Renewable Energy Sources (RES) have been massively integrated into the modern electric power system in the past few decades due to the environmental and sustainability concerns throughout the world. As a result, the power electronic converters are anticipated to acquire a steadily increasing role as they are the key element for the interface between RES and the grid. However, owing to the intermittency of the RES and the distinguished features of the Inverter-Based Resources (IBRs).

The main focus of this thesis is to develop optimal system operation strategies to maintain the security and stability of the grid while considering the fast and accurate control of the IBR units. To achieve this, we investigate challenges in different areas.

Regarding system frequency and low inertia issues, the main challenges are the incorporation of differential equation-based frequency dynamics into algebraic equation-based optimization problem as well as the optimal utilization of the frequency support from different sources. We first target on the optimal system scheduling on a transmission system level to achieve system operation cost minimization while maintaining the frequency security.

In addition, the frequency stability problem in microgrids after unintentional islanding events is also studied. We consider the frequency support from WTs, PV and storage systems as well as noncritical load shedding to ensure the microgrid frequency security after unintentional islanding events. Furthermore, a SCC-constrained Unit Commitment (UC) model is developed, maintaining a minimum SCC level at different locations in the system such that enough reactive current could be supplied during the fault to trigger the protection devices and maintain the post-fault voltages. Moreover, the static voltage stability in systems with high IBR penetration is also investigated considering the interactions among the IBR units and their reactive power support capability within rating limits.