How THD and TDD are calculated and judged
Current THD is the root sum of squares of the harmonic currents divided by the fundamental current. TDD uses the same numerator but divides by the maximum demand load current at the point of common coupling. Drives and UPS units draw a small fundamental at light load, so their THD can read 40% or more while the actual burden on the utility stays small, which is exactly why IEEE 519 judges compliance on TDD.
IEEE 519-2022 sets the current limit from the ratio of short-circuit current to maximum demand load current at the PCC: under 20 gives 5.0% TDD, 20 to 50 gives 8.0%, 50 to 100 gives 12.0%, 100 to 1,000 gives 15.0% and above 1,000 gives 20.0%. Voltage distortion at the PCC is limited to 5.0% THD with 3.0% on any single harmonic for systems at or below 69 kV. Individual harmonic current limits also apply by order, so check the full tables in the standard before signing off on a design.
Third harmonic is zero sequence and adds arithmetically in the neutral of a 3-phase 4-wire system, reaching up to three times the phase value and often forcing an oversized neutral. K-factor weights each harmonic by the square of its order and points to a K-4, K-13 or K-20 rated transformer or a capacity derate. Have a licensed power quality engineer review measured data before committing to filters or utility agreements.
Frequently Asked Questions
THD divides by the instantaneous fundamental current, TDD by the maximum demand current. At light load the fundamental shrinks and THD alone inflates, so TDD is the fair basis for judging effect on the system.
A 3-5% line reactor or DC link choke on a drive cuts 5th and 7th sharply, while 12-pulse rectifiers and active front ends go much further. Power factor capacitors can resonate with system inductance, so evaluate them only together with a detuning reactor.