I was working with Homer, trying to modelate a litle Wind Park (20 MW).  In the report of the systems never apears that the park reach the nominal power rate of 20 MW. Why?

HOMER multiplies the power curve by the air density ratio, which is correct only for stall-regulated wind turbines.  Unfortunately it is not able to do the air density correction properly for pitch-regulated wind turbines.  (Large wind turbines like the 2 MW turbine you are modeling are virtually all pitch-regulated.)  We intend to fix this problem in a future version of HOMER, but for now you can only solve it by doing some work in a spreadsheet.

You need to supply HOMER with the ‘effective wind speed’, which is the wind speed that would produce the same amount of power at sea level as the actual wind speed would produce at elevation.  Using a spreadsheet, you will need to calculate the effective wind speed for each time step according to the equation that appears below.  (This is an excerpt from the Windographer help file.)  Then supply that data as the wind data, and set the elevation to zero.   You should find that the wind turbine produces power up to its maximum rated capacity.