Electrical Stress Analysis for Vacuum on-Load Tap-Changer Transition Circuits under the Influence of Harmonic Currents

Abstract

With the breakthrough and performance improvement of vacuum switching technology in the field of research and development of on-load tap changer (OLTC) for large power transformers, converter transformers use vacuum OLTC to regulate their complex operating conditions to meet the safe, stable and efficient operation of the UHV DC transmission system. However, because the AC side of the converter has a special current waveform with high harmonic content, the electromagnetic transient waveform of the vacuum OLTC switching process is very complex. The switching characteristics and breaking capacity of different OLTC transition circuit structures are very different, which directly affects the operating characteristics and life loss of vacuum OLTC for the converter. Therefore, this paper proposes an analytical method for the electrical stress of vacuum OLTC vacuum tubes for converters with different harmonic combinations, focusing on the analytical formula of switching the electrical stress of single-resistance transition circuit vacuum tubes, and then exploring the influence mechanism of harmonics on the switching characteristics of OLTC. The method can provide theoretical support and references for the structural design, parameter selection, and operation, and maintenance optimization of converter on-load tap changer in actual DC projects.