Monitoring a transformer comprising a tap changer

A method for monitoring a transformer comprising a tap changer, said transformer having at least two magnetically coupled windings and the tap changer comprising at least one impedance element and a switch configured to gradually pass a load current through the at least one impedance element when changing between two tap changer positions during a tap change operation, the method being performed in a monitoring device and comprising:

• obtaining waveforms of measured current and voltages recorded at the first and second transformer sides,
• processing the recorded waveforms for obtaining at least one current difference waveform and at least one power loss waveform, each representing a tap change operation,
• wherein the processing of recorded waveforms further comprises
• determining a circulating current start time, a circulating current end time and circulating current amplitude from the at least one current difference waveform representing a tap change operation and
• extracting a tap change start time and a tap change end time from the at least one power loss waveform representing a tap change operation, and
• determining at least one transition current time and at least one circulating current time using the circulating current start time and the circulating current end time, the tap change start time and a tap change end time and
• extracting information indicative of the health of the tap change from said at least one current difference waveform and said at least one power loss waveform, each representing a tap change operation,
• wherein the extracted information comprises a circulating current amplitude and/or a circulating current time covering at least a part of the tap change operation represented by said at least one current difference waveform, and/or a transition current time covering at least a part of the tap change operation represented by said at least one power loss waveform.