Total harmonic distortion measures how far a voltage or current waveform departs from a clean sine wave. Non-linear loads such as variable frequency drives and switch-mode power supplies inject harmonics that overheat equipment and disturb the grid. This tool computes THD from the individual harmonic magnitudes and checks it against the IEEE 519 limits.
How it works
THD referenced to the fundamental (THD-F) is the root-sum-square of every harmonic divided by the fundamental:
THD = sqrt(H2² + H3² + H4² + ... + Hn²) / H1 × 100%If you enter the harmonics as percent of the fundamental, H1 is one hundred percent and the math is direct. If you enter absolute amps or volts, the tool divides the harmonic root-sum-square by the fundamental you supply. Either way the result is the same percentage.
Where harmonics come from
Harmonics enter an electrical system through non-linear loads — devices that draw current in pulses rather than smoothly following the voltage waveform. Common sources:
- Variable frequency drives (VFDs): 6-pulse drives produce a characteristic harmonic spectrum with large 5th and 7th harmonics.
- Switch-mode power supplies: computer equipment, LED drivers, and UPS systems draw current in short bursts near the voltage peaks.
- Arc furnaces and welding equipment: generate a broad, less predictable harmonic spectrum including inter-harmonics.
- Fluorescent and LED lighting with cheap drivers: produces significant 3rd harmonic current, which adds in the neutral conductor of three-phase systems.
IEEE 519-2022 voltage limits
IEEE 519 sets limits on voltage THD at the point of common coupling (PCC) — the point where the utility supply meets the customer’s system.
| Bus voltage | Individual harmonic limit | Voltage THD limit |
|---|---|---|
| 1 kV and below | 5.0% | 8.0% |
| 1 kV to 69 kV | 3.0% | 5.0% |
| 69 kV to 161 kV | 1.5% | 2.5% |
| Above 161 kV | 1.0% | 1.5% |
These limits tighten with voltage class because high-voltage transmission systems supply a much larger population of customers and distortion propagates further.
Current distortion and TDD
Current distortion is governed separately under IEEE 519 as Total Demand Distortion (TDD) — referenced to the maximum demand load current rather than the instantaneous fundamental. Allowed TDD also depends on the short-circuit ratio (ISC/IL) at the PCC: the higher the available fault current relative to load current, the more harmonic current is permitted. Evaluating current TDD compliance requires the system’s short-circuit data and is not automated in this tool.
Mitigation approaches
When voltage THD exceeds the limit at the PCC, or when measured current distortion is causing equipment problems, common mitigation strategies include:
- Passive harmonic filters — tuned LC filters that absorb specific harmonic orders (typically 5th, 7th, 11th, 13th)
- Active harmonic filters — inject equal and opposite harmonic currents in real time; effective across a broad spectrum
- 12-pulse or 18-pulse drives — use multiple phase-shifted rectifiers to cancel lower-order harmonics at the source
- Line reactors — add impedance ahead of a VFD to spread the current pulse and reduce peak harmonic injection; a practical first step
- Isolation transformers with delta-wye winding — trap triplen harmonics (3rd, 9th, 15th) in the delta winding, protecting upstream systems