Abstract:
An electrical power supply system and method for an aircraft. The system includes an electrical network including primary generators powering the electrical distribution channels, and an homopolar generator making it possible to create an artificial neutral.

Description:
TECHNICAL FIELD 
     The invention relates to an electrical power supply system and method for an aircraft, for example an airplane. 
     Hereafter, for simplification reasons, an aircraft of the airplane type will be considered. 
     BACKGROUND OF THE INVENTION 
     In aeronautics, the neutral state commonly used is the TN-C state, which means:
         T: neutral of the installation connected directly to the ground,   N: masses of the installation connected to the neutral conductor,   C: protective conductor for the masses of the installation and neutral conductor combined.       

     In an electrical power supply system onboard an airplane, the neutral of each primary generator G is connected to the fuselage  10 ′ of the airplane, at the engine pylons (1 meter of cable) in order to ensure the return of the unbalance current and faults. Furthermore, all of the masses of the equipment are connected to said fuselage. The fuselage therefore plays the role both of neutral and ground conductor.  FIG. 1  thus shows a three-phase generator G, the neutral N of which is connected to the fuselage  10 ′, connected to an electric core  11 ′ by its phases ph 1 , ph 2  and ph 3 , a phase-to-ground fault charge  12 ′ whereof the mass is connected to the fuselage  10 ′. A fault current id  13 ′ relative to this fault charge  12 ′, which passes toward the fuselage  10 ′, is also illustrated. 
     In such a configuration, the primary generators G perform the functions of the direct, reverse, and homopolar current component generators for example described in the document referenced [1] at the end of the description. 
     The neutral state as defined above makes it possible to achieve a significant mass gain while avoiding the addition of extra cables. However, the distribution of the neutral of the primary generators G causes an excess weight for two reasons:
         The connection of the neutral of each generator G to the fuselage allows the circulation of the harmonic  3  currents, i.e. the circulation of homopolar currents, which causes additional losses. To greatly reduce the value of these harmonic  3  currents, it is possible to use generators designed with a ⅔ pitch winding structure, which involves a slight attenuation of the fundamental. Consequently, the power-weight ratio of the generator decreases.       

     In the case of an airplane whereof the fuselage is made from carbon, the neutral of each generator G cannot be connected to the engine pylon. It is connected to the PVR (“Point Voltage Reference”) of the ESN (“Electrical Structural Network”)  10 , as illustrated in  FIG. 2 . The neutral and the mass of the charges are connected to the ESN. 
     The location of the primary generators G in the wings of an airplane  20  near each engine  21  and that of the electric cores  22  at the front of the airplane requires a non-negligible neutral cable length  23 , as illustrated in  FIG. 3 , such an “electric core” being a source—charge interconnect node including protective and contact members, the voltage of which is stabilized by an outside element. This results in a significant power-weight ratio in the case where the airplane  20  has four electric power supply channels, and therefore four neutral cables. 
     The invention aims to eliminate such neutral cables in order to decrease the volume and mass of the electrical power supply on-board the airplane, the primary generators keeping the role of generators of the direct and reverse components, but the homopolar generator function being performed by a specific piece of equipment. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The invention relates to an electrical power supply system of an aircraft including an electrical network comprising primary generators powering electrical distribution channels, electrical cores, and charges, characterized in that it comprises means for placing an artificial neutral at the electric cores including an homopolar generator, which is a piece of equipment of the electrical network. 
     It can comprise a transformer. This can be a transformer rectifier unit. Such a transformer rectifier unit includes:
         a three-phase transformer with galvanic isolation comprising:   a wye-connected primary, the neutral of which is grounded,   a delta-connected secondary,   a wye-connected secondary,   a three-column magnetic structure.   a twelve-phase rectifier stage to create the direct network.       

     The invention also relates to an electrical power supply method for an aircraft including an electrical network comprising primary generators powering electrical distribution channels, electrical cores, and charges, in which the direct and reverse components of the current are generated using these generators, characterized in that an artificial neutral is placed at the electrical cores and in that the homopolar component is generated using a specific piece of equipment, which is a piece of equipment of the electrical network. This equipment of the electrical network may comprise a transformer. This can be a transformer rectifier unit. 
     The invention thus makes it possible to eliminate the distribution of the neutral between the primary generators and the electrical cores using cables, with the placement of an artificial neutral at the electric cores, which makes it possible to save in terms of aircraft mass and sizing of said generators. The creation of an artificial neutral using an homopolar generator makes it possible, in fact, to convey the fault currents distributed over three phases in the form of three homopolar components and no longer only on the short-circuit phase alone. 
     Using a piece of equipment already existing in the electrical network as homopolar generator makes it possible to save in terms of mass while avoiding adding extra equipment. 
     Using a transformer rectifier unit TRU as homopolar generator makes it possible to mutualize the homopolar generator function on an existing piece of equipment by connecting the neutral of the primary of the transformer to the PVR of the airplane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 ,  2  and  3  illustrate an electrical power supply system for an airplane according to the prior art. 
         FIG. 4  illustrates the incorporation of a homopolar generator into an electrical network according to the invention. 
         FIG. 5  illustrates the embodiment of the homopolar generator illustrated in  FIG. 4  using a transformer whereof the primary is a wye-connected primary. 
         FIG. 6  illustrates an embodiment of the system according to the invention in which a transformer rectifier unit (TRU) is used as homopolar generator. 
         FIG. 7  illustrates the distribution of the currents in a transformer-rectifier as illustrated in  FIG. 6  in case of fault. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, the references used for the elements illustrated in  FIG. 2  are kept to illustrate similar elements in  FIGS. 4 ,  5  and  6 . The electrical power supply system of the invention, as illustrated in  FIG. 4 , thus comprises an electrical network made up of a primary generator G, an electrical core  11 , a fault charge  12 , and the ESN of the airplane. It also comprises an homopolar generator  30 . 
     As it is well known by those skilled in the art, an homopolar generator makes it possible to create an impeding neutral on an electrical network not having one, and to ground that network through said impedance. The impedance is calculated so as to limit the fault current to a determined value. The value of the fault current depends on the network one wishes to protect. 
     Such an homopolar generator  30  is characterized by:
         a weak homopolar impedance to facilitate the circulation of the homopolar components,   a strong direct impedance so that the device does not behave as a charge.       

     In the invention, such an homopolar generator  30  makes it possible to create an artificial neutral that makes it possible to eliminate the neutral cables coming from the primary generators. 
     Thus, as shown in  FIG. 4 , in the case of a phase-to-ground fault charge  12 , the fault current i d  is conveyed toward the three-phase network via the homopolar generator  30  in the form of three homopolar components 
                 i   o     =       i   d     3       ,         
which are divided over the three phases.
 
     In one advantageous embodiment of the invention, illustrated in  FIG. 5 , the homopolar generator function can be performed with a piece of equipment existing in the electrical network of the airplane, which makes it possible to save on mass. However, it is possible to add an additional piece of equipment that makes it possible to create an artificial neutral and to thereby perform the homopolar generator function. This equipment can, for example, be a transformer  40 . The secondary of this transformer may or may not be connected to functional loads  41 . This transformer  40  must have the following characteristics:
         galvanic isolation between primary and secondary, necessary condition for the electrical charges  41  connected to the secondary of the transformer  40  not to be disrupted by the rise of the fault currents,   a wye or zigzag connected primary,   the neutral of the primary grounded,   a three-column magnetic structure in order to minimize the homopolar impedance.       

     The invention thus makes it possible to reduce the mass of the airplane, due primarily to the elimination of the neutral cable. 
     The invention also makes it possible to:
         increase the power-weight ratio of the generators owing to the increase in the value of the fundamental,   improve the quality of the electrical network by attenuating harmonics  5  and  7  a bit more.       

     In fact, no longer connecting the neutral of the generators prevents the circulation of the harmonics  3 . The ⅔ pitch winding structure, which has the drawback of attenuating the fundamental of the currents, is therefore no longer necessary to eliminate the harmonics  3 . It is therefore possible to use another type of winding, for example a ⅚ pitch winding, which makes it possible to minimize harmonics  5  and  7  while obtaining a fundamental value of 10% more relative to a ⅔ pitch winding. 
     The invention lastly makes it possible to obtain a distribution of the fault currents on three phases in the form of three homopolar components, contrary to the system of the prior art, in which all of the fault currents circulate on the short-circuit phase. Thus, in the invention, at a given fault current, the generator must only provide ⅔ of the fault current on the short-circuit phase instead of its entirety, before the protections are triggered. The sizing of the generators therefore becomes less restrictive relative to the short-circuit currents. 
     Example of Application 
     In the example of application, the rise of the fault and unbalance currents is ensured by an artificial neutral done with a transformer rectifier unit TRU operating as an homopolar generator. 
     Such a transformer rectifier unit TRU  59 , used to create a direct network from an alternating three-phase network, is made up of:
         a three-phase transformer with galvanic isolation comprising:   a wye-connected primary  60 ,   a delta-connected secondary  62 ,   a wye-connected secondary  61 ,   a three-column magnetic structure  63 .   a twelve-phase rectifier stage  64  to create the direct network.       

     In this embodiment, the neutral N′ of the primary of the transformer  60  is connected by a connector  50  to the ESN  10  of the airplane, as illustrated in  FIG. 6 . The rise of the homopolar currents to the primary does not disrupt the DC network at the output of the TRU  59 . The galvanic isolation between the primary  60  and the two wye-connected and delta-connected secondaries  61  and  62  without a distributed neutral does not allow the circulation of homopolar components. 
       FIG. 7  illustrates the distribution of the currents in the transformer  59  in case of fault. One thus has:
         At the primary  60  of the transformer  59 , the direct three-phase currents i pA , i pB , i pC  conveying the power toward the DC network and the homopolar currents i o  coming from the unbalances on the electrical network overlap.   At the secondaries  61  and  62  of the transformer  59 , the direct three-phase currents i sa , i sb , i sc , and i sa ′, i sb ′, i sc ′ conveying the power toward the DC network are the only ones to circulate toward the twelve-phase rectifier  64 .       

     REFERENCES 
     
         
         [1] “Protection des réseaux-généralités” by Claude Corroyer (Techniques de l′ingénieur D4800, pp 1-16, 1991).