Abstract:
An electrical power protection system, includes a generator configured for supplying Direct Current (DC) power to a load bus, the load bus in electrical communication with a bus circuit; a generator control unit being configured for regulating the output voltage supplied by the generator; a bus contactor in serial communication with the bus circuit, the bus contactor including logic circuits configured for detecting an overcurrent in the bus circuit, the overcurrent representative of a ground fault in the bus circuit; and a capacitor bank coupled to the generator for selectively supplying an excitation voltage through a diode switch to the generator during the ground fault in the bus circuit.

Description:
FIELD OF INVENTION 
       [0001]    The subject matter disclosed herein relates generally to the field of electrical power systems and, particularly, to an aircraft&#39;s electrical power system having a capacitor source for supplying power to a self-excited generator in order to maintain the generators field during a fault clearing mode. 
       DESCRIPTION OF RELATED ART 
       [0002]    Typically, an aircraft&#39;s electrical power system includes a DC power generator as a primary power source with batteries serving as an emergency backup power source. The DC power generator is used to start an aircraft&#39;s engine and, once started, the engines cause power generation through the power generators resulting in electrical power being supplied to load busses in the electrical power system. A typical twin engine generator power system would consist of two generators, one per engine. A first generator would be used for starting the first engine and for providing, for example, electrical power to the left hand busses. A second generator would be used for starting the second engine and providing power to the right hand busses. In the event that the first generator was to fail, the power system would compensate by providing power to all buses through the remaining generator. In some cases, the remaining load busses can overload the remaining generator, resulting in its failure as well. Most power systems, therefore, also include a battery backup for providing supplemental power to if one or both of the generators fail. The battery feeds power to the emergency busses and the essential busses. 
       BRIEF SUMMARY 
       [0003]    According to one aspect of the invention, an electrical power protection system includes a generator configured for supplying Direct Current (DC) power to a load bus, the load bus in electrical communication with a bus circuit; a generator control unit being configured for regulating the output voltage supplied by the generator; a bus contactor in serial communication with the bus circuit, the bus contactor including logic circuits configured for detecting an overcurrent in the bus circuit, the overcurrent representative of a ground fault in the bus circuit; and a capacitor bank coupled to the generator for selectively supplying an excitation voltage through a diode switch to the generator during the ground fault in the bus circuit. 
         [0004]    According to another aspect of the invention, a method of clearing a ground fault in an electrical power protection system includes supplying Direct Current (DC) power, via a self-excited generator, to a main bus circuit; regulating, via a generator control unit, the output voltage supplied by the generator; detecting, via a bus contactor, an overcurrent in the main bus circuit, the overcurrent representative of a ground fault in the bus circuit; and selectively supplying, via a capacitor bank coupled to the generator, an excitation voltage through a diode switch to the generator during the ground fault in the bus circuit. 
         [0005]    Other aspects, features, and techniques of the invention will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0006]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0007]      FIG. 1  illustrates a schematic block diagram of the DC electrical power system according to an embodiment of the invention; and 
           [0008]      FIG. 2  illustrates a schematic block diagram of an algorithm used to clear a fault utilizing a capacitor as a backup power source according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Embodiments of an aircraft&#39;s electrical power protection system includes a DC electrical power system having a capacitor bank for selectively applying power to the system when back up battery power is not available to clear a ground fault. The system includes a capacitor bank connected to a field line of a self-excited generator for supplying hold-up power to the generator during a fault clearing mode. Additionally, the capacitor bank supplies power to the logic circuits of a bus tie contactor for maintaining its logic during the fault clearing mode as well as for selectively opening the logic circuit and isolating the fault from the main circuit. 
         [0010]    Referring now to the drawings,  FIG. 1  illustrates a schematic block diagram of an aircraft&#39;s electrical power protection system  100  having a plurality of engines  106 ,  108  including generators  105 ,  110  for selectively applying power to the power protection system  100  in order to clear a ground fault according to an embodiment of the invention. Particularly, the electrical power protection system  100  includes, in an embodiment, a plurality of power sources such as, for example, self-excited DC generators  105 ,  110  connected in parallel to Direct Current (“DC”) bus circuits  102 ,  104 . In embodiments, the power sources  105 ,  110  may be permanent magnet generators, or three-phase AC generators having rectifiers for receiving power from the generators  105 ,  110  and distributing 28 VDC power to the DC bus circuits  102 ,  104  respectively. The engine  106  includes a DC self-excited generator  105 , which feeds electrical power to the main circuit  102  and to equipment connected to the load bus  145  while engine  108  includes DC self-excited generator  110 , which feeds electrical power to the main circuit  104  and to equipment connected to the load bus  150 . Also, self-excited generator  105  is connected to a generator control unit  125  and generator line contactor  115  while self-excited generator  110  is connected to generator control unit  130  and generator line contactor  120 . The generator control units  125 ,  130  are microprocessor controlled devices and include an overvoltage sensing circuit that regulates the output voltage being supplied by the respective DC self-excited generators  105 ,  110 . In particular, the generator control unit  125  continually regulates the output voltage being supplied by self-excited generator  105  by feeding the output voltage back to an excitation circuit (not shown) in the self-excited generator  105  with a return line  135  in order to excite the field of the self-excited generator  105 . Additionally, the generator control unit  125  may disconnect the power source  105  from the main circuit  102  if the voltage exceeds a preset level. Similarly, generator control unit  130  continually regulates the output voltage being supplied by self-excited generator  110  and feeds the output voltage back to an excitation circuit (not shown) in the self-excited generator  110  with a return line  140  in order to excite the field of the self-excited generator  105 . Additionally, the generator control unit  125  may disconnect the power source  110  from the main circuit  104  if the voltage exceeds a preset level. Each generator control unit  125 ,  130  energizes its respective generator line contactor  115 ,  120  for electrically connecting the self-excited generators  105 ,  110  to the main circuits  102 ,  104  when the generator outputs from each self-excited generator  105 ,  110  are within specified limits. The generator control units  125 ,  130  are preprogrammed to sense when a short circuit occurs in the load busses  145 ,  150  and supply hold-up power to the self-excited generators  105 ,  110  in order to clear the fault caused by the short circuit. In embodiments, the system  100  includes backup batteries  175 ,  180  that are connected to the respective generators  105 ,  110  through diode switches (not shown) in order to excite the field of the generators  105 ,  110  during a ground fault. In operation, in the event of a short circuit, the generator control units  125 ,  130  provide power to self-excited generators  105 ,  110  through capacitor banks  185 ,  190  respectively as well as energizing the bus tie contactors  155 ,  160 . The capacitor banks  185 ,  190  provide voltage to the self-excited generators  105 ,  110  in order to maintain the voltage level of the generator&#39;s field when the self-excited generators respective batteries  175 ,  180  are not available in order to clear the fault, as is shown and described below with reference to  FIG. 2 . 
         [0011]    Also shown in  FIG. 1 , the power protection system  100  includes bus contactors  155 ,  160  electrically connected to load buses  145 ,  150 . In an embodiment, load buses  145 ,  150  include respective current limiting devices  165 ,  170  such as, for example a fuse or a thermal “trip” device in order to provide fault protection on each of the load buses  145 ,  150  during a short circuit. In an embodiment, the current limiting device is an 80 Ampere fuse or thermal device that is rated to “open” or “trip” at a predetermined I 2 t rating (i.e., a time-current thermal value or “trip curve”). Other current limiting devices having a different I 2 t rating may also be utilized in embodiments. In an example, the bus tie contactors  155 ,  160  include hall-effect sensors (not shown) for monitoring the bidirectional current values traversing through the main circuits  102 ,  104 . The bus tie contactors  155 ,  160  include logic circuits that are programmed, in embodiments, with predetermined I 2 t curves for “tripping” or opening for currents (i.e., interrupt the circuit) exceeding the rated values in either direction of the contactors  155 ,  160  such as, for example, in the event of a ground fault or loss of one or more of the self-excited generators  105 ,  110 . The bus tie contactors  155 ,  160  selectively open their contacts if the measured bidirectional currents exceed a predetermined current value in either direction through the contactors  155 ,  160 . It is to be appreciated that while only two power sources  105 ,  110  are shown in electrical communication with the load busses  145 ,  150 , additional power supplies or additional load busses may be connected to the main circuits  102 ,  104  without departing from the scope of the invention. 
         [0012]      FIG. 2  illustrates a schematic block diagram of an algorithm for clearing a fault utilizing a capacitor bank as a backup power source according to an embodiment of the invention. Although, a description of the algorithm for clearing a fault in engine  106  is shown, this algorithm provides an adequate description of the algorithm used for clearing a fault in engine  108  ( FIG. 1 ) and in load bus  150  connected to power source  110  and its circuit  104  ( FIG. 1 ). The generator control unit  125  energizes the generator line contactor  115  for electrically connecting the output from the self-excited generator  105 , to the main circuit  102  when the generator outputs from the generator  105  is within specified limits. The generator control unit  125  is preprogrammed to sense when a ground fault occurs in the load bus  145  and supplies hold-up power to the generator  105  during a ground fault. In an embodiment, in the event of a “ground fault”  205  (for example, in the order of a 5 milliohm resistance to ground) in load bus  145  such as, for example, a bare copper wire that makes contact with a larger bus wire causing a “ground fault”, an excess ground current will flow from self-excited generator  105  through the main circuit  102  and into load bus  145 . The excess ground current collapses the field in self-excited generator  105 , thus lowering the output voltage from the self-excited generator  105  and selectively opening the generator line contactor  115 . The generator control unit  125 , which continually monitors the output voltage being supplied by the self-excited generator  105 , connects the capacitor bank  195  through the diode  210  to the field line  135  in the event of a failure of the backup battery  175  ( FIG. 1 ). The capacitor bank  195  sources voltage to the self-excited generator  105  in order to maintain the field of the generator  105  for a predetermined amount of time and energize the main circuit  102  by closing the generator line contactor  115 . The generator  105  feeds current to the main circuit  102  for the predetermined time needed to burn through the bare copper wire that is causing the ground fault, thereby clearing the fault. In other embodiments, the generator control unit  105  cycles the generator  105  for a predetermined number of times in order to supply a burst of current during each cycle of the generator and clear the ground fault. Also, the generator control unit  125  supplies power to the logic circuits in the bus tie contactor  155  in order to maintain their logic during the time utilized by the generator to clear the fault. Further, the capacitive bank  195  provides power to the bus tie contactor  155  for sensing the short circuit current caused by the ground fault, where the contactor  155  may selectively open its contact and isolate the load bus  145  from the main circuit  102 , thereby preventing the generator  105  and devices connected to the load bus  145  from being damaged. 
         [0013]    The technical effects and benefits of exemplary embodiments include a DC electrical power protection system having a capacitor bank for selectively applying power to the system in order to clear a fault. The system includes a capacitor bank connected to a field line of a self-excited generator for supplying hold-up power to the generator during the fault clearing mode. Additionally, the capacitor bank supplies power to the logic circuits of a bus tie contactor for maintaining its logic during the fault clearing mode as well as for selectively opening the logic circuit and isolating the fault from the main circuit. 
         [0014]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while various embodiment of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.