Power capacitor and use and method related thereto

Details Power capacitor and use and method related thereto Power capacitor and use and method related thereto

2002-12-09

2004-10-19

Dinkins, Anthony (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S524000, C361S530000, C361S318000

Reexamination Certificate

active

06807046

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a power capacitor of the type described in the preamble to claim
1
. The power capacitor in accordance with the invention is primarily intended for a rated voltage exceeding 1 kV, e.g. 5 kV, preferably at least 10 kV.
Power capacitors are important components in systems for the transmission and distribution of electric power. Power capacitor installations are used primarily to increase the power-transmission capability through parallel and series compensation for voltage stabilisation by means of static var-systems and as filters for the elimination of harmonics.
Second and third aspects of the invention relate to use of the type described in claim
21
, and to a method of the type described in claim
23
.
Capacitors have a phase angle close to 90°, and therefore generate reactive power. By connecting capacitors in the vicinity of the components that consume reactive power, the desired reactive power can be generated there. Cables can thus be utilised to the full for transmitting active power. The consumption of reactive power in a load may vary and it is desirable to constantly generate a quantity of reactive power corresponding to the consumption. For this purpose, a plurality of capacitors are connected via series and/or parallel connection in a capacitor bank. The number of capacitors required to correspond to the consumed reactive can be connected in. Compensating for consumed power by utilising capacitors in the manner described above is known as phase compensation. For this purpose a capacitor bank in the form of a shunt battery is arranged in the vicinity of the components consuming reactive power. Such a shunt battery consists of a plurality of capacitors connected together. Each capacitor comprises a plurality of capacitor elements. The structure of such a conventional capacitor is described below.
A shunt battery usually comprises a number of chains of a plurality of capacitors connected in series. The number of chains is determined by the number of phases, usually three. The first capacitor in a chain is thus connected to a cable for transmitting electric power to the consuming component. The cable for transmitting is arranged a certain distance from the ground or from points in the surroundings with earth potential. This distance is dependent on the voltage of the cable. The capacitors are then connected in series from the first capacitor, which is connected to the cable, and downwards. A second capacitor arranged at the opposite end of the chain of series-connected capacitors is connected to earth potential or to a point in the electrical system having zero potential (e.g. non-earthed 3-phase system). The number of capacitors and their design are determined so that the permissible voltage (rated voltage) over the series-connected capacitors corresponds to the voltage of the cable. A plurality of capacitors are therefore series-connected and arranged in stands or on platforms insulated from earth potential. Such a capacitor bank thus includes a plurality of different components and requires relatively large quantities of material. It also requires a relative robust construction so that the stand/platform can withstand the effects of wind, earthquakes, etc. Considerable work is thus required to construct such a capacitor bank. This problem is particularly noticeable when the capacitor bank consists of a large number of capacitors. The capacitor bank also takes up a relatively large area on the ground.
Long cables for alternating voltage are inductive and consume reactive power. Capacitor banks for series-compensation are therefore arranged with regular spacing along such a cable in order to generate the necessary reactive power. A plurality of capacitors is connected in series to compensate the inductive voltage drop. In a capacitor bank for series-compensation, as opposed to a shunt battery, the series-connection of capacitors usually only takes up part of the voltage in the cable. The chains of series-connected capacitors included in the capacitor bank for series compensation are also arranged in series with the cable to be compensated.
A conventional capacitor bank comprises a plurality of capacitors. Such a capacitor in turn comprises a plurality of capacitor elements in the form of capacitor rolls. The capacitor rolls are flattened and stacked one on top of the other to form a stack 1 m tall, for instance. A very large number of dielectric films with intermediate metal layers will be arranged in parallel in the vertical direction of the stack. When a voltage applied over the stack increases, the stack will be compressed somewhat in vertical direction, due to Coulomb forces that act between the metal layers. For the same reason, if the voltage decreases the stack will expand somewhat in vertical direction. The stack formed has a specific mechanical resonance frequency or natural frequency, which is relatively low. The mechanical resonance frequency of the stack is amplified by specific frequencies of the current, which may produce a loud noise. The mains frequency constitutes such a frequency. However, amplification of the mechanical resonance frequency can also be effected by harmonics in the current.
An example of a power capacitor of this known type is described in U.S. Pat. No. 5,475,272. A high-voltage capacitor constructed from a plurality of capacitor elements stacked one on top of the other and placed in a common container, is thus described here. The container is made of metal in conventional manner. The electrical lead-throughs are made of porcelain or polymer. The publication also describes various alternative couplings for connecting the capacitor elements in series or in parallel.
DESCRIPTION OF THE INVENTION
In known capacitors of this type the capacitor elements are impregnated with oil. The oil is also arranged to surround the capacitor elements and thus fill up the space between these and the wall of the container. Oil is satisfactory from the insulation aspect but entails a number of drawbacks. Damage to the container or defective sealing may result in oil leakage which may damage the function of the capacitor as well as contaminating the environment.
Against this background, the object of the present invention is to overcome the problem of oil leakage from a power capacitor of the type under consideration.
From a first aspect of the invention this object is achieved by a power capacitor of the type described in the preamble to claim
1
comprising the characteristic features defined in the characterizing part of the claim. The insulating medium in the form of a dielectric fluid, e.g. an oil comprising a gelling component. The dielectric fluid may be electrically insulating oil to which gelling components have been added. In this context it should be understood that the component may consist of a mixture of part-components. The gel surrounding the capacitor elements in the container thus replaces the oil normally used for this purpose. Any damage to the container will not therefore result in oil leakage since no liquid oil is present. The consistency of the dielectric fluid prevents the formation of drops and it is therefore unable to leak out. Since the container is made of a polymer material and therefore yields to a certain extent and is negligibly sensitive to cracking, it has properties of significance in combination with the enclosed gel. The material combines good insulation ability with other desired features such as strength, manageability and cost. A design in accordance with the invention also offers favourable conditions for overcoming the problem of thermal conduction and insulation around the edges of the capacitor windings, which is a particular problem with power capacitors for high voltage.
It is known per se to gel an oil for use in electrical arrangements. PCT/SE 98/02314, for instance, describes the arrangement of an electrical arrangement comprising an electric conductor and an insulation system with a porous, fibre-based or laminated structure. The structure is impregnate

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