A control technique for doubly fed induction generators to solve flicker problems in wind power generation

The paper deals with wind power plants working connected to weak network. For high power applications, at present, doubly fed induction machines are often used. Their use is widespread because they make possible the: speed regulation, even if in a limited range, without the need high power electronic converters. Wind energy conversion systems can produce torque fluctuations at the generator shaft. These fluctuations represent a serious limitation for weak networks in which flicker effect can emerge. In the paper a full modelling of the doubly fed induction machine used in wind energy systems and of the flicker phenomenon is done. In particular, a new analytical function suitable to represent the magnetic behaviour of the induction machine basing only on some steady state data has been proposed and verified, by means of experimental tests. It is shown that rotor feeding influences flicker amplitude. In order to reduce flicker effect a proper rotor feeding algorithm has been proposed and tested by means of numerical simulations.