FLOW electrification is known to occur at the surfaces of
solid insulators exposed to the oil flow in oil-immersed power
transformers. The solid insulators are usually negatively
charged while the insulating oil is positively charged [1] by
this flow electrification. Static electrification of insulating oil
and solid insulators is known to cause this flow electrification.
Recently, the flow electrification was found more likely to
occur in aged power transformers. Deteriorations of insulating
oil and solid insulators such as pressboard due to aging were
found to facilitate the static electrification of these insulating
materials. Oxidation of insulating oil and pressboards and
adsorption of polar substances contained in insulating oil onto
solid insulators are postulated as major changes affecting
aging [2-5]. This static electrification has paid more attention
because lots of aged power transformers exceeding their
design lifetime are still in service.
Electrostatic charging tendency (ECT) has been used as an
evaluation index of insulating oil in Japan to prevent failures due
to flow electrification.