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Variable-Speed Wind Power System with Improved Energy Capture via Multilevel Conversion

National Renewable Energy Laboratory

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Technology Marketing Summary

Power converters are used to convert alternating current (AC) electric power from a fixed-frequency and fixed-voltage to different frequencies and different voltages for powering loads, such as electric motors. They are also used in reverse to convert variable-frequency, variable-voltage, AC electric power to fixed-frequency, fixed-voltage electric power. Electric generators driven by wind turbines rotate at different speeds, depending on wind conditions, so they produce variable-frequency, variable-voltage, AC electric power. Therefore, power converters are used to convert such wind-generated electric power to fixed-frequency, fixed-voltage, AC power to match public utility and similar AC power systems. However, wind turbine power systems also spend lots of time operating at light loads or fractions of their rated power capacities, whereas standard, state-of-the-art, power converters are designed to operate most efficiently at full-rated power all, or nearly all, of the time. Further, standard power converters do not work at low voltages. Therefore, when wind turbine-driven generators are operating in low wind, light load, conditions, standard power converters are inefficient and may not work at all.


Engineers at the National Renewable Energy Laboratory (NREL) have developed a system that improves existing variable-speed, wind power technology by achieving high efficiency at low wind speeds and capturing more wind energy. This is done by providing improvements in AC power converter technologies to enable more efficient and reliable conversion of variable-frequency, variable-voltage, AC power to fixed-frequency, fixed-voltage, AC power and vice versa. The system also provides more efficient and reliable power converters for wind power generator applications in which generated AC power varies over wide ranges of frequencies, voltages, and load levels, for converting such AC power to fixed-frequency, fixed-voltage, AC power for public utility and similar AC power systems.

  • Improves over existing technology to provide higher efficiency at low wind speeds
  • Captures more wind energy
  • More reliable and efficient power conversion
Applications and Industries
  • Wind power systems
  • Wind energy
Patents and Patent Applications
ID Number
Title and Abstract
Primary Lab
Patent 6,900,998
Variable-speed wind power system with improved energy capture via multilevel conversion
A system and method for efficiently capturing electrical energy from a variable-speed generator are disclosed. The system includes a matrix converter using full-bridge, multilevel switch cells, in which semiconductor devices are clamped to a known constant DC voltage of a capacitor. The multilevel matrix converter is capable of generating multilevel voltage wave waveform of arbitrary magnitude and frequencies. The matrix converter can be controlled by using space vector modulation.
National Renewable Energy Laboratory 05/31/2005
Technology Status
Technology IDDevelopment StageAvailabilityPublishedLast Updated
NREL ROI 01-50DevelopmentAvailable03/14/201603/14/2016

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To: Erin Beaumont<>