On-load tap-changer of converter transformer is not only the core component to realize voltage regulation of converter transformer, but also the key component that has not been localized. It plays an important role in ensuring the voltage stability of power system. The remarkable difference between converter transformer and ordinary AC transformer is that its on-load tap-changer has high operation frequency. With the increase of voltage regulation times, its fault rate increases accordingly. With the development of flexible DC transmission technology, the on-load tap-changer switching the load current with harmonic component is becoming more and more prominent. The switching operation is often carried out under the complex working condition of multiple harmonic superposition. Therefore, the load current will have a large current slope (di/dt), which significantly increases the switching load of the tap-changer. In particular, it poses a great challenge to the current transfer capability of the vacuum interrupter in the vacuum type on-load tap-changer. In recent years, the failure rate in the application field has gradually increased, so it puts forward higher requirements for the reliability of the tap-changer. At present, IEC and Chinese national standards are based on the research of conventional DC system, and do not put forward the requirements of load current switching test with harmonic current. Combined with the actual operating conditions of ± 800kV converter transformer, this paper introduces the basic conditions of switching load current of the vacuum type on-load tap-changer of converter transformer, analyzes the difference between harmonic current switching and conventional switching of on-load tap-changer of converter transformer through electromagnetic transient simulation modeling. This paper reveals the particularity of the operation condition of converter transformer tap-changer, and analyzes the influence of harmonic current on the switching capability of vacuum interrupter in on-load tap-changer. This paper will have important reference significance for the design, type selection, operation and maintenance of on-load tap-changer of converter transformer in DC transmission project.

on-load tap-changer,harmonic current switching,vacuum interrupter,converter transformer