Contents I. Introduction
Nowadays, the doubly-fed induction generator (DFIG) system is extensively used for the variable speed wind energy conversion systems (WECS). In DFIG system, the stator side circuit of the induction machine is directly connected to the grid and the rotor circuit of the slip-ring is directly connected to the stator circuit through a back-to-back (B2B) converter [1]. This arrangement provides flexibility of operation in both sub-synchronous and super-synchronous speed ranges in motoring and generating modes, respectively. The speed range of the generator is normally limited to 30% [2] of the synchronous speed. In a WECS, the slip power flows from the rotor to the grid and vice-versa by keeping the DC-link voltage constant. For a smooth integration between grid and generator power demands the regulation available captured wind power. The nonlinearity of wind turbine systems coupled with the highly fluctuating wind energy rate complicates the regulation of captured wind power. In most wind energy systems, captured wind power is regulated by estimating the wind speed by using the output power measurement for tracking the maximum power point [3]. Generally, the maximum power point tracking (MPPT) algorithm for wind turbine is implemented by varying the rotor speed so as to achieve the nominal tip speed ratio (TSR) by using power verses rotor speed characteristic curve. However, for fast varying wind speed, it is very difficult to achieve the maximum power point [4]. The variable speed variable pitch (VSVP) wind turbine is used to extract more wind energy within the wide range of wind speed.
