Detection of overpressure inside the transformer by a pressure controller.
Detection of oil leakage by proper float.
Both these functions operate directly the protection fuses. In oil-immersed full sealed distribution, transformers parameters to be checked are:
internal pressure which must remain under the safety level.
1. Internal pressure
1a. In operating conditions, the internal pressure depends on heating of live part caused by current going throw them and load of the equipment. A protection able to take out the transformer from the line in case of overheating avoids greater consequences.
1b. Localized anomaly in insulation level could origin gas by dissociation of oil and therefore pressure increasing inside the tank. This increasing, when caused by a modest phenomenon, brings slowly the pressure to the safety level, and allows the operation of proper protection able to take out the transformer from the line avoiding greater consequences.
2. Oil Level
The transformer is placed in service with the correct oil level and with the guaranteed tightness of the tank. During the life, the tightness could decrease due to mechanical shocks or due to aging of critical point as welding or others. In these condition, with decreasing of oil level, dielectric features could change with high risk of electrical discharge, up to an explosion.
A protection able to de-energize the transformer as soon as the oil level goes down, avoid to get worst dielectric features and risk of explosion with destruction of transformer, and allow to repair the damage.