AUWiring

Harmonic current TDD calculator

Calculate total demand distortion from entered harmonic current magnitudes and demand current for Australian power-quality records.

  • Calculator
  • Power quality
  • Australia
Use a switchboard, feeder, analyser or PQ report reference.
A
Enter the demand-current basis used for this TDD record.
A
Enter RMS current for the third harmonic from the same record.
A
Enter RMS current for the fifth harmonic from the same record.
A
Enter RMS current for the seventh harmonic from the same record.
A
Enter RMS current for the eleventh harmonic from the same record.
A
Enter RMS current for the thirteenth harmonic from the same record.
A
Enter combined RMS current for other included harmonics if the record provides one.
%
Enter the project, report, equipment or engineering review threshold.
Name the source of the review threshold before interpreting the TDD value.
I_h_rms = sqrt(I3^2 + I5^2 + I7^2 + I11^2 + I13^2 + I_other^2); TDD_% = I_h_rms / I_demand x 100
  • Harmonic current values must be RMS values from one current record.
  • The demand-current basis must be entered by the user and kept visible.
  • The entered threshold is a project, equipment, report or engineering review value, not a universal hidden limit.
Formula variables
VariableMeaningUnitUse
I_demandDemand currentAUser-entered demand-current basis for the TDD record.
I3Third harmonic currentAEntered RMS current for the third harmonic.
I5Fifth harmonic currentAEntered RMS current for the fifth harmonic.
I7Seventh harmonic currentAEntered RMS current for the seventh harmonic.
I11Eleventh harmonic currentAEntered RMS current for the eleventh harmonic.
I13Thirteenth harmonic currentAEntered RMS current for the thirteenth harmonic.
I_otherOther harmonic RMS currentACombined RMS current for included harmonics not shown separately.
I_h_rmsHarmonic current RMSARoot-sum-square of the entered harmonic current magnitudes.
TDD_%Total demand distortion%Harmonic current RMS divided by demand current.
More

Harmonic current TDD calculator technical guide

Calculate total demand distortion from entered harmonic current magnitudes and demand current for Australian power-quality records.

Use this calculator when a harmonic current report provides RMS current values and a demand-current basis. It turns those rows into harmonic current RMS and total demand distortion while keeping the threshold source visible.

TDD is a current-record worksheet. It is adjacent to THD, neutral current and power-quality measurement records, but it should not be collapsed into those pages. The demand-current basis is the key difference.

Demand-Current Basis

TDD record fields
FieldWhy it mattersReview note
Demand currentDenominator for the TDD percentage.Must belong to the same review basis.
Harmonic current rowsNumerator through root-sum-square arithmetic.Use RMS current values, not percentages.
Other harmonic RMSCarries included rows not listed separately.Use only when the report provides a combined value.
Threshold basisExplains the comparison value.Keep project, equipment or report source visible.

The same harmonic current set can give a different TDD percentage when the demand-current basis changes. That is why this page is separate from the existing THD calculator.

Result Interpretation

TDD outputs
OutputMeaningPractical reading
Harmonic current RMSCombined RMS current from entered harmonic rows.Shows the magnitude being compared with demand current.
TDD percentageHarmonic current RMS divided by demand current.Main worksheet result.
Dominant harmonicLargest entered harmonic current row.Points to the row driving the simple record.
Threshold basisSource of the entered review value.Prevents a hidden-limit impression.

If the value is above the entered threshold, review the demand-current basis and report source before using the result. If it is below the entered threshold, keep it as a record only; another loading state may produce a different value.

Boundary With Related Calculators

Which page owns the question
QuestionCorrect ownerInput basis
What TDD follows from current harmonics and demand current?Harmonic current TDD calculatorHarmonic current RMS rows and demand current.
What THD follows from harmonic magnitudes and a fundamental?Harmonic THD calculatorFundamental RMS magnitude and harmonic RMS values.
What neutral current follows from phase currents?Neutral current calculatorEntered phase currents for linear-load arithmetic.
What fields belong in a PQ record?Power-quality measurement record tableInstrument, location and measurement context.

Worked Records

Harmonic current TDD examples
SituationInputsResult patternInterpretation
Switchboard current record200 A demand, selected harmonic current rows17.03 A harmonic RMS, 8.51% TDDSlightly above the entered review threshold.
Higher demand-current basisSame harmonic rows, 320 A demand5.32% TDDDemand-current basis changes the percentage.
Zero harmonic entry checkPositive demand current, zero harmonic rows0% TDD with record noteConfirm zeros are measured values, not missing rows.

Stop Points

  • The harmonic rows are percentages rather than RMS current values.
  • The demand-current basis is missing or belongs to another record.
  • Voltage harmonics are being entered into a current TDD worksheet.
  • The threshold has no stated project, equipment, report or engineering basis.
  • The result is being used as a source diagnosis without load and measurement review.

Switchboard current harmonic record

A power-quality report gives demand current and selected harmonic current magnitudes for one switchboard record.

Reference
PQ-TDD-01
Demand current
200 A
5th harmonic current
12 A
7th harmonic current
9 A
  1. Harmonic current RMS17.03 A
  2. TDD8.51%
  3. Dominant harmonic5th
Total demand distortion8.51%

17.03 A harmonic RMS divided by demand current.

The TDD value is just above the entered review threshold, so the demand-current basis should be checked.

  • Harmonic currents are RMS values.
  • Demand current is entered by the user.
  • The threshold source remains visible.

Higher demand-current basis

The same harmonic current set is compared with a larger demand-current basis.

Reference
PQ-TDD-DEMAND
Demand current
320 A
5th harmonic current
12 A
7th harmonic current
9 A
  1. Harmonic current RMS17.03 A
  2. TDD5.32%
  3. Dominant harmonic5th
Total demand distortion5.32%

17.03 A harmonic RMS divided by demand current.

The same harmonic RMS current produces a lower TDD percentage when the demand-current basis is higher.

  • The harmonic record is unchanged.
  • Demand-current basis is documented.
  • No network threshold is embedded.

Zero harmonic entry check

A record with no harmonic current magnitudes should stay traceable instead of silently implying a complete report.

Reference
PQ-TDD-ZERO
Demand current
180 A
5th harmonic current
0 A
7th harmonic current
0 A
  1. Harmonic current RMS0 A
  2. TDD0%
  3. Dominant harmonicNone
Total demand distortion0%

0 A harmonic RMS divided by demand current.

The calculator returns zero TDD but asks the user to confirm that zeros are measured values rather than missing data.

  • Zero entries come from the record.
  • Demand current is positive.
  • Measurement completeness remains external.

Questions

Is TDD the same as THD?

No. TDD uses demand current as the denominator. THD uses a fundamental magnitude.

Can I enter voltage harmonics here?

No. This page is for current harmonic RMS values only. Use harmonic THD for voltage or current THD records.

What if my report lists percentages?

Convert the report rows to matching RMS current values before using this worksheet.

Does the calculator set a TDD threshold?

No. The threshold is entered by the user from project, equipment, report or engineering criteria.