Patent Application
20100285968
The present invention relates generally to thermal contraction compensation for cables, and more particularly to an apparatus and method for compensating for thermal contraction of superconducting and cryo-resistive cables.
Short length demonstrator superconducting cable systems have been assembled and tested in a number of countries worldwide. Recent demonstrator cables have been of the high temperature superconductivity (HTSC) type with lengths typically less than 100 meters and up to a maximum of 200 meters. Earlier demonstrator cables were of the low temperature superconducting type and the cryo-resistive type. To be economically acceptable, commercial HTSC installations are required to have similar reel lengths (500 meters to 1,500 meters) to conventional cables to reduce the number of site assembled joints, which increase the risk of unreliable operation and which are expensive to assemble.
Conventional electric power cables with copper or aluminum conductors are typically installed at ambient temperature, e.g. 15° C. When carrying the rated current, they are designed not to exceed the specified operating temperature which is typically 90° C., a rise of 75° C. Although this increase is comparatively moderate, the expansion forces, if constrained, generate high thermomechanical forces. For example a stranded copper conductor having a cross sectional area of 2000 millimeters2 (mm2) can generate 60 kilonewtons (kN) of force. Accordingly, accessories such as joints, termination, and support structures have to be designed to withstand these forces and protect the cable system against damage. As a result, early cable systems were beset with problems of thermomechanical failure of conductor connectors in “rigidly constrained” systems and fatigue failure of a cable's metallic sheath at preferential positions in “unconstrained” systems subjected to cyclic loading.
Superconducting and cryo-resistive cables experience high thermal contraction strain when they are cooled down to their operating temperatures. The forces that are developed are sufficient to damage the cable, joints, and terminations. The conductor in a superconducting cable, particularly of the HTSC type is comprised of a large number of small and fragile elements which have low tensile strength and are difficult to connect together in a straight joint or termination in a sufficiently robust manner to withstand long term tensile forces of high magnitude. Additionally, the magnitude of current that can be carried safely in a superconducting state is limited by mechanical strain.
Insulation surrounding the conductor also contracts during cool-down and its electrical integrity is dependent upon the absence of mechanical disturbance and damage. The presence of thermal contraction strain in the cable conductor and the insulation directly impacts the feasibility and economics of HTSC cable systems in reducing the cable reel length, increasing the number of joints, reducing the current carrying capacity, and increasing the risk of electrical failure of the insulation.
Present superconducting and cryo-resistive cables operate in the temperature range of −200° C. to −270° C. The cables are installed at near room temperature, for example 15° C., and so they are required to cool-down through a large temperature drop of 215° C. to 285° C., this being some 3-4 times greater than the temperature rise experienced by conventional cables. Thus, it is apparent that the prospective thermal contraction forces experienced by the cable, joints, and terminations will be 3-4 times greater than those in conventional cable system components. Because of these forces, the amount of damage that may occur to the conductor, insulation, outer cable layers, vacuum cryostat, and coolant pressure pipes can make superconducting and cryo-resistive cables unsuitable for medium to long length commercial applications, i.e. to more than several hundred meters.
These and other shortcomings of the prior art are addressed by the present invention, which provides a method to eliminate or reduce the magnitude of thermomechanical contraction forces that are generated in a superconducting or cryo-resistive cable systems when cooled down from installation temperatures to operating temperatures.
According to one aspect of the present invention, a method of compensating for thermal contraction of superconducting and cryo-resistive cables, includes the steps of determining a compensation length for a cable such that when the cable is subjected to an operating temperature, the cable is in or near a stress-free state, and installing the cable into a pipe such that portions of the cable extend outwardly past ends of the pipe. The method further includes the steps of marking each end of the cable such that the determined compensation length is visibly shown, forcing the cable into the pipe at an ambient installation temperature such that a cable pattern is formed therein, and maintaining the cable in the pipe to prevent the cable from being pushed out of the pipe.
According to another aspect of the present invention, a cable insertion tool includes a guide tube adapted to engage a pipe and to prevent a cable positioned therein from buckling; a cross-bar adapted to move along the guide tube and engage an end of the cable positioned therein; and a cable drive apparatus operably connected to the cross-bar, the cable drive apparatus being adapted to move the cross-bar such that the cable positioned in the guide tube is forced into the pipe.
According to another aspect of the present invention, a cable includes a duct formed by a central mandrel; a tensile member positioned in the duct; at least one conductor laid upon the central mandrel; and at least one insulation layer.
The subject matter that is regarded as the invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which:
Referring to the drawings, a conventional uncompensated system is shown in
As shown, if the cable lengths had not been jointed together they would be free to contract a total length Δx, Equation 1, this being equivalent to a retraction of Δx/2 at the end of each cable span. In a practical cable system the cables are jointed together and are not free to contract, so at operating temperature the contraction strain Δx/LS is locked into the cable and joints throughout the circuit length and generates high thermomechanical tensile forces. The total length Δx is calculated as follows:
Δx=LS·α·Δθ Equation 1
Where Δx is the total prospective thermal retraction length in meters (m) of the cable in one pipe span; LS is the initial length in meters (m) of cable at the installation ambient temperature; α is the effective coefficient of thermal expansion of the cable (K−1); and Δθ is the temperature change from installation to operating temperature in Kelvin (K).
As shown in
F=K·α·Δθ Equation 2
Fsw=F/RB Equation 3
Where F is the thermomechanical axial force acting in a cable or cable element expressed in newtons (N); Fsw is the thermomechanical sidewall force acting in a cable or cable element expressed in newton-meters (Nm−1); K is the effective axial stiffness of a cable or cable component expressed in newtons (N); and RB is the radius of a bend in a cable expressed in meters (m).
Referring to
As illustrated in
In some cable applications, a compromise with other aspects of cable design may prohibit the attainment of a completely stress-free cable system. In such applications the compensation length is used to reduce the tensile and sidewall forces to a magnitude that can be safely withstood by the cable and accessories during the required service life.
The method described above may be used to achieve a calculated compensation length in multiple applications. For example, the method may be used with a single cable installed into a pipe or duct system,
To install the cable 16 into the pipe 11, the cable is forcefully pushed into the duct, pipe, or sheath (hereinafter collectively referred to as pipe) by the pre-calculated compensation distance, Δx, and formed into a pre-designed cable pattern. The cable 16 may be pushed into the pipe 11 by Δx from one joint position or preferably by Δx/2 at the two joint positions on each end of the pipe span to induce the cable 16 to store the compensation distance into a loose pipe. The compensation being stored in the form of pre-calculated lateral deflections of the cable 16 in relation to the axis of the pipe 11. For example, the cable 16 may be stored in a helical sinusoid pattern,
The cable absorption length that can be stored is controlled by the pipe radial clearance ΔRcp and the wavelength LW. The helical pattern requires the highest insertion force, but is capable of absorbing a long cable compensation length. The helical sinusoid requires a lower insertion force, with a medium capability to store cable compensation length. The lateral deflection of the cable in a pipe bend requires the lowest insertion force and is capable of absorbing a high cable compensation length. The length that can be stored in pre-formed pipe bends,
To reduce the amount of force needed to push the cable into a pipe, the outer surface of the cable and the inner surface of the pipe may be coated with a combination of materials to minimize longitudinal friction, and achieve a unified distribution of stored compensation strain in the cable along the pipe span length. The inner surface of the pipe may be coated with a low friction material containing poly-tetra-flouro-ethylene (PTFE). Alternatively, or in combination, the cable may be provided with a polymeric extruded jacketing material of high shore hardness, such as a medium to high density grade of polyethylene (PE), a polypropylene jacket, a nylon jacket or a nylon co-extruded jacket on a base of PE. The surfaces may also be lubricated at the time of installation with, for example, a synthetic or natural wax based solution or a graphite based dry powder.
To aid in inserting a cable into a pipe, as described above, cable insertion tools,
Referring to
In use, after installation into a pipe 130, the cable 112 is cut and terminated with the end cap 113, similar to that shown in
The above process is preferably performed simultaneously at both ends of the pipe 130. If the process is only performed at one end of the pipe 130, then the cable 112 is clamped at the opposite end of the pipe 130 to prevent outwards movement. After cable insertion is completed, the ends of the cable 112 are clamped to the pipe 130 to prevent compressive forces in the cable 112 from pushing the cable 112 back out of the pipe 130. A clamp 131 is located between the pipe 130 closure flange 132 and the insertion tool end plate 124, such that after tightening the clamp 131 to grip the cable 112, the guide tube 110 may be removed. The joint between the adjacent cable lengths is then assembled. The clamps 131 may then either be removed or left permanently in position to protect the joint from movement due to any differential cable forces that may occur in service operation.
As shown in
Referring to
A block and tackle pulley insertion tool is shown in
Referring to
In addition to using insertion tools, like those disclosed above, other means may be used to aid the process of inserting a cable into a pipe. For example, vibration may be applied to the pipe during or after the cable insertion operation. The use of vibrations help reduce the insertion force needed and help uniformly distribute the compensation length. Based on finite element analysis, it has been shown, for example, that a vibration of frequency 13 Hz and magnitude 0.032 mm is sufficient to momentarily lift a cable from the constraint of gravity and friction and permit stored compressive strain to move the cable along the pipe. The vibration may be applied by a powered vibration tool applied to the pipe at a number of positions according to the span length. The vibration tool may be, for example, an electronically powered transducer or a motor driven eccentrically loaded rotor disc.
In another example, an impulsive shock may be applied to the cable during or after the cable insertion process to reduce the insertion force and uniformly distribute the compensation length. A pulse of electric current applied along a conductor in the cable produces a momentary electro-mechanical impulsive force of sufficient magnitude to momentarily lift the cable from the constraint of gravity and friction and permit stored compressive energy to move the cable along the pipe. A parallel return conducting path is required to return the pulse of current, such as along two or more cables in one pipe, or in the case of one cable in the pipe, a second temporary cable laid either inside or outside the pipe.
The impulsive force results from the interaction of the magnetic field from one conductor with the current in the second conductor. The advantages of this are that the cable is lifted throughout the length of the pipe and it is unnecessary to provide external access to the pipe or to distribute mechanical vibrating machines along the pipe. The magnitude of the current may be calculated to suit the particular cable unit weight. For example, a simultaneous current pulse of peak magnitude 28 kA and 5×10−3 seconds duration applied to three heavy 230 kV, 2500 kcmil transmission class cables is sufficient to momentarily lift them and permit longitudinal movement. The current pulse may be applied by either discharging a charged capacitor, discharging the cable insulation, or short circuiting the output of a spinning generator along the cable through a suitable series impedance.
Referring to
The flexible tensile member 601 is tightened after installation and before jointing to provide the cable compensation length by longitudinally compressing the cable 600 construction and forcing the cable 600 to initiate the formation of lateral patterns, thereby relieving the cable 600 and its joints and terminations from experiencing an unacceptable magnitude of tensile load at cable operating temperature. The tightening of the tensile member 601 may be performed in combination with the cable insertion process described above to reduce the insertion force and achieve a uniform distribution of patterns. Clamps are applied to the cable 600 at each end of the pipe 630 to prevent the compressed cable 600 from moving. The tensile member 601 is then released at one or both cable ends and the cables are jointed together. During cool-down the stored cable compensation length is released to cancel the thermal contraction strain and to eliminate tensile thermomechanical forces.
The cable 600 construction is designed such that it may be axially compressed during the cable insertion process and extended during the cable cool-down. The cable's 600 low axial modulus is achieved by using a flexible central mandrel upon which inner and outer conductors 606, 608 are laid, both longitudinally radially. The conductors 606 and 608 are applied at short lay angles with low compaction and with discrete interstitial gaps between the wires and tapes. The inner and outer conductors 606, 608 and their internal successive layers of wires or tapes are applied in a uni-lay direction, and insulation 607 is formed of a longitudinally deformable material, for example, an extruded elastomeric insulation with a low temperature, low modulus of elasticity such as a silicone elastomer or an EPR elastomer, or a taped construction of laminate or Kraft paper insulation separated by suitably sized butt gaps. A metallic sheath 609 with a thin corrugated wall and a short pitch, for example, 1 mm thick sheet stainless steel is also used.
In addition to achieving an axially compressible construction, the uni-lay conductor 600 produces a non-torque balanced cable that will readily form spiral helices under the action of low compressive force. During installation into the pipe the cable is protected from being extended by restricting the pulling tension to a minimal value suitable for cable guidance only and by the use of a cable engine or distributed engines as described above. The use of the cable construction with a tensile member 601 inside a duct 610 of the mandrel 602 permits the cable 600 to be pulled into the pipe from its leading end without risk of extension. During the pull the tensile member 601 is connected to the cable ‘pulling bolts’ (pulling eyes) connected to each end of the cable, such that the member 601 withstands the tension and protects the cable 600 from extension. The cable 600 is then clamped, the internal member is released, and the cables are jointed together.
Referring to
The cable may alternatively be installed without being forcefully inserted outwards into the pre-formed bends. In this instance, the cable is inserted a sufficient distance to lie on the neutral axis of the pipe A-A′ such that 50% of the prospective distance is released when the cable moves to the inner wall of the bend, C-C′ during cooldown. The advantages of this method are that the pipe bends control the uniform distribution of bending along the cable length, a comparatively small pipe diameter may be selected, and longitudinal cool-down stresses in the pipe are reduced.
It should also be appreciated that the pipe may be elastically deformed during installation into temporary bends. The initial increase in cable length compared to the straight span length provides the compensation length. During cool-down the elastic pipe deformation is progressively released such that the pipe and cable become straight. This allows the pipe to temporarily accommodate the compensation distance by increasing the pipe length, such that the distance A, B, C, D . . . Q is greater than the straight span length LS. This can be advantageous when the cable is required to be a tight fit in the pipe or metallic cable sheath. This is particularly suited for cable applications that provide accessibility to the pipe, for example, those above ground or in a tunnel. The maximum amplitude of the patterns is dictated by the safe bending strain for the pipe. The amplitude may be increased by use of a corrugated pipe or metallic cable sheath exhibiting low bending stiffness or the insertion into the pipe at regular intervals of longitudinal flexible bellows. The pipe amplitude may be achieved by the use of a conventional pipe bending tool in which the pipe is anchored at two closely spaced half wavelength points. The pipe is then deflected outwards by a hydraulic ram pressing on a curved shoe. The pipe system is held in the deformed shape by temporary distributed clamps.
The cable is next installed into the deformed pipe. The difference between the length A . . . Q and LS provides the compensation length. The joints are then made to connect the ends of the cables together and the pipe is closed. During cool-down the positions of the temporary pipe clamps are progressively relaxed to permit the pipe patterns to move elastically inwards back to the original straight length. When the cable is required to be re-heated to ambient temperature, the temporary cleats are re-applied and pulled back to their original positions as the temperature rises. Alternatively the cleats may be left permanently in position and be restrained by steel springs, such that the positions of the temporary pipe clamps are progressively relaxed to permit the pipe patterns to move elastically inwards back to the original straight length. When the cable is required to be re-heated to ambient temperature, the temporary cleats are re-applied and pulled back to their original positions as the temperature rises. Alternatively, the cleats may be left permanently in position and be restrained by steel springs, such that upon cooling the pipe straightens and extends the springs and upon re-heating, the pipe expands and is pulled back into the original pattern.
The foregoing has described an apparatus and method for compensating for thermal contraction of superconducting and cryo-resistive cables. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.
