Electric capacitor

Abstract: An electric capacitor comprising at least two capacitor elements (2) composed of electrodes (5) which are separated by one or more dielectrics (6). When the capacitor is being periodically charged, periodical electrostatic forces give rise to periodically changing mechanical forces between the electrodes in the capacitor. These forces give rise to vibrations which radiate as sound from the surface of the capacitor. According to the invention at least one active or passive spring element (11) is arranged between a pair of adjacent capacitor elements in a package (10) of compressed capacitor elements (2), which spring element is adapted to change the natural frequencies of the capacitor. In this way, the oscillation resistance of the capacitor may be changed at the frequency ranges where the driving force is acting. The vibrations of the surface layer of the capacitor decrease, resulting in reduced sound radiation from the capacitor.

Description: TECHNICAL FIELD The invention relates to an electric capacitor according to the precharacterising part of claim 1. Such a capacitor is especially used as a power capacitor for high voltages. The capacitor elements are wound from electrically conducting and intermediate electrically insulating foils. Such power capacitors are used, inter alia, in filters where, together with reactors and resistors, they form a resonance circuit to filter away harmonics from the electric distribution network. Usually, a large number of power capacitors are stacked in high stands with different planes, insulated towards ground, where each plane represents a certain potential. BACKGROUND ART A power capacitor of the above-mentioned kind is built up of a number of sub-capacitors, so-called capacitor elements. Each element comprises a plurality of very thin layers of electrodes of thin aluminium foils separated by films of a dielectric material and wound into a roll, which is flattened to be able to be optimally stacked into a package. Packages are placed in a capacitor tank filled with an electrically insulating fluid. The elements are connected together into a matrix and connected to two insulated bushings, the number of series-connected and parallel-connected elements, respectively, being determined by the desired capacitance of the capacitor. Usually, a fuse is also connected in series with each element. In addition to elements and fuses, the capacitor also includes special resistors for discharging residual charges. When two separated, electrically conducting plates are charged with charges of opposite potentials, an electrostatic field is generated. In this field a mechanical force arises between the two plates, which attracts the plates towards each other. When the charge is discharged, the attractive force ceases whereas it again arises if the plates are charged with the reverse polarity. When a charge is applied to the plates at a certain frequency, each time with an alternating potential, an alternating mechanical force therefore arises between the plates. This force gives rise to a mechanical movement of the plates, the frequency of this movement (vibration) being twice the frequency of the applied alternating charge. For a wound capacitor element consisting of a very large number of layers, the movement of each inner layer is added to the outer, whereby the movement in the outer layer may become very large. A drawback with such a capacitor element is, therefore, that it starts oscillating when being periodically charged, whereby vibrations of the surface layer of the capacitor element cause radiation of unwanted sound or noise. A common measure for reducing noise from capacitors of the above-mentioned kind, comprising a plurality of elements which when periodically charged generate vibrations which are transported to the outside of the capacitor tank from where they cause sound radiation, is to screen the capacitors or enclosing them completely or partially. A problem which arises when screening the capacitors is that, since the capacitors often have a considerable potential difference in relation to ground, the screening must be placed at a certain safety distance. A larger distance between the screen and the sound source results in inferior screening effect and, in the case of enclosed capacitors, large casings. This increases both weight and consumption of material. Therefore, these measures are costly. Within the audio technique, a capacitor is known from patent JP 60-16 739 (1985), which comprises a capacitor element wound from electrodes and plastic films and which is placed in a casing with an inner and an outer layer. The inner layer may be either of elastic or non-elastic material. The outer layer may also either be of elastic or non-elastic material but must be of a kind opposite to that of the inner layer. This capacitor design does not solve the acoustic problem of the capacitor under discussion, which comprises several capacitor elements and which is so large that a layer of the above-mentioned kind would have no damping effect on the sound generation. SUMMARY OF THE INVENTION The invention aims to provide a capacitor of the kind described above, wherein the internal oscillations of the capacitor, when periodically charged, are suppressed, the capacitor thus generating less sound. To achieve this aim the invention suggests a capacitor according to the introductory part of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the invention are characterized by the features of the additional claims. According to the invention one or more weak spring elements are arranged between capacitor elements or bundles of capacitor elements inside a capacitor tank, thus changing the natural frequencies of the capacitor body. A spring element comprises mass and spring. A spring element may also be fixed to the end of a bundle of capacitor elements. The oscillation resistance of the capacitor body can thus be increased at the frequency ranges within which the driving force is acting, whereby the capacitor generates less sound. Such a spring element may be formed as an evacuated, tight container, preferably of metal. The spring element may comprise a passive or an active resilient (or spring) device. By an active spring device, a vibration-absorbing element is meant, the stiffness of which may be adjusted by externally controllable energy supply. Such an active spring device can also be put into a vibrating state by externally controllable supply of energy, for example from a magnetic field into magnetostrictive material in the spring device, whereby the frequency of the magnetic field and thereby the frequency of the spring device may be changed. BRIEF DESCRIPTION OF THE DRAWINGS By way of example, the invention will now be described in greater detail with reference to the accompanying drawings showing in Figure 1 a partially cut-open power capacitor, Figure 2 a capacitor element, Figure 3 schematically how a plurality of capacitor elements are connected together, Figure 4a the design of a known capacitor package, and its mechanical equivalent, Figure 4b the design of a capacitor package with a weak spring element according to the invention, and its mechanical equivalent, Figure 5 a diagram with vibration velocity versus frequency of a known capacitor package and of a capacitor package comprising a weak spring element, Figure 6 a cross section of a spring element according to the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a power capacitor comprising a capacitor tank 4 which is provided with bushings 1 and which encloses a plurality of capacitor elements 2, impregnated and surrounded by electrically insulating fluid, the capacitor elements 2 being maintained insulated from the capacitor tank 4 by an insulating enclosure 3. Figure 2 shows how a capacitor element 2 is wound from two electrically conducting metal foils 5, preferably of aluminium, separated by three electrically insulating films 6 of dielectric material. The capacitor elements 2 comprise very long foils and films, respectively, which are wound into a roll. To save space in the tank 4, the rolls are compressed and stacked in a row on one another to form a package 10. Figure 3 shows how the elements 2 have been electrically interconnected by means of fuses 7 and discharge resistors 8 and have been connected to the bushings 1. It is known that sound generation from a mechanical system, such as, for example, a capacitor body, depends partly on the applied force, partly on the dynamic properties of the system. If the frequency of the mechanical driving force coincides with any of the natural frequencies of the system, huge periodical deflections arise. These vibrations reach via the insulating fluid the capacitor tank from which the sound is radiated. The intensity of the sound being generated depends on the intensity of the driving force and on the structural damping of the vibration at the natural frequency. Inversely, if the frequencies of driving force and resonance can be mismatched such that they do not coincide, less sound will be generated in that the resistance of the system to oscillate at the frequency of the driving force is increased. By damping the system while at the same time aiming for the natural frequencies and the frequencies of the driving force being located far away from each other, the sound can be reduced. In most situations the frequency pattern of the driving force is given, and therefore a modification of the structural behaviour of the system is the ultimate solution. The natural frequencies of a composite system, however, are difficult to estimate. By numerous acoustic measurements of capacitors under specific load, a mathematical model has been developed for calculating sound radiation from a capacitor. In that connection it has been found that the mechanical properties of the capacitor are dominated by a resonant behaviour which can be directly traced to the fundamental natural frequency for longitudinal waves in the capacitor package 10. The natural frequency is proportional to the square root of the quotient of the stiffness of the capacitor package and the density thereof and inversely proportional to the length of the package. When the package is divided and a weak spring element is placed between the parts, the natural frequency for the package is changed. By choosing the stiffness of the spring, the natural frequency can be altered and suitably chosen in relation to the frequencies of the driving force. Figure 4a schematically shows a package or row of capacitor elements and its corresponding mechanical equivalent. The capacitor package 10 consisting of a number of capacitor elements 2 can be viewed mechanically as two large masses 12 interconnected by a spring 13. Figure 4b shows the same capacitor package 10, which according to the invention is divided into two parts 10a, 10b, separated by a weak spring element 11. Figure 4b also shows the mechanical equivalent circuit diagram of this divided capacitor package. Since the original capacitor package is divided by a weak spring element 11, it can, from a mechanical point of view, be regarded as two smaller capacitor packages according to the above, connected together by a weak spring. The smaller capacitor packages are thereby given smaller masses 14 and, because of the shorter package, a stiffer spring 16. The diagram in Figure 5 shows vibration velocity as a function of frequency. The two curves in the diagram constitute the calculated result of the vibration velocity at the outer ends of the capacitor package at an applied driving force, sweeping by frequency. The dash-dotted curve represents the homogeneous capacitor package 10 which has a resonance at 1000 Hz. The unbroken curve represents the capacitor package 10a, 10b, divided by a weak spring element 11. It has two resonances, one at 350 Hz and one at 1750 Hz. The result also shows that in the latter case an anti-resonance occurs at 500 Hz, which is a result of the mechanical system, divided by spring elements, receiving more degrees of freedom. At such an anti-resonance, the oscillation resistance is infinitely great at the end surfaces of the capacitor package and hence the movement is, in principle, zero whereas the spring elements take up the whole movement. Figure 6 shows an embodiment of a spring element 11. It consists of an evacuated plane-parallel, tight container of metal, the upper side and underside of which in Figure 6 are made of thick, stiff plates 20, and the edges 21 of which on all sides are made of thin sheet strips of semicircular cross section fixed to each plate 20. The completed spring element has a shape like a parallelepiped formed with rounded edges and with the same plane dimensions as the capacitor packages. With knowledge of the driving forces to which a capacitor will be subjected, the mechanical construction of a capacitor can be chosen in an optimum way, by the introduction of one or more weak springs, such that the natural frequencies are located far away from the frequencies of the critical force components. An optimum mechanical embodiment is then obtained when the introduced spring elements are dimensioned such that anti-resonances occur at the same frequencies as the dominating driving forces. As spring elements or vibration-absorbing elements, a plurality of materials and embodiments are possible, both with passive and active resilient properties. Passive spring elements are such devices where the resilient properties are determined by the shape and material of the spring and cannot be changed by external influence. An active spring element may be built up in the same way as a passive element but of a material whose properties, such as volume or shape, may be changed by external energy supply, for example magnetostrictive or piezoelectric materials. As in the embodiment shown, the spring element may be constructed as a hollow parallelepiped, the walls of which are tight (impermeable for gas), of some material corrodibly resistant to the insulating fluid, such as, for example, metal or plastic. The spring element, instead of consisting of a hollow body, may also consist of a solid body the material of which exhibits elastic properties and is resistant to the surrounding medium. The spring element comprises the desired vibration-absorbing properties and may either itself constitute the resilient device, or a device with the desired properties may be enclosed in a container which constitutes the spring element.

Claims: Electric capacitor comprising at least two capacitor elements (2) which are composed of electrodes (5) separated by one or more dielectrics (6) and which capacitor elements are arranged in a row to form a capacitor package (10), characterized in that at least one spring element (11) is arranged between a pair of adjacent capacitor elements in said row or fixed at the outside of the capacitor element at one end of said row and that the stiffness of said spring element is adapted or adaptable such that the external vibrations of the capacitor, caused by the oscillating driving forces in the capacitor elements, are reduced, resulting in reduced sound radiation from the capacitor. Electric capacitor according to claim 1, characterized in that each capacitor element (2) comprises a roll wound from metal foils (5) and dielectric films (6) placed between the metal foils. Electric capacitor according to claim 1 or 2, characterized in that the capacitor elements (2) and the at least one spring element are surrounded by an electrically insulating fluid enclosed in a capacitor tank (4). Electric capacitor according to claim 3, characterized in that said capacitor tank comprises bushings (1) and protective insulation (3). Electric capacitor according to any of the preceding claims, characterized in that said at least one spring element (11) comprises an evacuated, tight container of metal or plastic. Electric capacitor according to any of the preceding claims, characterized in that said spring element (11) comprises a passive spring device determined by material and shape. Electric capacitor according to any of claims 1 to 6, characterized in that said spring element (11) comprises a active spring device which is actively operable by external energy supply.

Inventors: JOHANSSON CLAES-GOERAN, WINROTH OLLE, JOHANSSON, CLAES-GOERAN, WINROTH, OLLE, Johansson, Claes-Göran
Applicants: ASEA BROWN BOVERI, ASEA BROWN BOVERI AB
Application year: 1995
Main classification: H01G2
IPC codes: H01G2, H01G4, G10K11