Control Strategy To Mitigate Secondary Frequency Dips For Dfig With Virtual Inertial Control

The rotor of doubly-fed induction generator (DFIG) is connected to the grid through converters and the active power can be regulated by controlling the converters. Thus DFIG can quickly response to frequency fluctuations with virtual inertial control, releasing kinetic energy stored in rotors. However, it must exit from frequency regulation mode when rotor speed decreases to the minimum allowable speed, limited by finite kinetic energy stored in rotor, and then its output power will drop rapidly, leading to a secondary frequency dip. This paper proposes a new method to alleviate the secondary dip where DFIG can exit smoothly so that sharp decrease in system frequency can be avoided. Compared to the traditional recovering strategy, sudden change in output power is avoided under this control, so the frequency changes smoothly at the cost of a comparatively wider recovering time scale. The influence of parameters is analyzed and methods of parameter determination are given accordingly. Simulation shows that the proposed strategy can efficiently reduce the secondary frequency dip.