Total Demand Distortion Limit Check (IEEE 519-2022)
The distortion limit that loosens as the supply gets stiffer, the number a utility actually holds you to at the PCC: IEEE 519 limits total demand distortion (harmonic current over the maximum DEMAND load), not THD or a flat 5%. ratio = Isc/IL sets the limit: < 20 -> 5%, 20-50 -> 8%, 50-100 -> 12%, 100-1000 -> 15%, > 1000 -> 20%. Isc 10,000 A / IL 400 A is a ratio of 25 -> 8% limit, so a measured 6% TDD PASSES; on a soft service (Isc 6,000 A -> ratio 15) the limit tightens to 5% and the identical 6% FAILS. The distortion did not change; the supply stiffness did. Even/individual sub-limits also apply. A screening aid; the utility agreement and a measurement study govern.
Formula and source
ratio = Isc / IL; limit = ratio < 20 ? 5.0 : ratio < 50 ? 8.0 : ratio < 100 ? 12.0 : ratio <= 1000 ? 15.0 : 20.0 (%); pass = measured_TDD <= limit.
The IEEE 519-2022 Table 1 current-distortion limits (total demand distortion at the point of common coupling), by name; the utility agreement and a measurement study govern.
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