DC bus voltage build up and control in stand-alone wind energy conversion system using direct vector control of SCIM

This paper presents the start up and operation of a stand-alone wind energy conversion system (WECS) using a squirrel cage induction machine (SCIM). The proposed scheme obviates the use of either excitation capacitor or batteries in DC bus. The DC bus comprising of a combination of normal electrolytic capacitor and few super capacitors, is interfaced with the SCIM through a current controlled inverter. The super capacitors having higher self-discharge time, provide initial voltage to the inverter to start the voltage build up process and the electrolytic capacitor filters out the ripple. Direct field oriented control (DFOC) of induction generator (IG) is performed to have good dynamic response and an adaptive controller is designed to control the DC bus voltage at a constant value irrespective of the variation in wind speed and electrical load. Rotor flux is maintained constant up to the base speed of the machine, which allows the terminal voltage to increase with wind speed and hence increases the power extraction at higher wind speed. The overall system modeling and the analytical control design is presented. Predictions made from analysis are completely validated through simulation using realistic converter model.