This invention relates to protective devices for transformers. More particularly, the invention comprises an apparatus for monitoring the condition of a dielectric in a sealed transformer.
When a transformer is operating, the temperature of the windings increases to a greater or lesser extent depending upon the load. This causes the temperature of the dielectric cooling liquid to also increase; the volume of the dielectric thus increases with increasing temperature. When the transformer stops operating, the temperature of the windings and of the dielectric decrease and the volume of the dielectric liquid also decreases. This phenomena also exists when the load on a constantly operating transformer is varied, e.g. on a daily basis. Thus, for example, transformer load increases during the day when more energy is being used and decreases at night when less energy is being used.
In early transformers, an air space between the dielectric liquid and the transformer cover permitted expansion without the risk of overflow; in this manner, the transformer "breathed". This, however, permitted moisture in the ambient air to mix with the dielectric liquid, thereby reducing the dielectric strength of the liquid and enhancing oxidation and deterioration of the transformer winding insulation materials.
Various solutions to this problem have been attempted, one of which is the hermetically sealed transformer which does not "breathe". These transformers are generally filled with an oil or askarel (chlorinated liquid of specific density 1.6) dielectric.
A total fill sealed transformer is completely filled at around 20.degree. C. and hermetically sealed so that no air comes in contact with the dielectric liquid. Since the transformer no longer breathes, as the dielectric liquid expands in volume and concurrently increases in temperature (under increased load conditions), an overpressure builds up in the transformer case. Excessive overpressure and/or excessive temperature increases can result in transformer breakdown. In addition, internal incidents in the transformer case may result in gas being released due to the decomposition of the insulators; this results in decreased dielectric levels and the potential risk of flashover between the high voltage connections at the transformer bushings in the case.
It is an object of the present invention to provide a protective device for use with sealed transformers.
It is a further object to provide a protective device which continuously monitors the condition of the dielectric in a sealed transformer.
It is a further object to provide a protective device which permits both visual and electronic monitoring of dielectric conditions.
It is a further object to provide a protective device to enable the transformer dielectric conditions to be monitored from a remote station.