Autonomous power system for island or grid-connected wind turbines in distributed generation Grzegorz Iwanski * ,y and Wlodzimierz Koczara Institute of Control and Industrial Electronics, Warsaw University of Technology, 75 Koszykowa St., 00-662 Warsaw, Poland SUMMARY Modern power generation systems for wind turbines are often based on the rotor fed slip-ring machines. Power electronics converter provides the slip power, and also the reactive power for excitation of the generator during standalone operation. This way the isolated load can be supplied even if the grid has failed. Stator voltage in an autonomous operation is controlled using vectorial phase locked loop (PLL) structure; therefore the information about mechanical speed or angular position of the rotor is eliminated from the control method. The second PLL is also used for synchronization of the generated voltage with the grid voltage. Voltages synchronization is necessary for soft connection and protection of the supplied load from the rapid change of the supply voltage phase. The grid-connected doubly fed induction generator (DFIG) can be useful after grid fault; however, the mains outage detection methods are necessary for fast disconnection after grid failures. Copyright # 2008 John Wiley & Sons, Ltd. key words: distributed generation; variable speed generation; wind energy; standalone generation; grid synchronization; mains outage detection 1. INTRODUCTION The concept of distributed power generation assumes that the power system can operate in grid-connected mode as well as in the standalone mode [1]. The active and reactive power delivered to grid is controlled during grid connection operation [2], whereas the output voltage amplitude and frequency should be controlled during standalone operation to supply an isolated load [3]. The operation modes are selected using grid connection switch (GCS) (Figure 1). Autonomous operation can be selected during grid disturbances (breaking of transmission line or permanent short circuit in the power network). The issues of grid connection and disconnection processes are common for many types of the power sources. Standalone operation may also be used in an island power systems, which are never connected to the distribution grid, and have to provide continuously a fixed amplitude and frequency of the generated voltage for isolated load. EUROPEAN TRANSACTIONS ON ELECTRICAL POWER Euro. Trans. Electr. Power 2008; 18:658–673 Published online 27 March 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/etep.256 *Correspondence to: Grzegorz Iwanski, Institute of Control and Industrial Electronics, Warsaw University of Technology, 75 Koszykowa St., 00-662 Warsaw, Poland. y E-mail: iwanskig@isep.pw.edu.pl Copyright # 2008 John Wiley & Sons, Ltd.