Abstract:
The protection device for providing protection against short-circuits upstream from an electrical power supply module having an inlet filter with at least one capacitor and an inductor and having a converter with components associated with a plurality of freewheel diodes includes at least one auxiliary winding and a dissipator element associated with the inlet filter, thereby making it possible to provide an inlet filter of small size without over dimensioning the freewheel diodes.

Description:
TECHNICAL FIELD AND PRIOR ART 
       [0001]    The invention lies in the field of electrical power supply modules, in particular for electrical power distribution racks used in aircraft. 
         [0002]    An electrical power supply module, referred to more simply as a “power module”, is powered from a direct current (DC) electrical energy source, possibly obtained by rectification and via an alternating current (AC) transformer, and including an inlet filter having at least one capacitor. The power module comprises at least one converter, which itself includes in particular inductors and freewheel diodes, possibly incorporated in switch components. 
         [0003]    Usually, a plurality of optionally identical power modules are operated from a common electricity source and they may be associated in parallel in a power distribution rack in order to power a common load. 
         [0004]    When the volume of an inlet filter is minimized, a converter is found to be particularly vulnerable to short-circuit at its inlet, where such a short-circuit constitutes a common mode of failure in power distribution racks. A short-circuit can cause excessive current to flow in the power module, and in particular in the freewheel diodes of the converter. 
         [0005]    In order to remedy that problem, it is general practice to add cut-off members such as fuses, contactors, or circuit breakers. In addition to the complexity due to those additional members, that leads to operation being interrupted, which can be inconvenient and might even affect safety. 
         [0006]    The extent to which the size of an inlet filter can be reduced, as is necessary to enable it to be incorporated in a power distribution rack, is limited for the capacitor C, since it needs to be dimensioned to be capable at least of supplying the current required by the converter, and is limited for the inlet inductor L by the minimum inductance authorized in particular because of constraints on emitting interference in the electromagnetic compatibility (EMC) sense. Under such circumstances, the current produced by a short-circuit at the inlet is given by: 
         [0000]      = U/√{square root over (L/C)}   
         [0007]    A small value for L contributes to reducing the volume of the inlet filter, thereby increasing the current produced by a short-circuit. 
         [0008]    This current flows through the freewheel diodes for a considerable length of time, which means that the diodes need to be over dimensioned. 
         [0009]    There thus exists a need to provide an inlet filter that is of small size, while ensuring that, in the event of a short-circuit at the inlet of the filter, the current that flows through the freewheel diodes of a converter of a power module is not too great, and that it does not flow for too long. 
       OBJECT AND DEFINITION OF THE INVENTION 
       [0010]    The invention seeks to remedy the above-mentioned drawbacks and to make it possible in particular to protect power modules against inlet short circuits in a manner that is simple and reliable, and without leading to an untimely interruption of the operation of the power module. 
         [0011]    In order to resolve the above-mentioned problems, there is provided a protection device for providing protection against short-circuits upstream from an electrical power supply module having an inlet filter with at least one capacitor and an inductor, and having a converter with a plurality of freewheel diodes, the device being characterized in that it comprises at least one auxiliary winding coupled to said at least one inductor of the inlet filter and a dissipator element connected to said at least one auxiliary winding, said dissipator element being adapted to dissipate the energy stored in said at least one capacitor of the inlet filter in the event of a short-circuit occurring at the inlet of said inlet filter. 
         [0012]    More particularly, the dissipator element may be constituted by a resistor of the braking resistance type. 
         [0013]    In a particular embodiment, a switch member, such as an active component or a magnetic amplifier, is configured to allow the current to flow in said auxiliary winding only when the switch member has detected a malfunction due to the short-circuit, such as a sudden drop in the voltage across the terminals of the capacitor of the inlet filter or an overcurrent within the converter. 
         [0014]    The invention is particularly applicable to power modules for providing an electrical power supply in aircraft. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Other characteristics and advantages of the invention appear from the following description of particular embodiments given as examples with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  is a diagrammatic view of an embodiment of a device in accordance with the invention for providing protection against short circuits upstream from power modules; 
           [0017]      FIG. 2  is a more detailed view of another embodiment of the invention; 
           [0018]      FIG. 3A  plots curves showing how the voltage across the terminals of an inlet filter capacitor of a power module varies as a function of time in the event of a short-circuit at the inlet, respectively with and without the protection device of the invention; 
           [0019]      FIG. 3B  plots curves showing how the current at the inlet of a power module, written Is 1  in  FIG. 2 , varies as a function of time in the event of an upstream short-circuit, respectively with and without the protection device of the invention; 
           [0020]      FIG. 4A  plots curves showing how the current at various points of the  FIG. 2  circuit varies as a function of time in the event of a short-circuit at the inlet, for a first selected resistance value; 
           [0021]      FIG. 4B  plots curves showing how the voltage at various points of the  FIG. 2  circuit varies as a function of time in the event of a short-circuit at the inlet for the first selected resistance value; 
           [0022]      FIG. 5A  plots curves showing how the current at various points of the  FIG. 2  circuit varies as a function of time in the event of a short-circuit at the inlet, for a second selected resistance value; 
           [0023]      FIG. 5B  plots curves showing how the voltage at various points of the  FIG. 2  circuit varies as a function of time in the event of a short-circuit at the inlet for the second selected resistance value; 
           [0024]      FIG. 6  shows an example of a protection device of the invention with overcurrent detection and with switching by a magnetic amplifier; and 
           [0025]      FIG. 7  shows another example of a protection device of the invention with overcurrent or under-voltage detection and with switching by an active component. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]      FIG. 1  is a diagram of a device of the invention for providing protection against short-circuits upstream from power modules  160 ,  160 ′ that are designed for electrically powering equipment, in particular within an aircraft where electrical power supply devices need to be capable of powering more and more equipment at ever higher powers. 
         [0027]    In  FIG. 1 , there can be seen a diagram of two power modules  160 ,  160 ′, each comprising in particular at least one converter  120 ,  120 ′ including amongst other components inductors and switch components associated with freewheel diodes  118 ,  118 ′ that may optionally be integrated with the switch components. Each power module  160 ,  160 ′ is powered from a DC source via lines  101 ,  102 ;  101 ′,  102 ′ and via an inlet filter comprising inductors  104 ,  105 ;  104 ′,  105 ′ that are optionally coupled with capacitors  160  to  108 ;  106 ′ to  108 ′ that are optionally connected to the frame  109 ,  110 ;  109 ′,  110 ′. 
         [0028]      FIG. 1  also shows an auxiliary winding  131  that is connected to a dissipator element  132  and a diode  133 , and that is coupled to one of the inductors  105 ′ of the inlet filter. 
         [0029]    The dissipator element  132  may be constituted by a resistor of the braking resistance type or by some other dissipator device that is capable of dissipating the energy stored in a capacitor of the inlet filter by means of the auxiliary winding  131 , which comes into action when a short-circuit  103  arises at the inlet of the filter, e.g. a short-circuit between the lines  101  and  102  as shown by way of example in  FIG. 1 . 
         [0030]      FIG. 3A  shows how the voltage across the terminals of the capacitor  106  or  106 ′ varies in the event of the short-circuit  103  appearing at an instant t 1 , firstly when the auxiliary winding  131  and the dissipator element  132  are not used (curve  141 ), which constitutes the prior art, and secondly in the presence of the auxiliary winding  131 , the dissipator element  132 , and the diode  133  in accordance with the invention (curve  142 ). 
         [0031]    Furthermore,  FIG. 3A  shows instants t 2 , t 3  that correspond to the beginning of conduction by the freewheel diodes of the converter  120 ,  120 ′, respectively without and with the auxiliary winding and the dissipator element. It can be seen that the drop in the voltage across the terminals of the capacitor  106  or  106 ′ is much slower with the protection device of the invention (curve  142 ) than without the device (curve  141 ). 
         [0032]      FIG. 3B  shows how the current to the power module  160 ,  160 ′ varies in the event of a short-circuit  103  appearing at an instant t 1 , firstly when the auxiliary winding  131  and the dissipator element  132  are not used (curve  151 ), which constitutes the prior art, and secondly in the presence of the auxiliary winding  131 , the dissipator element  132 , and the diode  133  of the invention (curve  152 ).  FIG. 3B  also shows the instants t 2 , t 3  that correspond to the beginning of conduction by the freewheel diodes of the converter  120 ,  120 ′ respectively without and with the auxiliary winding of the dissipator element. It can be seen that the short-circuit current is much more limited with the protection device of the invention (curve  152 ) than without the device (curve  151 ). 
         [0033]    In more particular manner, the short-circuit current without the auxiliary winding  131  (curve  151 ) presents a value equal to: 
         [0000]      − U/√{square root over (L/C)} 
 
         [0000]    where L is the inductance of the inductor  104 ,  105 ;  104 ′,  105 ′ of the inlet filter, and where C is the capacitance of the capacitor  106 ,  106 ′ of the inlet filter. 
         [0034]    In contrast, the short-circuit current with an auxiliary winding  131  having a turns ratio of n relative to the number of turns of the inductor  105 ;  105 ′, and with a dissipator element  132  having a resistance R, (curve  152 ) presents a value equal to: 
         [0000]      − U /( R/n   2 )
 
         [0000]    that, with suitably selected values for R and n, can be very significantly less than the current without the protection device  131  to  133 , assuming that there exist constraints on selecting the values L and C for the inlet filter, which values are associated with the operation of the converters  120 ,  120 ′ of the power modules  160 ,  160 ′, which constraints do not apply for the auxiliary protection circuit  131  to  133 . 
         [0035]      FIG. 1  shows two power modules  160 ,  160 ′, but naturally the invention applies to a greater number of power modules powered from parallel power supplies from a common DC voltage source. 
         [0036]      FIG. 2  shows a particular embodiment of a protection device of the invention with measurement means for characterizing the effect of the protection device of the invention. 
         [0037]    In  FIG. 2 , the primary  15   a  of a transformer  15  is powered using switches  13  controlled by a control circuit  14  for alternating between connection to the power supply  11 ,  12  and operating with an upstream short-circuit; the power supply is made up of DC voltage sources  11 ,  12  (e.g. each at 300 volts (V)). 
         [0038]    The secondary  15   b  of a transformer  15  is connected to a series of elements contributing to limiting current, such as inductors  21 ,  25 , resistors  22 ,  23 , and a Zener diode  24 . 
         [0039]    In accordance with the invention, a secondary constituting an auxiliary winding  15   b  is associated with the primary  15   a  of the transformer  15 . A power module with its inlet filter and its converter are represented symbolically by a capacitor  17 , a diode  18 , and a load resistance  19  that form a parallel connection with one of its ends connected to the terminal of an inductor  16  that is not connected to the primary  15   a  of the transformer  15 , and with its other end connected to the switches  13 . 
         [0040]    In order to take measurements, a current probe  31  is connected in series with the DC voltage sources  11 ,  12  in order to measure a current Idc, a current probe  32  is connected between the inductor  16  and the capacitor  17  to measure a current Is 1 , and a current probe  33  is connected between the capacitor  17  and the resistance  19  in order to measure a current Isd. 
         [0041]    The secondary  15   b  of the transformer  15  having a primary  15   a  to secondary  15   b  transformation ratio of 1:n, the resistor  22  of resistance R, and the diode  24  perform the same functions as the elements  131  to  132  respectively of  FIG. 1 . 
         [0042]    The diode  18  represents the freewheel diodes  118 ,  118 ′ included in the converters  120  and  120 ′ of  FIG. 1 . 
         [0043]      FIGS. 4A and 4B  plot curves showing how the current and the voltage measured at various points of the  FIG. 2  circuit vary over time, with a short-circuit being simulated at instant t 1 , for a resistor  22  having a resistance of 40 ohms and corresponding to a peak value for Isd of 200 amps (A). 
         [0044]      FIGS. 5A and 5B  plot curves showing how the current and the voltage measured at various points of the  FIG. 2  circuit vary over time, with a short-circuit being simulated at instant ti, for a resistor  22  having a resistance of 4 ohms and corresponding to a peak value for Isd of 1600 A. 
         [0045]    The curves  211  and  221  in  FIGS. 4A and 5A  show the current Is 1  measured using the probe  32 . 
         [0046]    The curves  212  and  222  of  FIGS. 4A and 5A  show the current Isd measured using the probe  33 . 
         [0047]    The curves  213  and  223  of  FIGS. 4A and 5A  show the current Isecondary measured at the secondary of the transformer  15 . 
         [0048]    The curves  214  and  224  of  FIGS. 4B and 5B  show the voltage Vcapa across the terminals of the capacitor  17 . 
         [0049]    The curves  215  and  225  of  FIGS. 4B and 5B  show the voltage Vcrowbar applied to the inlet of the power module. 
         [0050]    In the event of a short-circuit at the inlet filter formed by the primary  15   a  of the transformer and the capacitor  17 , it can be seen that by virtue of the measures recommended by the present invention (namely adding a single auxiliary winding  15   b  and a dissipator resistor  22 , associated with a diode  24 , to the primary  15   a  of the transformer  15 ) the repercussion on the current Isd passing through the diode  18  is greatly attenuated. 
         [0051]      FIG. 6  shows an example of a protection device of the invention, as described above with reference to  FIG. 2 , including detection of overcurrent for the current Is 1  as measured by the probe  32  and switching of the elements  15   b  and  21  to  25  as performed by a magnetic amplifier  40 . Elements in common in the circuits of  FIGS. 2 and 6  are given the same reference numbers. 
         [0052]      FIG. 7  shows another example of a protection device of the invention, as described above with reference to  FIG. 2 , including detection of overcurrent for the current Is 1  measured by the probe  32 , or of under-voltage across the terminals of the capacitor  17  by logic components  52  to  54  including an OR gate  52 . The elements  15   b  and  21  to  25  are switched by an active component  51 , which may for example be a thyristor having its gate connected to the output of the OR gate  52 . Once more, elements in common in the circuits of  FIGS. 2 and 7  are given the same reference numbers. 
         [0053]    The invention is not limited to the embodiments described, but extends to any variant coming within the ambit of the claims. 
         [0054]    In particular, although the dissipator element is advantageously constituted by a braking resistance, it is possible to use other dissipator devices.