Driven by clean energy goals, power systems are undergoing a transformative shift with the increasing integration of renewable energy resources. Traditional centralized power plants, which interface with the grid through synchronous machines, are progressively being replaced by inverter-based resources (IBRs). As this transition accelerates, power grids are evolving from machine-dominated to inverter-dominated systems, resulting in weakened transmission and distribution networks in certain regions.

Reduced system strength and inertia could challenge the stable operation of conventional grid following (GFL) inverters as they are primarily designed to operate in the presence of a strong grid. Unstable operation or large-scale disconnection of GFL IBRs following disturbances challenges the reliability of both transmission and distribution systems. Real-world incidents reported by system operators such as the Electric Reliability Council of Texas (ERCOT) and the Australian Energy Market Operator (AEMO) underscore these concerns.

Grid-forming (GFM) IBR has been proposed as one of the solutions to address weak grid challenges. In the near term, they are primarily considered in transmission systems with low fault current and rotational inertia, and in distribution inverter-based microgrids. Transmission-connected GFM IBRs have been shown as beneficial in addressing weak grid challenges. Some transmission system operators (e.g., Hawaiian Electric, FINGRID) have started requiring GFM capability from the transmission-connected IBRs. However, the potential need for and benefits of GFM inverters in distribution systems under grid-connected (blue-sky) operating conditions remain insufficiently explored.

Previous work by the authors [1] has demonstrated that GFM distributed energy resources (DERs) can enhance local stability by supporting nearby DER operations. Building on this foundation, this paper extends the analysis to a broader, system-wide perspective, focusing on scenarios where DERs contribute a substantial share of total generation capacity. Such high-penetration scenarios are already a reality in regions like South Australia, where distributed PV met over 90% of demand during certain periods in 2021 and 2022. Using electromagnetic transient (EMT) simulations in PSCAD/EMTDC, this study evaluates how high DER penetration affects system-wide oscillation damping and frequency nadir in a simplified transmission system integrated with real distribution feeders. By comparing GFL and GFM DER control methods, we evaluate the extent to which GFM DERs can contribute to stabilizing system-wide oscillations and improving frequency response.

In addition to potential benefits, the paper also examines the potential adverse impacts of GFM DERs on distribution system operations. These include challenges to protection coordination, risks of unintentional islanding, among others. The findings aim to inform both transmission and distribution system operators about the value of GFM DERs, while equipping distribution system operators with a clear understanding of potential adverse impacts on distribution networks and the corresponding mitigation strategies.

[1] A. Venkataramanan, W. Wang, D. Ramasubramanian, A. Huque, T. Key, and A. Mehrizi-Sani, ‘‘Grid-forming distributed energy resources: Value in high renewable penetration and weak grid scenarios,’’ in Proc. 22nd Wind Sol. Integr. Workshop (WIW), vol. 2023, Sep. 2023, pp. 321–330.