1. Field of the Invention
This invention relates to a capacitor element and to a power capacitor built up of at least one such capacitor element. Usually a power capacitor comprises a plurality of capacitor elements which are connected in series and/or parallel so as to obtain the desired capacitor data.
2. Description of the Prior Art
A known capacitor element is often of the wound type, i.e. it is built up of several wound turns of metal foils, serving as electrodes, and a solid dielectric, usually in the form of polymer films and paper, arranged therebetween. After drying, the capacitor is impregnated with a suitable dielectric fluid in order to fill up all existing voids in the capacitor units with fluid and to suppress partial discharges. In this connection, the paper layers act as wicks for sucking the dielectric fluid into the inner portions of the capacitor elements. Capacitors with a solid dielectric of only polymer film are also known.
In the power capacitors which occur most frequently on the market, in the or each capacitor element, the polymer films have greater width than the metal foils. Films and foils are thereby arranged so that longitudinal edges of the metal foils are located inside longitudinal edges of the polymer films and are exposed to the end surfaces of the capacitor element between adjacently located polymer films. In such capacitor elements, the foils are arranged in electrical contact with separate current tap strips, which from the interior of the capacitor element protrude outside one or both end surfaces of the capacitor element, i.e. outside one or both longitudinal edges of the polymer films. The purpose of the current tap strip is to connect the metal foils to external connection conductors. Each metal foil electrode in the element is thereby provided with such a number of current tap strips as is required for supply and discharge of electric current of the intended magnitude to and from the capacitor units. Often the number of current tap strips is small and the current tap strips arranged at great mutual distances, so that they only cover a few percent, or less, of the length of each metal foil.
According to a known, similarly commercial modification of the capacitor described above, a capacitor element is provided in which there are first and second metal foils, the first metal foil being provided with both its longitudinal edges folded inwardly (i.e. at the two end surfaces of the capacitor element) and the second metal foil having unfolded longitudinal edges. Also the edges at the beginning and end of the wound metal foils may be transversely folded in, i.e. folded across the longitudinal direction of the foils. The folding results in improved partial discharge properties, which is due to a folded edge becoming relatively even and rounded and without sharp points and irregularities, which may occur on an edge which is cut in the usual way.
The present invention seeks to provide a capacitor element or capacitor elements which when incorporated in a capacitor markedly improves the partial discharge properties of the capacitor, the or each capacitor element comprising wound metal foils separated by solid dielectric material comprising polymer films and, at each end surface of the capacitor element, one of the metal foils being arranged with a folded edge inside the edge of the polymer films, and one of the metal foils being arranged with an unfolded edge. Current tap strips are also arranged to protrude outside one or both end surfaces of the capacitor element. The improvement of the partial discharge properties can be achieved with retained, or with only a negligible increase of the thickness of the capacitor. To the same extent as the partial discharge properties of the capacitor are improved, the performance and reliability of operation of the capacitor are improved since these properties are dependent on the partial discharge properties.