Harmonic distortion, commonly measured as Total Harmonic Distortion (THD), is a power quality metric that describes how much an electrical waveform (voltage or current) deviates from a pure sinusoidal shape due to harmonics—frequency components at integer multiples of the fundamental frequency (50 Hz in most of Europe). In EV charging, THD matters because power electronics used in chargers can introduce harmonics, affecting grid compliance, efficiency, and site reliability.
What Is THD?
THD expresses the combined magnitude of harmonic components relative to the fundamental frequency:
– THD (current) indicates distortion in current drawn by a device
– THD (voltage) indicates distortion present on the supply voltage
Higher THD means more waveform distortion, which can increase losses, heating, and interference in electrical equipment.
Why THD Matters in EV Charging
EV chargers (and vehicles’ onboard chargers) rely on rectifiers and switching electronics that can generate harmonic currents. At scale—multiple chargers on one site—this can impact:
– Transformer and cable heating due to higher RMS current
– Nuisance tripping or stress on protective devices
– Reduced efficiency and higher electrical losses
– Interference with sensitive equipment in commercial buildings
– Compliance with grid connection and EMC requirements
For site owners and installers, managing THD becomes more important as charger density and total connected power increase.
How Harmonics Are Created
Harmonics commonly arise from non-linear loads—devices that draw current in pulses rather than smoothly:
– Rectifiers and switching power supplies
– Variable speed drives
– LED lighting drivers
– EV onboard chargers and some EVSE power supplies
In EV charging, harmonic generation depends on charger design, switching strategy, and the vehicle’s onboard charger behavior.
Typical Impacts of High Harmonic Distortion
High THD can lead to operational and maintenance issues:
– Overheating of transformers, neutral conductors, and distribution panels
– Increased losses and reduced capacity in cables and switchgear
– Higher risk of resonance with power factor correction capacitors
– Voltage distortion that can affect charger stability and uptime
– Communication noise and measurement errors in some metering systems
In three-phase systems, certain harmonics can accumulate in the neutral, increasing neutral current beyond expected levels.
THD and EV Charger Compliance
EV chargers are usually designed to meet relevant power quality and electromagnetic compatibility requirements. In deployments, compliance can be affected by:
– Weak grids with higher impedance (more voltage distortion for the same harmonic current)
– Large numbers of chargers operating simultaneously
– Poor upstream power quality or incompatible site equipment
– Incorrect installation practices and cable routing
Power quality assessment during commissioning can identify whether THD is within acceptable limits for the site and utility requirements.
How to Reduce Harmonic Distortion at Charging Sites
Common mitigation measures include:
– Selecting chargers with good input current quality and low harmonic emissions
– Using active power factor correction (PFC) where applicable
– Installing harmonic filters (passive or active) for high-density sites
– Ensuring proper transformer sizing and avoiding overloaded neutrals
– Applying load management to reduce simultaneous peak operation
– Separating sensitive loads and improving electrical layout
Mitigation strategy depends on site scale, grid strength, and local requirements.
Related Glossary Terms
Power Quality
Harmonics
Power Factor Correction (PFC)
EMC (Electromagnetic Compatibility)
Flicker Emission
Neutral Conductor
Transformer Sizing
Load Balancing
Grid Services
Commissioning Documentation