1. Field of the Invention:
This invention relates, in general, to electrical inductive apparatus and, more specifically, to insulation structures for transformer windings.
2. Description of the Prior Art:
Electrical distribution transformers are usually manufactured in relatively large quantities on a production assembly line. The manner in which these transformers are constructed makes it desirable to have a transformer design which may be manufacutred on an assembly line in the shortest possible time. It has always been important to transformer engineers to design the insulation system of a transformer with this objective in mind.
The insulation system is important in controlling the transformed properties and the manufacturing time of distribution transformers. Enough insulation between the transformer winding turns and other components of the transformer must be provided to give the transformer the ability to withstand normal and overvoltage operating conditions and impulse voltages. However, the amount of insulation must be kept to a minimum amount possible in order to save space, material, and manufacturing time. Generally, more insulation in the transformers requires longer degassing and liquid dielectric impregnating cycles during construction. Consequently, it has always been the desire of transformer engineers to keep the amount of insulation in transformers, and in particular the thickness of the insulation, at a practicable minimum.
Various methods have been used to reduce the amount and thickness of insulation in transformers apart from any change in the composition of the insulating material itself. Devices or members which more evenly distribute the voltage stresses along or across the insulation structures have been used to permit more efficient use of the transformer insulation. Other types of grading or distributing arrangements have been used to shape the voltage stresses to change the insulation failure patterns between creep failure and puncture failure to achieve the greatest overall benefit of the insulation material contained within the transformer.
While the methods used to enhance the ability of the solid insulation to perform properly in a transformer system are numerous, almost universally it has been the tendency of transformer engineers to either increase the amount of insulation or change the voltage stress in a region where the insulation was known to be failing under actual field use or during laboratory testing. With either approach, the problems of complexity and economy are detrimentally affected. Therefore, it is desirable, and it is an object of this invention, to provide a transformer insulation structure which performs satisfactorily with a minimum of solid insulating material and stress grading or shaping members.