Multi-Frequency Acoustic Signal Under Short-Circuit Transient and Its Application on the Condition Monitoring of Transformer Winding

The large current flowing through transformer windings during external short-circuit events causes strong vibration, and the tank emits multi-frequency sound. This paper investigates the transient acoustic signal theoretically and experimentally and proposes to monitor the winding condition with the spectrum of the acoustic signal. A two-wire model is first built as a basic unit of winding structure. The dynamic equation of the model is constructed based on the principle of least action. The coupling action between the electromagnetic force and the motion of the wires is described as the second-order term of the wire spacing in the equation. The solution shows that the spectrum of the winding vibration contains half power frequency and its multiples in some cases. If the winding is loosened and deformed, the motion space and vibration amplitude would be enlarged, and the natural frequency of winding would decrease. As a result, the vibration spectrum would contain half frequency and its odd multiples. Then, the acoustic signal of a 110-kV transformer is measured on a multiple short-circuit test platform. The acoustic time-frequency spectrums are consistent with the theoretical analysis. The half frequency ratio (HFR) of the acoustic spectrum raises along with the increase of short-circuit times. After 21 short-circuits, the HFR raises obviously, indicating serious deformation of the winding, while no obvious change is found in the short-circuit reactance (SCR). The results prove that the HFR of acoustic signal is a more sensitive indicator than SCR, which can characterize the cumulative process of short-circuit on the winding.