Method and arrangement for damping a shaft oscillation

It is described a method for damping an oscillation of a rotational speed (167) of a shaft (129, 135) of a wind turbine (101) having a converter (141) connected between a generator (137) and a wind turbine output terminal (152), the method comprising: measuring the rotational speed (É, 167) of the shaft (129) or the generator (137); determining a DC-link voltage reference (Udc_ref, 163) varying in dependence with the rotational speed (É, 167); controlling a DC-link voltage (Udc) of a DC-link (145) of the converter (141) according to the DC-link voltage reference (Udc_ref, 163).