Control of Doubly Fed Induction Generator under Symmetrical Voltage dips Jesús López, Pablo Sanchis, Eugenio Gubía, Alfredo Ursúa and Luis Marroyo, Xavier Roboam Department of Electrical and Electronic Engineering, Public University of Navarra, Spain jesus.lopez@unavarra.es Abstract-Today, Doubly Fed Induction Generator (DFIG) is largely used in wind generators. It provides variable speed characteristics in a cost-effective way. The main drawback of this machine is its sensitivity to grid disturbances. Voltage dips cause overvoltages to appear in the rotor that can surpass the converter limits. Additionally, variations of the grid voltage cause the stator flux to oscillate at the grid frequency. This paper proposes a novel control strategy to overcome these problems; furthermore, it reduces the rotor voltage, improving the control of the rotor current and it accelerates the damping of the flux oscillations. I. INTRODUCTION Variable speed turbines have become a standard for wind turbines of 1 MW and above. Among the different alternatives to obtain variable speed, the Doubly-Fed Induction Generator (DFIG) has become the most popular due to the small size of the power converters. However, wind turbines based on the DFIG are very sensitive to grid disturbances, especially to voltage dips. An abrupt drop of the grid voltage causes overvoltages and overcurrents in the rotor windings that can destroy the converter if no protection elements are installed. Traditionally, the solution used by manufacturers to protect the rotor converter has been to short-circuit the rotor windings by means of resistances with the so-called crowbar [1]. Nevertheless, as a response to the increasing wind power contribution, new grid codes are being approved that toughen the requirements for the wind turbines behavior in case of voltage dips [2]. According to the new grid codes, the wind turbine cannot be disconnected from the grid, as it was commonly done with older turbines. In addition to this, the wind turbine must inject a reactive power to assist the grid on recovering its rated voltage. In order to fulfill the new requirements, modifications to the conventional DFIG architecture for ride-through have become necessary. The behavior of the DFIG in case of three-phase voltage dips was previously analyzed by the current authors in a recent paper [3]. In that paper a theoretical study was proposed for understanding the causes of the overcurrents that appear during a network fault. The study was based on the decomposition of the stator flux into two terms: the steady-state flux, rotating synchronously and the so-called “natural flux”, a transient flux fixed to the stator caused by the voltage change. The present article bases the study of the interactions between the DFIG and its converter on the natural flux concept. As a result of this study, a new control strategy that improves the machine behavior under these faults is developed. II. SYSTEM DESCRIPTION The basic configuration of a wind turbine based in DFIG is shown in Fig. 1. As it is shown in the figure, the stator of the machine is directly coupled to the grid and only the rotor power must be handled by the converters. DC bus grid DFIG vector control vector control Ps* Qs* Vbus* Qf* if, vg ir Vbus rotor converter front-end converter Fig. 1. Wind turbine scheme with DFIG. The rotor converter; controls the DFIG to generate the desired stator active and reactive power. The front-end converter regulates the DC bus voltage and can additionally generate an independent reactive power that will be injected into the grid. Overruling these controls, there is a supervisor that calculates the optimum power to be extracted from the wind and controls several others subsystems, i.e. the pitch angle, the yaw control, among others. This paper will focus on the interactions between the rotor converter and the generator. The front-end converter will not be included in the study. The DIFG is usually controlled by means of a vector controller. Assuming that the rotor currents are transformed to a synchronous frame where the q-axis is aligned with the grid voltage and neglecting the small effect of the stator resistance, the relation between rotor currents and stator active and reactive power is decoupled as the following equations illustrate: s s grid rd s m grid s rq s m grid s L V i L L V Q i L L V P ω 2 − ⋅ = ⋅ = (1)