Energy discharge capacitors constructed with metallized electrodes are smaller, lighter and have a greater energy density than those using discrete metal foils this is because the weight and diameter of the wound capacitors element is much smaller than comparable designs using discrete metal foils. In addition, the metallized design can be operated at much higher voltage stresses, which allows the use of thinner dielectric layers. These factors contribute to the higher energy density of metallized designs.

The high voltage stress of the metallized design is the result of the "selfhealing" or "clearing action" which takes place when a fault or momentary short circuit occurs in the dielectric. Clearing takes place when a shorting current vaporizes and removes the metallization in the immediate vicinity of the breakdown in the dielectric. The vaporization of electrode continues, ideally in a circular pattern, until the current path at the point of the breakdown becomes too great, stopping the short-circuiting between electrodes. While this clearing action is beneficial, in that the capacitor returns to an operable condition almost instantaneously, it has a major drawback in high energy capacitors. The resulting deterioration of the dielectric in the immediate area adjacent to the fault, if severe enough, could trigger additional breakdown points which would in turn clear and so on. This chain reaction or avalanche is signaled by a loss of capacitance and low insulation resistance, leading eventually to the inability to store the desired energy or to a

For about the last twenty years, Sprague Electric has been supplying a composite dielectric system which, to a large extent, inhibits its damaging clearing effect. By coating polyester with a proprietary lacquer, a metallized energy discharge capacitor can e constructed which allows an effective clearing action to take place that does not have th deleterious carbonization and destruction polyester alone (Figure 3). This proprietary film allows a higher voltage stress, and thus capacitors of higher energy density than could be otherwise achieved, When a clearing takes place, both the metallization and the coating are vaporized from the area adjacent to the fault, but the puncture hole in the polyester is kept to a minimum.

A useful dielectric configuration for high voltage applications is to convolutely wind together plain plastic and metallized paper dielectrics to form a multi-layer capacitor winding. These windings are then usually impregnated. This construction type takes advantage of high breakdown strength and dielectric constant of the plastic, the clearability of metallized paper and the wicking action of the impregnant throughout the paper to further enhance the breakdown voltage (Figure 4).

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