The invention is concerned with a monitoring device, method and computer program product for monitoring a transformer comprising a tap changer. The transformer has at least two magnetically coupled windings and a tap changer comprising impedance elements and a changeover switch configured to gradually pass the impedance elements when changing between two tap changer positions during a tap change operation. The method is performed in the monitoring device and comprises: obtaining (50) waveforms of measured power transmission properties recorded at the first and second transformer sides, processing (52, 54, 56) the recorded waveforms for obtaining at least one waveform (Ploss) representing a tap change operation, and extracting (56, 60) information indicative of the performance of the tap change from the at least one waveform that represents the tap change operation.