Harmonic loading is the additional electrical and thermal stress imposed on power system components by harmonic currents from non-linear loads. In EV charging sites, harmonic loading becomes important when many chargers operate simultaneously, as harmonics increase RMS current, create additional losses, and can reduce the usable capacity and lifetime of transformers, cables, and switchgear.
What Is Harmonic Loading?
When current contains harmonics, it is no longer a pure sine wave. Harmonic components contribute to:
– Higher RMS current for the same delivered real power (kW)
– Increased I²R losses (heat) in conductors and equipment
– Additional losses in transformers (eddy currents, stray losses)
– Potential voltage distortion that affects other loads
This “extra burden” on the electrical system is referred to as harmonic loading.
Why Harmonic Loading Matters for EV Charging
EV charging involves power electronics (in EVSE auxiliaries and, especially, vehicle onboard chargers) that can generate harmonic currents. At scale, harmonic loading can:
– Overheat transformers and reduce their available kVA capacity
– Increase heating in cables, busbars, and distribution panels
– Raise neutral currents in three-phase systems (triplen harmonics)
– Increase nuisance tripping and stress protective devices
– Degrade power quality and impact charger stability and uptime
For commercial buildings and fleet depots, harmonic loading is a key factor in designing a reliable electrical infrastructure for multiple charge points.
Common Sources of Harmonic Loading at Charging Sites
Typical contributors include:
– EV onboard chargers drawing non-linear current
– Charger internal power supplies and control electronics
– Other non-linear building loads (VSD drives, UPS systems, LED lighting)
– Mixed-load sites where EV charging is added to an already distorted network
Even if each charger is compliant individually, the combined effect can be significant at high utilization.
How Harmonic Loading Affects Electrical Components
Transformers
Harmonics increase transformer losses and heating, potentially requiring:
– Derating of the transformer’s usable capacity
– Use of K-rated transformers or designs intended for non-linear loads
– Additional cooling or improved thermal margin
Cables and switchgear
Harmonics increase RMS current and heating in:
– Feeders and distribution cables
– Busbars and panelboards
– Protective devices and connection points
Neutral conductors
In three-phase systems, certain harmonics (especially 3rd, 9th, 15th) can add in the neutral, causing:
– Neutral currents higher than expected
– Overheating if neutrals are undersized or shared improperly
How Harmonic Loading Is Assessed
Harmonic loading is typically evaluated through:
– Power quality measurements of THD (current and voltage)
– Harmonic spectrum analysis (which harmonic orders dominate)
– Transformer/cable thermal calculations and derating checks
– Site studies that account for simultaneous charger operation and other loads
This is often part of a broader power quality or connection feasibility assessment.
How to Reduce Harmonic Loading
Mitigation strategies include:
– Selecting chargers and equipment with low harmonic emissions and good input current quality
– Using harmonic filtering (active or passive) for high-density sites
– Separating EV charging feeders from sensitive loads where possible
– Proper transformer sizing and avoiding overloaded neutrals
– Applying load management to reduce simultaneous peak operation
– Improving overall site power quality and grounding practices
The best approach depends on site scale, grid strength, and the existing harmonic environment.
Related Glossary Terms
Harmonic Distortion (THD)
Harmonic Filtering
Harmonics
Power Quality
Neutral Conductor
Transformer Sizing
K-rated Transformer
EMC (Electromagnetic Compatibility)
Load Balancing
Commissioning Documentation