CONTROL METHOD, COMPUTER PROGRAM PRODUCT, CONTROL SYSTEM & USE

A method for controlling an output voltage of at least one transformer in a power grid. The comprises providing the least one transformers with a power electronics converter and an output voltage controller, determining a grid frequency, comparing the grid frequency with a reference value, producing an error signal, applying the error signal as feedback to the controller, and generating a control action to cause the controller to change the output voltage of the transformer comprising a power electronics converter. The power electronics converter is an AC-AC converter, and the method comprises connecting the AC-AC converter to either a primary side or a secondary side of the transformer. The AC-AC converter has a first AC side and a second AC side and the method comprises connecting two terminals on the first AC side in-parallel to a winding of the transformer, and connecting the second AC side to an external circuit line.