Electricity: electrical systems and devices – Electrostatic capacitors – With protection or compensating means
Reexamination Certificate
1999-04-02
2001-04-03
Kincaid, Kristine (Department: 2831)
Electricity: electrical systems and devices
Electrostatic capacitors
With protection or compensating means
C361S275100, C361S301400
Reexamination Certificate
active
06212055
ABSTRACT:
The present invention relates to a self-healing power capacitor comprising at least one capacitor unit, each capacitor unit comprising at least one winding, made of at least two films of insulating material on which a metal coating has been applied, said windings of each capacitor unit being provided with a first and a second connection electrode, said windings being surrounded by an encapsulation material and housed in a first casing, said capacitor having at least one protection element for each capacitor winding, said protection element being mounted in a second casing of which one side is formed by a membrane, said protection element being provided for activating, upon a pressure exerted on said membrane by a gas produced by a short-circuit within said windings, a current interruption element connected in series with one of said electrodes, said protection element being separated from an end-face of said windings.
Such a self-healing power capacitor is known from GB-A-2 204 996. The power capacitor is i.a. used in electrical networks to compensate reactive power (correct cos &phgr;) or as parts of filters to absorb harmonic currents and thereby lower harmonic voltage distortion on the networks. The known power capacitors are manufactured by first evaporating a metallic material on a film and then wind the metallized film. A first and a second electrode are applied on the extremities of the metallized film, in order to provide electrical connectors. The windings are surrounded by an encapsulation or filling material, such as for example resin, oil or a gel. The windings and their encapsulation material are housed in the first casing.
When the metallized film of a self-healing capacitor breaks down, the short-circuit is removed automatically by the following process. As a short-circuit is established, the current through the failed spot will increase rapidly. This high current will create a very high current density in the thin evaporated electrode around the failed spot. The current will rapidly become so high that the evaporated electrode will transform into a gaseous plasma and blow away from the film around the failed spot. Without the electrode around the failed spot, insulation will be re-established. The capacitor has self-healed and may continue to operate normally. The process of self-healing is very short (microseconds) and the area of insulation created around the failed spot is very small (a few square millimeters). Since the area of one capacitor winding is typically ten's of square meters, the relative loss in capacitance resulting from one self-healing operation is very small. Many thousands of self-healing operations may occur without any noticeable effect on the capacitor or its performance. As the dielectric system ages a situation will develop where the thermal and dielectric load of the system become too high for the mechanism to function. At this point in time an avalanche of self-healings will occur and create a short-circuit of the capacitor element. Self-healing capacitors however do not always create short-circuits with a low voltage drop, i.e. a low short-circuit impedance. The short-circuit impedance may vary between low values and quite high values.
In order to protect self-healing power capacitors against such short-circuits, protection elements are built-in in the casing of the power capacitor. With the known capacitor, the protection element is formed by an overpressure disconnector. The principle of such an overpressure disconnector is that gases, produced by a short-circuit within the windings of the capacitor, will accumulate in the space between the windings and the second casing and cause an overpressure to build up. This overpressure bulges the membrane of the protection element, which is applied above the windings.
In the known capacitor the membrane comprises a wire which is connected in series with one of the electrodes. The overpressure created by the gas causes the membrane to bulge. This bulging causes the wire to break and thus the current flow in the winding to be interrupted.
A drawback of the known self-healing power capacitor is that the second casing, housing the protection element, is fixed to the upper end wall of the first casing. This signifies that the protection element is not physically separated from the end walls of the first casing. Deformation or damages to the first casing could thus also cause damages to the protection element. Moreover an erroneous mounting of the second casing on the upper end wall of the first casing could lead to a malfunctioning of the protection element. Also a leakage in the upper wall of the first casing could lead to a malfunctioning of the protection element.
It is an object of the present invention to realise a self-healing power capacitor wherein the protection element is mounted in a less vulnerable manner.
For this purpose a self-healing power capacitor is characterised in that said second casing is lodged inside said first casing, separated from the top and/or bottom walls belonging to said first casing, said second casing being lodged in said encapsulation material and having at least one wall separated by a narrow gap from said windings. Since the second casing is lodged separated from the side walls of the first casing, the protection element and the windings form separate elements which are together mounted in the first casing, which no longer needs to be provided with an upper end wall. The encapsulation of the protection element and the one of the windings enable to mount them in the first casing in a reliable and easy manner. As the protection element is independent of the side walls, it is less vulnerable for damages caused to the first casing. As windings and protection element are both encapsulated but separated from each other by the narrow gap, the gas can only accumulate in the narrow gap. The dimension of the gap and the fact that the protection element is lodged in the encapsulation material, will cause the gas to remain in the gap and thus exert enough pressure on the membrane, and thus providing a reliable operation of the protection element.
A first preferred embodiment of a self-healing power capacitor according to the invention is characterised in that said current interruption element comprises a fuse and said protection element comprises a switch, provided to be switched by said membrane, said switch being connected in parallel with said first and second electrode. In this embodiment the membrane acts as a switch, which short-circuits the first and the second electrode. As the fuse is connected in series with one of the electrodes, a short-circuit of the electrodes will melt the fuse and so doing, disconnect the capacitor unit.
A second preferred embodiment of a self-healing power capacitor according to the invention is characterised in that said current interruption element comprises a fuse wire connected in series with one of said electrodes and placed in front of a cutting member, which is part of said protection element, said cutting member being provided to be moved by said membrane towards said wire in order to cut said wire, when said pressure is exerted on said membrane. With this embodiment, the movement of the membrane, caused by a gas pressure applied thereon, will cause the cutting element to be moved towards the fuse wire. Once the cutting element reaches the wire, it will cut the latter, thereby disconnecting the capacitor unit. The travel distance of the cutting element is calibrated in such a manner as to enable a reliable protection element.
Preferably said membrane is a bi-stable membrane. A bi-stable membrane has the advantage that it provides either a connection or a disconnection of the capacitor unit, which enables to contribute to a reliable operating protection element.
The invention will now be described in more detail by means of the annexed drawings which show some examples of a self-healing power capacitor according to the present invention.
REFERENCES:
patent: 3475659 (1969-10-01), Buice et al.
patent: 4635163 (1987-01-01),
Bonhomme Henri
Lovkvist Thomas
Monni Cipriano
Asea Brown Boveri Jumet S.A. (ABB)
Jacobson Price Holman & Stern PLLC
Kincaid Kristine
Thomas Eric
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