Abstract:
Enhanced superconducting power cable of at least one phase, characterized by two tubular sections, the first section being a flexible superconducting core, with a stainless steel tape mesh and copper tape layers overlaid at an angle of 0° to 45° followed by two or more superconducting material layers placed overlaid and a second application of two or more superconducting material layers in opposite direction with regard to the previous ones; the second tubular section is an annular space of vacuum thermal insulation formed by a flexible corrugated pipe covered with multilayer insulations, including also a pipe with a stainless steel mesh to adhere an internal semiconducting shield made of polyethylene with insulation followed by a second metal shield based on copper tapes and a polyethylene protecting cover.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The instant invention relates to the conduction of electric power and particularly to the manufacturing of a superconducting power cable of at least one phase, characterized by a central core based on a superconducting tape material BSCCO of 22233 (Bi 2  Sr 2  Ca 2  Cu 3  O x ) commercial composition giving a minimum current density of 7 KA/cm 2  under the criteria of 1 μV/cm. It also includes an annular space of thermal insulation system wherein the corrugated casing of the system presents a vacuum pressure below 10 mPa (milliPascals) permitting the thermal insulation to maintain operating temperatures of 77°K (temperature of liquid nitrogen under atmospheric pressure) throughout the cross section of the cable in its superconducting part. 
   2. Previous Art 
   The invention relates to the transportation of electric power in superconducting conditions, zero resistance in direct current. This invention replaces the use of power cables for distribution and transmission in voltages from 15 kV upwards because it presents lower conduction losses. 
   High temperature superconductors can be important aspects of technological advances, because equipment and devices could have superconducting parts in their components. An obvious application in superconducting state is the use of zero resistance properties to the passage of direct current and low power losses in the electricity transmission. In the present transmission lines, electric power is lost through heat when the current passes through normal conductors. If electricity is transmitted through superconducting cables, said losses can be reduced or eliminated with the subsequent savings in the energy costs. This can be applied to any electric component having cooper leads, for examples, motors, transformers, generators and any equipment involved with electric power. 
   Some US and Japanese companies have manufactured and evaluated superconducting cable models of up to 5000 cm obtaining current values not exceeding 1700 A to 2000 A. Tests conducted in 5000 cm long segments have shown problems related to current distribution among layers. Said distribution tends to be irregular because of electromagnetic problems related to the lead itself. 
   Patent WO 00/39813 describes a superconducting cable using high temperature superconducting materials HTS with flexible core. However it applies to a traditional coaxial design with insulated HTS tape layers and cold design. 
   Japanese Patent 06239937 A2 describes a superconducting cable with HTS materials and flexible core but involving a traditional DC (direct current) design and insulation between each HTS tape layer. 
   U.S. Pat. No. 5,929,385 describes a superconducting cable similar to the object of the instant invention but only as far as the type of materials used is concerned. U.S. Pat. No. 5,952,614 also describes a superconducting cable similar as far as the use of HTS materials and flexible core are concerned but with a coaxial design, in cold conditions and with HTS tape traditional design. For these reasons, said inventions are different from the characteristics of the instant invention. 

   
     DESCRIPTION OF THE INVENTION 
     Hereinafter the invention will be described in connection with the drawings of  FIGS. 1  to  6 , wherein: 
       FIG. 1  is a perspective view with cross section showing the different layers of the superconducting power cable. 
       FIG. 2  is a cross section view of FIG.  1 . 
       FIG. 3  is a perspective view with cross section of the vacuum section of the central core thermal protection. 
       FIG. 4  is a perspective view with cross section of  FIG. 3  showing the opposite wall of the thermal insulation. 
       FIG. 5  is a perspective view with cross section of the superconducting power cable core. 
       FIG. 6  is a perspective view with longitudinal cross section of  FIG. 1 , showing the annular space of thermal insulation. 
   

   The invention is related to the transportation of electricity in superconducting conditions, (zero resistance in direct current). This invention replaces the use of power cables for distribution and transmission in voltages of 15 kV or more because it presents lower conduction power losses, considering that for a Cu lead with a current density of 1-4 A/mm 2  and a resistivity of 2×10 −8  Ωm, the transmission losses are on the order of 20-80 mW/Am. To better compare with superconducting cables, losses caused by the heating of superconducting materials have to be taken into account. At cryogenic temperatures, said losses are defined by a Carnot factor divided between the efficiency of the cooling system. In the case of liquid nitrogen, this factor is between 10 and 20. Thus, in a superconductor losses will be lower than 5 mW/Am. The flow of liquid nitrogen fills the longitudinal cavity  21 ,  FIG. 5 , of the flexible corrugated pipe  1  of 304 or 316 stainless steel. Said pipe can have an external diameter between 2 cm and 6 cm and an internal diameter between 1 cm and 4 cm wherein the depth of the corrugation can vary between 0.5 cm and 1 cm. The corrugation pitch can be between 0.8 and 1.5 cm for a corrugation depth between 0.4 and 0.5 cm. As another alternative for a depth between 0.4 and 0.6 cm, the corrugation pitch can be between 1.6 and 3 cm. On this pipe, a 304 or 316 stainless steel mesh is placed in order to obtain a relatively flat surface. On this mesh a stainless steel tape layer  2  is placed, between 4 and 5 cm wide and between 0.0005 and 0.006 thick. They are placed on the corrugated pipe with spacing between 0.15 and 0.2 cm. Then one or two additional stainless steel tapes, 2.5 to 4 cm wide and 0.001 to 0.002 cm thick, are placed with spacing between the tapes of 0.1 to 0.15 cm. After a first layer of Cu tapes  3  is placed, from 0.25 to 0.40 cm wide and from 0.025 to 0.030 cm thick, with a cabling length between 2 cm and 100 cm depending on the design of the first layer of superconducting tapes to be applied. Said layer of Cu tapes can be laid at an angle ranging from 0° to 45° depending on the cable design. The superconducting material to be used is made of tapes of a 22233 (Bi 2  SR 2  Ca 2  Cu 3  O x ) composition commercial product BSCCO. Said tapes range in width between 0.38 and 0.42 cm and in thickness between 0.018 and 0.022 cm, which gives a minimum current density of 7 kAcm 2  under the criteria of 1 μV/cm, (microvolt/centimeter). With this superconducting material, two or more layers of tapes are laid with a cabling lay length between 20 cm and 300 cm, at an angle ranging from 0° to 45° depending on the design of each layer with a direction that can be right or left  4 ,  5 ,  6 . And two or more layers of superconducting material tape with a lay length between 20 cm and 300 cm with an angle ranging from 0° to 45° depending on the design of each layer with a direction that can be right or left with regard to the cabling, in the opposite direction of the previously placed layers  7 ,  8 ,  9 . Finally, a wrapping tape made of insulating material  10 , with a thickness ranging between 0.005 and 0.01 cm and a width ranging between 2 and 4 cm is laid. 
   In order to protect the central core, the superconducting power cable object of the instant invention is also characterized because it includes a vacuum thermal insulation system consisting of a flexible corrugated pipe  11  made of 304 or 316 stainless steel, to hold the superconducting cable and liquid nitrogen. Said pipe can have an external diameter ranging between 4 cm and 8 cm and an internal diameter ranging between 3 cm and 7 cm, the corrugation depth varying between 0.5 cm and 1 cm. The corrugation pitch can vary between 1 cm and 2 cm for a corrugation depth between 0.5 and 0.8 cm. Then, on the periphery of this pipe, a multi layer thermal insulation (ρ a )  12  is applied, which can have a thickness ranging between 0.0005 cm and 0.005 cm which is calculated according to the following formula: 
    ρ a =( S   s +ρ r   t   r )( N/Δx ) 
   wherein: 
   
       
       ρ a  Thickness of the insulating layer 
       S s  Mass of the material per area unit 
       ρ r  Insulating material density 
       t r  Thickness of the anti-radiation casing 
       N/Δx Layer density 
     
  
   Concentrically around the flexible corrugated pipe  11 , covered with the insulating material  12 , a second corrugated pipe  13  is placed, creating the vacuum thermal insulation space  20 , FIG.  6 . 
   To ensure the adequate functioning of the thermal insulation system at a temperature of 77°K, a vacuum pressure below 10 mPa. (milliPascals) is required. 
   Said second corrugated pipe  13 , which creates the vacuum space, is made of 304 or 316 stainless steel which can have and external diameter ranging between 8 cm and 10 cm and an internal diameter ranging between 6 and 7 cm, wherein the depth of the corrugations may vary between 0.5 cm and 1.5 cm. The corrugation pitch can be between 1 and 2 cm for a corrugation depth between 0.5 and 1 cm. 
   The thermal insulation system includes also on the external wall of the corrugated pipe  13 , a braided stainless steel mesh  14 ,  FIGS. 1 and 3 , offering a uniform surface to the external wall structure of the helical or spiral shaped corrugated pipe. 
   Around the uniform mesh surface  14 , an internal semiconducting shield  15  is applied, which is made of low density thermoplastic polyethylene or any other thermoplastic or thermosetting semiconducting material. The conductivity of said shield should not exceed 1000 Ω m when it is measured at room temperature, said shield having a thickness of at least 0.006 cm. On this semiconducting shield the electric insulation of the cable  16  is placed. Said electric insulation is based on low, medium or high density, thermoplastic or thermosetting or crossed chain polyethylene and/or Ethylene Propylene (EP), the thickness of the insulation being between 0.229 cm and 0.976 cm depending on the operation voltage level of the cable. On this electric insulation, a second semiconducting shield  17  made of the same materials as the internal semiconducting shield  15  is placed, FIG.  4 . However, in this case, the thickness of the shield must be at least 0.0129 cm and has to fulfill a maximum volume resistivity of 500 Ωm when measured at room temperature. On this layer, a metal shield made of Cu tape  18  is placed, which must be at least 0.0635 cm thick, having a cross section area of at least 0.1 mm 2/ mm. On this metal shield  18 , a protective casing  19  is placed, possibly made of polyethylene or polyvinyl chloride (PVC) depending on the application of cable, said casing having a thickness ranging between 0.203 and 0.279 cm. 
   According to the technical requirements, the basic superconductor design parameters used were as follows:
         Tape Width (cm): 0.4±0.02   Tape Thickness (cm): 0.02±0.002   Critical current Density (kA/cm 2 )&gt;7 (criterion of 1 μV/cm)   Filamentary section thickness inside the tape 2 b   sc (cm): 0.018   Critical current in the bending deformation voltage value: 0.1%—not below 95% or 0.2%—not below 90%.   About 20% reduction in the critical current when the field is between 0T and 0.1T.       

   The basic equations to compute the number of superconducting tapes and the design parameters are as follows:
 
Number of tapes per layer (Ni) 
       Ni   =       π   ⁢           ⁢   x   ⁢           ⁢   Diox   ⁢           ⁢   COS   ⁢           ⁢   β   ⁢           ⁢   i       2   ⁢     ai   ⁡     (     1   +   Si     )               
 
wherein:
         D io =average diameter of the i layer   2 ai =Tape Width of the i layer   S i =Relative space between the tapes of the i layer   β i =Laying angle of the superconducting tapes
 
Lay of the tapes in a layer (Pi) 
       Pi   =       π   ⁢           ⁢   x   ⁢           ⁢   Dio       tan   ⁢           ⁢   β   ⁢           ⁢   i           
 
Relative spacing between the tapes of a layer: (Si) 
       Si   =       π   ⁢           ⁢   x   ⁢           ⁢   Diox   ⁢           ⁢   Cos   ⁢           ⁢   β   ⁢           ⁢   i       2   ⁢   aix   ⁢           ⁢   Ni           
 
Relative deformation voltage ε i  regarding the superconductor in bending conditions of the tapes is:
 
ε i =2 bscx senβ/Dio
       

   The model base of the superconducting high temperature cable has been developed, which consists of the design of the superconducting core itself, as well as the development of insulation based on known and previously developed materials for use in medium and high voltage power cables. 
   EXAMPLE 1 
   Under the design conditions, the superconducting tape VAC (Germany) was chosen. Said tape presents a critical current of 59.8 A to 64.7 A depending on the combination of thickness and width of the superconducting tape. Based on these variations, the criteria of linearity of the critical current density used for the cable optimization and calculation is not very congruent, and thus a value of the critical current density in the external magnetic field equals to cero is accepted as 113 A/cm for cable calculations. Taking into account said variations, the followings values were taken as parameters for the calculation.
         External Diameter of the core D fe =5.5 cm;   Tape thickness 2 b   t =0.002 cm;   Filament section thickness inside the tape 2 b   sc =0.018 cm;   Relative space between each tape in each layer S=0.05.       

   The minimum lay of the tapes (maximum angle of tape laying) is selected based on the limitations imposed by the deformation voltage threshold with regard to bending, for a superconductor when the tapes in one layer are bent on a diameter D i  and the tape laying angle β i  is at a maximum permissible value (ε&lt;0.2%, wherein ε=2 b   sc  cos β i /D i ). The critical current of the cable is expected to be between 6 kA and 10 kA, under the criteria of 1 μV/cm and the approximate values of the magnetic field induction on the surface of the sixth layer being between 0.04 T and 0.07 T. For this reason for every 0.001 T increase, the critical current reduction of the tape is expected to be 2% its initial value. 
   The influence of the deformation voltage on the superconductor with regard to the value of the tape critical current during the manufacturing of cable is described in the comments on Table No. 1. 
   
     
       
             
           
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
           
         
             
               TABLE NO. 1 
             
           
           
             
                 
             
             
               Expected manufacturing results 
             
             
               (2 a  = 0.38 cm, 2b sc  = 0.018 cm) 
             
           
        
         
             
               Layer 
               D i   
                 
                 
               I c   
                 
                 
               I maxi   
                 
             
             
               Num- 
               Bending 
               ε 
                 
               Tape 
               I ci   
                 
               REAL 
               S i   
             
             
               ber 
               Mm 
               % 
               N i   
               A 
               A 
               I ci /I co   
               A 
               Real 
             
             
                 
             
           
        
         
             
               1 
               13.32 
               0.135 
               40 
               42.22 
               1688.8 
               0.1667 
               1672.3 
               0.0377 
             
             
               2 
               17.52 
               0.103 
               42 
               41.54 
               1744.7 
               0.1722 
               1727.6 
               0.0399 
             
             
               3 
               42.03 
               0.043 
               44 
               40.85 
               1797.4 
               0.1771 
               1779.8 
               0.0446 
             
             
               4 
               51.72 
               0.035 
               45 
               40.17 
               1807.7 
               0.1784 
               1790.0 
               0.0325 
             
             
               5 
               15.82 
               0.114 
               42 
               39.49 
               1658.6 
               0.1637 
               1642.3 
               0.0457 
             
             
               6 
               9.97 
               0.181 
               37 
               38.80 
               1435.6 
               0.1417 
               1435.6 
               0.0484 
             
             
               Σ 
                 
                 
                 
                 
               10132.8 
               1 
               10047.6 
             
             
                 
             
           
        
       
     
   
   According to the above table, it can be seen that the current value depends on the maximum deformation voltage if and only if it does not exceed the deformation value of 0.2% which is the critical value of the current. From the results obtained in the above table, we observe that there is uniform current distribution in every layer, which gives a current distribution factor I ci /I co =1 and a real maximum critical current value of I MAX REAL =10047 A. 
   EXAMPLE 2 
   However, in Table No. 2, the optimization results of the cable are presented as the criteria to reach the peak critical current value and the minimization of the energy losses under the influence of the flow and axial magnetic field. 
                                                                                       TABLE NO. 2                   Optimization Results                D i                                     Exterior   Tape laying   J ct     β i     P f     I calc         Layer Number   Cm   direction   A/cm   degrees   cm   I i /I o     J i                      1   5.554   L/1   111.11   24.6   37.89   0.1671   1.0000       2   5.588   L/1   109.31   18.6   51.96   0.1716   0.9938       3   5.632   L/1   107.51   7.70   130.35   0.1765   0.9859       4   5.676   R/−1   105.71   6.30   160.89   0.1769   0.9941       5   5.720   R/−1   103.91   21.2   46.15   0.1648   0.9967       6   5.764   R/−1   102.11   35.3   25.48   0.1431   0.9985       Σ   5.764                   1.0000   0.9948               Maximum current reached I MAX  = 10028.5        Total sum of the utilization coefficient in the six layers K MAX  = Σ ji  = 5.96689             
Maximum current reached I MAX =10028.5
 
   Total sum of the utilization coefficient in the six layers
 
K MAX=Σ   ji =5.96689
 
And according to the above mentioned criteria, current distribution is uniform in all the cable layers, and the losses caused by the axial magnetic field are minimized.
 
Wherein:
         D i  exterior=external diameter of the i layer       

   J ci =Density of the lineal critical current for the tapes of the i layer 
   β i =Tape laying angle for the tapes of the i layer 
   P i =Tape lay for the tapes of the i layer 
   N i =Number of tapes in the i layer 
   I ci =Total critical current of all the tapes in the i layer (current i layer) versus the total number of tapes (sum of the critical currents of all the tapes) in the model. 
   I calc =I i /I o  Current distribution in the i layer of the total current. 
   N i /N o =I ci /I co =Number of tapes in the i layer (critical current in the i layer) versus the total number of tapes (sum of the critical currents of all the tapes) in the model. 
   I max  REAL=Real value of the current peak in the i layer when the current reaches its critical value in at least one of the layers. 
   J i =Superconductor utilization coefficient in the i layer.