The power quality report arrives, and one number immediately gets everyone’s attention:
Thdi: 32%.
The next questions usually come quickly:
Is 32% THDi too high?
Do we need an active harmonic filter?
If we do, what AHF size should we specify?
A high THDi reading deserves attention, but it does not answer any of those questions by itself.
It tells you that the current waveform was significantly distorted at a particular measurement point and under a particular operating condition. It does not tell you how much harmonic current must be compensated, whether the whole facility has the same harmonic condition, or whether an Active Harmonic Filter is necessarily the right solution.
That distinction is important because an AHF is ultimately sized in amperes of compensation current, not in percentage points of THDi.
If you already have a power quality report in front of you, don’t start with the filter catalogue. Start with the report.
In This Guide
- Where was the THDi measured?
- What was the operating load?
- How much harmonic current is actually present?
- Which harmonic orders dominate?
- Are harmonics really causing the reported problem?
- What is happening to THDv?
- What is the actual mitigation target?
Start With the Report, Not the AHF Rating
Suppose your analyzer report shows:
| Parametre | Measured Value |
|---|---|
| System voltage | 400 V |
| RMS current | 180 A |
| Thdi | 32% |
| THDv | 3.1% |
| Güç faktörü | 0.95 |
| Dominant harmonics | 5th and 7th |
İlk bakışta, 32% THDi looks like the most important number.
But there is not enough information here to specify an AHF.
We still do not know where the analyzer was connected, what equipment was operating at the time, whether 180 A represents light load or normal production, or what electrical problem the project is actually trying to solve.
So the first job is not to calculate an AHF size. It is to understand what the measurement actually represents.
1. Where Was the High THDi Measured?
The same THDi percentage can mean very different things depending on where the analyzer was connected.
- an individual VFD input;
- a motor control center;
- a production-line distribution board;
- a transformer secondary;
- a main LV switchboard;
- or the Point of Common Coupling.
Örneğin:
THDi = 38% at the input of one VFD
is useful information about that VFD.
But it does not mean:
THDi = 38% for the entire facility at the PCC.
So instead of recording only “THDi = 38%,” the report should also tell us: “THDi = 38%, measured at the VFD input.” That extra sentence can completely change how the result should be interpreted.
Before moving on, confirm three things: where the measurement was taken, what loads are downstream of that point, and whether we are evaluating one machine, one production line, or the entire electrical system.
If the report does not clearly identify the measurement point, that should be clarified before equipment sizing begins.
2. What Was the Load When the THDi Was Recorded?
A high THDi percentage should never be separated from the operating condition in which it was measured.
Consider the same production line at two different times.
Low-load operation: Yük akımı 90 A, Thdi 34%.
Normal production: Yük akımı 420 A, Thdi 18%.
If we look only at the percentage, the first condition appears worse. But that does not automatically mean the AHF should be sized from the 34% reading.
THDi is expressed relative to the fundamental current present during the measurement. When the fundamental current falls during light-load operation, the THDi percentage can increase even when the absolute harmonic current has not increased proportionally.
The more useful question is not “When was THDi highest?” It is: Under which operating condition does the system produce the harmonic current that actually needs to be compensated?
If plant load changes significantly between startup, normal production, peak production, night operation or batch changes, one analyzer screenshot may not represent the condition that matters most.
3. Stop Looking Only at Percentages—Look at Harmonic Current in Amperes
This is one of the most common points of confusion in an AHF project.
A customer sends: “THDi = 30%. Please quote an AHF.”
But an Active Harmonic Filter is not rated in percentage points of THDi. AHF capacity is normally discussed in compensation-current ratings such as 50 A, 100 A, 150 A or 200 A.
So the project eventually has to move from percentage to harmonic current in amperes.
System A: Yük akımı 80 A, Thdi 30%.
System B: Yük akımı 600 A, Thdi 30%.
The THDi percentage is identical. The compensation requirement clearly is not.
That is why “THDi is 30%, therefore use a 100 A AHF” is not a complete engineering sizing method.
The better question is: How much harmonic current exists during the relevant operating condition, and how much of it needs to be compensated?
AHF capacity calculation becomes useful only after the measurement has been properly understood.
4. Don’t Stop at THDi—Open the Harmonic Spectrum
THDi is a summary number. The harmonic spectrum tells you what is inside that number.
Two systems may both show THDi = 25% while having very different harmonic profiles.
- One system may be dominated by the 5th and 7th harmonics.
- Another may contain significant 3rd, 5th, 7th, 11th and 13th harmonics.
Looking at individual harmonic orders helps engineers understand what types of nonlinear loads may be contributing and what the compensation equipment will actually need to handle.
Instead of sending only “THDi = 27%,” a more useful report would say: “THDi = 27%. The 5th and 7th harmonics are dominant. Harmonic spectrum attached.”
If the analyzer can export the individual harmonic currents or spectrum chart, that information should be included in the engineering review.
5. Are Harmonics Actually Causing the Problem?
Finding high THDi and identifying the root cause of a plant problem are related—but separate—tasks.
Suppose the maintenance team reports: “Our transformer is overheating.” The power quality analyzer also reports high THDi.
- excessive loading;
- poor ventilation;
- high ambient temperature;
- phase imbalance;
- connection problems;
- neutral current;
- harmonic losses;
- or several factors acting together.
It may be tempting to conclude: Transformer overheating → High THDi → Install AHF.
But the transformer may also be affected by the factors above.
The same caution applies to capacitor failures, nuisance tripping and poor power factor. Harmonics may be involved, but they should not automatically be treated as the root cause.
A useful power quality analysis should answer two separate questions: What abnormal electrical condition can we measure? Ve is that condition actually causing the problem the customer wants to solve?
Sometimes the answer leads to an Active Harmonic Filter. Sometimes it leads to another solution. And sometimes it shows that more measurement is needed before equipment should be purchased.
6. Read THDv Alongside THDi
Current distortion is only part of the power-quality picture. If the report contains THDi but no THDv, voltage distortion should also be checked.
THDi describes distortion of the current waveform. THDv describes distortion of the voltage waveform.
Consider two plants: Site A has THDi 25% and THDv 2.0%; Site B has THDi 25% and THDv 7.0%.
The current-distortion percentage is the same. The wider system condition is clearly different.
This does not mean THDv alone determines whether an AHF is needed. It means the more useful engineering picture is: Thdi + THDv + harmonik spektrum + load condition + measurement point.
7. Define the Target Before You Define the AHF Size
Before discussing whether the project needs a 100 A or 150 A filter, one question should be answered first: Why are we mitigating the harmonics?
- Uygunluk — The project has an IEEE 519, utility or customer harmonic-performance requirement.
- Equipment reliability — Sensitive equipment may be affected by power-quality conditions.
- Transformer or cable loading — The customer wants to reduce harmonic-current-related electrical stress.
- Plant expansion — Additional VFDs, UPS Sistemleri, rectifiers or other nonlinear loads will be added.
- Process stability — The plant is investigating electrical disturbances associated with production interruptions.
- EPC or end-user requirement — The project specification defines a target power-quality level.
These are not identical projects. Before asking “What AHF size do we need?” the project should first define: What electrical condition are we trying to achieve?
Without a target, even a technically correct sizing calculation may solve the wrong problem.
From High THDi to an Engineering Decision
A practical decision process should look like this:
Notice that AHF sizing appears near the bottom. That is deliberate. Sizing should be the result of diagnosis, not the first step.
Does High THDi Automatically Mean You Need an Active Harmonic Filter?
HAYIR. Bir Aktif harmonik filtre is one option for harmonic mitigation.
Depending on the electrical system, engineers may also evaluate:
- line reactors;
- DC chokes;
- passive harmonic filters;
- active front end drives;
- load redistribution;
- system configuration changes;
- or combinations of these measures.
An AHF becomes particularly attractive when the installation contains changing nonlinear loads and requires dynamic harmonic compensation. But even then, the question should not begin with “Which AHF model should we buy?” It should begin with: What does the system actually require us to compensate?
What Should You Send Before Requesting an AHF Quote?
If you want an engineering recommendation rather than only a catalogue quotation, provide as much of the following information as possible，You can also use our AHF sizing service to provide the key system data for engineering review.
| Information | Why It Matters |
|---|---|
| System voltage | Defines electrical connection requirements |
| 3P3W / 3P4W | Defines system configuration |
| Single-line diagram | Shows loads and measurement locations |
| Measurement point | Explains what the harmonic data represents |
| RMS current | Shows actual load level |
| Profili yükle | Shows how the system changes over time |
| Thdi | Gives current-distortion context |
| Harmonic spectrum | Shows individual harmonic orders |
| THDv | Adds voltage-distortion context |
| Güç faktörü | Shows concurrent reactive-power conditions |
| Peak and normal load | Prevents sizing from an unusual snapshot |
| Required target | Defines what the solution must achieve |
| Future expansion | Allows future loading to be considered |
You may not have every item. That is fine. The purpose is not to delay the project until every measurement is perfect. It is to avoid making a hardware decision from one percentage.
What If You Only Have One Analyzer Screenshot?
That situation is common. First check what information the screenshot already contains:
- Gerilim;
- akım;
- Thdi;
- THDv;
- güç faktörü;
- individual harmonics;
- timestamp;
- measurement location.
hen identify what is missing. Örneğin, Thdi 31% and current 76 A may look alarming. But if normal production current is usually around 350 A, the first question becomes: Was the 31% THDi measured during a light-load condition?
If nobody knows, repeating the measurement during representative production may be more useful than immediately requesting several AHF quotations.
One Question That Can Improve Your AHF RFQ
Instead of sending this:
Our THDi is 30%. Please quote an Active Harmonic Filter.
send something like:
We measured high THDi at the main LV bus during normal production. Attached are the load current, Thdi, THDv, harmonic spectrum and measurement conditions. Please review how much harmonic current requires compensation and whether an AHF is appropriate for the required target.
The difference is important. The first question asks: What equipment can you sell us?
The second asks: What does our electrical system actually require?
A useful engineering proposal should explain what measurements were used, which operating condition was selected, how much compensation current is expected, what target the system is designed to achieve, what assumptions were made, and why the proposed AHF capacity is appropriate.
If the only explanation is “THDi is high, so use a larger filter,” there are probably still unanswered questions.
Before You Specify an Active Harmonic Filter
A high THDi reading is worth investigating. But it is the beginning of the analysis, not the end.
Before AHF sizing, make sure you understand where the high THDi was measured, how heavily the plant was loaded, how much harmonic current is actually present, which harmonic orders dominate, what is happening to voltage distortion, whether harmonics are actually causing the reported problem, and what performance target the project needs to achieve.
Once those questions have been answered, Active Harmonic Filter sizing becomes much more meaningful.
If you already have a power quality report but are unsure how to translate those measurements into an AHF requirement, send the report together with the system voltage, load current, operating condition and single-line diagram for engineering review.
The first question should not be: “How large an AHF can we install?”
It should be: “What does this electrical system actually need?”
SSS
Is 30% THDi always a serious problem?
It indicates substantial current waveform distortion at the point and operating condition where it was measured. Fakat, 30% THDi alone does not tell you the condition of the entire facility, the compensation current required, or whether an AHF is necessary.
Can I calculate AHF size from THDi alone?
THDi alone is not sufficient for a reliable engineering selection. Load current is essential, and a better assessment also considers the operating condition, harmonik spektrum, measurement location, THDv and mitigation target.
Why does THDi sometimes increase when plant load decreases?
THDi is expressed relative to the fundamental current under the measured operating condition. When the fundamental current falls, the THDi percentage can increase even if harmonic current does not increase proportionally.
Should I measure harmonics at the VFD or at the PCC?
It depends on what you are trying to determine. A VFD measurement helps evaluate that individual load. A distribution-board measurement helps evaluate part of the installation. For system-level harmonic assessment, the relevant PCC or main system measurement point becomes especially important.
What information is most useful for AHF sizing?
Start with system voltage, wiring configuration, RMS load current, Thdi, harmonik spektrum, THDv, measurement location and operating condition. A single-line diagram, project target and information about future loads make the assessment more useful.
Can an AHF compensate both harmonics and reactive power?
Some active power-quality equipment can perform harmonic mitigation and reactive-power compensation depending on its configuration and available capacity. The harmonic and reactive-power requirements should still be quantified before deciding how the equipment should be configured.
