Method for classifying the status of the winding clamping of a power transformer

1. A method for classifying a status of a winding clamping of a power transformer immersed in an oil filled transformer tank, the power transformer including at least one transformer coil with at least one clamped electrical winding arranged on a transformer core, the method comprising:

applying a mechanical force impulse on an impact area of the transformer tank, so that the at least one clamped electrical winding is mechanically excited to vibration and a voltage is induced within the at least one clamped electrical winding therewith;

measuring the induced voltage of the at least one clamped electrical winding for a period of time during vibration to generate measurement data;

transferring the measurement data from the time domain into the frequency domain and providing a respective dataset comprising the measurement data in the frequency domain; and

searching for local maxima within the measurement data in the frequency domain of the dataset, the searching comprising:

classifying the status of the winding clamping of the at least one clamped electrical winding as defective if there is no distinct maximum within the local maxima; and

classifying the status of the winding clamping of the at least one clamped electrical winding as at least possibly sufficient if there is exactly one distinct maximum within the local maxima.

2. The method according to claim 1 , wherein the frequency domain covers a maximum frequency of at least 1.5 kHz.

3. The method according to claim 1 , wherein the power transformer comprises several windings,

wherein a respective dataset is provided synchronously for at least two of those windings; and

wherein the status of the winding clamping of those at least two windings is classified.

4. The method according to claim 3 , wherein the power transformer is a 3-phase transformer.

5. The method according to claim 4 , wherein the induced voltage within a respective winding is measured in between a respective phase conductor and either a real or virtual neutral point.

6. The method according to claim 3 , wherein, in case of at least two datasets with a respective distinct maximum, a cross reference in between the frequencies of the distinct maxima is done when classifying the status of a respective winding clamping.

7. The method according to claim 1 , wherein the power transformer comprises a tap changer and the method is repeated sequentially with different tap changer settings.

8. The method according to claim 1 , wherein the method is repeated at least two times, and wherein the impact area of the respective mechanical force impulse is subject to vary or not.

9. The method according to claim 8 , wherein at least one impact area is located on a side wall of the transformer tank across from the at least one clamped electrical winding to be analyzed.

10. The method according to claim 1 , wherein an energy of the mechanical force impulse is in a range of less than 500 J.

11. The method according to claim 1 , wherein the mechanical force impulse is applied in a reproducible manner by respective standardized impulse means.

12. The method according to claim 1 , wherein a local maximum in the frequency domain is classified as one distinct maximum if it is as least twice as great than respective other maxima.

13. The method according to claim 1 , wherein two adjacent local maxima are classified as one local maximum if their frequency differs not more than 30 Hz.