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Abstract-This paper presents a new control strategy for the rotor-side converter (RSC) of wind turbines (WTs) based on doubly fed induction generators (DFIG) that intends to improve its low-voltage ride through capability. The main objective of this work is to design an algorithm that would enable the system to control the initial overcurrents that appear in the generator during voltage sags, which can damage the RSC, without tripping it. As a difference with classical solutions, based on the installation of crowbar circuits, this operation mode permits to keep the inverter connected to the generator, something that would permit the injection of power to the grid during the fault, as the new grid codes demand. A theoretical study of the dynamical behavior of the rotor voltage is also developed, in order to show that the voltage at the rotor terminals required for the control strategy implementation remains under controllable limits. In order to validate the proposed control system simulation, results have been collected using PSCAD/EMTDC and experimental tests have been carried out in a scaled prototype.

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This paper presents first a review of the conventional active and reactive power theory, valid for the steady-state analysis and them t h e instantaneous power theory, introduced by Akagi et a1 [ l , 21, is presented. This instantaneous theory is, valid for steady-and-transient states and for generic voltage and current waveforms. Some examples explaining the physical meaning of the new concepts will be presented. Also, by using the well known concepts of symmetrical components together with the new theory the powers in an unbalanced system will be analyzed, including the zero-sequence instantaneous power. It will be presented an example showing how this theory can be used to design and control an active power filter. Some simulation results a.re presented and discussed.

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