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Film Technology Extends Lifetime In Green Energy Applications

Wed, 02/02/2011 - 9:08am
Chris Reynolds, AVX Corporation, www.avxcorp.com
AVX's capacitor product imageAs worldwide power consumption continues to increase hand-in-hand with directives to reduce dependence on fossil energy, the need to accelerate the adoption of clean, renewable energy solutions is becoming more vital.

Two major areas of renewable energy generation are solar and wind, while EV / HEV (electric and hybrid electric vehicle) development is at the forefront of reducing dependence on fossil fuel.

Another significant area to reducing fossil fuel consumption is increasing energy efficiency in the industrial sector. Variable frequency drives (VFDs) can now reduce energy consumption substantially versus “standard” AC drives. Bearing in mind that nearly a third of all US energy is consumed by motors for blowers, compressors, fans and pumps, VFD efficiency is having a major impact in these applications versus “standard” AC drives.

If we first look at wind energy, manufacturers, such as AVX, have developed advanced ‘controlled self healing’ DC power capacitors used in inverters for these applications, achieving long lifetimes due to the unique technology of the metallized dielectric film. This film technology combines high rms current capability with high reliability and fail-safe operation, especially in harsh environments that experience a wide range of temperature, humidity and transients (from lightning strike to grid).

Solar energy growth may be even greater than wind. Given their track record in wind applications, film capacitors have been established as the most reliable and safe solution for the DC bus in inverters for solar energy manufacturers.

AVX-Figure1

Adjustable speed drives (ASDs) use speed control instead of throttling to control power output at maximum efficiency. VFDs are AFDs that use an electronic power converter to convert constant frequency AC power into a variable frequency output, so that the AC motor speed varies in proportion to the drive output frequency.

In VFDs, as the inverter IGBTs move to higher switching frequencies, the low loss factor, low ESR and low ESL characteristics of polypropylene film dielectric make the capacitor technology ideally suited for the pre-charge capacitor bank for ripple smoothing between the DC bus and inverter.

In all the above cases, one common way to further boost efficiency is to migrate to higher system voltages. The simple equation below shows the relation between voltage and power, where:

P = VI

P = power, Watts
V = voltage
I = current, Amps

Increasing the voltage allows decreasing the current for the same power output.

Taking wind power as an example, DC power film capacitors are increasingly used to reduce power loss in the alternator and converter of the wind turbine.
In this application, the capacitors are located inside the converter.

Wind-power generators once used capacitors with a working voltage of about 650 VDC, but today’s “sweet spot” ranges from 800 V to 1,350 VDC (and even higher with direct power conversion), depending on the alternator’s AC voltage output. Higher voltages yield lower rms current for the same power. In this area, dry metallized film and non-gas impregnated film capacitors offer improved ripple and longer lifetime than aluminium electrolytic technology.

AVX's FFVE Series Power Film CapacitorIn all the applications discussed, the common advantage for film capacitors is their ability to overcome internal defects.

The dielectric films used for DC filter capacitors are coated with a patterned, thin metallic layer, referred to as “segmentation”. Should a defect occur, the metal will evaporate locally and, depending on pattern design, isolate the defect, effectively self-healing the capacitor. Because wind and solar power systems are normally located in remote locations, and industrial systems have to maintain high throughput rates, this feature can significantly reduce maintenance costs and ensure higher efficiency for the installed system.

Segmentation design is critical to the lifetime and performance of these advanced systems. Over the past 30 years, the technology has been proven in a wide range of applications, from automotive to alternate energy, traction and industrial. The main advantages of segmented technology include: 
• Proven field reliability with zero catastrophic failures even under severe usage; 
• A design that maintains the highest electrical and mechanical specifications; 
• Long lifetime expectancy.

Since 1980, significant improvements have been made to DC filter capacitor technology using a combination of metallized plastic films and different segmentations of the metallization on those film dielectrics. Film manufacturers have continued to develop thinner films with improved segmentation techniques used on the metallization, which has reduced the volume and weight by a factor of three or four. In addition, these advancements have greatly improved the performance of such capacitors, making film capacitors a reliable, safe and energy-efficient solution.
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