Subsynchronous oscillations (SSO) have re‑emerged as one of the most critical stability challenges in modern electric power systems. The SSOs are distinct from classical low‑frequency electromechanical oscillations and can lead to severe operational and mechanical consequences. Early SSO events were primarily associated with synchronous generators connected to series‑compensated transmission networks, where the interaction between electrical resonance and turbine‑generator (TG) shaft dynamics produced catastrophic failures. The well‑documented incidents at the Mohave Generating Station in 1970, the Navajo Project in 1976, and subsequent events at Shangdu demonstrated the destructive potential of subsynchronous resonance (SSR) and motivated extensive research into analytical methods, protection schemes, and mitigation strategies.

However, the nature of SSO has evolved significantly with the rapid growth of power‑electronic devices and inverter‑based resources (IBRs). Modern grids increasingly rely on HVDC transmission, STATCOMs, SVCs, flexible AC transmission systems (FACTS), and large‑scale wind and solar plants. These devices introduce fast control loops, nonlinear behaviors, and frequency‑dependent impedance characteristics that fundamentally alter system dynamics. As a result, new forms of subsynchronous resonances (SSR) and subsynchronous interactions (SSI) have emerged, extending beyond classical generator-network resonance.

The global expansion of renewable energy has further amplified these challenges, with a higher share of wind power capacity and renewables, many regions now operate grids with high shares of IBR-dominated generation. China, Europe, and the United States have experienced rapid growth in wind and solar installations, leading to significant structural changes in grid dynamics. While power‑electronic devices offer enhanced controllability and flexibility, their widespread deployment has introduced new stability concerns, including emerging subsynchronous torsional interaction (SSTI) and subsynchronous control interaction (SSCI) modes that were not captured by traditional SSR theory. Consequently, system planners and operators must now address a broader and more complex spectrum of SSO mechanisms.

Given the diversity of modern SSO mechanisms, a comprehensive review is firstly needed to consolidate knowledge, classify interaction types, and evaluate analytical methods. Practical guidance is also essential for engineers who must assess SSO risk in real‑world networks with limited model availability and numerous operating conditions.

To this end, this paper addresses these gaps first by providing a comprehensive review of SSO phenomena. It recalls representative real‑world SSO events involving synchronous machines, HVDC systems, FACTS devices, and IBR‑based renewable energy sources. Then, it summarizes evaluation strategies and offers practical considerations for SSO assessment in emerging IBR‑dominated grids, providing engineers and researchers with a unified framework for understanding and addressing SSO in both legacy and future power systems. At last, according to the network frequency perturbation (NFP) concept, a $P/f$ and $Q/f$ evaluation and analysis based on the test signal injection approach for analyzing the GFM's role to mitigate the risk of SSOs and analyze the damping torque is offered and investigated.