Condition Monitoring of On-Load Tap-Changers in Converter Transformers Based on Vibration Method

The proper operation of On-Load Tap-Changer (OLTC) in converter transformer is essential for the safety and stability of HVDC system. Since a well-defined series of vibration signals are produced as its signature, it's extremely desirable to indicate the inner connections and differences among these signals of each condition. The mechanical vibration theories of OLTC are analyzed first. Based on an improved EMD method aimed at reducing aliasing effect in the analysis of nonstationary and nonlinear signals, the frequency of OLTC vibration would be effectively extracted. Meanwhile, some common faults such as contact looseness have been set and the characteristics of each condition are clearly identified both in time and frequency domain. The results can distinguish the fault pattern from the normal one effectively and efficiently, which is also applied in the real OLTC in converter transformer. It will provide theoretical basis and practical guidance for condition monitoring and fault diagnosis of OLTC.