TDD — Total Demand Distortion (IEEE 519)
Converts total harmonic current distortion into total demand distortion, TDD = THD_I × I₁ ÷ I_L, where I₁ is the fundamental current at the moment of measurement and I_L the installation's maximum demand current. The difference matters a lot: THD has the instantaneous fundamental in its denominator, so a drive running at 20% load can read 60% THD while injecting negligible harmonic current into the grid. IEEE 519 sets its limits in TDD for exactly this reason, referencing everything to maximum demand — typically 5% TDD for installations with an I_sc/I_L ratio below 20, which is the criterion the utility enforces. Enter the measured current THD, the fundamental current at measurement and the maximum demand current.
Result
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TDD vs THD: why IEEE 519 measures against peak demand
The utility letter arrives claiming the plant injects harmonics above the allowance, or commissioning a bank of inverters calls for a power quality report. The analyser went on the point of common coupling, logged a week and returns current THD every ten minutes. IEEE 519, though, sets no limit whatsoever on current THD: every figure in its tables reads as TDD. Skip the conversion and the report fails an innocent installation — a lightly loaded rectifier logs enormous THD precisely because the fundamental in its denominator went small.
The conversion reads TDD = THD_I × I₁ ÷ I_L. THD_I is measured distortion as a percentage of the fundamental; I₁ is the fundamental current at that instant; I_L is maximum demand current at the measuring point, usually the average of monthly maximum demands over the previous twelve months. Both quantities start from the same harmonic sum: THD divides by the momentary fundamental, TDD divides by I_L, which stays fixed. With the defaults — 12% THD, 180 A fundamental, 400 A demand — the result reads 5.400%. Table 2 of IEEE 519 for systems at or below 69 kV allows 5% when the I_sc/I_L ratio falls under 20, rising to 8%, 12%, 15% and 20% as the supply gets stiffer.
First trap: the THD convention. Analysers report either as a percentage of the fundamental or of total RMS, and IEEE 519 wants the former; at 12% the gap stays under 0.1 point and hardly matters, yet at 50% it grows to 5 points and flips the verdict. Second trap: the measuring point. I_L means maximum demand at the common coupling, never the nameplate current of the drive making the harmonics. Measuring THD at a drive terminal and dividing by whole-plant demand yields a flattering, worthless TDD. Finally, the total limit covers half the requirement — each harmonic order carries its own cap.
Frequently asked questions
The defaults give a TDD of 5.400%. Does that fail the standard?
How do I find the maximum demand current I_L?
Will the calculator accept any combination of values?
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