THD vs TDD: Harmonic Limits at the Point of Common Coupling

Harmonic reports often go wrong before anyone reaches the limits table. A technician records 28% current THD at a lightly loaded panel, calls it a failure, and recommends a filter. Another report shows 4% voltage THD at a machine input and concludes that the utility is responsible. Neither conclusion is defensible until the measurement point, load condition, calculation reference, and applicable requirement are clear.

This guide explains the practical difference between THD and TDD, why the point of common coupling matters, and how to build a harmonic survey that an engineer can actually use. It is written for plant electricians, commissioning teams, consultants, and buyers of power quality analyzers. It does not reproduce proprietary standards tables or turn a diagnostic measurement into a compliance certificate.

Start with the question the survey must answer

"Measure the harmonics" is not a complete job instruction. Harmonic measurements may be used to investigate overheated neutrals, transformer loading, nuisance trips, capacitor failures, distorted voltage, drive operation, or a contractual requirement at the facility boundary. Each question points to a different measurement location and a different set of quantities.

• If electronic loads are overheating a neutral, record phase and neutral current together with the individual harmonic spectrum.
• If several variable-frequency drives appear to distort a bus, record current, voltage, power, operating state, and selected harmonic orders through a production cycle.
• If a project asks for IEEE 519 evaluation, identify the user point of common coupling and the current-demand basis before comparing any number with a limit.
• If equipment is malfunctioning, measure at the equipment input as well as upstream. A local reading can locate a problem without being the point where a system-level limit applies.

Write the decision at the top of the job sheet. A good example is: "Determine whether the fifth and seventh harmonic currents from the drive group are associated with voltage distortion on the main 480 V bus during maximum production." That sentence tells the technician what to record and tells the reviewer what the results can support.

THD is a ratio to the fundamental

Total harmonic distortion expresses the combined RMS contribution of harmonic components relative to the fundamental component. For voltage, voltage THD is the RMS combination of the harmonic voltages divided by the fundamental voltage. Current THD uses the harmonic currents divided by the fundamental current at the time of measurement.

The denominator matters. A current waveform can produce a high current-THD percentage when the fundamental load current is small, even though the actual harmonic amperes are modest. When the same equipment carries normal load, the fundamental current rises and the THD percentage can fall. The equipment has not necessarily become cleaner; the reference has changed.

That is why a single current-THD screenshot taken during idle production is weak evidence. Record the fundamental current, total RMS current, individual harmonic currents, load state, and time. If the analyzer displays only percentages, export enough data to recover the operating context.

TDD uses a demand-current reference

Total demand distortion was introduced to make current-distortion assessment less dependent on the instantaneous load at the moment of the reading. Instead of dividing harmonic current by the current fundamental measured at that instant, TDD uses a defined demand-current reference. In an IEEE 519 project, that reference and the observation period must follow the edition and project method being applied.

TDD therefore answers a different question from current THD. Current THD describes the present waveform relative to its present fundamental. TDD relates harmonic current to a demand basis intended for system planning and evaluation at the PCC. The two values can be very different at light load.

The PCC is an electrical boundary, not simply the main panel

IEEE 519 describes the interface between sources and loads as the point of common coupling. The applicable PCC for a real facility must be established from the one-line diagram, service arrangement, transformer ownership, other connected users, and the project or utility agreement. It should not be selected merely because one panel is easy to access.

A plant may have useful diagnostic measurement points downstream of the PCC. Those points help locate which load group produces a harmonic current or which branch experiences distorted voltage. They do not automatically inherit the system-level limits applied at the PCC. Label every dataset with the exact location and intended use: compliance point, upstream reference, or diagnostic point.

If there is doubt, have the facility engineer and utility or authority agree on the PCC before the survey. A week of excellent data from the wrong boundary can still fail to answer the contractual question.

Voltage distortion and current distortion tell different parts of the story

Nonlinear loads draw nonsinusoidal current. As those harmonic currents flow through the system impedance, they contribute to harmonic voltage drops. The voltage waveform observed at a bus therefore depends on both the current spectrum and the source and distribution impedance. A facility may have substantial current distortion without severe voltage distortion when the source is stiff. The same loads on a weaker source can produce a different voltage result.

Do not treat voltage THD and current THD as interchangeable scores. Measure both, and retain the individual orders. Fifth and seventh harmonics may suggest one family of loads, while triplen harmonics in a four-wire system raise different neutral-current questions. The spectrum, phase relationship, operating sequence, and network arrangement are more informative than a total alone.

Do not copy a familiar 5% number into every report

Harmonic requirements depend on the applicable document, edition, voltage level, system strength, current-demand relationship, measurement point, aggregation method, and observation period. Equipment emission standards, utility interconnection rules, industrial compatibility levels, and IEEE 519 system goals do not all answer the same question.

The current IEEE standards page describes IEEE 519-2022 as setting steady-state voltage and current distortion goals at the user PCC for facilities containing harmonic-producing loads. A revision project is active, so a report should identify the exact edition named by the contract rather than saying only "per IEEE 519." IEC 61000-4-7 addresses harmonic and interharmonic measurement instrumentation and grouping; it is not a substitute for deciding which system limit applies.

Before field work, create a requirement sheet with the document title and edition, PCC, voltage level, short-circuit information if required, demand-current definition, measurement duration, aggregation method, exclusions, and responsible reviewer. If any item is missing, mark the survey as diagnostic until the owner supplies it.

A harmonic analyzer must preserve context, not just calculate THD

For industrial surveys, a useful instrument should record synchronized voltage and current on the required phases, show individual harmonic orders, retain timestamps, and trend power and load current alongside distortion. Current sensors must fit the conductors and maintain suitable accuracy across the actual current range. A sensor selected only for maximum bus current may provide poor information on a lightly loaded feeder.

The EK-F500 Power Quality Analyzer is specified with four voltage and four current channels, true-RMS measurements, phase harmonics through the 50th order, harmonic histograms, voltage and current THD, power, energy, power factor, unbalance, waveform capture, and long-term recording. These functions support diagnostic harmonic surveys. They do not, by themselves, prove that the instrument meets a particular measurement class or that a dataset complies with IEEE 519.

The EK-F523A Touch Screen Power Quality Analyzer is presented with three voltage channels, four current channels, 600 V measurement, waveform and electrical-parameter recording, and an 8 GB TF card. Its public specification is brief. Before using it for a contractual harmonic study, request the current manual and confirm harmonic order range, aggregation, sensor options, uncertainty, export format, safety rating, and any declared conformity.

Use the power quality analyzer buying checklist to confirm channels, sensors, recording time, event modes, software, and documentation before purchase.

Build the survey around operating states

Harmonics change as converters, drives, UPS systems, welders, chargers, furnaces, and capacitor banks change state. A survey should cover the operating cycle that matters, not an arbitrary hour. Interview operators before installation. Ask when the largest lines start, which products create the heaviest load, when capacitor stages switch, and whether failures occur during shifts, weekends, or generator operation.

1. Review the one-line diagram and identify the PCC and downstream diagnostic points.
2. Record nominal voltage, wiring arrangement, transformer data, major nonlinear loads, and expected current range.
3. Select voltage leads and current sensors for the actual site. Document model, range, ratio, direction, and phase assignment.
4. Connect under the site's approved electrical-safety procedure. Verify wiring mode and phase mapping with the phasor display before recording.
5. Trend voltage, current, active power, power factor, voltage THD, current THD, and the dominant individual orders.
6. Record production state, drive status, capacitor switching, generator operation, alarms, and maintenance activity.
7. Run long enough to capture representative minimum and maximum demand and the condition linked to the complaint.
8. Back up raw files before changing thresholds, ranges, or names in analysis software.

Check phase mapping before trusting power and TDD

A reversed current sensor or a voltage lead paired with the wrong phase can produce negative kW, implausible power factor, or misleading phase totals. The distortion spectrum may still look believable, which makes the error easy to miss. Before leaving the site, verify phase sequence, sensor arrows, channel labels, voltage-current pairing, and expected direction of real power.

If the analyzer displays negative active power on a load that should be importing energy, stop and resolve the wiring or configuration before using the recording. Our field guide on negative kW, CT polarity and phase mapping provides a step-by-step diagnosis.

Turn raw data into an engineering record

A useful report can be reviewed months later by someone who was not on site. It should identify the facility, measurement point, one-line reference, instrument and firmware, current sensors, wiring configuration, recording interval, clock basis, survey period, operating states, and applicable requirement. State whether results are diagnostic or intended for formal evaluation.

Show a load profile before presenting distortion. Then show voltage THD, current THD, TDD if correctly established, and the dominant individual orders on a common timeline. Include representative spectra at low and high load. Explain what changed when a load switched. Do not hide inconvenient intervals or average a short event into a smooth trend.

The conclusion should separate observation from interpretation. "Fifth harmonic current rose from 18 A to 46 A when Drive Group B started" is an observation. "Drive Group B is the sole cause of PCC voltage distortion" is a stronger claim that may require upstream comparison, impedance information, or controlled switching evidence.

Mitigation starts after the source and operating condition are known

Filters are not the first step. First verify the measurement, locate the dominant sources, review transformer and conductor loading, check capacitor-bank interactions, and understand how the process operates. A passive filter designed for one system can interact with system impedance or future capacitor changes. An active filter must be sized from the harmonic current and operating envelope it will actually see.

Possible actions include changing drive configuration, using line reactors or suitable chokes, separating sensitive loads, reviewing transformer or neutral capacity, detuning capacitor banks, applying passive or active filtering, and correcting system resonance. The engineering choice depends on the spectrum, load variability, source impedance, equipment requirements, and project economics. Measure again after the change using the same points and operating states.

Common reporting errors

• Calling current THD and TDD the same quantity.
• Comparing a light-load current-THD screenshot with a TDD limit.
• Using equipment nameplate current as the demand reference without approval.
• Applying a PCC limit to every downstream branch panel.
• Reporting only a total and omitting individual harmonic orders.
• Ignoring current-sensor range, orientation, conductor position, or saturation.
• Combining data from different operating states without documenting the changes.
• Claiming compliance from an analyzer feature list rather than a defined method and declared instrument capability.
• Recommending a filter before confirming the source and system conditions.

Frequently asked questions

Can current THD be high when harmonic current is low?

Yes. At light load, the fundamental-current denominator becomes small, so current THD can rise even when the harmonic amperes are not large. Record actual current, load state, and individual harmonics rather than judging the percentage alone.

Should TDD be measured at every panel?

TDD is principally used for the defined system-evaluation point and demand basis. Downstream panels are valuable diagnostic locations, but do not assume the same limits apply. Label the measurement purpose and confirm the PCC.

Does a 50th-harmonic display prove IEEE 519 compliance?

No. Harmonic order capability is only one part of the measurement chain. The PCC, demand reference, instrument method, aggregation, duration, current sensors, and applicable edition also matter.

How long should a harmonic survey run?

Long enough to include the operating states relevant to the complaint or requirement. A continuously loaded process may need a shorter representative period than a plant with weekly production changes. Define the operating cycle before choosing the duration.

References and next step

Use the current project copy of the applicable standard. The official references are IEEE 519, Harmonic Control in Electric Power Systems and IEC 61000-4-7, harmonic and interharmonic measurement instrumentation. For a complete field sequence, see the power quality survey and report workflow.

If you are selecting an analyzer for a harmonic study, send EK Instruments the one-line diagram, nominal voltage, wiring arrangement, expected current range, conductor dimensions, suspected loads, survey duration, required data export, and the exact standard edition. That information is more useful than a request for a generic "IEEE 519 meter."