Hybrid Renewable Energy System Using DFIG and Multilevel Inverter

The rotor of a doubly fed induction generator (DFIG) driven by a wind turbine needs rotor excitation so the stator can supply a load or feed the grid. In a variable-speed wind energy conversion system (WECS), the mechanical frequency of the generator varies, and in order to keep the stator voltage and frequency constant, the rotor voltage and its frequency have to be varied. Thus the system requires a power conversion unit to supply the rotor with a variable frequency voltage that keeps the stator frequency constant irrespective of the wind speed. In the scheme proposed, the rotor of the DFIG draws power either from three-phase ac mains or from a set of photo voltaic (PV) panels depending on the availability of the solar power. Maximum power point tracking techniques have been used for power extraction from both the wind turbine and solar panels. A multi-level inverter is used to convert the rectified voltage from ac mains and dc voltage from PV panels to a variable-frequency voltage to supply the rotor.