES) curve of prohibition grazing is given. The hydrological results show that the observed and simulated data have a good fit, in which the Nash–Sutcliffe efficiency, balance error, and explained variance are 0.69, -0.13, and 0.62, respectively;the model can simulate the majority of peaks well;the source code of Spatial Modelling Environment (SME) needs to be deeply understood to improve the algorithms of interpolation and conflux;the PES result shows that prohibition grazing will occur completely when the price of PES reached to 17.42 yuan, and the increased amount of ecosystem services is 2.13 × 108 m3.The main purpose of this paper is to build a better understanding of developing a meaningful integrated model in the study area to solve its ecological and economic problems.
文摘Software engineering＇s lifecycle models havc proven to be very important for traditional software development. However, can these models be applied to the development of Web-based applications as well？ In recent years, Web-based applications have become more and more complicated and a lot of efforts have been placed on introducing new technologies such as J2EE, PhP, and .NET, etc., which have been universally accepted as the development technologies for Web-based applications. However, there is no universally accepted process model for the development of Web-based applications. Moreover, shaping the process model for small medium-sized enterprises （SMEs）, which have limited resources, has been relatively neglected. Based on our previous work, this paper presents an expanded lifecycle process model for the development of Web-based applications in SMEs. It consists of three sets of processes, i.e., requirement processes, development processes, and evolution processes. Particularly, the post-delivery evolution processes are important to SMEs to develop and maintain quality web applications with limited resources and time.
