Wind farm stability analysis in the presence of variable-speed generators M. Dicorato, G. Forte * , M. Trovato Politecnico di Bari, DEE - Dipartimento di Elettrotecnica ed Elettronica, via E. Orabona 4, 70125 Bari, Italy article info Article history: Received 3 June 2011 Received in revised form 24 October 2011 Accepted 24 November 2011 Available online 22 December 2011 Keywords: Doubly-fed wind generator Permanent-magnet wind generator Steady-state analysis Small signal stability Transient stability analysis abstract Variable-speed technologies have been adopted to improve the efficiency of electricity generation in wind farms. Among these technologies, doubly-fed induction generators have widely spread and their stability issues have been experienced and analysed. In recent years, permanent-magnet synchronous generators have been also exploited, connected to the grid through full-scale converters, ensuring high flexibility, reducing losses and weight. The presence of these technologies heightens the necessity of deeply investigating wind turbine behaviour under different disturbances. In this paper, a small- and large-disturbance stability analysis is carried out in order to compare dynamic characteristic of wind farm equipped with the aforementioned generation technologies. To this purpose, a detailed model of wind turbine, together with relevant control schemes, is adopted for each generator of a real-sized wind farm. The compliance of the plant with fault-ride-through requirements imposed by transmission system operator is also verified. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Wind turbines represent the most mature technology for renewable electricity generation. Several solutions have been exploited during the last decades, aiming to obtain energy from wind speed in optimal conditions. Due to wind speed variability, variable-speed turbine technologies have been employed, in particular doubly-fed induction generators (DFIGs) and synchro- nous generators equipped with converters [1]. Wind turbines are generally equipped with blade pitch control [2], in order to realize aerodynamic regulation. Mechanical connection between turbine and generator shaft in DFIGs is realized by means of a gearbox, due to the great difference in nominal speed. Moreover, doubly-fed machines are equipped with a frequency converter, connected to wound rotor of the induction generator, designed to carry up to 30% of total power [3,4]. In this way, reactive power can be controlled, and variable- speed drive can be realized, with a range of roughly 30% around synchronous speed. Synchronous-machine wind generators are equipped with a full-rated converter which carries the total generated power. This solution enables a wider control of reactive power exchange and the complete decoupling from the grid [5]. Moreover, the employment of permanent-magnet synchronous generators (PMSGs) allows to operate at low speed omitting the gearbox [6]. This reduces weight and dimensions of nacelle equipment, along with power losses in conversion and maintenance requirements [7,8]. These features allow the recent diffusion of this technology, notwithstanding higher generator costs. Therefore, though dynamics of DFIGs have deeply been experi- enced in actual operation and investigated by means of validated models [3,9,10], suitable modelling of PMSG-based wind turbine dynamic and control is still under investigation [11]. Various dynamic models for PMSGs have been proposed in literature. Among them, in [12], a control model of the converters including proportional-integral (PI) loops is adopted. In particular, the generator-side converter controls the output active power of the PMSG and minimizes power loss of the generator. The grid-side converter controls both the DC-link voltage and the terminal voltage of the wind turbine system. A similar control scheme is adopted in [13] and [14], where additional lead-lag blocks are considered, in order to improve the stability of the model. A different strategy is described in [15], where the generator-side converter controls the output voltage and the grid-side converter controls active and reactive power. However, most of the stability analysis developed so far represent the wind farm as a sole equivalent generator. In this paper, mathematical models for wind turbines based on PMSG and DFIG technologies are analysed, together with relevant control schemes. The study is carried out on a wind farm including 14 generators, in order to assess the influence of small and big disturbances on wind farm behaviour under different wind conditions. * Corresponding author. Tel.: þ39 80 5963422; fax: þ39 80 5963410. E-mail address: forte@deemail.poliba.it (G. Forte). Contents lists available at SciVerse ScienceDirect Energy journal homepage: www.elsevier.com/locate/energy 0360-5442/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2011.11.050 Energy 39 (2012) 40e47