TDD vs THDi is one of the most important distinctions in an IEEE 519 harmonic study. THDi shows how distorted the current waveform is at the moment of measurement, while TDD compares the harmonic current with the system’s maximum demand current. That difference explains why a lightly loaded facility can show high THDi without necessarily failing at the point of common coupling (PCC), and why a low THDi reading alone cannot prove compliance.
Quick answer: use THDi to diagnose equipment and operating conditions; use TDD, the short-circuit ratio and the limits specified for the PCC to evaluate current harmonic compliance. A valid conclusion also requires the measurement location, demand-current basis, operating period and voltage distortion results. The applicable contract, utility requirement and edition of the standard must always be confirmed for the project.
| Metric | Formula | Denominator | Best use |
|---|---|---|---|
| THDi | √(I₂² + I₃² + …) / I₁ × 100% | Present fundamental current I₁ | Diagnosing the waveform at a load or feeder |
| TDD | √(I₂² + I₃² + …) / IL × 100% | Maximum demand load current IL | Evaluating current distortion at the PCC |
The harmonic numerator is similar in both calculations: the root-sum-square of individual harmonic currents, normally excluding the fundamental. The denominator changes the meaning. THDi divides by the fundamental current measured during the same interval. If the load falls, the fundamental becomes small and THDi may rise even when the absolute harmonic amperes have barely changed. TDD uses a demand-current reference, so the result is more stable and better aligned with the effect of the customer load on the shared electrical system.
This is also why a portable meter screenshot that shows “THD-I = 35%” is not enough to approve or reject a site. The value may describe a single variable-frequency drive at partial load, a panel downstream of the real PCC, or a short time window that misses the plant’s highest production state. For a fuller explanation of voltage and current distortion, read THDi vs THDv: What Is the Difference in Power Quality?
Assume a facility records 60 A RMS of harmonic current at the PCC. During a lightly loaded measurement interval, the fundamental current is 150 A. The instantaneous THDi is therefore 60/150 = 40%. The maximum demand load current used for the study is 600 A, so TDD is 60/600 = 10%.
Both results are correct, but they answer different questions. The 40% THDi warns that the present current waveform is highly distorted relative to the small fundamental component. The 10% TDD describes the harmonic current relative to the facility’s demand capacity. Whether 10% meets the project limit cannot be decided from that number alone. The engineer must also determine the ratio of available short-circuit current to maximum demand current at the PCC, identify the voltage class, review individual harmonic orders and check any even-harmonic or interharmonic requirements that apply.
The PCC is the electrical point where the utility and other users could be affected by the customer’s harmonic current. It is not automatically the terminals of a VFD, UPS, rectifier or active harmonic filter. In a large industrial plant, engineers may also set internal limits at transformers and major distribution boards, but those internal limits serve design and troubleshooting goals. The formal compliance location must be agreed with the utility or defined in the project specification.
Moving the instrument downstream can produce a much higher THDi because the measurement captures one nonlinear load before its current is diluted by other linear loads. Moving it upstream may lower the percentage, but it also changes the short-circuit strength and the harmonic current seen by the network. Record the single-line diagram, transformer ratings and impedances, CT ratios, meter configuration and the exact measurement point before comparing results.
IL is not simply the highest current seen during a five-minute site visit, nor should it be replaced automatically by transformer full-load current. It represents the maximum demand load current at the PCC under the definition adopted by the applicable study and standard. In practice, the engineer should review utility demand records, power-quality logger data and production schedules over a representative period. New facilities require an engineering estimate based on realistic simultaneous loading, with assumptions clearly stated and updated after commissioning.
An active harmonic filter (AHF) injects compensating current for selected harmonic components. Its rating should therefore be based on harmonic amperes, spectrum, phase imbalance, load diversity and the required residual current—not by multiplying total load current by a THDi percentage from a single snapshot. For example, a feeder with 800 A fundamental current and 12% harmonic current does not automatically require a 96 A filter. Some harmonic orders may already be acceptable, future loads may increase the requirement, and the AHF may need headroom for imbalance or reactive-current functions.
Follow the process in our Active Harmonic Filter Sizing Guide for VFD Loads: measure before selection, calculate the current that must be cancelled, apply realistic diversity, verify the CT and connection architecture, and define a performance target at the same measurement point. If reactive power is also a concern, compare AHF, SVG and hybrid compensation instead of assuming one device must perform every function.
Start with a one-line diagram and identify the candidate PCC. Collect transformer impedance, utility fault-level information, nonlinear load schedules and existing capacitors or filters. Install a suitable power-quality analyzer with verified CT polarity and scaling. Measure long enough to include the operating patterns that matter, then calculate THDi, TDD, individual harmonic currents, THDv and individual harmonic voltages. Compare the results with the limits required by the project, investigate resonance or capacitor stress, model proposed corrections when needed, and repeat measurements after commissioning.
The IEEE 519 standard page is the authoritative place to confirm the published edition. For international projects, engineers may also need the relevant IEC power-quality and electromagnetic compatibility documents, local grid codes and the utility interconnection agreement. A manufacturer should support the study with application data, but the final compliance boundary and acceptance method belong in the project specification.
Yes. This is common at partial load because THDi uses the present fundamental current as its denominator, while TDD uses maximum demand current. The same harmonic amperes can therefore produce a high THDi and a lower TDD.
No. Compliance requires the correct PCC, demand-current reference, short-circuit ratio, individual harmonic evaluation and voltage distortion review. A low value measured at the wrong point is not sufficient evidence.
Provide the one-line diagram, system voltage and frequency, transformer data, CT location and ratio, fundamental and harmonic currents by order, THDi/TDD trends, load type and duty cycle, existing capacitor banks, expansion plans and the required result at a defined point. Our power quality report guide explains how to organize these measurements.
The useful question is not whether TDD or THDi is the “better” number. Each has a specific job. THDi helps locate and understand nonlinear loads; TDD supports a network-level assessment at the PCC. When the location, denominator and operating window are documented, the data can guide a defensible choice among load-side correction, an AHF, an SVG, a hybrid system or no additional equipment. For a review of your measurements and compensation target, contact Elumotive with the study files and single-line diagram.
