Behaviour of the doubly fed induction generator exposed to unsymmetrical voltage sags

PROJECT TITLE :

Behaviour of the doubly fed induction generator exposed to unsymmetrical voltage sags

ABSTRACT :

This study analyses the dynamic behaviour of the doubly fed induction generator exposed to unsymmetrical voltage sags, providing insights into wind turbine fault ride-through capability. The sags are assumed to be caused by faults. An analytical approach assuming that the rotor-side converter will keep constant the rotor current in the synchronous reference frame during the event is used. The voltage limit of the rotor-side converter is additionally thought of in order to determine the things where the rotor current can be controlled. The effects of sags on grid-connected equipments rely on the sag characteristics (period and depth), but also on the fault clearing method. During this method, the fault isn't cleared instantaneously, as within the case of abrupt sags (which is the standard approach in the literature), but within the successive natural fault-current zeros, leading to a voltage recovery in many steps referred to as discrete sags. The effects of each abrupt and discrete sags on the doubly fed induction generator behaviour are compared. This comparison reveals that the discrete clearing process smoothes the sag effects on the generator.

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