PNNL

Abstract

To ensure safe and reliable operations, electric utilities must understand how the dynamics of the power grid are evolving, as the existing synchronous machine-dominated systems continue to incorporate increasing amounts of inverter-based resources (IBRs). A pressing concern is understanding if and how the well-known inter-area modes of oscillation will change due to increasing inverter penetration. To address this question, this paper derives an analytical expression to identify the major factors influencing changes in inter-area mode properties of bulk power grids when synchronous machines are replaced by IBRs. The IBRs are assumed to implement droop-based grid forming (GFM) control, but the analysis can be extended to other inverter control methods. It is concluded that the strongest factor dictating mode changes is the location where synchronous machines are removed and/or IBRs are added, and the observed changes can be mitigated to some extent by tuning two of the GFM inverter control parameters, namely (a) the active power-frequency droop coefficient, and (b) the time constant of the active power measurement low-pass filter. The analytical conclusions are validated using full dynamic simulations of the IEEE 39-bus benchmark system and the 2031 heavy-winter planning model of the US Western Interconnection. Conclusions from this study will help utilities understand if/which inter-area modes will continue to be of concern in their footprints in an IBR-dominated future, and identify areas where the displacement of synchronous machines are likely to significantly alter the properties of existing modes.