Purpose/Aim
Cellulose particles are polarized in electric field and migrating towards high electric field region due to the dielectrophoretic (DEP) force, eventually a bridge is formed, which may induce partial discharge or even insulation breakdown. Almost all the existing researches are carried out in static transformer oil, but the oil in transformer is circulating due to the cooling systems, for which the accumulation characteristics of cellulose particles will be changed. In order to clarify the effect of oil flow on cellulose particle accumulation, the images of accumulation in DC electric field at different flow rates and voltages were recorded, and the time of bridging two electrodes for the first time was recorded at the same time in this paper.
Experimental/Modeling methods
The circulating flow device of transformer oil and the particle observation system were established. The oil circulated through an oil pump, and the test electrode was a pair of copper spherical electrodes with a diameter of 40mm. Apply DC voltages to the electrodes, the accumulation process of cellulose particles with a diameter less than 100μm was recorded by the high-speed camera, the time of bridging two electrodes for the first time was recorded at the same time, and the change of leakage current during the process was collected by the electrometer.
Results/discussion
The accumulation characteristics of cellulose particles in the gap of spherical electrodes under different voltages and oil flow velocities were obtained. As in static oil, cellulose particles also accumulated to form a bridge in flowing oil under DC voltage. With the increase of voltage, the cellulose bridge became thicker, the leakage current increased, and the time of bridging two electrodes decreased. With the increase of flow velocity, the bridge became thinner and its position moved along the direction of oil flow, the leakage current decreased, and the time of bridging two electrodes decreased first and then increased, that is, the low-speed oil flow could accelerate the formation of cellulose bridge.
Conclusions
Voltage increase leads to the thickening of cellulose bridge and the shortening of bridge forming time. Low-speed oil flow accelerates the formation of bridge but inhibits its thickening while high-speed oil flow slows down the formation of bridge and inhibits its thickening. The above results can be used as one of the theoretical basis for oil flow velocity design in transformer oil channel, for example, the velocity range that accelerating the formation of bridge should be avoided.

Accumulation Characteristics,Cellulose Particles,Flowing Transformer Oil,DC Electric Field