Transformer Harmonic Derating Factor (Thdf) - AEMC 725 User Manual

THDF (Transformer Harmonic Derating Factor)
THDF is accessed through A~. Push the (yellow) Shift button three times
to sequence to THDF.
THDF is a method of calculating transformer derating established by the
Computer and Business Equipment Manufacturers Association
(CBEMA) for phase-to-neutral loads. This derating factor can be applied
to electrical systems with high third harmonic current and high crest fac-
tor, and can be helpful where large portions of the load consist of single-
phase switch mode power supplies.
THDF= (1.414 x RMS current) ÷ Peak current
kVA (derated) = kVA (nameplate rating) x THDF value
This equation will provide a value between 0 and 1.0. This method is
based on the fact that most complex waveforms have a crest factor greater
than 1.414.
THDF Example:
Measurements indicate actual phase currents to be 250 Arms. The Peak
currents indicate values of 400. THDF will indicate a value of .88, indi-
cating that the specific transformer should not be loaded to any more
than 88% of its nameplate kVA rating.
For more detailed information about this derating factor, contact
CBEMA.
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Clamp-On Harmonic Probe Model