A Three-Part Blog Series by ZTZ Services
The Cost of “Free”
Renewable power sources are attractive because they convert “free” environmental energy into electricity. However, their variable and weather-dependent nature creates new operational realities that utilities must adapt to.
Part 1 of this series discussed the fundamental challenges that inverter-based resources pose to connected substations. Transformers are the workhorses of the national power grid and bear much of the impact from IBR-induced rapid capacity shifts and sustained fault energy.
Why IBRs Stress Transformers
Four primary mechanisms directly affect transformer health in high-IBR environments.
Thermal Cycling. Renewable power output variability creates repeated load swings that cause expansion and contraction of winding insulation. Per IEEE C57.91, even a sustained hotspot temperature rise of just 6 to 8°C can double the insulation aging rate. Transformers operating above design temperatures are aging faster right now, every hour at elevated temperature.
Harmonic Loading. Solar inverters are a major source of unwanted harmonics. In high-density photovoltaic areas, aggregate total harmonic distortion at the feeder level can reach 3 to 5% before mitigation is typically required[1]. Harmonic currents increase eddy current losses in proportion to the square of frequency. Higher-order harmonics therefore generate disproportionately more heat in windings and core structures than conventional thermal models typically account for.
OLTC Wear. Bidirectional power flows from distributed and utility-scale solar have increased on-load tap changer (OLTC) operations by 2 to 5 times in high-DER areas. A typical OLTC is designed for a finite number of mechanical operations. Accelerated operation leads to faster contact erosion and oil contamination. A 2025 review in Electrical Engineering (Springer) highlighted this as an emerging diagnostic challenge with rising renewable penetration[2].
Correlated Weather Stress. Because renewable output is weather-driven, regional stress events are no longer statistically independent. A heat dome can simultaneously spike air-conditioning load, suppress wind generation, and reduce transformer cooling capability, compounding thermal stress exactly when generation margins are tightest.
Good Data Feeds Correct Analysis
Renewable variability imposes several hidden stresses on the grid that go beyond simple load swings. Understanding these mechanisms and the identifiable signatures they leave in monitoring data is critical for properly maintaining transformers, provided the data is collected and analyzed with the right framework.
Part 3, the final article in this series, reveals three underexplored connections between IBR operating conditions and transformer monitoring data, and explains why analytical frameworks must keep pace.
References
[1] IEEE Std 519-2022; Afor Energy. (March 10, 2026). Understanding Solar Inverter THD Requirements: A Complete Guide. https://www.aforenergy.com/understanding-solar-inverter-thd-requirements-a-complete-guide/
[2] Springer Nature. (November 26, 2025). OLTC condition assessment and future trends: an overview. Electrical Engineering, Vol. 108.
