Research on Vibration Saltation of the Transformer Core Caused by DC Bias of Based on Empirical Mode Decomposition

Abstract

This paper presents a time–frequency analysis of the vibration of transformer under direct current (DC) bias through Hilbert–Huang transform (HHT). First, the empirical mode decomposition (EMD) process, which is the key in HHT, was introduced. The results of EMD, namely, intrinsic mode functions (IMFs), were calculated and summed by Hilbert transform (HT) to obtain time-dependent series in a 2D time–frequency domain. Next, the theory of DC bias for the transformer was analyzed. In consideration of the DC bias effect and in combination with the existing transformer vibration-related mechanism, the electromagnetic force equations of the transformer core were deduced. Lastly, a test system of vibration measurement for the transformer was set up. Three direction (x, y, and z axes) components of core vibration were measured. Decomposition of EMD and HHT spectra showed that vibration strength increased, and odd harmonics were produced with DC bias. This method illustrates the most obvious vibration distortion in the z-axis direction when the transformer is DC biased. Among them, the distortion of IMF3 has increased by more than 5 times. However, the distortion in the x-axis and y-axis directions also exists, but it is not obvious. Especially, 50 Hz component appeared in z-direction, 50 Hz component increased twofold in y-direction, and 150 Hz component increased threefold in z-direction. Results indicated that HHT can not only provide the occurrence time of DC bias but can also obtain the signal change components of the transformer vibration. Thus, HHT is a viable signal processing tool for transformer health monitoring.