Optimized Demagnetizing Control of DFIG Power Converter for Reduced Thermal Stress During Symmetrical Grid Fault

Direct linking between the stator of a doubly fed induction generator (DFIG) and the power grid makes this type of generator sensitive to disturbances in the grid voltage, which may lead to high voltage and current on the rotor side. Moreover, modern grid codes, which specify stringent requirements on reactive power compensation, challenge fault ride-through operation even more. In this paper, based on conventional demagnetizing current control, the capability of a DFIG rotor-side converter to ride-through a symmetrical grid fault is calculated in accordance with its current and voltage ratings. Afterwards, an optimized demagnetizing coefficient is designed to guarantee the same rotor current amplitude between the instants of the fault occurrence and the reactive current injection. A reduction of the junction temperature of the power device can thereby be achieved. It is concluded that, regardless of the rotor speed, the demagnetizing coefficient is related only to the dip level. Compared with traditional vector control, a simulation of 2 MW DFIG system agrees with the reduced thermal stress during the fault period, and experimental results in a down-scale DFIG system verify the feasibility of the proposed control strategy as seen from the electrical characteristics.