According to an embodiment of a power network device, the device includes a computer configured to estimate a plurality of parameters internal to a transformer, including estimating a turns ratio of the transformer. The computer performs the parameter estimation based on an equivalent circuit model of the transformer and current and voltage samples which correspond to current and voltage measurements taken at primary side and secondary side terminals of the transformer. The computer indicates when one or more of the estimated parameters deviates from a nominal value by more than a predetermined amount. The computer can be part of an intelligent electronic device configured to acquire analog or digital signals representing the primary side and secondary side current and voltage measurements, or located remotely from the intelligent electronic device e.g. in the control room or substation controller.