Transformer Core-Vibration Analysis: Coupling Paths

Vibration analysis is a promising way to detect damage and faults in transformers in a nondestructive and retrofittable manner. Current existing vibration models assume a quadratic voltage functional relationship between the 100 Hz vibration amplitude and the applied voltage to account for the magnetostriction in core vibrations. However, the core vibrations measured at the transformer tank differ from the signal emitted by the core inside the tank. This results from the influence of the tank structure and the different coupling path of the vibrations from the core to the tank surface.This paper analyzes how the effects of coupling paths and tank structure can be included in a core-vibration model. The effects are determined in a test setup with distribution transformers. The findings from these tests are then translated into an adapted core-vibration model. For the tests, vibration sensors are positioned at the middle limb of the core and at the top and side surface of the tank. The tests show that the impact of the coupling path is small when vibrations at the transformer side surface are considered. However, for vibration amplitudes measured at the top of the tank, the existing vibration models cannot reproduce the vibrations for all top tank positions sufficiently. We propose a model that has not a constant quadratic voltage functional relationship but allows for a variable functional relationship between the voltage and the vibration amplitude (e.g. quadratic, cubic or quartic). It is shown that for the centered top tank position, a model based on a quartic functional relationship can model the 100 Hz vibration amplitudes with a 51% lower root mean squared error than a model based on quadratic voltage functional relationship.