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Passive Harmonic Filter

Passive Harmonic Filter: The robust solution for stable harmonic pollution

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.

In brief: What you need to know about passive filtering

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.

Who is this technology relevant to?

The correct application of passive filters is primarily relevant for professionals responsible for the continuity of heavy, energy-intensive installations.

  • Electrical Engineers: who are looking for a robust solution to comply with the grid code or EN50160 without complex control electronics.
  • Installation managers (IVs): Those dealing with transformers that get too hot or circuit breakers that trip inexplicably.
  • Technical Managers in Industry: who need to manage budgets and weigh up the lower investment of passive filters versus the flexibility of active filters.
  • OEM manufacturers: Who want to deliver their machines (e.g. large variable speed drives) 'clean' to the end customer to avoid subsequent claims.

What is a Passive Harmonic Filter?

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).

Why is harmonic filtering necessary?

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:

  • Overheating of transformers and cables: Harmonic currents cause additional losses (skin effect and eddy currents). This can overload a transformer even if the rated power is not exceeded.
  • Shortened equipment life: The extra heat generated makes insulation materials age faster. The rule of thumb is that every 10 degrees rise in temperature halves the lifetime of electronics.
  • Unexplained faults: Control electronics (PLCs) can be upset by 'zero-crossing faults' if the voltage form is too disturbed.
  • Penalties and compliance: Grid operators impose requirements on the maximum pollution you are allowed to feed back (Grid Code). In addition, standards such as EN 50160 and IEEE 519 prescribe limits that your installation must meet.

How do you recognise the need for a filter?

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:

  • Noise: Transformers or main distribution boards make a clearly audible humming or buzzing noise.
  • Tripping circuit breakers: Circuit breakers trip without a short circuit or nominal overload.
  • Hot cables: Power cables feel hot, while the current clamp indicates a value well within specifications.
  • Flickering lighting: When heavy machinery is switched on, the lighting dims or starts flashing(Flicker).
  • Communication failures: Bus systems (such as Profibus or Modbus) randomly fail.

Note: These symptoms can also have other causes. A Power Quality measurement is the only way to identify harmonics as the definitive cause.

What causes this contamination?

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:

  • HVAC systems (large fans and chillers).
  • Water management (heavy pumps running continuously).
  • Marine applications (propulsion and thrusters).
  • Industrial extruders and presses.

Passive versus Active Harmonic Filter: which is the best choice?

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)

  • Operation: Static. Filters fixed frequencies (e.g. 5th and 7th).
  • Advantages: Robust, no moving parts, lower initial investment, high efficiency.
  • Disadvantages: Risk of overload if background grid pollution rises. Risk of resonance. Less flexibility for expansions.
  • Best for: Installations with one or a few large, stable polluters.

Active Harmonic Filter(AHF)

  • Operation: Dynamic. Measures continuously and injects opposing current (anti-noise principle).
  • Advantages: Filters all harmonics simultaneously, adapts to varying loads, no risk of resonance, can also correct imbalance and reactive power (cos phi).
  • Disadvantages: Higher investment, more complex technology.
  • Best for: Complex installations with many different, varying loads and stringent requirements.

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.

Common mistakes in passive filtering

Installing a passive filter without preliminary testing is risky. We regularly see the following mistakes in practice:

  • No simulation of resonance: This is the most critical fault. Every passive filter, together with the transformer inductance, forms a resonance point. If this point coincides with a frequency present in the grid, the voltage is extremely excited. This leads to equipment failure and even fire hazard.
  • Forgetting grid impedance: A passive filter works on the basis of impedance ratios. If the grid impedance is not known (or changes due to grid switching), the filter's performance is unpredictable.
  • Overload from background pollution: It is assumed that the filter only filters its own machines. However, a passive filter is 'blind' and will attract all harmonic currents from the entire grid (including from outside). Without overdimensioning, the filter will fail.
  • Tuning due to ageing: Capacitors lose capacity over time. This shifts the filter's tuning frequency. A filter tuned to the 5th harmonic (250 Hz) may shift to a point where it is less effective or even cause resonance.
  • Placement in the wrong place: A filter placed too far from the source unnecessarily loads the intermediate cabling with reactive current.

Roadmap: Implementing a filter safely

Want to reduce harmonic pollution with a passive filter? Then always follow these steps to eliminate risks:

  1. Measurement (Zero Measurement): Insert a Power Quality analyser to map the exact spectrum (THDi, THDu and individual harmonics) over a period of at least a week.
  2. Data analysis: Analyse measurement data. Is the load stable? Which harmonics are dominant? What is the background contamination from the grid?
  3. Simulation: Use simulation software to model the filter in your specific network configuration. Check where the resonance point is and whether this is a safe frequency.
  4. Selection & Engineering: Choose the right components (L and C) to withstand thermal and electrical stress.
  5. Installation & Verification: Install the filter and perform a verification measurement to demonstrate that harmonic reduction has been achieved and no resonance occurs.

When do you call in a specialist?

Not every situation requires the intervention of HyTEPS, but in complex issues, expertise is indispensable. Consider support in the following situations:

  • You are considering passive filters for an installation that must also be able to run on emergency power (generators). (Note that frequency fluctuations in generators make passive filters complex).
  • You face strict requirements from the grid operator and must provide a compliance report.
  • You are in doubt about possible resonance with existing capacitor banks.
  • You seek assurance through simulation before investing in hardware.

Want to know more about Power Quality?

Delve further into the subject matter via these related pages:

Harmonic pollution

Blinding power

Voltage dips

Supraharmonic

Certainty about your Power Quality?

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.

Call: 0492 37 12 12
Mail: office@hyteps.nl

HyTEPS

Beemdstraat 3

5653 MA Eindhoven