Abstract:
A high voltage direct current (HVDC) power distribution system comprises at least one power bus; at least one load conductor; and a hybrid contactor for interconnecting the at least one power bus and the at least one load conductor and through which inductive energy passes upon disconnection of the at least one load conductor from the at least one power bus. A first portion of the inductive energy passes through the hybrid contactor as an arc. A second portion of the inductive energy passes through the hybrid contactor as resistively dissipated heat.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention is related to commonly-assigned application Ser. No. 11/968,314 entitled “HYBRID HIGH VOLTAGE DC CONTACTOR WITH ARC ENERGY DIVERSION” filed Jan. 2, 2008 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to control of electrical power distribution and more particularly, employment of high voltage direct current (HVDC) contactors for switching under conditions in which a DC load may be present in a circuit. 
         [0003]    There is a growing need for HVDC power distribution systems in vehicles. For example, use of HVDC electrical power on aircraft such as the so-called More Electric Aircraft (MEA) is potentially attractive for low-loss distribution while maintaining relatively low electrical system weight. 
         [0004]    Some of the challenges associated with the use of HVDC systems include improving the reliability and reducing size and weight of key components in the power distribution system, such as electric load control units (ELCUs) or remote power controllers (RPCs) for load control and feeder protection, and primary bus switching contactors, which mainly operate on an electromechanical principle. These current interrupting devices generally include a pair of mechanical contacts between the HVDC source and a load path which can rapidly separate either by means of electromechanical force upon an open command, or when mechanically “bouncing” during a closing transition upon a close command. When the contacts become separated, an electric arc may form as a result of the inductive energy stored in the connected circuit. Unlike AC applications, where the arc is self extinguished due to zero crossing of the AC current, the arc generated in an HVDC contactors will continue to carry current until the current eventually ceases as result of further opening of the contacts. This generates heat in the contact area and gradually erodes the surface of the contacts after repeated application. Use of higher operating voltages exacerbates this phenomenon. Various methods have been developed for HVDC contactors to suppress arcing using different arc chamber configurations and materials, which are structured to rapidly increase arc voltage. Also hybrid HVDC contactor concepts have been proposed whereby semiconductor switching devices are connected in parallel with the main electromechanical power switching contacts to bypass (or absorb) the entire energy generated during the switching transients, which would, otherwise, cause an arc. Some prior art HVDC contactors may employ positive temperature coefficient (PTC) materials connected in parallel with the main electromechanical contacts to convert the arc energy generated during contactor switching operation into heat dissipated in the PTC device. 
         [0005]    As can be seen, there is a need to provide improved hybrid HVDC contactors and HVDC circuit interruption techniques. In particular there is a need to provide for circuit interruption with controlled arc energy 
       SUMMARY OF THE INVENTION 
       [0006]    In one aspect of the present invention, a high voltage direct current (HVDC) power distribution system comprises at least one power bus; at least one load conductor; a hybrid contactor for interconnecting the at least one power bus and the at least one load conductor and through which inductive energy passes upon disconnection of the at least one load conductor from the at least one power bus; wherein a first portion of the inductive energy passes through the hybrid contactor as an arc; and wherein a second portion of the inductive energy passes through the hybrid contactor as resistively dissipated heat. 
         [0007]    In another aspect of the present invention, a hybrid HVDC contactor comprises a main contactor having movable contacts; a controlled solid state switch for shunting inductive energy from the contacts during arcing between the contacts; and wherein an amount of shunted inductive energy is insufficient to extinguish an arc between the contacts, whereby inductive energy is distributed between shunted inductive energy and arc energy. 
         [0008]    In still another aspect of the present invention, a method for mitigating arcing effects in an HVDC contactor comprising the steps of: separating contacts of a contactor in a HVDC circuit; sensing an amount of inductive energy that develops at the contacts at the time of the separation of the contacts; shunting a first portion of the inductive energy away from the contacts; and passing a second portion of the inductive energy between the contacts as arc energy. 
         [0009]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a block diagram of a power distribution system for HVDC in accordance with an embodiment of the present invention; 
           [0011]      FIG. 2  is a set of graph lines portraying a relationship between arc current and arc voltage for various arc lengths in accordance with an embodiment of the present invention; 
           [0012]      FIG. 3  is a flow chart of a method for operating a hybrid HVDC contactor in accordance with an embodiment of the present invention; and 
           [0013]      FIG. 4  is a flow chart of another method for operating a hybrid HVDC contactor in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0015]    Various inventive features are described below that can each be used independently of one another or in combination with other features. However, any single inventive feature may not address any of the problems discussed above or may only address one of the problems discussed above. Further, one or more of the problems discussed above may not be fully addressed by any of the features described below. 
         [0016]    Broadly, embodiments of the present invention generally may provide a system for interrupting HVDC circuits while mitigating the effects of arcing between contacts of a mechanical contactor. More particularly, embodiments of the present invention may provide a system in which a first portion of arc energy is shunted away from the contacts during opening of the contacts while a second portion of the arc energy is allowed to pass between the opening contacts. Embodiments of the present invention may be useful in distributing HVDC power in an aircraft. 
         [0017]    Referring now to  FIG. 1  there is shown a block diagram of portion of a HVDC power system  10  which may comprise a hybrid HVDC contactor  12 . The power system  10  may also comprise a power bus  14 , a load conductor  16  and a line current sensor  18 . The hybrid HVDC contactor  12  (hereinafter “the hybrid contactor  12 ”) may be positioned so that it may interconnect the power bus  14  and the load conductor  16 . The hybrid contactor  12  may connect the load conductor  16  to the power bus  14  or disconnect the load conductor  16  from the power bus  14  upon receipt of an appropriate external ON/OFF command  20 . The contactor may also disconnect or re-connect the power bus  14  and the load conductor  16  responsively to an internally-generated ON/OFF command  22  (hereinafter the internal command  22 ). An internal command  22  may be generated responsively to an over-current condition sensed by the line current sensor  18 . In that case, the internal command may be an OFF command and the hybrid contactor  12  may disconnect the load conductor  16  from the power bus  14 . 
         [0018]    The hybrid contactor  12  may comprise a main contactor  24  with contacts  24 - 1  and  24 - 2 . The main contactor  24  may be a conventional coil-driven electromechanical contactor constructed with spring-actuated contacts. The main contactor may be provided with a conventional degree of tolerance for arcing. The hybrid contactor  12  may further comprise a contactor engine  26 , a solid state power switch  28 , a positive temperature coefficient (PTC) resistor element  30  (hereinafter “the PTC  30 ”), an arc voltage sensor  32  and an arc current sensor  34 . The contactor engine  26  may be a digital signal processor (DSP) based controller. The solid state switch  28  may be a conventional solid state switch and may operate responsively to a determined duty-cycle or a determined gate drive voltage. 
         [0019]    Upon connecting or disconnecting the power bus  14  and the load conductor  16 , arcing may develop between the contacts  24 - 1  and  24 - 2  as a result of inductive energy. In an exemplary embodiment of the invention, the hybrid contactor  12  may shunt some HVDC current to the solid state switch  28  and the PTC  30  during separation of the contacts  24 - 1  and  24 - 2 . In this regard, the hybrid contactor  12  may be considered to shunt a portion of the inductive energy to the PTC  30 . The PTC  30  may absorb the shunted inductive energy and dissipate the energy as resistively dissipated heat. However, as will be explained later herein, only a portion of the inductive energy may be shunted away from the contacts  24 - 1  and  24 - 2 . Arcing may be allowed to continue in the main contactor  24  at an energy level lower than that which would otherwise be present without shunting. 
         [0020]    As a consequence of apportioning the inductive energy, arcing erosion of the contacts  24 - 1  and  24 - 2  may be mitigated. Because a portion of the inductive energy may pass through the contacts  24 - 1  and  24 - 2 , the solid state switch  28  and the PTC  30  may shunt only a portion of the inductive energy. The portion of inductive energy shunted to the PTC  30  may be controlled at a level that may be tolerated by the PTC  30  and the solid state switch  28 . 
         [0021]    Referring now to  FIG. 2 , a series of graph lines show various combinations of arc voltages Va and arc currents la at which arcing may be initiated and sustained between contacts  24 - 1  and  24 - 2  of the hybrid contactor  12  during interruption of current. Inductively-induced surge voltages and currents may arise in the hybrid contactor  12  during such an interruption. A graph line  100  may represent an arc-initiation relationship between voltage Va and current Ia when the contacts  24 - 1  and  24 - 2  are separated by a first distance d 1  (e.g. 0.5 millimeter [mm]). A graph line  102  may represent an arc-initiation relationship between Va and Ia when the contacts  24 - 1  and  24 - 2  are separated by a second distance d 2  (e.g. 1.0 mm). In other words, to use graph line  100  as an example, at a contact spacing of d 1 , an arc may develop at a surge voltage at a graph-line- 100  value of Va and a surge current at a graph-line- 100  value of Ia. An arc, at the spacing d 1  may develop at any combination of Va and Ia on the graph line  100 . There always exists a minimum arc power point, e.g. point  100 - 2 , on the graph line  100  such that the product Va*Ia, the arc power, at any other point on the graph  100  is always greater than that at point  100 - 2 . 
         [0022]    It may be seen that as spacing between the contacts  24 - 1  and  24 - 2  increases, the product Va*Ia, i.e. the arc power, must become larger in order for an arc to remain sustained. 
         [0023]    Referring again to graph line  100 , a point  100 - 1  may represent a minimum voltage Va for the graph line  100 . Typically, an uncontrolled arc between the contacts  24 - 1  and  24 - 2  may initiate at this minimum voltage Va represented by the point  100 - 1 . It may be noted that the arc current Ia at the point  100 - 1  is relatively high, as compared to arc current at points to the left of point  100 - 1 . As a result, the arc power, as well as its energy contained between the contacts  24 - 1  and  24 - 2  at the relatively high current value of point  100 - 1  may be greater than arc power at the minimum arc power point  100 - 2 . Such an uncontrolled arc may be represented to progress through a series of points  102 - 1 ,  104 - 1  and  106 - 1  as the contacts  24 - 1  and  24 - 2  may continue to separate from one another. 
         [0024]    A conventional HVDC main contactor such as the main contactor  24  may be capable of withstanding a number of repeated operations during which uncontrolled arcing may occur. However, repeated uncontrolled arcing may eventually produce failure or reduced reliability of the main contactor  24 . But if arcing were to occur at a lower arc power level, then the useful life of the main contactor  24  may be extended. In other words, useful life of the main contactor  24  may be extended if arc power associated with every contact spacing were made lower than at points  100 - 1 ,  102 - 1 ,  104 - 1  and  106 - 1 . For example, if arcing were to occur at a points  100 - 2 ,  102 - 2 ,  104 - 2  and  106 - 1 , then potential damage to the contacts  24 - 1  and  24 - 2  may be reduced. 
         [0025]    In an exemplary embodiment of the present invention, the hybrid contactor  12  may produce controlled arcing that may occur at lower current than in the uncontrolled arcing represented by the points  100 - 1 ,  102 - 1 ,  104 - 1  and  106 - 1 . 
         [0026]    Referring back to  FIG. 1  and further to  FIG. 2  the contactor engine  26  may be programmed to calculate a distance between the contacts  24 - 1  and  24 - 1  during separation. Based on a determined distance, the contactor engine  26  may perform a comparison with a Va versus la relationship for that distance (e.g., the graph lines  100 ,  102 ,  104 ,  106  and  108 ). The contactor may then determine an amount of arc current that must be shunted around the main contactor in order to produce a condition in which arcing occurs at a point to the left of a minimum arc voltage point for that distance. For example, at the distance d 1  the contactor engine  26  may determine how much current must be shunted so that arcing occurs at a point to the left of point  100 - 1 . In an ideal case, the determined shunted current may result in arcing at a point such as the point  100 - 2  where arcing energy is at a minimum. The contactor engine  26  may be programmed to produce this ideal condition if possible. 
         [0027]    However, if the load conductor  16  is carrying a particularly high current load or if a load has a particularly high inductive component this ideal case may not be achievable. Therefore, the contactor engine  26  may also be programmed so that the PTC  30  is not subjected to a level of shunting current that may exceed its capability for dissipating energy. In other words, the contactor  26  may be programmed so that shunting current level is chosen to minimize arc energy or to limit shunting current at a level that may be tolerated by the PTC  30 . The contactor engine  26  may be programmed to select, whichever of these shunt current levels is lower. It may be noted that, irrespective of which level is selected by the contactor engine  26 , any diminishment of arcing current may be beneficial for extending the useful life of the main contactor  24  even if arcing does not occur at the minimum-energy set of points  100 - 2 ,  102 - 2 ,  104 - 2  and  106 - 1 . 
         [0028]    The contactor engine  26  may calculate distance between the contacts  24 - 1  and  24 - 1  by performing a calculation in accordance with the following expression. 
         [0000]        d= ½ ε*{A/∫i*v dt}*v   2    equation (1) 
         [0029]    where: 
         [0030]    d=distance between contacts 
         [0031]    ε=permittivity of material between contacts (e.g. vacuum or nitrogen) 
         [0032]    A=contact area 
         [0033]    i=monitored arc current at the arc current sensor  34   
         [0034]    v=monitored arc voltage at the arc voltage sensor  32 . 
         [0035]    Alternatively, the contactor engine  26  may be programmed with a first order approximation of contact spacing vs. time relationship for the main contactor  24 . For example, a particular type of main contactor may have a travel time of about 5 milliseconds to achieve contact separation spacing of about 2 mm. In that case, the shunting current may be determined as a function of elapsed time from initial contact separation, with each millisecond corresponding to 0.4 mm of distance between contacts. This first order approximation may produce results that may be less precise than those attained by calculation based on equation 1. However, processing time in the contactor engine  26  may be lower if the equation 1 calculations are not performed. 
         [0036]    Referring now to  FIG. 3 , there is illustrated an exemplary operation of the hybrid contactor  12 .  FIG. 3  shows a method  300  in which the hybrid contactor  12  may mitigate arcing effects in the event of an overcurrent condition in the load conductor  16 . In a step  302 , an overcurrent condition may be detected (e.g. the line current sensor  18  may detect an overcurrent condition in the load conductor  16 ). In a step  304 , an OFF command may be generated (e.g., the contactor engine  26  may generate an internal OFF command  22  to de-energize a coil of the main contactor  24 ). In a step  306 , contacts of the main contactor may begin separating (e.g. the contact  24 - 1  and  24 - 2  may move away from one another). 
         [0037]    In a step  308  arc voltage and arc current may be sampled (e.g., the arc voltage sensor  32  may sense a voltage drop across the main contactor  24  and the arc current sensor  34  may sense arc current). In a step  310  distance between the contacts (i.e., arc length) and desired shunt current may be determined (e.g., the contactor engine  26  may calculate distance between the contacts  24 - 1  and  24 - 1  in accordance with equation 1 using the arc voltage and the arc current sensed in step  308  as variables and an amount of shunt current needed to minimize arc power may be determined). 
         [0038]    In a step  312 , a solid state switch may be operated, responsively to signals from the contactor engine, to allow the desired amount of shunting current to pass through a PTC (e.g. the contactor engine  26  may generate a duty cycle for the switch  28  that corresponds to a shunting current flow that is sufficient to reduce arcing current so that arcing through the contacts may occur at the points  100 - 2 ,  102 - 2 ,  104 - 2  and  106 - 1 ). 
         [0039]    In a step  314 , a determination may be made as to whether a transition of fault current from the main contactor to the shunt path through the PTC is completed (e.g. the contactor engine  26  may determine whether or not arc current is present at the arc current sensor  34 ). In the event that arc current is present, steps  306  through  312  may be repeated. In the event that arc current is not present, a step  316  may be performed to produce a command to turn off the solid state switch. 
         [0040]    Referring now to  FIG. 4 , there is illustrated another exemplary operation of the hybrid contactor  12 .  FIG. 4  shows a method  400  in which the hybrid contactor  12  be employed to safely activate a circuit without causing excessive arcing due to contactor bouncing. In a step  402  the solid state switch may be turned on (e.g., the switch  28  may be turned on to allow current to pass through the PTC  30 ). In a step  404 , a determination may be made as to whether a line overcurrent condition is present (e.g., the line current sensor  34  may sense current passing through the PTC  30  and the contactor engine  26  may receive the line current signal and then determine whether or not the line current is excessive). In the event of a determination of overcurrent is made, a step  406  may be initiated to turn off the solid state switch. In the event that a determination is made that an overcurrent does not exist, a step  408  may be initiated to begin a delay period after which a step  410  may be initiated to close contacts in the main contactor. Since a main current path may already be established through the PTC  30  upon closing of the main contactor  24 , potential arcing due to bouncing of the contacts  24 - 1  and  24 - 2  may thus be minimized as a result of low level of initial current passing through the main contactor  24 . 
         [0041]    It may be seen that as a result of the sequence of steps of the method  400 , initial (or inrush) current to a load may pass through the PTC  30 . The PTC  30  may suppress potentially excessive current flow to the load conductor  16 . The contactor engine  26  may programmed with data that is representative of characteristics of the PTC  30 . Consequently, the contactor engine  26  may determine if excessive line current may potentially develop. This determination may be made even though actual excessive line current never commences. 
         [0042]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.