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Abstract-This paper focuses on modeling and oscillatory stability analysis of a wind turbine with doubly fed induction generator (DFIG). A detailed mathematical model of DFIG wind turbine with vector-control loops is developed, based on which the loci of the system Jacobian's eigenvalues have been analyzed, showing that, without appropriate controller tuning a Hopf bifurcation can occur in such a system due to various factors, such as wind speed. Subsequently, eigenvalue sensitivity with respect to machine and control parameters is performed to assess their impacts on system stability. Moreover, the Hopf bifurcation boundaries of the key parameters are also given. They can be used to guide the tuning of those DFIG parameters to ensure stable operation in practice. The computer simulations are conducted to validate the developed model and to verify the theoretical analysis.Index Terms-Doubly fed induction generator (DFIG), eigenvalue sensitivity, Hopf bifurcation, stability.

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SUMMARYIn this paper, we derive the discrete-time model for the power-factor-correction (PFC) buck-boost converter in terms of a stroboscopic switching map. Fast-scale instability is analysed through a fold diagram, which exposes the periodicity of the operation as well as the locations of the critical phase angles of the line voltage at which instability begins to occur along a half-line cycle. The asymmetrical locations of the critical phase angles along a half-line cycle is explained in terms of 'underdeveloped' bifurcation. Border collision bifurcations are observed and analysed in detail.

Fluorinated polyethylene propylene (FEP) ferroelectrets with an air-filled parallel-tunnel structure and regularly distributed bipolar space charges can stimulate integration of ferroelectrets in transducers due to their unique piezoelectric and mechanical properties. Such FEP ferroelectret films exhibit not only large longitudinal piezoelectric activity but also a strong transverse piezoelectric effect which allows for application in flexible transducers either working in 33 mode or in 31 mode. In this article, the transducer relevant properties of FEP ferroelectret films have been characterized. The experimental results show that the directly measurable, quasi-static piezoelectric d 33 -and g 31 -coefficients of the FEP films reach up to 4700 pC N -1 and 4.3 Vm N -1 , respectively, and dynamic values in the frequency range from 10 to 100 Hz are 2600-1500 pC N -1 and 1.2-0.78 Vm N -1 , respectively. No significant reduction of d 33 -coefficient is observed at strains up to 2.1%, showing a high elasticity of the film. Both isothermal decay of d 33 -coefficient and short circuit thermally stimulated discharge (TSD) current spectra show that the ferroelectric FEP films exhibit excellent thermal stability. After 14-day of storage in an atmosphere with a relative humidity of 99%, 75% of the initial piezoelectric d 33 -coefficient was still retained. Also a mechanical test with 1.6 million cycles does not harm the piezoelectric coefficients indicating that air-filled parallel-tunnel FEP ferroelectret films are promising candidates for a variety of flexible transducer applications.