Modern grid codes determine that wind generation plants must not be disconnected from the grid during some levels of voltage sags and contribute to network stabilization. Wind energy conversion systems equipped with the doubly fed induction generator (DFIG) are one of the most frequently used topologies, but they are sensitive to grid disturbances due to the stator direct connection to the grid. Therefore, many efforts have been done in the last few years in order to improve their low-voltage ride-through capability. This paper analyzes the behavior of the DFIG during symmetrical voltage sags using models in the frequency domain. A new strategy, the machine magnetizing current control, is proposed in order to enhance the system response during balanced dips. The method is derived on a theoretical basis and numerically investigated by means of simulation. Experimental results are presented and validate the proposed strategy. Finally, the practical aspects of the use of this strategy are discussed.

Modern grid codes worldwide determine specific requirements for the connection of wind energy conversion systems (WECSs) to the grid. One of the requirements is the ride‐through fault capability (RTFC) or low‐voltage ride‐through (LVRT), which defines that the plants must not be disconnected from the grid at certain levels of voltage sags and also must contribute to network stabilisation. The LVRT of the doubly‐fed induction generator (DFIG) technology is a well‐known problem, mainly during unbalanced grid voltage conditions. The present study extends the studies presented in the literature, analysing the DFIG behaviour during asymmetrical voltage sags using a frequency‐domain modelling. A modified resonant controller is proposed to improve the system response during voltage sags, and the analysis of the RTFC based on the converter limits is carried out. The proposed strategy is validated with simulation results of a 2 MW‐WECS and experimental results of a 25 kW scaled test bench.

Due to the increasing number of wind power plants, several countries have modified their grid codes to include specific requirements for the connection of this technology to the power system. One of the requirements is the ride-through fault capability (RTFC), i.e., the system capability to sustain operation during voltage sags. In this sense, the present paper intends to investigate the behavior of a full-converter wind generator with a permanent magnet synchronous machine during symmetrical and asymmetrical voltage sags. Two solutions to improve the low voltage ride-through capability (LVRT) of this technology are analyzed: discharging resistors (brake chopper) and resonant controllers (RCs). The design and limitations of these solutions and the others proposed in the literature are discussed. Experimental results in a 34 kW test bench, which represents a scaled prototype of a real 2 MW wind conversion system, are presented.