How to Size an Active Harmonic Filter: THDi, Load Current and Capacity

Summary. Learning how to size an active harmonic filter correctly usually decides whether a project passes its THDi target or needs a second unit six months later. Sizing is not a kVA exercise: it starts from the measured harmonic spectrum, converts individual harmonic currents into a required compensating current, then adds headroom for load growth, ambient temperature and reactive duties. This guide walks through the calculation an engineer should perform before issuing a purchase order: reading THDi and TDD properly, summing harmonic currents in RMS rather than arithmetically, converting a spectrum into an AHF current rating, and verifying the result against a worked 400 V distribution board example. It also explains the four sizing mistakes that account for most under-performing installations and the commissioning checks that prove the unit is doing the job it was specified for.

What Does Sizing an Active Harmonic Filter Actually Involve?

Sizing is the process of determining the compensating current an active harmonic filter must inject to hold distortion at the point of common coupling inside a defined limit. It requires three inputs: the load current at the compliance point, the harmonic spectrum of that load, and the limit you intend to meet. The output is a filter current rating, a mounting format and a control configuration, in that order.

The first technical distinction is between THDi and TDD. THDi is the ratio of harmonic RMS current to fundamental RMS current at the moment of measurement, so it rises steeply when the fundamental falls at light load. TDD compares harmonic current to the maximum demand current, which is the quantity most grid codes limit. A board that shows 45% THDi at 30% load may be perfectly compliant on a TDD basis, and sizing a filter to cancel the light-load THDi figure is a common and expensive over-reaction.

The second distinction is RMS summation. Harmonic currents at different orders add as the square root of the sum of squares, not arithmetically. A load with 30 A of 5th, 18 A of 7th and 9 A of 11th harmonic carries approximately 36 A of total harmonic current, not 57 A. Getting this wrong by adding orders directly inflates the filter by 50% or more, and it is one of the most frequent errors in consultant specifications that copy a total THD figure without a spectrum behind it.

Third, sizing must recognise that an AHF compensates current, not distortion percentage. If the load grows from 200 A to 300 A with the same percentage spectrum, the harmonic current grows proportionally and a filter sized for the original condition becomes undersized even though nothing about the load type changed. That is why the recommended practice is to size against maximum demand plus a growth allowance, not against today's average reading.

Why Correct Sizing Decides the Project Outcome

1. Undersizing fails the acceptance test. A filter running at 100% of rated current has no reserve for the peaks that dominate a compliance assessment. When the measured THDi lands at 8% instead of the specified 5%, the installer carries the cost of a second unit, and the consultant loses confidence in the supplier. Twenty to thirty percent headroom is the difference between a passing report and a negotiation.

2. Oversizing wastes capital and switchgear space. A unit two frame sizes larger than necessary costs more, draws more auxiliary power, occupies more cabinet space and, on weak networks, can interact more aggressively with the supply impedance. Buyers comparing an oversized quote with a correctly sized one usually choose the cheaper unit, so oversizing loses the order as well as the margin.

3. Four-wire loads need the neutral sized explicitly. Triplen harmonics add in the neutral of a four-wire system. A commercial board feeding single-phase rectifiers can carry a neutral current above the phase current, and a three-wire filter simply cannot address it. Selecting a 30A four-wire AHF module at design stage avoids a retrofit that costs more than the original device.

4. Reaction with existing compensation must be checked. Plants that already run capacitor banks or detuned reactors have a resonant frequency determined by the capacitor size and transformer impedance. If the AHF is sized without checking that resonance, the filter may amplify a band it was meant to remove. Sizing therefore includes a short resonance check, not just a current calculation.

5. Future load changes the answer. EV chargers, additional drives or a new cooling plant can double harmonic current within a year. A modular frame allows capacity to follow load: start with a 50A rack-mounted module and parallel a second unit when the load arrives, rather than replacing the whole device.

6. Ambient and fault level change the effective size. A module rated 50 A at 40 °C may deliver only 42 A at 50 °C, and a weak network with limited fault level forces lower control gain, which reduces achievable compensation. Both effects belong in the sizing sheet as derating factors, not as surprises found during commissioning.

How to Calculate AHF Capacity Step by Step

Step 1 — Define the compliance point and limit. Confirm whether the target is THDi, TDD, or a utility-specific emission limit, and whether it applies at the transformer secondary, the board, or the point of common coupling. The measurement point changes the answer materially, because downstream loads that are not compensated still appear in a local reading.

Step 2 — Capture the spectrum under representative load. A one-week survey with a Class A analyser should record individual harmonic orders, neutral current, phase imbalance and the load profile. Size against the maximum demand interval, not the average, and note whether the dominant orders are characteristic (5th, 7th, 11th, 13th) or include triplen and even orders.

Step 3 — Compute the required compensating current. Take the harmonic currents you intend to cancel, sum them in RMS, then apply a margin for ambient derating and control headroom.

With 37 A of harmonic current and a 1.3 design factor for headroom and derating, the specification becomes approximately 48 A, which points to a 50 A module rather than a 30 A unit. Note that the arithmetic sum of the four orders would have suggested 63 A and an unnecessarily large frame.

Step 4 — Add reactive and balancing duties. If the device will also correct power factor or balance phases, size the frame for the larger of the harmonic and reactive requirements, then confirm the combined duty is within the continuous rating. A multifunctional 50A–600A active power filter is specified this way: the harmonic current sets the floor, and the reactive requirement sets the final frame.

Step 5 — Verify with a post-installation measurement. Re-measure THDi, TDD, power factor and neutral current under the same load condition used for the survey. If THDi lands above target, check CT placement, headroom and whether the compliance point is downstream of uncompensated loads before concluding that the unit is undersized.

Worked example. A 400 V board feeds 180 A of six-pulse drive load at 28% THDi. The harmonic current is about 50 A, RMS-summed across the measured spectrum, and an 80 A measured peak pushes the design figure to roughly 65 A. A 75A floor-standing cabinet covers the requirement with margin, and the same calculation run for a 100 A data hall load points instead to a 100 A rack-mounted unit.

Two final checks belong in every sizing sheet: ambient temperature at the installation point (most modules derate above 40 °C, and a sealed plant room in summer is often 45 °C) and available fault level, since very weak networks require gain limits that reduce effective compensating current. You can compare frame sizes and mounting options across the full Active Harmonic Filter range before finalising the schedule.

What belongs in the sizing document. A useful sizing sheet is short and auditable: the compliance point and target, the measured spectrum with date and load condition, the RMS sum calculation, the design factor and its justification, the selected frame with its derating curve, and the expected post-installation THDi. Consultants accept this document because every number can be traced back to a measurement. Without it, two suppliers will quote different frame sizes for the same board and the buyer has no way to judge which is right.

Peak versus continuous duty. Some loads, such as resistance welding or a press line, draw harmonic current in bursts that are far above the average. Sizing on the continuous average under-specifies the unit for the bursts, while sizing on the instantaneous peak over-specifies for the whole day. The practical compromise is to size on the RMS value over a 200 ms window at maximum production rate, then confirm that the module's short-term overload capability covers the peak within that window.

Matching the filter to the transformer. Transformer impedance sets both the available fault level and the natural resonance with existing capacitors. A 1,000 kVA transformer at 6% impedance feeding a 400 V board behaves very differently from a 630 kVA unit at 4%, even with identical loads. Record transformer rating, impedance and any existing capacitor kVAr in the sizing sheet: the filter frame may be the same, but the control settings and gain limits will differ, and on lightly damped networks the difference shows up immediately as oscillation at commissioning.

A note on measurement uncertainty. Analysers disagree, especially on high-order components and on neutral current in a distorted waveform. Where a specification depends on the measured spectrum, ask for the instrument class and the calibration date, and re-measure with a second device if the two surveys differ by more than ten percent on the dominant order. Sizing on a single unverified capture is how projects end up arguing about whether the filter failed or the survey was wrong.

FAQ

Can I size an AHF from THDi alone?

No. THDi is a percentage without the current behind it. You need the load current and the individual harmonic orders to convert distortion into amperes. Sizing from a percentage alone produces either an oversized frame or a unit that cannot reach the target.

How much headroom should I specify?

Twenty to thirty percent above the calculated harmonic current is typical. The allowance covers ambient derating, control headroom and short-term load growth. Sites with planned expansion should consider a modular frame rather than a larger single unit.

Does load growth require a bigger filter?

Yes, if the harmonic spectrum stays similar. Harmonic current scales with fundamental current, so doubling load roughly doubles the required compensating current. Paralleling an additional module is the cheapest way to follow that growth.

Should the neutral be included in the calculation?

On four-wire systems, yes. Triplen harmonics add in the neutral, and a filter that ignores neutral current will not resolve overheating. Specify a four-wire unit and include measured neutral current in the sizing sheet.

What if the measured spectrum has even harmonics?

Even orders usually indicate half-wave rectifiers, DC drives or a measurement problem. Confirm with a second instrument, then ensure the filter can target even orders — many units allow programmable order selection, which passive solutions do not.

Is the sizing calculation different for 690 V systems?

The arithmetic is identical, but insulation, enclosure protection and thermal design differ. Currents are lower for the same power, so a 690 V mining installation often needs a smaller current frame than an equivalent 400 V load, with more attention to environmental protection.

Conclusion

Sizing an active harmonic filter is a measurement-driven calculation, not a catalogue look-up. Capture the spectrum at maximum demand, sum harmonic currents in RMS, apply a 1.3 design factor, add reactive duty if required, and confirm ambient conditions at the installation point. Doing this before the purchase order turns a compliance risk into a predictable engineering outcome, and it prevents the two most expensive errors in this product category: a filter too small to pass the test, and one too large to win the order.

Send us your power quality report and we will return a sizing sheet. Provide the load list, measured spectrum and compliance target, and our engineers will confirm the frame size, mounting format and expected post-installation THDi. OEM and ODM projects with custom ratings are welcome. Contact our engineering team for a sizing proposal.