In our September Watt’s Current edition, the EPE team previewed ERCOT’s proposed Planning Guide Revision Request (PGRR120) aimed at reducing Subsynchronous Oscillation (SSO) risks for new and modified generation resources. On November 6, 2025, the PUCT approved PGRR120, with an effective date of December 1, 2025. 

Key Requirements under PGRR120: 

• New Interconnections: Generators with an original SGIA executed on or after April 1, 2026 cannot interconnect if an N-1 contingency would make them radial to a series capacitor.  
• Modifications: Projects that could become radial to a series capacitor must demonstrate no SSO risk, or show ERCOT that mitigation is fully in place before energization.  
• Operations: If SSO is observed, ERCOT may halt generator operations until mitigation is verified. 

Security Screening Study Updates: 

ERCOT will now: 

• Assess SSR vulnerability during the Security Screening Study. 
• Include in its report: 
• The number of Credible Single Contingencies that could make the generator radial to a series capacitor, and 
• A determination of whether the project complies with the new SSO requirements. 

As a reminder, Interconnecting Entities have 180 days after receiving the screening study results to elect to proceed to the FIS, or the GIM is considered withdrawn. 

Why This Matters 

These changes enhance grid reliability by preventing interconnections that pose SSO risks under N-1 conditions. At EPE, we are ready to support developers in considering how these rules may affect timelines and viability for projects located near series-compensated transmission paths.

With deep expertise in modeling and interconnection processes, EPE remains a trusted partner in navigating ERCOT’s evolving regulatory landscape. Contact our team using the form below to find out how we can help you with this process today.