Dynamic interactions between the power system and DERs - modeling and case studies

DERs in general have completely different dynamics than the traditional rotating machinery (generators and motors) based power system. Present DER technology uses converters for interfacing DERs to the power grid. These interfaces are inertialess and limit the electric current during faults and transients. At the same time, the converters have sophisticated feedback control loops that may interact with the transient of the electric power system with the end effect of generating unintended modes of oscillations. These characteristics of DERs impact the dynamics of the integrated power system with DERs. Present day models are inadequate to realistically simulate the new paradigm of the integrated model. We present a new modeling methodology for the integrated system based on quadratization of the model equations and quadratic integration of the resulting dynamical equations. The new approach captures the salient features of the integrated system. The paper presents the utilization of this model for transient simulation of the integrated system as well as for analyzing the modes of oscillations of the system using small signal analysis. The methodologies are illustrated with case studies. The case studies illustrate the unique dynamic behavior of the integrated system as the penetration level of DERs increases as well as the dynamic interactions between the system and the converter controls of the DERs which generate additional unexpected modes of oscillation