Influence of Structure and Interface Charges on DC Breakdown Initiation and Polarity Effect of Multilayer Oil-Paper Composite Insulation

To investigate the breakdown characteristics and polarity effect of multilayer oil-paper composite insulation (OPCI) under DC voltage, experiments are conducted to obtain the positive and negative breakdown voltages of various representative OPCI structures. Light and current signals during breakdown are measured, and their onset time difference is compared. The surface morphology of the oil-impregnated paper (OIP) after breakdown is observed. The combined analysis of these two results enables the determination of the breakdown initiation within OPCI. An improved extended Debye equivalent circuit model is proposed and applied to calculate the voltage distribution and interface charge density of OPCI. Special attention is given to the roles of structure and interface charges. The results indicate that the breakdown characteristics of OPCI are determined by the insulation matching between the voltage across each layer and their breakdown voltage. Optimizing the interface charge density enhances insulation matching and shifts the breakdown initiation from the oil gap to OIP, increasing the breakdown voltage. Both interface charges and insulation matching contribute to the polarity effect of breakdown voltage. In symmetric structures, the breakdown consistently initiates in OIP, and the polarity effect is negligible. These findings provide a theoretical reference for optimizing OPCI design to improve its dielectric strength and reliability.