Method and device for estimating the clamping force on a winding package of a transformer or a reactor

1. A method for estimating a clamping force on a winding package of a transformer or a reactor, the winding package comprising at least one winding with connections and an iron core, wherein the method comprises the steps of:

supplying a current pulse to the connections of the winding in order to excite the winding in a vibration mode,

measuring vibrations of the winding package generated by the current pulse,

calculating a mechanical resonant frequency based on the measured vibrations, and

estimating the clamping force on the winding package based on the calculated mechanical resonant frequency and a predetermined relation between expected mechanical resonant frequency and clamping force.

2. The method according to claim 1 , wherein said vibration mode is an axial symmetric vibration mode with fixed ends for a winding package of a transformer.

3. The method according to claim 1 , wherein said winding package of the transformer comprises a first and a secondary winding with connections, the method further comprising the step of electrically arranging the connections of the secondary winding so that the windings are excited in the vibration mode, and the current pulse is supplied to the connections of the first winding.

4. The method according to claim 3 , wherein the connections of the secondary winding are arranged so that the secondary winding is short-circuited.

5. The method according to claim 1 , wherein the current pulse has a rise time that is close to or shorter than the rise time of the expected mechanical resonant frequency.

6. The method according to claim 1 , wherein the predetermined relation between the expected mechanical resonant frequency and the clamping is estimated mathematically based on the rated power and rated voltage of the transformer.

7. A device for determining a mechanical resonant frequency of a vibration mode for a winding package of a transformer or a reactor, the winding package comprising at least one winding with connections and an iron core, characterized in that the device comprises:

a current pulse generator connected to the winding and configured to generate a current pulse sufficient to excite the winding,

a sensor for measuring vibrations of the transformer or the reactor generated by the current pulse, and

a computing unit configured to calculate said mechanical resonant frequency based on the measured vibrations and to estimate a clamping force on the winding package based on the calculated mechanical resonant frequency and a predetermined relation between expected mechanical resonant frequency and clamping force.

8. The device according to claim 7 , said winding package of the transformer comprising a first and a secondary winding with connections, wherein the connections of the secondary winding are electrically arranged so that the windings are excited in the desired vibration mode, and the current pulse is supplied to the connections of the first winding.

9. The device according to claim 8 , wherein the connections of the secondary winding are arranged so that the secondary winding is short-circuited.

10. The device according to claim 7 , wherein said sensor is an accelerometer.

11. The device according to claim 7 , wherein the pulse generator comprises an electrical circuit to be connected to the winding, including a capacitor, a DC voltage supply arranged to charge the capacitor, and a switch configured, upon activation, to generate a current pulse by discharging the capacitor.

12. The device according to claim 11 , wherein a diode is arranged in parallel with the capacitor.

13. The device according to claim 11 , wherein the capacitor inductance together with the winding inductance forms an electrical resonant circuit and, the capacitor is selected so that the resonant frequency of the electrical resonant circuit is close to or higher than the expected mechanical resonant frequency of the windings.