Method and system for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid

A control system for controlling a DC link voltage of a power converter connecting an electric generator of a wind turbine with a power grid, the control system comprising:

• a first determining unit for determining the rotational speed of a rotor of the electric generator,
• a second determining unit for determining a strength of harmonic components of the rotational speed of the rotor, wherein the harmonic components have frequency components being different from a fundamental rotational frequency of the electric generator,
• a third determining unit for determining a damping reference signal as a function of the determined strength of the harmonic components,
• a measurement unit to measure a DC link voltage in a DC link capacitor of a DC link of the power converter that connects the electric generator of the wind turbine with the power grid to obtain a measurement signal indicative of the actual DC link voltage,
• a filter for filtering the measurement signal indicative of the actual DC link voltage, the filter effective to suppress at least partially frequency components corresponding to mechanic resonance frequencies of a drive train of the wind turbine, and
• a control unit for controlling a time-dependent variation in the DC link voltage of the DC link capacitor in the DC link of the power converter that connects the electric generator of the wind turbine with the power grid in response to the determined damping reference signal and further in response to the filtered measurement signal, the control of the variation in the DC link voltage of the DC link capacitor effective to damp the harmonic components of the rotor without feeding harmonic power to the grid.