Method and controller for stably operating a wind turbine in different strength of grid

It is described a method for adjusting a reactive power output current (123, 323) of a wind turbine (322) which is connected to a utility grid (328), the method comprising: receiving a voltage error signal (109, 209) indicative of a deviation between an output voltage (105, 205) of the wind turbine and a reference output voltage (107, 207) of the wind turbine; estimating a system gain (129, G(s), k) related to the output voltage of the wind turbine and the reactive power output current of the wind turbine; determining a reference reactive power output current signal (111, 211) related to a reactive power reference output current based on the voltage error signal and the system gain; and outputting, by the wind turbine, the reactive power output current (123, 223), in order to adjust the output voltage of the wind turbine to the reference output voltage of the wind turbine.