Impact of Moisture Content on the Bridging Characteristics of Cellulose Particles in Flowing Transformer Oil

Abstract

The contamination of damp cellulose particles highly degrades the insulation performance of oil-immersed power transformers. At present, the bridging behavior of damp particles remains unclear. In this article, the studies on bridging dynamics of particles with different moisture contents at different oil flow velocities under both DC and AC voltages were carried out. The results indicated that under DC voltage, particles with 1%–7% moisture contents formed complete bridge which can connect the high and grounded electrodes at flow velocities of 0–0.04 m/s except the case when the flow velocity was relatively high. As the moisture content increased, the bridging time was shortened, and the width of stabilized bridge increased. Under AC voltage, only partial bridge could be observed, which could not connect the two electrodes. The increase of moisture content accelerated bridging and the growth of partial bridge length on the electrode surface. Low-velocity oil flow tended to promote the development of partial cellulose bridges, while high-velocity oil flow inhibited it. And the range of low-velocity oil flow that expanded with the increase of moisture content. At 0%–1% moisture contents, the accelerated bridging phenomenon primarily occurred when the flow velocity was below 0.02 m/s, while this range expanded to below 0.04 m/s as the moisture contents increased to 4%–7%. The schematic of the solid–liquid two-phase flow model was constructed to better analysis from the force. It was found that the increasing cellulose moisture content increased dielectrophoretic forces on particles, enhancing the resistance of particles to the oil flow. More particles converged and bridged to the high field strength region at the electrodes center. As the cellulose moisture content changed, the bridging behavior is altered under both AC and DC voltages. This article is helpful to understand “bridge breakdown” theory in moving liquid dielectrics.