This paper presents new developments on a previously proposed reduced-order model for wind turbine generators (WTGs) and explores the steps towards a simplified dynamic model for wind farms. WTGs based on doubly-fed induction generators (DFIGs) are considered. Using bifurcation and sensitivity analysis, little interaction between WTGs and system's synchronous generators (SGs) is found. Focusing on the most relevant modes and variables, the model order is reduced using selective modal analysis (SMA). A general formulation for applying SMA is shown and two reduction examples are presented: one with one relevant mode and another with three relevant modes. In the simplest case, the reduced-order model is conceptualized as a negative load. In addition, the impact of WTGs on power system operation is studied through eigenvalue sensitivity analysis. Finally, a preliminary aggregated model for wind farms is presented with satisfactory results. The steps towards a general model for wind farms are addressed where other aspects such as the use of a supervisory control should be considered. (C) 2011 Elsevier B.V. All rights reserved.