A Practical Guide to Active Harmonic Filter Types for Your Facility

Harmonics are one of the biggest challenges in modern electrical systems, affecting power quality. The presence of harmonics leads to issues like:

• Overheating of equipment
• Malfunctioning of sensitive devices
• Reduction in efficiency
• Higher energy costs

To tackle these problems, Active Harmonic Filters (AHFs) have emerged as an effective and reliable solution.

AHFs:

• Mitigate harmonics.
• Improve the power factor.
• Balance load currents.
• Enhance electrical networks' overall stability and performance.

This blog explores the most common types of active harmonic filters used, how they work, and the factors influencing their selection.

Most Common Types of Active Harmonic Filters

Here are the most widely used active harmonic filter types across commercial, industrial, and utility applications:

Shunt Active Harmonic Filters

The most common type of AHF used for commercial and industrial applications. These filters operate by injecting harmonic currents that have equal magnitude but are opposite in phase to the harmonic currents produced by the non-linear load to which they are connected.

Key Features

• Connected in parallel between the load (non-linear) and the power supply
• Compensate for current harmonics as a controlled current source
• Most effective for current-source (non-linear load) type harmonic problems
• Can simultaneously provide reactive power compensation

Shunt AHFs constantly measure the load current, extract the harmonic components via signal processing algorithms, and produce the necessary compensating current using power electronic converters. They commonly employ Insulated Gate Bipolar Transistors (IGBTs) or other fast-switching devices to ensure accurate current injection.

Shunt AHFs are ideal for applications where there are multiple non-linear loads or when the load conditions change significantly throughout the day. They are typically used for manufacturing facilities, hospitals, data centres, and any type of commercial building where variable frequency drives and other power electronic devices create harmonic distortion.

Also Read: How Does an Active Harmonic Filter Work: A Detailed Exploration

Series Active Harmonic Filters

Not as widely used as shunt filters. These filters are particularly suited for applications where voltage harmonics are the main/primary issue.

Key Features

• Connected in series with the power line, operating as a controlled voltage source
• Compensate for voltage imbalances and voltage harmonics
• Function as a harmonic isolator between the load and the source
• Typically need lower current ratings compared to shunt filters

Series AHFs measure the supply voltage, detect harmonic components, and inject compensating voltages to make sure a clean sinusoidal voltage is delivered to the load. They are especially effective in isolating critical sensitive loads that may experience voltage distortion from the utility supply or other loads on a shared electrical system.

The primary challenge with series AHFs is that they must carry the full load current. They are ideal for medium-power applications that demand precise voltage quality.

Hybrid Active Harmonic Filters

Utilise both active and passive filtering technologies in order to leverage the benefits of both while reducing the drawbacks of each approach. This hybrid combination provides an economical approach to harmonic mitigation for applications with high power.

Common Hybrid Configurations

• Active Filter With Passive Filter in Series: The passive filter deals with the majority of the lower-order harmonics (5th and 7th), and the active filter addresses the higher-order harmonics as well as compensates for the limitations of the passive filter.
• Active Filter With Passive Filter in Parallel: Both the active and passive filters independently operate in parallel, with the passive filter providing basic-level compensation and the active filter addressing dynamic changes and remaining harmonics.

Key Advantages

• More cost-effective than pure active filters in high-power applications
• Lower rating requirements for active filters
• Better efficiency for harmonic compensation at steady state
• Higher reliability via complementary operation

Hybrid filters are especially popular in heavy industrial applications where harmonic currents are high/significant and economic considerations are a concern.

Multi-Level Active Harmonic Filters

Represent a sophisticated topology that utilises multiple voltage levels to synthesise the compensating current waveform. Rather than switching between two voltage levels — positive and negative DC bus — these filters can switch between three or more voltage levels.

Key Features

• Create smoother output waveforms with minimised switching frequency
• Reduced EMI (electromagnetic interference)
• Minimised stress on switching devices
• Better accuracy of harmonic compensation

The typical configurations include — cascaded H-bridge converters and three-level neutral point clamped (NPC) converters. Multi-level topologies are being increasingly used in medium-voltage applications where standard two-level converters may fall short.

Modular Active Harmonic Filters

A collection of smaller filter units that can be assembled together to achieve the required compensation capacity — providing benefits in terms of both scalability and redundancy.

Key Advantages

• Easy capacity expansion via adding modules
• N+1 redundancy for enhanced reliability
• Reduced downtime for maintenance (individual modules can be serviced)
• Scalable deployment for growing facilities

Modular designs have become popular in hospitals, data centres, and mission-critical facilities where system availability is paramount. If one module fails, the remaining modules remain operational — ensuring power quality management continues uninterrupted.

Up next — key aspects that guide AHF selection.

Factors to Consider While Selecting AHFs

• Load Type: The magnitude and type of the harmonics generated by the loads greatly influence the filter selection. Shunt filters work best for current harmonics, while series filters mitigate voltage harmonics.
• Voltage and Power Levels: For higher voltage and power applications, hybrid or multi-level configurations may provide a better economic and technical justification.
• Existing Infrastructure: Facilities with existing passive filter installations would likely find the hybrid configurations the most cost-effective option for upgrades.
• Space and Installation Constraints: The modular option provides adaptability in tight spaces and supports gradual, phased deployment.
• Budget and Economics: While pure active filters provide the best performance, hybrid configurations offer adequate compensation with minimised costs for high-power applications.
• Future Expansion: When load growth is anticipated, consider using modular designs.

Generally, shunt active filters or hybrid active filters are preferred due to their scalability, enhanced performance, and affordability. Therefore, selecting the right filter is not just a technical choice but a strategic investment tailored to your specific operational and financial goals.

Making the Right Choice for Financial and Operational Stability

Ultimately, matching the active harmonic filter types to your specific business risk enables you to effectively transform a capital expenditure into a predictable profit driver, offering tangible financial security and long-term savings.

At Nantech Power Systems Pvt. Ltd — one of the leading UPS manufacturers in Chennai — we help you secure your power supply with effective active harmonic filters. Connect with us today for expert consultation tailored to your needs!