Research on the Movement Characteristics and Local Accumulation Mechanism of Rubber Particles in Converter Transformer Mineral Oil

Rubber particles shed from aging seals in converter transformers can distort local electric fields and degrade insulation. As a critical but understudied impurity in converter transformers, the migration and accumulation mechanisms of rubber particles remain unclear. This study investigates the movement and accumulation behaviors of nitrile butadiene rubber (NBR) particles under a DC electric field. High-speed camera observations were conducted under DC voltages up to 6 kV to capture the migration of 100–200 µm NBR particles under parallel-plate and sphere-plate electrodes. A multiphysics model was developed to simulate field gradients and particle trajectories. Results show that particles form dynamic bridges via reciprocating migration in uniform fields, whereas they exhibit staged movement under non-uniform fields. A dielectrophoretic (DEP) potential well model reveals that high-gradient regions (e.g., electrode edges) form steep wells that trap particles when their kinetic energy falls below the well depth. Quantitative analysis indicates that particles with a radius below the critical value (e.g., 150 µm) are prone to local accumulation under applied voltages above 3.5 kV. Above this critical value, accumulation is suppressed, depending on the voltage and oil velocity conditions. These findings clarify the critical criteria for rubber particle accumulation, supporting insulation design in converter transformers.