Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1999-02-08
2001-06-12
Hess, Bruce H. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S379000, C174S02300R, C174S1050SC, C174S1020SC, C174S12100B, C174S1210SR, C174S12000C, C174S1200SC
Reexamination Certificate
active
06245426
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electric device which comprises one or more current- or voltage-carrying bodies, i.e. conductors, and a porous electrical insulation, arranged between or around the conductors, the insulation comprises an open porosity and is impregnated with a dielectric fluid. The present invention relates in particular to an electric device used in high voltage application with a porous electrical conductor insulation comprising a fiber-based material, especially a material containing cellulose-based fibers.
BACKGROUND ART
For a known electric device comprising insulated conductors operating at a high voltage, i.e. a voltage above 100 kV, such as a high-voltage transmission or distribution cable or a power transformer or reactor used in a network for transmission or distribution of electrical power it is known to either use an essentially solid insulation comprising a polymeric material or a porous material impregnated with a dielectric fluid, e.g. an insulation based on cellulose fibers and impregnated with an electric insulating oil. In this application, cellulose fibers mean pulp fibers which contain cellulose and to a varying extent lignin and hemi-cellulose.
Conventional cellulose-based electrical insulations consists of wound or spun layers of tape or of preformed bodies manufactured by dewatering and/or pressing a slurry comprising the cellulosic fibers, commonly known as pressboard. Both wound and preformed insulations are impregnated with an electrically insulating fluid, a dielectric fluid, usually an organic fluid such as an oil. This impregnation is normally carried out prior to, in connection to or after the insulation have been applied around the conductor or between conductors. The active part of the insulation is the cellulose fibers in the paper or the board. The oil protect the insulation against moisture pick-up and fills all pores and voids, whereby the dielectrically weak air is replaced by the oil. It is also known to use porous tapes and boards containing polymer-based man-made fibers in such insulations and also impregnate porous fiber-based insulations with similar dielectric fluids.
The impregnation of these porous fiber-based insulations is time consuming and in case of large volumes to be impregnated such as for long high-voltage direct current transmission cables these impregnation processes are carried out for days or weeks using a strictly controlled temperature cycle to ensure a complete and even impregnation of the fiber-based insulation.
To ensure a good impregnation result, a fluid exhibiting a low-viscosity is desired. But the fluid shall be viscous at normal operation conditions for the electrical device to avoid migration of the fluid in the porous insulation, and especially away from the porous insulation. Darcy's law is often used to describe the flow of a fluid through a porous media.
According
to
Darcy
'
s
law
:
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 media, &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. Thus the flow velocity of a fluid within a porous media will be essentially reciprocally proportional to the viscosity. A fluid exhibiting a low-viscosity or a highly temperature dependent viscosity at operating temperature will thus show a tendency to migrate under the influence of temperature fluctuations naturally occurring in an electric device during operation and also due to a temperature gradient building up across a conductor insulation in operation and might result in the formation of unfilled voids in the insulation. Both temperature fluctuations and temperature gradients in conductor insulation will be more expressed in high-voltage direct current devices such as HVDC cables than for most other electric insulations. Unfilled voids will in an insulation operating under an electrical high-voltage direct current field constitute a site where space charges tends to accumulate, thus risking the initiation of dielectric breakdown through discharges which will degrade the insulation and ultimately might lead to its breakdown. Unfilled voids in the insulation as a result of a poor impregnation will have the same effect as described in the foregoing. Thus a dielectric fluid is required that exhibit a low-viscosity under impregnation and is highly viscous under operation conditions.
Conventional dielectric fluid used for impregnating a porous conductor insulation comprised in an electric device, such as a cable, transformer or reactor used in an installation for high-voltage direct current transmission exhibit a viscosity that decreases essentially exponential as the temperature increases. Thus in the high temperature range for impregnation, the temperature has to be increased substantially to gain the required decrease in viscosity due to the low temperature dependence of the viscosity at these temperatures. In comparison the temperature dependence of the-viscosity; as at temperatures prevailing during operation conditions, is very high. Thus small variations in impregnation or operation conditions might have detrimental effect on the performance of the dielectric fluid and the conductor insulation. When using such dielectric fluids they can be chosen such that they are sufficiently viscous at normal operation temperatures to be essentially fully retained in the insulation also under the temperature fluctuations that occurs in the electric device during operation and also that this retention is unaffected of the temperature gradient that normally builds up over a conductor insulation for an electric device comprising conductors at high-voltage. This will mean that the impregnation will have to be carried out at a temperature substantially higher than the operation temperature the insulation is designed to operate at. The high impregnation temperature is needed to ensure that the insulation will be essentially fully impregnated. Such high impregnation temperatures are however disadvantageous as they risk effecting the insulation material, the surfaces properties of the conductor and promotes chemical reactions within and between any material present in the device which insulation is being impregnated. Also energy consumption during production and overall production costs will be negatively affected by a high impregnation temperature. Another aspect to consider is the thermal expansion and shrinkage of the porous insulation which implies that the cooling rate during cooling must be controlled and slow, adding further time to the already time consuming process. For a conventional insulating oil to exhibit a sufficient temperature dependent change in viscosity, a base oil in which a conventionally used polymer, e.g. polyisobuthene, is disolved in exhibits a highly temperature dependent viscosity. This can only be achieved for highly aromatic oils, such as the base oil of T2015 from Dussek Campbell. Such oils exhibits, however, poorer electric properties in comparison with more naphtenic oils, which are oil types suitable for us as insulation oil in an electric device according to the present invention. A more aromatic oil must additionally normally be treated with bleaching earth to exhibit acceptable electric properties. Such processing is costly and there is a risk that small sized clay-particles remains in the oil if not a careful filter- or separation-processing is carried out after this treatment. Alternatively an oil as disclosed in U.S. Pat. No. 3,668,128 can be chosen for its low viscosity at low temperatures. The oil described in U.S. Pat. No. 3,668,128 comprise additions of from 1 up to 50 percent by weight of an alkene polymer with a molecular weight in the range 100-900 derived from an alkene with 3, 4 or 5 carbon atoms, e.g. polybutene
Kornfeldt Anna
Kronberg Bengt
ABB Research Ltd.
Dykema Gossett PLLC
Gray J. M.
Hess Bruce H.
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