Active Dynamic Filtering - What is an ADF?

The development of electronics ensures that compensation techniques are becoming more intelligent. In recent years, we have seen that an  active dynamic filter (ADF) is increasingly being used to solve power quality problems. But what is an ADF? What are the benefits and what should we consider when purchasing? 
This white paper discusses these topics.

White paper - Active Dynamic Filtering - fortop

Power Quality: a brief introduction

Power Quality describes the quality of voltage and current. Poor power or voltage quality can lead to equipment failure, plant overload or claims and penalties. Therefore, requirements are set for current drawn by equipment as well as the voltage supplied. There are standards for public networks, industrial non-public networks and connected equipment.

Agreements between grid operator and user

The EN-50160 describes the standards that apply as agreements between the grid operator (supply of voltage) and the user (user of electricity). However, electricity networks of end-users of electricity have to deal with the EN-61000 series of standards. This series includes standards for both voltage (networks up to 35 kV) and power quality (equipment). 
Power Quality, normen en verantwoordelijkheden

Figure 1 - Power Quality, standards and responsibilities

In some cases, the characteristics of equipment or the electrical infrastructure cause problems with power quality. These problems may be related to the voltage level, harmonic contamination or a bad cos-phi. These problems can lead to failure or malfunction of equipment. On the other hand, standards can also be exceeded. In all these cases, it is necessary to take measures.



The development of electronics in compensation techniques

The price of power electronics has fallen drastically in recent decades. The application of these electronics in, for example, household appliances, lighting, cars and in industry, has therefore increased explosively. The application of these electronics gave developers more freedom, allowed for more compact construction and also saved energy. Where previously electric motors were controlled using a relay or star-delta switch, it is now noticeable that speed controls are applied to every size of motor. From small central heating pumps of 50 W to large drives of up to 20 MW. Also in lighting, the use of high-frequency ballasts or LEDs is replacing the conventional chokes.

Besides the application of electronics, it has also been shown that business processes change more quickly and become more dynamic. Companies and institutions must be able to adapt faster to changing market conditions.

De ontwikkeling van elektronica en compensatietechnieken
Figure 2 - The development of electronics and compensation techniques

From the point of view of power quality, this also means a change. Where previously fixed capacitors could be used to compensate for a constant load and a constant process, it turns out that in many cases thyristor-controlled capacitors do not offer the desired flexibility or speed.

As with the development of electronics, it turns out that it is also beneficial for compensation techniques in terms of flexibility and speed. With the help of semiconductor techniques, the active filter was developed.

The operation of an active dynamic filter

An active dynamic filter (ADF) can be compared to an anti-noise system. The sine wave is scanned at a high frequency. If deviations from the ideal sine wave are found, the active filter reacts. The filter compares the deviation with the ideal sinusoidal waveform (50 Hz fundamental waveform) and injects a countercurrent which is shifted by 180° in relation to the deviation. The result is that at the connection point of the active filter in the installation, the 50 Hz basic form of the current is restored. 

Een actief filter compenseert vervuiling met een “tegenstroom”

Figure 3 - An active filter compensates for pollution with a "counterflow".


Because the current from the active filter to, for example, the transformer does not contain any harmonic contamination, it can also no longer influence the voltage. The voltage therefore remains free of pollution and can no longer cause damage to equipment.

The operation of an active filter
An active filter is a freely programmable power source that can be used in any type of installation. By setting the parameters, it is possible to choose what type of pollution the filter must concentrate on and to what extent it must eliminate it. This is a significant difference to passive compensation systems, such as cos-phi or harmonic compensation using capacitors. With passive compensation systems, the system is dimensioned according to the degree of contamination. If the system is subsequently modified, there is a chance that a passive system could be overloaded.


  • Fully dynamic
  • Not overloadable
  • Very fast reaction time
  • Immune to resonances
  • Automatically adapts to changing conditions
  • Maximum savings potential is exploited
  • All possible PQ problems are eliminated
  • Pauses when load is low
  • Electronics as a weapon

The speed of the filter is in many cases an important factor in choosing an active filter. When a filter reacts too late to, for example, reactive power or harmonic contamination, over- or under-compensation may occur. This often leads to new problems.

Because a filter adapts to changing circumstances, the filter is very effective during its entire life span. As a result, the higher investment costs are generally earned back quickly.

Applications of active filters
Because an active filter is fast and intelligent, it can compensate for many types of mains pollution. This makes it widely applicable and it can be used for many different problems and applications:


  • Data centres
  • Welding machines
  • Lighting
  • Improved installation efficiency through balanced loading

Cos phi
  • Data centres
  • UPS machines
  • Dynamic loads
  • Capacitive loads
  • Solar farms
  • Hospitals
  • Improves cos-phi in dynamic loads
  • Improves capacitive grids
  • Provides stable voltage by fast reactive compensation
  • Improves reliability by reducing voltage dips

The use of active filters in a mini-smart grid

The changing energy market, the incorporation of more electronic components and the intensification of the use of electrical energy will only increase the demand for higher availability of the electrical infrastructure. At the same time, companies need to be able to change configurations more quickly, while the electrical infrastructure may not be suitable or have sufficient capacity to do so.

The above changes within companies mean that there is a great demand for the state of the electrical infrastructure in terms of availability. The ability to grow, shrink, renew and change machines, for example, can only take place if there is sufficient insight. This insight can be provided by placing Power Quality Analysers in the electrical infrastructure. With the help of these analysers, continuous insight can be generated into the actual availability of the installation.

Het mini smart grid is een intelligent net dat zich voortdurend aanpast aan de veranderende omstandigheden
Figure 4 - The mini smart grid is an intelligent grid that continuously adapts to changing circumstances

The use of active filters in a mini-smart grid means that the system automatically adjusts itself to changing conditions. Whether it is a change in power consumption, harmonic pollution, cos-phi, flickering or imbalance, all power quality variables are automatically compensated for.

Active communication

Through active communication between the power analysers and the filter, the capacity of the filter is used where it is needed. This concept can be applied, for example, within companies where production processes change rapidly or where the load of the company is at the limit of the electricity connection. The entire concept is integrated with each other so that meters, software and ADFs work together.

The use of active filters in generator operation

The use of generators reduces the strength of the grid. As a result, polluting loads result in a much poorer voltage quality. This can lead to problems, particularly in situations where a switch is made from grid operation to generator operation (such as in hospitals and data centres). The poorer voltage quality leads to an increase in harmonic pollution, reactive power, zero currents, etc.

Het omschakelen van netbedrijf naar generatorbedrijf leidt tot een verslechtering van de spanningskwaliteit 
Figure 5 - Switching from grid operation to generator operation results in a deterioration of voltage quality

An active filter will automatically adjust the filter current after the system has been switched from mains to generator operation. As a result, the voltage quality is kept more or less constant during the changeover. This means that reliable operation of the installation is guaranteed, even after switching to generator operation.

Active filters of Low Harmonic Drives

A Low Harmonic Drive is a frequency converter and active filter combined in one module. The Low Harmonic Drive continuously regulates the harmonic suppression according to the load and the mains conditions without affecting the connected motor. Low Harmonic Drives are a hot topic in the industry and have the great advantage of an integrated design. However, the use of an active filter separate from the frequency converter has a number of major advantages:

  • If the inverter breaks down, replacing the inverter is cheaper than a Low Harmonic Drive.
  • Stand-alone inverters are much more compact and can be supplied more quickly
  • The system structure of active filters and independent frequency converters is more flexible.
  • Loose frequency drives have a higher system efficiency
  • Low Harmonic Drives generate more current contamination in the higher frequency spectrum

Installation and management of active filters

When applying an active filter, it is important to know the environmental conditions first. It must be known exactly which Power Quality issues are at play. That is why measurements need to be taken first. The specialists at fortop will be happy to help you carry out a Power Quality scan. Based on the measurement data, insight can be gained into the interference present in the grid and where it occurs. The type and extent of the pollution are input for the dimensioning calculation. In addition, the level to which the pollution must be reduced must be known. Should specific standards be met, should only interference-free operation be guaranteed, or should room be created in the capacity of the electrical infrastructure? Based on the answers to these questions, the size and type of the active filter can be determined.

Once it is known what type, where and how many active filters are needed to compensate for the pollution, the planning can be drawn up. This installation work can be carried out by your own in-house installer under the supervision of the specialists from fortop. After the system has been put into operation by the specialists from fortop, a report and control measurement are carried out. With the aid of this report, the operation and result of the ADF is demonstrated and tested.

Maintenance agreement

It is possible to enter into a maintenance agreement with fortop. This means that fortop annually tests and checks the operation of your installation, performs firmware updates and advises you on possible follow-up actions. The step-by-step plan is as follows:

Onderzoek, installatie en gebruik

Delivery program Comsys active filters

The Comsys ADF family of active filters is used in the delivery programme. These active filters are manufactured in Sweden and are among the fastest and most accurate active filters on the market. The wide range and modular design make the system easy and flexible to use.

The modular concept means that each ADF is made up of a standard module (power processors), control computer and housing. If additional compensating power is required, modules can be expanded using the same control computer. This modular system makes it possible to meet the flexibility requirements of the end users. The modular concept allows the customer to expand the ADF from 50 kVAr, for a single AC drive, to about 6 MVAr, for wind and solar farms, for example. Each type of ADF uses the same basic modular technology and is therefore widely applicable.


View active dynamic filters in the webshop

Case study 1: Harmonic compensation

A printing company had regular problems with its lighting. They had to replace the high-frequency ballasts of this lighting once every six months. Someone was working on this two days a week. In addition, the main switches regularly tripped, causing the printing press to break down, with a consequent loss of production. This also caused the control system of the printing presses to malfunction. All this led to considerable financial damage.

In order to find a solution, the quality of the power supply was first measured: the Power Quality Health Check. This investigation showed that the problems were caused by the frequency converters of the printing presses. Based on the measurements and problem analysis, a suitable solution was offered to relieve the customer of its technical problems. The solution consists of six Active Dynamic Filters (ADF). These were placed at four main distribution boxes feeding the printing presses, two cooling machines and the compressed air compressors.

The payback time for this solution will be within two years. The savings consist of avoided costs due to early replacement of electronic components, production interruptions, damage to the control system of the presses, less overtime and reduction of energy costs. The ability to meet delivery obligations also results in higher customer satisfaction.

Case study 2: Resonance problems

In a data centre of an office building in The Hague, there were problems with the UPS and static switch. The equipment failed because the static switch interfered several times a day, causing it to switch frequently. The purpose of this switch is to be able to change the power supply for the IT equipment without interrupting the power supply, so that the equipment does not break down in the event of a failure. However, this switch was not built for so many switching actions and therefore often broke down. The extra costs involved were approximately €45,000 per two years.

Measurements showed that there was a relationship between a resonance in the electrical installation, a tone-frequency signal (for switching the public lighting) from the grid manager and the times when the failure occurred. During the measurement, it was recorded that the disturbances corresponded to the times of the tone frequency signal when a resonance occurred.

Using a special broadband filter from Comsys, the problem was solved. The broadband filter ensures that this customer's network becomes less sensitive to this tone-frequency signal by removing the resonance. As a result, the tone frequency signal still exists, but no longer causes resonance. This filter is capable of eliminating any type of contamination in the 50 to 5000 Hz range and is therefore ideal for resolving resonance problems.

White paper on stress dips - fortop

Read more white papers

Read more about specific substructures in one of the white papers below. On the corresponding page, you can request the white paper free of charge as a PDF.


Go to the complete overview of white papers