A Three-Part Blog Series by ZTZ Services 

Just the Facts 

The fast growth of inverter-based resources (IBRs) such as wind and solar have delivered clear benefits in cleaner energy and greater supply diversity. At the same time, these resources introduce new operating conditions that are changing the stresses placed on our substation transformers. Fortunately, IBR systems have now been operating on the grid for years, giving utilities valuable real-world data on their actual performance and impacts. 

It is no longer business as usual. 

Two key 2025 reports highlight a fundamental challenge facing the industry: the power grid is now operating under conditions that its original protection systems, thermal models, and maintenance practices were never designed to handle. 

In July 2025, FERC approved new reliability standards requiring inverter-based resources to ride through voltage and frequency disturbances. This was a clear regulatory acknowledgment that the existing framework was written for a predominantly synchronous generation fleet that no longer exists. ^[1] 

That same month, NERC’s 2025 State of Reliability report noted improving outage severity and restoration times overall, yet warned that emerging risks driven primarily by the rapid IBR transition are outpacing the development of effective mitigation strategies. ^[2] 

A New Type of Inertia and Variability 

Synchronous generators provide more than just real power. They deliver rotational inertia that helps stabilize frequency during disturbances. IBRs contribute little to no equivalent inertia. In ERCOT, for example, data shows synchronous inertia has dropped more than 50% during periods of high renewable output in recent years. The result is a grid that responds more quickly to changes but with significantly less natural damping. 

This shift manifests in extreme ramp rates. Utility-scale PV plants can experience power ramp rates exceeding 10% of rated capacity per minute under passing clouds ^[3]. In CAISO, the evening net-load ramp routinely exceeds 13 GW within roughly three hours ^[4]. This daily operational stress cycle did not exist a decade ago. 

Fault behavior has also changed. IBRs typically deliver lower fault currents than synchronous machines, weakening the signals that traditional overcurrent relays rely on. Unadjusted relays may trip late, fail to trip, or operate incorrectly. Any of these outcomes allows fault energy to remain in the transformer longer, accelerating damage to windings and insulation. 

The Supply Chain Constraint 

The transformer supply chain challenges of today are well known. Lead times for certain power transformers now exceed 200 weeks, with an average of approximately 128 weeks. Wood Mackenzie projected a roughly 30% supply deficit for power transformers that will likely persist well into the 2030s ^[5].  

In this environment, an unplanned transformer failure is no longer a short-term recovery issue. It becomes a multi-year planning problem. Against this backdrop, managing IBR-induced stresses on transformers is essential. With the right engineering controls and condition-based asset management, medium and large power transformers can still reach their intended service life in high-renewable hybrid grids. 

Part 2 of this series examines the specific stress mechanisms IBRs place on transformers. 

References 

[1] Federal Energy Regulatory Commission. (July 24, 2025). FERC Approves Grid Reliability Standards Applicable to Inverter-Based Generators. 

[2] North American Electric Reliability Corporation (NERC). (June 12, 2025). 2025 State of Reliability — Overview. 

[3] Lappalainen et al. (2020), Applied Energy 

[4] CAISO Fast Facts PDF 

[5] Wood Mackenzie. (August 14, 2025). Power transformers and distribution transformers will face supply deficits of 30% and 10% in 2025.