GB2055254A - A high voltage electric cable line - Google Patents

Abstract

An oil-filled high voltage electric cable line incorporates a filter in an external cooling duct which interconnects the opposite ends of a cable run and also incorporates a pump for forced circulation of the oil through the cable and a heat exchanger to effect the cooling. The filter comprises two metal meshes 15, 20 with the hollow space between filled with adsorbent material 30 and a filter paper 29 lining the mesh 15. Oil entering at 33 passes through mesh 20, material 30, filter paper 29 and then mesh 15 before issuing at 34. The meshes remove the larger particles and the adsorbent material removes the smaller particles or powders from the cable oil. <IMAGE>

Description

SPECIFICATION A high voltage electric cable line The present invention relates to a high voltage electric cable line (carrying at least 275 kV) provided with a cooling system. The present invention relates especially to an electric cable line formed by one or more oil filled cables (O.F.

cables), each essentially comprising a conductor, a stratified insulation (for example paper) impregnated with an insulating liquid (fluid oil) under pressure (for example, a mineral oil or a synthetic alkyl-derivative having very low viscosity), the whole contained within a metal sheath.

A preferred cooling system is an internal one which utilizes the fluid oil as a cooling medium, ducts being provided within the cable for a forced circulation of the oil, a system being provided external of the cable and comprising means for pumping, cooling and filtering said fluid oil.

As known, nowadays the trend in the field of electric power transmission is, for obvious technical and economical reasons, to install very high power electric lines. This aim can be pursued on condition that the conductor(s) is (are) prevented from exceeding temperatures of 85--900C, above which the safety and the average life-time of the cable itself are compromised.

In practice, the internal cooling is carried out by a forced and continuous circulation of the insulating fluid within a longitudinal channel or duct within the conductor, or within channels adjacent the conductor, extending along a cable run between two closed ends (stop joints, sealing ends, transformer inputs, and so on).

The system external of the cables, previously mentioned, comprises at least a pump for admitting fluid at low temperature (preferably at room or ambient temperature) into an end of a cable run, a heat exchanger for cooling the hot fluid issuing from the opposite end of the cable run to the desired temperature, as well as a filter to filter the oil which has already passed through the cable.

This filter is of particular importance since one of the problems linked to the cooling of O.F. cables is related to the fact that the fluid oil circulating within the cable becomes progressively impure; this, as is clearly understandable, produces an impairment of the dielectric properties of the oil itself. Therefore it is necessary to subject the fluid oil to appropriate treatment which, eliminating the pollutants, maintains the oil itself at an acceptable level of purity.

As it is known, the pollutants accumulating in the fluid oil have different natures and origins. The gaseous pollutants (CO2, CO, CH4, etc.) or the liquid pollutants (H20) form in consequence of the ageing of the insulation; these pollutants remain in solution in the fluid oil and in practice do not reach dangerous concentrations during the lifetime foreseen for the cable. The solid pollutants, in the form of particles generally having dimensions of a few micron and therefore remaining in suspension in the moving fluid oil, are of two kinds. The first comprises essentially soaps and organic copper salts: since these products are present on the conductor as residual products of the lubricants used for drawing the conductor itself, their concentration in the fluid oil rapidly increases in the initial periods of lifetime of the cable, but reach quite soon a maximum concentration.The second comprises metal powders originating in consequence of corrosion effects from the conductors, pumps, heat exchangers, etc.: the concentration of the metal powders in the fluid oil increases with the lifetime of the cable and with number of associated apparatus and depends on the quality of these.

All the solid products (but particularly the metal powders) constitute a danger for the cable. In fact, they are conductive substances which reduce the dielectric strength of the insulations and increase the dielectric loss tangent (tan S) of the fluid oil; moreover said substances tend to accumulate in particular zones of the electric line (for example in the restricted ducts of the cable joints) favouring the occurrence of ionisation phenomena, starting of discharges, etc. Therefore it is absolutely necessary to provide for elimination of these impurities or at least to contain their concentration within limits.

The easiest solution is that of installing a filter in the external pumping and cooling circuit.

According to known technique, said filter comprises nets of metal meshes (for example of steel). However, this is a partial solution since the nets of metal meshes are not able to retain very small particles. In practice, the filtering power of said nets is about of 100 , whilst a good proportion of the solid particles in suspension in the fluid oil have dimensions of a few micron only.

Sometimes attempts to obviate this drawback have been made by associating to said filter a magnetic trap for retaining said metal powders.

However, also in this case the results have not been completely satisfactory; moreover, the magnetic trap presents a considerable technical complication.

On the other hand, attempts made by us to use filters already employed for filtering operations of other kind, and therefore for performances different from those required in the case of an electric line provided with O.F. cables, have been with negative results: sintered steel filters have a filtering power of a few micron become obstructed very rapidly; filtering paper sheets require, in order to be efficient, a considerable thickness of the filtering pack and, consequently, a high pressure difference across the filtering pack itself. Filters made with folded paper are more efficient, but the resinous materials used for stiffening the folds (generally epxoy resins) dissolve in the oil thus impairing the oil itself.

In accordance with the present invention, there is provided a high voltage electric cable line, comprising at least one length of cable extending between first and second sealed ends, said cable comprising at least one conductor, insulation impregnated with an insulation liquid under pressure, and an enclosing sheath, and a duct external of the cable and connecting to each other said first and second ends and including a pump, a heat exchanger and a filter in series to one another, said filter having a first net of metal mesh and a second net of metal mesh, the second net being upstream of and spaced from the first net to form with the first net a hollow space, said hollow space, being taken by at least one filtering paper sheet laid on said first net and a layer of adsorbent material arranged between said filtering paper sheet and said second net.

An embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: FIGURE 1 is a schematic diagram of a high voltage electric cable line provided with a cooling system in accordance with the present invention; and FIGURE 2 is a vertical section through a preferred embodiment of filter.

Figure 1 shows a section or run C of an O.F.

cable, extending between first and second stop joints G1 and G2. As known to technicians, a stop joint connects electrically, but not hydraulically, contiguous O.F. cable lengths.

The O.F. cable comprises a conductor having a longitudinal channel for the flow of fluid oil (preferably alkylbenzene, alkylnaphthene, etc.) and is insulated with paper (or other stratified insulation) impregnated with the same fluid oil; the whole is covered with various containment and protection coverings (metal sheath, extruded plastic materials, etc.).

The ends of the length C of O.F. cable, or better the ends of the longitudinal channel of the conductor, are connected, through the first and second stop joints G 1 and G2, to the ends of a duct or header TC, in which a pump P, a heat exchanger SC and a filter F are arranged in series to one another.

The circulation of the fluid oil is as follows. The oil fluid is admitted to the cable, through the first stop joint G 1, at a certain pressure and at low temperature (for example room or ambient temperature); in passing through said length C of O.F. cable, the fluid is heated by the Joule effect, i.e. by heat produced in the conductor, and at the same time it undergoes a drop in pressure; the fluid oil issues from the cable, through the second stop joint G2, at a lower pressure and at a higher temperature with respect to the pressure and the temperature which it had when it entered the cable length C.

The pump P and the heat exchanger SC have the function of bringing again the pressure and the temperature of the fluid oil to desired values. The fluid oil before being re-admitted to the cable passes through the filter F, leaving therein the impurities accumulated during its passage through the cable itself. An expansion tank SE guarantees that the circuit be constantly filled with fluid oil.

Clearly, the same duct or header TC comprising the pump P, the heat exchanger SC and the filter F, can be utilized to serve (instead of only one O.F.

cable length as in Figure 1) a plurality of O.F. cable lengths, through suitable hydraulic connections.

Anyhow, the inlet and the outlet of the fluid oil must be connected respectively to the stop joints arranged at the opposite ends of each length.

The position of the filter F downstream of the heat exchanger SC is preferred, because in this position the filter works at low temperature, and the filtering, even if slower, is more effective.

However, this does not prevent the filter according to the present invention from working effectively at high temperatures (85~90 C); and this is useful when the filter must, for any reason, necessarily be placed upstream of the heat exchanger.

The filter F is preferably constituted as shown in Figure 2. It comprises a metal container 10 of cylindrical shape open at the top, having within, and along its axis X-X, a rod 11 which is integral with the container 10 and has a threaded top end 12. Within the container 10, and coaxially with it, there are first and second cylinders 13 and 14.

The first cylinder 13 is formed by a first net of metal mesh 15 (preferably of stainless steel) wound so as to form the side wall of the cylinder, said first net being fixed at the bottom to a flange 16 and at the top of an annular flange 17. Said flange 16 has a central hole 18 of a diameter a little greater than the diameter of the rod 11; said annular flange 17 has one or more openings, as 19, whose function will be described later on. Said flange 17 has an inner circumference of such a diameter so as to permit the passage of the second cylinder 14 and therefore its housing inside the first cylinder 13.

The second cylinder 14 is formed by a second net of metal mesh 20 (preferably of stainless steel too) wound so as to form the side wall of the cylinder, the second net being fixed at the bottom to a flange 21 and at the top to a flange 22. Said flange 21 has a central hole 23 of a diameter a little greater than the diameter of the rod 11: said upper flange 22 has a central hole 24 aligned with the central hole 23 of the flange 21 and an opening 25 for the inlet of the fluid oil to be filtered.

The first and second cylinders 13 and 14, coaxially arranged within the container 10, provides first, second and third hollow spaces respectively 26, 27 and 28. At least one filtering paper sheet 29 is laid in the second hollow space 27 in contact with the first net of metal mesh 15 (for reasons of safety against eventual ruptures or tearings of the filtering paper sheets it is preferred to overlap two or more sheets): the second hollow space 27 is filled with adsorbent earths 30, inserted through the opening 19.

The container 10 is closed at its top by a cover 31 which has a central hole 32 of diameter a little greater than the diameter of the rod 11; the cover 31 moreover has a first opening 33 in correspondence with the opening 25 in the flange 22 and communicating with the first hollow space 26 and a second opening 34 communicating with the third hollow space 28.

A nut 35 screwed on the threaded end 12 of the rod 11 fastens the cover 31 to the container 10, whilst a set of sealing rings (for example, rubber rings of the O-ring type) of different dimensions (36, 37, 38, 39, 40 and 41) ensures a hydraulic seal between the various parts forming the filter and maintains hydraulically separated the first hollow space 26 from the third hollow space 28. In this way the fluid oil, to pass from the first hollow space 26 to the third hollow space 28, must cross the second hollow space 27, delimited by the nets of metal mesh and containing the filtering paper sheet and the adsorbent earths.

The assembling of the filter shown in Figure 2 is carried out as follows. The sealing ring 36 with the first cylinder 13, placed over the rod 11 through the hole 18 of the flange 16, is disposed on the inner bottom of the container 1 0. A filtering paper sheet 29 has been previously laid on the inner wall of the first cylinder 13, i.e. in contact with the first net of metal mesh 15 (as already mentioned, it is recommendable, for safety reasons, to lay a plurality of filtering paper sheets).

The sealing ring 37 together with the second cylinder 14, placed over the rod 11 through the hole 23 of the flange 21 and the hole 24 of the flange 22, is arranged on the inner bottom of the first cylinder 13.

The hollow space 27, formed between the first and second cylinder, is filled with adsorbent earths introduced through the opening 19 (and other similar ones) in the annular flange 17 of the first cylinder 1 3.

The three sealing rings 38, 39, 40 having progressively increasing diameters, are inserted and the container 10 is closed with the cover 31 and the nut 35 is applied to press on the sealing ring 41.

Oil to be filtered is admitted through the openings 33 and 25, respectively of cover 31 and flange 22, into the hollow space 26. The sealing rings 38 and 39 arranged in the upper part of the container and the rubber rings 36 and 37 arranged in the lower part, prevent the fluid oil from passing directly into the hollow space 28 without crossing the hollow space 27 and therefore without crossing the filtering layers.

Under the pressure existing in the header TC, in consequence of the action of pump P (see Figure 1), the fluid oil enters the second hollow space 27 passing through the second net of metal mesh 20; then the oil crosses the layer of adsorbent earths 30 and reaches, through the filtering paper sheet 29 and the first net of metal mesh 15, the third hollow space 28. From this, the fluid oil issues from the filter through the opening 34 in the cover 31. The sealing ring 40 ensures that the fluid oil does not escape in the contacting zone between the cover 31 and the container 10; the sealing ring 41 ensures that the fluid oil admitted into the hollow space 26 does not escape from the filter in the zone where the rod 11 passes through the central hole 32 of the cover 31.

In filtering a fluid oil of the type generally used in O.F. cables (alkylbenzene, alkylnaphthene, etc.) and for pollutants of the above mentioned type, the components of the filter have the following characteristics: the nets of metal mesh are of stainless steel and have a filtering power of about 100 microns, the filtering paper sheet has a thickness of about 0.8 mm and a filtering time of about 30 seconds (in this specification the term "filtering time" means the time necessary for filtering 500 cc of water at room temperature with a pressure difference of 1 atm. and through a filtering surface of 12.5 cm2; said "filtering time" is therefore a chemical-physical characteristic of the paper: thickness, porosity, etc.); the adsorbent earths, forming a layer of about 5 cm of thickness, are constituted by granules whose dimensions are on the average between 0.7 and 1.5 mm and are based on silicon dioxide and magnesium oxide; the container is of an antirust material (for example: an alloy of iron and cadmium). A filter having the general characteristics reported above can work at high temperature (80~90 C) and with pressure differences between inlet and outlet of 10-20 atm. However, the filter is able to work at low temperatures and therefore it is placed preferably downstream of the heat exchanger.

From the above it is apparent that the advantages of the filter in use on the line comprising O.F. cable derive from the combination of various filtering elements according to a suitable arrangement.

In fact, the nets of metal mesh retain the impurities having large dimensions (higher than about 100 microns) and constitute at the same time a suitable seat for containing the adsorbent earths.

The filtering paper sheet enables the use of adsorbent earths formed by small granules without the risk that these granules can obstruct the nets of metal mesh; on the other hand, the quite small granulation of the adsorbent earths and their association with the filtering paper sheet enable retention of very small impurities (especially metal powders) without requiring large differences of pressure between the inlet and outlet of the filter. This situation lasts for long periods of time, permitting prolonged periods of service of the filter.

In particular, the preferred embodiment of filter described above has the further advantages of a large filtering surface in a relatively reduced volume, and a very easy and rapid assembling (and therefore substitution during use).

Claims (8)

1. A high voltage electric cable line, comprising at least one length of cable extending between first and second sealed ends, said cable comprising at least one conductor, insulation impregnated with an insulating liquid under pressure, and an enclosing sheath, and a duct external of the cable and connecting to each other said first and second ends and including a pump, a heat exchanger and a filter in series to one another, said filter having a first net of metal mesh and a second net of metal mesh, the second net being upstream of and spaced from the first net to form with the first net a hollow space, said hollow space being taken by at least one filtering paper sheet laid on said first net and a layer of adsorbent material arranged between said filtering paper sheet and said second net.

2. An electric cable line according to claim 1, in which said first and second nets of metal mesh are of stainless steel.

3. An electric cable line according to claim 1 or 2, in which said first and second nets of metal mesh have a filtering power of about 100 microns.

4. An electric cable line according to any one of the preceding claims, in which said filtering paper sheet has a thickness of about 0.8 mm and a filtering time of about 30 seconds.

5. An electric cable line according to any one of the preceding claims, in which said layer of adsorbent material comprises silicon dioxide and magnesium oxide.

6. An electric cable line according to claim 5, in which said layer of adsorbent material is about 5 cm thick and comprises granules of dimensions comprise between 0.7 and 1.5 mm.

7. An electric cable line according to any one of the preceding claims, in which said filter comprises a cylindrical container having an open top and provided with a rod along its axis and projecting from the top of the container and having a threaded top end; a first cylinder having a diameter smaller than the inner diameter of said container and formed by said first net of metal mesh and fixed at its bottom to a flange provided with a central hole through which said rod extends and being fixed at its top to an annular flange provided with one or more openings, said first cylinder being housed coaxially within said container; a second cylinder having a diameter smaller than the diameter of the inner circumference of said annular flange and formed by said second net of metal mesh and fixed at its bottom to a flange provided with a central hole through which said rod extends and fixed at its top to a flange provided with a central hole through which said rod extends and an opening, said second cylinder being housed coaxially within said first cylinder so that first, second and third hollow spaces are formed respectively between said rod and said second cylinder, between said first cylinder and the inner wall of said container, said opening being in communication with said first hollow space; a cover disposed on said container and having a central hole through which said rod extends and an opening for incoming fluid communicating with said opening of said flange of said second cylinder communicating with said first hollow space, and an opening for outgoing fluid communicating with said third hollow space; sealing rings; and a nut screwed on the threaded end of said rod, the tightening of said nut rendering said sealing rings effective to separate said first and third hollow spaces and to seal said container to said cover.

8. A high voltage electric cable line, substantially as herein described with reference to the accompanying drawings.