Experimental and theoretical studies on static electrification phenomena owing to oil flow in EHV power transformers

In this paper, the static electrification phenomena owing to oil flow in core type forced oil cooled EHV power transformers are simulated by means of a multi-layer oil duct model made of the oil-impregnated pressboards. The effect of oil velocity, oil flow state, oil temperature, AC field application as well as the method of oil duct arrangement on the static electrification has been worked out, and the leakage behavior of the electrostatic charge accumulated in the oil duct has also been investigated. Based on the studies, a mathematical model is presented. The combined effects of flow-induced charging and AC energization-induced charging have been taken into consideration. The model is established for analyzing the static electrification under the condition of the turbulent flow state. It is capable of predicting the charging level to some extent in the complicated oil duct structure of the transformer. The results obtained are of significance for the designs of the insulation structure and oil duct system, as well as the operation of forced oil cooled EHV transformers.