When to use: Use this simulator to estimate current distortion produced by a VFD or other nonlinear load and check it against IEEE 519 current distortion limits. THD (%) = √(ΣIₕ²) / I₁ × 100 is distortion relative to the fundamental; TDD (%) is the same RMS harmonic sum normalized to the maximum demand load current (IL)at the point of common coupling — the metric IEEE 519's Table 1 limits actually specify. Select a preset harmonic profile or edit each harmonic order directly.
IEEE 519 Table 1 sets a looser TDD limitat higher short-circuit ratios because a "stiff" system (high Isc relative to the load) has low source impedance — the same harmonic current produces much less harmonic voltage distortion elsewhere on the system. A "weak" system (low Isc/I_L) must hold nonlinear loads to a tighter current-distortion limit to protect neighboring customers sharing the same point of common coupling (PCC).
Harmonic distortion is the deviation of a voltage or current waveform from a pure 60 Hz sine wave, caused by nonlinear loads — VFDs, rectifiers, UPS systems, and switch-mode power supplies — that draw current in pulses rather than smoothly. In VFD-heavy commercial and industrial distribution systems, uncontrolled harmonics degrade power quality, overheat transformers and neutral conductors, and cause nuisance tripping of protective devices. This tool is built for electrical engineers, power quality specialists, and PE Power exam candidates who need a working feel for IEEE 519 THD/TDD calculations and compliance limits.
A harmonic is a sinusoidal component of a distorted waveform at an integer multiple of the fundamental frequency (60 Hz in North America). The 5th harmonic is 300 Hz, the 7th is 420 Hz, and so on. Linear loads (resistive heaters, incandescent lamps, standard motors running near rated load) draw current that follows the applied voltage waveform shape and produce essentially no harmonics. Nonlinear loads — VFDs, DC drives, rectifiers, electronic ballasts, and switch-mode power supplies — draw current in short pulses timed to the conduction of internal semiconductor switches, and that pulsed current waveform, decomposed by Fourier analysis, contains a fundamental component plus a series of harmonic components.
The number of rectifier pulses per cycle determines which harmonics dominate. A standard 6-pulse VFD front end (the most common industrial drive topology) produces characteristic harmonics at orders h = 6k ± 1 — the 5th, 7th, 11th, 13th, 17th, 19th, and so on — with magnitude generally decreasing as order increases. A 12-pulse converter, which combines two 6-pulse bridges through a phase-shifting transformer, cancels the 5th and 7th harmonics through destructive interference, leaving the 11th and 13th as the dominant residual distortion. This is why higher pulse-number equipment is a standard harmonic mitigation strategy for large drives.
THD (Total Harmonic Distortion) and TDD (Total Demand Distortion) are frequently confused, but they are normalized against different denominators and are not interchangeable. THD (%) = √(ΣIₕ²) / I₁ × 100 expresses the RMS sum of all harmonic magnitudes as a percentage of the fundamental component (I₁) — typically the instantaneous or rated fundamental value at the moment of measurement. THD is the metric most commonly used for voltage distortion (%THDv) and in casual power-quality discussion.
TDD (%) = √(ΣIₕ²) / I_L × 100 uses the identical RMS harmonic sum in the numerator, but divides by I_L — the maximum demand load current at the point of common coupling, averaged over a defined demand period (typically 15 or 30 minutes), measured at rated/nameplate load. IEEE 519's current distortion limits (Table 1) are specified in TDD, not THD, precisely because TDD is stable against the load's instantaneous operating point: a lightly loaded nonlinear load can show a very high THD (small I₁ in the denominator inflates the percentage) while contributing a modest, well-behaved TDD relative to the system's actual maximum demand. Using THD where TDD is specified overstates distortion at light load and understates it at full load — always confirm which metric a stated percentage refers to before comparing it against an IEEE 519 limit.
IEEE 519 Table 1 (current distortion limits for general distribution systems, 120 V–69 kV) bands its maximum allowable TDD by the short-circuit ratio Isc/I_L — the ratio of the available short-circuit current at the point of common coupling (PCC) to the maximum demand load current of the customer being evaluated. Representative bands follow this general structure: Isc/I_L < 20 → 5.0% TDD limit (individual harmonics further limited); 20–50 → 8.0%; 50–100 → 12.0%; 100–1000 → 15.0%; and ≥ 1000 → 20.0%.
The logic is straightforward: Isc/I_L is effectively an inverse measure of source impedance relative to load size. A high ratio means a "stiff" system — the utility source is large relative to the customer's load, so source impedance is low, and a given harmonic current produces very little harmonic voltage drop across that impedance. A low ratio means a "weak" connection — a large nonlinear load relative to a comparatively small available fault current, where the same harmonic current produces much larger voltage distortion that propagates to every other customer sharing that PCC. IEEE 519 therefore holds weaker connections to tighter current limits to protect shared infrastructure, while allowing more headroom to nonlinear loads connected to genuinely strong points on the system.
When a calculated TDD exceeds the applicable IEEE 519 limit, several established mitigation approaches are available, generally in increasing order of cost and effectiveness. Passive harmonic filters (tuned LC filter banks, typically tuned near the dominant 5th harmonic) provide a low-impedance path that shunts specific harmonic currents away from the source — inexpensive but limited to the harmonics they are tuned for and sensitive to system frequency variation. Active harmonic filters (AHFs) use power electronics to inject a real-time canceling current across the full harmonic spectrum, adapting automatically to changing load conditions — more expensive but far more flexible and effective across broadband distortion.
Higher pulse-number drives (12-pulse, 18-pulse, or multi-pulse VFD front ends) cancel their own lowest and most troublesome harmonics through phase-shifted rectifier bridges before they ever reach the distribution system, as discussed above. Isolation transformers with appropriate winding configurations (delta-wye) block triplen (3rd, 9th, 15th) harmonic currents from passing between windings. Line reactors (input chokes) add series impedance ahead of a VFD, which smooths the pulsed input current waveform and meaningfully reduces THD/TDD at low cost, and are frequently the first and most cost-effective mitigation step specified for individual drives. Combining moderate mitigation at the load (line reactors, 12-pulse fronts) with system-level correction (filters at the service entrance) is standard practice on VFD-heavy industrial and commercial feeders.
THD is a measure of how much a voltage or current waveform deviates from a pure sine wave, expressed as the RMS sum of all harmonic components divided by the fundamental (60 Hz) component, times 100. THD (%) = √(ΣIₕ²) / I₁ × 100. It applies to both voltage (%THDv) and current, and quantifies the same underlying distortion phenomenon, but the denominator (the fundamental, which varies with instantaneous loading) makes THD less stable as a compliance metric than TDD.
THD normalizes the RMS harmonic sum against the fundamental current or voltage at the moment of measurement. TDD (Total Demand Distortion) normalizes the identical RMS harmonic sum against the maximum demand load current (I_L) — a fixed, averaged reference measured at rated/full load over a defined demand interval. IEEE 519's current distortion limits are specified in TDD specifically because it stays meaningful regardless of how lightly or heavily loaded the equipment happens to be at the moment of measurement; a lightly loaded VFD can show a deceptively high THD while its TDD (relative to full-load demand) remains modest and compliant.
A VFD converts incoming AC to DC through a rectifier bridge (typically a 6-pulse diode or SCR bridge), then re-synthesizes variable-frequency AC for the motor through an inverter stage. The rectifier draws current only during short conduction intervals near each AC voltage peak rather than continuously, producing a pulsed, non-sinusoidal input current waveform. Fourier decomposition of that pulsed waveform reveals harmonic content at characteristic orders determined by the rectifier's pulse number — h = 6k ± 1 (5th, 7th, 11th, 13th...) for a standard 6-pulse front end.
Exceeding IEEE 519 TDD limits does not carry an automatic regulatory penalty by itself — IEEE 519 is a recommended practice, not a mandatory code, though many utilities incorporate it into interconnection agreements and tariffs with contractual enforcement. Practically, excessive harmonic distortion causes real equipment problems: transformer overheating and accelerated insulation aging (harmonic eddy-current losses scale with the square of harmonic order), neutral conductor overload from triplen harmonics that add arithmetically rather than canceling, capacitor bank overheating and premature failure, nuisance tripping of protective devices misreading distorted waveforms, and voltage distortion that propagates to and disturbs neighboring customers sharing the same PCC.
Common mitigation strategies, roughly in order of typical first application: add a line reactor (input choke) ahead of the VFD to smooth the pulsed input current at low cost; specify a 12-pulse or higher pulse-number drive front end that cancels its own low-order harmonics through phase-shifted rectifier bridges; install a passive tuned harmonic filter targeted at the dominant harmonic (commonly the 5th) for a specific problem load; or install an active harmonic filter at the service entrance for broadband, load-adaptive correction across an entire feeder serving many nonlinear loads. The right choice depends on whether the problem is a single large drive or an aggregate of many smaller nonlinear loads on a shared feeder.
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