Harmonic THD calculator

Field use cases

Harmonic distortion work usually starts with a measurement record, not with a design conclusion. A technician may have a power-quality analyser report for a main switchboard. An electrician may be checking a VSD feeder where current harmonics are materially higher than the voltage distortion seen upstream. An engineer may need a quick arithmetic check before deciding whether the record belongs with equipment data, DNSP discussion, filtering review or further logging.

This calculator is built for that first record. It takes a fundamental RMS voltage or current magnitude and the entered harmonic RMS magnitudes, then calculates the root-sum-square harmonic magnitude and THD percentage. It keeps voltage and current records separate because they tell different stories. It also keeps the review threshold visible so the result does not look like a hidden site rule.

The useful result is not only the percentage. The record should show whether the input is voltage or current, which harmonic magnitude is largest, what the fundamental value was, and where the review threshold came from. A THD value with no threshold basis is weaker than the same value tied to equipment data, DNSP/report criteria or engineering instruction.

Measurement basis

The calculator expects RMS magnitudes. If a report lists harmonic percentages instead of magnitudes, convert those rows to matching voltage or current magnitudes before entering them. Do not mix voltage and current rows. Do not combine measurements from different boards or different logging windows into one calculation.

Australian low-voltage context commonly appears around 230/400 V a.c. and 50 Hz, but the THD formula itself uses the measured fundamental magnitude entered by the user. That is important when a report uses a logged fundamental value rather than a nominal value.

Result interpretation

The formula squares each entered harmonic magnitude, sums the squares, takes the square root, and divides by the fundamental magnitude. This is why one large harmonic row can dominate the result even when several smaller rows are present.

If the THD value is above the entered threshold, review the record and its source rather than treating the value as an automatic cause finding. The follow-up path may include power-quality logging, equipment manufacturer data, VSD or rectifier loading, neutral-current review, filter review, transformer loading, voltage distortion at the point of supply, or DNSP/project requirements.

If the THD value is within the entered threshold, it is still only a record for the entered measurement basis. It does not prove that other operating states, other feeders or other time windows have the same distortion level.

Review path

Start by naming the record boundary. A current THD value from a VSD feeder has a different meaning from a voltage THD value at the main switchboard. Then confirm whether the harmonic rows are RMS magnitudes or percentages. Finally, record the threshold basis before using the comparison result.

Sequence components, interharmonics and flicker are related power-quality topics, but they are not calculated here. They need suitable analyser data and a measurement method that records those quantities directly.

Boundary with related calculators

Harmonic THD often appears beside other power-quality issues, but those issues should not be collapsed into one page.

A site can show high current THD on one feeder while the voltage THD at the main switchboard remains lower. A site can also have voltage unbalance or voltage dip concerns that are not harmonic distortion questions. Treat the calculations as adjacent evidence, not interchangeable labels.

Worked field example

A switchboard voltage record uses a 230 V fundamental. The entered harmonic magnitudes are 4 V for the 3rd harmonic, 6 V for the 5th harmonic, 2 V for the 7th harmonic and 1 V for other included harmonics. The entered threshold is 5% from the project review basis.

The record is within the entered threshold, so the practical outcome is that the row can be kept as a project THD record. It does not remove the need to review equipment data, logging duration, instrument settings or site conditions where those details matter.

Stop points

• The fundamental and harmonic rows are not from the same analyser record.
• Voltage and current harmonic rows are being mixed.
• Harmonic percentages are entered as RMS magnitudes.
• Values from different operating states are treated as one record.
• The threshold has no stated project, equipment, DNSP/report or engineering basis.
• The result is being used to identify harmonic source without load and measurement review.
• The concern is actually voltage unbalance, phase-current imbalance, motor-start voltage dip, flicker, interharmonics or neutral-current heating.

When one of those stop points appears, the calculator still has a useful role. It shows where the arithmetic ends and where the power-quality review needs better records.