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Posted on 01 March 2019

Film Technology has Wind in its Sails

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Film capacitors overcome internal defects

Aluminium electrolytic capacitors present serious deficiencies for the higher voltages required in the very latest wind power applications. In contrast, film technology offers significantly improved life expectancy, environmental performance and power handling.

By Gilles Terzulli and Craig Hunter, AVX

 

Wind power is a fast growing market around the world. Higher prices for fossil fuels and concerns over environmental impact have been two of the main driving factors. Further to this, improved efficiency of windmill power generation is also developing at a rapid pace. One element of this is higher voltages in generator systems, where the capacitors are actually located inside the converter.

Below is a typical circuit diagram showing the location of the DC filtering capacitors, used in a windmill application.

The location of the DC filtering capacitors

Historically many wind power systems have used capacitors with voltages around 500VDC, but today the “sweetspot” is in the voltage range 600VDC to 1350VDC depending on the output ac voltage from the alternator. This is because higher voltages are increasingly used to reduce power loss in the alternator and converter of the windmill. Higher voltages allow lower rms current for the same power. In this area, non-gas impregnated film capacitors offer significant technical advantages over previously used electrolytic capacitors.

DC filtering capacitor function

The DC filtering function smoothes voltage wave form and limits the magnitude of ripple voltage.

DC filtering function smoothes voltage wave form

AVX power film capacitors are able to provide a real boost to wind power stations, with extremely high capacitance values of up to 48,000 μF available.

Controlled self-healing

One major advantage is the ability of film capacitors to overcome internal defects. The latest dielectric films used for DC filter capacitors are coated with a very thin metallic layer. In the case of any defect, the metal evaporates and therefore isolates the defect, effectively self-healing the capacitor. As wind power systems are normally located in remote locations, this feature can significantly reduce ongoing maintenance costs and ensure higher efficiency of usage in the installed system.

Technology comparison Film vs. Aluminum

With today’s dry film technology the gradient of voltage can reach more than 500V/um for discharge applications and 250V/um for DC filtering applications. These film capacitors are designed to withstand CEI 1071 standards. This means they are able to handle multiple voltage surges of up to twice the rated voltage, without significantly decreasing product lifetime. It also means the designer need only account for nominal voltage requirements when specifying his system.

By comparison, due to the process technology, the thickness of aluminium foil used in electrolytic capacitors is key to reaching higher voltages. However, there is a trade-off: the higher the voltage, the lower the available capacitance. In addition, higher voltage (500V) electrolyte conductivity reaches 5kohms/cm compared to around 150kohms/cm for lower voltage versions. This also limits rms current values to about 20mA per µF, compared to 1A per µF for film capacitors. A major requirement for DC link capacitors is its ability to handle ripple current. Here film capacitors have a major advantage. Using aluminium electrolytics would require banks of several capacitors being used. Not because the capacitance value is required, but simply to handle the current. Using film capacitors would mean the designer need only consider the minimum capacitance value required for his system. As a result, designs which use film technology frequently save space.

In order to reach the necessary higher voltages of the systems being designed and deployed today, it would be necessary to connect multiple electrolytic capacitors in series. It would then also be necessary to balance the voltage. This would require connecting a resistor to each capacitor because the insulation resistance of each individual device will vary.

Another concern in using electrolytic capacitors would be that if a reverse voltage or over voltage higher than 1.5 times rated voltage occurs, it would cause a chemical reaction. Should it last long enough, the capacitor may suffer complete failure. This would be where the capacitor might explode or suffer a pressure release where the electrolyte may evaporate. To overcome this, the system designer would need to connect a diode in parallel to erase the potential problem.

This means that, although it is technically possible for aluminium electrolytics connected in series to attain the necessary higher voltage levels required by today’s wind power applications, there are some very important deficiencies which would require the use of additional components or require careful design to ensure they operate successfully.

Another issue is that of surge voltage. The capability of aluminium electrolytics to withstand surge voltages is limited to approximately 1.2 times the nominal voltage. This means designers must take surge voltage into account when specifying these types of capacitors.

Higher voltage film capacitors (above 1200V) utilize non-toxic organic oil filled technology and can operate up to 100kV. These capacitors along with the dry film technology discussed earlier can be considered environmentally friendly solutions, because they do not use acids and therefore do not represent a risk to the system itself.

Main advantage of film capacitors is life expectancy

Both types of film capacitors can also be stored without concern, as unlike electrolytic capacitors, they have no “dry out” / wear-out mechanism.

Life expectancy

However, the main advantage of film capacitors is life expectancy. Our internal data shows that AVX controlled self-healing DC filtering capacitors exhibit a capacitance fall of a maximum of just 2% after 100,000 hours operation. When added to the fact that compared to aluminium electrolytics, complete device failure is very unlikely to occur. This means that during the full lifetime of an installed wind power system it is not necessary to change the capacitors. This represents a major maintenance saving for the user.

Film capacitor developments

Since the early 80s, significant improvements have been made in the application performance of DC filter capacitors. This has been achieved through the use of either combinations of metallized films or by using different segmentations of metallization on the dielectric films. In fact, power film capacitor manufacturers continue to develop much thinner films and improved segmentation techniques, which will result in the continued release of superior performance devices.

FFLC series film capacitor for DC filtering.

Summary

System voltages are continuing to increase in wind power/windmill applications. As these voltage requirements have risen they have passed the 600V barrier which represents a major hurdle for aluminium electrolytics. These are limited in voltage and require connection in series to successfully address this application, which can add significant cost in terms of space, as well as being much more complex to design and install.

Film capacitors (both dry and non-toxic organic oil-filled), offer significant technological advantages, including superior life expectancy and environmental performance as well as the ability to handle the various types of “in-application” technical issues (over-voltage and reverse voltage) which can easily occur.

When considering the fact that these systems are often deployed in remote locations and would require the minimum amount of maintenance and down-time, the advantages of using of film capacitors in these kinds of DC filtering applications appears to be overwhelming.

 

 

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