History of Danish Wind Power - Wind Power for the World: The Rise of Modern Wind Energy - page 68

tower. And like many others they experienced vibration problems
due to the blades passing in the “wind shadow” behind the tower.
This was a contributing cause to the fact that the wind turbine
was not able to reach the projected maximum of 40 rpm. In this
case, vibration problems were avoided by limiting the speed of the
turbine to a lower level. As the turbine was running at variable
speeds and the electricity from the AC-generator had to pass
a converter to obtain grid frequency of 50 Hz, it gave no extra
technical problems. But the production was of course reduced,
because only about half of the generator capacity could be used.
However, the goal was never to obtain a maximum production
of electricity for export to the grid—but to get enough power
and heating for the schools. And here, the 900 kW that could be
obtained was more than needed. Reduced speed gave an additional
advantage in a lower stress and a longer lifetime for mechanical
components.

Figure 3.29

The Tvind turbine with its red and white decoration, applied
for the 25th birthday in 2003 and designed by the Danish
architect Jan Utzon, son and partner of the late Jørn Utzon, who
designed the Sydney Opera House (Photo: Benny Christensen,
DVS).