Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2000-08-03
2002-05-07
Kelly, Cynthia H. (Department: 2831)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S372000, C428S375000, C428S396000, C174S02300R, C174S02300R, C174S1200SR, C174S1100PM, C427S117000, C427S118000, C427S434600, C427S439000
Reexamination Certificate
active
06383634
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a dielectric gelling composition comprising a dielectric fluid and a gelling additive, in particular an electrical insulation oil to which one or more gelling additives, gelators, i.e. compounds that impart a gelling behaviour in the dielectric fluid, have been added. In particular the invention relates to such a gelling composition exhibiting a thermo-reversible transition between the easy flowing fluid state at high temperatures and a highly viscous and elastic gelled state at low temperatures, a thermo-reversible liquid-gel transition.
The present invention relates in another aspect to the use of such a gelling composition as part of an electrical insulation system for an electric device.
In a particular aspect the present invention relates to an insulated electric direct current cable, an insulated DC-cable, with an insulation system comprising such a dielectric gel with a thermo-reversible liquid-gel transition. The present invention also relates to a method for manufacturing such DC-cable. The insulated DC cable is suited for transmission and distribution of electric power. The insulation system comprises a plurality of functional layers, such as an inner semi-conductive shield, an insulation and an outer semi-conductive shield, wherein at least the insulation comprises a porous, fibrous and/or laminated body impregnated with a dielectric fluid.
BACKGROUND ART
Electrical insulation oils and other dielectric fluids are used in electric insulation systems for devices such as transformers, capacitors, reactors, cables and the like. The dielectric fluids are typically used in combination with a porous, fibrous and or laminated solid part, which is impregnated with the dielectric fluid, the electric insulating oil, but also as encapsulants to prevent water penetration. The active part of an impregnated insulation is the solid part. The oil protects the insulation against moisture pick-up and fills all pores, voids or other interstices, whereby any dielectrically weak air in the insulation is replaced by the oil. Impregnation is typically a time consuming and delicate process carried out after the solid part of the insulation has been applied and needs to be carefully monitored and controlled. For example, the impregnation of a DC-cable intended for a long distance transmission of electric power, where several kilometres of a cable are treated, typically exhibits a process cycle time extending over days or weeks or even 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. The impregnation of other insulation systems comprising dielectric fluids such as transformers, capacitors and the like is, although not as time consuming as the impregnation of a DC-cable, a sensitive process and specific demands are put on the impregnant, the medium to be impregnated and the process variables used for impregnation.
To ensure a good impregnation result, a fluid exhibiting a low-viscosity is desired. The fluid shall also preferably be viscous at operation conditions for the electrical device to avoid migration of the fluid in the porous insulation. Darcy's law (1) is often used to describe the flow of a fluid through a porous or capillary medium.
(
1
)
:
v
=
k
Δ
P
μ
L
In this law v is the so called Darcy velocity of the fluid, defined as the volume flow divided by the sample area, k is the permeability of the porous medium, &Dgr;P is the pressure difference across the sample, &mgr; is the dynamical viscosity of the fluid and L is the thickness of the sample. The flow velocity of a fluid within a porous medium is essentially reciprocally proportional to the viscosity. A fluid exhibiting a low-viscosity or a highly temperature dependent viscosity at operating temperature will have a tendency to migrate under the influence of temperature fluctuations naturally occurring in an electric device during operation and also due to any temperature gradient building up across a conductor insulation in operation and might result in unfilled voids being formed in the insulation. Temperature fluctuations and temperature gradients are present in a high-voltage DC cable, and thus any problem associated with migration of the dielectric fluid must be carefully considered. Unfilled voids or other unfilled interstices or pores in an insulation operating under an electrical high-voltage direct current field constitute deficiencies where space charges tend to accumulate. Accumulated space charges might under unfavorable conditions initiate dielectric breakdown through discharges which will degrade the insulation and ultimately might lead to its breakdown. The ideal dielectric fluid should exhibit a low-viscosity under impregnation and be highly viscous under operation conditions.
Conventional dielectric oils used for impregnating a porous, fibrous or laminated conductor insulation in an electric device such as a DC cable exhibit a viscosity that decreases essentially exponential as the temperature increases. The impregnation temperature must therefore be substantially higher than the operation temperature to gain the required decrease in viscosity due to the low temperature dependence of the viscosity at high temperatures. In comparison, the temperature dependence of the viscosity at temperatures prevailing during operation conditions is high. Small variations in impregnation or operation conditions affect the performance of the dielectric fluid and the conductor insulation. Oils are therefore selected such that they are sufficiently viscous at expected operation temperatures to be essentially fully retained in the insulation also under the temperature fluctuations that occur in the electric device during operation. The retention shall also be essentially unaffected of any temperature gradient building up over an insulation. This typically leads to a high impregnation temperature being used to ensure that the insulation will be essentially fully impregnated. However, a high impregnation temperature is disadvantageous as it risks effecting the insulation material, the surface properties of the conductor, and promoting chemical reactions within and between any material present in the device being impregnated. Also energy consumption during production and overall production costs are negatively affected by a high impregnation temperature. Another aspect to consider is the thermal expansion and shrinkage of the insulation which implies that the cooling must be controlled and slow, adding further time and complexity to an already time consuming and complex process. Other types of oil impregnated cables employ a low viscosity oil. However, these cables then comprise tanks or reservoirs along-the cable or associated with the cable to ensure that the cable insulation remains fully impregnated upon thermal cycling experienced during operation. With these cables, filled with a low viscosity oil, there is a risk for oil spillage from a damaged cable. Therefore an oil exhibiting a highly temperature dependent viscosity and with a high viscosity at operating temperature is preferred.
To impart a suitable increased temperature dependency in the viscosity for a conventional mineral oil, it is known to add and dissolve a polymer, e.g. polyisobuthene, in the oil. This can only be achieved for highly aromatic oils, but oils of this kind typically exhibit, poorer electric properties in comparison with more naphtenic oils. These latter are oil types suitable for use in electric insulations. A more aromatic oil must typically be treated with bleaching earth to exhibit acceptable electric properties. Such processing is costly and there is a risk that small sized clay-particles remain in the oil if not a careful f
Bergkvist Mikael
Felix Johan
Kornfeldt Anna
Nordberg Per
Schütte Thorsten
ABB AB
Dykema Gossett PLLC
Gray J. M.
Kelly Cynthia H.
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