Operation of doubly fed induction generator in ocean wave energy conversion system by stator phase sequence switching

This paper presents operation of a Doubly Fed Induction Generator (DFIG) to extract oscillatory power from ocean wave energy converter (WEC). DFIG has the advantage of generating power at both sub-synchronous and super-synchronous speed with only slip power to be handled by power converter at rotor side. At super-synchronous speed the rotor generates slip power along with the stator to take the net power generation above the generator power rating. Due to power generation capability above its rating, DFIG is a good candidate for WEC system. However, to operate DFIG in an oscillatory system where direction of rotor speed keeps changing it requires controlled supply from stator side as well. In this paper, a rotor side control method is proposed to operate the DFIG in oscillatory system with a low cost stator phase sequence switching circuit to adapt to the change of direction of the rotor speed. A suitable control scheme is proposed to control the machine flux during start up and speed reversal to eliminate current inrush at the stator side. The DFIG current is controlled in vector control method with control axes orientated along stator flux axis. The current controllers are designed to control the rotor side currents to produce power and to regulate machine flux in accordance with the wave energy converter dynamics. The system is modeled and simulated in MATLAB-Simulink platform to validate the proposed scheme. All the corresponding results are presented and discussed.