The increasing penetration of electric vehicles (EVs) in low-voltage distribution networks has introduced new challenges related to harmonic emissions, particularly under AC charging. These emissions often result in harmonic unbalance across the three phases, primarily due to the heterogeneous integration of single-phase and three-phase EVs, along with diverse charger configurations operating within the same network. Additionally, recent measurement-based studies have demonstrated that the harmonic distortion of on-board chargers increases when the charging current is reduced from its rated level.

In this study, a computational framework is proposed to evaluate the Individual Harmonic Unbalance (IHU) and Aggregated Harmonic Unbalance (AHU) indices under three representative charging scenarios, categorised as small, medium, and large deployments, considering both constant-rate and smart (variable-rate) charging profiles. Real-world charging and harmonic data from three EV models are utilised: the Nissan Leaf e+ (single-phase), and the Peugeot e-2008 and Renault Zoe R90 (three-phase).

The IHU and AHU metrics are computed using an extended Fortescue transformation, wherein the balanced component is redefined as the characteristic harmonic of a given sequence, positive sequence for harmonics of the form 3n+1 and negative sequence for harmonics of the form 3n+2, while the unbalanced components capture the deviations from this characteristic behaviour, comprising the remaining negative- and zero-sequence harmonics. In alignment with existing power quality standards, the allowable thresholds for both IHU and AHU are set at 100%.

Simulation results indicate that these limits are exceeded for several harmonic orders, notably the 3rd, 9th, and 21st. Furthermore, in scenarios involving smart charging with dynamic load variations, the 95th percentile of IHU and AHU values consistently surpasses the prescribed threshold across nearly all harmonic orders. To the best of the authors’ knowledge, this work constitutes the first systematic assessment of harmonic unbalance stemming from EV charging, and provides critical insights for distribution system operators and network planners in managing harmonic impacts in future power systems.