It is presented a method executed in a transmission system, the transmission system comprising an AC grid, a DC grid and at least two AC/DC converters connected between the AC grid and DC grid. The method comprises the steps of: obtaining set points for each one of the at least two AC/DC converters, each set point comprising a magnitude and direction of power through the respective AC/DC converter during normal operation generating, based on the set points, a virtual AC grid, the virtual AC grid corresponding to AC behaviour of the DC grid, as viewed from each AC side of the at least two AC/DC converters; and controlling the at least two AC/DC converters to mimic a behaviour in accordance with the virtual AC grid. A corresponding transmission system is also presented.