Control of DFIG based wind generation systems under unbalanced network supply

This paper develops a dynamic model and control scheme for DFIG systems to improve the performance and stability under unbalanced grid conditions. A dynamic DFIG model containing the positive and negative sequence components is presented using stator voltage orientation. The proposed model accurately illustrates the active power, reactive power and torque oscillations, and provides a basis for DFIG control system design during unbalanced network supply. Various control targets such as eliminating the oscillations of the torque, active/reactive power are discussed and the required rotor negative sequence current for fulfilling different control targets are described. Performance of a DFIG-based wind turbine under unbalanced condition using the proposed control method is evaluated by simulation studies using Matlab/Simulink. The proposed control scheme significantly attenuates the DFIG torque or active power oscillations during network unbalance whereas significant torque/power oscillations exist with the conventional control schemes

Dynamical Response of Fermi Condensate to Varying Magnetic Fields

We investigate the dynamical response of strongly interacting ultra-cold fermionic atoms near Feshbach resonance to varying magnetic fields. Following the experimental practices, we calculate the response of the atoms to oscillating and to linearly ramped magnetic fields respectively. For oscillating magnetic fields, depending on the frequencies and the amplitudes of the oscillations, the response of the pair excitation gap shows either linear or rich non-linear behaviour. In addition, both the spectral studies through the linear response theory and the time-domain simulations suggest the existence of a resonant frequency corresponding to the pair dissociation threshold. For linearly ramped magnetic fields, the response of the excitation gap shows damped oscillations. The final value of the excitation gap depends on the rate of the field sweep.Comment: 6 pages, 6 figure

K-essence Explains a Lorentz Violation Experiment

Recently, a state of the art experiment shows evidence for Lorentz violation in the gravitational sector. To explain this experiment, we investigate a spontaneous Lorentz violation scenario with a generalized scalar field. We find that when the scalar field is nonminimally coupled to gravity, the Lorentz violation induces a deformation in the Newtonian potential along the direction of Lorentz violation.Comment: 8 pages, the final version, discussion and references adde

Absence of topological insulator phases in non-Hermitian PT-symmetric Hamiltonians

In this work we consider a generalization of the symmetry classification of topological insulators to non-Hermitian Hamiltonians which satisfy a combined $PT$-symmetry (parity and time-reversal). We show via examples, and explicit bulk and boundary state proofs that the typical paradigm of forming topological insulator states from Dirac Hamiltonians is not compatible with the construction of non-Hermitian $PT$-symmetric Hamiltonians. The topological insulator states are $PT$-breaking phases and have energy spectra which are complex (not real) and thus such non-Hermitian Hamiltonians are not consistent quantum theories.Comment: 5 pages, 1 figur