Improving the transient performance of doubly fed induction generators when submitted to voltage sags

It is proposed in this article an optimal solution to improve the transient behavior of doubly-fed induction generators in wind parks connected to the electrical grid, when they are submitted to voltage sags due to the occurrence of faults in the electrical network. The recommended procedure in these cases is to maintaining the wind generator connected to the electrical network during and after the fault period without disconnecting the rotor-side converter. To achieve this objective, in the case of large voltage sags occurring at the wind generator bus, the suggested procedure is to insert a by-pass resistance in the rotor winding during the fault period in order to reduce the over-current and mainly to avoid high over-voltage in the rotor circuit. During the post-fault period it is proposed in this article the use of optimal PI controllers´ for the rotor-side converter, which gains are adjusted by a genetic algorithm methodology. This proposed strategy has demonstrated to be efficient in increasing the DFIG ride-through capability as well as the system transient stability margin when compared with the formal methodology of designing PI controllers via poles placement technique. For small voltage sags at the wind generator bus the solution used to improve the transient behavior is based only on the optimal PI controllers´ action, without using the by-pass resistor.