Electronic Thesis and Dissertation Repository

Thesis Format



Master of Engineering Science


Electrical and Computer Engineering


Firouz Badrkhani Ajaei


The majority of the Distributed Energy Resources (DERs), i.e., Energy Storage Systems (ESSs) and Renewable Energy Systems (RESs), utilize inverters to convert the Direct Current (DC) power to the Alternating Current (AC) power needed by the majority of the consumers. Proliferation of the inverter-based DERs has caused significant changes in the operation of the modern electric power systems. Inverters lack the mechanical inertia that is inherent in the traditional power generators, i.e., rotating electrical machines. As a result, the emerging inverter-dominated power systems suffer from lower stability margins, excessive frequency deviations, and poor dynamic response to disturbances. This issue has adversely affected the integration of the highly advantageous inverter-based renewable energy systems in microgrids and active distribution systems. Appropriate inverter control can be used to emulate virtual inertia by imitating the behavior of traditional generation units. Based on this idea, the concept of virtual synchronous generator (VSG) has been proposed.

VSGs suffer from the transient stability issues that affect the operation of the Synchronous Generators (SGs). They can become unstable due to prolonged faults. Unlike the SGs that can handle significant over-current stress, VSGs have limited overcurrent capacity. The studies conducted in this research indicate that the current limiting strategy of the VSG significantly impacts its transient stability. The impacts of different inverter current limiting strategies on the performance of the VSG are investigated and the one that leads to the largest transient stability margin is identified.

Summary for Lay Audience

The penetration of renewable energy sources into power system has been rising in recent years. Proliferation of these technologies has caused significant changes in the design and operating principles of the modern electric power systems. Due to the physical characteristics of these resources and the fact that they are typically interfaced to the power grid by power electronics converters, i.e., inverters, their interaction with the grid is substantially different from those of the traditional power generation plants. While the rotating parts in the traditional power generators inherently provide inertia to the system, inverter-interfaced renewable energy systems lack the mechanical inertia. This lack of inertia can cause instability issues. This thesis identifies the best technical practices and develops innovative solutions to improve the stability of the modernized grid in presence of renewable energy sources.