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
大规模风电场并网对电力系统的安全稳定运行造成了较大影响,改善含风电场电力系统的小干扰稳定性日益重要。针对多机电力系统,提出一种联合双馈风机(doubly-fed induction generator, DFIG)与超导储能(superconducting magnetic energy ...大规模风电场并网对电力系统的安全稳定运行造成了较大影响,改善含风电场电力系统的小干扰稳定性日益重要。针对多机电力系统,提出一种联合双馈风机(doubly-fed induction generator, DFIG)与超导储能(superconducting magnetic energy storage,SMES)协调控制改善电力系统小干扰稳定性的方法。从风电场在电网的实际接入位置出发,通过对含风电场电力系统的节点电压方程进行修正和收缩处理,推导出同步机电磁功率改变量与DFIG接入的关联关系。在此基础上,依据互补群惯量加权平均角度相对值变换法则分析了风电场并网点的功率特性对系统振荡模式的阻尼影响,给出了提高系统小干扰稳定性的DFIG-SMES功率补偿协调控制策略。仿真结果表明,在系统小扰动动态过程中通过对SMES合理调节交换功率,增强了风电场并网点对系统的正阻尼作用,达到了提升系统小干扰稳定性的效果。展开更多
The formulas of the energy gap and superconducting critical temperature appropriate for systems with both odd and even number of electrons are derived; the bases of the derivations are BCS theory and energy level stat...The formulas of the energy gap and superconducting critical temperature appropriate for systems with both odd and even number of electrons are derived; the bases of the derivations are BCS theory and energy level statistics. Numerical results qualitatively agree with the experimental phenomena. i.e., the superconductivity of small metallic grains will first enhance then decrease to zero when the grain are getting smaller and smaller. The calculations indicate that the above phenomena happen in the metallic grains belonging to Gaussian Orthogonal Ensemble (GOE) and Gaussian Unitary ensemble (GUE) with zero spin; The superconductivity of small metallic grains in Gaussian Symplectic Ensemble (GSE) will monotonically decrease to zero with the decreasing of the grain size. The analyses suggest that the superconductivity enhancements come from pairing and the balance of the strengths between spin-orbital coupling and external magnetic field. In order to take the latter into account, it is necessary to include the level statistics given by Random Matrix Theory (RMT) in describing small metallic grains.展开更多
Theoretical and experimental results show that the state of minimum Ginsburg Landau free energy of small superconducting particles contains no flux line if the applied field is smaller than its critical field. For suc...Theoretical and experimental results show that the state of minimum Ginsburg Landau free energy of small superconducting particles contains no flux line if the applied field is smaller than its critical field. For such micron sized particles, after being cooled in a small magnetic field (e.g. less than 100 μT), the moment remaining during subsequent zero field warming contains almost no contribution from trapped flux and is dominated entirely by the paramagnetic Meissner effect (PME) of the particles. A systematic study of such a moment has been carried out to reveal its characteristic behavior of temperature, magnetic field and cooling or warming rate dependence. Methods for removing and recovering the PME of small superconducting particles are also reported.展开更多
Here we review recent small-angle scattering studies of the vortex lattice in a range of type-II superconductors carried out by our group. Emphasis is placed on providing examples of the kind of information which can ...Here we review recent small-angle scattering studies of the vortex lattice in a range of type-II superconductors carried out by our group. Emphasis is placed on providing examples of the kind of information which can be obtained by such measurements, focusing in particular on studies of the vortex lattice structure and form factor in LuNi2B2C, TmNi2B2C, CeCoIn5 and Ba（Fe0.93Co0.07）2As2.展开更多
A range of powdered Bi:2 212 samples exhibiting the paramagnetic Meissner effect (PME) are systematically examined. Interpretation of the results is made in terms of a phenomenological model in which there is a concen...A range of powdered Bi:2 212 samples exhibiting the paramagnetic Meissner effect (PME) are systematically examined. Interpretation of the results is made in terms of a phenomenological model in which there is a concentration within the material of small local moments that can be polarized during a field cooling. Information about the magnitudes of these local m0oments and their distribution are deduced. Relations between the local moments and the particle sizes, the weak link, oxygen content and the interactions between the local moments are also discussed. Comparison of the results from small particles and bulk samples shows that conclusions obtained from small particle experiments are reliable and universal.展开更多
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。
文摘大规模风电场并网对电力系统的安全稳定运行造成了较大影响,改善含风电场电力系统的小干扰稳定性日益重要。针对多机电力系统,提出一种联合双馈风机(doubly-fed induction generator, DFIG)与超导储能(superconducting magnetic energy storage,SMES)协调控制改善电力系统小干扰稳定性的方法。从风电场在电网的实际接入位置出发,通过对含风电场电力系统的节点电压方程进行修正和收缩处理,推导出同步机电磁功率改变量与DFIG接入的关联关系。在此基础上,依据互补群惯量加权平均角度相对值变换法则分析了风电场并网点的功率特性对系统振荡模式的阻尼影响,给出了提高系统小干扰稳定性的DFIG-SMES功率补偿协调控制策略。仿真结果表明,在系统小扰动动态过程中通过对SMES合理调节交换功率,增强了风电场并网点对系统的正阻尼作用,达到了提升系统小干扰稳定性的效果。
基金supported by the National Natural Science Foundation of China(NNSF)(Grant Nos.10147207 and 10375001)China Shanghai Foundation for Developing Science and Technology(Grant No.0252nm082)+1 种基金China Shanghai Foundation for Developing Science and Technology in Universities(Grant No.03DZ03)the Third Phase Foundation of Theoretical Center in China Lanzhou National Key Laboratory.
文摘The formulas of the energy gap and superconducting critical temperature appropriate for systems with both odd and even number of electrons are derived; the bases of the derivations are BCS theory and energy level statistics. Numerical results qualitatively agree with the experimental phenomena. i.e., the superconductivity of small metallic grains will first enhance then decrease to zero when the grain are getting smaller and smaller. The calculations indicate that the above phenomena happen in the metallic grains belonging to Gaussian Orthogonal Ensemble (GOE) and Gaussian Unitary ensemble (GUE) with zero spin; The superconductivity of small metallic grains in Gaussian Symplectic Ensemble (GSE) will monotonically decrease to zero with the decreasing of the grain size. The analyses suggest that the superconductivity enhancements come from pairing and the balance of the strengths between spin-orbital coupling and external magnetic field. In order to take the latter into account, it is necessary to include the level statistics given by Random Matrix Theory (RMT) in describing small metallic grains.
文摘Theoretical and experimental results show that the state of minimum Ginsburg Landau free energy of small superconducting particles contains no flux line if the applied field is smaller than its critical field. For such micron sized particles, after being cooled in a small magnetic field (e.g. less than 100 μT), the moment remaining during subsequent zero field warming contains almost no contribution from trapped flux and is dominated entirely by the paramagnetic Meissner effect (PME) of the particles. A systematic study of such a moment has been carried out to reveal its characteristic behavior of temperature, magnetic field and cooling or warming rate dependence. Methods for removing and recovering the PME of small superconducting particles are also reported.
基金Acknowledgements M.R. Eskildsen was supported by the U. S. National Science Foundation through grant DMR-0804887. Collaboration and stimulating discussions are acknowledged with： A. B. Abrahamsen, T. M. Artemova, E. D. Bauer, A. D. Bianchi, T. D. Blasius, S. L. Bud＇ko, P. C. Canfield, P. Das, L. DeBeer-Schmitt, J. M. Densmore, C. D. Dewhurst, Z. Fisk, E. M. Forgan, J. L. Gavilano, S. Gerber, A. I. Coldman, M. Ichioka, R. Ikeda, N. Jenkins, M. Kenzelmann, V. G. Kogan, J. Kohlbrecher, A. Kreyssig, M. Laver, K. Machida, J. Mesot, R. Movshovich, N. Ni, T. O＇Brien, C. Petrovic, R. Prozorov, K. Rovira, J. L. Sarrao, I. S. Veshchunov, L. Ya. Vinnikov. J. S. White and M. Zolliker.
文摘Here we review recent small-angle scattering studies of the vortex lattice in a range of type-II superconductors carried out by our group. Emphasis is placed on providing examples of the kind of information which can be obtained by such measurements, focusing in particular on studies of the vortex lattice structure and form factor in LuNi2B2C, TmNi2B2C, CeCoIn5 and Ba（Fe0.93Co0.07）2As2.
文摘A range of powdered Bi:2 212 samples exhibiting the paramagnetic Meissner effect (PME) are systematically examined. Interpretation of the results is made in terms of a phenomenological model in which there is a concentration within the material of small local moments that can be polarized during a field cooling. Information about the magnitudes of these local m0oments and their distribution are deduced. Relations between the local moments and the particle sizes, the weak link, oxygen content and the interactions between the local moments are also discussed. Comparison of the results from small particles and bulk samples shows that conclusions obtained from small particle experiments are reliable and universal.
