A novel control strategy for the rotor side control of a doubly-fed induction machine

The prospect of using the doubly fed induction machine (DFIM) drive for renewable energy sources like wind and wave energy has been well established. The state-of-the-art technology uses two back-to-back IGBT converters for controlling the machine and line side interface. However, IGBTs are costly compared to thyristors, especially at high power. In this paper, a novel machine side converter (MSC) and associated control strategy for the DFIM is proposed which will replace one of the IGBT converters. The proposed converter consists of a thyristor bridge and a boost/buck-boost DC-to-DC interface. This converter is capable of bidirectional power flow and thus the machine can be controlled in both the super-synchronous and subsynchronous speed ranges. The proposed converter has the ability for successful commutation of the thyristors for all speeds of operation. Through the suitable control of the IGBT based front end converter (FEC), the composite system interfaces with the grid at nearly unity power factor. The FEC supports the power flow between the rotor circuit and the utility and in addition, it is made to work as an active filter to compensate for any additional harmonics injected by the stator of the machine to the utility. With the proposed power converter combination, the potential cost of the system is reduced without any major sacrifice in the system performance.