A Novel Online Monitoring Strategy for the Localized Grounding Insulation Defect of Converter Transformers Based on Converter Switching States Control

Power electronic converters are operated under rapid and frequent switching actions. Large electrical stresses and various current harmonics have critical effects on insulation ageing and reliable operation of converter transformers. In this article, a novel online monitoring strategy for the localized grounding insulation defect is proposed based on the control of converter switching states (CSS). First, a transfer function (TF) measurement is developed using switching transient voltages. Analytical expression of the TF is derived that can separate the effects of defect location and ageing severity. Then, a CSS control method is proposed by inserting six designated CSS sequences, which can identify the phase with defect and evaluate defect location and ageing severity. Next, the monitoring strategy is established in the procedure of CSS control, testing, data processing, and evaluation. Finally, online experiments are performed to validate the effectiveness of the proposed CSS-control-based monitoring strategy. The resonance trace is shown as a powerful tool whose slope and magnitude are utilized to evaluate location and severity, respectively. The main advantage of the proposed strategy is that it can be implemented conveniently and reliably, with no extra hardware requirement and little effect on the normal operation of converter system.