The retirement of conventional synchronous generators and the integration of inverter-based renewable energy sources (RES) are transforming power systems in unprecedented ways. Understanding the impact and consequences of this resource mix transition is crucial to maintaining reliable grid operation. In this regard, this paper examines the impact of increased renewable energy sources on the oscillatory behavior of the U.S. Eastern Interconnection. From our analysis, we found that no single trend could be used to correlate the impact of increased RES penetration levels on the damping ratio of system modes. In fact, the impact of the increased RES penetration levels on a system mode was influenced by the group of synchronous machine models that were replaced with RES machine models. For example, replacing the synchronous generators in Florida significantly increased the damping ratio of the 0.21 Hz mode, whereas the impact was relatively small on the mode when synchronous machines were replaced in the North-eastern areas of the EI.

With an increase in the oscillation events observed in the U.S. Eastern Interconnection (EI), it has become increasingly important to have a good understanding of the EI system oscillatory behavior. However, as compared to the Western Interconnection system (WI), not much work has been done in this regard for the EI system. Therefore, in this paper, a thorough analysis is carried out to identify inter-area modes and their properties for the EI system using a 80000+ bus real EI model. Multi-channel Prony method is used in this paper for estimating system modes and mode-shapes.