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, from a waveform memory of a waveform recorder, waveforms of measured current and voltages recorded at a first and second transformer sides;
• processing, by a current difference handling block of the monitoring device, 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, by the current difference handling block of the monitoring device, 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, wherein determining the circulating current start time and the circulating current end time comprises processing, by the current difference handling block of the monitoring device, the recorded waveforms and obtaining a first current deviation waveform and a second current deviation waveform from the current difference waveform, and wherein obtaining the first current deviation waveform and the second current deviation waveform further comprises determining, by the current difference handling block of the monitoring device, a current difference waveform that sets out a difference between the current on the two sides of the transformer based on the measured current, predicting a first predicted current waveform, forward in time, and a second predicted current waveform, backwards in time, from the current difference waveform, obtaining the first current deviation waveform as a difference between the difference waveform and the first predicted current waveforms and obtaining the second current deviation waveform as a difference between the difference waveform and the second predicted current waveform;
• and
• extracting, by the monitoring device, 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, by the monitoring device, 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, by the monitoring device, 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;
• and
• analyzing, by an information analyzing block of the monitoring device, the extracted information to estimate wear of the tap changer and, based on the estimated wear, determining by the information analyzing block of the monitoring device, a service window for performing maintenance.

A monitoring device for monitoring a transformer comprising a tap changer, said transformer having at least two magnetically coupled windings arranged at a first and a second transformer side and the tap changer comprising at least one impedance element and a switch configured to gradually pass load current through the at least one impedance element when changing between two tap changer positions during a tap change operation, the monitoring device comprising a waveform analyzer comprising:

• a waveform recorder comprising a waveform memory;
• a current difference handling block;
• a memory;
• and
• an information analyzing block,
• the waveform analyzer operative to:
• obtain, from the waveform memory of the waveform recorder, a plurality of waveforms of measured current recorded by the waveform recorder at a first and second transformer sides;
• based on the recorded waveforms, generate, by the current difference handling block of the monitoring device, at least one current difference waveform representing a tap change operation, wherein the generating of the at least one current difference waveform comprises obtaining, by the current difference handling block of the monitoring device, a first current deviation waveform and a second current deviation waveform, wherein the obtaining of the first current deviation waveform and the second current deviation waveform further comprises determining, by the current difference handling block of the monitoring device, a current difference waveform that sets out a difference between the current on the two sides of the transformer based on the measured current, generating, by the current difference handling block of the monitoring device, a first predicted current waveform, forward in time, and a second predicted current waveform, backwards in time, from the current difference waveform, and generating, by the current difference handling block of the monitoring device, the first current deviation waveform as a difference between the difference waveform and the first predicted current waveform and the second current deviation waveform as a difference between the difference waveform and the second predicted current waveform;
• refine, by the current difference handling block of the monitoring device, the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing a steady component based on load current;
• extract, by the monitoring device, information indicative of the health of the tap changer from said at least one waveform that represents the tap change operation, wherein the extracted information comprises a circulating current time covering at least a part of the tap change operation and determined from a circulating current start time and a circulating current end time of the tap change operation, and/or a circulating current amplitude of said at least one waveform representing the tap change operation;
• store, in the memory, the information indicative of the health of the tap changer;
• and
• analyze, by the information analyzing block of the monitoring device, the extracted information to estimate wear of the tap changer and, based on the estimated wear, determine, by the information analyzing block of the monitoring device, a service window for performing maintenance.

A computer program product comprising program code stored on a non-transitory computer readable medium which, when the program code is executed by a processor of a waveform analyzer, causes the waveform analyzer to execute 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, from a waveform memory of a waveform recorder, a plurality of waveforms of measured current recorded by the waveform recorder at a first and second transformer sides;
• based on the recorded waveforms, generating, by a current difference handling block of the monitoring device, at least one current difference waveform representing a tap change operation, wherein the generating of the at least one current difference waveform comprises obtaining, by the current difference handling block of the monitoring device, a first current deviation waveform and a second current deviation waveform, wherein the obtaining of the first current deviation waveform and the second current deviation waveform further comprises determining, by the current difference handling block of the monitoring device, a current difference waveform that sets out a difference between the current on the two sides of the transformer based on the measured current, generating, by the current difference handling block of the monitoring device, a first predicted current waveform, forward in time, and a second predicted current waveform, backwards in time, from the current difference waveform, and generating, by the current difference handling block of the monitoring device, the first current deviation waveform as a difference between the difference waveform and the first predicted current waveform and the second current deviation waveform as a difference between the difference waveform and the second predicted current waveform;
• refining, by the current difference handling block of the monitoring device, the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing a steady component based on load current;
• extracting, by the monitoring device, information indicative of the health of the tap changer from said at least one waveform that represents the tap change operation, wherein the extracted information comprises a circulating current time covering at least a part of the tap change operation and determined from a circulating current start time and a circulating current end time of the tap change operation, and/or a circulating current amplitude of said at least one waveform representing the tap change operation;
• storing, in a memory, the information indicative of the health of the tap changer;
• and
• analyzing, by an information analyzing block of the monitoring device, the extracted information to estimate wear of the tap changer and, based on the estimated wear, determining, by the information analyzing block of the monitoring device, a service window for performing maintenance.

A non-transitory computer readable medium having stored thereon computer-executable instructions that, when executed by a processor of a waveform analyzer comprising a waveform analyzer, a current difference handling block, a memory, and an information analyzing block, causes the waveform analyzer to perform operations comprising:

• obtaining, from a waveform memory of a waveform recorder, waveforms of measured current recorded at a first and second transformer sides of a transformer, wherein the transformer includes a tap changer;
• processing, by the current difference handling block, the recorded waveforms for obtaining at least one current difference waveform representing a tap change operation, wherein the processing of recorded waveforms comprises obtaining a first current deviation waveform and a second current deviation waveform, wherein the obtaining of the first current deviation waveform and the second current deviation waveform further comprises determining a current difference waveform that sets out a difference between the current on the two sides of the transformer based on the measured current, predicting a first predicted current waveform, forward in time, and a second predicted current waveform, backwards in time, from the current difference waveform, and obtaining the first current deviation waveform as a difference between the difference waveform and the first predicted current waveform and the second current deviation waveform as a difference between the difference waveform and the second predicted current waveform;
• refining, by the current difference handling block, the current difference waveform by adding the first current deviation waveform to the second current deviation waveform and removing a steady component based on load current;
• extracting, by a monitoring device, information indicative of the health of the tap changer from said at least one waveform that represents the tap change operation, wherein the extracted information comprises a circulating current time covering at least a part of the tap change operation and determined from a circulating current start time and a circulating current end time of the tap change operation, and/or a circulating current amplitude of said at least one waveform representing the tap change operation;
• storing, in the memory, the information indicative of the health of the tap changer;
• and
• analyzing, by the information analyzing block, the extracted information to estimate wear of the tap changer and, based on the estimated wear, determining a service window for performing maintenance.