Passives for Green Power
30 June 2010
Power conditioning circuitry for clean energy generation must not only be highly efficient but must also offer extreme durability and competitive pricing to satisfy all the demands of this increasingly commercial sector. Alistair Winning reports.
Renewable energy sources such as solar, wind and waves are becoming an increasingly important aspect of national power-generation policies around the world. The USA is working on an ambitious climate change bill, and an Ernst & Young survey has found that countries such as Brazil and Japan have dramatically increased their targets for renewable energy generation in recent energy reviews while China now has more than 150 GW of installed capacity. Recent figures from the EU name Sweden as the leader in Europe; since 2005 the country has derived over 40% of its energy from renewables.
As prices for fossil fuels such as oil and gas continue to rise, the green-energy industry is focusing intensely on reducing the cost per Watt from renewable sources. In the drive to realise the full market and environmental potential of renewables by achieving price parity with traditional fuels, every Joule harvested is precious. This is driving producers of components such as power semiconductors and passives to deliver new generations of low-loss devices capable of allowing a meaningful proportion of the harvested energy to be added to the grid in the form of line-quality AC electricity.
More than miserly electronics
In addition to efficiency, a number of other criteria are becoming important in the design of suitable components. One is the need for reliability and life expectancy beyond the specification of normal off-the-shelf components, recognising that maintenance costs can be extremely high for large-scale wind, solar and wave farms installed in remote or offshore locations.
The success of renewable energy generation has also triggered a move towards a more distributed power infrastructure comprising large numbers of micro generators owned and operated by small businesses and some householders, able to sell their excess energy back to the grid. This segment of the market will demand low-cost components that at the same time deliver acceptable levels of efficiency and reliability.
Passives for power conversion
The power conditioning circuitry for a renewable-energy installation is required to convert the raw electrical output from the wind turbine, solar cells or wave generator, into an AC sinusoid of the correct voltage and frequency. The waveform must have a low harmonic content and high power factor. Figure 1 shows the main power conditioning functions necessary to connect a photovoltaic-cell array to an AC-powered load or to a grid system. The topology comprises a DC link to feed the low-voltage output from the PV cells to an inverter, followed by filtering stages to ensure a supply of clean, high-quality power.
Wind turbines, on the other hand, are typically built using an AC induction generator. The raw AC output has high harmonic content and transients, particularly when there are large fluctuations in wind speed. As Figure 2 shows, the generator output is filtered and converted to DC, before being fed via a DC link to an inverter with output filtering.
Passive components such as capacitors are used extensively for filtering, decoupling of the DC link, and snubber circuitry to protect inverter IGBTs, and have an important influence on the efficiency, lifetime expectancy and cost of the installation. In a wind turbine application, for example, the output filter is typically implemented using a switched-capacitor bank containing large electrolytic or power-film capacitors to maintain the best-case power factor when connecting to the grid. High-voltage ceramic capacitors and film capacitors may also be used in the ‘front end’ of the system, while the associated control circuitry tends to use both ceramic and tantalum capacitor technologies. Power inductors having low DC resistance and the ability to withstand high transient current spikes are also used throughout the power conversion and conditioning stages.
Application-optimised construction
A number of established manufacturers of passive components, such as Kemet, NIC and Vishay, have responded to the emerging requirements placed on components for renewable energy generation. NIC, which positions its NPI and NPIS series power inductors and the NACZ and NACK liquid-electrolyte capacitors for alternative energy applications, has also added the NSPE electrolytic capacitor family incorporating hybrid technology that combines the low ESR and high current ratings achievable using solid polymer with the high voltage and temperature ratings and high transient-withstand capabilities of liquid-electrolyte capacitors.
Switched-capacitor banks used for power-factor correction in wind turbine plants can be exposed to high temperatures, especially when large numbers of capacitors are installed within a small cabinet. Capacitor families such as the NIC NACVF or NRBX series are able to withstand operating temperatures up to 105°C, and also provide long lifetimes as well as high voltage and current ratings. To achieve these high temperature ratings, component designers have focused attention on finding optimal filling materials to maximise thermal transfer from the capacitor element to the casing, from where heat can be dissipated into a heatsink or removed by forced-air cooling. An inert gas is often favoured as the filling medium, to maximise heat transfer.
Vishay has developed an alternative biodegradable, vegetable-based oil for use in its ESTAprop MKP capacitors targeting PFC applications. The oil has around seven times better thermal conductivity than standard fillings, and has the advantage of imposing no special handling requirements at the capacitor’s end of life. Slim case dimensions also promote heat transfer, allowing these devices to achieve typical life expectancy of over 150,000 hours depending on how the ambient temperature and operating conditions influence the capacitor case temperature. Vishay’s ESTAprop capacitors, as well as the ESTAdry gas-filled family, have already accumulated many years of reliable service in the wind turbine industry.
Since PFC capacitors in wind turbine applications tend to be switched in and out of circuit much more frequently than in any conventional switched capacitor bank, power conditioning presents specific inrush-control challenges. In particular, standard coiled-wire inductors tend to generate excessive heat under frequent switching. To combat this, Vishay’s oil-filled and dry capacitors instead feature integrated pre-resistors on the contactors, which are specially developed to suit capacitor switching.
Hazards facing PFC capacitors include unpredictable transients and harmonics resulting from the motion of the rotor and resonance phenomena, which can lead to repeated over-voltages. To withstand these conditions, the self-healing properties of electrolytic capacitors can deliver an important reliability gain. As a failsafe in the event of excessive internal pressure caused by large and repeated over-voltages, capacitors such as the ESTAprop/ESTAdry MKP series feature all-phase overpressure tear off fuse systems that quickly disconnect the capacitor completely from the grid with no potentially harmful arc.
Lifetime-Extension Techniques
Kemet is able to fulfil 95% of all possible capacitance solutions for alternative energy applications, and has supplied components to wind, solar, wave, geothermal and electric-vehicle projects. Film capacitors such as the C4A and C44x are used in snubber circuitry and AC filters. Custom stacked-film capacitors and aluminium electrolytic devices are optimised to deliver long lifetimes in DC-link applications.
To meet exacting customer specifications, particularly in relation to life calculations for electrolytic capacitors, Kemet provides samples that include thermocouples permitting measurements to be taken in situ to ensure that the real-world operating conditions match those of the customer’s theoretical design. This allows customers to be sure that equipment will satisfy the minimum lifetime specifications, which are extremely important for equipment destined to be installed in remote locations. Customised power-film capacitors are chosen for applications demanding high operating voltages and a wide temperature range.
Conclusion
The success of the clean-energy sector is winning government and public support the world over. As demand for renewable-energy generating equipment gathers pace, capacitors and inductors optimised for alternative energy applications will define the leading edge in terms of the efficiency, durability and cost effectiveness of passive components.
Alistair Winning works for Farnell
Figure 1 Shows the main power conditioning functions necessary to connect a photovoltaic-cell array to an AC-powered load or to a grid system.
Figure 2 The generator output is filtered and converted to DC, before being fed via a DC link to an inverter with output filtering.
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