Effect of duct configuration on oil activity at liquid/solid dielectric interfaces

Abstract

A test cell has been designed to allow simultaneous determination of the currents flowing at the surfaces of two spacers in a 5mm oil gap. The spacers were asymmetrically disposed so their surfaces were at different distances from the bulk oil. Electrical measurements and Schlieren observations showed that the currents and activity in the oil at the spacer surface are influenced to a considerable degree by the length of duct: the current increasing by an order of magnitude in the longer duct. The technique tends to reduce the difficulties often experienced when attempting to compare results from direct voltage tests carried out in different cells. During the application of 50Hz alternating voltages to the system, different phenomena were observed at the spacers. These are considered to be due to the effect of dielectric losses in the spacer material. At higher stresses (of the order of 4 × 10 6V/m) EHD effects appeared to be present in certain regions resulting in movement of the oil: an increase in partial discharge activity was recorded for this condition. Higher frequency tests, up to 1000Hz, and similar stresses indicated that no EHD effects were present: it is postulated that the changes observed were due to losses in the oil itself. The work is continuing in an attempt to explain in more detail the various phenomena and to determine their possible significance in technology.