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A variable speed wind turbine is provided. The wind turbine includes a generator, a power converter for converting at least a portion of electrical power generated by the generator, a power dissipating unit operatively coupled to the DC link, and a controller. The power converter includes a generato

A variable speed wind turbine is provided. The wind turbine includes a generator, a power converter for converting at least a portion of electrical power generated by the generator, a power dissipating unit operatively coupled to the DC link, and a controller. The power converter includes a generator side converter, a grid side converter, and a DC (direct current) link therebetween. The controller is adapted to determine a DC link voltage error signal, the DC link voltage error signal being the difference between a function of an actual DC link voltage and a function of a predefined reference DC link voltage, determine a DC link error power based on the DC link voltage error signal, determine a feed forward power, and generate a duty ratio for operating the power dissipating unit based on the DC link error power and the feed forward power.

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1. A variable speed wind turbine comprising: a generator;a power converter for converting at least a portion of electrical power generated by the generator, the power converter comprising a generator side converter, a grid side converter, and a DC (direct current) link therebetween;a power dissipati

1. A variable speed wind turbine comprising: a generator;a power converter for converting at least a portion of electrical power generated by the generator, the power converter comprising a generator side converter, a grid side converter, and a DC (direct current) link therebetween;a power dissipating unit operatively coupled to the DC link; anda controller, wherein the controller is adapted to: determine a DC link voltage error signal, the DC link voltage error signal being the difference between a function of an actual DC link voltage and a function of a predefined reference DC link voltage;determine a DC link error power based on the DC link voltage error signal;determine a feed forward power; andgenerate a duty ratio for operating the power dissipating unit based on the DC link error power and the feed forward power. 2. The variable speed wind turbine of claim 1, wherein the power dissipating unit comprises at least a switch and one resistor. 3. The variable speed wind turbine of claim 1, wherein the DC link voltage error signal is the difference between the squares of the actual DC link voltage and the predefined reference DC link voltage. 4. The variable speed wind turbine of claim 1, wherein the controller further comprises a PI (Proportional-Integral) controller for determining the DC link error power based on the DC link voltage error signal. 5. The variable speed wind turbine of claim 1, wherein the controller is further adapted to determine the feed forward power based on the difference between a power supplied to the generator side converter and a power transferred by the grid side converter. 6. The variable speed wind turbine of claim 5, wherein the controller is further adapted to estimate the power supplied to the generator side converter based on the difference between power extracted from wind and power losses in the generator and in a drive train of the wind turbine. 7. The variable speed wind turbine of claim 1, wherein the controller is further adapted to generate the duty ratio by: determining a power to be dissipated by the power dissipating unit;determining a maximum power that can be dissipated by the power dissipating unit; anddetermining the ratio of the power to be dissipated and the maximum power, thereby obtaining the duty ratio. 8. The variable speed wind turbine of claim 1, wherein the controller is further adapted to: determine power extracted from the wind, power supplied by the wind turbine and power loss in the power dissipating unit;determine the difference between the power extracted from the wind and the sum of the power supplied by the wind turbine and the power loss in the power dissipating unit; andactivate the power dissipating unit when the difference between the power exceeds a predefined power difference threshold. 9. A method for operating a power dissipating unit in a wind turbine, the wind turbine comprises a power converter for converting at least a portion of electrical power generated by a generator wherein the power converter comprises a generator side converter, a grid side converter,. and a DC (direct current) link therebetween, the power dissipating unit is operatively coupled to the DC link, the method comprising: obtaining a DC link voltage error signal, the DC link voltage error signal being the difference between a function of an actual DC link voltage and a function of a predefined reference DC link voltage;determining a DC link error power based on the voltage error signal;determining a feed forward power; andgenerating a duty ratio for operating the power dissipating unit based on the DC link error power and the feed forward power. 10. The method of claim 9, wherein the DC link voltage error signal is determined from the difference between the squares of the actual DC link voltage and the predefined reference DC link voltage. 11. The method of claim 9, wherein the DC link error power is determined based on the DC link voltage error signal using a PI (Proportional-Integral) controller. 12. The method of claim 9, wherein the feed forward power is determined based on the difference between a power supplied to the generator side converter and a power transferred by the grid side converter. 13. The method of claim 12, wherein the power generated by the generator is estimated based on the difference between power extracted from wind and power losses in the generator and in a drive train of the wind turbine. 14. The method of claim 9, wherein generating the duty ratio comprises: determining a power to be dissipated by the power dissipating unit;determining a maximum power that can be dissipated by the power dissipating unit; anddetermining the ratio of the power to be dissipated and the maximum power, thereby obtaining the duty ratio. 15. The method of claim 9, further comprising: determining power extracted from the wind, power supplied by the wind turbine and power loss in the power dissipating unit;determining the difference between the power extracted from the wind and the sum of the power supplied by the wind turbine and the power loss in the power dissipating unit; andactivating the power dissipating unit when the difference between the power exceeds a predefined power difference threshold.