Modern non-linear loads, LED drivers, VFDs, UPS, computer power supplies, draw current in pulses, not sinusoids. The result is harmonic distortion that overheats transformers, trips capacitor banks, and trips SEC penalties. IEEE 519 and IEC 61000-3-2 set the limits. Here's how to meet them.
Four ideas to carry into the next specification conversation you have.
Square root of sum of squares of harmonic currents (or voltages), divided by fundamental. Reported as a percentage. < 5% V-THD = clean grid.
Two metrics. ITHD describes what your equipment injects. VTHD describes what the grid presents. Both matter, to different stakeholders.
Point of Common Coupling = the grid interface. VTHD < 5% for general use, < 3% for hospitals and labs. ITHD limits scale with short-circuit ratio.
Each kills a different problem. Get the right tool. A capacitor bank without detuning + LED loads = resonance and explosion.
Per IEEE 519-2022 and IEC 61000-3-2 / 61000-3-4.
| Limit | Threshold | Where enforced |
|---|---|---|
| VTHD general user (PCC) | < 5% | IEEE 519 |
| VTHD sensitive load (hospital, lab) | < 3% | IEEE 519 |
| ITHD low Isc (residential) | < 5% | IEEE 519 |
| ITHD high Isc (industrial) | < 15% | IEEE 519 |
| Individual harmonic limit (h<11) | < 4% | IEEE 519 |
| LED driver THD (full load) | < 10% | IEC 61000-3-2 Class C |
| VFD input THD (no filter) | 30-80% | Untreated |
| VFD with passive filter | < 10% | Mitigated |
Five symptoms and their root causes.
Triplen harmonics (3rd, 9th, 15th) from LED drivers and computer loads add, they don't cancel like the fundamental does. The neutral can carry 1.7× phase current. Spec a 200% rated neutral for any LED-heavy or IT-heavy installation.
K-factor describes how much extra heat a transformer suffers from harmonics. Standard transformer = K-1. Modern IT/LED building needs K-13 or K-20. Using K-1 on a non-linear load = 30-40% derate.
Capacitors look like a short circuit to high-frequency harmonics. If the system resonance frequency happens to match the 5th or 7th harmonic, current amplifies 5–10×. Solution: detuned reactor in series with the bank, tuning to 189 Hz (below the 5th).
Symptom of high VTHD. Even SEC-supplied voltage can be polluted if neighbouring industrial customers are heavily harmonic. Active Harmonic Filters (AHF) at your incomer inject counter-harmonics that cancel both your loads' contribution and the grid's.
Lab equipment, medical monitors, precision balances all assume sinusoidal mains. > 3% VTHD causes measurement drift, lock-out, and false alarms. Hospital ICUs and pharma cleanrooms now spec dedicated UPS-fed sub-mains with < 1% VTHD.
We measure with Class-A analysers (Fluke 435-II, Hioki PW3198), mitigate with passive filters, AHFs and detuned banks, and certify to IEEE 519 at the PCC.
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