Control Dynamics of a doubly fed induction generator under sub- and super-synchronous modes of operation

Depending on wind speed, a doubly fed induction generator (DFIG) based variable speed wind turbine is capable of operating in sub- or super-synchronous mode of operation using a back to back PWM converter. A smooth transition between these two modes of operation is necessary for reliable operations of the wind turbine under fluctuating wind. This paper presents the analysis and modeling of DFIG based variable speed wind turbine and investigates the control dynamics under two modes of operation. A battery energy storage (BESS) with a bidirectional DC-DC converter is added for a smooth transition between the modes. Mathematical analysis and corresponding modeling results show that the power flow in the rotor circuit under two modes can be controlled by changing current and voltage phase sequence through the rotor side converter (RSC) and line side converter (LSC). A coordinated control among RSC, LSC and DC link storage system ensure variable speed and maximum power extraction from the fluctuating wind and reduce the possibility of instability around synchronous speed. Extensive simulations have been conducted to investigate control dynamics under the two modes of operation and during transitions.