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Exploring Active Harmonic Filters: A Comparative Study with Passive Filters

The difference between active and passive harmonic filter is illustrated with an EMERSON Active Harmonic Filter.

Harmonic filters, much like superheroes, protect your power systems. They fight against harmful harmonics that can damage the quality of your power supply. There are two main types of these protectors: active harmonic filters and passive harmonic filters. This article offers a detailed comparison between the two, underlining their respective strengths and weaknesses. Grasping this could greatly improve your business by boosting power quality and efficiency. Additionally, you'll learn about the factors to consider when choosing the right filter, with insights from experts like Nantech Power Systems, offering advanced active harmonic filters in Chennai to safeguard your power systems."

Understanding Harmonics and Their Impact on Power Systems

Harmonics in electrical power systems refer to currents or voltages with frequencies that are whole multiples of the fundamental power frequency. For example, if the base frequency is 60 Hz, the second harmonic would be at 120 Hz, and the third at 180 Hz. The electricity supplied to a property may not always be 'clean', and properties can experience harmonics. These harmonics come from non-linear loads, such as electronic devices that draw current in pulses rather than a smooth wave.

This sudden change in current flow injects harmonic currents into your power system, which can cause various power quality issues. These issues can appear as voltage dips and swells, imbalances in voltage or current across electrical phases, and flicker effects caused by repetitive switching of electrical loads. You can notice these problems through signs like flickering lights, overheating transformers, and breakers that trip frequently.

The state of harmonics in your system is represented by Total Harmonic Distortion (THD), a measure of all harmonic effects. It's typically measured up to the 50th multiple of the base frequency of the power system, which is 3kHz or, per some guidelines, the 40th multiple (2.4kHz).

Poor power quality due to harmonics can cause several problems, such as:

  • Increased power consumption leading to higher installation and utility bills.
  • Overheating of equipment.
  • Decreased profitability.
  • Potential damage to your equipment.
  • Overheating in neutral conductors and distribution transformers.
  • Decreased equipment reliability and lifespan.
  • Increased maintenance requests and downtime.
  • Higher electricity costs.

These additional frequencies distort the AC sine wave in an electrical circuit and can have serious consequences, including reducing the life of your equipment. Now that we've discussed the effects of harmonics on your power systems let's discuss how harmonic filters can help tackle these issues.

Introduction to Harmonic Filters

In understanding harmonic filters, you will explore the basic principles of active and passive harmonic filters. You will gain insight into their unique features and how they function. Let's start this informative journey.

Active Harmonic Filter Basics

Active Harmonic Filters (AHFs) present a modern answer to the issue of harmonic distortions in power systems. They employ cutting-edge technology to detect and study the harmonics in the network. A Central Processing Unit (CPU) then generates a harmonic current that opposes the measured spectrum. It introduces this counteractive current into the system in real time, effectively neutralising all existing harmonics.

We can categorise active filters into three types, each with its unique benefits:

  • Shunt active filters: These connect parallel to the load and estimate the harmonic current from the load. They generate a compensating current that neutralises the harmonic component.
  • Series active filters: These connect in series to the power system and inject voltage that cancels out the harmonic voltage in the system. This ensures that the load receives a stable voltage.

The primary benefits of active filters lie in their ability to enhance the power factor. They supply both capacitive and inductive reactive power, making them a sophisticated solution for harmonic filtering. Active filters can adapt to changing harmonic influences and filter out multiple harmonic frequencies simultaneously. They employ sophisticated power electronics and control algorithms to dynamically decrease harmonic distortion, injecting compensating currents into the power system. This results in a cleaner, more stable power supply.

Active filters hold several advantages over passive filters:

  • They can eradicate multiple harmonics simultaneously.
  • They adapt to changes in the power system frequency and the harmonic spectrum.
  • They don't create resonance problems in the power system, unlike passive filters.
  • They actively generate a reverse compensation current that cancels out various harmonic components, thereby improving power quality parameters such as voltage regulation and unbalance.

AHFs, also known as Active Power Filters (APFs), represent a new breed of power electronic equipment. They employ high-speed DSP devices and modern power electronics technology. They actively suppress harmonics and compensate for reactive power. AHFs are adaptive and can respond to a wide range of harmonic frequencies, making them a versatile solution for various power system configurations. Voltage fluctuations and harmonics can trigger grid disturbances and result in overheating and escalated energy bills. An AHF can mitigate these problems, producing a result better than 5% Total Harmonic Distortion (THD) across the load range, improving the power factor and balancing the load on all three phases if required.

With their adaptive nature and superior technology, Active Harmonic Filters provide an effective solution to the challenges posed by harmonic distortions in power systems. We will now look into the basics of Passive Harmonic Filters and their comparison with Active Harmonic Filters.

Passive Harmonic Filter Basics

Passive Harmonic Filters (PHFs) work on the principles of basic electrical circuit theory. They use resistors, inductors, and capacitors to eliminate unwanted frequencies. These components work together in different ways to create a specific filtering effect.

The success of a PHF in removing harmonics heavily depends on its design and position within the power system. Designing involves choosing the right values of resistance, inductance, and capacitance to create the desired filtering effect. For the best harmonic elimination, you should place the filter near the harmonic source.

A PHF's job is to allow certain frequencies and block others. It accomplishes this by using the unique responses of capacitors and inductors to different frequencies. In power systems, PHFs are designed to 'trap' harmonic frequencies, letting only the base frequency pass. This is achieved by making the filter's impedance high at the harmonic frequencies and low at the base frequency.

There are various types of PHFs, each with its unique response to frequency and circuit setups. These include:

  • Single-tuned filters
  • Double-tuned filters
  • High-pass filters

Although PHFs are relatively simple and cost-effective, they don't offer the flexibility of Active Harmonic Filters (AHFs). Changes in system operation can also affect their performance.

PHFs use passive components like reactors and capacitors, which are tuned to a specific frequency to filter out a particular harmonic part and reduce the resulting harmonics. They also compensate for reactive power, improving the power factor. Let's now compare these to their active counterparts.

Active Harmonic Filter vs Passive: A Scientific Analysis

The world of harmonic filters presents two primary types: active and passive. Each carries unique strengths and weaknesses, and your selection hinges on multiple factors. You need to evaluate their effectiveness in diminishing harmonics, their cost-efficiency, the upkeep they demand, and their flexibility and adaptability under varying power system conditions. Now, let's commence this comparative exploration.

Efficiency Comparison

Several factors can influence the efficiency of both Active Harmonic Filters (AHFs) and Passive Harmonic Filters (PHFs), including the specific harmonic frequencies in the system, the load conditions, and the design and location of the filters. Both AHFs and PHFs play a crucial role in reducing harmonics in power systems, but they function differently and offer varying levels of efficiency. Let's explore how these differences affect their efficiency.

Here are some key differences between AHFs and PHFs:

  • AHFs can reduce multiple frequencies simultaneously.
  • PHFs typically filter individual harmonics.
  • AHFs can actively adjust to changes in the power grid's harmonic current within a certain range.
  • PHFs can only reduce the harmonics of fixed orders (3,5,7) within a certain frequency range.

When it comes to safety, AHFs avoid overloading when the active filter limit is reached, unlike PHFs. Moreover, if an AHF becomes incapacitated, it won't affect the motors it optimises power for. This brings us to an important consideration when comparing the two.

Thus, while both AHFs and PHFs play their roles and offer benefits, it's clear that AHFs offer superior efficiency and safety in managing power system distortions.

Cost Comparison

Deciding between Active Harmonic Filters (AHFs) and Passive Harmonic Filters (PHFs) is vital for your business because they come with different cost implications. Let's examine their costs.

At first look, AHFs might seem pricier for a single-drive application. However, their cost-effectiveness grows as the number of non-linear loads increases. This happens because one AHF can correct multiple loads, thus making it a more budget-friendly choice in the long term.

Conversely, PHFs are typically more cost-effective for applications with a large, single, non-linear load. The capital expenditure for one AHF is usually higher than that of a PHF. But, if constant output holds the utmost importance, AHFs are the suggested solution. Despite the higher initial cost, AHFs can significantly reduce energy expenses and ensure consistent output, enhancing your business's net income.

You should consider factors like the specific harmonic frequencies in your system, the load conditions, and the design and placement of the filters when deciding between AHFs and PHFs.

While you must consider the initial investment and future maintenance costs, it's also crucial to take into account the specific needs of your power system when deciding between AHFs and PHFs. Let's shift our focus to the maintenance aspect of these filters.


Both active and passive harmonic filters present unique advantages and disadvantages regarding maintenance. External harmonics can affect Passive Harmonic Filters (PHFs), causing overheating and posing challenges in sizing due to the unpredictability of their impact.

Some modern PHFs, which technicians install on the cold side of wye-connected electrical sub-panels and ground applications, are passive and inductive. These devices do not require any third-party electrical components, eliminating the need for maintenance.

Active Harmonic Filters (AHFs), also known as harmonic correction units (HCUs), offer a more sophisticated solution for managing power system distortions. Unlike passive filters, AHFs can adapt and react to a broad spectrum of harmonic frequencies. This adaptability makes them a flexible solution for various power system configurations.

An active harmonic filter system consists of three main parts:

  • A module that detects harmonic
  • A control module
  • An inverter bridge module

However, maintaining AHFs is not as simple. While AHFs are generally more sophisticated and adaptable than PHFs, their complexity and the incorporation of power electronics might require more frequent maintenance. The specific frequency of this maintenance largely depends on the AHF model and the manufacturer's instructions.

On the other hand, passive filters employ a slower method known as contactor switching. Any changes in the frequency of the passive filters can alter the resonance point, diminishing the harmonic filtering effect.

In terms of operating costs:

  • Active harmonic filters might incur higher costs due to the necessity for constant monitoring and maintenance. Regular checks and adjustments are crucial to ensure the filter continues to operate optimally as the conditions of the power system change over time.
  • Installing a Passive Harmonic Filter on each drive can be a less intrusive compensation method as it does not require any ongoing maintenance.

After discussing the maintenance aspects, the next consideration is the adaptability and flexibility of these filters.

Flexibility and Adaptability

Active Harmonic Filters (AHFs) and Passive Harmonic Filters (PHFs) both have a role in reducing harmonic distortion in power systems. However, their flexibility and adaptability vary greatly. AHFs use power electronics and control algorithms to dynamically fight against harmonic distortion. They inject opposing currents into your power system, neutralising unwanted harmonics. The result? A cleaner, more stable power supply.

Conversely, Passive Harmonic Filters (PHFs) usually comprise reactive components like inductors, capacitors, and resistors. External harmonics from the mains can influence these components, potentially causing them to overheat. This unpredictable interference of external harmonics can make the task of sizing these passive harmonic filters quite challenging.

Active Harmonic Filters come with the following advantages:

  • They can adapt to a broad spectrum of harmonic frequencies, making them a flexible solution for different power system configurations.
  • Across the load range, Active Harmonic Filters can bring down Total Harmonic Distortion (THD) to less than 5%.
  • If needed, they also have the potential to enhance the power factor and evenly distribute the load across all three phases.

Therefore, given the adaptability of AHFs versus the specificity of PHFs, AHFs offer more flexibility. The choice between the two will hinge on the specific needs of your power system.

Choosing the Right Harmonic Filter: Factors to Consider

The process of selecting the appropriate harmonic filter for your power system isn't a minor task. It demands a comprehensive understanding of the specific requirements of your system and consultation with experts in the field. So, let's begin discovering the ideal harmonic filter for your needs.

Understanding Your Power System

Comprehending the specific requirements of your power system is an essential part of choosing a harmonic filter. This process involves determining the extent of harmonic distortion that needs addressing. For instance, when your power system includes a variety of load types, such as UPS, VFD, and DC drives, active filters often emerge as the best option.

Harmonic filters work by redirecting specific harmonic currents. Interestingly, engineers design some filters specifically to redirect harmonics of certain frequencies.

In this scenario, it's worthwhile to mention the IEEE 519-2022 standard. This guideline aids in designing electrical systems containing linear and nonlinear loads. It describes the voltage and current waveforms across the system, establishes waveform distortion goals for system designers, and explains the connection between sources and loads.

Considering the variety of power systems, understanding the differences between active and passive harmonic filters is crucial:

  • Active Harmonic Filters: These filters are mainly used in situations where the harmonic component and size change frequently and the harmonic composition is complex.
  • Passive Harmonic Filters: These filters are more appropriate for scenarios where the harmonic components and size remain relatively stable and the harmonic components are less complex.

Consulting Experts

Choosing the correct harmonic filter is essential for your business operations. Professionals from Nantech, the top UPS Dealers in Chennai, can offer helpful advice, providing a wide variety of services, including Active Harmonic Filters.

They offer a variety of power system solutions, such as:

  • Online UPS
  • Servo Stabilizers
  • Isolation Transformers
  • Line Interactive UPS
  • Power Factor Controllers
  • CVT
  • Inverters
  • Batteries

These top-performance products serve domestic and commercial customers, providing extended maintenance-free service. Their safe and customised solutions can boost your business operations' efficiency. They also plan and set up UPS systems that merge smoothly with your existing power system. So, consider contacting experts from Nantech to assist you in choosing the right harmonic filter for your power system. This choice might be the secret to enhancing your power system's performance.

Wrapping It Up

You've studied the scientific basis of active and passive harmonic filters, understanding their strengths and weaknesses. You've evaluated their efficiency, cost-effectiveness, maintenance needs, and adaptability. It's essential to keep in mind that your power system's specific requirements take precedence when choosing the right filter. For a knowledgeable decision, seek expert guidance or contact Nantech Power Systems. As the saying goes, "Electric power quality is not an Investment but an Expense." Hence, invest wisely in the correct harmonic filter to ensure your power system operates smoothly and efficiently.


Is an active harmonic filter better than a line reactor?

Active Harmonic Filters and Line Reactors have roles in managing harmonics in power systems, yet their methods differ. Active Harmonic Filters balance out harmonic distortion by generating currents that counteract it. They can function without Line Reactors, but issues might occur if the filter and the load lack sufficient impedance. In contrast, Line Reactors protect Variable Frequency Drives from power spikes and manage the inrush current flowing into the drive's rectifier. Whether you choose an Active Harmonic Filter or a Line Reactor depends on your power system's unique requirements.

What are the key differences between active and passive harmonic filters?

Active Harmonic Filters and Passive Harmonic Filters both control harmonics, yet their methods differ. Active Harmonic Filters eliminate harmonic components using power electronics. They add active power, matching the harmonic frequency but in the opposite phase, thereby neutralising the harmonic. Conversely, Passive Harmonic Filters employ passive elements such as resistors, capacitors, and inductors. These filters adjust to a particular frequency. Choosing between an Active Harmonic Filter or a Passive Harmonic Filter hinges on your power system's specific requirements.

What are the advantages of using an active harmonic filter instead of a passive filter?

Active Harmonic Filters (AHFs) outperform Passive Harmonic Filters in many ways. AHFs adapt more and perform better. They manage to keep the Total Harmonic Distortion (THD) under 5%, even with a load as low as 10%. In contrast, not every Passive Filter can meet the 8% or 5% THD IEEE-519 specification, even at maximum load. Unlike passive filters, AHFs do not lead to a power factor without load. You can put AHFs anywhere in the lineup, while passive filters require installation at each Variable Frequency Drive (VFD). When handling multiple VFD loads, AHFs are more cost-effective and save more space than passive filters. Want to delve deeper into the advantages of active over passive harmonic filters? Head over to our blog “Why Choose Active Harmonic Filters Over Passive Harmonic Filters?

How can an active harmonic filter improve power quality?

Active Harmonic Filters, or AHF, enhance power quality by eliminating harmonics. These harmonics can trigger protective devices, interfere with delicate equipment, and distort voltage. AHFs counteract these unwanted harmonics by releasing counteractive currents, purifying and stabilising the power supply. This action not only protects delicate electronic equipment but also improves energy efficiency and minimises the risk of costly downtime. Moreover, AHFs contribute to the power factor of the electrical system. They lessen both thermal and electrical stress on the electrical infrastructure, paving the way for long-term energy efficiency and cost savings.

What maintenance is required for active harmonic filters?

Maintaining Active Harmonic Filters (AHFs) regularly is key to their optimal performance. This involves a few critical steps.

  • Inspect the AHF's physical condition regularly to identify any damage, loose parts, or signs of wear and tear.
  • Stay alert for software updates from the manufacturer. These updates enhance the AHF's performance and rectify any issues or bugs.
  • Verify the AHF's calibration to ensure its accurate reading and response to electrical conditions.
  • Monitoring the operational temperature of the AHF is also crucial. Overheating can impair its performance and potentially cause damage.

Following these steps, you can ensure your AHF remains current and accurate, offering dependable power quality management.

How can you quantify the return on investment from installing an active harmonic filter?

An Active Harmonic Filter (AHF) investment can offer several benefits.

  • Energy Savings: AHFs boost power quality, which translates to significant energy savings over time.
  • Reduced Equipment Downtime: AHFs prevent equipment failure and cut downtime, resulting in cost savings.
  • Longer Equipment Lifespan: AHFs extend your equipment's lifespan by reducing the heat and electrical stress from harmonics.
  • No Penalties: AHFs help you comply with power quality standards, saving you from utility company fines.

Remember, the exact return on investment will vary based on your unique situation and power system.

What factors should be considered when selecting an active harmonic filter supplier?

When you need to choose a supplier for an Active Harmonic Filter (AHF), remember these key factors:

  • Inverter Topology: Many current AHFs build on a 3-level NPC inverter topology. It's a step up from the older 2-level topology and brings numerous benefits.
  • Losses: The losses an AHF might have can swing wildly. It hinges on both the design and the topology of the AHF.
  • Response Time: Some power quality problems can crop up in a flash. That's why the AHF's response time needs to beat that speed.
  • Harmonic Compensation Capacity: How well an AHF can balance a specific harmonic order matters a lot. It can heavily influence the whole system's performance.
  • Electromagnetic Compatibility (EMC): A few countries enforce stringent rules about EMC. It's crucial to remember this when you're picking an AHF.

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Author: Nantech Team
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