Comparison between active power filters and conventional solutions: [Part 2/5: Active harmonic filters (AHF)]

Active harmonic filters

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.

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.

Comparison with conventional solutions

There are several solutions competing with AHFs that can be applied to a certain installation once that the power quality and energy efficiency problems and their magnitudes are identified. They provide different levels of problem mitigation depending on their technical complexity and the financial investment.

These solutions can be divided into six groups depending on the technology used (passive or active) and their connection to the problematic equipment (shunt connection or series connection). There are also some VSDs in the market with special designs that mitigate power quality problems.

Comparison with passive harmonic filters

Passive harmonic filters are designed to take care of installations’ harmonic distortion problems and can be connected directly to the low or high voltage electric power system. All passive harmonic filter configurations have a capacitive character as they are built with inductive, capacitive and resistive elements configured and tuned to control harmonics. The technical approach of these filters is to provide a low impedance path to harmonic currents at certain frequencies.

They are an efficient and economical solution for certain applications, for example, if specific harmonic frequencies usually produced by a specific piece of equipment need to be mitigated. But they are not efficient when working at partial loads and they possess the risk of causing resonances within the system if not properly sized.

The use of passive harmonic filters for certain installations can offer some disadvantages including:

• Design challenges as they can act as magnet for existing harmonics in the upstream system.
• They are system-specific solutions, therefore any minor changes in the installation can increase the harmonic distortions in the system.
• Monitoring is needed to ensure that they will not become overloaded or that overcompensation will not raise the system voltage significantly.

AHF operation compared to conventional tuned passive harmonic filter

Comparison with K-factor transformers

Nonlinear loads generate harmonic currents that cause transformers and system neutrals to overheat. The K-factor relates to the capability of transformers to supply power to varying degrees of nonlinear loads without exceeding the rated temperature rise limits of the transformer.

K-factor transformers are specially designed to withstand the harmonics generated by nonlinear loads and operate at full load without derating, a situation that a standard transformer could not adequately handle due to overheating. They are not designed to directly eliminate harmonics (even if they offer line reactance that could help on this), but to withstand their negative effects without damage or loss of performance.

K-factor transformers are preferred over oversized conventional transformers because they are specifically designed to supply nonlinear loads and are likely to be smaller, lighter and cheaper.

Comparison with isolation transformers

Isolation transformers are similar to K-factor transformers in that they reduce the amount of harmonic currents that are allowed to flow to the loads and prevent harmonics from moving upstream into the power source. They can provide a moderate reduction in voltage and current harmonics by contributing to source reactance.

Features comparison

Total cost of ownership comparison

Summary

There is an increasing number of low and high voltage electrical equipment with nonlinear voltage-current characteristics connected to the electric power system. Such equipment introduces harmonic currents to the system which cause harmonic voltages across the network impedance which add to the fundamental system voltage resulting in voltage distortion.

The best solution for a certain installation depends on the nature and the power demand of the problematic equipment, and the severity of the power quality and energy efficiency problems. Selection factors to consider should include:

• Compactness or overall space required.
• Simplicity in operation.
• Harmonic mitigation effectiveness.
• Energy efficiency.
• Cost-effectiveness for the application.

It is very important to notice that the total cost of ownership of active harmonic filters compared to the TCO of conventional solutions for a certain application it is very much dependent on the topology of the installation, the design of the whole system, the ratings of the device needed for taking care of the problems of the application and the requirements of the end user.

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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.