Effect of Superimposed Harmonics on Partial Discharge Characteristics of Insulation in Power Electronic Transformers

With the rapid development of power electronics, some power electronic devices in flexible DC transmission system make the harmonic problem in power system more prominent, such as rectifier and inverter and so on. Affected by harmonics, internal electric field distribution and partial discharge (PD) characteristics of transformer insulating materials will be more complicated, and insulation deterioration will accelerate. In addition, the operation of transformers in harmonic environment for a long time will produce a lot of loss and heat, making the insulation aging accelerate and even destroyed. In view of the influence of harmonics on PD characteristics of power electronic transformer (PET) insulating materials, this study builds a PD test platform of polyimide (PI) under the superposition of different harmonics, and deeply analyses characteristic quantities of PD including the number and amplitude of PD and phase resolved partial discharge (PRPD) pattern. The results show that the number and amplitude of PD increase with the content and order of superimposed harmonics, which promote PD. With the content and order of superimposed harmonics increasing, the PRPD patterns change from single peak to multi-peak in positive and negative half periods. Studying the PD characteristics under the superposition of harmonics can help to reveal the action mechanism of harmonics on PET insulating materials and provide a theoretical basis for improving the reliability of PET insulation.