WO2001001423A1

WO2001001423A1 - Method for protection of a current-carrying cable against overheating and cables

Info

Publication number: WO2001001423A1
Application number: PCT/DK2000/000334
Authority: WO
Inventors: Kristian GLEJBØL; Manfred DÄUMLING
Original assignee: Nkt Research A/S
Priority date: 1999-06-28
Filing date: 2000-06-22
Publication date: 2001-01-04
Also published as: AU5391100A

Abstract

In a method of protecting superconducting cables and power DC or AC cables, a substance having different phase states at different temperatures is provided in the vicinity of the current-carrying conductors of the cable. Hereby, an unintentional temperature change can be counteracted, as the substance absorbs or liberates heat by its phase change. A temperature change causing destruction of the cable is prevented for a certain period of time. The invention also relates to cables including a substance having different phase states at different temperatures in the vicinity of the current-carrying conductors of the cable.

Description

Method for protection of a current-carrying cable against overheating and cables

The invention relates to a method of protecting a current-carrying cable against harmful temperatures.

The invention moreover relates to cables.

A first type of power cables is usually divided into DC cables and AC cables. This type of cables, which are provided with various insulating and reinforcing layers around a centrally positioned current conductor, e.g. of copper, according to their use, has been used for many years for the transport of electrical energy, e.g. from power plants to the individual connected consumers. The cables are typically dug into the ground or are laid on the sea bed.

A more recent type of cable for the transfer of power current is the superconducting cable which requires cooling m order to carry current m comparison with the above-mentioned power cables. The advantage of this more recent type of cable is primarily that the losses m the operational state of the superconducting cable are negligent m comparison with the ordinary cables operating at ambient temperature.

Normally, power cables are dimensioned such that they can liberate the energy which is dissipated m the cable m operation without any problems.

In particular, considerable amounts of energy, so-called heat losses, are dissipated m the conventional DC or AC cables, whereas the losses in the superconducting cables are negligent.

On the other hand, it is necessary that the superconduct- ing cables are superconducting all the time, since, if the superconducting state disappears, they will assume an ordinary oh ic state which will cause rapid melting of the current conductor of the cable.

If a current above a certain critical value I thus runs in superconducting cables, the cable will have an ohmic resistance causing a strong power dissipation in the cable itself, which may lead to evaporation of the cooling medium and consequently destruction or even bursting of the cable itself.

Accordingly, an object of the invention is to provide a method wherein, in case of an excess current (and thereby overheating) of a power cable, which may be a conven- tional DC or AC cable or a superconducting cable, for some reason, it is ensured that such overheating does not damage the cable.

The object of the invention is achieved by a method of the type stated in the introductory portion of claim 1, which is characterized in that a substance having different phase states at different temperatures is provided in the vicinity of the current-carrying parts of the cable.

An excess current, if any, will hereby supply energy to the endothermic process that causes the phase change in the substance, instead of resulting in a fatal heating of the current-carrying part of the cable. An expedient way of realising the phase state change is by the use of a substance which, as stated m claim 2, changes from solid to liquid state upon an increase m the temperature.

Like m a phase state change from solid to liquid state, it is possible to find substances with an endothermic magnetic transition from non-magnetic to e.g. ferromagnetic or antiferromagnetic state.

As mentioned, the invention also relates to cables.

A first cable of the superconducting type is characterized, as stated m claim 6, m that the superconducting tapes are embedded m a substance which has different phases at different temperatures.

A second cable of the alternating current type is characterized, as stated m claim 8, m that the conductor is hollow and filled with a substance having an endothermic phase transition which is between the nominal temperature of the conductor and the permissible maximum operational temperature of the original dielectric.

Finally, a third cable of the direct current type is characterized m that the conductor is hollow and filled with a substance having an endothermic phase transition which is between the nominal temperature of the conductor and the permissible maximum operational temperature of the conductor.

Expedient embodiments of the cables are defined m claims 7, 9 and 11. The invention will now be explained more fully with reference to the figures, m which:

Fig. 1 is an end view of a superconducting cable for the transfer of alternating current with protection against superheating according to the invention,

Fig. 2 is an end view of a conventional power cable for the transfer of alternating current between two points,

Fig. 3 shows the power cable of fig. 2, but with protection against overheating according to the invention,

Fig. 4 is an end view of a conventional power cable for the transfer of direct current,

Fig. 5 shows the power cable of fig. 4 with protection against overheating according to the invention, while

Fig. 6 shows an example of a temporal/temperature course for the conducting tapes m a superconducting cable with and without superheating protection according to the invention.

Fig. 1 shows a superconducting cable having a former 1 which is hollow as shown at 2 to provide a duct for the transport of cooling medium, such as liquid nitrogen.

A plurality of superconducting tapes 3 are wound exter- nally on the former 1, said tapes being embedded m a substance that serves as an energy-absorbing medium, cf. the following description. Externally on the superconducting tapes, there is a thermal insulation 4 which is m turn surrounded by an electrically conducting polymer 5, a dielectric m the form of a PE (polyethylene) material 6, and a further layer of electrically conducting polymer 7.

If the former 1 is made of e.g. a corrugated steel pipe, no cooling medium can penetrate out between the superconducting tapes.

If the superconducting cable is made so as to leave a free volume between the superconducting tapes 3, this free volume, during cooling, may be filled with a substance that has a phase transition at a temperature which is between the nominal operational temperature of the cable and the boiling point of the cooling medium used.

A possible substance is propene which has a boiling point of 226K and a melting point of 87K. This substance can therefore be filled m gas form m the cable during cooling, which ensures that all free volumes between the superconducting tapes 3 will be filled.

In case of an excess current, energy will dissipate m the superconducting tapes. This dissipated energy will increase the temperature of the tapes until the melting point of the substance embedding the superconducting tapes is reached.

Then the temperature of the superconducting tapes will be constant until the solid substance has melted, following which the temperature will increase additionally. As will be appreciated, the substance embedding the superconducting tapes will protect against undesired temperature increases in the cable for a certain period of time, thereby allowing prevention against damage to the cable to be implemented.

As regards substances with magnetic transition, several substances may be used. For example, the Neel temperature (the temperature at which magnetic transition takes place) of Cud: is 70K. The substance α-Mn has the Neel temperature 95K, while MnF; has a Neel temperature between 72K and 75K.

These substances may be placed together with substances having a solid-liquid phase transition or alone.

The advantage of using magnetic substance is inter alia that there is no risk of evaporation.

Additionally, it is an advantage to use substances with magnetic phase transition if the operational temperature drops to e.g. 30K, because there are not many substances which have a phase transition from solid to liquid state at 30K.

Fig. 2 shows a conventional PEX insulated power cable for the transfer of alternating current. The cable innermost has an electrical conductor 8 which frequently consists of an extruded aluminium rod wound with pre-shaped copper rods. The electrical conductor is surrounded by a layer of polymer having a low conductivity. This layer is intended to smooth concentrations in the electrical field that may cause breakdown in the cable. The electrically conducting polymer layer has externally extruded thereon a dielectric 10, frequently of a cross- linked polyethylene (PEX) , which prevents galvanic contact between the electrical conductor 8 and the surround- mgs .

On top of the dielectric there is a further layer of polymer 12 having a low electrical conductivity. This layer smoothes concentrations m the electrical field and prevents breakdown.

Fig. 3 shows a cable according to the invention which differs from the one shown m fig. 2 m that the innermost aluminium rod is replaced by a hollow pipe 13 of e.g. stainless steel.

This pipe 12 is filled with a substance 14 which has the property that it has an endothermic phase transition between the nominal and permissible maximum operational temperatures of the cable.

It is hereby possible to draw a greater current than the nominal one of the cable for a shorter period of time, without the dielectric being destroyed because of the thermal load.

Fig. 4 shows a conventional cable for the transfer of direct current. The cable innermost has an electrical conductor 15, which may consist of a central copper rod wound with pre-shaped copper rods 16.

The electrical conductor is wound with an oil-impregnated paper layer 17 having a low electrical conductivity. This layer smoothes concentrations m the electrical field which might otherwise cause breakdown m the cable.

The electrically conducting paper layer 17 has externally wound thereon a dielectric 18 of oil-impregnated paper which prevents galvanic contact between the electrical conductor and the surroundings .

The dielectric 18 has externally wound thereon a layer of oil-impregnated paper 19 having a low electrical conductivity. This layer smoothes concentrations m the electrical field and thereby prevents undesired breakdown.

Fig. 5 shows a cable which differs from the one shown m fig. 4 m that the innermost copper rod is replaced by a hollow pipe 20 e.g. of stainless steel.

This pipe 20 is filled with a substance 21 which has the property that it has an energy-consuming phase transition between the ambient temperature and the nominal operational temperature of the cable.

It is hereby ensured that heating and cooling of the dielectric take place during a time span of a duration which ensures that the dielectric will not be broken down because of sudden temperature changes.

Fig. 6 shows a graphical representation of the development of the temperature because of a fault current partly for a conventional power cable, e.g. of the superconducting type, and partly for a power cable according to the invention which has received a substance with a phase transition between the nominal operational temperature of the cable and the boiling point of the cooling medium used, as has been explained above.

22 designates a curve which shows the temperature in- crease in a conventional power cable which is subjected to a much too great current relative to the nominal maximum operational temperature of the cable.

By providing the cable with a substance, as explained above, the temperature increase in the cable will be limited for a period of time. This is illustrated by the curve 23, it being shown at 24 that the substance, because it has an endothermic phase transition, can absorb the dissipated power, thereby preventing a destructive temperature increase in the cable for a given period of time .

Some examples of the use of the principles of the invention are given below.

Example 1

An oil/paper insulated cable for the transfer of direct current has an electrical conductor formed of an inner copper pipe with an internal diameter of 25 mm.

The inner pipe is wound with copper rods until a conductive area of 120 mnr has been reached, and then the pipe is wound with conducting and insulating paper.

The cable is then impregnated in a bath containing hot oil. After the impregnation of the cable, but while it is still positioned in the oil bath, the interior of the copper pipe is filled with a molten hydrocarbon, e.g. C10H12.

C10H12 has a melting point of 325K and a melting heat of 202 J/cm³.

In operation, the enclosed volume of C10H12 can thus absorb 40400J per metre of cable by heating above 325K and liberate 40400J by cooling of the cable below 325K.

This will allow rapid start and termination of the cable, since thermal tensions in the insulation layer do not have to be taken into consideration, like in conventional oil/paper insulated cables.

Of course, a large number of other substances than C10H12 may be contemplated for use, depending on the temperature at which the cable must be kept during a cooling procedure .

One possibility is to replace the melting substance by an immiscible system of two or more substances having melting points which do not coincide.

This will ensure additional smoothing of temperature variations because of sudden start or termination of the cable .

Alternatively, the central pipe may be filled with a sub- stance that melts at a temperature above the nominal operational temperature of the cable. This will allow overloading of the cable until the material in the central pipe has melted. Example 2

A superconducting cable is cooled with liquid nitrogen.

The superconducting cable is made by winding a corrugated former of stainless steel with superconducting tapes made of silver and a superconducting ceramic material.

A net made of a polymer is arranged on the superconduct- ing tapes. This net ensures mass transport along the longitudinal axis of the cable, if a pressure gradient occurs along it.

In addition to the net, a further steel pipe is laid, and then thermal insulation and dielectrics are applied to the pipe, as described m connection with fig. 1.

The cable is then laid between 2 points. Following laying of the cable, nitrogen is blown through the free volume around the superconducting tapes. The blowing is continued until other gases have been displaced. Then the blowing is repeated with propene until the gas around the superconducting tapes essentially consists of propene.

Now access to the free volume is closed at one end of the cable, and cooling of it is begun by passing liquid nitrogen through the former of the cable from one end to the other.

As the temperature of the cable reaches the saturation point of propene, this will saturate to form a liquid. By ust admitting propene at one end of the cable and cooling the cable by passing nitrogen through the former from the other end, it is ensured that the free volume around the superconducting tapes is filled completely without occurrence of gaps m the free volume.

Filling of propene continues until the free volume around the tapes is filled completely, and then the end is closed. Cooling of the cable is now continued with liquid nitrogen until the cable reaches the operational temperature .

In case of heating of the cable, e.g. because of service, the cable is heated by circulating hot gases through the former. During heating, propene is discharged from the cable at the end where the hot gas is blown m.

It is hereby ensured that no trapped gas bubbles are formed m the free volume along the superconducting tapes that may cause bursting of the pipe.

Although the invention has particularly been explained m connection with conventional power cables of the AC or DC type and of the superconducting type, nothing prevents the principles of the invention from being used m related fields within the scope defined by the claims.

Thus, there is great freedom m applying the principles of the invention m connection with all transmission lines of electrical power where the transmission takes place by transfer of electrical charge.

Claims

P a t e n t C l a i m s :

1. A method of protecting a current-carrying cable against harmful temperatures, c h a r a c t e r i z e d m that a substance having different phase states at different temperatures is provided m the vicinity of the current-carrying parts of the cable.

2. A method according to claim 1, c h a r a c t e r i z e d m that the phases of the substances change from solid to liquid state upon an increase m the temperature .

3. A method according to claim 1 or 2, c h a r a c t e r i z e d m that the substances consist of an organic substance.

4. A method according to claim 1, c h a r a c t e r - l z e d m that the substances consist of magnetic materials which, at a given temperature, change from being ferromagnetic or antiferromagnetic, respectively, to being paramagnetic.

5. A method according to claims 1 - 4, c h a r a c t e r i z e d m that the substances consist of combinations of metal or metal alloys and of magnetic materials.

6. A cable consisting of a hollow core m which a cool- mg medium is transported, said core having wound thereon a plurality of superconducting tapes, c h a r a c t e r i z e d m that the superconducting tapes are embedded m a substance which has different phases at different temperatures .

7. A cable according to claim 6, c h a r a c t e r i z e d m that the substance is propene.

8. A cable according to claims 1 - 6, consisting of a conductor (3) surrounded by four layers which, m sequence, consist of a polymer layer (4), a semi-conducting layer (2), a second layer of polymer (1) and a second semi-conducting layer (2), c h a r a c t e r i z e d m that the conductor is hollow and filled with a substance having an endothermic phase transition which is between the nominal temperature of the conductor and the permissible maximum operational temperature of the conductor.

9. A cable according to claim 8, c h a r a c t e r i z e d m that the cable fully or partly consists of a pipe of stainless steel, aluminium, copper or an alloy thereof.

10. A cable according to claims 1 - 6, consisting of a conductor having externally thereon four layers which, m sequence, consist of a layer of oil-impregnated paper having a low electrical conductivity, a layer of dielectric of oil-impregnated paper, and outermost a further layer of oil-impregnated paper of low conductivity, c h a r a c t e r i z e d m that the conductor is hollow and filled with a substance having an endothermic phase transition which is between the nominal temperature of the conductor and the permissible maximum operational temperature of the conductor.

11. A cable according to claim 9, c h a r a c t e r i z e d m that the conductor, fully or partly, is formed by a pipe of stainless steel or copper. Method for protection of a current-carrying cable against overheating and cables

ABSTRACT

In a method of protecting cables, such as superconducting cables, power DC or AC cables, a substance having different phase states at different temperatures is provided in the vicinity of the current-carrying conductors of the cable. Hereby, an unintentional temperature change in the cable can be counteracted, as the substance can absorb or liberate heat by its phase state change, whereby a temperature change with destruction of the cable can be prevented for a certain period of time.

In a first embodiment, the cable is a superconducting cable having a core on which a plurality of superconducting tapes is wound. These tapes are embedded in a substance which changes phase state at a given temperature between the nominal and maximum temperatures of the cable .

In two other embodiments of a power cable of the DC or AC type, respectively, the conductor is formed by a hollow steel pipe whose cavity accommodates a substance which changes phase state at a given temperature. This substance may be organic, a metal or a metal alloy, or consist of a magnetic material.

Fig. 1 should be published.