Field distribution anomalies and dielectric failure mechanisms associated with core surface defects in liquid-insulated power transformers

Core burr defects are latent sources of electric field distortion and insulation degradation in oil-immersed power transformers. To elucidate their physical mechanism and quantitative impact, this study combines finite element analysis and field testing to investigate the electric field distribution and discharge behavior induced by core burrs. A two-dimensional axisymmetric core–oil–winding model was established in COMSOL to analyze three defect types: core-center, core-top, and elongated top burrs. Simulation results reveal pronounced local field enhancement in regions near the grounded shield and the winding–core interface. The grounded shield–core interface is the most sensitive, where an elongated top burr increases the local field strength by up to 219% compared with normal conditions. The grounded shield field rises moderately by 51.6%, while the winding field remains nearly unaffected (<4%). The winding–core interface exhibits strong coupling with burr geometry and position, with both middle and elongated top burrs producing over 100% enhancement. The field distortion originates from geometric protrusion, dielectric discontinuity, and proximity coupling between the core, oil, and winding. The burr tip acts as a micro-electrode that concentrates equipotential lines, generating intense local gradients. Field tests on a 220 kV transformer confirmed strong partial discharge activity consistent with simulation predictions. These results clarify how core burrs distort electric fields and trigger discharges, providing theoretical guidance for insulation design optimization, core surface quality control, and condition-based monitoring of power transformers.