1. A method for handling resonances in a resonance handling device of a power transmission system, the method comprising: in the resonance handling device: obtaining system measurements from measurement devices in the power transmission system; applying the measurements in at least one state space model of the system; determining modal resonance frequencies in the system based on application of the system measurements in said state space model; determining modal activity of at least one of the resonance frequencies; and outputting a control signal for counteracting power oscillations to the power transmission system based on the determined modal activity of at least one of the resonance frequencies.