A passive harmonic filter is a proven electrical engineering component used to filter out specific harmonic frequencies from an electrical installation. In environments with large, static power electronics, this type of filter is a cost-effective and reliable method to ensure voltage quality(Power Quality). However, the application requires precise engineering beforehand: an incorrectly sized passive filter can lead to dangerous resonance with the mains impedance. This page tells you exactly how a passive filter works, when to apply it and why pre-simulation is necessary.
Short on time? Here are the key points you need to know:
Function: An LC circuit (coil and capacitor) that forms a low impedance for one specific frequency (e.g. the 5th or 7th harmonic), thus dissipating this current and not flowing into the grid.
Application: Ideal for installations with a stable and predictable load profile, such as heavy industrial pumps, compressors or extruders.
Biggest risk: Resonance. Without grid analysis and simulation, a passive filter can actually amplify pollution instead of attenuating it.
Difference from active: A passive filter is static and robust; an active filter is dynamic and flexible.
Requirement: Preliminary measurement and simulation are essential for safe operation.
The correct application of passive filters is primarily relevant for professionals responsible for the continuity of heavy, energy-intensive installations.
A passive harmonic filter consists of a composition of passive components: coils (inductance, L) and capacitors (capacitance, C), sometimes supplemented by resistors. These components are calculated and switched so as to form a very low resistance (impedance) for one specific frequency. This is called a suction circuit.
Its operation is based on the law of least resistance. Electric current preferentially chooses the path with the lowest impedance. By tuning the filter to the frequency of the pollution - e.g. 250 Hz for the 5th harmonic - the filter 'attracts' this specific current to itself. As a result, the harmonic current does not flow back to the main distributor and transformer, but is absorbed into the filter.
Analogy: Compare it to a traffic situation. Harmonic currents are heavy trucks that you do not want on your main road (the installation). A passive filter is a special exit that is just wide enough for one type of truck. As long as only that type of truck is driving, the exit ramp works perfectly and the main road remains clear. If other vehicles suddenly arrive, or the road structure changes (grid impedance), the exit ramp may become clogged or even cause accidents (resonance).
Harmonic pollution is not a theoretical problem, but a physical load on your installation. When non-linear loads such as variable speed drives or LED lighting generate harmonic currents, the sinusoidal form of the voltage distorts. This directly affects operational reliability:
Filtering is often not thought of until there are problems. The following symptoms often indicate an excessive level of harmonic pollution where a passive filter can be the solution:
Note: These symptoms can also have other causes. A Power Quality measurement is the only way to identify harmonics as the definitive cause.
Passive filters are typically used to compensate for contamination of so-called 6-pulse rectifiers. This is the standard input stage of most conventional variable frequency drives (VFDs).
When a 6-pulse regulator is loaded, harmonic currents occur in the orders. This means that especially the 5th (250 Hz) and 7th (350 Hz) harmonics are dominant. To a lesser extent, you will also see the 11th and 13th harmonics.
Because these frequencies are predictable and directly related to the load, a passive filter can be effectively dimensioned accordingly. You see this a lot in:
The choice between a passive and an active filter depends entirely on your installation characteristics and budget. HyTEPS advises in both pathways, but the application areas differ fundamentally.
Passive Harmonic Filter (PHF)
Active Harmonic Filter(AHF)
Nuance: A passive filter does not look at the rest of the installation. If your neighbour (or the utility grid) delivers a lot of pollution on the 5th harmonic, your passive filter will try to suck it up ('sponge effect'). This can lead to overloading your filter. An active filter does not have this problem.
Installing a passive filter without preliminary testing is risky. We regularly see the following mistakes in practice:
Want to reduce harmonic pollution with a passive filter? Then always follow these steps to eliminate risks:
Not every situation requires the intervention of HyTEPS, but in complex issues, expertise is indispensable. Consider support in the following situations:
Delve further into the subject matter via these related pages:
Symptoms are often subtle until things go wrong. Look out for unexplained machine failures, flickering lights, cables getting hot or transformers buzzing. Also, if electronics (PLCs, drivers) fail earlier than the service life indicates, chances are that the power quality is insufficient. A Power Quality measurement provides the answer.
This is possible, provided you have a high-quality Power Quality Analyzer (according to IEC 61000-4-30 Class A) and the knowledge to interpret the data. Collecting data is easy; analysing the correlation between events, harmonics and your specific business processes requires specialist engineering knowledge. We are happy to support you in the analysis.
Not by definition. NEN-EN 50160 describes the minimum requirements for voltage at the grid operator's transfer point. However, modern equipment can be more sensitive and malfunction even if the voltage is within this standard. We therefore look beyond the standard: we look at the compatibility between your power supply and your connected load.
Peace of mind, certainty and insight. You get a clear diagnosis of the 'health' of your electrical installation. We pinpoint the cause of faults, enabling you to avoid unplanned downtime and reduce fire risks or unnecessary energy losses. You receive a concrete advisory report with practical points for improvement.
No, that is a misconception. A filter is a powerful tool, but not a panacea. Sometimes the solution lies in changing transformer settings, redistributing loads or adjusting cabling. HyTEPS always recommends a thorough analysis and simulation before we recommend hardware, to avoid unnecessary investments.
Yes, significantly. Solar panel inverters and LED lighting drivers are non-linear loads that cause harmonics and sometimes supraharmonics. This can lead to interference with other equipment or overloading of the neutral conductor. When renovating or preserving, a Power Quality check is essential to ensure operational reliability.
We call this phenomenon 'nuisance tripping'. Often the cause is not the total amount of current, but the distortion of the current (harmonics) or short peak currents that your measuring equipment misses. This contamination can extra heat up thermal protections or confuse electronic protections, causing them to switch off wrongly. A specialised measurement can find out exactly why a protection reacts.
For a reliable picture, we usually measure at least one to two weeks. This is necessary to capture a full duty cycle, including weekends and peak loads. For specific acute failures, we can also take short-term measurements or deploy 'continuous waveform recording' to capture transients.
Your installer is an expert in installation and maintenance (the 'general practitioner'). HyTEPS is the specialist (the 'Power Quality Doctor'). We have advanced measuring equipment, simulation software and in-depth knowledge of theoretical electrical engineering and regulations. We often work together with installers to solve complex puzzles that fall outside standard knowledge.
After the measurement, you receive a report with conclusions in understandable language as well as technical details. If necessary, we simulate the possible solutions in our software. So you know exactly what the effect of a measure will be in advance. We then supervise the implementation and verify the result with a follow-up measurement.
Do you doubt whether a passive harmonic filter is the right choice for your situation, or are you concerned about possible resonance? Our engineers will be happy to look at it with you. We can simulate your situation or map it directly with a measurement. Speak to an engineer for clear advice.
HyTEPS
Beemdstraat 3
5653 MA Eindhoven
