Power Capacitors For Wind Turbines

By Josef Reindl, Vishay ESTA; Theo van de Steeg, Vishay BCcomponents; Frans van Roemburg, Vishay BCcomponents

The adoption of renewable energy sources, such as the use of wind turbines, is advancing quickly as increases in oil prices are establishing price parity between power from renewables and fossil fuels. Additionally, there is a fast-growing, world-wide demand for energy, particularly in rapidly developing nations such as China and India. This demand is coinciding with the maturing of energy conversion technologies, which can now generate practical quantities of electrical power from the wind—in the range of several Megawatts per turbine. The final pieces of the jigsaw puzzle are now falling into place. These pieces include techniques to ensure adequate power quality to drive domestic or industrial loads, Power Factor Correction (PFC) to avoid unnecessary reactive power loading of the transmission lines and network transformers, and increased efficiency of the power generation and transmission process.

Wind Turbine Configurations
A number of generator technologies, which lend themselves to particular modes of use, are currently operational. Large induction generators, for example, require no maintenance over long periods because they use no brushes. This makes them suitable for use in inaccessible locations, such as offshore wind farms. Power factor correction by separate capacitor banks is also essential to ensure best case unity power factor when connecting to the grid. Using synchronous generators is another approach to converting the raw electricity generated at rotor speed to mains frequency, and to feed the rotor by DC link with large aluminum electrolytic capacitors, to smooth the DClink voltages of the frequency converter. These typically have high voltage and current ratings, and therefore are physically large. They also have high ripple current capabilities in order to withstand load differences and perform reliably over a long lifetime in service. Capacitors such as the Vishay BCcomponents 102 series have been used successfully in such applications, assembled in capacitor bank configurations that frequently contain more than 100 capacitors in total. Experience has shown that all the capacitors within such a bank must display a similar rate of change in parameters over their lifetimes. Designers need to consider these capacitor banks as a whole, and ensure that all components are as similar as possible. Wind turbines for use with such a power electronic converter are typically built without a gearbox, yielding a cost saving. However, in order to compensate for the sometimes large fluctuations in wind speed, the pitch of the turbine rotors is adjustable. Further motion controls, which make use of smaller aluminum capacitors for smoothing of power delivered to the pitch control rotors, are implemented to provide this.

Capacitor Design and Construction for Power Factor Correction:
The requirements applicable to PFC circuits for use with wind turbines, based on the induction generator architecture— particularly those arising from wind speed fluctuations, heat build-up, overvoltage tolerance, and self-healing— demand capacitors that display a particular combination of characteristics.

Inrush Control Techniques:
Continuously changing rotor speed means that capacitors are switched in and out of circuit much more frequently than in any conventional switched capacitor bank. Coiled wire inductors, the traditional established inrush control technology, generate excessive heat under these conditions. Integrated pre-resistors on the contactors, specially developed to suit capacitor switching, represent a better solution.

Package Design:
Devices designed for wind turbine applications must ensure optimal can dimensions to maximize the ratio of surface area to volume, in order to maximize cooling. The arrangements of the winding elements play an important role in determining the shape of the can. Materials for filling the can are also of importance, in order to maximize the efficiency of heat transfer. Inert gas has become a popular choice, but the biodegradable, vegetable-based oil used in standard Vishay ESTA capacitors (designed specifically for PFC applications) displays around seven times better thermal conductivity. Because of their compact and slim design, ESTA capacitors also provide the best heat dissipation in the category of dry capacitors in gas design. These measures to maximize heat transfer help typical life expectancy to exceed 150,000 hours, depending on how the ambient temperature and operating conditions influence the capacitor case temperature.

Vishay ESTA PFC capacitors have already shown their long term reliability in the wind turbine industry, in oil and dry design as well, for many years. Countermeasures to Internal Fault and Overpressure Unpredictable transients and harmonics, resulting from the motion of the rotor and resonance phenomena, may lead to repeated overvoltages and continuous self-healing processes, which will also increase the internal pressure within the capacitor. Techniques to combat this include allphase overpressure tear off fuse systems, which have performed well in many applications for a number of years. These systems disconnect the faulty capacitor completely from the grid if the pressure inside the device reaches a level sufficient to expand the case, causing tear-off of the internal fuses. The active capacitor elements are thus cut off from the source of supply. The pressure within the casing separates the breaking point so rapidly that no harmful arc can occur. Self-healing properties are important in capacitors destined for wind turbine applications.

Figure 1: Control and Switching Cabinet for Induction Generator Wind Turbine, Showing Capacitors

Cabinet Design Guidelines
The layout of power conditioning cabinets has an appreciable effect on performance, efficiency, and longevity. For example, experience has shown that mounting capacitors near the top of the cabinet increases the risk of overheating. Therefore cabinets should be designed to mount the capacitors in the cooler air near the floor (Figure1). In addition, special capacitor terminal designs make it easier to connect large capacitor banks for power factor correction (Figure 2). These “feed through” terminals allow capacitors to be quickly “daisy-chained,” connected by cables with up to 25mm2 cross-section or 50mm, depending on the type of can diameter. Vishay ESTA always provides the highest cross-sections capacity in each range.

Figure 2: Capacitor Terminals for PFC Applications in Induction Generator Wind Turbines, Facilitating “Daisy Chaining” with Large Cross-section Cables.

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