The Importance of Passive Components in Today’s Power Applications: Renewable Energy and Smart Grid Technology
Continual developments in renewable energy technology have caused the cost of renewables to decrease significantly since the early 1990s, and the introduction and evolution of smart grid technologies has allowed renewables to legitimately contend with fossil fuels. The one major advantage that fossil fuels like coal, crude oil, and natural gas have over renewable energy sources is the potential energy they store, which allows fossil fuels to be stored without difficulty and used on demand.
Continual developments in renewable energy technology have caused the cost of renewables to decrease significantly since the early 1990s, and the introduction and evolution of smart grid technologies has allowed renewables to legitimately contend with fossil fuels. The one major advantage that fossil fuels like coal, crude oil, and natural gas have over renewable energy sources is the potential energy they store, which allows fossil fuels to be stored without difficulty and used on demand. Alternately, since renewables like solar and wind power — the two fastest growing renewable energy sources — exhibit uncontrolled power output due to changing sunlight and wind conditions, they provide spontaneous power to the grid, which can cause load balance problems if they’re relied on like fossil fuel power. As such, engineers have developed new smart grid technology, advanced capacitors, and other energy storage methods designed to help balance the unstable power outputs from renewable energy 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, which allows them to charge and discharge faster than batteries and other storage technologies, and makes them good solutions for “instant” power. This so-called “instant” power can be discharged when a grid dips below rated power in order to maintain proper power flow. Power film capacitors are a particularly advantageous solution for high voltage power grid applications, as they exhibit high efficiency, long lifetimes, excellent reliability, limited temperature effects, and a soft end-of-life, have no moving parts, and, unlike other energy storage devices, require little to no maintenance.
Power Film Capacitors
Of all the power films capacitors used in high voltage power applications, polypropylene film tends to be most common due to their high dielectric strength, low volumetric mass, and extremely low dielectric constant (tanδ). Polypropylene film capacitors also experience low losses, and can be made with either smooth or hazy surfaces (the latter of which is favorable for oil impregnation) depending on the application. For example, AVX uses three primary technologies for their power film capacitors: one wet, oil-impregnated aluminum metallization film, and two dry, segmented film and no-free-oil silicone film impregnation.
Oil-impregnated power film capacitors are usually designed for discharge applications, high voltage DC or AC filtering, and power factor correction. Segmented film is designed for medium power, snubber, AC and DC filtering, induction heating, surface mounted device (SMD), and EMI applications, amongst many others; and no-free-oil film impregnated capacitors are used to close the gap between low voltage and high voltage products, but can also be made for use in dry high voltage applications.
Advancements in Power Film Capacitor Technology
New, hybrid and high crystalline dielectric, non-polar polypropylene power film capacitors exhibit a higher temperature range and higher current handling capabilities with new dielectric film technologies than both previous generations and competing technologies (e.g., aluminum electrolytics), providing high reliability, signal protection, and filtering in a variety of applications within the renewable energy and smart grid electronics market. These power film capacitors also effectively handle both AC and DC voltages and, as a result of thinner films, exhibit higher volumetric efficiency; although, the latter comes at a cost of limited current handling capabilities. To surmount these limitations, thin films engineers have designed capacitors with two film bobbin elements in parallel, as the total of four terminals effectively allows for higher current handling. In addition, these capacitors can be arranged in a series to provide a higher overall voltage level. Overall, though, these recent advancements in power film dielectrics have enabled capacitors to not only keep pace with, but to continue enabling the ever-changing power electronics market.
Figure 1: AVX’s four-terminal TRAFIM capacitor features four terminals and a proprietary internal design to allow for higher current handling
With regard to oil-impregnated power film capacitors, both wet and dry versions used to feature a high crystalline hazy polypropylene dielectric impregnated with vegetable oil, but now typically feature paper or aluminum film foil impregnated with mineral oil if intended for use in high power applications. Vegetable oil has good thermal and dielectric properties, and is also environmentally friendly. Additionally, when combined with a hazy polypropylene film, vegetable oil effectively disperses between the film layers, which better extinguishes arcing. Alternately, mineral oil is less environmentally friendly, and provides less effective extinguishing capabilities when used with foil technology. Moreover, advanced no-free-oil capacitors can be used 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.
Power Film Capacitor Design Limitations
The decreased thickness of the layers of metallized film in newer, thinner film power capacitors is the reason behind their limited current-carrying capabilities; as layers that typically measure only 0.02 to 0.05µm physically limit the current in these layers. However, the use of wider thin film can increase the current carrying capabilities of these capacitors, as well as increase their capacitance due to the larger surface area (C=ϵA/d). The use of various metallization processes, like double edge metallization, has also enabled higher current carrying capabilities for thin film power capacitors.
Additionally, high electrical currents running through a capacitor play a significant role in its lifetime, as higher current can increase the hotspot temperatures in the capacitor, damaging it and decreasing its overall lifetime. Hotspot temperature can be calculated with the equation θ_HS=θ_amb+(P_j+P_d)×R_th, in which P_j=Rs×I_rms^2 and P_d=((I_rms^2)/(C*2*π*f))×tgδ0. Depending on the needs of the application, solutions for achieving longer lifetimes include the implementation of a cooling system or a higher rated capacitor.
Figure 2: AVX’s FFHV Series no-free-oil capacitors provide a dry solution for high voltage applications in which impregnated capacitors are restricted.
Other design aspects to consider 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, what mechanical stresses and/or vibrations it will be exposed to, and how long it will need to endure spike currents or voltages occur for.
Power Film Capacitor Benefits
Power film capacitors are available in a wide range of specifications and provide a safer solution than aluminum electrolytics, which have a limited voltage range and a high risk of explosion, and a number of other technologies that are unable to safely and effectively handle high voltage and high current at feasible capacitances due to their physical limitations. Table one provides a summary of how power film capacitors compare to aluminum electrolytic capacitors.
Table 1: Performance comparison of power film capacitors vs. aluminum electrolytic capacitors
Power Film Capacitor Applications
Power film capacitors are employed in power grids to filter and smooth the unwanted current and voltage noise created by the varying voltage and current levels typical of most alternative energy sources in order to maintain continuity and prevent other components from damage. Advanced film capacitors effectively prevent variable power outlets from damaging digital systems in alternative energy and smart grid applications by decoupling the AC-DC or DC-DC converters in the power systems.
Power film capacitors are critical in high voltage AC power grid applications, as they provide reactive power to the grid, which is important because many other components in these systems — such as motors, converters, and power lines — consume reactive power, which makes the system less efficient. Power film capacitors also correct out-of-phase current and voltage, which prevents motors and generators from having to compensate for the lags caused by reactive power and effectively improves system efficiency.
In high voltage DC power grid applications, power film capacitors are configured in parallel to achieve high capacitance that can be used as energy storage to stabilize the system’s voltage level. For example, a short in the DC output energy could quickly be compensated for via capacitor discharge in order to maintain the system’s voltage level. Other DC applications that power power film capacitors are frequently employed in include both DC-AC and DC-DC converters and motor drives.
Summary
In sum, advanced power film capacitors are a critical component for the continual evolution of high voltage renewable energy sources and smart grid technologies due to the fact that they exhibit high efficiency, long lifetimes, excellent reliability, limited temperature effects, and a soft end-of-life, in addition to the ability to deliver highest power density of comparable technologies and balance the unstable power outputs from renewable energy — all with minimal to no maintenance — which improves overall system efficiency and, subsequently, market adoption and the eventual triumph of alternative energy over fossil fuels.
