IEEE Energy2030 Atlanta, GA USA 17-18 November, 2008 Doubly-Fed Induction Generator Control Under Voltage Sags K. Lima*, A. Luna ♠ , P. Rodriguez ♠ , E.Watanabe*, R. Teodorescu ♣ and F. Blaabjerg ♣ *FEDERAL UNIVERSITY OF RIO DE JANEIRO Power Electronics Laboratory Rio de Janeiro – BRAZIL ♠ TECHNICAL UNIVERSITY OF CATALONIA Department of Electrical Engineering Barcelona – SPAIN ♣ AALBORG UNIVERSITY Institute of Energy Technology DK-9220 Aalborg - DENMARK Abstract – This paper proposes a new control technique to improve the fault-ride through capability of doubly fed induction generators (DFIG). In such generators the appearance of severe voltage sags at the coupling point make rise to high over currents at the rotor/stator windings, something that makes necessary to protect the machine as well as the rotor side power converter. As a difference with the most extended solution, that reduces these currents by means of the connection of a crowbar circuit, this works intends to enhance the DFIG’s response in such conditions without introducing extra hardware in the system. To this end, the proposed control system feedback the stator currents as the rotor current reference during the fault, until they are driven to their nominal values. The feasibility of this proposal has been proven by means of mathematical and simulation models, based on PSCAD/EMTDC. I. INTRODUCTION The increasing capacity of the installed wind power generation facilities world wide, as well as the high scale penetration of such systems in the next future, have made necessary to renew the grid code requirements regarding the operation of wind farms. For the wind power case these grid codes demand control requisites similar to those demanded in conventional power plants, which are basically focused on achieving a stable and reliable operation of the electricity network. One of these requirements is the capacity of wind power generators to remain connected to the grid in case of severe grid voltage sags. This feature, also known as fault ride- through capability (FRT), is specially critical in DFIG based wind turbines (WT). Several studies have been carried out in order to improve the ride through capability in this kind of WTs [1-8]. The main objective of these works has been focused on avoiding the disconnection of the system during the sag. Among the different proposed solutions, those based on limiting the over currents at the machine and at the power converter, as well as, over voltages at the DC bus of the converters by means of auxiliary circuits stand out. However, this kind of solution implies the installation of extra devices in the system, something that finally increases the costs and hinders its reliability, as the control system should became more complex. These circuits do not only reduce the reliability of the system, but require an extra control effort. In addition the design of such protection is calculated considering the features of the wind power system, and hence any future change of its configuration would affect its performance. Regarding this solution several crowbar topologies can be selected, as it is explained in [9-12]. Other alternatives to crowbars, as the inclusion of static keys between the stator and the electrical grid, presented by Petterson [13], or the stator flux damping proposed by Næss [14] have been also published. However all these techniques require also the installation of extra active/passive elements in the power system. The aim of this paper is to propose a new control technique that would permit to guarantee the controllability of DFIG WTs during severe voltage sags near the point of common coupling (PCC) without adding extra hardware. This solution will be based on the feedback of the real statoric currents to the current setpoint of the rotor converter. The feasibility of this proposal will be analyzed in the following sections and its performance will be tested by means of simulation models based on PSCAD. II. MODELLING OF THE GENERATOR UNDER FAULT CONDITIONS Considering the classical single cage fifth order model of a DFIG [13] the voltage and magnetic flux of the stator can be written as in (1), and, in an analogous way, the equations that describe the dynamics of the voltage and magnetic flux in the rotor as in (2). ds ds s ds s qs qs qs s qs s ds ds s ds m dr qs s qs m qr d v Ri dt d v Ri dt Li Li Li Li λ ω λ λ ω λ λ λ = + − = + + = + = + (1) Considering that the system described by (1) and (2) is linear, something that assumes that the magnetic circuit of the DFIG is linear, it is possible to obtain the following statoric currents, (3)-(4), in the synchronous reference frame [14]. Authorized licensed use limited to: IEEE Xplore. Downloaded on March 13, 2009 at 06:16 from IEEE Xplore. Restrictions apply.