Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2000-08-03
2002-05-21
Kelly, Cynthia H. (Department: 2831)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S375000, C428S379000, C428S377000, C427S117000, C427S118000, C427S434600, C427S439000, C156S048000, C156S053000
Reexamination Certificate
active
06391447
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for manufacturing an insulated electric device comprising at least one current- or voltage-carrying body, i.e. a conductor, and a dielectric such as an electrically insulating dielectric system. The dielectric system comprises an electrically insulating solid part that is porous, fibrous, fibrous and/or laminated and which is impregnated with a dielectric fluid, such as oil or a mass.
The present invention relates in particular to a method for manufacturing an insulated electric DC cable comprising a conductor and an impregnated insulation system. The impregnated insulation system comprises a plurality of layers, such as an inner semi-conducting layer, an insulation and an outer semi-conduction layer. The present invention also relates to the manufacture of other static electric machines, such as transformers, capacitors and the like. In another aspect the present invention relates to a porous, fibrous and/or laminated body to be used in such a process for manufacturing an electric device comprising a conductor and an impregnated, porous, fibrous and/or laminated insulation system and the use of such body in an electric device.
BACKGROUND ART
Many of the first electrical supply systems for transmission and distribution of electrical power were based on DC technology. However, these DC systems were rapidly superseded by systems using alternating current, AC. The AC systems had the desirable feature of easy transformation between generation, transmission and distribution voltages. The development of modern electrical supply systems in the first half of this century was exclusively based on AC transmission systems. By the 1950s there was a growing demand for long transmission schemes and it became clear that in certain circumstances there could be benefits by adopting a DC based system. The foreseen advantages include a reduction of problems encountered in association with the stability of the AC-systems, a more effective use of equipment as the power factor of the system is always unity and an ability to use a given insulation thickness or clearance at a higher operating voltage. Against these very significant advantages has to be weighed the cost of the terminal equipment for conversion of the AC to DC and for inversion of the DC back again to AC. However, for a given transmission power, the terminal costs are constant and therefore, DC transmission systems are economical for schemes involving long distances, such as for systems intended for transmission from distant power plants to consumers but also for transmission to islands and other schemes with transmission distances where the savings in the transmission equipment exceeds the cost of the terminal plant.
An important benefit of DC operation is the virtual elimination of dielectric losses, thereby offering a considerable gain in efficiency and savings in equipment. The DC leakage current is of such small magnitude that it can be ignored in current rating calculations, whereas in AC cables dielectric losses cause a significant reduction in current rating. This is of considerable importance for higher system voltages. Similarly, high capacitance is not a penalty in DC cables.
As in the case of AC transmission cables, transient voltages is a factor that has to be taken into account when determining the insulation thickness of DC cables. It has been found that the most onerous condition occurs when a transient voltage of opposite polarity to the operating voltage is imposed on the system when the cable is carrying full load. If the cable is connected to an overhead line system, such a condition usually occurs as a result of lightning transients.
A typical DC-transmission cable include a conductor and an insulation system comprising a plurality of layers, such as an inner semi-conductive shield, an insulation body and an outer semiconductive shield. The cable is also complemented with casing, reinforcement, etc., to withstand water penetration and any mechanical wear or forces during, production installation and use. Almost all the DC cable systems supplied so far have been for submarine crossings or the land cable associated with them. For long crossings the mass-impregnated solid paper insulated type cable is chosen because there are no restrictions on length due to pressurizing requirements. It has to date been supplied for operating voltages of up to 450 kV. The voltage is likely to be increased in the near future. To date a wound body comprising an essentially all paper tape, i.e. a tape based on paper or cellulose fibers, is used but application of laminated tape materials such as a laminated polypropylene paper tape is being persued. The wound body is typically impregnated with an electric insulation oil or mass. A commercially available insulated electric DC-cable such as a transmission or distribution cable designed for operation at a high voltage, i.e. a voltage above 100 kV, is typically manufactured by a process comprising the winding or spinning of a porous, fibrous and/or laminated solid insulation based on cellulose or paper fiber followed by the impregnation with the electric insulating oil. The impregnation, that typically is carried out in batches after the insulation has been applied around the conductor, is time consuming and needs to be carefully monitored and controlled. For impregnation of a DC-cable, where several kilometers of cable is impregnated with a typically viscous fluid, the process has a process cycle time extending over days or even weeks or months. In addition, this time consuming impregnation process is made according to a carefully developed and strictly controlled process cycle with specified ramping of both temperature and pressure conditions in the impregnation vessel used during heating, holding and cooling to ensure a complete and even impregnation of the fiber-based insulation.
A transformer or a reactor for use in a DC-transmission network, at a power generating utility or at a large consumer installation such as an industrial plant also typically comprises porous, fibrous and/or laminated insulating bodies disposed around and between conductors. Typically preformed bodies, such as so called pressboards, manufactured by dewatering and/or pressing a slurry comprising the fibers are used. The bodies are impregnated by a dielectric fluid to impart the desired electrical properties needed. The impregnation of these bodies in a transformer, although not time consuming, is a sensitive process and specific demands are put on the fluid, the medium to be impregnated and the process variables used for impregnation.
A capacitor exhibits a laminated structure with a dielectric medium comprising one or more polymeric films disposed between two electrodes. Typically films of polyolefin or thermoplastic polyester are used. The capacitor is typically impregnated with a dielectric fluid. The impregnation of the laminated structure of a capacitor, although not time consuming, is a sensitive process and specific demands are put on the fluid, the medium to be impregnated and the process variables used for impregnation.
The active part of the impregnated insulation systems described in the foregoing is the solid part, such as the cellulose fibers, the polymeric films or any laminate or tape used. The dielectric fluid protects the insulation against moisture pick-up and fills all pores, voids or other interstices, whereby any dielectrically weak air contained in the insulation is replaced by the dielectric fluid.
To ensure a good impregnation result, a fluid exhibiting a low-viscosity is desired. But the fluid shall also preferably be viscous at operation conditions for the electrical device to avoid migration of the fluid. Darcy's law (1) is often used to describe the flow of a fluid through a porous, fibrous or capillary medium.
(
1
)
:
v
=
k
Δ
P
μ
L
Where v is the so called Darcy velocity of the fluid, defined as the volume flow divided by the sample area, k
Bergkvist Mikael
Felix Johan
Kornfeldt Anna
Nordberg Per
ABB AB
Dykema Gossett PLLC
Gray J. M.
Kelly Cynthia H.
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