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Technology / Engineering guide
Wind Power Harmonics & THD Measurement Records
A THD percentage describes a defined comparison of harmonic content. It is incomplete without the measured quantity, denominator and analysis scope. Keep voltage and current results separate, and retain their spectrum and operating conditions.
Ask which waveform and denominator the percentage uses.
The same percentage at different operating points need not represent the same absolute harmonic magnitude.
01 / Quantity identity
Separate voltage distortion from current distortion.
Voltage THD and current THD describe different waveforms at an identified point.
Voltage THD and current THD describe different waveforms at an identified point. A low value for one does not establish a low value for the other. State the phase or path represented instead of using an unqualified label such as system THD.
Also retain whether a result concerns generator-side conditions, converter output or the site connection. These locations can contain different contributions. A site measurement is not automatically attributable entirely to the turbine.
02 / THD definition
Keep the harmonic set and reference together.
A common THD-F definition uses the square root of the sum of squared included harmonic RMS magnitudes, divided by the fundamental RMS magnitude.
A common THD-F definition uses the square root of the sum of squared included harmonic RMS magnitudes, divided by the fundamental RMS magnitude. The harmonic orders included in that sum must be stated.
HIOKI distinguishes fundamental-referenced THD-F from RMS-referenced THD-R. The labels refer to different denominators. Retain the instrument’s actual definition rather than assuming that every percentage titled THD uses the same calculation.
03 / Spectrum context
Preserve absolute magnitudes alongside percentages.
A combined percentage hides how distortion is distributed across orders.
A combined percentage hides how distortion is distributed across orders. Retain the relevant harmonic spectrum, its units and whether values are absolute or relative. Different spectra can produce the same THD number.
In a hypothetical comparison, one included harmonic of 1 A is 10% of a 10 A fundamental but 50% of a 2 A fundamental. This illustrates a changing denominator, not a MARSTON measurement or a compliance threshold.
04 / Operating point
Keep generation, load and background conditions.
Identify the output level, connected load and other sources represented by the result.
Identify the output level, connected load and other sources represented by the result. Current distortion expressed relative to a small fundamental can look different from a result taken at a higher output, even where absolute harmonic components are similar.
Retain any relevant background record and the assessment’s attribution method. A change after adding equipment may warrant review, but it does not by itself identify the sole source of every harmonic.
05 / Analysis scope
Identify what the measurement can resolve.
Retain the instrument, measurement-chain identity, included order range, reporting period and applicable aggregation method.
Retain the instrument, measurement-chain identity, included order range, reporting period and applicable aggregation method. A missing order in a plot may be outside the analysis range rather than an observed zero.
Averaged THD, a maximum and a snapshot are different statistics. Keep those labels visible. The time-record guide helps identify the periods represented without prescribing measurement operations.
06 / Evidence decision
Compare against the actual project requirement.
Harmonic review uses requirements applicable to the specific connection, equipment and assessment boundary.
Harmonic review uses requirements applicable to the specific connection, equipment and assessment boundary. This guide supplies no universal limit, standard pass value or product harmonic rating.
Use the power-quality overview to connect the result to the complete assessment. No MARSTON harmonic compliance, filter provision or correction capability is established here. Keep an actual report and configuration with any claim.
Continue the review with Active, Reactive & Apparent Power in Wind Projects.
Continue the review with Wind Turbine EMC: Emissions, Immunity & Assembly Scope.
Continue the review with Wind Power Meter CT Ratios & Data Scaling.
General background: HIOKI: THD-F and THD-R denominator distinctions. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Prepare the information for this review.
Use the checklist to organise the project information. Mark unavailable items and retain document references.
| Review item | Information to provide | Purpose |
|---|---|---|
| Waveform | Voltage or current, phase/path and location | Identify the measured quantity. |
| Definition | THD-F/THD-R, denominator and included orders | Interpret the percentage. |
| Spectrum | Absolute and relative harmonic magnitudes | Retain detail hidden by the total. |
| Conditions | Output, load, other sources and background context | Compare applicable operating points. |
| Report scope | Instrument, period, statistic and requirement | Keep a claim within its evidence. |
Engineering questions.
Is voltage THD the same as current THD?
No. They describe different waveforms.
Can two spectra share one THD value?
Yes. A total percentage does not uniquely identify its harmonic components.
Why can current THD rise at lower current?
The fundamental denominator may decrease; review absolute magnitudes as well.
Are THD-F and THD-R identical?
No. Their reference denominators differ.
Does a site THD value prove the turbine caused it?
No. Background and other equipment contributions need assessment.
Does this page provide a MARSTON THD limit or pass result?
No. Applicable limits and configuration-specific evidence must come from the actual assessment.
Technical enquiry
Bring your project information to the discussion.
A THD percentage describes a defined comparison of harmonic content. Share the available information and the equipment scope you want reviewed in a technical quotation.