Power factor correction capacitors, known as power capacitors, are made up of individual capacitor windings. Each winding consists of at least two electrode foils separated by at least one dielectric spacer film. The windings are placed together and encircled by wraps of dielectric insulating film.
The windings of a power capacitor must be insulated from the metal case of the capacitor, both for the safety of the personnel who may come in contact with the case, and for preventing breakdown of the capacitor when high overpotentials, such as lightning on the feeders or switching impulses, are imposed on the capacitor.
Once in the case, the individual capacitor windings are connected internally in series/parallel combinations so that the required voltage and KVAR rating can be obtained. Finally, the dielectric spacer films are impregnated with a dielectric fluid as disclosed in the U.S. Pat. No. 4,117,579 granted to Shaw et al.
In the past, the dielectric spacer and insulating films were entirely of dielectric impregnated paper. Due to the anticipated advantages of more uniform product quality, smaller dielectric constant, and higher voltage stress characteristics, it has long been believed that it would be desirable to replace all the paper in a power capacitor with synthetic resin film.
With recent advances in dielectric impregnation technology, it has been found to be possible to manufacture workable capacitors having all synthetic resin spacer films as shown in U.S. Pat. No. 3,363,156 granted to Cox.
However, despite the change to synthetic resin spacer films in the windings, those skilled in the art were unable to successfully replace the paper dielectric insulating films with synthetic resin films between the windings and the case. While some of the paper dielectric insulating films could be changed to a synthetic resin, some paper was still required within the capacitor; attempts to replace all of the paper with all synthetic resin films have been unavailing.
For example, when taking advantage of the high voltage stress characteristics of a synthetic resin of the polyolefin family, polypropylene, and replacing paper with comparable thickness polypropylene in thinner total thickness because of polypropylene's higher voltage stress capability, the all polypropylene capacitors would fail under the Basic Impulse Level (BIL) test which simulates high impulse overpotentials.
Other experiments have led those skilled in the art to believe that an all synthetic resin film capacitor was not possible. For example, when nine layers of ten mils thickness poylpropylene directly replaces the conventional thirteen layers of seven mils thickness paper for a 95 KVBIL capacitor, current draw or failure occurs at withstand voltages which are 10% low. Even when additional layers of polypropylene are added to increase the total thickness by 60%, a proportional increase in BIL withstand voltage capability could not be achieved.
Thus, there has been a long halt to progress in developing an all synthetic resin film power capacitor in which there is no paper present.