THD and TDD become useful engineering records only when the electrical location, current reference, and operating state are retained with the value. A local VFD input, a process feeder, and a common coupling boundary are not interchangeable measurement points, even when a meter displays the same type of percentage.
Start with the guides sur la qualité de l'énergie for broader planning context. This article explains how to keep a THD/TDD comparison reviewable; it does not establish a plant limit, equipment size, or project result.
Réponse directe
Compare THD and TDD only after naming the decision boundary and retaining the reference current used for the calculation. The number gains meaning from where it was measured, what was running, and which reference basis belongs to the report.
Partie 1. Commencer par la frontière de décision
Before collecting values, write down the engineering question. A team may be characterizing one drive, comparing a production feeder, or preparing data for a supply-side assessment. Each question calls for a different electrical location and a different set of coincident loads.
Put the meter point, voltage reference, and CT orientation on the one-line diagram. Retain the CT ratio, capture interval, time base, and the responsible reviewer. A later reader should be able to identify whether the exported record represents a local input or a shared feeder.
Important : Do not treat a local measurement as proof of a wider boundary. The assessment location, operating condition, and acceptance method must be agreed together (Aperçu de la norme IEEE 519).
| Decision to support | Suitable record boundary | Record that must travel with it |
|---|---|---|
| Diagnose one drive group | Defined drive input or local feeder | Drive identity, operating state, CT and voltage reference |
| Compare a process area | Départ de distribution de processus | Running loads and feeder one-line position |
| Prepare a system discussion | Defined supply-side or common boundary | Project boundary, time window, and review owner |
Part 2. Keep the denominator with the number
THD and TDD are not labels to detach from the report. The comparison should state the current reference basis used by the meter or calculation and preserve the associated configuration. Without that context, a percentage can look more or less severe simply because the production state changed.
For practical review, retain a short definition note beside every export. Do not copy a value from a screen into a quotation request without its denominator, location, time interval, and load state. Those omissions make it difficult to decide whether two records are comparable.
| Review question | Information to retain | Pourquoi c'est important |
|---|---|---|
| What index is shown? | Meter label and current-reference setting | Avoids comparing differently based values as though they are identical. |
| Which interval produced it? | Start time, duration, and aggregation | A transient and a representative operating interval are not the same record. |
| What was energized? | Process, VFD, capacitor, and other-load state | The feeder composition may have changed. |
| What is the decision? | Monitoring, troubleshooting, or RFQ purpose | Keeps a percentage from becoming an unsupported equipment instruction. |
The existing guide to décisions relatives au facteur de puissance et à la distorsion harmonique is useful when the project team is deciding which electrical objective is actually under review.
Part 3. Place meters for the question being asked
Meter placement is not a paperwork detail. The placement defines the loads included in the current record and the voltage source used for the comparison. A drive terminal can help answer a local troubleshooting question, while a feeder record can include several drive groups and other coincident loads.
Draw a simple boundary diagram before the record begins. Mark the incoming source, outgoing circuits, CT locations, voltage taps, and any bypass or parallel path that can change the included load. If the plant has multiple operating modes, make a separate entry for each mode instead of averaging unlike conditions into one anonymous value.
La Aperçu de la norme NEMA MG 1 provides motor-system context, but it does not define the correct plant measurement boundary or select mitigation equipment for a specific installation.
Part 4. Log states that make readings comparable
An electrical trend without an operating log is hard to interpret. Name the process state, production line, bypass condition, starts and stops, and major coincident loads. A comparable data set is usually smaller and better labeled than a large folder of undated screenshots.
| Operating-state item | Log beside the electrical trend | Examiner l'utilisation |
|---|---|---|
| Production state | Product run, speed range, and shift | Identifies whether records describe similar work. |
| Nonlinear loads | Active drive groups, rectifiers, and UPS loads | Shows which sources may be included. |
| Switching changes | Capacitor, bypass, transfer, or maintenance state | Explains changes that may not be steady-state behavior. |
| Supply condition | Known source or feeder configuration change | Helps keep supply-side events separate from a local theory. |
For an RFQ-ready baseline, use metering before an RFQ as a companion checklist. It is a planning aid, not a compliance certificate.
Part 5. Separate index review from equipment selection
Treat the index review as a scope-definition step rather than a device instruction. Before discussing an AHF/APF option, define the harmonic-current objective, connection point, CT arrangement, representative load record, protection coordination, installation limits, and acceptance method.
La Filtre Actif Harmonique AHF page is a product-line starting point for a defined harmonic-current scope. It is not a promise that a particular percentage, plant limit, or operational outcome will be achieved.
This fit boundary is important. A voltage event, a reactive-power-only objective, a protection concern, or a record without a defined electrical boundary may require a different investigation. The IEEE 519 assessment context should be reviewed with project-specific responsibilities rather than used as a generic device-size shortcut.
Part 6. Send a reviewable measurement package
A clear inquiry lets a supplier or engineering team understand what the numbers represent. Send a compact record set rather than only a single percentage from a screen.
| L'acheteur devrait fournir | Pourquoi c'est important |
|---|---|
| Schéma unifilaire et caractéristiques nominales du système | Establishes the electrical boundary and supply context. |
| Empositions du compteur, du transformateur de courant et de la référence de tension | Lets reviewers locate the actual measurement point. |
| THD/TDD exports with stated current reference | Preserves the calculation basis. |
| Operating-state log and load inventory | Connects trends to the loads that were running. |
| Existing mitigation and protection details | Identifie les contraintes d'intégration. |
| Space, cooling, cable, communication, and acceptance requirements | Tests whether a defined solution scope is feasible. |
CNBYG can discuss product-line fit when the harmonic-current objective and plant boundary are defined. Use the measurement package to begin that discussion; a general article cannot replace a plant study or guarantee an electrical result.
Foire aux questions
Is THD the same as TDD?
No. Both labels need the reported current reference, measurement location, and operating condition beside the value. Do not compare percentages from different bases as though they describe the same record.
Where should plant current distortion be measured?
Measure at the electrical location that matches the decision being made, and show that location on the one-line diagram. A drive input and a common feeder are different boundaries.
Can a VFD input reading represent a plant feeder?
Not automatically. A VFD input record describes that local location, while a feeder can include several connected loads and supply arrangements.
Why record the operating state?
The load mix may change by shift, product, speed, switching state, or maintenance condition. The operating log makes two electrical records comparable.
Does IEEE 519 select a device size?
No. It supplies harmonic-control context, while a project still needs its own boundary, measured evidence, integration scope, and acceptance criteria.
What belongs in a measurement-led RFQ?
Send the one-line diagram, nominal data, meter and CT positions, stated current reference, representative exports, operating log, load inventory, existing equipment, and installation constraints.
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