Active or passive harmonic filter: which to choose

Short answer

The passive filter is a tuned reactor-capacitor circuit that gives one or two harmonics a low-impedance path and, along the way, supplies reactive power at the fundamental frequency. The active filter is an electronic converter that measures the load current and injects the opposite harmonics in real time, without creating resonances. Passive fits stable loads with known dominant harmonics; active fits variable loads, many harmonic orders or little reactive power to compensate. The right decision comes from a measurement and a study, not from the catalog.

What a harmonic filter is and why it is installed

Variable frequency drives, rectifiers, chargers and inverters do not draw sinusoidal current: they take current in pulses and generate harmonics, currents at multiples of the fundamental frequency (50 or 60 Hz). In a plant, those harmonics heat transformers and cables, trip protections, damage capacitors and distort the voltage that all other equipment receives. When the distortion exceeds what the utility or the process itself tolerates, a harmonic filter is installed.

There are two families: the passive filter and the active filter (AHF, for active harmonic filter). They solve the same problem with different principles, and choosing wrong is expensive: a filter that falls short, or one that creates a new resonance. This guide explains how each one works, when each fits and what must be done before buying. If you do not yet know how much distortion you have, start with the harmonic study to IEEE 519-2022.

How the passive filter works

A tuned passive filter is a reactor in series with a capacitor, connected in shunt. At the tuning frequency, the reactance of the reactor and that of the capacitor cancel and the filter offers a very low-impedance path: the harmonic current of that order flows into the filter instead of going up to the grid. It is normally tuned slightly below the target harmonic, so that component tolerances and capacitor aging do not push it into an exact resonance.

Three consequences you need to know:

It supplies reactive power. At the fundamental frequency the filter behaves as a capacitor, so it also corrects power factor. That is an advantage if the plant needs reactive power and a risk if the load drops and the plant ends up overcompensated.
It creates a parallel resonance. Together with the network inductance, every passive filter has a parallel resonant frequency below its tuning. The design must keep it away from the harmonics present.
It also filters what comes from outside. The filter cannot tell your plant’s harmonics from those already in the grid voltage. If the grid is distorted, the filter takes on foreign harmonics and can become overloaded.

There are variants: single-order filters (5th, 7th), multi-branch combinations, damped high-pass filters and broadband passive filters installed at the input of a drive. Do not confuse a filter with a detuned bank: a bank with a detuning reactor corrects power factor and avoids resonance, but it absorbs few harmonics. See capacitor bank. More on how to classify and monitor power quality phenomena in power quality monitoring under IEEE 1159.

How the active filter works

The active filter is an electronic converter with IGBT transistors and a DC bus. Its current transformers measure the load current, the controller calculates the harmonic content and the converter injects an equal and opposite current. The grid sees an almost sinusoidal current.

What sets it apart:

It adapts to the load. If drives start or stop, the filter follows the new current in real time.
It covers many orders at once, up to the order and with the response each manufacturer declares.
It does not create resonance with the grid at the fundamental frequency, because it is not a capacitor.
It has a current limit. It is specified in amperes of compensation. If the load produces more harmonics than its capacity, the filter compensates up to its limit and the rest passes to the grid.
It can do more than filter. Many models also compensate reactive power and unbalance, but those functions share the same rated current.
It has higher losses and needs ventilation, maintenance and a suitable room, like any power electronics equipment.

Quick comparison

Criterion	Passive filter	Active filter
Principle	Tuned reactor and capacitor	Converter that injects the opposite current
Orders it filters	Those of its tuning (one or a few)	Many, depending on the manufacturer
Variable load	Loses effectiveness or overcompensates	Adapts in real time
Resonance with the grid	Always exists; must be studied	Creates no resonance at the fundamental
Reactive power	Always supplies it	Optional, shares capacity
Harmonics coming from the grid	Absorbs them and can become overloaded	Compensates only what its CTs measure
Losses and maintenance	Low losses; check capacitors	Higher losses; ventilation and electronics
Future expansion	Requires a redesign	Modules added in parallel

When each one fits

Passive is usually a good option when the plant has large, stable loads (for example, a rectifier or a group of drives that run at constant duty), when the dominant harmonic is clear and when it also needs reactive power. Also at medium voltage, where a well-designed passive filter is a robust solution for large loads.

Active is usually better when the load changes a lot through the shift, when there are many harmonic orders, when power factor is already good (because modern drives draw little reactive power) or when a resonance cannot be risked. It is common at low voltage, at the drive panel or at the motor control center.

Hybrid solutions exist: a detuned bank for reactive power and an active filter for harmonics, or a passive filter for the 5th and an active one for the rest. And sometimes the best solution is not a filter: line or DC-link reactors on the drives, 12-pulse or higher rectifiers or active-front-end drives solve the problem at the source.

Before buying: what you need to know

How much distortion there is and where. It is measured with a Class A power quality analyzer over several days, at the connection point and at the panels with the nonlinear loads.
Which limits you must meet. IEEE 519-2022 sets voltage and current limits at the point of common coupling (PCC) with the grid, not at each piece of equipment. Some utilities require them at the connection. For the environment inside the plant, the IEC 61000 series defines compatibility levels for industrial installations.
How the grid responds. The impedance of the grid and of its transformer decides where the resonances fall. Without that data, a passive filter is a gamble.
Which capacitors exist. A capacitor bank without a reactor on the same bus completely changes the filter’s behavior.
How the plant will grow. A filter sized to the limit stops being enough with the next drive.

With that data the network is modeled, the alternatives are simulated and the one that complies with margin is chosen. IEEE 1531 is the reference for specifying passive filters; for active ones, the specification comes from the measured spectrum and the operating conditions. For the IEEE 1531 criteria for applying and specifying passive filters, see harmonic filters and the IEEE 1531 criteria.

Frequent mistakes

Buying from the catalog, with the transformer rating as the only data.
Installing a passive filter next to a bank without a reactor and creating a resonance worse than the original problem.
Placing the current transformers of the active filter badly: the filter only compensates what those CTs see.
Not checking for overcompensation of the passive filter when the plant runs at low load.
Not measuring afterwards. Without a verification measurement you do not know whether the filter complies.

We do your study

Our team measures the power quality of your plant, models the network with the utility impedance and your capacitors, compares the filtering alternatives and delivers the filter specification and the verification that it does not create resonances. We do the modeling and the study remotely, worldwide, with your data and measurements; field measurements are done in Colombia, or elsewhere on request. See our power quality service within engineering design and studies and send us your single-line diagram and measurements through contact.

References

IEEE 519-2022, IEEE Standard for Harmonic Control in Electric Power Systems.
IEEE 1531, IEEE Guide for the Application and Specification of Harmonic Filters.
IEEE 3002.8-2018, IEEE Recommended Practice for Conducting Harmonic Studies and Analysis of Industrial and Commercial Power Systems.
IEC 61000-2-4, Compatibility levels in industrial plants for low-frequency conducted disturbances.
IEC 61000-4-30 and IEC 61000-4-7, power quality and harmonic measurement methods.

Frequently asked questions

Which is better, an active or a passive harmonic filter?

Neither is better in general. The passive filter is robust and supplies reactive power, but it only filters the orders it is tuned for and can resonate with the grid. The active filter adapts to the load and covers many orders, but it has a current limit and higher losses. The decision depends on your load profile, the utility network and the limits you must meet at the connection point.

Does a harmonic filter improve power factor?

A passive one does, because at the fundamental frequency it behaves as a capacitor and supplies reactive power; so you must make sure it does not overcompensate when the plant is lightly loaded. Many active filters can also compensate reactive power and unbalance, but that capability shares the rated current of the unit with the filtering.

Why do capacitors burn out when there are drives in the plant?

Because the capacitor bank and the network inductance form a resonant circuit. If the resonant frequency falls near a harmonic the drives produce, the current and voltage at that order are amplified and overload the capacitors and their fuses. It is avoided with a detuned bank, a properly tuned filter or an active filter, depending on the study.