Harmonic THD in Australian power-quality checks
How total harmonic distortion uses fundamental RMS, harmonic RMS rows, dominant harmonic and an entered threshold basis.
What Harmonic THD Means
Harmonic THD means total harmonic distortion. It is a ratio between the combined harmonic RMS magnitude and the fundamental RMS magnitude from the same voltage or current measurement record.
In Australian 230/400 V a.c., 50 Hz sites, THD can appear in main switchboard analyser reports, VSD feeder records, UPS output records, generator output checks and plant-room investigations. Voltage THD and current THD are different records and should stay separate.
Formula Fields
The worksheet combines selected harmonic RMS values using a root-sum-square relationship:
H_rms = sqrt(H2^2 + H3^2 + H5^2 + H7^2 + H11^2 + H13^2 + other harmonic RMS^2).
THD_% = H_rms / H1 x 100.
H1 is the fundamental RMS value from the same analyser record. The 2nd, 3rd, 5th, 7th, 11th and 13th harmonic rows, plus any other harmonic RMS value, must come from the same measurement basis.
| Field | Use it for | Why it matters |
|---|---|---|
| Measurement type | Voltage THD or current THD. | The two records answer different questions. |
| `H1` fundamental RMS | Base value for the percentage. | THD is calculated relative to this value. |
| Harmonic RMS rows | 2nd, 3rd, 5th, 7th, 11th, 13th and other RMS. | Missing rows change the root-sum-square result. |
| Dominant harmonic | Largest entered harmonic row. | Helps focus the measurement review. |
| Threshold basis | Project, DNSP, report or equipment criterion. | The calculator does not create a universal limit. |
Worked THD Example
A switchboard voltage record has H1 = 230 V, with harmonic RMS values of 0 V, 4 V, 6 V, 2 V, 0 V, 0 V and 1 V for the selected rows.
H_rms = sqrt(0^2 + 4^2 + 6^2 + 2^2 + 0^2 + 0^2 + 1^2) = 7.55 V.
THD_% = 7.55 / 230 x 100 = 3.28%.
| Item | Example value | Reading |
|---|---|---|
| Measurement point | Main switchboard analyser record. | THD is tied to a location and time window. |
| Measurement type | Voltage THD. | Do not compare directly with current THD. |
| Dominant harmonic | 5th harmonic at 6 V. | The largest row helps guide review. |
| Result | 3.28% THD. | Compare only with the entered threshold basis. |
Related Power-Quality Terms
THD is not voltage unbalance, phase balancing, motor voltage dip, flicker, interharmonics or a neutral-current review. Those may be related in an investigation, but they use different inputs.
For VSD feeders, UPS outputs, generator outputs or plant-room records, keep the analyser setup, loading condition, threshold basis and equipment data with the THD value.
Next checks
- Use harmonic THD when the fundamental RMS value and harmonic RMS rows come from one analyser record.
- Use the harmonic THD formulas chart when the root-sum-square relationship needs review.
- Use the power-quality measurement table to keep location, loading condition, analyser setup and threshold basis visible.
Boundaries
- This page does not set harmonic limits or diagnose distortion source.
- It does not replace waveform analysis, equipment review or supply investigation.
- DNSP requirements, equipment limits, analyser settings, measurement conditions and qualified investigation remain controlling inputs.