John Gallipeau, Technical Marketing Manager, Power Film Capacitors, AVX
The cost of renewable energy technology has significantly decreased since the early 1990s due to continual developments in enabling technologies, and the relatively recent evolution of smart grid technologies has allowed renewable energy sources to legitimately contend with fossil fuels. The primary advantage that fossil fuels like coal, crude oil, and natural gas have over renewable energy sources is the potential energy they store, which allows them to be stored without difficulty and used on demand. Alternately, renewable energy sources like solar and wind power exhibit uncontrolled power output due to changing environmental conditions, and, as a result, provide spontaneous power to the grid, which can cause load balance problems. In response, engineers have developed new smart grid technology, next-generation capacitors, and other advanced energy storage methods designed to help balance renewables’ unstable power outputs and maintain their ability to contend with, and hopefully one day outpace, fossil fuels.
In general, capacitors have the highest power density of comparable technologies. This allows them to charge and discharge faster than batteries and other storage technologies, and makes them effective solutions for instant power, which can be discharged when a grid dips below rated power to provide proper power flow. Power film capacitors are an especially advantageous solution for high voltage power grid applications due to the fact that they exhibit high efficiency, long lifetimes, excellent reliability, limited temperature effects, and a soft end-of-life. They also don’t have any moving parts and, unlike other energy storage devices, require little to no maintenance.
Power Film Capacitors
Polypropylene power film capacitors are frequently employed in high voltage power applications due to their high dielectric strength, low volumetric mass, and extremely low dielectric constant (tanδ). These capacitors also experience low losses and, depending on application demands, can be made with either smooth or hazy surfaces — the latter of which is favorable for oil impregnation.
Power film capacitors can be made from wet, oil-impregnated aluminum metallized film, from dry, segmented film, and from dry, segmented film impregnated with no-free-oil silicone. Oil-impregnated power film capacitors are generally used for discharge applications, high voltage DC or AC filtering, and power factor correction. Dry, segmented film capacitors are employed in a very wide range of applications, including, but not limited to: medium power, snubber, AC and DC filtering, induction heating, surface mount device (SMD), and EMI applications, while dry, segmented film capacitors that are imbued with no-free-oil silicone are generally used to close the gap between low voltage and high voltage products. Although, they can also be made for use in dry high-voltage applications.
New hybrid and high crystalline dielectric film technologies have allowed engineers to develop new non-polar polypropylene film capacitors that exhibit a higher temperature range and higher current handling capabilities than both previous generations of this same technology and competing technologies, like aluminum electrolytics. These features allow them to provide high-reliability signal protection and filtering in a variety of applications within the renewableenergy and smart grid electronics markets. These new power film capacitors also effectively handle both AC and DC voltagesand, by virtue of employing thinner films, exhibit higher volumetric efficiency. However, achieving this higher volumetric efficiency generally comes at a cost of limited current handling capabilities.
To surmount these limitations, engineers have developed thin film capacitors with two film bobbin elements in parallel. The total of four terminals that this design achieves effectively enables higher current handling while simultaneously retaining the part’s high volumetric efficiency. These capacitors can also be arranged in a series to provide higher voltage. These and other recent advancements in power film dielectric technologies have allowed capacitors to not only keep pace with the rapidly evolving power electronics market, but to continue enabling new, next-generation technologies like alternative energy and smart grid solutions.
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Figure 1: AVX’s six-terminal FFLC Series power film capacitor is ideal for DC filtering in wind turbines.
In the past, both wet and dry oil-impregnated power film capacitors utilized hazy, high crystalline polypropylene dielectric impregnated with vegetable oil. Newer power film capacitors typically utilize paper or aluminum film foil, and generally only impregnate the dielectric with mineral oil if it’s intended for use in high-power applications. Vegetable oil has good thermal and dielectric properties, is environmentally friendly, and, when combined with hazy polypropylene film, effectively disperses between the film layers to mitigate arcing. Mineral oil is less environmentally friendly, and has less effective arc extinguishing capabilities when used with foil technology, but can be useful with paper films. Some newer, more advanced power film capacitors also utilize no-free-oil technology to achieve the thermal and dielectric benefits that wet oil-impregnated capacitors provide. These are especially useful in high power applications that prohibit the use of oil-impregnated capacitors due to safety regulations, as they have zero risk of explosion and are thus a safe alternative to wet power film capacitors.
Recently, thinner polypropylene (PP) films have been combined with complex metallization patterns to enable smaller packages especially designed for use in today’s ever more compact power electronics. Smaller package sizes have been trending for years, but are becoming even more commonplace due to the mass production and employment of wide bandgap (WBG) technologies designed for the power electronics market. WBG technology allows for higher switching frequencies, voltages, and temperatures, but, in doing so, creates demand for smaller, more complex components.
New film power capacitors tend to utilize thinner layers of metallized film than their predecessors to help meet the reduced size and weight demands that affect nearly every aspect of the electronic design industry. These layers now typically measure only 2 to 5µm in thickness, which physically limits their current-carrying capabilities. One way design engineers can surmount this limitation is to use wider than average thin films, as these can help increase both current carrying capacitance capabilities by virtue of having a larger surface area (C = eA/d). The use of various metallization processes, including double-edge metallization, has also enabled higher current carrying capabilities in new power film capacitor designs.
The high electrical currents that run through a capacitor play a significant role in its lifetime. Higher rms current can increase hotspot temperatures in a capacitor, which can damage it and decrease its overall lifetime. Hotspot temperature can be calculated with the equation Øamb +(Pj + Pd )XRth in which Pj = RsXI2 / rms and Pd = (I2rms/C*2*π*f) xtgδ0. Depending on the demands of the particular application at hand, solutions for achieving longer lifetimes can include the implementation of a cooling system or a capacitor with higher ratings.
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Figure 2: AVX’s FFLI HV Series no-free-oil capacitors provide a dry solution for high voltage applications up to 3.8kVdc.
Additional design elements that can affect the performance of power film capacitors in high voltage applications with extreme conditions include: mounting conditions, how much (if any) surface area will be exposed to forced air, the level of mechanical stress and/or vibration it will have to endure, and how long it will need to withstand spike currents or voltages.
Handling High Hot Spot Temperatures
Power capacitors are a defined function of three primary correlated parameters: hot spot temperature, working voltage gradient, and lifetime expectancy. In order to increase energy density, adjustments must be made to the film material, metallization, and controlled self-healing behavior, and all while taking these several parameters into account. Resultant developments will be a correlation of the capacitor’s dielectric nature, metallization, and segmentation.
If an engineer’s target is to increase epsilon g² (in which epsilon = dielectric constant and g = voltage gradient), the volume of the capacitor will be reverse proportional to the square of the voltage gradient.
To improve a power film capacitor design’s film material, engineers should select a high crystalline polypropylene designed to exhibit fewer amorphous phases than alternative options. These films enable capacitor operation at temperatures up to 105°C for long-life applications and up to 115°C for applications that don’t require especially lengthy lifetimes. To improve a power film capacitor design’s metallization and segmentation, engineers should aim to avoid generating an avalanche effect. This can be achieved by managing a safety area in term of I²t when self-healing occurs.
Available in a wide range of configurations and performance specifications, power film capacitors provide safer solutions than aluminum electrolytics, which have a limited voltage range and a high risk of leakage, and several other technologies that are physically unable to safely and effectively handle high voltage and high current at useful capacitance values. Table 1 provides a summary of how power film capacitors compare to aluminum electrolytic capacitors.
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Table 1: Performance comparison of power film capacitors vs. aluminum electrolytic capacitors
Renewable Energy Applications
Since varying voltage and current levels are typical of most alternative energy sources, power film capacitors are frequently employed in power grids to filter and smooth the unwanted current and voltage noise created by these imbalanced loads. Often used to decouple the AC-DC or DC-DC converters in such power systems, power film capacitors effectively prevent variable power outlets from damaging digital systems in alternative energy and smart grid applications, help maintain the overall continuity of the system, and help to prevent other components from damage resulting from current or voltage spikes.
Power film capacitors are extremely useful in high-voltage AC power grid applications due to the fact that they provide reactive power to the grid. This is important because many other components in these systems, including motors, converters, and power lines, consume reactive power, which makes the overall system less efficient. Power film capacitors can also correct out-of-phase current and voltage, which improves system efficiency by preventing motors, generators, and other connected devices from having to compensate for the lags caused by reactive power.
Additionally, in high-voltage DC power grid applications, power film capacitors can be configured in parallel to achieve energy storage levels of high capacitance, which, in the event of an occurrence like a short in DC output energy, can help stabilize a system’s voltage level by quickly discharging to compensate for the variance. Power film capacitors are also often employed in both DC-AC and DC-DC converters and motor drives.
Advanced power film capacitors are a critical enabling technology that actively supports the continual evolution of high-voltage renewable energy and smart grid technologies. Their ability to deliver high efficiency, long lifetimes, excellent reliability, limited temperature effects, the highest power density of comparable technologies, and soft end-of-life performance — and all while requiring minimal to no maintenance — makes them an ideal solution for balancing the unstable power outputs from renewable energy sources, which can significantly improve the overall efficiency of power systems and, subsequently, help to hasten both market adoption and the eventual triumph of alternative energy sources like solar and wind power over fossil fuels.