Early Detection of Subsynchronous Risks in Grid Siting and Development Studies

As the grid becomes more complex, shaped by the rapid build-out of renewable generation, conventional assets, and the explosive growth of data center loads, new risks are emerging in places previously considered to be routine. Among them are subsynchronous phenomena, including subsynchronous resonance (SSR) and subsynchronous ferroresonance (SSFR), which can trigger harmful oscillations that threaten both project-level equipment and broader system stability. 

For developers and planners, the message is clear: anticipating these dynamics during siting and interconnection studies is no longer optional. 

Why Subsynchronous Risks Matter Now 

Historically, SSR was mostly a concern in large thermal plants near series-compensated transmission lines. Today the combination of grid-connected renewables, inverter-based resources, and large nonlinear loads has expanded the risk landscape. Subtle interactions that once seemed rare are becoming more common, and the cost of overlooking them can be steep, ranging from transformer damage to forced outages and compliance violations.  

A Framework That Looks Deeper 

At EPE, we have developed a study framework that brings subsynchronous risks into focus at an earlier stage. Unlike traditional SSR analysis, our approach explicitly models the nonlinear saturation characteristics of main power transformers (MPTs). This allows us to capture ferroresonant effects, behaviors that are often invisible to conventional models. 

By running fault simulations across varying types, impedances, and locations, we can evaluate system behavior over subsynchronous frequency ranges. This enables us to test how oscillations might be initiated, how they may be damped, and whether certain configurations could lead to sustained instability. 

What the Results Reveal 

Our findings underscore an important point: magnetic nonlinearities matter. Transformer saturation curves, when paired with transmission series capacitors, can, under specific contingencies, create the conditions for self-sustaining oscillations. Ignoring these nonlinearities risks underestimating vulnerabilities and leaves both projects and the grid exposed. 

The Case for Early Screening 

For developers, utilities, and operators, the takeaway is straightforward: build SSR/SSFR screening into siting studies. This provides three key benefits: 

• Early insight into potential vulnerabilities before significant capital is committed. 
• Alignment with evolving interconnection standards, which increasingly require more rigorous electromagnetic transient (EMT) and stability analyses. 
• Cost-efficient design and mitigation, allowing for targeted solutions rather than expensive retrofits after the fact. 

As the energy transition accelerates, the line between traditional and nontraditional risks continues to blur. By proactively addressing subsynchronous phenomena at the siting stage, the industry can avoid costly surprises, safeguard reliability, and accelerate the path toward a resilient and flexible grid. 

At EPE, we believe early detection is not just good engineering, it’s good business. To find out how we can help you prepare your project for success, contact our team of experts using the form below.