Causes

The “SF6 in oil – Bushing seal defects” criticality is mainly caused by sealing defects between the insulator (insulated with oil) and the metallic casing (containing SF6) that encloses the top of the insulator itself.

Sealing defects can result from physical chemical degradation of materials, electrical, thermal and/or mechanical stress. Taken together, these factors lead to a migration of SF6 in insulator oil. If, on the one hand, this leads to the presence of SF6 in the oil (oil compartment of the insulator), on the other hand, it causes contamination of the SF6 in the metal casing enclosing the upper part of the isolator by part of the oil. This latter condition is far more serious than the opposite condition; in particular the oil mist in the part in SF6 makes the latter lose its dielectric properties, endangering the accessory insulation.

The seriousness of this criticality lies in the fact that, without particular signs/symptoms, it can give rise to electrical failures with power arcs up to explosion and fire in the insulator, with catastrophic consequences. In fact, following an explosion of an insulator, pieces of porcelain and metal components are projected up to 200 metres from the point of installation with a high risk to people, and are likely to trigger a domino effect on surrounding equipment. In the case of fire, the severity of the event is as serious given the high probability of propagation to the transformer casing. The consequence of a similar event could also cause explosion (and/or fire) of the transformer, with a very serious impact on people, things and the environment.