1. Field of the Invention
The present invention relates to a power capacitor primarily intended for a rated voltage in excess of 1 kV, .e.g. 5 kV, preferably at least 10 kV. The present invention also relates to a capacitor bank having a plurality of power capacitors.
2. Description of the Background Art
Power capacitors are important components in systems for transmission and distribution of electric power. Power capacitor installations are used primarily to increase power transmission capability through parallel and series compensation, for voltage stabilisation through static var-systems, and as filters for eliminating harmonics.
Capacitors have a phase angle that is close to 90° and therefore generates reactive power. By connecting a capacitor in the vicinity of the components that consume reactive, power, the desired reactive power can be generated there. Cables can thus be utilized to the full extent for transmitting active power. The consumption of reactive power by the load may vary and it is desirable to constantly generate a quantity of reactive power equivalent to the consumption. For this purpose a plurality of capacitors are connected via series and/or parallel connection in a capacitor bank. A requisite number of capacitors can be connected corresponding to the reactive power consumed. Compensating for the consumed power by utilizing capacitors in the manner described above is called phase compensation. A capacitor bank in the form of a shunt battery is arranged for this purpose in proximity of the components consuming reactive power. Such a shunt battery consists of a plurality of capacitors connected together. Each capacitor in its turn comprises a plurality of capacitor elements. The structure of such a conventional capacitor is explained below.
A shunt battery usually comprises a number of chains of a plurality of series-connected capacitors. The number of chains is determined by the number of phases, which is 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 electric power is arranged a certain distance from the ground or from points in the surroundings having earth potential. This distance is dependent on the voltage in the cable. The capacitors are thus connected in series from the first capacitor, which is connected to the cable, and downwards. A second capacitor, arranged at an end of the chain of series-connected capacitors opposite to the first capacitor, is connected to earth potential or to a point in the electric system that has zero potential (e.g. unearthed 3-phase system).
The number of capacitors and their design are determined so that the permitted voltage (rated voltage) over the series-connected capacitors corresponds to the voltage in the cable. A plurality of capacitors is thus connected in series and arranged in stands or on platforms insulated from earth potential. Such a capacitor bank thus comprises a plurality of different components and demands relatively much material. Furthermore, a relatively robust construction is required if the stand/platform is to withstand external influence in the form of wind, earthquakes, etc. Considerable work is thus required to construct such a capacitor bank. This problem is particularly pronounced 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 spaced along such a cable in order to generate the necessary reactive power. A plurality of capacitors is series-connected 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 a 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, which is defined by the fundamental frequency of the current and is usually 50 Hz. 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. Cylindrical capacitor containers are also known through EP Patent No. 0190 621, EP Patent No. 0 416 164 and EP Patent No. 0 702 380. None of these, however, relates to a power capacitor for high voltage.
One problem with a capacitor of known type, e.g. of the type described in U.S. Pat. No. 5,475,272 mentioned above, is that the capacitor elements included must be insulated from the container. The insulation must withstand voltage stresses considerably higher than the rated voltage of the capacitor. The aim is to fill the container with capacitor elements as efficiently as possible. Their external, flattened shape is unfavorable with regard to electric field amplification due to protruding foils, small radii, etc. They must also be connected together via internal connecting wiring in a manner that often creates further local irregularities in the electric field. This leads to considerable electric strength demands on the insulation against the container. If the capacitor is of a type that lacks fuses, short-circuiting between a capacitor element and the container may result in large amounts of energy being discharged at the defective point. The consequence may be an explosion with major damage.
Another problem with conventional power capacitors is the sound that is generated. The sound generation is strongest when the vibrations generated by the electrical voltage load coincide with the mechanical resonance frequency of the capacitor. The resonance frequency is proportional to the square root of the quotient between the rigidity of the capacitor package perpendicular to the electrode layers and inversely proportional to the extension of the package perpendicular to the electrode layers.