Reduction of fast dU/dt impact for protection of distribution transformers

Transformers operating in power networks are subjected to lightning, network switching phenomena like protection action and selected failure stages. Those effects may result in additional stresses on insulation system of the windings in form of external overvoltages or internal resonances in windings caused by fast dU/dt transients. This paper presents the method for reduction of dU/dt based on specially designed upstream choke device. The new concept employs the upstream impedance placed on high voltage line and application dependent surge capacitance. The upstream choke mitigation impact will be presented for induced lightning stroke producing the fast surge with high dU/dt on the transformer terminals. The comparison of insulation stress for various transients with different rise time properties will be shown. The simulation results along with real measurements of choke operation will be compared.