CA1072646A - Concomitant cooling arrangement for underground gas pipe-type cables - Google Patents
Concomitant cooling arrangement for underground gas pipe-type cablesInfo
- Publication number
- CA1072646A CA1072646A CA256,489A CA256489A CA1072646A CA 1072646 A CA1072646 A CA 1072646A CA 256489 A CA256489 A CA 256489A CA 1072646 A CA1072646 A CA 1072646A
- Authority
- CA
- Canada
- Prior art keywords
- cooling
- medium
- gas pipe
- pipe
- cables
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B9/00—Power cables
- H01B9/06—Gas-pressure cables; Oil-pressure cables; Cables for use in conduits under fluid pressure
- H01B9/0677—Features relating to the enclosing sheath of gas-pressure cables
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B7/00—Insulated conductors or cables characterised by their form
- H01B7/17—Protection against damage caused by external factors, e.g. sheaths or armouring
- H01B7/29—Protection against damage caused by extremes of temperature or by flame
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B7/00—Insulated conductors or cables characterised by their form
- H01B7/42—Insulated conductors or cables characterised by their form with arrangements for heat dissipation or conduction
- H01B7/428—Heat conduction
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02G—INSTALLATION OF ELECTRIC CABLES OR LINES, OR OF COMBINED OPTICAL AND ELECTRIC CABLES OR LINES
- H02G5/00—Installations of bus-bars
- H02G5/10—Cooling
-
- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02G—INSTALLATION OF ELECTRIC CABLES OR LINES, OR OF COMBINED OPTICAL AND ELECTRIC CABLES OR LINES
- H02G9/00—Installations of electric cables or lines in or on the ground or water
- H02G9/06—Installations of electric cables or lines in or on the ground or water in underground tubes or conduits; Tubes or conduits therefor
Landscapes
- Laying Of Electric Cables Or Lines Outside (AREA)
- Gas Or Oil Filled Cable Accessories (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
The invention relates to a concomitant cooling arrangement for underground gas pipe-type cables, each having at least one electrical conductor, and at least one cooling pipe arranged concomitantly therewith. In accordance with the invention, at least one medium which has a low specific thermal resistance and which is at least to some extent plastic and adhesive, more particularly above substantially 40°C, is arranged at least between the casing of the relevant gas pipe type cable and at least one cooling pipe arranged concomitantly therewith. The medium is also characterized by a specific thermal resistance of below 50°C cm/W, and it is at least to some extent plastic and adhesive, in the temperature range of 40°C to 100 C. Further the medium consists of at least one substance in which at least one metallic substance is dispersed, the at least one metallic substance being in the form of chips, wool or netting.
The invention relates to a concomitant cooling arrangement for underground gas pipe-type cables, each having at least one electrical conductor, and at least one cooling pipe arranged concomitantly therewith. In accordance with the invention, at least one medium which has a low specific thermal resistance and which is at least to some extent plastic and adhesive, more particularly above substantially 40°C, is arranged at least between the casing of the relevant gas pipe type cable and at least one cooling pipe arranged concomitantly therewith. The medium is also characterized by a specific thermal resistance of below 50°C cm/W, and it is at least to some extent plastic and adhesive, in the temperature range of 40°C to 100 C. Further the medium consists of at least one substance in which at least one metallic substance is dispersed, the at least one metallic substance being in the form of chips, wool or netting.
Description
I.l?~t;'.~
The invention relates to a concomitant cooling arrange-ment ~or underground ga~ pipe-type cables having at least one electrical conductor each.
It is the purpose of the invention to improve the cooling of gas pipe-type cables, and thus cable cooling in general and, at the same time, to simplify the manufacture of such concomitant cooling arrangements. ~liS would also simplify, and lower the cost of any necessary inspection of the cable and/or the cooling means.
The transfer of heat through two gaseous or fluid media separated by a partition is known to consist of three individual processes, namely two convection processes and one conduction process through the wall~ If heat is to be transferred, a tempera~
ture difference must exist between the surface of the partition and the adjacent medium. Thus, if we consider a medium inside and outside a partition wherein the temperature of ~he medium inside the partition is viand the temperature on the inside wall of the partition is Vl, and the temperature on the outside wall of the partition is ~2 and the temperature on the outside medium is Va. If heat is to pass from the inside to the outside, the average temperature of the medium Vi inside must be higher than the partition temperature Vl inside, Vi must be higher than the partition temperature V2 outside, and finally V2 must be hig~er than the average temperature of the medium Va outside Vi ~ Vl 7 V2 ~ Va The transfer of heat from the medium inside to the wall, and from the wall to the medium outside is a highly comple~ pro- -cedure. In order to simplify the mathematical analysis of this procedure, the amount of heat Q , Q being the amount of heat and z z the time, passing per unit of time from the inside medium to the inside wall, i.e. the flow of heat Q = ~ is made proportional to the difference (~i ~ Vl), which gives ~ ~i A (Vi - Vl) ' ' ' ,, . :, :IV'~ S
A being the partition area through which the hea-t transfer takes place and ~, being the proportionality factor known as the heat- ~ -transfer coefficient. Thus ~i~ measured in kcal/m2h is the amount of heat transferred in one hour through l m2 at a temperature difference of l degree. It is known that ~i may vary within wide limits, since this value is dependent upon the type and velocity of the flow of the medium, the type of medium, i~s pressure, its temperature, etc. In this case the heat is transferred by the gas or liquid particles on the partition and proceeds in a thin ; lO boundary layer.
The following then applles to the transfer of heat from the partition to the medium:
a A ( V2 - Va~ II
The above heat-conduction process proceeds in the partition of a thickness ~, the flow of heat ~ passing from the higher-temperature locations Vl to the locations at a distance ~ at the lower temperature V2 Experience has shown that the .
~ flow of heat is directly proportional to the temperature . , ~;~ difference (Vl - V2) and to area A, and inversely proportional ;~ 20 to the thiF]cness ~ of the partition This therefore gives:
A (Vl - V2) III
the empirically deflned proportlonality factor being a material constant which identlfies the partition and which is known as the heat conductivity or coefficient of heat transfer ~ .
..
The heat conductivity measured in kcal/h m degree, or ;~
the coefficient of heat transfer, corresponds numerlcally -to the amount of heat flowing in l hour through l m2 of a partition l m thick, when the~temperature difference between the two surfaces of the partition is 1.
The invention relates to a concomitant cooling arrange-ment ~or underground ga~ pipe-type cables having at least one electrical conductor each.
It is the purpose of the invention to improve the cooling of gas pipe-type cables, and thus cable cooling in general and, at the same time, to simplify the manufacture of such concomitant cooling arrangements. ~liS would also simplify, and lower the cost of any necessary inspection of the cable and/or the cooling means.
The transfer of heat through two gaseous or fluid media separated by a partition is known to consist of three individual processes, namely two convection processes and one conduction process through the wall~ If heat is to be transferred, a tempera~
ture difference must exist between the surface of the partition and the adjacent medium. Thus, if we consider a medium inside and outside a partition wherein the temperature of ~he medium inside the partition is viand the temperature on the inside wall of the partition is Vl, and the temperature on the outside wall of the partition is ~2 and the temperature on the outside medium is Va. If heat is to pass from the inside to the outside, the average temperature of the medium Vi inside must be higher than the partition temperature Vl inside, Vi must be higher than the partition temperature V2 outside, and finally V2 must be hig~er than the average temperature of the medium Va outside Vi ~ Vl 7 V2 ~ Va The transfer of heat from the medium inside to the wall, and from the wall to the medium outside is a highly comple~ pro- -cedure. In order to simplify the mathematical analysis of this procedure, the amount of heat Q , Q being the amount of heat and z z the time, passing per unit of time from the inside medium to the inside wall, i.e. the flow of heat Q = ~ is made proportional to the difference (~i ~ Vl), which gives ~ ~i A (Vi - Vl) ' ' ' ,, . :, :IV'~ S
A being the partition area through which the hea-t transfer takes place and ~, being the proportionality factor known as the heat- ~ -transfer coefficient. Thus ~i~ measured in kcal/m2h is the amount of heat transferred in one hour through l m2 at a temperature difference of l degree. It is known that ~i may vary within wide limits, since this value is dependent upon the type and velocity of the flow of the medium, the type of medium, i~s pressure, its temperature, etc. In this case the heat is transferred by the gas or liquid particles on the partition and proceeds in a thin ; lO boundary layer.
The following then applles to the transfer of heat from the partition to the medium:
a A ( V2 - Va~ II
The above heat-conduction process proceeds in the partition of a thickness ~, the flow of heat ~ passing from the higher-temperature locations Vl to the locations at a distance ~ at the lower temperature V2 Experience has shown that the .
~ flow of heat is directly proportional to the temperature . , ~;~ difference (Vl - V2) and to area A, and inversely proportional ;~ 20 to the thiF]cness ~ of the partition This therefore gives:
A (Vl - V2) III
the empirically deflned proportlonality factor being a material constant which identlfies the partition and which is known as the heat conductivity or coefficient of heat transfer ~ .
..
The heat conductivity measured in kcal/h m degree, or ;~
the coefficient of heat transfer, corresponds numerlcally -to the amount of heat flowing in l hour through l m2 of a partition l m thick, when the~temperature difference between the two surfaces of the partition is 1.
- 2 -,. - . . . , . . ~ . .;
: ' . - - ` ~ ' - ' ! . ~
~ 26~
Thus the flow o~ heat ~ passing from one side of the partition to the other satisfies the above three equations (I, II, III~. If Vl and V2 are now calculated from the two first equations (I, II) as V = V. - ~
~l A and V2 = Va + (~
~ A
a and if these values are inserted into equation II, this gives-= ~ A ~Vi ~ ~ A ~ ~ a ~ or + + 1 ~ = A (V V ) ~ a ) The sum of the two coefficients of heat-convection resistance ~1 and ~1 , together with the coefficient of i a heat-conduction resistance ~_' , is known as the coefficient of heat-transfer resistance - :
:, + ~ + 1 k ~ a . .
The reciprocal: -k =
I + ~ ~ 1 a is known as the coefficient of heat transfer.
Thus in the case o heat transfer considered above, :- :
:: ~ . : , ., the following applies to the~flow of heat~
~ ( i a) . .
wherein A is the area of one of the two partition surfaces, V
s the averag~e temperature of~the medium inside, and Va is the average temperature of the medium outside.
In the aase of laminated partitions, or a plurality of partitions arranged one behind -the other, it may easily be ....
-................................ .. .
~ -:
~,, .
shown that:
k =
- 1 + 1 + 2 + ..... + n +
~ '2 ~Yn a .~ 1 - - _ h + ;~ i +
~ a it being necessary to take into account a 1 for each lamination or partition.
If one writes:
= k (Vi - Va) ..
' A
and if one inserts k and the calculation is made, this produces:
= Vi ~ Va i ~ ?~A a A . :
wherein the expressions~
A ~a A
are the heat-convection resistances and the expression.
', : ~ , _ ' '~'' '' '"-''>~ . ..
. A . ~. ' is the heat-conduction resistance, . ~.
: This shows that in the case of a flow of heat ~, in addition to the temperature difference between the media, the heat-convection resistance:and~the heat-conduction resistance (the heat-;conduction resistance in the case of a laminated partition or aplurality of partitions) are also deciding factors, x being the heat-convection coefficient and ~'the heat-conductlon coefficient.
. ~ ' ':
.
.
Zt;~;
Now it is already known, in the case of electrical cables, to leave the core running in the axial direction ope~
and to eliminate, as directly as possible, the Joule heat arising - in the cable, by passing a cooling medium therethrough.
It has furthermore also been proposed to fit the cabLe into a pipe larger in cross section, and to fill the space left between the cable and the inside wall of the pipe with a flowing cooling medium (Electrical Review, July 20th, 1973, page 86).
Now the first of the said designs, in which the cooling :
~0 medium, in this case oil or water, cools the tubular inner wall of the cable without the interposition of any insulating material, :
represents the favourable case of direct cooling of the condùctor.
Thus the system in question- consists as a whole of a flowing cooling medium which takes over, in the interior of the cable, a consider~
able portion of the heat lost, through the conductor insulation ~ :
and the protective sheath of insulation, to the cable environment, i.e. the ground, Now if, for the purpose of simplifying all further estimates, the cylindrical or tubùlar shape of the conductors, ~ 20 the lnsulation, the protectlve caslng, etc~ is disregarded, then -:
:~ in this last-mentioned system, in addition to the temperature. : .
: difference between the tubular cable conductor and the oil or water flowing therein, and the temperature difference between ~ . .
:this cable conductor and the~ground surrounding the cable, the :
: deciding factors are the heat-convection resistance between the -conductor pipe and the oil or water, the heat-conduction ~.~
.~ .
resistance of the conductor insulation surrounding the said con-ductor pipe, the~heat-conduction resistance of the protective sheath o~ insulation, and the~:heat-conduction resistance of the ~-.
~: 30 ground surrounding the cable~. If any insulation is provided : ~between the conductor pipe and the o~il or water, then the heat- .
~ conduotlon reslstance of this 1nsulatLon should a~so be taken into .
: ~ _ 5 ~
';:
' :~Q'~646 account. In short, this is an insulated conductor with liquid cooling which can release, through the additional layers surround-ing the cable, a portion of the lost heat to the ground, the heat-conduction resistance of which is largely undetermined.
- Again in the case of the known direct-cooling system mentioned above in the second case, and conslsting of an insulated cable running inside a pipe, with cooling by means of a liquid medium, such as water, flowing between the cable and the outer pipe, it is essential to take into account, on the one hand, the tempera-ture difference between the cable conductor and the medium and, on the other hand, the temperature difference between the said conductor and the ground surrounding the outer pipe. Since the insulated cable lies, at least along one pipe generatrix or Zone, upon the inside of the pipe, it is necessary to take into account the heat- ;
convection resistance of the cable insulation, the heat-conduction resistance of the boundary layer of water located in this zone bet- -ween the cable insulation and the outer pipe, the heat-conduction resistance of the wall of the outer pipe, and the hard-to-define - heat-conduction resistance of the ground in the vicinity of this zone of the pipe. For the major portion of this system, however, account must be taken of the cooling jacket of water located between the cable insulation and the inside wall of the pipe. It is, there-fore also necessary to take into account the heat-convection resist-ance between the cable insulation and the water, the heat-convection resistance between the water and the inside wall, and the heat~con-duction resistance of the outer pipe. ~ere again, the heat-con-duction resistance of the ground surrounding the outer pipe is largely undefined.
Now known composite systems of the kind described above, and consisting of a cable with integrated liquid or gas cooling, are considerably larger in diameter than the cable alone, and this results in a marked increase in excavating costs for the laying of such systems, as compared with normal cables. Designing the cable and cooling in one also complicates an arrangement of this ~ 26'~
kind and makes it more expensive to lay. If a composite system of this kind should be damaged, the necessary repair work is usually difficult and time consuming. If, as previously indicated, the system also comprises oil cooling, there is also a danyer of polluting sub-soil water.
However, the original conventional layin~ of the cable directly in the ~round has been found unsatis-factory, since the load-carrying ability and temperature of such a cable is highly dependent upon the thermal xesistance of the ground surrounding it.
The few values given below indicate the extent to which the specific thermal resistance of the ground may vary:
M terialSpecO thermal resistance i~
Slag 500 - 900 C cm/W
Concrete 65 - 130 - " -Sand, dry - 300 _ " _ Sand, 10% humidity100 - " ~
Sand, moisture saturated 55 - " _ Ground, dry 130 - " -Ground moist 40 - 70 - " -In order to provide a connection between the heat-conduction resistance and the specific conduction resistance ~ a or specific thermal resistance ~ , it is pointed out that the reciprocal of thermal conductivity or the coefficient of heat transfer ~ of the specific heat-conduction resistance or specific thermal resistance ~i5:
1 = ~2 spec. thermal resistance As indicated in the preceding table, the thermal resistance of the ground is highly dependent upon its moisture ; content. Cables subjected to considerable heat are ]~nown to have a tendency to elimlnate moisture from the ground around the cable.
In order to prevent the drying-out of the ground from increasing the specific thermal resistance, it has already been proposed to t;~
lay, parallel with the cable, a plastic irrigation pipe carrying water, by means of which the ground ~an be kept con-stantly moist tElectrical Review, August lLth, 1972, page 190).
. The water thus added to the ground serves to lower the heat conduction resistance thereof.
An arrangement of this kind has now been found to present a variety of pr~blems, since soil conditions are determined by the actual type of composition of the soil, the time of year, existing weather conditions, and many other factors.
One of the main problems has been the drop in pressure in the irrigation line, and the water consumption is very high (German - Patent 1,174,386).
Now if a comparison is made between the cable-cooling system described above, in which the cable is cooled internally or externally by means of a flowing cooling medium, with the irrigation-pipe system described above, it is found that, with the irrigation system, the heat is dissipated, not by a flowing medium at a definite temperature, but by the ground, to which varying amounts of water are added, which has a variable thermal resistance and therefore a variable heat-conduction resistance, -and this has been found to be very unreliable.
For this reason, action was taken which now pertains to the state of the art, this action consisting of laying, - .
parallel with the electrical cable, an appropriate number of metal or plastic pipes, through which fresh or arti~icially-cooled water, for example, is passed. These water pipes are designed to remove from the cable environment at least a portion of the heat developed by the cable, thus preventing excessive heating of the cable and the cable environment (Rlectrical Review, July 20th, lg73, page 86, and German Patent 1,174 ,386) .
Although as compared with the systems so fa.r described, indirect.cooling by means of ~ater pipes running parallel with , ................. . .
-. .. . - - . . .-: . . . . . .
the cable is simpler and less expensive, it is less efficient.
The reason for this is that, in the case of such indirectly cooled cables, the heat-conduction resistance of the ground between the - cable and the cooling pipe has its effect, the said heat-conduction resistance being dependent not only upon the stretch ~ of ground between the cable and the cooling pipe, and upon th~ effective cross-sectional area of this ground through which the transfer of heat takes place, but also upon the specific thermal resistance 1 . However, as already indicated, this specific thermal res1stance J~ is to no small degree a function of the moisture content of the ground, the heterogeneity of which also increases the difficulty of determining y ~ . -It has therefore already been proposed, in order toovercome this latter disadvantage, to replace the earth excavated from the immediate vicinity of the cable with a special back-fill material. This is usually a mixture of sand and clay of a specific grain size and having a relatively low specific thermal resistance (between 70 and 100C cm/W), which does not dry out irreversibly, but even these systems have the disadvantage of being dependent upon moisture content. Moreover, large volumes of the back-fill materiaI
are required, the production of which requires a great deal of care -~and thoroughness in order to achieve a cable bed of uniform quality throughout.
In order to eliminate as far as possible this dependence upon ground moisture content, it has also been proposed, in the case of indirectly-cooled systems, to fill up the space between the cable and the cooling pipe with poor-quality concre-te. ~owever the ; difference between the thermal expansion of the cable and that of the cooling pipe tends to cause the formation of multiple cracks in the concrete, and this increases the heat-conduction resistance of this matarial quite considerably.
_ g _ ' ,:
.. . . . - , .
~y~
Finally, in a borderline case b~tween indirect and direct cooling disclosed in U.S. Patent 3,409,731, issued November 5, 1968, inventors Lester H. Fink et al, preferably three insulated conductors are laid in a pipe in a manner such that each of the three cables comes into contact with the inner wall of the plpe alon~ one wall generatrix, -the space between the cables and the pipe being filled, for instance, with oil or a gas under pressure. Running parallel with this piEe are two coo:ling pipes which rest thereagainst at two outer wall generatrices spaced apart by the diameter of the pipe, and held in the relevant positions by means of metal strips around the pipe and the cooling pipes. ~his arrangement, consisting essentially of three cables, a pipe, and two cooling pipes is placed inside another pipe, the space between the two pipes being filled with plastic-foam insulation, and the last pipe being surrounded by another medium, for instance the ground.
This arrangement provides only a small amount of surface contact, between the two cooling pipes and the pipe -surrounding the three cables, along the pipe generatrices, and a minimal area of connecting metal, while the remaining space between the cooling pipes and the pipe is filled with a heat-insulating foam material. Apart from the complexity of the system as a whole, it is not difficult to realize that the heat-transfer conditions are very complex, since in -this case it is essential to take into account the heat-conduction resistance of the cable insulation, the heat-convection resistance between the cable insulation and the oil or gas, the heat-convection resistance between the oil or gas and the inside wall of the pipe, the heat-conduction resistance of the pipe wall, the undefined heat-convection resistance in the boundary layers between the pipe and the cooling pipes, the heat-conduction resistance of the plastic-foam material between the pipe and the cooling pipes, - the heat-conduction resistance of the foam material between the - 1 0 "
~ 6 pip~ and the outside pipe, the heat-conduction resistance of the said outside pipe and, finally, the heat-conduction resistance of the surrounding ground. The use of a contact metal between the pipe and the cooling pipes - which will be discussed further hereinafter - does not affect the thermal-resistance conditions enumerated above, because this contact metal is not arranged directly between the cables and the cooling pipes.
Now it is the purpose of the invention, in the case of a system consisting of gas pipe-type cables with concomitant cooling arrangements, more particularly with coolin~ pipes running parallel with the gas pipe-type cables, to reduce the thermal resistances existing between the electrical conductors and the cooling media to à minimum remaining as constant as possible and, furthermore, to keep the flow of heat from such system to the environment optionally small.
According to the invention, a concomitant cooling arrangement forunderground gas pipe-type cables, each having at least one electrical conductor, and at least one cooling pipe arranged concomitantLy therewith, is characterized by at least one medium which has a low specific thermal resistance and which is at least to some extent plastic and adhesive, more particularly above substantially 40C, the said medium being arranged at least between the casing of the relevant gas pipe-type cable and at least one cooling pipe arranged concomitantly therewith.
It is particularly desirable for this medium to consist of a substance which is made of asphalt or bitumen which is at least to some extent plastic and adhesive, and in which at least one metallic substance is dispersed in the form of chips, wool, or netting, forming a framework.
In this connection, the use of metallic substances made of copper, aluminum, iron, or steel is reco~nended.
" '~, , ~
.. ,. - : . . . - . . , - .. :- : . . . ~
~ Zt;~
The main advantages obtained by the use of the invention, as compared with the direct-cooled systems described above, if it is at all comparable therewith, is i-ts great simplicity. In con-- trast to the said systems, the construction of the cable or cables is in no way complicated by the cooling, and the l~ying and possible repairing of the cable system and coolin~ systern accord~
ing to the invention is relatively simple and inexpensive. .
As compared with systems cooled by the ~round, especially with special back-fill material made of sand and clay or by ground irrigation, the medium surrounding the cables to be cooled is independent of moisture~ . :..... .-As compared with electrical cables embedded in poor-quality concrete, and therefore relatively independent of ..... .
ground moixture, the invention is noted for its medium which is ade~uately plastic and adhesive at normal operating temperatures from about 40 to 80C, and which adapts itself to the thermal expansions of the cable and cooling pipes, with no change in its heat-conduction resistance~ -Above all, the invention provides a solution of the problem of indirect cooling of gas pipe-type cables, the cooling of which must be substantially forced in view of the high transfer currents, apart from the fact that, in the case of known indirect cooling arrangements, there is no assurance of satisfactory and reliable thermal contact between the cables and the cooling .. :.
. .
pipes, as a result of which cooling arrangement of this kind are limited in their use and effectiveness. This applies particularly to the object of U.S. Patent 3,409,731, in which the intermediate --layer between the pipe surrounding the three insulated conductors, on the one hand, and, on the other hand, the two cooling pipes, consists partly of plastic-foam insulating material and partly of a narrow layer of a connecting metal which cannot be trusted to provide a permanent joint because of the alternating thermal . . . . . ;
~ 6 and mechanical stresses, and because of corrosion. In addition tothis, the production o~ this joint, and of the entire system, is complex and therefore time-consuming and expensive. Furthermore, considerable expense is involved in the event of a break, because of the difficulty of making the necessary repairs.
In short, the invention combines the advantage of the satisfactory efficiency of directly cooled systems using sand-clay mixtures, or poor-quality concrete, as a heat-transfer material between the cable and the cooling pipe, while eliminating the disadvantages of the two latter systems~ Moreover the invention possesses properties, in addition to the plasticity and aclhesiveness of its heat-transfer medium, which will be discussed hereinafter, and these are not suygested by any known system for cooling electrical cables.
An embodiment of the invention is described hereinafter in greater detail and is illustrated in the drawings, wherein:
FIGURE l is a diagram explaining the heat--transfer condi-~ions in media separated by a partition, FIGURE 2 is a cross section through a particularly preferred embodiment of the invention having a three-phase cable FIGURE 3 is a cross section through a three-phase example of the invention, with individual phase guidance, two cooling pipes being arranged concomitantly with each phase, FIGURE 4 is a cross section showing three separate cables with a total of only two cooling pipes, --FIGURE ~a ls a cross section through the embodiment according to Figure 4, with the advantages of an asymmetrical arrangement of the relevant cooling pipes located between two cables, FIGURE 5 is a cross section through a design according -1~7~fi~:~6 to Figure 4, with an additional cooling pipe adjacent each of the two ou-ter cables of the three cables running side by side, .
FIGURE 6 is a cross section through an advantageous design havin~ three separate cables, two cooling pipes being arranged between each two cables.
In Figure 1, the thickness x of the media in question is ~arked off on the abscissa of the diagram and the temperature V
of the said media on the ordinate. In this case, the inner li~uid or gaseous medium i is separated by a partition of thickness d from an outer gaseous or liquid medium a. The average temperature V
of the hot inner medium i drops, in its boundary layer, to the temperature Vl of the inner boundary surface of the partition. The wall d itself shows the temperature drop visible in Figure 1. The temperature V2 of the outer boundary surface of the wall then drops, in the boundary layer of the outer colder medium a, to the average temperature Va thereof, thus fulfilling the heat-transfer require-ments.
Vi ~ Vl > V2 ~ Va mentioned in the preamble to the specification.
Figure 2 shows a particularly advantageous example of the invention in a section at right angles to the cable. This is a high-tension under~round cable, such as is being used with increasing frequency in heavily populated areas. Although most conventional cables use oil-paper or oil-pIastic insulation with a specific thermal resistance 1 o~ between 300 and 600C cm/W, the insulation used in the example in Figure 2 for high-power transmission is preferably based upon the use of SF6 gas as the insulating agent. The reason for this is that the internal thermal resistance of gas pipe-type cable 2, i.e. the heat-convection resistance between electrical conductors 2b and SF6, the heat--~lV~
conduction reslstance of the SF6, and the heat-convec-tion resistance between the SF6 and tubular casing 2a is much lo~er (about 1/~) than in conventional cables. In addition -to the - relatively low thermal resistance of gas pipe-type cable 2, there is the heat-conduction resistance ~ of tubular casing 2a, which is directly proportional to specific thermal resistance _ _ of the casing thickness and the pipe thickness and is approximately inversely proportional to the average pipe-casing area. A
of about 1.4C cm/W may be used for the casing which is generally madeof iron or steel. Arranged between gas-pipe-type cable 2 and cooling tubes 3 is plastic and adhesive medium 1, the specific thermal resistance of which is below 50C cm/W, while the heat-conduction resistance thereof may also be regarded as relatively low. In order to establish the total heat-txansfer resistance hn h i=i ~iAl ~ i i the followlng must be added~ the heat-conduction resistance of the walls of cooling pipes 3, the heat-convection resistance of the cooling medium with cooling pipes 3, the heat-conduction reslstance of the thermal shunt afforded by filler material 5, ..
disregarding the ground 6. In this connection there is sub-stantially only one ~uestion regardlng pipe-type cable loading, cooling-pipe and cooling dimensions and arrangement, the dimension-ing of agent 1, and the specific thermal resistance of filler material 5, namely: what percentage of the system consisting of gas pipe-type cable 2, agent 1, and cooling pipes 3 constitutes a closed system, and whether it is at least the equivalent of, or better than, the directly cooled systems described at the beginning hereof, from the point of view of efficiency.
B The medium 1 located between gas pipe-type cable ~ and ~ .:
coo]ing pipes 3 is best made of asphalt (German Industrial Standard 55 946) from the point of view of plasticity and adhesive-. . - .:
~ 6 ness, i.e. of a mixture of bitumen and mineral substances which is generally produced industrially. Pure asphalt is a mixture of high-molecular-weight polycyclic hydrocarbon compounds containing small amounts of oxygen-, sulphur-, and nitrogen-compounds. This classification is not homogeneous and the resin limits are no-t sharply defined. Bitumen (GIS 55 946) is to be understood as mean-ing the high-molecular weight hydrocarbon mixtures obtained by the careul processing of mineral oils and those parts o~ natural asphalt which are soluble in carbon disulphide. The adhesiveness and plasticity of bitumen make it suitable mainly as a bonding agent.
Bitumens are particularly noted for minimal swelling in water and minimal permeability to water and water vapour. They are therefore very useful in sealing and water-repellant materials. They also have very low electrical conductivity. Thus medium 1 located between gas pipe-type cable 2 and cooling pipes 3 has adequate plasticity and ductility at normal operating tempera-tures of between 40 and 80C, and is therefore not affected by thermal -expansion in gas pipe-type cable 2 or cooling pipes 3. ~n the temperature range of between 40 and 100C, the range of interest, it is found that, in the case of asphalt, ade~uate plastic deforma-tion is provided by the inherent weight of the asphalt compound, so that this, in conjunction with the adhesiveness of asphalt, definitely prevents the formation of cracks between medium 1 and pipes 2 and 3 which expand and then contract. Asphalts also have the advantage that their specific thermal resistance is practically - independent of the moisture content of the surrounding filler -' material 5 and the adjacent ground 6. Now in order to reduce the specific thermal resistance of about 120C cm/W of the portion of medium 1 consisting of asphalt, an asphalt-like substance, or an appropriate resin, substances having good thermal conductivity are added to this portion. Most suitable for this purpose are metal substances, the specific thermal resistance of which is about 100 y~
times lower than that of earth or sand, as shown by the following brief review of the most important metals under consideration for this purpose:
Al ... .................. 0,3 - 0,5 C cm/W
Cu .................... 0,27C cm/W
Fe ... ~ C cm/W
The substances added in the form of metal chips, steel or aluminum wool, or netting to the asphalt, and consti-tuting the remainder of medium 1, form in the said asphalt a grid or frame-~ork having good thermal conductivity, by means of which the specific thermal resistance of medi.um 1 achieves a value of less than 30C cm/W. Medium 1, consisting of a mixture of aluminum chips or aluminum wool and asphalt, also has the additional special advantage that, because of -the approximately equal specific weights of the components among other things, there is no segrega-tion, and therefore no change in the specific thermal resistance of medium 1, even at the relatively high temperatures at which the asphalt is already highly plastic.
Thus the general idea of the invention, in connection with medlum 1, is to produce, from a mixture of at least two :
readily available and inexpensive substances, a substance having . .
all of the favourable properties described above, and the combina- :
tion of these substanees must have properties, such as constancy of mixture proportions and mixture topology, and therefore thermal .
resistance, in excess of their individual properties.
Thus the concomitant cooling arrangement according to the invention is not only highly efficient and inexpensive, but .is also particularly easy to produce and xepair. .
The cable is laid by progressively excavating the ground 6, i.e. the soll, in the form of a trench, using a mobile excavator. Already available filler material 5, or the earth ~ -~
excavated, is filled in to the height required to support cable 2, : and the said cable is laid continuously. At the same time, - 17 - ~:
~ 2~
earth or filler material 5 is added continuously up to approximately the height of the horizontal diameter of the gas pipe-type cable. Formers 4, for instance planks, are then installed and medium 1 is shaken continuously into the trough formed by the upper half of casing 2a and formers 4. Cooling pipes 3 are then laid continuously in medium 1. Finally, the excavated earth, or filler material 5, is filled in up to level 5a, i.e. ground level 6a,`continuously.
If the soil conditions are good, it is economical to re-use the soil instead of filler material 5, and formers 4 may also be omitted. On the other hand, in the case of particularly high transmission powers, gas pipe-type cable 2 may be completely embedded in medium 1 In the embodiments illustrated in Figures 3 to 6 and described hereinafter, items corresponding to those in Figure 2 bear ~
the same reference numerals. -Figure 3 is a cross section through a design having three separate single-phase gas pipe-type cables 2, the design corresponding in all other respects to that described in connection with Figure 2. In this case, individual electrical conductors 2b are each cooled by means of two pipes 3 which project slightly out of agent 1. This has the advantage that the said cooling pipes are easily located as soon as any repairs or additions are found "
necessary, Figure 4 is a cross section through a variant of the invention having three single-phase gas pipe-type cables and a total of only two cooling pipes. In this case, formers 4 are provided only between cables 2 and, incidentally, between medium 1 and filler material 5. They may, if necessary be omitted.
This provides a concornitant cooling arrangement in which the heat sink provided by the two cooling pipes 3 is connected to the source of heat formed by the gas pipe-type cables :~'7Z~6 2, by medium 1 which has particularly high -~hermal conductivity.
The thermal shunt leading from the source of heat, through filler material 5, to the heat sink, and the flow of heat into ground 6, may in this design, as in the preceding designs, be largely dis-regarded.
As a rule, only between 25 and 50% of the casing of the cable is definitely cooled by medium 1. In spite oE this, the temperature distribution at the periphery of the casing is almost uniform, since the flow of heat from the hottest locations in the casing flows, through the casing itself (wall thickness, for example, 10 mm Al), to the coldest location on the periphery thereof, with only a small portion flowing through the thermal shunt, i.e. through the parallel path formed by excavated earth 6 to the said casing.
Any dry~ out of the soil takes the form, under the most unfavourable circumstances, of an irreversibly dried-out layer surrounding the cable up to a thickness of about 50 cm, and the said layer may assume the characteristics of a baked tile. What is feared, in this case, is an increase in the specific thermal resistance of this layer, for e~ample from 70 to 300C cmjW.
Figure aa shows a cross section through a preferred design according to Figure 4, with cooling pipes 3 arranged as~mmetrically between gas pipe-type cables 2. In this case, cooling pipe 3 associated with two cables 2 is arranged closer to outer cable 2, in order to ensure uniform cooling of all three cables~
According to Figure 5, additional cooling pipes 3 are associated with the two outside gas pipe-type cables 2, the free cross section of these additional pipes being hal~ that of the remaining cooling pipes 3 of the system, in order to achieve more uniform temperature gradients therein.
., - ' -.' , . . . . .
~ 3~$
Finally, Figure 6 shows a variant of the invention which is particularly to be preferred, in which the heat source 2 and the heat sink 3 are in thermal communication with each ot~er largely through medium 1. If necessary, casing parts 2a, which are still free of medium 1, may be sheathed therein, in order to achieve favourable heat-transfer conditions. In this case, the - area of the radial cross section of such a sheath, not shown in Figure 5, may increase approximately in direct proportion to the distance ~'d" between the cross-sectional points and the nearest cooling pipe 3, the thickness of the said sheath, at the minimal cross section thereof and measured in the radial direction, being at least enough to ensure that the transfer of heat to filler material 5 or to the ground 6 does not exceed an optional value.
Obviously gas pipe-type cables 2 of this kind, sheathed in agent 1, may also be enclosed in a heat-insulating layer of high specific thermal resistance, in order to prevent almost entirely the release of any heat to the filler material or ground.
In the embodiments illustrated in Figures 4 to 6, the shortest distance between each cooling pipe and the gas pipe-type cables associated therewith may be less than the diameterof said cooling pipes.
: ' . - - ` ~ ' - ' ! . ~
~ 26~
Thus the flow o~ heat ~ passing from one side of the partition to the other satisfies the above three equations (I, II, III~. If Vl and V2 are now calculated from the two first equations (I, II) as V = V. - ~
~l A and V2 = Va + (~
~ A
a and if these values are inserted into equation II, this gives-= ~ A ~Vi ~ ~ A ~ ~ a ~ or + + 1 ~ = A (V V ) ~ a ) The sum of the two coefficients of heat-convection resistance ~1 and ~1 , together with the coefficient of i a heat-conduction resistance ~_' , is known as the coefficient of heat-transfer resistance - :
:, + ~ + 1 k ~ a . .
The reciprocal: -k =
I + ~ ~ 1 a is known as the coefficient of heat transfer.
Thus in the case o heat transfer considered above, :- :
:: ~ . : , ., the following applies to the~flow of heat~
~ ( i a) . .
wherein A is the area of one of the two partition surfaces, V
s the averag~e temperature of~the medium inside, and Va is the average temperature of the medium outside.
In the aase of laminated partitions, or a plurality of partitions arranged one behind -the other, it may easily be ....
-................................ .. .
~ -:
~,, .
shown that:
k =
- 1 + 1 + 2 + ..... + n +
~ '2 ~Yn a .~ 1 - - _ h + ;~ i +
~ a it being necessary to take into account a 1 for each lamination or partition.
If one writes:
= k (Vi - Va) ..
' A
and if one inserts k and the calculation is made, this produces:
= Vi ~ Va i ~ ?~A a A . :
wherein the expressions~
A ~a A
are the heat-convection resistances and the expression.
', : ~ , _ ' '~'' '' '"-''>~ . ..
. A . ~. ' is the heat-conduction resistance, . ~.
: This shows that in the case of a flow of heat ~, in addition to the temperature difference between the media, the heat-convection resistance:and~the heat-conduction resistance (the heat-;conduction resistance in the case of a laminated partition or aplurality of partitions) are also deciding factors, x being the heat-convection coefficient and ~'the heat-conductlon coefficient.
. ~ ' ':
.
.
Zt;~;
Now it is already known, in the case of electrical cables, to leave the core running in the axial direction ope~
and to eliminate, as directly as possible, the Joule heat arising - in the cable, by passing a cooling medium therethrough.
It has furthermore also been proposed to fit the cabLe into a pipe larger in cross section, and to fill the space left between the cable and the inside wall of the pipe with a flowing cooling medium (Electrical Review, July 20th, 1973, page 86).
Now the first of the said designs, in which the cooling :
~0 medium, in this case oil or water, cools the tubular inner wall of the cable without the interposition of any insulating material, :
represents the favourable case of direct cooling of the condùctor.
Thus the system in question- consists as a whole of a flowing cooling medium which takes over, in the interior of the cable, a consider~
able portion of the heat lost, through the conductor insulation ~ :
and the protective sheath of insulation, to the cable environment, i.e. the ground, Now if, for the purpose of simplifying all further estimates, the cylindrical or tubùlar shape of the conductors, ~ 20 the lnsulation, the protectlve caslng, etc~ is disregarded, then -:
:~ in this last-mentioned system, in addition to the temperature. : .
: difference between the tubular cable conductor and the oil or water flowing therein, and the temperature difference between ~ . .
:this cable conductor and the~ground surrounding the cable, the :
: deciding factors are the heat-convection resistance between the -conductor pipe and the oil or water, the heat-conduction ~.~
.~ .
resistance of the conductor insulation surrounding the said con-ductor pipe, the~heat-conduction resistance of the protective sheath o~ insulation, and the~:heat-conduction resistance of the ~-.
~: 30 ground surrounding the cable~. If any insulation is provided : ~between the conductor pipe and the o~il or water, then the heat- .
~ conduotlon reslstance of this 1nsulatLon should a~so be taken into .
: ~ _ 5 ~
';:
' :~Q'~646 account. In short, this is an insulated conductor with liquid cooling which can release, through the additional layers surround-ing the cable, a portion of the lost heat to the ground, the heat-conduction resistance of which is largely undetermined.
- Again in the case of the known direct-cooling system mentioned above in the second case, and conslsting of an insulated cable running inside a pipe, with cooling by means of a liquid medium, such as water, flowing between the cable and the outer pipe, it is essential to take into account, on the one hand, the tempera-ture difference between the cable conductor and the medium and, on the other hand, the temperature difference between the said conductor and the ground surrounding the outer pipe. Since the insulated cable lies, at least along one pipe generatrix or Zone, upon the inside of the pipe, it is necessary to take into account the heat- ;
convection resistance of the cable insulation, the heat-conduction resistance of the boundary layer of water located in this zone bet- -ween the cable insulation and the outer pipe, the heat-conduction resistance of the wall of the outer pipe, and the hard-to-define - heat-conduction resistance of the ground in the vicinity of this zone of the pipe. For the major portion of this system, however, account must be taken of the cooling jacket of water located between the cable insulation and the inside wall of the pipe. It is, there-fore also necessary to take into account the heat-convection resist-ance between the cable insulation and the water, the heat-convection resistance between the water and the inside wall, and the heat~con-duction resistance of the outer pipe. ~ere again, the heat-con-duction resistance of the ground surrounding the outer pipe is largely undefined.
Now known composite systems of the kind described above, and consisting of a cable with integrated liquid or gas cooling, are considerably larger in diameter than the cable alone, and this results in a marked increase in excavating costs for the laying of such systems, as compared with normal cables. Designing the cable and cooling in one also complicates an arrangement of this ~ 26'~
kind and makes it more expensive to lay. If a composite system of this kind should be damaged, the necessary repair work is usually difficult and time consuming. If, as previously indicated, the system also comprises oil cooling, there is also a danyer of polluting sub-soil water.
However, the original conventional layin~ of the cable directly in the ~round has been found unsatis-factory, since the load-carrying ability and temperature of such a cable is highly dependent upon the thermal xesistance of the ground surrounding it.
The few values given below indicate the extent to which the specific thermal resistance of the ground may vary:
M terialSpecO thermal resistance i~
Slag 500 - 900 C cm/W
Concrete 65 - 130 - " -Sand, dry - 300 _ " _ Sand, 10% humidity100 - " ~
Sand, moisture saturated 55 - " _ Ground, dry 130 - " -Ground moist 40 - 70 - " -In order to provide a connection between the heat-conduction resistance and the specific conduction resistance ~ a or specific thermal resistance ~ , it is pointed out that the reciprocal of thermal conductivity or the coefficient of heat transfer ~ of the specific heat-conduction resistance or specific thermal resistance ~i5:
1 = ~2 spec. thermal resistance As indicated in the preceding table, the thermal resistance of the ground is highly dependent upon its moisture ; content. Cables subjected to considerable heat are ]~nown to have a tendency to elimlnate moisture from the ground around the cable.
In order to prevent the drying-out of the ground from increasing the specific thermal resistance, it has already been proposed to t;~
lay, parallel with the cable, a plastic irrigation pipe carrying water, by means of which the ground ~an be kept con-stantly moist tElectrical Review, August lLth, 1972, page 190).
. The water thus added to the ground serves to lower the heat conduction resistance thereof.
An arrangement of this kind has now been found to present a variety of pr~blems, since soil conditions are determined by the actual type of composition of the soil, the time of year, existing weather conditions, and many other factors.
One of the main problems has been the drop in pressure in the irrigation line, and the water consumption is very high (German - Patent 1,174,386).
Now if a comparison is made between the cable-cooling system described above, in which the cable is cooled internally or externally by means of a flowing cooling medium, with the irrigation-pipe system described above, it is found that, with the irrigation system, the heat is dissipated, not by a flowing medium at a definite temperature, but by the ground, to which varying amounts of water are added, which has a variable thermal resistance and therefore a variable heat-conduction resistance, -and this has been found to be very unreliable.
For this reason, action was taken which now pertains to the state of the art, this action consisting of laying, - .
parallel with the electrical cable, an appropriate number of metal or plastic pipes, through which fresh or arti~icially-cooled water, for example, is passed. These water pipes are designed to remove from the cable environment at least a portion of the heat developed by the cable, thus preventing excessive heating of the cable and the cable environment (Rlectrical Review, July 20th, lg73, page 86, and German Patent 1,174 ,386) .
Although as compared with the systems so fa.r described, indirect.cooling by means of ~ater pipes running parallel with , ................. . .
-. .. . - - . . .-: . . . . . .
the cable is simpler and less expensive, it is less efficient.
The reason for this is that, in the case of such indirectly cooled cables, the heat-conduction resistance of the ground between the - cable and the cooling pipe has its effect, the said heat-conduction resistance being dependent not only upon the stretch ~ of ground between the cable and the cooling pipe, and upon th~ effective cross-sectional area of this ground through which the transfer of heat takes place, but also upon the specific thermal resistance 1 . However, as already indicated, this specific thermal res1stance J~ is to no small degree a function of the moisture content of the ground, the heterogeneity of which also increases the difficulty of determining y ~ . -It has therefore already been proposed, in order toovercome this latter disadvantage, to replace the earth excavated from the immediate vicinity of the cable with a special back-fill material. This is usually a mixture of sand and clay of a specific grain size and having a relatively low specific thermal resistance (between 70 and 100C cm/W), which does not dry out irreversibly, but even these systems have the disadvantage of being dependent upon moisture content. Moreover, large volumes of the back-fill materiaI
are required, the production of which requires a great deal of care -~and thoroughness in order to achieve a cable bed of uniform quality throughout.
In order to eliminate as far as possible this dependence upon ground moisture content, it has also been proposed, in the case of indirectly-cooled systems, to fill up the space between the cable and the cooling pipe with poor-quality concre-te. ~owever the ; difference between the thermal expansion of the cable and that of the cooling pipe tends to cause the formation of multiple cracks in the concrete, and this increases the heat-conduction resistance of this matarial quite considerably.
_ g _ ' ,:
.. . . . - , .
~y~
Finally, in a borderline case b~tween indirect and direct cooling disclosed in U.S. Patent 3,409,731, issued November 5, 1968, inventors Lester H. Fink et al, preferably three insulated conductors are laid in a pipe in a manner such that each of the three cables comes into contact with the inner wall of the plpe alon~ one wall generatrix, -the space between the cables and the pipe being filled, for instance, with oil or a gas under pressure. Running parallel with this piEe are two coo:ling pipes which rest thereagainst at two outer wall generatrices spaced apart by the diameter of the pipe, and held in the relevant positions by means of metal strips around the pipe and the cooling pipes. ~his arrangement, consisting essentially of three cables, a pipe, and two cooling pipes is placed inside another pipe, the space between the two pipes being filled with plastic-foam insulation, and the last pipe being surrounded by another medium, for instance the ground.
This arrangement provides only a small amount of surface contact, between the two cooling pipes and the pipe -surrounding the three cables, along the pipe generatrices, and a minimal area of connecting metal, while the remaining space between the cooling pipes and the pipe is filled with a heat-insulating foam material. Apart from the complexity of the system as a whole, it is not difficult to realize that the heat-transfer conditions are very complex, since in -this case it is essential to take into account the heat-conduction resistance of the cable insulation, the heat-convection resistance between the cable insulation and the oil or gas, the heat-convection resistance between the oil or gas and the inside wall of the pipe, the heat-conduction resistance of the pipe wall, the undefined heat-convection resistance in the boundary layers between the pipe and the cooling pipes, the heat-conduction resistance of the plastic-foam material between the pipe and the cooling pipes, - the heat-conduction resistance of the foam material between the - 1 0 "
~ 6 pip~ and the outside pipe, the heat-conduction resistance of the said outside pipe and, finally, the heat-conduction resistance of the surrounding ground. The use of a contact metal between the pipe and the cooling pipes - which will be discussed further hereinafter - does not affect the thermal-resistance conditions enumerated above, because this contact metal is not arranged directly between the cables and the cooling pipes.
Now it is the purpose of the invention, in the case of a system consisting of gas pipe-type cables with concomitant cooling arrangements, more particularly with coolin~ pipes running parallel with the gas pipe-type cables, to reduce the thermal resistances existing between the electrical conductors and the cooling media to à minimum remaining as constant as possible and, furthermore, to keep the flow of heat from such system to the environment optionally small.
According to the invention, a concomitant cooling arrangement forunderground gas pipe-type cables, each having at least one electrical conductor, and at least one cooling pipe arranged concomitantLy therewith, is characterized by at least one medium which has a low specific thermal resistance and which is at least to some extent plastic and adhesive, more particularly above substantially 40C, the said medium being arranged at least between the casing of the relevant gas pipe-type cable and at least one cooling pipe arranged concomitantly therewith.
It is particularly desirable for this medium to consist of a substance which is made of asphalt or bitumen which is at least to some extent plastic and adhesive, and in which at least one metallic substance is dispersed in the form of chips, wool, or netting, forming a framework.
In this connection, the use of metallic substances made of copper, aluminum, iron, or steel is reco~nended.
" '~, , ~
.. ,. - : . . . - . . , - .. :- : . . . ~
~ Zt;~
The main advantages obtained by the use of the invention, as compared with the direct-cooled systems described above, if it is at all comparable therewith, is i-ts great simplicity. In con-- trast to the said systems, the construction of the cable or cables is in no way complicated by the cooling, and the l~ying and possible repairing of the cable system and coolin~ systern accord~
ing to the invention is relatively simple and inexpensive. .
As compared with systems cooled by the ~round, especially with special back-fill material made of sand and clay or by ground irrigation, the medium surrounding the cables to be cooled is independent of moisture~ . :..... .-As compared with electrical cables embedded in poor-quality concrete, and therefore relatively independent of ..... .
ground moixture, the invention is noted for its medium which is ade~uately plastic and adhesive at normal operating temperatures from about 40 to 80C, and which adapts itself to the thermal expansions of the cable and cooling pipes, with no change in its heat-conduction resistance~ -Above all, the invention provides a solution of the problem of indirect cooling of gas pipe-type cables, the cooling of which must be substantially forced in view of the high transfer currents, apart from the fact that, in the case of known indirect cooling arrangements, there is no assurance of satisfactory and reliable thermal contact between the cables and the cooling .. :.
. .
pipes, as a result of which cooling arrangement of this kind are limited in their use and effectiveness. This applies particularly to the object of U.S. Patent 3,409,731, in which the intermediate --layer between the pipe surrounding the three insulated conductors, on the one hand, and, on the other hand, the two cooling pipes, consists partly of plastic-foam insulating material and partly of a narrow layer of a connecting metal which cannot be trusted to provide a permanent joint because of the alternating thermal . . . . . ;
~ 6 and mechanical stresses, and because of corrosion. In addition tothis, the production o~ this joint, and of the entire system, is complex and therefore time-consuming and expensive. Furthermore, considerable expense is involved in the event of a break, because of the difficulty of making the necessary repairs.
In short, the invention combines the advantage of the satisfactory efficiency of directly cooled systems using sand-clay mixtures, or poor-quality concrete, as a heat-transfer material between the cable and the cooling pipe, while eliminating the disadvantages of the two latter systems~ Moreover the invention possesses properties, in addition to the plasticity and aclhesiveness of its heat-transfer medium, which will be discussed hereinafter, and these are not suygested by any known system for cooling electrical cables.
An embodiment of the invention is described hereinafter in greater detail and is illustrated in the drawings, wherein:
FIGURE l is a diagram explaining the heat--transfer condi-~ions in media separated by a partition, FIGURE 2 is a cross section through a particularly preferred embodiment of the invention having a three-phase cable FIGURE 3 is a cross section through a three-phase example of the invention, with individual phase guidance, two cooling pipes being arranged concomitantly with each phase, FIGURE 4 is a cross section showing three separate cables with a total of only two cooling pipes, --FIGURE ~a ls a cross section through the embodiment according to Figure 4, with the advantages of an asymmetrical arrangement of the relevant cooling pipes located between two cables, FIGURE 5 is a cross section through a design according -1~7~fi~:~6 to Figure 4, with an additional cooling pipe adjacent each of the two ou-ter cables of the three cables running side by side, .
FIGURE 6 is a cross section through an advantageous design havin~ three separate cables, two cooling pipes being arranged between each two cables.
In Figure 1, the thickness x of the media in question is ~arked off on the abscissa of the diagram and the temperature V
of the said media on the ordinate. In this case, the inner li~uid or gaseous medium i is separated by a partition of thickness d from an outer gaseous or liquid medium a. The average temperature V
of the hot inner medium i drops, in its boundary layer, to the temperature Vl of the inner boundary surface of the partition. The wall d itself shows the temperature drop visible in Figure 1. The temperature V2 of the outer boundary surface of the wall then drops, in the boundary layer of the outer colder medium a, to the average temperature Va thereof, thus fulfilling the heat-transfer require-ments.
Vi ~ Vl > V2 ~ Va mentioned in the preamble to the specification.
Figure 2 shows a particularly advantageous example of the invention in a section at right angles to the cable. This is a high-tension under~round cable, such as is being used with increasing frequency in heavily populated areas. Although most conventional cables use oil-paper or oil-pIastic insulation with a specific thermal resistance 1 o~ between 300 and 600C cm/W, the insulation used in the example in Figure 2 for high-power transmission is preferably based upon the use of SF6 gas as the insulating agent. The reason for this is that the internal thermal resistance of gas pipe-type cable 2, i.e. the heat-convection resistance between electrical conductors 2b and SF6, the heat--~lV~
conduction reslstance of the SF6, and the heat-convec-tion resistance between the SF6 and tubular casing 2a is much lo~er (about 1/~) than in conventional cables. In addition -to the - relatively low thermal resistance of gas pipe-type cable 2, there is the heat-conduction resistance ~ of tubular casing 2a, which is directly proportional to specific thermal resistance _ _ of the casing thickness and the pipe thickness and is approximately inversely proportional to the average pipe-casing area. A
of about 1.4C cm/W may be used for the casing which is generally madeof iron or steel. Arranged between gas-pipe-type cable 2 and cooling tubes 3 is plastic and adhesive medium 1, the specific thermal resistance of which is below 50C cm/W, while the heat-conduction resistance thereof may also be regarded as relatively low. In order to establish the total heat-txansfer resistance hn h i=i ~iAl ~ i i the followlng must be added~ the heat-conduction resistance of the walls of cooling pipes 3, the heat-convection resistance of the cooling medium with cooling pipes 3, the heat-conduction reslstance of the thermal shunt afforded by filler material 5, ..
disregarding the ground 6. In this connection there is sub-stantially only one ~uestion regardlng pipe-type cable loading, cooling-pipe and cooling dimensions and arrangement, the dimension-ing of agent 1, and the specific thermal resistance of filler material 5, namely: what percentage of the system consisting of gas pipe-type cable 2, agent 1, and cooling pipes 3 constitutes a closed system, and whether it is at least the equivalent of, or better than, the directly cooled systems described at the beginning hereof, from the point of view of efficiency.
B The medium 1 located between gas pipe-type cable ~ and ~ .:
coo]ing pipes 3 is best made of asphalt (German Industrial Standard 55 946) from the point of view of plasticity and adhesive-. . - .:
~ 6 ness, i.e. of a mixture of bitumen and mineral substances which is generally produced industrially. Pure asphalt is a mixture of high-molecular-weight polycyclic hydrocarbon compounds containing small amounts of oxygen-, sulphur-, and nitrogen-compounds. This classification is not homogeneous and the resin limits are no-t sharply defined. Bitumen (GIS 55 946) is to be understood as mean-ing the high-molecular weight hydrocarbon mixtures obtained by the careul processing of mineral oils and those parts o~ natural asphalt which are soluble in carbon disulphide. The adhesiveness and plasticity of bitumen make it suitable mainly as a bonding agent.
Bitumens are particularly noted for minimal swelling in water and minimal permeability to water and water vapour. They are therefore very useful in sealing and water-repellant materials. They also have very low electrical conductivity. Thus medium 1 located between gas pipe-type cable 2 and cooling pipes 3 has adequate plasticity and ductility at normal operating tempera-tures of between 40 and 80C, and is therefore not affected by thermal -expansion in gas pipe-type cable 2 or cooling pipes 3. ~n the temperature range of between 40 and 100C, the range of interest, it is found that, in the case of asphalt, ade~uate plastic deforma-tion is provided by the inherent weight of the asphalt compound, so that this, in conjunction with the adhesiveness of asphalt, definitely prevents the formation of cracks between medium 1 and pipes 2 and 3 which expand and then contract. Asphalts also have the advantage that their specific thermal resistance is practically - independent of the moisture content of the surrounding filler -' material 5 and the adjacent ground 6. Now in order to reduce the specific thermal resistance of about 120C cm/W of the portion of medium 1 consisting of asphalt, an asphalt-like substance, or an appropriate resin, substances having good thermal conductivity are added to this portion. Most suitable for this purpose are metal substances, the specific thermal resistance of which is about 100 y~
times lower than that of earth or sand, as shown by the following brief review of the most important metals under consideration for this purpose:
Al ... .................. 0,3 - 0,5 C cm/W
Cu .................... 0,27C cm/W
Fe ... ~ C cm/W
The substances added in the form of metal chips, steel or aluminum wool, or netting to the asphalt, and consti-tuting the remainder of medium 1, form in the said asphalt a grid or frame-~ork having good thermal conductivity, by means of which the specific thermal resistance of medi.um 1 achieves a value of less than 30C cm/W. Medium 1, consisting of a mixture of aluminum chips or aluminum wool and asphalt, also has the additional special advantage that, because of -the approximately equal specific weights of the components among other things, there is no segrega-tion, and therefore no change in the specific thermal resistance of medium 1, even at the relatively high temperatures at which the asphalt is already highly plastic.
Thus the general idea of the invention, in connection with medlum 1, is to produce, from a mixture of at least two :
readily available and inexpensive substances, a substance having . .
all of the favourable properties described above, and the combina- :
tion of these substanees must have properties, such as constancy of mixture proportions and mixture topology, and therefore thermal .
resistance, in excess of their individual properties.
Thus the concomitant cooling arrangement according to the invention is not only highly efficient and inexpensive, but .is also particularly easy to produce and xepair. .
The cable is laid by progressively excavating the ground 6, i.e. the soll, in the form of a trench, using a mobile excavator. Already available filler material 5, or the earth ~ -~
excavated, is filled in to the height required to support cable 2, : and the said cable is laid continuously. At the same time, - 17 - ~:
~ 2~
earth or filler material 5 is added continuously up to approximately the height of the horizontal diameter of the gas pipe-type cable. Formers 4, for instance planks, are then installed and medium 1 is shaken continuously into the trough formed by the upper half of casing 2a and formers 4. Cooling pipes 3 are then laid continuously in medium 1. Finally, the excavated earth, or filler material 5, is filled in up to level 5a, i.e. ground level 6a,`continuously.
If the soil conditions are good, it is economical to re-use the soil instead of filler material 5, and formers 4 may also be omitted. On the other hand, in the case of particularly high transmission powers, gas pipe-type cable 2 may be completely embedded in medium 1 In the embodiments illustrated in Figures 3 to 6 and described hereinafter, items corresponding to those in Figure 2 bear ~
the same reference numerals. -Figure 3 is a cross section through a design having three separate single-phase gas pipe-type cables 2, the design corresponding in all other respects to that described in connection with Figure 2. In this case, individual electrical conductors 2b are each cooled by means of two pipes 3 which project slightly out of agent 1. This has the advantage that the said cooling pipes are easily located as soon as any repairs or additions are found "
necessary, Figure 4 is a cross section through a variant of the invention having three single-phase gas pipe-type cables and a total of only two cooling pipes. In this case, formers 4 are provided only between cables 2 and, incidentally, between medium 1 and filler material 5. They may, if necessary be omitted.
This provides a concornitant cooling arrangement in which the heat sink provided by the two cooling pipes 3 is connected to the source of heat formed by the gas pipe-type cables :~'7Z~6 2, by medium 1 which has particularly high -~hermal conductivity.
The thermal shunt leading from the source of heat, through filler material 5, to the heat sink, and the flow of heat into ground 6, may in this design, as in the preceding designs, be largely dis-regarded.
As a rule, only between 25 and 50% of the casing of the cable is definitely cooled by medium 1. In spite oE this, the temperature distribution at the periphery of the casing is almost uniform, since the flow of heat from the hottest locations in the casing flows, through the casing itself (wall thickness, for example, 10 mm Al), to the coldest location on the periphery thereof, with only a small portion flowing through the thermal shunt, i.e. through the parallel path formed by excavated earth 6 to the said casing.
Any dry~ out of the soil takes the form, under the most unfavourable circumstances, of an irreversibly dried-out layer surrounding the cable up to a thickness of about 50 cm, and the said layer may assume the characteristics of a baked tile. What is feared, in this case, is an increase in the specific thermal resistance of this layer, for e~ample from 70 to 300C cmjW.
Figure aa shows a cross section through a preferred design according to Figure 4, with cooling pipes 3 arranged as~mmetrically between gas pipe-type cables 2. In this case, cooling pipe 3 associated with two cables 2 is arranged closer to outer cable 2, in order to ensure uniform cooling of all three cables~
According to Figure 5, additional cooling pipes 3 are associated with the two outside gas pipe-type cables 2, the free cross section of these additional pipes being hal~ that of the remaining cooling pipes 3 of the system, in order to achieve more uniform temperature gradients therein.
., - ' -.' , . . . . .
~ 3~$
Finally, Figure 6 shows a variant of the invention which is particularly to be preferred, in which the heat source 2 and the heat sink 3 are in thermal communication with each ot~er largely through medium 1. If necessary, casing parts 2a, which are still free of medium 1, may be sheathed therein, in order to achieve favourable heat-transfer conditions. In this case, the - area of the radial cross section of such a sheath, not shown in Figure 5, may increase approximately in direct proportion to the distance ~'d" between the cross-sectional points and the nearest cooling pipe 3, the thickness of the said sheath, at the minimal cross section thereof and measured in the radial direction, being at least enough to ensure that the transfer of heat to filler material 5 or to the ground 6 does not exceed an optional value.
Obviously gas pipe-type cables 2 of this kind, sheathed in agent 1, may also be enclosed in a heat-insulating layer of high specific thermal resistance, in order to prevent almost entirely the release of any heat to the filler material or ground.
In the embodiments illustrated in Figures 4 to 6, the shortest distance between each cooling pipe and the gas pipe-type cables associated therewith may be less than the diameterof said cooling pipes.
Claims (18)
1. A concomitant cooling arrangement for underground gas pipe-type cables, each having at least one electrical conductor, and at least one cooling pipe arranged concomitantly therewith, characterized by at least one medium which has a low specific thermal resistance and which is at least to some extent plastic and adhesive, more particularly above substantially 40°C, the said medium being arranged at least between the casing of the relevant gas pipe-type cable and at least one cooling pipe arranged concomitantly therewith.
2. A cooling arrangement according to claim 1, character-ized in that the medium has a specific thermal resistance of below 50°C cm/W.
3. A cooling arrangement according to claim 1, character-ized in that the medium is at least to some extent plastic and adhesive, in the temperature range of 40°C to 100°C.
4. A cooling arrangement according to claims 1, 2 or 3, characterized in that the medium consists of at least one substance in which at least one metallic substance is dispersed.
5. A cooling arrangement according to claims 1, 2 or 3, characterized in that the medium consists of at least one substance in which at least one metallic substance is dispersed, the at least one metallic substance being in the form of chips, wool or netting.
6. A cooling arrangement according to claims 1, 2 or 3, characterized in that the medium consists of at least one substance in which at least one metallic substance is dispersed, in which the at least one metallic substance forms a framework in the at least to some extent plastic and adhesive substance.
7. A cooling arrangement according to claims 1, 2, or 3, characterized in that the at least to some extent plastic and adhesive substance is made of asphalt or bitumen.
8. A cooling arrangement according to claims 1, 2 or 3, characterized in that the medium consists of at least one substance in which at least one metallic substance is dispersed, the metallic substance being copper (Cu) or aluminum (Al) or iron (Fe) or steel.
9. A cooling medium according to claim 3 including a single cable having a surrounding casing and characterized in that the medium is arranged on the side of the casing facing the surface of the ground, and in that the cooling pipes are at least partly embedded in said medium.
10. A cooling medium according to claim 3 including a plurality of cables each having a surrounding casing and charac-terized in that the medium is arranged on the side of each casing facing the surface of the ground, and in that the cooling pipes are at least partly embedded in said medium.
11. A cooling medium according to claim 3 including a plurality of cables each having a surrounding casing and charac-terized in that the medium is arranged on the side of each casing facing the casings of adjacent cables, and in that the cooling pipes are at least partly embedded in said medium.
12. A cooling arrangement according to claims 9, 10 or 11, characterized in that the cooling pipes are arranged on the side of the casing facing the surface of the ground, at a distance from, and concomitantly with, the said gas pipe-type cable.
13. A cooling arrangement according to claims 10 or 11, characterized in that the cooling pipes are arranged between the gas pipe-type cables adjacent the sides of the casings facing each other, at a distance from, and concomitantly with, the said gas pipe-type cable.
14. A cooling arrangement according to claims 9, 10 or 11, characterized in that the cooling pipes are arranged on the side of the casing facing the surface of the ground, at a distance from, and concomitantly with, the said gas pipe-type cable, and further characterized in that the free cross-section of each cooling pipe is relatively small.
15. A cooling arrangement according to claims 10 or 11, characterized in that the cooling pipes are arranged between the gas pipe-type cables adjacent the sides of the casings facing each other, at a distance from, and concomitantly with, the said gas pipe-type cable, and further characterized in that the free cross-section of each cooling pipe is relatively large.
16. A cooling arrangement according to claims 10 or 11, characterized in that the cooling pipes are arranged between the gas pipe-type cables adjacent the sides of the casings facing each other, at a distance from, and concomitantly with, the said gas pipe-type cable, and further characterized in that the cooling pipes are arranged closer to the outer gas pipe-type cables.
17. A cooling arrangement according to claims 10 or 11, characterized in that the cooling pipes are arranged between the gas-pipe type cables adjacent the sides of the casings facing each other, at a distance from, and concomitantly with, the said gas pipe-type cable, and further characterized in that the plurality of gas pipe-type cables are arranged side by side and connected together by the medium, the free cross-section of the cooling pipe arranged near the outside of the relevant outermost gas pipe-type cable being about half as large as the free cross-section of the cooling pipe located between the gas pipe-type cables.
18. A cooling arrangement according to claims 10 or 11, characterized in that the cooling pipes are arranged between the gas pipe-type cables adjacent the sides of the casings facing each other, at a distance from, and concomitantly with, the said gas pipe-type cable, and further characterized in that the shortest distance between each cooling pipe and the gas pipe-type cables associated therewith is less than the diameter of the said cooling pipes.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CH887875A CH601899A5 (en) | 1975-07-08 | 1975-07-08 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1072646A true CA1072646A (en) | 1980-02-26 |
Family
ID=4345771
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA256,489A Expired CA1072646A (en) | 1975-07-08 | 1976-07-07 | Concomitant cooling arrangement for underground gas pipe-type cables |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPS528488A (en) |
| CA (1) | CA1072646A (en) |
| CH (1) | CH601899A5 (en) |
| DE (2) | DE7523180U (en) |
Families Citing this family (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4409431A (en) * | 1981-08-07 | 1983-10-11 | Harvey Hubbell Incorporated | Oil well cable |
| USRE32225E (en) * | 1981-08-07 | 1986-08-12 | Harvey Hubbell Incorporated | Oil well cable |
| JPS5862510U (en) * | 1981-10-21 | 1983-04-27 | 住友電気工業株式会社 | power transmission line |
| JPS58112204A (en) * | 1981-12-26 | 1983-07-04 | 住友電気工業株式会社 | Cooled power cable line |
| US4454377A (en) * | 1982-06-21 | 1984-06-12 | Harvey Hubbell Incorporated | Oil well cable |
| US4454378A (en) * | 1982-12-08 | 1984-06-12 | Harvey Hubbell Incorporated | Arcuate armored cable |
| FR2740919B1 (en) * | 1995-11-07 | 1997-11-28 | Gec Alsthom T & D Sa | COOLING DEVICE FOR UNDERGROUND ELECTRICAL TRANSPORT LINE |
| EP3248512B1 (en) * | 2015-01-19 | 2019-05-22 | Bridgestone Corporation | Seat pad |
| US20200079259A1 (en) * | 2016-12-22 | 2020-03-12 | Bridgestone Corporation | Seat pad |
| DE102017208144A1 (en) * | 2017-05-15 | 2018-11-15 | Siemens Aktiengesellschaft | Gas-insulated pipe and method for operating a gas-insulated pipe |
-
1975
- 1975-07-08 CH CH887875A patent/CH601899A5/xx not_active IP Right Cessation
- 1975-07-21 DE DE19757523180 patent/DE7523180U/en not_active Expired
- 1975-07-21 DE DE19752532534 patent/DE2532534A1/en not_active Withdrawn
-
1976
- 1976-07-06 JP JP7954476A patent/JPS528488A/en active Pending
- 1976-07-07 CA CA256,489A patent/CA1072646A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| DE2532534A1 (en) | 1977-01-27 |
| CH601899A5 (en) | 1978-07-14 |
| JPS528488A (en) | 1977-01-22 |
| DE7523180U (en) | 1977-07-07 |
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