Provided is a premagnetization device for a converter system that is connectable to a three-phase electrical grid that has a three-phase power transformer, a converter connected to the power transformer and a circuit-breaker on the grid side of the power transformer. The premagnetization device is premagnetizes the power transformer in a state isolated from the grid. The premagnetization device uses a single-phase reference voltage (Uref) that has a fixed relationship to the grid voltage (Ugrid) with regard to the voltage parameters, in order to generate, based on the measured instantaneous reference voltage (Uref) and the known fixed parameter relationships, by actuating the converter, a three-phase alternating voltage synchronous to the grid voltage (Ugrid) on the grid side of the power transformer.