Behavior of the charge accumulation at the pressboard/oil interface under DC external electric field stress

Mineral oils are commonly used for cooling and electrical insulation purposes in electrical power transformers. The physicochemical process appearing at the Pressboard/Oil interface, leads to the formation of an Electrical Double Layer (EDL). Therefore the interface is polarized by the EDL, the charge of one sign is generated within the solid medium and the opposite charge remains in the liquid medium. The process of flow electrification is due to the convection of the diffuse layer inside the liquid. The test loop was recently optimized with the addition of an energized wrapped copper electrode in the sensor. This modification allows studying the effect of an external electric field on the charge production and the distribution of the EDL : in the liquid via the streaming current (global charge production) and in the pressboard via the capacitive current. This capacitive current allows quantifying the accumulated charge within the pressboard plate (local charge production). Therefore, the dynamics of the accumulated charge refer to two EDL affected in the same way by the DC electric field. This paper proposes to show the influence of an external DC electric field on the accumulated charge within the pressboard as well as on the streaming current.