Total Harmonic Distortion (THD) is a power quality metric that measures how much a voltage or current waveform deviates from a pure sine wave due to harmonics (multiples of the fundamental frequency, e.g., 50 Hz in Europe). In EV charging, THD is used to assess how chargers and other power electronics affect the electrical network, helping prevent overheating, nuisance tripping, metering errors, and reduced equipment life.
What Is THD?
THD quantifies the magnitude of harmonic components compared to the fundamental component.
– Voltage THD (THD-V) indicates distortion in the supply voltage at a point in the network
– Current THD (THD-I) indicates distortion in the current drawn by a device (such as an EV charger)
In simple terms:
– Higher THD means “dirtier” power with more harmonic content
– Lower THD means a waveform closer to an ideal sine wave
Why THD Matters in EV Charging Infrastructure
EV chargers use power electronics (onboard chargers in vehicles, and control/aux systems in EVSE) that can introduce harmonics into the grid. When many chargers operate on the same feeder—especially in workplaces, depots, and apartment complexes—harmonics can accumulate and create site-level issues.
THD management supports:
– Compliance with grid code and utility interconnection requirements
– Stable operation of protective devices (RCDs, breakers) and reduced nuisance trips
– Reliable metering and billing accuracy (especially where fiscal metering applies)
– Reduced heating in cables, transformers, and neutral conductors
– Better performance of sensitive equipment on the same electrical installation
How THD Is Measured
THD is measured using a power analyzer or power quality meter at the relevant point:
– At the charger input (to evaluate the charger’s impact)
– At the Point of Common Coupling (PCC) (to evaluate the whole site’s impact on the grid)
– On individual phases and the neutral (important for three-phase sites)
Measurements typically include:
– THD-V (%) and THD-I (%)
– Individual harmonic levels (3rd, 5th, 7th, etc.)
– Frequency spectrum and time trends (peak periods vs off-peak)
Common Harmonic Issues in Charging Sites
Typical problems linked to high harmonic levels include:
– Neutral overheating due to triplen harmonics (3rd, 9th, 15th) summing in the neutral
– Transformer and cable heating (higher losses)
– Interference with monitoring or communication equipment
– Protective device misoperation (especially in complex multi-load installations)
– Voltage distortion that can affect multiple tenants or building systems
Managing THD in EV Charging Deployments
Practical mitigation approaches depend on the site and the severity:
– Specify chargers with good input current shaping (often supported by power factor correction (PFC))
– Balance loads across phases to reduce phase-specific distortion
– Increase short-circuit strength at the PCC (where feasible) through proper connection design
– Install passive harmonic filters or active filters if required by the grid impact study
– Separate sensitive loads from charging circuits using proper distribution design
– Monitor power quality continuously in large sites (depots, hubs, multi-tenant buildings)
THD vs Power Factor
THD and power factor are related but not the same:
– Power factor indicates how effectively current is converted into useful power
– THD indicates how much the waveform is distorted by harmonics
A site can have acceptable power factor while still having problematic harmonic distortion, especially under partial load or mixed equipment conditions.
Limitations and What to Watch For
– THD numbers vary with load: distortion may worsen at low or high utilization depending on equipment design
– Compliance targets differ by region and utility (site-level requirements may be stricter than device-level specs)
– THD is only part of power quality—also consider flicker, voltage dips, phase imbalance, and transients
– Measure at the right location: charger THD-I can look fine while PCC THD-V becomes problematic due to weak grid impedance
Related Glossary Terms
Harmonic
Power quality
Power analyzer
Power factor
Power factor correction (PFC)
Passive harmonic filters
Neutral conductor
Phase imbalance
Point of Common Coupling (PCC)
Grid impact study