Active or passive harmonic filters: Which solution should you choose?

In the world of electrical systems, managing harmonic distortion is crucial to ensuring efficient and reliable operations. Harmonic filters—both active and passive—are key tools used to mitigate these distortions, but they function in different ways and are suited to different applications. Here’s a breakdown of their differences to help you understand which might be best for your needs.

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What are harmonics?

Harmonics are voltage or current distortions that can cause inefficiencies, equipment malfunctions, and even damage to electrical systems. They typically result from non-linear loads, such as Variable Frequency Drives (VFDs), power supplies, and other electronic devices. Harmonic filters are employed to combat these distortions.

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Merus® A2 is a scalable, versatile, and durable active harmonic filtering solution designed and manufactured in Finland using innovative Merus® technology.

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Active harmonic filters (AHFs) briefly

• Performance and flexibility: Active harmonic filters are sophisticated devices that dynamically respond to changes in the electrical system. They monitor the system in real-time and inject counter-harmonic currents to cancel out the distortions. This makes them highly effective across a wide range of operating conditions, ensuring that Total Harmonic Distortion (THD) remains within acceptable limits, even under varying loads.
• Installation versatility: One of the key advantages of active harmonic filters is their flexibility in installation. They can be installed at various points in the electrical system—at the point of common coupling (PCC), at the main distribution board, or near sensitive equipment. This flexibility allows for optimal placement depending on the system’s specific needs.
• Power factor improvement: Unlike passive filters, active filters do not contribute to a leading power factor when the system is under no load. This is important because a leading power factor can sometimes lead to overvoltage conditions, which can be harmful to equipment.

Merus Power designs advanced active harmonic filters, our Merus® A2 filter is engineered to deliver exceptional THD control, flexibility, and power factor management for modern electrical systems. Click the link below to learn more about our technology.

Passive harmonic filters (PHFs) in a nutshell

• Simple design: Passive harmonic filters are simpler and typically less expensive than their active counterparts. They consist of inductors, capacitors, and resistors designed to block specific harmonic frequencies from propagating through the system. Passive filters are generally tuned to target particular harmonics, making them suitable for applications with stable, predictable loads.
• Installation constraints: Passive filters must be installed close to the source of the harmonics—typically at each VFD. This can lead to increased installation complexity and cost, especially in systems with multiple VFDs. Additionally, passive filters are less effective when dealing with variable loads, as their performance is optimized for specific conditions.
• Potential for power factor issues: A notable drawback of passive filters is their tendency to cause a leading power factor at no load. This can reduce system efficiency and potentially create challenges in maintaining voltage stability.

Which one should you choose?

The choice between active and passive harmonic filters depends mainly on your specific application needs:

• Choose active harmonic filters if you need a flexible solution that can adapt to varying loads and ensure consistent performance across the system. They are ideal for complex systems with multiple VFDs, where space and installation flexibility are crucial. Check out Merus Power’s Active Harmonic Filter.
• Choose passive harmonic filters if you have a simpler system with predictable loads. Passive filters are best suited for applications with well-defined and stable harmonic levels.

Merus Power also understands that some customers need a solution beyond these two options. For these cases, we offer the Merus® HPQ, a hybrid power quality compensator that combines the strengths of passive and active filters in one compact system.

Merus® A2 – Active Harmonic Filter

Merus® A2 is a scalable, versatile, and durable active harmonic filtering solution designed and manufactured in Finland using innovative Merus® technology.

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Merus Power’s active harmonic filter expertise at your service!

Merus Power offers specialized active harmonic filters designed to improve the efficiency and reliability of your electrical systems. Our expertise lies in our proprietary design and technology, which we tailor to your specific needs to effectively reduce Total Harmonic Distortion (THD), ensuring your operations run smoothly and efficiently.

We understand that every application has unique needs, and our team is here to help you select the best active harmonic filtering solution for your business. Focusing on delivering practical and reliable power quality solutions, Merus Power is committed to supporting you with expert advice and tailored solutions.

Conclusion

Harmonic distortion is a significant challenge in modern electrical systems, but choosing the right filtering solution can make all the difference. Active harmonic filters provide superior performance and adaptability, making them ideal for complex and variable systems, while passive filters are better suited for simpler, more stable environments.

By carefully considering your system’s specific needs—such as load variability and installation requirements—you can select the harmonic filter that best aligns with your operational goals.

With Merus Power’s expertise in power quality, you can ensure your system operates efficiently and reliably with the right advanced active harmonic filter tailored to your business.

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Good day Guys and Gals.

My understanding of harmonics is that they over heat the power source (transformer) and the neutral (grounded) conductor. I work at a wastewater treatment plant that has numerous VFD that is supplied by 3 phase 480 volt systems. None of these systems have the neutral conductor connected to any of the loads because the neutral conductor is not needed for proper operation. I am assuming that the harmonic (triplen) currents are seen on the phase (ungrounded) conductors if the neutral is not connected. Is this a safe assumption?

I am asking because we have an ATS that one set of the contacts is burning up and causing a single phase condition every 2-3 years. We hardly ever run at full load (usually 30-50% percent of capacity) of the equipment. Could harmonics be a source of heat on the phase that is burning up and failing in the ATS? Just trying to figure out why we cannot get the life out of these ATS that we should be getting. Sound more like a connection issue but connections are torqued to manufactrure specs and monitored with thermal camera every 90 days. Over time one of the phases start to show signs of overheating on the images and soon fails. The failure does not occur at the termination point of the feeder conductor but in the ATS contact that open and closes.

Any ideas would be appreciated. Neutral current due to harmonics has to do with single phase loads that are non-linear, such as power supplies, HID lighting, and now LED lamps, because each one has a power supply (albeit small).

But all power electronics produce harmonics, some more than others, and harmonic current has real effects on things like contacts, especially in that the “load” that you are observing may not show the harmonic current. The VFDs would be producing odd-order non-triplen harmonics, which would be generally equal between phases. If your facility has a preponderance of single phase non-linear loads that are not balanced between the 3 phases, that could cause one phase to be carrying more harmonic current than the other two. So the first thing I would look at would be your load balancing.

Then also, do a harmonic current study. If you have more than 10% THD-I, you should be looking into some form of mitigation. If the VFDs are all mitigated already, then you might want to look into an Active Harmonic Filter installed at the PCC (Point of Common Coupling). An AHF does not “care” where the harmonics come from, it measures it and reacts to correct it, usually on each phase individually. As mentioned above, the loading on the different phases should be balanced as well as possible.
Is the set of contacts that's burning up on the same phase each time that it has happened? If so, it might be useful to rotate the phases of the load on the ATS and document what was done. Then if the problem occurs again you can see if the failure moves with the same phase or if it stays at the same physical switch contact.

In the meantime, you might check the voltage drop across the switch contacts and the associated currents on a regular basis. This could provide an earlier warning than checking the temperature would of any degradation of the switch contacts.
If it's not already being done, perhaps some periodic use of contact cleaner and application of appropriate lubricant such as at the link below could be done. There are likely others on this forum that have specific experience in what would be the best monitoring and maintenance procedures.
Since you mentioned that it's occurring in a wastewater treatment plant, environmental factors including humiidity, etc. may be involved in degrading the contacts. And so more frequent preventative maintenance as mentioned above could be helpful.

https://www.gordonelectricsupply.com/p/Square-D-Swlub-Lubricant-Tube/

Neutral current due to harmonics has to do with single phase loads that are non-linear, such as power supplies, HID lighting, and now LED lamps, because each one has a power supply (albeit small).

But all power electronics produce harmonics, some more than others, and harmonic current has real effects on things like contacts, especially in that the “load” that you are observing may not show the harmonic current. The VFDs would be producing odd-order non-triplen harmonics, which would be generally equal between phases. If your facility has a preponderance of single phase non-linear loads that are not balanced between the 3 phases, that could cause one phase to be carrying more harmonic current than the other two. So the first thing I would look at would be your load balancing.

Then also, do a harmonic current study. If you have more than 10% THD-I, you should be looking into some form of mitigation. If the VFDs are all mitigated already, then you might want to look into an Active Harmonic Filter installed at the PCC (Point of Common Coupling). An AHF does not “care” where the harmonics come from, it measures it and reacts to correct it, usually on each phase individually.

Thank for the reply Jraef.

I took these pictures of the display on the ATS. Current THD% is definitely higher than 10%. That is with just 2 pumps running. On a rainy day we could have 4 or more pumps running. What do you think about these readings?

Thank for the reply Jraef.

I took these pictures of the display on the ATS. Current THD% is definitely higher than 10%. That is with just 2 pumps running. On a rainy day we could have 4 or more pumps running. What do you think about these readings?

What is the ampere loading on the ATS relative to its nameplate? If the load is only a small % of the nameplate, then the total demand distortion is more important. If the load is very small, the acceptable THD can be higher.

Total demand distortion (TDD) is the per-phase harmonic current distortion against the full load nameplate of the electrical system. TDD indicates the impact of harmonic distortion in the system.

What is the ampere loading on the ATS relative to its nameplate? If the load is only a small % of the nameplate, then the total demand distortion is more important. If the load is very small, the acceptable THD can be higher.

Total demand distortion (TDD) is the per-phase harmonic current distortion against the full load nameplate of the electrical system. TDD indicates the impact of harmonic distortion in the system.

Thanks for the reply Ron.

The current rating is 800A. Thats the feeder breaker size. The ATS is rated for 800A as well. Transformer is rated a lot higher because it feeds.. several other builidngs as well. The transformer is located in a substation owned by us.

Each pump draws on average about 110 to 140 amps according to what speed it is running. We hardly ever run more than 2-3 pumps at a time.

Thanks for the reply Ron.

The current rating is 800A. Thats the feeder breaker size. The ATS is rated for 800A as well. Transformer is rated a lot higher because it feeds.. several other builidngs as well. The transformer is located in a substation owned by us.

Each pump draws on average about 110 to 140 amps according to what speed it is running. We hardly ever run more than 2-3 pumps at a time.

The load is small, so likely not a big deal.
This is a good video lesson and they discuss TDD around 10 minute mark

The load is small, so likely not a big deal.
This is a good video lesson and they discuss TDD around 10 minute mark

The load is small, so likely not a big deal.
This is a good video lesson and they discuss TDD around 10 minute mark

Thanks for the reply Ron.

My understanding from the video and your response, correct me if I am wrong, is that the combination of the normal load and harmonic load currents should not be an issue with overheating unless the combination of those two load currents exceeds the rated current of the equipment. Also, if that was the case, our breakers protecting the feeder would be tripping as well. Is my understanding correct? How much harmonic current would I expect to see if the normal load is around 300A total from two six pulse drives? It is obviously less that 800A or my breakers would be tripping. Correct?