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Harmonic Distortion / IEEE 519 Quick Check

Verify whether a UPS install will pass IEEE 519-2022 Table 2 current distortion limits at the point of common coupling. Rectifier topology + PCC short-circuit ratio → verdict.

Report details for the PDF
IEEE 519-2022 verdict
Pass
5.0% TDD vs 12.0% limit
Tick = 12.0 % TDD limit for the 50 ≤ Isc/IL < 100 band
Margin to limit
+7.0%
Headroom available
UPS current I₁ / max demand I_L
72.2 / 144.3 A
TDD = THDi 10% × 0.50
PCC short-circuit current
14.4 kA
10.0 MVA — utility figure
Isc / IL ratio
100.0
50 ≤ Isc/IL < 100 → 12.0% TDD limit

IEEE 519-2022 Table 2 — where this PCC sits

5 %8 %12 %15 %20 %20501001000Isc/IL →this site: 100.0

Log scale. The stronger the service relative to the site's maximum demand, the more distortion it tolerates: a weak PCC below 20 gets 5 % TDD, a stiff one above 1000 gets 20 %.

How IEEE 519 reads the room

The limit isn't a single number — it depends on how strong the PCC is relative to the load. A weak service with Isc/IL < 20 gets only 5% TDD allowance; a strong service above 1000 gets 20%. Stronger services tolerate more distorted loads because the source impedance soaks up less voltage distortion from the same harmonic current.

Important notes

  • • TDD is referenced to maximum demand (IEEE 519 IL = UPS at 100 %), so at 50% loading it reads 0.50× the rectifier THDi. If the UPS will not reach full load, enter the true site maximum demand instead.

Catch the harmonic problem before commissioning.

Power Stack keeps a service history per UPS — tickets, PM visits, notes — so a commissioning-time harmonic check has somewhere to live besides a technician’s phone, and the next engineer on site can find it.

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What IEEE 519 actually requires

IEEE 519 sets limits on harmonic current that a customer can inject into the utility's grid at the point of common coupling. The 2022 revision tightened a few definitions and introduced clearer enforcement language, but the structure is unchanged: pick a row in Table 2 by the short-circuit ratio Isc/IL, then compare your installation's total demand distortion (TDD) against the row's limit. The result is a pass/fail, and non-compliance gives the utility a basis to require filters or refuse interconnection.

UPS rectifiers as the dominant source

For a typical commercial install, the UPS rectifier is the single largest source of harmonic current — VFDs run a close second. A 6-pulse SCR rectifier with no filter is the worst case at ~30% THDi; a passive 5th/7th harmonic filter cuts that to ~10%; a 12-pulse rectifier holds the same ~10% without a filter. IGBT (AFE) rectifiers stay below 5% — usually under 3% in practice. The right architecture for the load is the cheapest way to comply.

Why this is a quick check, not a final report

A formal IEEE 519 study models the full upstream impedance, all non-linear loads on the same bus, and the cumulative effect across harmonic orders. This calculator approximates with a single-load assumption: the UPS dominates the PCC. That holds for small-to-medium commercial installs where the UPS is the largest non-linear load. For data centres with multiple UPS rectifiers, VFDs, or LED drivers contributing, commission a full study.

Strengthening a weak PCC

If the verdict is “Fail” and the rectifier upgrade isn't economical, the alternative is to strengthen the PCC: a larger transformer, a higher-voltage primary, or moving the PCC closer to the utility's strong bus. Each option drops the source impedance and increases Isc/IL, which both directly increases the TDD allowance and indirectly reduces the actual harmonic-current draw.

Questions engineers ask

What is the difference between THDi and TDD?

THDi is the rectifier’s harmonic current divided by its own fundamental current. TDD, which IEEE 519 limits, divides the same harmonic current by the site’s maximum demand current. A UPS running at 50 % of a bus whose maximum demand is the UPS at full load has a TDD of half its THDi.

Which Isc/IL band am I in?

Isc is the short-circuit current at the point of common coupling, from the utility or derived from transformer kVA ÷ Z %, and IL is the maximum demand load current. The ratio picks a row of IEEE 519-2022 Table 2: below 20 the limit is 5 % TDD, above 1,000 it is 20 %.

Is this enough for a compliance submission?

No. It is a single-load screen that assumes the UPS dominates the PCC. A formal study models the upstream impedance, every non-linear load on the bus and each harmonic order. Use this to decide whether a filter or an IGBT rectifier is worth pricing before you commission that study.

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Engineering disclaimer: Power Stack provides this calculator as a general engineering estimate. Final design must be verified by a qualified electrical engineer and reconciled with manufacturer datasheets, the applicable national wiring regulations (NEC, BS 7671, IEC 60364, or your local equivalent), and site-specific conditions. Power Stack accepts no liability for design decisions made from this output.