How to choose the right active power filter for your application [Part 4/7: Active harmonic filters (AHF)]

After the introduction of active power filters in the first two articles of this series, this fourth article will discuss features and applications of active harmonic filters (AHF).

Active harmonic filters (AHF for short) have been around since the beginning of the 1990s. Description of their topology and operating principle can be found as far back as 1990. They were developed as a customised design of shunt active power filters (APF for short) to take care of the increasing harmonic problems in the electric power system caused by the widespread use of nonlinear equipment like variable speed drives (VSD for short) or switched-mode power supplies (SMPS for short) that conventional passive solutions like reactors, passive harmonic filters (PHF for short), K-factor transformers and isolation transformers, or conventional active solutions like active front ends (AFE for short) could not handle.

Functions

AHFs eliminate waveform distortions from the loads like harmonics, interharmonics and notching, by injecting in real-time in the electric power system the distorted current of same magnitude but opposite in phase. They can also work as harmonic generators for harmonic injection testing purposes.

Modern AHFs can take care of several power quality problems and support the development of clean energy by combining different control functions in a single device.

Markets and applications

AHFs can be applied to small, medium or large applications in a wide range of segments. Their versatility, instantaneous response time and numerous benefits compared to conventional solutions together with their price decrease in recent years make that nowadays AHFs are considered as a natural replacement for passive harmonic filters in many applications.

They have many low and high voltage potential applications where their use offers many benefits.

• Equipment using variable speed drives (VSD).
• Arcing devices: Electric arc furnaces (EAF), ladle furnaces (LF) and arc welders.
• Switched-mode power supplies (SMPS): Computers, printers, photocopiers, air cons, PLCs, TVs, etc.
• Battery chargers (including EV charging stations).
• Double conversion or rotary UPS systems.
• Medical devices: MRI scanners, CT scanners, X-rays machines and linear accelerators.
• Lighting devices: LED, fluorescent, mercury vapor, sodium vapor and ultraviolet (UV) lamps.
• Solar inverters and wind turbine generators.
• Modulated phase controllers, cycloconverters and thyristor-controlled AC voltage regulators.
• Saturable or rotating devices: Induction heaters, transformers, generators, reactors and motors.

Design

An AHF is a power electronics-based device connected in parallel with the equipment generating the power quality problems or that has issues to comply with grid code and energy efficiency requirements. The AHF behaves as a controlled current source providing any kind of current waveform (in terms of phase, amplitude and frequency) in real time (typical reaction time is under 50 microseconds and typical overall response time is under 100 microseconds).

By their connection capability to the electric power system, AHFs can be classified in 3-wire or 4-wire devices.?

• 3-wire AHFs are typically used for industrial and generation applications where there are VSDs or other nonlinear generators and loads present.
• 4-wire AHFs are typically used for applications in facilities where there are nonlinear loads like switched-mode power supplies and information technology equipment. They can filter triplen harmonics in the neutral conductors.

The most common operating voltage range for AHFs is 200 V up to 690 V as they are built using low voltage IGBT switches. It is possible to connect them to higher voltages using a suitable step-up transformer.

Operating principle

AHFs monitor the currents of the equipment generating the problems or that has issues to comply with energy efficiency or grid code requirements and compensate produced harmonic and interharmonic currents by generating a compensation current for each selected harmonic order in phase opposition. The result is a reduction on the levels of harmonics and interharmonics in the system to the limit requested by the end user.

If other problems or requirements from the application need to be taken care of like system unbalances, voltage fluctuations (flicker), notching, voltage variations, low power factor or high energy losses, the output current of the AHF can be adjusted for taking care simultaneously of those extra issues in real time.

The most common requirements from end users consist of reducing THDi under 5% and THDv under 3%, improving the power factor to unity and reducing both current and voltage unbalance under 3%. Fulfilling these requirements guarantees the continuous, reliable and safe operation of their equipment and processes, reducing operation and maintenance costs. It also ensures that the facility complies with international or local regulations on power quality and energy efficiency, reducing the overall energy consumption.

Features

The most typical features of AHFs that can be found nowadays in the market can be classified into the following categories.

Benefits

The most typical benefits of AHFs that can be found nowadays in the market can be classified into the following categories.

Total cost of ownership (TCO)

TCO is an important metric when evaluating active harmonic filters, especially if they are going to be used for critical process industries or mission critical facilities, as they operate 24 hours a day and 365 days a year. Owners and operators are increasingly concerned with the excessive costs of operating different types of industries and facilities and continuously looking for implementing solutions with the lowest possible total cost of ownership. More and more end users, consultants and engineers are requesting TCO analyses for projects than ever before, thus proving the importance of choosing a solution that provides cost saving potential over its useful lifetime.

A well-crafted TCO analysis clearly presents all costs associated with owning and operating an active harmonic filter. These costs are often described as capital expenditure or capital expense (CapEx for short) and operating expenditure or operating expense (OpEx for short) from an accounting perspective.

• Capital expenditure comprises the initial purchase, design, installation and commissioning costs of an active harmonic filter, as well as the costs of the necessary training for operators and maintenance personnel. The costs of the required room and floor space are usually also counted in here.
• Lifetime costs of an active harmonic filter can quickly exceed initial investments. When budgeting for an active harmonic filter, it is crucial to account for the operating expenses on power consumption, maintenance and reparations, upgrade and retrofit costs, and the costs for the owner or operator of any downtime, outage or failure.

The next article of this series will discuss features and applications of active load balancers (ALB).

If you would like to receive any of my publications on the topic or to explore how #ActivePowerFilters can benefit your application, feel free to reach me at [email protected].?

You are also welcome to join my running series of weekly #FreeWebinars for Asia-Pacific region on cutting edge #PowerElectronics solutions and their applications.

--------------------

About the author:

Pedro Esteban is a versatile, multicultural and highly accomplished marketing, communications, sales and business development leader who holds since 2002 a broad global experience in sustainable energy transition including renewable energy, energy efficiency and energy storage. Author of over a hundred technical publications, he delivers numerous presentations each year at major international trade shows and conferences. He has been a leading expert at several management positions at General Electric, Alstom Grid and Areva T&D, and he is currently working at Merus Power Plc.