A method of active flux control for an output stage of a power converter for an electric rail network, the output stage comprising a transformer connected to the power converter, the method comprising the steps of: deriving an estimated magnetisation flux (Φ̂) of the transformer using a closed-loop estimator comprising: a modelled magnetisation flux , wherein the output of the modelled magnetisation flux is an open-loop estimate of the magnetisation flux (Φ m ) of the transformer obtained from a magnetisation curve adapted to the transformer and an estimate of a magnetisation current (I m ) of the transformer , and a flux model comprising a dynamic model of the magnetisation flux of the transformer , wherein the output of the flux model is the estimated magnetisation flux (Φ̂) of the transformer and wherein the inputs of the flux model are a measured secondary current (I s ), a measured converter voltage (U s ), and an input derived (ΔΦ) from the difference between the output of the flux model and the output of the modelled magnetisation flux ; calculating a direct current DC component of the estimated magnetisation flux (Φ̂ dc ) of the transformer ; and carrying out current control by injecting a DC current component into the transformer , wherein the DC current component is calculated from the DC component of the estimated magnetisation flux (Φ̂ dc ) of the transformer . deriving an estimated magnetisation flux (Φ̂) of the transformer using a closed-loop estimator comprising: a modelled magnetisation flux , wherein the output of the modelled magnetisation flux is an open-loop estimate of the magnetisation flux (Φ m ) of the transformer obtained from a magnetisation curve adapted to the transformer and an estimate of a magnetisation current (I m ) of the transformer , and a flux model comprising a dynamic model of the magnetisation flux of the transformer , wherein the output of the flux model is the estimated magnetisation flux (Φ̂) of the transformer and wherein the inputs of the flux model are a measured secondary current (I s ), a measured converter voltage (U s ), and an input derived (ΔΦ) from the difference between the output of the flux model and the output of the modelled magnetisation flux ; a modelled magnetisation flux , wherein the output of the modelled magnetisation flux is an open-loop estimate of the magnetisation flux (Φ m ) of the transformer obtained from a magnetisation curve adapted to the transformer and an estimate of a magnetisation current (I m ) of the transformer , and a flux model comprising a dynamic model of the magnetisation flux of the transformer , wherein the output of the flux model is the estimated magnetisation flux (Φ̂) of the transformer and wherein the inputs of the flux model are a measured secondary current (I s ), a measured converter voltage (U s ), and an input derived (ΔΦ) from the difference between the output of the flux model and the output of the modelled magnetisation flux ; calculating a direct current DC component of the estimated magnetisation flux (Φ̂ dc ) of the transformer ; and carrying out current control by injecting a DC current component into the transformer , wherein the DC current component is calculated from the DC component of the estimated magnetisation flux (Φ̂ dc ) of the transformer . <EOS>