Study On Oil Flow Electrification Phenomena And Its Electrostatic Hazard In The Power Transformer

At a solid-liquid interface, the physical-chemical reactions lead to the formation of an electrical double layer at rest during a certain development time. Once the liquid is put in movement, a part of the electrical double layer (the diffuse layer) convected by the flow modify this static equilibrium, as a result, a streaming current develops and evolves to reach dynamic equilibrium. This is the flow electrification phenomena.In power transformers cooled by forced convection, the flow electrification phenomena occurring at the oil/pressboard surface has been suspected to be at the origin of the transformer failure and is becoming one of the important factors threating the safe performance of the large power transformers.For several years, with environmental problem caused by leakage and incidents now becoming extremely important, the transformer manufacturers propose the use of ester oils (vegetable or synthetic). Four ester oils have been appraised with an experimental set-up (the small flow loop facility) for electrostatic hazard following two arteries:the generation current and the accumulated charge. Then a comparative analysis of ester oils and mineral oils was developed.In this context, an oil duct model was established to determine the space charge density in the oil near the wall and to simulate the streaming current in the oil. The analysis of the transient state allows for better understanding of this phenomenon, so we examine the influence of the oil velocity, the temperature and the nature of the oil on the time constant of this transient. Two methods were proposed to calculate the time constant.