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A study of small signal behavior of power systems including wind generation units and energy capacitor systems (ECSs) is presented in this paper. Modeling of wind units with squirrel-cage induction generators connected to power system through a full-scale AC/AC converter and ECS unit are explained thoroughly. The ECS unit (considered in this paper) consists of electric double-layer capacitors (EDLC) and DC/AC converter. Small signal behavior of three test systems are studied and compared with one another using digital computer simulations. The three systems are: S1) WSCC 9-bus system with three conventional synchronous generators (SGs), S2) the WSCC system with one of the conventional SGs replaced by a wind energy system, and S3) the same as S2) but with the addition of an ECS unit. The study includes comparing of dominant eigenvalues, the participant state variables, and the related participation factors for the three systems. In addition, the effects of loading on the unstable modes of the system with conventional SG, wind and ECS units (system S3) are studied using the trajectory of these modes as loads are increased. [ABSTRACT FROM PUBLISHER]