The paper describes a method for obtaining the characteristics of the active part of power transformers online. The method is based on the processing of digital current and voltage wave-forms obtained at turning on of unloaded transformer with premagnetized core. The core of transformer, switched off by a circuit breaker, has residual induction value closing to nominal Bn = 1.5. Switching on of the transformer leads to addition of working AC field to residual one. Therefore full magnetic field may achieves saturation induction B = 3 T. In this case winding cur-rent is close to shorting one Idemag = (5-6)In. It let's to build magnetization curve – dependence of the magnetic flux on winding current. This dependence let's to get all the parameters of the active part of the transformers, which are usually measured on a disconnected transformer using an external voltage source.