Abstract:
Disclosed is a PTC (Positive Temperature Coefficient) current limiting device, which limits a current using PTC characteristics. The device includes a PTC element having the PTC characteristics; and upper and lower contact electrodes arranged to face each other with the PTC element being interposed therebetween, wherein, assuming that a distance from an end of the upper contact electrode to an end of the PTC element is a1, a distance from an end of the lower contact electrode to the end of the PTC element is a2, a thickness of the PTC element is b, and L=a1+a2+b, the following equations are satisfied: V/L&lt;10 and V/b&lt;50, where V is a rated voltage of the PTC current limiting device, a unit for a1, a2 and b is mm, and a unit for V is volt. This PTC current limiting device may prevent generation of flashover between electrodes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a current limiting device, and more particularly to a PTC (Positive Temperature Coefficient) current limiting device using a PTC element to prevent flashover between contact electrodes in the current limiting device.  
         [0003]     2. Description of the Related Art  
         [0004]     Generally, a circuit breaker is widely used for preventing a short circuit of a high or low voltage system. However, a conventional circuit breaker takes a long time for circuit breaking, and does not have a current limiting function against an estimated fault current value, so a ripple effect for the fault is lasting relatively longer. In addition, in case of failing to break a short circuit current, serious effects are given to surrounding power equipments and systems. Thus, there is an increased need for a current limiting device that is capable of effectively limiting a short circuit current of a system in a short time.  
         [0005]     The current limiting device is used for limiting overcurrent or short circuit current generated in a power system, and it may achieve its function using PTC (Positive Temperature Coefficient) materials generally in a low-voltage, low-current region.  
         [0006]     A material having the PTC characteristic has a relatively low resistance at a normal temperature to pass an electric current well. However, if a temperature of surroundings is increased or the material is heated by itself due to the introduction of a current over an allowable value, the resistance is abruptly increased several hundred times or more, thereby capable of limiting the current. Thus, if a circuit element is configured using the above material, various circuits may be protected when a temperature rises.  
         [0007]     In this connection, Japanese Patent Publication H10-321413 discloses a current limiting device using PTC. Referring to  FIG. 1  related to the above, the conventional PTC current limiting device includes a PTC polymer element  1  having PTC characteristics by mixing conductive particles therein, first electrodes  2 ,  3  arranged on both surfaces of the PTC polymer element  1  by welding, and second electrodes  4 ,  5  arranged on the surfaces of the first electrodes  2 ,  3  to be electrically connected thereto.  
         [0008]     At this time, the current limiting device has conditions that the PTC polymer element  1  has a surface area greater than the first electrodes  2 ,  3 , and the first electrodes  2 ,  3  have a surface area greater than the second electrodes  4 ,  5 . In this configuration, an interior short circuit occurring at both ends of the first electrodes  2 ,  3  may be effectively prevented.  
         [0009]     In this PTC current limiting device, an initial resistance of the PTC element  1  and the current density ensuring electric connection are decided according to the thickness of the PTC element  1  without tripping of the device, so the thickness of the PTC element I should be not so large in order to use the PTC current limiting device in a high-voltage, large-current power system. However, if the PTC element  1  has a small thickness, flashover is apt to be caused between the first electrodes  2 ,  3 . Thus, it is preferred to select a PTC element  1  with a thin thickness not causing any flashover between the first electrodes  2 ,  3 . That is to say, it is required to suggest optimal design conditions considering even a thickness factor of the PTC element  1 , not designing a PTC current limiting device by simple comparison between the surface area of the PTC element  1  and the surface area of the first electrodes  2 ,  3 .  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention is designed to solve the problems of the prior art, and therefore it is an object of the present invention to provide a PTC current limiting device capable of preventing generation of flashover between contact electrodes of the PTC current limiting device by considering a thickness factor of a PTC element as well as a contact area factor between the PTC element and the contact electrodes.  
         [0011]     In order to accomplish the above object, the present invention provides a PTC current limiting device, which limits a current using PTC characteristics, the device including a PTC element having the PTC characteristics; and upper and lower contact electrodes arranged to face each other with the PTC element being interposed therebetween, wherein, assuming that a distance from an end of the upper contact electrode to an end of the PTC element is a1, a distance from an end of the lower contact electrode to the end of the PTC element is a2, a thickness of the PTC element is b, and L=a1+a2+b, the following equations are satisfied:  
           V   L     &lt;     10   ⁢           ⁢   and   ⁢           ⁢     V   b       &lt;   50     ,       
 
 where V is a rated voltage of the PTC current limiting device, a unit for a1, a2 and b is mm, and a unit for V is volt. 
 
         [0012]     According to a preferred embodiment of the present invention, the PTC current limiting device may further include upper and lower current leads connected to the upper and lower contact electrodes respectively to electrically connect the contact electrodes to a system circuit.  
         [0013]     The PTC element may include at lease one polymer selected from the group consisting of HDPE (High Density Polyethylene), LDPE (Low Density Polyethylene), epoxy, silicone, and PVDF (Polyvinyl Difluoride); at least one type of conductive particles selected from the group consisting of carbon, metal and metal oxide; and an antioxidant.  
         [0014]     Preferably, the PTC element has a plate shape.  
         [0015]     In another embodiment of the present invention, the PTC current limiting device may further include a pressing means for pressing the contact electrodes toward the PTC element.  
         [0016]     Preferably, the pressing means gives a pressing force equal to or greater than an atmospheric pressure.  
         [0017]     The pressing means may include a housing that receives the PTC element, the contact electrodes and the current leads; and an elastic member elastically biased by an inner surface of the housing so as to press the current leads toward the PTC element.  
         [0018]     As an alternative, the pressing means may also include a pair of plates arranged so that the PTC element, the contact electrodes and the current leads are interposed therebetween; and a coupling member for coupling and fixing the pair of plates with each other. Preferably, the pressing member may further include an elastic member elastically biased by an inner surface of the plates so as to press the current leads toward the PTC element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     Other objects and aspects of the present invention will become apparent from the following description of embodiments with reference to the accompanying drawing in which:  
         [0020]      FIG. 1  is a sectional view showing a conventional PTC current limiting device;  
         [0021]      FIG. 2  is a perspective view showing a PTC current limiting device according to a preferred embodiment of the present invention;  
         [0022]      FIG. 3  is a sectional view showing the PTC current limiting device of  FIG. 2 ;  
         [0023]      FIG. 4  is a sectional view showing a PTC current limiting device according to another embodiment of the present invention;  
         [0024]      FIG. 5  is a sectional view showing a PTC current limiting device according to still another embodiment of the present invention;  
         [0025]      FIG. 6  is a graph showing an operation waveform of the PTC current limiting device when a current limiting action is failed; and  
         [0026]      FIG. 7  is a graph showing an operation waveform of the PTC current limiting device according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]     Hereinafter, preferred embodiments of the present invention will be described in detail referring to the accompanying drawings. Prior to the description, it should be understood that the terms used in the specification and appended claims should not be construed as limited to general and dictionary meanings, but interpreted based on the meanings and concepts corresponding to technical aspects of the present invention on the basis of the principle that the inventor is allowed to define terms appropriately for the best explanation. Therefore, the description proposed herein is just a preferable example for the purpose of illustrations only, not intended to limit the scope of the invention, so it should be understood that other equivalents and modifications could be made thereto without departing from the spirit and scope of the invention.  
         [0028]      FIG. 2  is a perspective view showing a PTC (Positive Temperature Coefficient) current limiting device according to a preferred embodiment of the present invention, and  FIG. 3  is a sectional view showing the PTC current limiting device of  FIG. 2 .  
         [0029]     Referring to  FIGS. 2 and 3 , the PTC current limiting device of this embodiment includes a PTC element  110 , and a pair of contact electrodes  121 ,  131  arranged to interpose the PTC element  110  between them.  
         [0030]     The PTC element  110  restrains an overcurrent in a power system by abruptly increasing its electrical resistance at a specific temperature value as a temperature of the surroundings rises, as mentioned above.  
         [0031]     The PTC element  110  have different properties according to a current value to be limited, but in this embodiment the PTC element  110  preferably has a specific resistance of 100 Ωm or below at 25° C., and the specific resistance at a switching temperature that Joule heat is generated due to the supply of current is preferably increased at least 10 5  times as great as that at 25° C. In addition, the PTC element  110  should be designed to endure a voltage of AC 100V or above with keeping electrical and thermal stability and not to generate flashover when an over-voltage of 30 kV or above per 1 cm is applied. Moreover, when being put into a circuit, the PTC element  110  should not be tripped at the time that an ordinary current, for example about 1A is applied thereto. In addition, when an overcurrent more than 10 times of a normal operation current is applied, the PTC element  110  should cause a rise of resistance within ½ cycle (here, one cycle is 16.7 ms) at a frequency of 60 Hz to limit the overcurrent. Moreover, the PTC element  110  is preferably fabricated so that an operation time should be faster as a magnitude of a short circuit current is greater, and also it may restore its initial state within several minutes after the overcurrent limiting operation.  
         [0032]     Preferably, the PTC element  110  has a plate structure, and it may have a circular, oval or polygonal shape. In addition, the present invention is not limited thereto, but its area and thickness are designed in consideration of use conditions of the PTC element  110 , namely various factors such as an ordinary current, an overcurrent to be limited, and an operation time, as described later.  
         [0033]     According to this embodiment, the PTC element  10  is preferably composed of polymer having PTC characteristics. In more detail, the PTC element  10  has a structure where conductive particles are impregnated in the polymer.  
         [0034]     The polymer may be at least one polymer selected from the group consisting of HDPE (High Density Polyethylene), LDPE (Low Density Polyethylene), epoxy, silicone, and PVDF (Polyvinyl Difluoride). In addition, the conductive particles may have at least one type of conductive particles selected from the group consisting of carbon, metal and metal oxide. In addition, an antioxidant may be further added to prevent oxidization of the PTC polymer.  
         [0035]     More preferably, an inorganic additive may be further added to the PTC polymer so as to improve a low resistance characteristic at a normal temperature and a high resistance characteristic at a high temperature further.  
         [0036]     The contact electrodes  121 ,  131  include an upper contact electrode  121  and a lower contact electrode  131  installed to top and bottom contact surfaces of the PTC element  110 , and they are adhered to the PTC element  110  as closely as possible so as to minimize a contact resistance.  
         [0037]     The contact electrodes  121 ,  131  may be composed of copper foil or other metal elements. In addition, the contact electrodes  121 ,  131  are preferably installed in a way of reducing a contact resistance to the minimum by using lamination or free contact as examples.  
         [0038]     At a short current fault, interfaces between the PTC element  110  and the contact electrodes  121 ,  131  may be separated by electron repelling force to cause arc and noise. If an arc is generated as mentioned above, the PTC element  110  is partially evaporated to form a conductive path, and flashover may be generated between the contact electrodes  121 ,  131  at both ends. In order to prevent the above, it is required to consider relations among the surface area of the PTC element  110 , the surface area of the contact electrodes  121 ,  131 , the thickness of the PTC element  110 , and a rated voltage. These are explained in detail as follows.  
         [0039]     First, the contact electrodes  121 ,  131  are designed to have a surface area smaller than that of the PTC element  110 . By this configuration, an insulating distance between both ends of the contact electrodes  121 ,  131  can be increased to prevent flashover.  
         [0040]     In addition, the PTC current limiting device according to the present invention is designed to satisfy the following equations 1 and 2 in addition to the above conditions.  
               V   L     &lt;   10           Equation   ⁢           ⁢   1                 V   b     &lt;   50           Equation   ⁢           ⁢   2             
 
         [0041]     As shown in  FIG. 3 , in the equations 1 and 2, L is a minimum value of the sum total of a distance a1 (mm) between an end of the upper contact electrode  121  and an end of the PTC element  110 , a distance a2 (mm) between an end of the lower contact electrode  131  and the end of the PTC element  110 , and a thickness b of the PTC element  110 . In addition, V is a rated voltage (Volt) of the PTC current limiting device.  
         [0042]     If the PTC element  110  and the contact electrodes  121 ,  131  are designed to satisfy the equations 1 and 2, the PTC current limiting device can conduct its current limiting action effectively without causing flashover between electrodes, as being understood by experimental examples described later.  
         [0043]     Preferably, the PTC current limiting device further includes current leads  122 ,  132  for electrically connecting the contact electrodes  121 ,  131  to a power system. The current leads  122 ,  132  are extended so that their one ends are electrically connected to the contact electrodes and the other ends are connected to an external circuit. In addition, the current leads  122 ,  132  are preferably made of metal materials and also preferably have size and thickness conforming to an applicable capacity of the system current.  
         [0044]     More preferably, the PTC current limiting device may further include a connection electrode (not shown) interposed between the contact electrodes  121 ,  131  and the current leads  122 ,  132 . This connection electrode is made of metal with a relatively lower resistance so that a current can be more easily applied from the power system to the PTC current limiting device.  
         [0045]      FIG. 4  shows a PTC current limiting device according to another embodiment of the present invention. In  FIG. 4 , the same reference numeral as in the former drawings designates the same component having the same function, and not described in detail.  
         [0046]     Referring to  FIG. 4 , the PTC current limiting device of this embodiment further includes a pressing means for pressing the contact electrodes  121 ,  131  toward the PTC element  110 . The pressing means includes a housing  440 , and elastic members  451 ,  452 .  
         [0047]     The housing  160  receives the entire PTC element  110 , the entire contact electrodes  121 ,  131 , an a part of the current leads  122 ,  132 . Thus, a part of the current leads  122 ,  132  is extended outward through the housing  440  and connected to a power system.  
         [0048]     The elastic members  451 ,  452  are supported against an inner surface of the housing  440  and configured to surround the outer circumference of the current leads  122 ,  132  and press the current leads  122 ,  132  toward the contact electrodes  121 ,  131 . Thus, the contact electrodes  121 ,  131  are pressed toward the PTC element  110 . Preferably, the elastic members  451 ,  452  may be prepared to any or both of the pair of current leads  122 ,  132 .  
         [0049]     Meanwhile, the elastic members  451 ,  452  are preferably designed to have a pressing force of at least 1 bar so as to cope with the separation of interfaces between the PTC element  110  and the contact electrodes  121 ,  131  caused by electron repelling force generated at a short circuit fault. In addition, it is also preferable that the pressing force of 1 bar or above is kept even when the thickness of the PTC element  110  is decreased to a half due to repeated current limiting operations.  
         [0050]     The elastic members  451 ,  452  may employ coil springs prepared to surround the outer circumference of the current lead  122  and/or  132 , for example. However, the present invention is not limited to the above, and various changes may be used within the scope of the invention by those skilled in the art.  
         [0051]      FIG. 5  shows a PTC current limiting device according to still another embodiment of the present invention. In  FIG. 5 , the same reference numeral as in the former drawings designates the same component having the same function, and not described in detail.  
         [0052]     Referring to  FIG. 5 , the pressing means of the PTC current limiting device according to this embodiment includes upper and lower plates  571 ,  572 , and a coupling member for coupling the upper and lower plates  571 ,  572 .  
         [0053]     The PTC element  110 , the contact electrodes  121 ,  131 , and the current leads  122 ,  132  are arranged between the upper and lower plates  571 ,  572 , and the upper and lower plates  571 ,  572  have a through hole  575  at their center so that the current leads  122 ,  132  are connected to an external circuit.  
         [0054]     The upper and lower plates  571 ,  572  have coupling holes  573 ,  574  in their edges, and thus the coupling members fix the upper and lower plates  571 ,  572  with each other through the coupling holes  573 ,  574 . Specifically, bolts  581  pass through the coupling holes  573 ,  574 , and nuts  582  are coupled to the bolts  581  to fix the upper and lower plates  571 ,  572  with each other.  
         [0055]     Preferably, the pressing means further includes elastic members  451 ,  452  that surround the current leads  122 ,  132 . The elastic members  451 ,  452  are supported against the inner side of the plates  571 ,  572 , and they are compressed and elastically biased along the outer circumference of the current leads  122 ,  132 . Accordingly, the contact electrodes  121 ,  131  press the PTC element  110 . A pressing force of the elastic members  451 ,  452  is substantially identical to that of the former embodiment.  
         [0056]     Meanwhile,  FIG. 5  shows that the elastic members  451 ,  452  are arranged to both current leads  122 ,  132 , but they may be arranged to any one of them when required.  
         [0057]     Though the detailed configuration of the pressing means has been explained in detail in the above embodiments, the present invention is not limited thereto, but it should be understood that various changes of a pressing means capable of pressing the contact electrodes  121 ,  131  toward the PTC element  110  may be used.  
         [0058]     Hereinafter, experimental examples are illustrated to facilitate better understanding of the present invention.  
         [0059]     PTC current limiting devices were made with changing the diameter of the PTC element  110 , the thickness of the PTC element  110  and diameters of the contact electrodes  121 ,  131  in various ways, and then a test voltage was changed to 100V to 500V. Detailed conditions of these experimental examples are shown in the following table 1.  
                                                                 TABLE 1                                   Diameter   Diameter       Thickness               of PTC   of Contact   a1   (b) of PTC   Test           element   Electrode   (=a2)   element   Voltage           (mm)   (mm)   (mm)   (mm)   (V)                                    Example 1   20   10   5   2.5   100       Example 2   20   10   5   2.5   200       Example 3   20   10   5   2.5   300       Example 4   30   14   8   5   100       Example 5   30   14   8   5   200       Example 6   30   14   8   5   300       Example 7   30   14   8   5   400       Example 8   30   20   5   5   100       Example 9   30   20   5   5   200       Example 10   30   20   5   5   300       Example 11   30   20   5   5   400       Example 12   45   20   12.5   10   300       Example 13   45   20   12.5   10   400       Example 14   45   20   12.5   10   500       Example 15   45   20   12.5   20   300       Example 16   45   20   12.5   20   400       Example 17   45   20   12.5   20   500                  
 
         [0060]     The following table 2 shows test results related to whether each experimental example satisfies the equations 1 and 2 and whether flashover is generated when the PTC current limiting devices made under the conditions of the table 1 were operated.  
                                                         TABLE 2                                   V/(a1 +       Satisfying Equations   Flashover between           a2 + b)   V/b   1 and 2   Contact Electrodes                                    Example 1   8.0   40   ∘   x       Example 2   16.0   80   x   ∘       Example 3   24.0   120   x   ∘       Example 4   4.8   20   ∘   x       Example 5   9.5   40   ∘   x       Example 6   14.3   60   x   ∘       Example 7   19.0   80   x   ∘       Example 8   6.7   20   ∘   x       Example 9   13.3   40   x   ∘       Example 10   20.0   60   x   ∘       Example 11   26.7   80   x   ∘       Example 12   8.6   30   ∘   x       Example 13   11.4   40   x   ∘       Example 14   14.3   50   x   ∘       Example 15   6.7   15   ∘   x       Example 16   8.9   20   ∘   x       Example 17   11.1   25   x   ∘                  
 
         [0061]     Seeing the table 2, it would be found that flashover is not generated between electrodes only when the equations 1 and 2 are satisfied.  
         [0062]      FIG. 6  is a graph showing an operation waveform of the PTC current limiting device when flashover is generated, and  FIG. 7  is a graph showing an operation waveform of the PTC current limiting device when flashover is not generated.  
         [0063]     Referring to  FIG. 6 , it would be understood that the PTC element trips after a fault, so a fault current is instantly decreased and then abruptly increased. This phenomenon is generated since an excessive voltage generated at both ends of the PTC element causes flashover between both electrodes and thus most of the fault current flows by means of the flashover. If the flashover is generated as mentioned above, the fault current instantly decreased is increased again, not allowing a proper current limiting action.  
         [0064]     Referring to  FIG. 7 , it would be understood that the PTC element trips a certain time after the fault to limit the fault current, thereby ensuring insulation between both electrodes and thus not causing flashover between electrodes. Thus, the current limiting action of the PTC element is lasting, thereby limiting the fault current to a very low value.  
         [0065]     If a PTC current limiting device is designed to satisfy the equations 1 and 2 by using the above experimental examples, it is possible to prevent flashover between the contact electrodes and thus ensure a proper current limiting action of the PTC element without a failure.  
         [0066]     The present invention has been described in detail. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.  
       APPLICABILITY TO THE INDUSTRY  
       [0067]     As described above, the PTC current limiting device according to the present invention may prevent flashover generated between contact electrodes even in a high-voltage and large-current power system in consideration of a thickness factor of the PTC element as well as a surface area factor of the PTC element and the contact electrodes, so it may protect the power system against an overcurrent more effectively.