Active Harmonic Filters – Buyer’s Guide

Active Harmonic Filters are growing in popularity as a method to mitigate power quality issues. There are several factors to consider when specifying an active harmonic filter. Typical applications for active filters are compensation of variable frequency drives and data-centers to reduce the load on UPS systems or compensating the effects of renewable energy sources on the grid.Active Harmonic Filter

What is an Active Harmonic Filter and what is its Application.

The general definition to describe this application is an analog or digital device that measures the power quality on the grid side. It then injects current to compensate any unwanted deviations from the standard 50 or 60 Hz supply. Deviations can be mitigated in full or partially.

What Factors to Consider when Specifying an Active Harmonic Filter

Sensor or sensorless control

There are suppliers that provide sensorless control eliminating the need for current transformers. This solution reduce the installation cost. Sensorless is not used in all applications so make sure to check the application with the supplier.

Losses

Depending on design, the filter has higher or lower losses. Check the losses as this will reduce the Life Cycle Cost on your investment. Some active filters have up to 1%-point lower losses, which depending on your user profile, means a potential for  considerable financial savings if calculated LCC over 5 years.

Harmonic Compensation Capacity

Harmonics are normally seen in the odd. Common capacity for active filters is 25th or 50th harmonic. Sometimes there is a claim of being able to mitigate the 51st harmonic, which has little value as harmonic order of 51 and above are normally not important.

Harmonics above the 50th are more difficult to measure as there are few PQ-meters that can handle such orders. There are however quite common sources such as Active Front End Drives that cause switching ripple above 3kHz, above the 60th harmonic (or above the 50th in 60 Hz systems).

There are a few Active Harmonic Filters capable of compensating such frequencies. Choose a filter according to the needs specified by your measurements.

A filter’s capacity to compensate a certain harmonic order is only part of the story. Another important factor is de-rating, discussed below.

Reponse Speed

Some power quality phenomena occur extremely fast requiring the mitigation to be even faster.  If your process is affected by fast flicker or transients, take special care to evaluate the response capacity of the filter. Flicker is a specific phenomena that normally requires special software to compensate flicker in a controlled environment.

Interharmonics

Interharmonics is commonly caused by syncronisation issues. If  your installation includes such interharmonic sources, the type of active filter changes and the vendor has to be consulted. This is a common issue on some types of older wind turbines.

EMC

In Europe there are strict guidelines regarding EMC. If you want to be sure that the active filter does not interfere, the filter must be fitted with a properly tuned EMC-filter.

De-rating

An Active Harmonic Filter’s rating is normally defined at nominal load, meaning at 50/60Hz. As the filter works further up the harmonics its capacity compared to nominal starts to de-rate. The de-rating curve is documented by all serious suppliers and should be available if you ask them.

A de-rating of 50%, at say the 13th harmonic, means that a 100A filter only has the capacity to compensate 50A at the 13th.  Naturally if you have harmonics of higher order it becomes more important to check the de-rating.

De-rating is a matter of how robustly the filter is designed. Some suppliers offer zero de-rating up to the 7th before capacity starts to fall.

Physical Footprint – How Much Cabinet Space is Required?

Most active filter suppliers offer several alternatives regarding installation. Wall mount, Cabinet and IP00 modules to install in cabinets. Efficient use of cabinet space translates to lower system cost. Some filters have a modular design and can be enhanced with further capacity without adding to the footprint.

Modularity

As mentioned, a modular design of your Active Harmonic Filter enables you to adapt the filter to potential changes in your future power compensation needs. The modular design means that you can easily add to the filter’s capacity within the existing cabinet, saving both cost and space.

Commissioning Software

Does the filter have built in commissioning software?  Commissioning and service of Active Filters can be quite time consuming. Ask for a review of the support software included in the machine. Some suppliers have an extra charge for the necessary software. Minimum required functionality should be that the system performs a self-check of Voltage and CT phase order, CT polarity check, self-diagnosis, and self-calibration. Such features will quickly find installation errors before they can cause problems and will also shorten the needed commissioning time.

If the filter does not have this type of support software the commissioning becomes much more complex and might even require external support adding to the system cost.

HMI

There are different HMI setups. Some have a very simple front HMI while others include graphs showing the current and voltage waveforms and many further functions. A great added value is to have at least a web-based interface allowing in-depth monitoring and control functionality. Then no extra PQ-meter is necessary.

Smart Grid Functionality

Active filters have a built in rudimentary power and power quality meter to calculate the required compensation. Some filter manufacturers make use of this fact and enable the user to connect all filters on site and company wide through a web based architecture.  An operator can then have an overview of the status of all connected cabinets and log them. This enables the possibility to log events that could or should have caused production disturbances, status monitoring of individual filters as well as remote control capability.  Email and text alerts to dedicated service personnel from the filter reduce response time dramatically.

IP/NEMA Class and Water Cooling

Water-cooled Active Filters enable very good cooling of the IGBTs, the most critical component in the Active Filter. Water-cooling reduces overheating immensely, which increases availability in the same way as for Variable Frequency Drives. The power density of the installation is also improved.

Voltage

Active Harmonic Filters are offered in a range of voltages.  Most common ranges are 380–415V, up to 480 V. Higher voltages up to 600 and 690V are also available without step-up transformer, reducing foot print. Some suppliers have the capacity to supply MV ratings as well, normally using a step-up transformer.

New Generation Passive Harmonic Filters – Ecosine Evo

Modular Passive Harmonic Filters from Schaffner – Ecosine Evo – the New Generation

The new PHF generation “ecosine evo” is designed for the most demanding harmonic mitigation tasks with front-end 6-pulse rectifiers.

Even partial load situations causing harmonic currents can be effectively mitigated. The introduction of a modular system allows the Ecosine Evo passive harmonic filter to deliver optimally tailored solutions for your particular installation. The solution is easily adapted by simply plugging in the suitable module where needed. For more information, watch this:

Active + Passive = Hybrid

Active Harmonic Filters are becoming cheaper and very competitive compared to other active mitigation solutions such as Active Front End, which we explained here.  In some applications that are not too dynamic, a passive harmonic filter makes perfect sense to reduce the investment. A combination of active and passive filters can be the best solution to reduce the investment cost while still being able to cope with dynamic loads. In such an application the passive harmonic filter focuses on the dominant harmonic component.  This solution is currently used by for example the German auto industry in their production lines.

PQ Nosswitz, a German power quality solutions firm, devised a system to allow a flexible combination of active harmonic filters and passive harmonic filters to enable the most flexible and cost efficient solution for every project.

 

Active Harmonic Filters Increase Productivity in the Paper Industry

The paper industry is known for its high availability targets and
multiple variable frequency drive applications. Enclosed is a case study
from Schaffner showing the advantages of applying an active harmonic
filter solution to reduce the grid harmonics. As the active filters are
in parallel there is no risk of stopping the machine if the filter fails.

Active Harmonic Filter Application in Ba Na Hills, Vietnam

Here is a great showcase of the effects of applying active harmonic filters on a cable car installation made by Power More in Vietnam. Ba Na Hills Mountain Resort are holders of three Guiness Records – Longest single rope cable car system, Longest distance between stations and Heaviest cable roll. For Ba Na Hills, power quality is a question of safety and service quality. With total harmonic distortion 5 times higher than the national regulated level, Ba Na Hills were facing several problems.

  • Station to station communcation was interrupted
  • Power factor penalties
  • Damaged PFC capacitor units
  • Inerference with PFC controller
  • Reduced motor effeciency in generator mode to 50%

Applying ADF Power Tuning active filters from Comsys to battle both power factor correction and harmonic filtering resulted in:

  • Eliminated risk of PFC failures
  • Eliminated power factor penalties
  • Increased motor efficiency to 100%
  • Improved cable speed

Active Filtering Just Got Simpler

Active Filtering just got simpler with Sensorless Control

Comsys has released a new solution for controlling the Active Harmonic
Filter. The sensorless active filter control reduces the cost and
complexity of the system as it does not require any current transformer
sensors.

In this case study they describe the retrofit on a small diesel
electric powered oil tanker, Fox Luna. The sensorless active filtering
approach is well equipped for protecting sensitive loads from dirty grids.

Sensorless Control Case Study

Schaffner use Active Harmonic Filters to Clean up Conference Center

In this case study of the Luzern Conference Center in Switzerland, Schaffner demonstrate the full utility of the active harmonic filter application. The active filters are distributed throughout the building to compensate not only harmonics but also power factor correction as well as load balancing.

Leave it to the Swiss to adapt to multifunctional tools.

Schaffner_AE_Ecosine_active_KKL_Lucerne

Vacon Launches NXC Low Harmonic Drive Based on Active Harmonic Filter Technology

Vacon Low Harmonic NXCVacon joins Danfoss as the second tier 1 drives manufacturer to integrate an active harmonic filter with a drive to lower harmonics. Vacons stand-alone NXC range previously offered 12 pulse as well as active front end drive solutions.

The Low Harmonic NXC was recently shown at a large exhibition in Sweden and is reportedly offered on a project by project basis but will become a standard offering in the near future. The system shown, verifies the smaller foot print and weight of the Active Filter drive system compared to the more common Active Front End drives. The active filter NXC also has a lower power loss, which is becoming more important as the EN 50598-2 standard is introduced.