Transformer Aging Under Harmonic Emissions from Electric Vehicle Smart Charging

The rise of electric vehicles (EVs) presents challenges for distribution transformers, mainly due to increased risk of overloading. Smart charging has emerged as a key solution to reduce grid stress and transformer strain. However, while its impact on reducing transformer loading is well-documented, the adverse effects of harmonic emissions remain understudied. This research explores the link between smart charging and harmonic emissions and their joint impact on transformer aging. A 160 kVA distribution transformer is evaluated under full-load conditions, which are achieved by varying the number of simultaneously charging EVs at fixed current levels ranging from 6A to 15A. Aging effects are assessed based on the IEEE C57.91 and IEEE C57.110 standards, considering both top-oil and hot-spot temperatures under harmonic-distorted loading. Results reveal that lower charging currents—despite allowing more EVs to connect—lead to higher harmonic losses, elevated hot-spot temperatures, and accelerated insulation aging. These findings highlight the trade-offs in smart charging and suggest strategies for improving transformer longevity.