What harmonics are

Utility voltage is a 60 Hz sine wave. Loads such as variable-frequency drives, DC drives, UPS systems, battery chargers and LED drivers don't draw a smooth sine-wave current. Their current can be described as the 60 Hz fundamental plus components at whole multiples of it, the harmonics. On a 60 Hz system the 5th harmonic is 300 Hz, the 7th is 420 Hz and the 11th is 660 Hz.

Three-phase 6-pulse rectifiers, the front end of most standard drives, mainly produce the 5th, 7th, 11th and 13th.
Single-phase electronic loads produce strong 3rd harmonics, which add up in the neutral of a 4-wire system rather than cancelling.

Harmonic current flows back through the supply's impedance and creates harmonic voltage distortion, which every other load on the system then sees.

THD vs TDD: the difference matters

THD (total harmonic distortion) compares harmonic content with the fundamental at that moment. A lightly loaded drive can show a very high current THD while drawing very little harmonic current in amps.
TDD (total demand distortion) compares harmonic current with the facility's maximum demand load current. IEEE 519 uses TDD for current limits precisely so that light-load readings don't look alarming. I_L is based on the average of the monthly maximum-demand currents over the preceding 12 months (with provisions for shorter histories and new installations), not the current measured during one survey.

Voltage distortion is normally stated as THD (THDv).

What IEEE 519 actually says

IEEE Std 519-2022, Standard for Harmonic Control in Electric Power Systems, establishes voltage and current distortion limits at the point of common coupling (PCC) for installations containing nonlinear loads. It does not establish equipment-terminal limits for an individual drive.

Where it applies: at the PCC, the point closest to the customer where the system owner could supply another customer. With a dedicated industrial service transformer it is frequently on the transformer's high-voltage side; with a shared transformer it is commonly on the low-voltage side. Confirm the PCC with the utility before choosing where to measure. A single drive "exceeding IEEE 519" is a common misreading.
Voltage limits depend on the bus voltage. For systems of 1 kV and below, the recommended limits are 5% for any individual harmonic and 8% THD. Higher-voltage systems have tighter limits.
Current limits include individual harmonic limits and TDD, both expressed relative to the maximum demand load current I_L. The table is selected by PCC voltage and the ratio I_SC/I_L (available short-circuit current over maximum demand current). For 120 V through 69 kV, the base TDD limits range from 5% to 20%; higher-voltage tables differ. In the 2022 edition, the 2nd, 4th and 6th harmonic limits are 50% of the corresponding table limits, and other even harmonics use the table limits.
How compliance is judged: statistically. Voltage uses weekly 95th-percentile 10-minute values against the table limits and daily 99th-percentile 3-second values against 1.5 times those limits. Current uses weekly 95th-percentile 10-minute values against the table limits, weekly 99th-percentile values against 1.5 times them, and daily 99th-percentile 3-second values against twice them. Measurements must include harmonics through at least the 50th order.
Generation: installations exporting power through inverter-based resources may fall under IEEE 1547 or IEEE 2800; IEEE 519-2022 explains how to decide applicability for mixed load and generation sites.

To check against IEEE 519 you therefore need measurements at the agreed PCC, your 12-month maximum demand current, and the utility's available short-circuit current, which the utility can provide.

What harmonics damage

Transformers run hotter, because harmonic currents add eddy-current and stray losses. K-factor rated or derated transformers are designed for this.
Neutral conductors overheat when triplen (3rd, 9th…) currents from single-phase electronics add up in the neutral.
Capacitor banks can form a parallel resonance with the supply inductance near a harmonic the plant produces. Harmonic current and voltage are then amplified, which blows capacitor fuses, fails capacitors and trips drives. This is why capacitor banks in plants with drives are often detuned, with the design checked against the actual harmonic spectrum (see power factor correction).
Breakers and relays can trip or misoperate, and some meters read incorrectly on distorted waveforms.
Sensitive equipment can malfunction if voltage distortion is high.

Mitigation options

Option	How it helps	Notes
Line reactor or DC link choke on drives	Smooths the drive's input current and substantially reduces its harmonic current	The simplest option, and often the first step. The effect depends on the drive and the supply.
Passive (tuned) filter	Gives specific harmonics (usually the 5th and 7th) a low-impedance path, and supplies some kVAR	Needs careful design against resonance, and is best for steady loads.
Multi-pulse drive (12- or 18-pulse)	Phase-shifting transformer windings cancel the 5th and 7th	Used on large drives.
Active front end (AFE) or low-harmonic drive	A controlled rectifier draws near-sinusoidal current	Higher drive cost, little external equipment.
Active harmonic filter	Measures harmonic current and injects the opposite	Adapts to changing loads and can also correct power factor. Higher cost.
Phase-shifting transformers across several drives	Cancels harmonics between groups of loads	Effective where many similar drives run together.
Detuned capacitor bank	Corrects displacement power factor while reducing the risk of harmonic amplification when properly selected	Selection must account for the harmonic spectrum and network impedance; not a guarantee against amplification, and not a harmonic filter in itself.

Which option is right depends on where the harmonics come from, the load profile, and whether the goal is equipment protection, power-factor correction, or meeting a utility limit at the PCC. That is why a measurement survey usually comes first.

Common signs it's worth measuring

Capacitor fuses blowing or capacitors failing early.
Transformers or neutrals running hot at normal load.
Drives tripping on overvoltage when capacitor steps switch.
A planned addition of large drives on an existing service.
A utility asking about harmonics after a complaint from a neighboring customer.

Working with us

If you suspect harmonics or resonance, send us what you have: bills, drive lists, any past measurements. We'll tell you what they show, and what has to be measured to know more. If filtering is needed, we design and build harmonic filter panels in our UL 508A shop. From the bill to the fix →

Harmonics and IEEE 519, explained