A case study on harmonics in electric vehicle (EV) charging stations reveals that the integration of non-linear power electronic converters can lead to significant power quality issues, particularly harmonic resonance and transformer degradation.

A prominent study focused on  dedicated cable system designed to feed EV fast chargers (150–350 kW) at highway rest areas.

  • Problem Identification:
    • EV fast chargers are ac-to-dc converters that generate harmonics.
    • Large capacitance in long underground cables (e.g., 1 km or more) can create harmonic resonance.
  • Findings:
    • Resonance Risk: When the cable length is approximately 1 km, the grid’s resonance frequency can coincide with the charger’s switching frequency (typically 2–9 kHz), causing large harmonic voltages and currents.
    • Suppression: Interestingly, the study found that increasing the number of rest area points (e.g., four points instead of one) can actually suppress this resonance phenomenon.
  • Operational Impact: If unmitigated, resonance can damage equipment like metal-oxide varistors.

Impact on Distribution Infrastructure

Beyond resonance, multiple case studies highlight the cumulative effect of EV charging on traditional residential and commercial grids.

  • Transformer & Cable Stress:
    • Current harmonics cause non-uniform current distribution, leading to additional heating in transformers and cables.
    • Simulation results indicate that 80% non-linear load can accelerate transformer aging by a factor of nearly 3.0.
    • Uncontrolled charging can increase a transformer’s hot spot temperature by up to 16°C, significantly reducing its lifespan.
  • Distortion Levels:
    • Total Current Harmonic Distortion (THDi) typically increases as the charging current decreases.
    • In fleet charging scenarios (e.g., a site with 300+ stations), unbalanced harmonic distortions can exceed individual phase limits, leading to equipment overloads.
  • Harmonic Cancellation: In some multi-vehicle scenarios, different charger types or power set-points can lead to harmonic cancellation, where the combined distortion is lower than the sum of individual parts. 

Mitigation Strategies

To address these issues, several technical solutions are employed:

  • Active Power Filters (APF): Shunt APFs are widely used to detect and cancel out harmonic currents.
  • Smart Charging: Adjusting charging rates based on grid capacity can mitigate rapid fluctuations and lower-order harmonics.
  • High-Quality Design: Using chargers with integrated filters and Power Factor Correction (PFC) circuits significantly reduces emissions. 

Would you like to explore specific mitigation technologies like active power filters or see industrial standards like IEEE 519 in more detail?