Abstract:
In one aspect, the present invention provides an electromagnetic circuit interrupter for use in a high voltage direct current (DC) aircraft power distribution system. The electromagnetic circuit interrupter comprises a contact mechanism operable to separate first and second electrical contacts by a first predetermined distance d 1  for a predetermined time τ so as to sustain an arc when the contact mechanism is opened. The contact mechanism is further operable to separate the first and second electrical contacts by a second predetermined distance d 2  after the predetermined time τ so as to extinguish the arc. The first predetermined distance d 1  is less than said second predetermined distance d 2 . By deliberately sustaining the arc for a relatively long period of time, this aspect of the present invention is particularly useful for extending the operational lifetime of the contacts and thereby of the electromagnetic circuit interrupter itself.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates generally to an electromagnetic circuit interrupter for a high voltage direct current (DC) aircraft power distribution system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recent developments in aircraft power distributions have involved a move towards the use of high voltage DC power distribution systems so as to permit a weight reduction for wiring harnesses used to distribute electrical power within an aircraft. 
         [0003]    However such high voltage DC systems give rise to additional problems when designing aircraft power distribution systems. The high DC voltages used can, for example, lead to a decreased component lifetime, particularly for electromagnetic switches used to interrupt circuitry from drawing power from the wiring harness. Such switches are preferred to solid state devices because of their higher power ratings and ability to resist the increased switching voltages. However, even these high power devices are not immune to the effects of contact sputtering caused by arcing of the switch contacts provided therein when such contacts are separated in order to break a circuit. 
         [0004]    Various devices and techniques have therefore been developed in an attempt to enhance the lifetime of such switchable contacts by mitigating the effects caused by the inductive energy that is stored in the circuit and which causes arcing once the contacts are separated. 
         [0005]    For example, various known techniques may employ conventional electromagnetic switches along with additional circuitry that is used to dissipate the inductive energy of the circuit so as to minimise the energy dissipated in the electromagnetic switches themselves [1-3]. Alternatively, various non-conventional electromagnetic switches have been produced which, for example, may seek to confine the physical position of arcs in an attempt to minimise contact erosion [4]. 
         [0006]    However, whilst such techniques can enhance the useful operational lifetime of electromagnetic switches, there is still a need in the art for high voltage DC electromagnetic circuit interrupters having a further enhanced operational lifetime, particularly when used for safety critical applications such as aircraft power distribution systems. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention has thus been devised whilst bearing the above-mentioned drawbacks associated with conventional high voltage DC electromagnetic switching devices in mind. 
         [0008]    According to one aspect of the present invention, there is thus provided an electromagnetic circuit interrupter for a high voltage DC aircraft power distribution system. The electromagnetic circuit interrupter comprises a contact mechanism operable to separate first and second electrical contacts by a first predetermined distance for a predetermined time so as to sustain an arc when the contact mechanism is opened. The contact mechanism is further operable to separate the first and second electrical contacts by a second predetermined, distance after the predetermined time so as to extinguish the arc. Additionally, the first predetermined distance is less than said second predetermined distance. 
         [0009]    Such an electromagnetic circuit interrupter contrasts with conventional devices as it does not seek to open the contacts widely as soon as possible, but rather enables the contacts to be separated for a relatively long time (e.g. several milliseconds compared to prior art devices opening in microseconds) in order that an arc is produced and sustained for a relatively long period. This has the advantage that much of the inductive energy stored in a circuit can be dissipated during the predetermined time period before the contacts of the electromagnetic circuit interrupter become hot enough to melt. Subsequently, the contacts can be further or fully opened to break the circuit, the arc having been extinguished, thereby minimising or substantially eliminating any contact sputtering. 
         [0010]    Hence, although the total switching time of the electromagnetic circuit interrupter is increased compared to conventional devices, the operational lifetime and reliability of the contacts can be greatly enhanced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    Various aspects and embodiments of the present invention will now be described in connection with the accompanying drawings, in which: 
           [0012]      FIG. 1A  shows an electromagnetic circuit interrupter for a high voltage direct current (DC) aircraft power distribution system in accordance with various embodiments of the present invention in a closed contact position; 
           [0013]      FIG. 1B  shows the electromagnetic circuit interrupter of  FIG. 1A  in an intermediate open contact position; 
           [0014]      FIG. 1C  shows the electromagnetic circuit interrupter of  FIG. 1A  in a fully open contact position; 
           [0015]      FIG. 2  shows temporal I-V curves for a low voltage DC circuit interruption; 
           [0016]      FIG. 3  shows an I-V characteristic graph for a low voltage arc; and 
           [0017]      FIG. 4  shows various high voltage arc voltage waveforms provided by operating various embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1A  shows an electromagnetic circuit interrupter  100  for a high voltage direct current (DC) aircraft power distribution system in accordance with various embodiments of the present invention in a closed contact position. 
         [0019]    The electromagnetic circuit interrupter  100  comprises a first electrical contact  120  and a second electrical contact  130  hermetically sealed in a housing  110 . The first and second electrical contacts  120 ,  130  are movable within the housing  110  between a closed position, an intermediate open contact position and a fully open contact position by activation of a contact mechanism  102 . These three positions are shown respectively in  FIGS. 1A-1C . The housing  110  may contain a fill gas. In various embodiments, the fill gas may comprise one or more of: dry air, nitrogen, argon, neon, krypton etc. In various preferred embodiments, nitrogen or another inert gas or gas mixture may be used. 
         [0020]    The first electrical contact  120  is formed with an electrically conductive projecting portion  122  which may be made of the same material as the main body of the first electrical contact  120 . Alternatively, the projecting portion  122  may be formed of dissimilar material, e.g. metal, from that of the main body of the first electrical contact  120 . Similarly, the second electrical contact  130  is formed with an electrically conductive projecting portion  132  which may be made of the same material as the main body of the second electrical contact  130 . Alternatively, the projecting portion  132  may be formed of dissimilar material, e.g. metal, from that of the main body of the second electrical contact  130 . The surfaces of the projecting portions  122 ,  132  may be shaped or substantially flat. 
         [0021]    In the closed contact position shown in  FIG. 1A , the projecting portions  122 ,  132  abut one another, or fit together depending upon their respective shapes, in order provide a low resistance electrical connection between the first and second electrical contacts  120 ,  130 . 
         [0022]      FIG. 1B  shows the electromagnetic circuit interrupter  100  in an intermediate open contact position. In the intermediate open contact position the contact mechanism  102  separates the surfaces of the projecting portions  122 ,  132  by a first predetermined distance d 1  for a predetermined time τ. Various methods for determining the first predetermined distance d 1  and the predetermined time τ for embodiments of the invention are discussed further below. 
         [0023]    When the first and second electrical contacts  120 ,  130  are supplied with a high voltage DC potential difference therebetween, an arc  150  is sustained between the projecting portions  122 ,  132  for a period substantially equal to the whole of the duration of the predetermined time τ. The arc  150  acts like a resistor in the circuit and dissipates stored inductive energy as heat energy causing the temperature of the proximal electrical contacts  120 ,  130  to rise. 
         [0024]    With fast (e.g. of the order of μS) full gap opening of contacts in conventional devices, the arc can heat the contacts up (through resistive I 2 R heating). This temperature rise may be enough to cause sputtering and intermittent restriking of the arc until enough inductive energy has been dissipated for this process to cease. 
         [0025]    However by selecting the predetermined time τ and the first predetermined distance d 1  to ensure that the temperature rise of the electrical contacts  120 ,  130  is limited to below the melting temperature of the materials from which they are formed, sputtering can be minimised and operational lifetime of the electromagnetic circuit interrupter  100  increased. 
         [0026]    The various parameters chosen depend upon the exact current, voltage and power rating of the electromagnetic switch, the fill gas used, and the contact materials, hence the first predetermined distance d 1 , the second predetermined distance d 2  and the predetermined time τ vary according to the specific embodiment that is used. 
         [0027]    One technique that can be applied to determine whether or not high voltage arcing will occur and/or various of the distance parameters involves finding the Paschen voltage for a particular electromagnetic circuit interrupter  100  embodiment. 
         [0028]    For parallel conducting plates, Paschen found that the breakdown voltage V b , (volts) could be described by the equation: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     b 
                   
                   = 
                   
                     
                       
                         k 
                         1 
                       
                        
                       
                         ( 
                         
                           P 
                           , 
                           d 
                         
                         ) 
                       
                     
                     
                       
                         ln 
                          
                         
                           ( 
                           
                             P 
                             , 
                             d 
                           
                           ) 
                         
                       
                       + 
                       
                         k 
                         2 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0000]    where P is the pressure of the gas between the two plates, d the separation distance between the two plates and k 1  and k 2  are constants dependant upon the specific gas or gas mixture used. 
         [0029]    Differentiating Equation 1 and setting the derivative to zero, gives: 
         [0000]        P·d=e   (1−k     2     )   Equation 2
 
         [0000]    which in turn enables the Paschen voltage V P =V bmin  to found from Equation 1. 
         [0030]    For example, for a high voltage application and so as to ensure arcing does actually occur, the operating high DC voltage of the electromagnetic circuit interrupter  100  must be greater than the Paschen voltage V P  for any particular gas and at any given temperature. For contacts in air at standard atmospheric pressure, for example, the following parameters may be selected: 1.5 mm&lt;d 1 &lt;2.5 mm with d 2 , for example, set such that d 2 ≈3 mm. 
         [0031]      FIG. 1C  shows the electromagnetic circuit interrupter  100  in a fully open contact position. In the fully open contact position the contact mechanism  102  separates the surfaces of the projecting portions  122 ,  132  by a second predetermined distance d 2 (where d 2 &gt;d 1 ) until such a time as the electromagnetic circuit interrupter  100  is switched back to the closed contact position. When switching back from the fully open contact position to the closed contact position, the contact mechanism  102  rapidly and directly moves the first and second electrical contacts  120 ,  130  together without any intermediate contact separation stages. 
         [0032]    As the first and second electrical contacts  120 ,  130  are fully opened from the intermediate open contact position, any arc  150  is rapidly extinguished. Additionally, since much of the stored inductive energy will already have been dissipated at this time, the arc  150  is highly unlikely to restrike and cause damage to the first and second electrical contacts  120 ,  130  or the projecting portions  122 ,  132 . 
         [0033]    In various embodiments, the contact mechanism  102  may include one or more solenoid actuators and/or mechanical arrangements for moving the first and second electrical contacts  120 ,  130  between the closed position, the intermediate open contact position and the fully open contact position. Various such embodiments would be readily envisaged by those skilled in the art of mechanical actuator design. 
         [0034]      FIG. 2  shows temporal I-V curves for a low voltage DC circuit interruption. The temporal I-V curves include a graphical depiction of a current (I) profile  210  and a graphical depiction of a voltage (V) profile  220  for a low voltage DC circuit interruption. 
         [0035]    At time t=5 mS, the circuit is interrupted and the current profile  210  shows a steady decrease in the circuit current from about 200 Amps to about 40 Amps over a period of about 5 mS as the stored inductive energy dissipates as heat. A rapid current decrease to zero Amps is observed after about t=10 mS with the current dropping rapidly from about 40 Amps to zero during an interval of about 1 mS. 
         [0036]    The voltage profile  220  shows how the potential between the contact electrodes varies over time. At t 0 , in this case equal to t=5 mS, circuit interruption begins and a potential of about 15 volts rapidly develops across the contact electrodes. At t 0 , the force holding the metallic electrodes together is reduced. This in turn increases the contact resistance resulting in increased heat. As the contact force is further reduced, the area over which current flows is reduced also increasing the contact temperature further. At the extreme limit, all of the circuit current passes through an infinitesimal surface area resulting in this area of the electrode melting and a controlled explosion occurs. 
         [0037]    Metal vapour or particles thus sputter from the contact electrodes, and between t 0  and t 1  (about 1 mS later) conduction through metalised air occurs. At t 1  the electrode gap becomes vacuous in nature and a vacuum arc develops. The voltage profile of the vacuum arc follows the exponential curve shown increasing initially from about 15-20 volts at t 1  to about 48 volts at a time when the current profile  210  reaches zero Amps. During this time period, i.e. from about t=6 mS to about t=11 mS, the inductive energy 
         [0000]    
       
         
           
             E 
             = 
             
               ( 
               
                 
                   1 
                   2 
                 
                  
                 
                   LI 
                   2 
                 
               
               ) 
             
           
         
       
     
         [0000]    stored in the circuit is converted to heat within the are and some is also dissipated by the load connected to the circuit interrupter. 
         [0038]      FIG. 3  shows an I-V characteristic graph  300  for the low voltage arc produced in  FIG. 2 . The fill gas is nitrogen.  FIG. 3  shows that as the current in a circuit that is being interrupted reduces, the arc voltage rises (negative impedance). Once the current is reduced to zero the arc voltage also reduces to zero volts. 
         [0039]    The arc voltage is also related to the gap over which the arc must traverse. If higher voltages are available and the circuit has enough energy stored, the arc may be drawn and higher arc voltages are observed. 
         [0040]      FIG. 4  shows various high voltage arc voltage waveforms  402  to  420  provided by operating various embodiments of the present invention. Voltage waveform  402  is substantially equivalent to the low voltage arc profile as per  FIG. 3 , described above. 
         [0041]    The y-axis (V arc ) is calibrated in volts. However, V arc  is also indicative of the temperature of the arc (T 2 ) relative to ambient temperature (T 1 ), such that 
         [0000]    
       
         
           
             
               V 
               
                 ar 
                  
                 
                     
                 
                  
                 c 
               
             
              
             α 
              
             
                 
             
              
             
               
                 
                   T 
                   2 
                 
                 
                   T 
                   1 
                 
               
               . 
             
           
         
       
     
         [0000]    The x-axis (F(I)) is a function of the current flowing in the arc. 
         [0042]    A predetermined time τ may thus be determined such that T arc &lt;T meltmin , where T arc  is the temperature generated by the arc and T meltmin  the lowest melting temperature of the materials from which the first and second electrical contacts are made. For example, τ may be determined such that 
         [0000]    
       
         
           
             
               
                 T 
                 
                   
                       
                   
                    
                   
                     ar 
                      
                     
                         
                     
                      
                     c 
                   
                 
               
                
               
                 &lt;&lt; 
                 
                   T 
                   meltmin 
                 
               
             
             , 
             
               
                 e 
                 . 
                 g 
                 . 
                 
                     
                 
                  
                 
                   T 
                   
                     ar 
                      
                     
                         
                     
                      
                     c 
                   
                 
               
               = 
               
                 
                   T 
                   meltmin 
                 
                 α 
               
             
             , 
           
         
       
     
         [0000]    where α=2, 5, 10, 20, etc. to minimise contact sputtering and may be from about 1 mS to about 10 mS, for example. 
         [0043]    An array of arc voltage waveforms possible in a circuit with higher voltages available is shown in  FIG. 4 . The second voltage waveform  404  has a profile equivalent to twice that of the low voltage arc profile of voltage waveform  402 . The third voltage waveform  406  has a profile equivalent to three times that of the low voltage arc profile of voltage waveform  402 . The fourth voltage waveform  408  has a profile equivalent to four times that of the low voltage arc profile of voltage waveform  402 . The fifth voltage waveform  410  has a profile equivalent to five times that of the low voltage arc profile of voltage waveform  402 . The sixth voltage waveform  412  has a profile equivalent to six times that of the low voltage arc profile of voltage waveform  402 . The seventh voltage waveform  414  has a profile equivalent to seven times that of the low voltage arc profile of voltage waveform  402 . The eighth voltage waveform  416  has a profile equivalent to eight times that of the low voltage arc profile of voltage waveform  402 . The ninth voltage waveform  418  has a profile equivalent to nine times that of the low voltage arc profile of voltage waveform  402 . The tenth voltage waveform  420  has a profile equivalent to ten times that of the low voltage arc profile of voltage waveform  402 . 
         [0044]    Each of the voltage waveform curves  402 - 420  is related to a given arc gap. The voltage is directly proportional to the gap size. Therefore for a higher voltage arc to be realised a greater gap size must be provided. For example, the first predetermined distance d 1  may be defined as: d 1 =m·λ, where m is a predetermined factor and λ a DC low voltage arc gap substantially equal to one electron mean free path between first and second electrical contacts. The second predetermined distance d 2  may then be equal to a conventional gap distance for an equivalently rated conventional electromagnetic circuit breaker. 
         [0045]    The mean free path λ may be defined such that: 
         [0000]    
       
         
           
             
               
                 
                   λ 
                   = 
                   
                     kT 
                     
                       p 
                        
                       
                           
                       
                        
                       σ 
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0000]    k being Boltzmann&#39;s constant, T being the arc temperature (e.g. 15,000 Kelvin), p the pressure of the gas between the contacts, and σ gas specific cross sectional area. 
         [0046]    In one embodiment, to interrupt a 270 volt circuit the following three stage process may be used in order to allow the circuit&#39;s inductive energy to be dissipated and prevent unwanted arc draw:
       1. Open the contacts to a distance about six to seven times the gap required for the low voltage arc  402  (e.g. m may lie in the range from about 6 to about 7). This provides an operating range for F(I) from about 8 to about 20 when V arc =270 volts, as can be seen in  FIG. 4 , and ensures an arc is sustained whilst also constraining the temperature rise of the contacts (proportional to V arc ) to below the peak values seen for the curves  412  and  414 ;   2. Hold the contacts for a period of time τ for a given energy interruption capability, or until the current reaches zero Amps; and   3. Open the contacts further to provide a dielectric withstand capability.       
 
         [0050]    For example, using Equation 3 with p=101321 Pa; T=6000 K, and σ=πr i   2  where r i  is the ionic radius for Nitrogen=30 nm, λ can be found. Multiples of λ can then be used to define the contact separation distances required. The contact predetermined opening time may be calculated by determining the time needed to dissipate an amount of energy ΔE, such that ΔE=V arc ·I·t, according to a specific device rating. 
         [0051]    The predetermined time τ may thus be chosen such that the inductive energy remaining in the circuit when the contacts are opened is not sufficient to increase the voltage across the contacts enough to enable the arc to restrike. An additional safety factor may be used such that E stored (τ)&lt;E rearc , e.g. τ is chosen such that 
         [0000]    
       
         
           
             
               
                 
                   E 
                   stored 
                 
                  
                 
                   ( 
                   τ 
                   ) 
                 
               
               = 
               
                 
                   E 
                   rearc 
                 
                 β 
               
             
             , 
           
         
       
     
         [0000]    where E stored (t) is the amount of inductive energy remaining in the circuit at a time t after the contacts are separated and the circuit broken at time t=0, E rearc  the energy needed to cause the arc to restrike when the first and second electrical contacts are separated by the first predetermined distance d 1 , and β a safety factor greater than one (e.g. β=2). 
         [0052]    Adopting such a release technique helps prevent the possibility of the arc re-striking should it be prematurely terminated. This contrasts with conventional devices in which if the metallic contacts are opened too fast, and the energy in the system is unable to sustain the original arc temperature, the arc quenches and current stops flowing. The still stored inductive energy in the system then increases the voltage across the contact gap until there is sufficient voltage available for breakdown to occur and thus re-strike the arc. 
         [0053]    For example, in various embodiments of the present invention, the predetermined time τ may be from about 1 mS to about 15 mS, or more preferably from about 5 mS to about 8 in mS. In contrast, conventional electromagnetic devices often open contacts to break a circuit over a time period that is several orders of magnitude faster than such embodiments, e.g. of the order of microseconds or tens of microseconds. 
         [0054]    Whilst various aspects and embodiments of the present invention have been described herein, those skilled in the art will also realise many embodiments of electromagnetic circuit interrupters falling within the scope of the claims may be made. Additionally, they will be aware that various techniques, both experimental and theoretical, may be used to determine certain operating parameters for such electromagnetic circuit interrupters, for example, in order to determine a first predetermined opening distance, a predetermined intermediate contact opening time and/or a second predetermined opening distance. Moreover, many versions of possible contact mechanism embodiments will also be apparent. 
       REFERENCES 
       [0000]    
       
         1. GB 1 333 685 (Hughes) 
         2. U.S. Pat. No. 4,249,223 (Shuey) 
         3. US 2008/0143462 (Belisle) 
         4. U.S. Pat. No. 5,004,874 (Theisen)
 
Where permitted, the content of the above-mentioned references are hereby also incorporated, into this application by reference in their entirety.