Definitions
| Fundamental | The signal's main frequency (e.g., 60 Hz) |
|---|---|
| Harmonic (n-th) | Integer multiple: 120, 180, 240, … Hz |
| Sub-harmonic | Below fundamental (e.g., 30 Hz) — rare |
| Interharmonic | Non-integer multiple |
| THD | √(Σ V_h² for h ≥ 2) / V_1 — as ratio or % |
| Triplens | 3rd, 9th, 15th … — especially bad in 3-phase (add in neutral) |
Typical THD in audio
| System | THD |
|---|---|
| Studio mic preamp | < 0.001% |
| Good home amplifier | < 0.01% |
| Consumer amp | < 0.1% |
| Guitar tube amp (clean) | 1–5% |
| Guitar tube amp (overdrive) | 10–50% |
| Dynamic microphone | < 1% (normal SPL) |
Power quality limits
| Standard | Application | THD limit |
|---|---|---|
| IEEE 519 (< 1 kV) | Utility bus — individual harmonic | 5% |
| IEEE 519 (< 1 kV) | Utility bus — total | 8% |
| EN 50160 | Supply voltage (95% week) | 8% |
| IEC 61000-3-2 Class A | Professional equipment | Per-harmonic mA limits |
| IEC 61000-3-2 Class D | PC, TV (< 600 W) | Per-harmonic proportional to P |
Mitigation
- Passive filters: LC-tuned traps at specific harmonics.
- Active filters: inject anti-harmonic currents.
- Line reactors: series inductance reduces harmonics from rectifier loads.
- 12-pulse or multi-pulse rectifiers: cancel low-order harmonics.
- Active PFC: forces input current to follow voltage — reduces harmonics at the source.
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