Modern plants run on drives, UPS units, LED drivers and electronic power supplies, and all of them distort the current they draw. Active harmonic filters clean that distortion at the source of the problem. For wider ways to cut waste in your plant, see our range of energy saving products and solutions.
This guide explains what harmonics do, how the filters work, when they beat passive options and how to size one without guessing.
The Short Version
• Harmonics come from non-linear loads and heat cables, transformers and capacitors.
• An active filter injects a cancelling current, so it copes with changing loads.
• Passive filters cost less on steady loads but are tuned to fixed frequencies.
• Always measure first, because sizing depends on harmonic current, not kVA.
What Are Harmonics And Why Do They Cost You Money?
Harmonics are currents or voltages at whole-number multiples of the 50 Hz supply frequency, created by non-linear loads such as drives, UPS units and LED drivers. They distort the waveform, heat equipment and can raise losses.
A non-linear load draws current in pulses instead of a smooth sine wave. Those pulses create extra frequencies, such as the 5th harmonic at 250 Hz and the 7th at 350 Hz, that flow through cables and transformers alongside the useful current.
Common effects in industrial buildings include:
• Overheated transformers, cables and neutral conductors
• Nuisance tripping and unexplained drive faults
• Failed capacitor banks
• Lower power factor and higher apparent power
• Metering errors on average-reading instruments
New electronic loads keep adding to the problem. EC fan retrofits for air handling units and cooling towers use electronic speed control, so count them when you list your non-linear loads. Even good efficiency projects change your harmonic profile, which is one more reason to measure before and after.
What Are Active Harmonic Filters And How Do They Work?
Active harmonic filters measure the load current, extract the harmonic part and inject an equal and opposite current, so the distortion cancels before it reaches the supply. They connect in parallel with the loads they protect.
Inside, a power electronic converter, usually built on IGBTs, creates the compensating current under the control of a fast digital processor. Because it measures continuously, it follows the load as it changes from minute to minute.
Many models can also supply reactive power and balance the phases, depending on the design and rating. Specify those functions if you need them, because not every unit includes them.
Should You Choose Active Or Passive Harmonic Filters?
Choose passive filters for steady loads with known harmonic orders, and active harmonic filters for varying loads, many drives or growing plants. Active units cost more but adapt on their own and scale with the load.
| Feature | Passive filter | Active filter |
|---|---|---|
| How it works | Tuned inductors and capacitors absorb chosen harmonic orders | Power electronics inject a cancelling current across many orders |
| Best for | Steady, predictable loads | Varying loads and many drives |
| Flexibility | Fixed tuning, designed for one load | Adapts as the load changes |
| Main risk | Resonance with the network if poorly designed | Undersizing, which leaves harmonics uncompensated |
| Cost | Lower | Higher |
One academic simulation, published in a UTM Elektrika journal study comparing active and passive power filters, found both filter types effective. The active filter cut current distortion on a diode rectifier from about 23% to 1.55%, against 2.55% for a single-tuned passive filter.
Treat that as a simulation, not a field guarantee. Real results depend on the network, the loads and correct sizing.
When Does Your Plant Need An Active Filter?
You need active harmonic filters when measurements show distortion above the limits that apply at your point of common coupling, or when harmonics are already damaging equipment. Symptoms and readings together make the case.
Look for these signs:
• Transformers or neutral cables running hot at modest loads
• Capacitor banks that fail early or keep tripping
• Drives or PLCs resetting without a clear cause
• Distortion readings above the limits in IEEE 519 at the PCC
• A fast-growing share of drives, UPS units and LED lighting
IEEE 519 sets recommended distortion limits at the point of common coupling, where your supply meets the utility. Your utility or regulator may apply its own rules, so confirm what applies to your connection.
How Do You Size And Specify An Active Filter?
Size active harmonic filters on the harmonic current you must cancel, not on the kVA rating of the loads. Measure that current by order across a typical operating period, then add margin for growth.
1. Measure: log harmonic current by order, power factor and load pattern at the main incomer and big feeders.
2. Set the target: decide the limit you must meet, such as IEEE 519 at the PCC.
3. Calculate: work out the harmonic current to compensate at peak.
4. Choose the location: central at the main bus, or local at large drive groups.
5. Check the details: neutral current, cooling, ambient temperature and communications.
An energy audit that logs voltage, current, power factor and harmonics together gives you the data for steps one to three.
Where Should You Install An Active Filter?
Install a central filter at the main low-voltage bus to protect the incomer, or local filters at large drive groups to protect the feeders. Central is simpler, while local protects more of the network.
• Central: one unit, one point of connection and a simple commissioning plan, but harmonics still flow through the feeders upstream of the drives.
• Local: each unit sits near its loads, so feeders and sub-boards stay cleaner, but you manage more units.
• Mixed: a central unit for the main bus plus local units on the dirtiest groups is common in larger plants.
Whichever you choose, leave space and ventilation around the unit and plan current transformer locations early. A neat layout saves commissioning time and reduces measurement errors.
Do You Need Other Power Quality Improvement Systems?
Active harmonic filters are one tool among several power quality improvement systems. Line reactors, DC chokes, detuned capacitor banks and good earthing each solve part of the problem at lower cost.
A line reactor protects a drive and trims some distortion, but on its own it may not meet IEEE 519 at the PCC. Capacitor banks can resonate with the network at harmonic frequencies, so check them before adding or keeping them.
Supply voltage matters too. Voltage optimisation lowers excess voltage on suitable loads, but it does not remove harmonics, so treat the two as separate fixes that work well together.
What Mistakes Should You Avoid?
The common mistakes are sizing by kVA, skipping measurement, ignoring resonance, picking the wrong location and forgetting future growth. Each is cheap to avoid when you plan before you buy.
• Sizing by kVA: harmonic current, not apparent power, sets the filter rating.
• No measurement: guessing the harmonic spectrum wastes money.
• Ignoring resonance: check existing capacitor banks first.
• Wrong location: a filter far from the problem loads cannot protect the feeders in between.
• No growth margin: extra drives and UPS units soon use up spare capacity.
How Do You Verify The Results?
Measure distortion at the same points before and after installation, under comparable load, and compare the readings with your target. Keep logging afterwards, because new loads can bring problems back.
IoT-based energy monitoring can track harmonics, power factor and demand continuously, so rising distortion shows up early.
Pair the work with an electrical safety audit. Thermal scans of panels and cables find hot joints that harmonics and overloads often cause, and the same visit can check earthing and neutral conductors.
Frequently Asked Questions
Can active harmonic filters save energy?
They reduce harmonic losses in cables and transformers, but the savings are usually modest. The main gains are reliability and compliance, so treat energy savings as a bonus.
Which loads create the most harmonics?
Variable frequency drives, UPS systems, rectifiers, LED drivers, welding sets and computer power supplies. The more such loads share one supply, the more distortion you will see.
Do I need a study before buying?
Yes. Measure harmonic current by order and load pattern first, because wrong sizing or location wastes money and can leave problems unsolved.
Can an active filter replace a line reactor?
Active harmonic filters handle distortion across many drives, while a line reactor alone may not meet IEEE 519 at the PCC. Many plants use both, each doing its own job.
Does an active filter fix low power factor?
Some models also supply reactive power, but not all do, so specify it if you need it. Otherwise fix power factor separately.
What data should I prepare?
Share your single-line diagram, transformer ratings, load list with drive ratings, 12 months of bills and any power quality logs. Better data narrows the filter size.
What Should You Do Next?
Start with measurement, then decide whether active harmonic filters, passive filters or simpler fixes suit your plant. Our team works Monday to Saturday, 9:30 AM to 6:30 PM, so contact ES Electronics for a free consultation and share your single-line diagram and any power quality readings.