Industrial Engineer · phase 13 of 16

THD

Total Harmonic Distortion, a ratio that summarizes harmonic content relative to the fundamental component.

The Explain card

Plain English: Total harmonic distortion is one number that sums up how much harmonic content a waveform carries compared with its fundamental. It is quoted separately for voltage and for current.
Example: A single six pulse drive can draw current with a THD around 80 percent. Adding a line reactor brings it near 35, and an active front end brings it under 5.
Why it matters: Standards like IEEE 519 set limits on current distortion at the point where the plant meets the utility, and voltage distortion limits for what the plant sees. THD is the number the harmonic study reports and the number the mitigation is bought against.
Hook: How far from a sine, in one percentage.

How the word is built, where it came from, and what it sits beside in memory.

In a sentence

During VFD commissioning, the technician records voltage THD and current THD at the drive input under full load.

Letter by letter

Where it came from

Origin: English technical initialism formed from total harmonic distortion
Entered the language: 20th century
What changed: First used in audio and radio testing, then adopted in power-quality analysis and variable frequency drive work

How it is spelled

Capitalisation: Always written THD, including mid-sentence; lowercase thd is nonstandard
Plural and possessive: Plural THDs, singular possessive THD's, and plural possessive THDs'; technical writing often prefers THD values
Out loud: Spelled out letter by letter.

Same family

Broken into chunks

What it sits beside

Same subject

Same shape

harmonic indices

distortion metrics

Phase 13: VFD Power Quality, Wiring, and Motor Protection

Harmonics, reflected waves, shielded cable and bearing currents: the side effects a drive inflicts on everything around it.