APF vs AHF vs SVG: Which Power Quality Device Do You Need?

Summary. Understanding apf vs ahf vs svg differences prevents the most expensive mistake in power quality procurement: buying a device that solves a problem the plant does not have. All three use IGBT converters and all three look similar in a catalogue, but an active power filter targets current distortion, an active harmonic filter targets harmonics and often reactive power as well, and a static var generator targets voltage support and reactive compensation on a much larger scale. This comparison explains the control objectives, typical ratings and measurement points of each device, shows where their functions overlap, and gives a selection path for plants that need more than one capability. It also covers hybrid arrangements where an AHF handles harmonics at the board while an SVG manages reactive power at the incomer.

What Are APF, AHF and SVG, Technically?

All three devices are voltage-source converters connected in parallel with the network. An active power filter (APF) measures load current, extracts the distortion component and injects an opposing current so that the supply current becomes sinusoidal. An active harmonic filter (AHF) performs the same function with explicit harmonic order selection, and in most modern product families also provides reactive current, load balancing and, in some models, flicker support at low voltage.

A static var generator (SVG) shares the converter topology but a different control objective. It regulates reactive current to hold voltage, improve power factor and suppress flicker. Its rating is expressed in kVAr or MVA rather than amperes of harmonic current, and it is commonly applied at medium voltage (6 kV to 35 kV) for renewable plants, substations and heavy industry. At low voltage the boundary between an AHF and an SVG becomes blurred, because a modern AHF with reactive capability can do much of what a small LV SVG does.

The practical difference is the quantity each device measures and controls. An AHF measures load current harmonics and regulates them; an SVG measures voltage and regulates reactive output. An APF is often the same hardware as an AHF with a simpler control target, which is why the two terms are used interchangeably in some markets and distinctly in others. When comparing quotations, ask what the device actually controls, at which measurement point, and what happens to that control when the other functions are enabled.

Ratings tell the rest of the story. Low-voltage AHFs are typically 15–600 A of compensating current at 380–690 V. Low-voltage SVGs cover tens to a few hundred kVAr. Medium-voltage SVGs reach tens of MVA in cascaded H-bridge configurations. A 100 A AHF at 400 V corresponds to roughly 69 kVAr of converter capacity, which illustrates why the two product families scale so differently.

Converter capacity is shared, not additive. A 100 A device at 400 V has roughly 69 kVAr of apparent converter capacity. If it injects 60 A of harmonic current and 40 A of reactive current, the vector sum must still stay inside the rating. Marketing sheets that list a maximum harmonic current and a maximum reactive current separately obscure this constraint, and it is the single most common reason a multifunction device underperforms its datasheet in a real plant.

Why Choosing the Wrong Device Is an Expensive Mistake

1. Voltage problems are not harmonic problems. A plant with a steady 0.86 power factor and stable voltage needs reactive compensation, not harmonic filtering. Installing an AHF to fix a power factor penalty wastes converter capacity on reactive current that a capacitor bank or SVG could supply more cheaply.

2. Harmonic problems do not disappear with reactive devices. A capacitor bank improves power factor and can amplify the 5th or 7th harmonic if the detuning is wrong. A plant that installed capacitors to fix a PF penalty and then measured worse distortion has learned this lesson expensively.

3. Measurement points decide the result. An AHF regulates current at its own CT location. If the CT is at the board but the utility meters at the transformer secondary, uncompensated upstream loads remain in the compliance measurement. An SVG at the incomer measures voltage, so its objective aligns naturally with utility metering. Matching device to measurement point is as important as matching function to problem.

4. Scale determines economics. For a 200 A harmonic load at 400 V, an APF rated 50A–600A is the straightforward answer. For a 20 Mvar reactive requirement at 35 kV, a medium-voltage SVG is the only practical option. Trying to cover a large reactive duty with stacked low-voltage filters is expensive and mechanically awkward.

5. Combined duties need explicit specification. A device that claims to do harmonics, PF and balancing must state the derating when all three operate simultaneously. Some units deliver full harmonic current only when reactive output is zero. Ask for the combined duty curve, not the maximum of each function.

6. Standards differ by function. Harmonic limits come from IEEE 519 or IEC 61000 series; voltage and flicker limits come from IEC 61000-3-3 and national grid codes. A device compliant for one objective may have no evidence for the other, which matters at inspection.

7. Lifecycle and spares differ by product family. Low-voltage filter modules are consumable items in a distributor's catalogue; medium-voltage SVG subassemblies are project components with long lead times. A plant standardising on LV filtering keeps one spare module and covers every board. A plant with an MV SVG needs a project-specific spares plan and, often, a service contract. Total cost of ownership comparisons that ignore this difference are misleading.

8. Grid-code evidence must match the declared function. Renewable interconnections increasingly require demonstrated reactive capability, voltage ride-through and harmonic emission limits at the point of connection. The evidence package is assembled per device type and per rating. Mixing functions across two devices is fine electrically, but the documentation must show which device delivers which requirement, and the utility will ask for it in exactly those terms.

How to Select Between APF, AHF and SVG

Step 1 — Characterise the problem before the product. Log THDi, individual orders, power factor, voltage variation, flicker and imbalance for a week. The dominant issue determines the device; if two issues are comparable, plan for a hybrid.

Step 2 — Check whether the neutral is involved. Four-wire boards with single-phase loads need triplen compensation. A rack-mounted modular APF for LV distribution cabinets with four-wire capability covers this case in a compact frame.

Step 3 — Decide on hybrid architecture for mixed problems. A common arrangement: an AHF at each problematic board for current distortion, and an SVG at the incomer for PF and voltage. The 150 kvar modular SVG cabinet and the low-voltage AHF for industrial plants are typically deployed together this way.

Step 4 — Confirm coordination and CT placement. Two fast converters on the same network can interact. Document the measurement point for each device, set the AHF to regulate current and the SVG to regulate voltage, and verify stability with both energised under step load.

Step 5 — Specify evidence for each function. List the standards you must meet and require test evidence per function: harmonic current limits, PF range, voltage regulation band and flicker reduction. A single compliance claim covering all functions is rarely enough for a utility review.

Step 6 — Compare total cost of ownership, not unit price. Include switchgear space, cable, losses, maintenance and spares. A multi-function AHF may cost more than a plain APF but remove a separate capacitor bank and controller, which frequently changes the ranking.

For plants at medium voltage where reactive power dominates, the decision usually sits between an SVG and a conventional capacitor bank with detuned reactors. Browse the SVG range for medium-voltage configurations, and compare against the AHF range when the dominant problem is current distortion rather than voltage support. Our engineering team can model both scenarios against your measured data before you commit.

Pilot before you scale. Where the site has several similar boards, filter one as a pilot for a month, measure the result and use the data to size the rest. This costs one additional commissioning visit and usually corrects at least one assumption in the original assessment, particularly about how much reactive current the converter can spare once harmonic duty is covered.

Common specification traps. The four that appear most often: quoting a harmonic current rating without stating the reactive duty at the same time; specifying a voltage regulation band without stating the fault level it applies to; assuming a four-wire unit is only needed in offices; and comparing a multi-function device with a single-function device on price alone. Each is easy to close with one extra line in the technical schedule.

Measure the starting point properly. A device comparison is only as good as the survey behind it. Record individual harmonic orders, PF at partial and full load, voltage variation over a shift change, flicker where welding or arc loads exist, and neutral current on four-wire boards. Ten minutes of data captured at the wrong time misleads every subsequent decision, and it is the most common reason a correctly specified device still disappoints.

Align the device with the metering tariff. Some utilities apply a reactive power charge measured on a 15-minute average, while harmonic limits may be assessed on much shorter windows. The two objectives can therefore be met by different devices operating at different points in the installation, and a single device is not automatically the cheapest way to satisfy both. Check the tariff structure before assuming that one multi-function unit closes every exposure.

FAQ

Is an APF the same as an AHF?

Often the hardware is identical and the terms are used interchangeably. Strictly, an AHF specifies explicit harmonic order selection and usually adds reactive and balancing functions, while an APF describes the broader category of active filtering. Confirm the control functions in writing.

Can an AHF replace a capacitor bank?

Partially. An AHF with reactive capability can supply leading or lagging reactive current and improve power factor, but the kVAr per ampere is limited by converter rating. Large reactive duties are more economical with an SVG or a properly detuned capacitor bank.

Do I need both an AHF and an SVG?

Only where both harmonic distortion and voltage or flicker problems are significant. Many industrial plants solve distortion at the board with AHFs and leave voltage support to the utility. Renewable plants and weak grids often need both.

Which device responds faster?

Both are converter-based and typically respond within milliseconds; the difference is the control objective rather than speed. An AHF reacts to load current changes, while an SVG reacts to voltage changes, so apparent speed depends on what is being measured.

Can an SVG also filter harmonics?

Some SVGs include harmonic compensation modes, but their rating is sized in kVAr for reactive duty, so harmonic capacity is limited. Where distortion is the main issue, a dedicated AHF uses the converter capacity more efficiently.

What about medium-voltage harmonic problems?

Medium-voltage filtering usually combines a stepped SVG or a tuned filter branch with LV filtering at the source. The most economical solution is normally to filter at low voltage where the nonlinear loads actually are, rather than at the MV incomer.

Conclusion

APF, AHF and SVG share a topology but not a purpose. Match the device to the measured problem: current distortion calls for filtering, voltage and reactive duty call for an SVG, and mixed problems justify a hybrid with clearly separated measurement points. Specify combined duty curves and per-function test evidence, and the choice becomes an engineering decision rather than a catalogue comparison.

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