THD vs TDD: How to Verify Harmonic Performance After AHF Installation
To verify an active Harmonic filter, measure harmonic performance at the agreed location under representative operating conditions. THDi shows current distortion relative to the fundamental current at the time of measurement; TDD uses a maximum demand reference current. A low THDi reading alone does not establish IEEE 519 compliance.
An Ahf is installed, its display looks healthy, and the commissioning team reports a lower distortion percentage. Is the project ready for acceptance? The answer depends on what was measured, where it was measured and which acceptance criteria apply. For plant engineers, panel builders and EPC contractors, these details turn a promising screenshot into a useful performance report.
This guide explains THD vs TDD, provides an illustrative calculation and sets out a practical verification checklist. If you are still specifying equipment, start with our active harmonic filter selection guide.
THD vs TDD: What Is the Difference?
Total harmonic distortion compares harmonic content with the fundamental component. For current, it is usually written THDi or THD-I. For voltage, it is THDv or THD-V. Total demand distortion, or TDD, compares harmonic current with a defined maximum demand load current, commonly written IL.
| Metric | Reference quantity | Useful question |
|---|---|---|
| THDv | Fundamental voltage | How distorted is the voltage waveform? |
| THDi | Fundamental current during the measurement | How large is harmonic current relative to the current operating load? |
| TDD | Maximum demand load current IL | How large is harmonic current relative to the installation's demand reference? |
Using IH for the root-sum-square harmonic current over the evaluated harmonic orders, the relationships are:
- THDi (%) = 100 × IH / I1
- TDD (%) = 100 × IH / IL
I1 is fundamental current, not automatically the same as total RMS current. The demand reference must be established using the applicable standard and project method. Do not substitute the AHF rating, transformer full-load current or a convenient instantaneous reading without a valid basis.
Why THDi Alone Can Be Misleading
Consider this simplified example, created for explanation rather than taken from an Intone Power customer project. Assume the same evaluated harmonic current and the same demand reference in both operating conditions.
| Quantity | Higher-load condition | Lower-load condition |
|---|---|---|
| Fundamental current I1 | 200 A | 50 A |
| Harmonic current IH | 10 A | 10 A |
| Demand reference IL | 250 A | 250 A |
| THDi | 5% | 20% |
| TDD | 4% | 4% |
THDi rises fourfold at lower load, although harmonic amperes and TDD stay unchanged. This does not prove that the filter failed. Conversely, a low THDi during heavy production does not establish acceptable performance across the full operating schedule.
Where Should Harmonics Be Measured?
IEEE 519-2022 addresses steady-state harmonic conditions at the PCC. A measurement close to a nonlinear load can help diagnose that load, but it cannot automatically represent conditions at the agreed PCC.[2]
A Practical AHF Verification Checklist
1. Agree on the acceptance criteria
Document the relevant standard edition, contractual targets, measurement boundary and required operating states. Avoid a specification that says only “harmonics below 5%.”
2. Capture a meaningful baseline
Record voltage, fundamental and RMS current, harmonic amperes, harmonic spectrum, distortion indices and production conditions before compensation.
3. Check installation and configuration
Confirm CT ratios, polarity, phase association and location against the selected AHF's installation instructions.
4. Measure representative operating states
Include minimum, typical and high production loads, relevant drive combinations and compensation-bank switching states.
5. Compare like-for-like results
Compare measurements from the same location with consistent settings and comparable load states.
What Should an AHF Test Report Include?
| Report section | Minimum useful content |
|---|---|
| Scope | Acceptance criteria, standard edition and operating modes |
| Electrical boundary | Annotated single-line diagram, measurement point |
| Results | THDv, THDi, TDD, harmonic amperes |
| Assessment | Criteria met, exceptions, follow-up actions |
How Intone Power Can Support Your Project
Intone Power offers active harmonic filter solutions. Send your project information to Intone Power to discuss the appropriate configuration and verification scope.