Harmonics in Industrial Systems: Understanding Total Harmonic Distortion (THD)

Variable frequency drives, UPS systems, rectifiers, switch-mode power supplies — the loads that make modern industrial operations efficient are also the primary generators of harmonic distortion.

What Are Harmonics?

Harmonics are voltage or current components at frequencies that are integer multiples of the fundamental frequency (60 Hz in North America). The 3rd harmonic is at 180 Hz, the 5th at 300 Hz, the 7th at 420 Hz, and so on. Total Harmonic Distortion (THD) is the ratio of the RMS value of all harmonic components to the RMS value of the fundamental, expressed as a percentage. A THD of 5% or below is generally acceptable; values above 10–15% indicate a problem requiring attention.

Sources of Harmonics

• Variable Frequency Drives (VFDs): primarily produce 5th and 7th harmonics
• UPS systems: front-end rectifiers produce characteristic harmonic spectra
• Switch-mode power supplies: present in all modern electronics
• Arc furnaces and induction heaters: generate both characteristic and non-characteristic harmonics
• Non-compliant LED/fluorescent drivers: contribute 3rd harmonic on neutral conductors

Effects on Equipment

Harmonics cause: transformer overheating via increased eddy current losses; capacitor bank resonance that can cause catastrophic failure; neutral conductor overloading (triplen harmonics sum rather than cancel); motor heating from negative-sequence rotating fields; and metering errors that affect billing accuracy.

IEEE 519-2022 Compliance

IEEE 519 establishes harmonic limits at the Point of Common Coupling (PCC). Current harmonic limits depend on the Isc/IL ratio at the PCC. Voltage THD limits are 5% total with 3% individual harmonic limits for systems below 1 kV. Compliance must be demonstrated through measurement, not assumption.

Mitigation Options

• Passive harmonic filters: tuned LC circuits for specific harmonic orders
• Active harmonic filters: inject equal and opposite harmonic currents
• 18-pulse or 24-pulse drive configurations: reduce 5th and 7th harmonic generation
• Line reactors on VFDs: reduce harmonic injection by 30–50%
• K-rated transformers: designed for harmonic loading environments

Frequently Asked Questions

Does my facility need a harmonic study?

Any facility where VFDs, UPS systems or large rectifiers account for more than 20% of total load should conduct a harmonic assessment. Transformer overheating, capacitor bank failures and neutral overcurrent are strong indicators of an existing problem.

Contact Argoe Group to discuss your power quality assessment needs. Our AEE-certified engineers are ready to help.