Harmonic Filters Selection Guide
Harmonic filters come in three-phase, single-phase and active types; a value like 7% is the reactance ratio and sets the step's resonance frequency.
This guide explains what to look for when choosing products in the Harmonic Filters category.
Types of harmonic filters
- Three-phase harmonic filters: reactors connected in series with three-phase capacitor steps.
- Single-phase harmonic filters: for single-phase capacitor steps; used in enclosures where the load is distributed unevenly across the phases and the phases are compensated separately.
- Active harmonic filters: power electronics modules referred to by their current (A) rating.
Some of the reactors connected in series with capacitors are listed in the harmonic filter reactors category; if the power, ratio or brand you are looking for is not here, check that category as well.
What the 7% on the product means
The percentage is the ratio of the reactor's reactance to the reactance of the series-connected capacitor at mains frequency, and it sets the step's resonance frequency. With a 7% reactor this frequency is 189 Hz on a 50 Hz network. The 5th harmonic is 250 Hz; because the resonance frequency stays below this, the step behaves inductively at the 5th and higher harmonics, and the capacitor does not go into resonance with these harmonics.
The third harmonic is 150 Hz, and 189 Hz is above it. With a 14% reactor the resonance frequency drops to 134 Hz. The Tibcon and Entes reactors in this category are 7%; models with a 14% ratio are listed in the harmonic filter reactors category. In facilities where single-phase nonlinear loads predominate and the third harmonic is dominant in the measurements, the reactor ratio is decided in the project according to the measured harmonic distribution.
Choosing the capacitor to go with the reactor
The products in the single-phase and three-phase groups are reactors; the capacitor is selected separately and matched with the reactor.
- Voltage: A series reactor raises the voltage across the capacitor terminals above the mains voltage. For this reason a capacitor with a rated voltage higher than the mains is used in the step; Entes pairs its 400 V 7% reactors with 450 V capacitors.
- Power: The kVAr in the reactor's name does not have to equal the capacitor's nameplate power. In the Entes table, a 25 kVAr reactor is paired with a 450 V 30 kVAr capacitor; when a capacitor operates below its rated voltage, it does not deliver its full nameplate power.
- Matching: Choose the reactor and capacitor according to the manufacturer's matching table; a capacitor of a different capacity shifts the step's resonance frequency.
The products that complete the step are in the power capacitors, power factor correction contactors and reactive power control relays categories.
How to choose an active harmonic filter
An active filter is connected in parallel with the network and prevents the load's harmonic currents from passing into the network by injecting a current of equal magnitude in opposite phase. Unlike passive circuits, it is not tuned to a specific frequency; it adapts to changing harmonic content. Parallel active filters can also supply reactive power and balance phase currents; verify these functions from the module's manufacturer data.
The module current (such as 50 A, 100 A and 150 A) is chosen according to the harmonic current measured at the connection point and, if applicable, the reactive power it is required to take over. For measurement, devices from the power quality analyzers category are used; the connection point and the placement of the current transformer are designed in the project according to the manufacturer's documentation.
Brands in this category
- Tibcon: 7% single-phase and three-phase harmonic filters; powers such as 1 kVAr, 10 kVAr and 20 kVAr.
- Entes: ENT.ERH series single-phase and three-phase harmonic filters; three-phase powers such as 8 kVAr, 20 kVAr and 65 kVAr.
- Tense: active harmonic filter modules with the TNS-AHF code.
Ordering notes
Products are sold per piece. For reactor-type filters, the kVAr power and number of phases are written in the product name. The reactor ratio appears as 7% in Tibcon names and, in Entes codes, together with voltage and power: the code ENT.ERH-E50-7-400-20 means 7%, 400 V and 20 kVAr. Entes single-phase models have 230 in the code; the manufacturer gives the operating voltage of these models as 230 V 50 Hz. You can reach a product directly by typing the manufacturer code (such as HRT-734020) into the search. Active harmonic filter modules are listed as made to order; for these products the lead time is confirmed before the order.
Comparison and selection table
| Selection Criterion | Detuned PFC Harmonic Filter Reactor | VFD AC Line / Input Choke Reactor | Active Harmonic Filter / Sine-Wave Filter |
|---|---|---|---|
| Operating Principle | Wired in series with capacitor bank to block resonance | Smooths VFD rectifier current spikes & reduces THDi | Injects counter-phase currents to cancel harmonics dynamically |
| Key Sizing Parameter | Detuning factor p = 7% (189 Hz), 5.67% or 14% (134 Hz) | 3% or 4% impedance (%Uk) matched to drive current (A) | Compensating current rating (30 A – 300+ A) |
| Paired Component | Capacitor step kVAr rating & 525 V voltage class | Variable frequency drive kW / rated input current | Main switchboard or sub-bus via CT feedback |
| Typical Application | Detuned power factor correction (PFC) panels | AC motor drive inputs and long motor cable outputs | Data centers, hospitals & heavy VFD industrial plants |
Frequently asked questions
What does a 7% harmonic filter mean?
It indicates that the reactor's reactance is 7 percent of the reactance of the capacitor it is connected in series with. This ratio sets the step's resonance frequency; for a 7% reactor it is 189 Hz on a 50 Hz network. Because the frequency stays below the 5th harmonic (250 Hz), the step behaves inductively at the 5th and higher harmonics, and the capacitor does not go into resonance with these harmonics. The ratio appears as 7% in Tibcon product names and as the digit 7 in Entes product codes.
Does a harmonic filter reactor come with the capacitor?
No. The products listed as single-phase and three-phase harmonic filters are reactors only; the capacitor is a separate product. The reactor is connected in series with the capacitor, and together they form a power factor correction step. Manufacturers give the suitable capacitor for each reactor in a matching table. A contactor and protection device are also required for the step.
What is the difference between a harmonic filter reactor and a shunt reactor?
A harmonic filter reactor is connected in series with the capacitor; its job is to protect the capacitor from harmonic currents by shifting the step's resonance frequency. A shunt reactor, on the other hand, is connected in parallel with the network and balances the facility's capacitive reactive power with inductive power. The two products are selected with different values and are not interchangeable; because both are referred to in kVAr, check the product type before ordering.
When is an active harmonic filter needed?
The job of a reactor step is to protect the capacitor; it is not designed to remove the harmonic currents produced by the load. An active filter is considered if the current harmonics in the measurements are above the limit required by the project or the grid connection conditions, if the harmonic content changes with the load, or if neutral current is causing problems. An active filter injects current in opposite phase to the harmonic currents; the module current is chosen according to the measured harmonic current.
Can a reactor be added later to an existing power factor correction enclosure without reactors?
Adding only a reactor is often not enough. Because a series reactor raises the capacitor voltage, the mains-voltage capacitors in the enclosure may not be suitable; Entes cites using 400 V capacitors with a reactor as a cause of capacitor failure and requires 450 V capacitors. The step power and the reactor-capacitor pairing also change. The conversion is carried out with a project based on harmonic measurement and the manufacturer's matching tables.