Small-Signal Modelling and Analysis of Doubly-Fed Induction Generators in Wind Power Applications

The worldwide demand for more diverse and greener energy supply has had a significant impact on the development of wind energy in the last decades. From 2 GW in 1990, the global installed capacity has now reached about 100 GW and is estimated to grow to 1000 GW by 2025. As wind power penetration increases, it is important to investigate its effect on the power system. Among the various technologies available for wind energy conversion, the doubly-fed induction generator (DFIG) is one of the preferred solutions because it offers the advantages of reduced mechanical stress and optimised power capture thanks to variable speed operation. This work presents the small-signal modelling and analysis of the DFIG for power system stability studies. This thesis starts by reviewing the mathematical models of wind turbines with DFIG convenient for power system studies. Different approaches proposed in the literature for the modelling of the turbine, drive-train, generator, rotor converter and external power system are discussed. It is shown that the flexibility of the drive train should be represented by a two-mass model in the presence of a gearbox. In the analysis part, the steady-state behaviour of the DFIG is examined. Comparison is made with the conventional synchronous generators (SG) and squirrel-cage induction generators to highlight the differences between the machines. The initialisation of the DFIG dynamic variables and other operating quantities is then discussed. Various methods are briefly reviewed and a step-by-step procedure is suggested to avoid the iterative computations in initial condition mentioned in the literature. The dynamical behaviour of the DFIG is studied with eigenvalue analysis. Modal analysis is performed for both open-loop and closed-loop situations. The effect of parameters and operating point variations on small signal stability is observed. For the open-loop DFIG, conditions on machine parameters are obtained to ensure stability of the system. For the closed-loop DFIG, it is shown that the generator electrical transients may be neglected once the converter controls are properly tuned. A tuning procedure is proposed and conditions on proportional gains are obtained for stable electrical dynamics. Finally, small-signal analysis of a multi-machine system with both SG and DFIG is performed. It is shown that there is no common mode to the two types of generators. The result confirms that the DFIG does not introduce negative damping to the system, however it is also shown that the overall effect of the DFIG on the power system stability depends on several structural factors and a general statement as to whether it improves or detriorates the oscillatory stability of a system can not be made

An asymptotic estimate of the variance of the self-intersections of a planar periodic Lorentz process

We consider a periodic planar Lorentz process with strictly convex obstacles and finite horizon. This process describes the displacement of a particle moving in the plane with unit speed and with elastic reflection on the obstacles. We call number of self-intersections of this Lorentz process the number V(n) of couples of integers (k,m) smaller than n such that the particle hits a same obstacle both at the k-th and at the m-th collision times. The aim of this paper is to prove that the variance of V(n) is equivalent to cn^2 (such a result has recently been proved for simple planar random walks by Deligiannidis and Utev)

Dynamical processes in galaxy centers

How does the gas get in nuclear regions to fuel black holes? How efficient is the feedback? The different processes to cause rapid gas inflow (or outflow) in galaxy centers are reviewed. Non axisymmetries can be created or maintained by internal disk instabilities, or galaxy interactions. Simulations and observations tell us that the fueling is a chaotic and intermittent process, with different scenarios and time-scales, according to the various radial scales across a galaxy.Comment: 6 pages, 3 figures, to appear in "The Central Kiloparsec in Galactic Nuclei: Astronomy at High Angular Resolution 2011", open access Journal of Physics: Conference Series (JPCS), published by IOP Publishin

Restoring Igbo dignity: Ike and Adichie on the University of Nigeria

Someone said that the University of Nigeria, which, fifty years on, remains one of the country's major achievements, was a dream come true. Envisioned many years before Independence, it eventually opened its gates on 7th October 1960 and classes began ten days later with an enrolment of 220 students and 13 academic members of staff. Since then, thousands of students and staff from all over the world have settled on its Nsukka and Enugu campuses to study, research and join in a unique experiment. This chapter considers the impact of UNN on Nigerian literature, focusing on Ike's Naked Gods (1970), and Adichie's Purple Hibiscus (2004) and Half of a Yellow Sun (2006), the only three Nigerian novels focusing on the University of Nigeria. It shows these texts as key documents, revealing UNN, the only Nigerian HE institution developed in a rural setting, as both a citadel of learning and a world in itself, whose influence permeated the whole region and extended far beyond. Whereas The Naked gods (1970) evokes the beginnings of the University, its main campus under construction and the negotiating of the University administrative structures between the British and the Americans under the critical eye of the side-lined indigenous staff and local traditional authorities, Adichie's novels, published in 2004 and 2006, complement Ike's picture as they paint a very different University, now totally manned by Nigerians and where expatriates are on the way out. They equally differ in other ways: whereas Ike chose to focus on the University as a workplace, Adichie presents it as a residential area, a village and a web of close-knit relationships. This comparative study highlights UNN's intellectual impact on both its students and staff and on the nation-building process