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This paper presents an analysis of the influence of the problem of Fault Induced Delayed Voltage Recovery (FIDVR), caused by the presence of induction motors, in the operation of a doubly fed induction generator (DFIG) connected to a distribution system operating under unbalanced conditions. The aim of this analysis is to determine the effect of the FIDVR problem in the oscillations of a DFIG with a comparison among results of balanced and unbalanced conditions. It was observed that, in the operation of the induction motor under unbalanced condition, the average torque produced by the induction motor is smaller (when compared to the balanced case) due to the negative sequence component of the electromagnetic torque. Thus, the FIDVR effect is aggravated by the unbalanced conditions, and this has a direct impact on the damping of the oscillations of the DFIG. The damping and frequency of the oscillations are determined by the Prony method. The results showed a decrease in damping of the oscillations of the generator operating under unbalanced conditions when compared to the balanced condition. Furthermore, in extreme cases can lead to the loss of stability of the generator.

As the wind power penetration continues to increase, wind turbines are required to provide Low Voltage Ride-Through (LVRT) capability, thus new grid codes for wind turbines are established. Most grid codes provide the LVRT curve only for the connection to the transmission system. Only few of them include the distribution system and yet the measuring of the terminal voltage do not consider unbalanced operating conditions. This paper analyses the fulfillment of LVRT requirements under unbalanced voltage conditions for doubly-fed induction generator (DFIG) wind turbines connected to the distribution system. Three different terminal voltage measurement schemes are used and analyzed. The simulations are carried out in EMTP-RV software and the results indicate that for highly imbalanced systems all three phases must be independently analyzed.