Pressure pulse diagnostics of an OLTC

A method, for monitoring a vacuum on-load tap changer, wherein the tap changer comprises:

• a housing filled with insulating fluid,
• a diverter switch arranged inside of the housing and comprising at least one movable contact and at least two vacuum interrupters comprising at least a first main vacuum interrupter and at least a first resistor vacuum interrupter for interrupting a current through the at least one movable contact,
• at least one pressure sensor which measures pressure in the housing and transmits an output signal to a processing unit,
• wherein the method comprises:
• continuously measuring the pressure in the housing with the pressure sensor and transmitting the output signal of the sensor to the processing unit for detecting opening and closing of the at least two vacuum interrupters, wherein the pressure increases when a respective one of the at least two vacuum interrupters is opened and drops when the respective one closes,
• processing the output signal of the sensor with the processing unit to detect at least a first pressure increase associated with an opening of the at least first main vacuum interrupter, a subsequent first pressure decrease associated with a closing of the at least first main vacuum interrupter, and to detect at least a second pressure increase associated with an opening of the at least first resistor vacuum interrupter, and a subsequent second pressure decrease associated with a closing of the at least first resistor vacuum interrupter,
• determining at least a first point in time associated with the opening of the at least first main vacuum interrupter, a subsequent second point in time associated with a closing of the at least first main vacuum interrupter, a third point in time associated with the opening of the at least first resistor vacuum interrupter, and a subsequent fourth point in time associated with a closing of the at least first resistor vacuum interrupter, and
• determining a total main vacuum interrupter opening time as a time elapsed between the first point in time and the second point in time, a total resistor vacuum interrupter opening time as a time elapsed between the third point in time and the fourth point in time, a circulating current time as a time elapsed between the second point in time and the third point in time, and/or a switch time as a sum of the total main vacuum interrupter opening time, the total resistor vacuum interrupter opening time and the circulating current time.