With the increasing requirements of power quality, adjusting voltage frequency and cumulative voltage regulation frequency are increasing for on-load tap-changing transformer, which makes the fault rate of on-load tap-changers(OLTC) in power transformer constantly raise. Contact loosing fault is a common fault for OLTC, which has been paid more attention in recent years. But so far, no effective diagnostic method has been found and the number of fault is increasing. So it is very important to find an effective on-line diagnostic method for OLTC. In this paper, for vibration signal made by contact closing, author applied wavelet packet energy entropy method to analyze the value of wavelet packet energy entropy for vibration signal during the period of contact closing when the on-load tap-changer works. The author found an apparent fault feature which was that the standard entropy of NO.2 node was clearly very high in wavelet packet energy entropy when the contact was loose (up to 3 times more than normal). Base on the theoretical analysis, the author experiment the OLTC which was widely used by Jiangsu Electric Power Company at working environment joined with HuaMing Power Equipment Manufacturing Co., Ltd. The result validates the feasibility of co-diagnosis algorithm. In addition, the diagnostic algorithm can not only discover the potential contact loose fault of OLTC quickly and effectively, but also can distinguish the contact off fault and contact burned fault to some extent, which provide an important reference for engineering staff to solve the problem timely and effectively.
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