Abstract:
A bus centering device for use in an aircraft electrical power distribution system that includes a positive bus rail, a negative bus rail, and a ground is described. The device includes a central node, a first and second switching component configured to couple the central node to the positive rail and the negative rail for a first and second predetermined duty cycle, respectively. The device includes an inductive component coupled between the central node and ground, and is configured to maintain a voltage at the central node substantially equal to ground, wherein a voltage between the positive rail and the central node is maintained substantially equal to a voltage between the negative rail and the central node. The device includes a first and second current limiting device configured to maintain a continuity of current from the inductive component when the first and second switching components are turned off.

Description:
BACKGROUND OF THE INVENTION 
     The present application relates generally to aircraft power systems, and, more particularly, to a method and apparatus for electrical bus centering. 
     Aircraft power systems provide electrical power to numerous components. A current trend is to include more electrical components in aircraft. This trend results in an increased power demand from the aircraft electrical power distribution system. At least some known aircraft power distribution systems minimize electrical feeder weight by increasing the electrical distribution, or bus, voltage level. In some known aircraft, the aircraft bus voltage exceeds 270 volts direct current (VDC). 
     As the aircraft bus voltages increase, a concomitant increase in risk to maintenance personnel and other aircraft components is created due to an increased risk of accidental electrical discharge. Moreover, a risk of undesirable corona discharge is increased at high aircraft bus voltages. Accordingly, in some known aircraft systems, aircraft bus voltages exceeding 270 VDC are provided using a bipolar bus that is centered about aircraft chassis ground, rather than a unipolar bus. However, at least some known bipolar bus implementations are less efficient than unipolar bus implementations. Additionally, at least some known bipolar bus implementations increase the weight of the aircraft over unipolar bus implementations. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In one embodiment, a bus centering device for use in an aircraft electrical power distribution system that includes a positive bus rail, a negative bus rail, and a ground is provided. The bus centering device includes a central node, a first switching component that is configured to couple the central node to the positive bus rail for a first predetermined duty cycle, and a second switching component that is configured to couple the central node to the negative bus rail for a second predetermined duty cycle. The device also includes an inductive component that is coupled between the central node and the ground. The inductive component is configured to maintain a voltage at the central node substantially equal to ground potential, wherein a voltage between the positive bus rail and the central node is maintained substantially equal to a voltage between the negative bus rail and the central node. The device also includes a first current limiting device that is coupled to the first switching component and the inductive component. The first current limiting device is configured to maintain a continuity of current from the inductive component when the first switching component is turned off. The device includes a second current limiting device that is coupled to the second switching component and the inductive component. The second current limiting device is configured to maintain a continuity of current from the inductive component when the second switching component is turned off. 
     In another embodiment, an aircraft electrical bus system that includes a positive bus rail, a negative bus rail, and a ground is provided. The system includes a power source, a load, and a bus centering device. The bus centering device includes a central node, a first switching component that is configured to couple the central node to the positive bus rail for a first predetermined duty cycle, and a second switching component that is configured to couple the central node to the negative bus rail for a second predetermined duty cycle. The device also includes an inductive component that is coupled between the central node and the ground. The inductive component is configured to maintain a voltage at the central node substantially equal to ground potential, wherein a voltage between the positive bus rail and the central node is maintained substantially equal to a voltage between the negative bus rail and the central node. The device also includes a first current limiting device that is coupled to the first switching component and the inductive component. The first current limiting device is configured to maintain a continuity of current from the inductive component when the first switching component is turned off. The device includes a second current limiting device that is coupled to the second switching component and the inductive component. The second current limiting device is configured to maintain a continuity of current from the inductive component when the second switching component is turned off. 
     In another embodiment, a method for centering an aircraft electrical power distribution system that includes a positive bus rail, a negative bus rail, and a ground is provided. The method includes providing a power source and a load that is coupled to the power source, and coupling a bus centering device to the positive bus rail, the negative bus rail, and the ground. The bus centering device includes a central node, a first switching component, a second switching component, and an inductive component. The inductive component is coupled between the central node and the ground. The method also includes coupling the central node to the positive bus rail for a first predetermined duty cycle, coupling the central node to the negative bus rail for a second predetermined duty cycle, and maintaining a voltage at the central node substantially equal to ground potential, wherein a voltage between the positive bus rail and the central node is maintained substantially equal to a voltage between the negative bus rail and the central node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary aircraft power distribution system. 
         FIG. 2  is a schematic diagram of a portion of a prior art power distribution system that may be used with the aircraft power distribution system shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram of an exemplary bus centering device that may be used with the aircraft power distribution system shown in  FIG. 1 . 
         FIG. 4  is a schematic diagram of an alternative bus centering device that may be used with the aircraft power distribution system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of an exemplary aircraft power distribution system  10 . System  10  includes at least one power source  20  that is coupled to at least one load  40  via an aircraft electrical distribution line, or bus,  50 . In the exemplary embodiment, power source  20  is a synchronous, three phase alternating current generator that includes a rotor and a stator (not shown). In the exemplary embodiment, power source  20  includes a rectifier circuit (not shown) to convert the three phase alternating current to direct current (DC), which is used to energize bus  50 , and ultimately load  40 . Power source  20  may generate DC power that is floating with respect to an aircraft chassis ground (not shown). As such, a bus centering device  30  may be coupled to bus  50  to substantially center bus  50  about ground. As used herein, the term “centering” refers to adjusting a first voltage rail and a second voltage rail such that the first voltage rail has a voltage level that is a positive level above a reference point, and the second voltage rail has a voltage level that is substantially equal to the first voltage level, but negative with respect to a reference point. For example, a bus may have a first voltage rail of 300 VDC, and a second voltage rail of 100 VDC. Centering this bus about a ground level of 0 VDC would result in the first voltage rail being +100 VDC and the second rail being −100 VDC. 
       FIG. 2  is a schematic illustration of a portion of a prior art power distribution system  100 . Prior art system  100  includes a positive terminal  102  and a negative terminal  104 . Terminals  102  and  104  are configured to be connected to an aircraft electrical bus such as bus  50  (shown in  FIG. 1 ). Positive and negative terminals  102  and  104  are coupled to aircraft chassis ground  106  via capacitors  108  and  110 , respectively. Prior art system  100  also includes a generator  112  such as a three phase wye-connected stator or transformer for use in generating power. Generator  112  includes a first, second, and third winding  114 ,  116 , and  118  that are coupled together at a common center point  120 . Center point  120  is coupled to chassis ground  106 , which facilitates centering prior art system  100  about ground  106 . An inductor  140  may be coupled between center point  120  and chassis ground  106 , as described below. A terminal  122  of first winding  114  is coupled to positive terminal  102  via diode  128  and to negative terminal  104  via diode  134 . A terminal  124  of second winding  116  is coupled to positive terminal  102  via diode  130  and to negative terminal  104  via diode  136 . A terminal  126  of third winding  118  is coupled to positive terminal  102  via diode  132  and to negative terminal  104  via diode  138 . 
     During operation, generator  112  produces three-phase alternating current electrical power. More specifically, a rotor (not shown) induces alternating magnetic fields into first, second, and third windings  114 ,  116 , and  118 , respectively. The magnetic fields cause alternating electrical currents to flow through windings  114 ,  116 , and  118  at phase offsets of substantially 0, 120, and 240 degrees. Diodes  128 ,  130 ,  132 ,  134 ,  136 , and  138  facilitate converting the alternating current provided by windings  114 ,  116 , and  118  to direct current. The phase offsets of current flowing through windings  114 ,  116 , and  118  facilitate providing a substantially uniform power to one or more loads (not shown) that may be coupled to terminals  102  and  104 . Due to the wye configuration of windings  114 ,  116 , and  118 , one or more high amplitude third harmonic currents may result in the center point  120  connection to chassis ground  106 . To minimize these currents, inductor  140  is provided. However, to properly reduce the harmonic currents, inductor  140  must have a sufficiently high inductance. As a result, inductor  140  may add substantial weight to the power distribution system. If multiple generators are provided in an aircraft power distribution system, multiple inductors  140  must be provided. Accordingly, the prior art system  100  may significantly and undesirably increase the weight of the aircraft power distribution system. 
       FIG. 3  is a schematic illustration of a bus centering device  200  in accordance with an exemplary embodiment of the present invention. In the exemplary embodiment, device  200  includes a positive bus terminal  202  and a negative bus terminal  204 . Terminals  202  and  204  are configured to be connected to a primary electrical distribution line such as electrical bus  50  (shown in  FIG. 1 ). More specifically, in the exemplary embodiment, positive terminal  202  is coupled to a positive DC voltage rail (not shown) and negative terminal  204  is coupled to a negative DC voltage rail (not shown) of bus  50 . Positive terminal  202  is coupled to a positive node  222 , negative terminal  204  is coupled to a negative node  224 , and a chassis ground  206  is coupled to a ground node  226 . A first capacitor  208  is coupled between positive node  222  and ground node  226 . A second capacitor  210  is coupled between negative node  224  and ground node  226 . A collector of a first switching device  212  is coupled to positive node  222 , and a drain of first switching device  212  is coupled to a common node  228 . A collector of a second switching device  214  is coupled to common node  228 , and a drain of second switching device  214  is coupled to negative node  224 . An inductor  216  is coupled between common node  228  and ground node  226 . 
     In the exemplary embodiment, first switching device  212  includes a transistor Q 1  and a diode  218  that is coupled in parallel to transistor Q 1 . More specifically, transistor Q 1  is an insulated gate bipolar transistor (IGBT). In the exemplary embodiment, second switching device  214  includes a transistor Q 2  and a diode  220  that is coupled in parallel to transistor Q 2 . More specifically, transistor Q 2  is an IGBT. Diodes  218  and/or  220  are configured to maintain a continuity of current from inductor  216  when transistors Q 1  and/or Q 2  are switched to an “off” state. In an alternative embodiment, each of first and second switching devices  212  and  214  includes a different transistor type, or any other switching device that may operate as described herein. As described herein, device  200  facilitates centering bus  50  without requiring the use of a third harmonic suppression inductor, such as inductor  140  (shown in  FIG. 2 ). Rather, a design of device  200  facilitates centering bus  50  with inductor  216  having a smaller inductance than inductor  140 . As such, inductor  216  may be provided with a comparatively smaller core and/or winding, which facilitates reducing a weight of device  200 . 
     During operation, a first DC voltage Vp is applied to positive terminal  202  and a second DC voltage Vn is applied to negative terminal  204  via bus  50  such that Vp is more positive than Vn. First switching device  212  is switched by a first external control device at a first duty cycle. Second switching device  214  is switched by a second external control device at a second duty cycle. In the exemplary embodiment, the first duty cycle is approximately 50%, and the second duty cycle is approximately 50%, but a turn-on of second switching device  214  is phase-delayed from a turn-on of first switching device  212  by approximately 180 degrees. As such, first switching device  212  and second switching device  214  switch at alternating times. More specifically, when first switching device  212  is switched to an “on” state, second switching device  214  is switched to an “off” state. When first switching device  212  is switched to an “off” state, second switching device  214  is switched to an “on” state. As such, a voltage at common node  228  alternates between the first DC voltage Vp and the second DC voltage Vn in a substantially square wave pattern. As such, the average voltage at common node  228  is approximately equal to (Vp+Vn)/2. Because an inductor generally enables a DC current to flow through the inductor substantially unimpeded, inductor  216  forces the voltage at common node  228  to be approximately equal to the voltage at ground  206 . As such, bus centering device  200  substantially centers Vp and Vn, and bus  50 , about ground  206 . More specifically, first DC voltage Vp is forced to substantially (Vp +Vn)/2 and the second DC voltage Vn is forced to substantially −(Vp+Vn)/2 with respect to ground  206 . 
       FIG. 4  is a schematic illustration of a bus centering device  300  in accordance with an alternative embodiment of the present invention. In the alternative embodiment, device  300  includes a positive bus terminal  302  and a negative bus terminal  304 . Terminals  302  and  304  are configured to be connected to a primary electrical distribution line such as electrical bus  50  (shown in  FIG. 1 ). More specifically, in the alternative embodiment, terminal  302  is coupled to a positive DC voltage rail (not shown) and terminal  304  is coupled to a negative DC voltage rail (not shown) of bus  50 . Positive terminal  302  is coupled to a positive node  358 , negative terminal  304  is coupled to a negative node  360 , and an aircraft chassis ground  306  is coupled to a ground node  362 . Positive terminal  302  is coupled to ground  306  via a first capacitor  308 . Negative terminal  304  is coupled to ground  306  via a second capacitor  310 . A collector of each of a first switching device  312 , a third switching device  316 , and a fifth switching device  320  are coupled to positive node  358 . A drain of each of a second switching device  314 , a fourth switching device  318 , and a sixth switching device  322  are coupled to negative node  360 . A drain of first switching device  312  and a collector of second switching device  314  are coupled to a first common node  366 . A drain of third switching device  316  and a collector of fourth switching device  318  are coupled to a second common node  368 . A drain of fifth switching device  320  and a collector of sixth switching device  322  are coupled to a third common node  370 . Device  300  further includes an inductor  332  and a three leg interphase transformer  324  that includes a first winding  326 , a second winding  328 , and a third winding  330 . A first terminal  334  of first winding  326  is coupled to first common node  366 . A first terminal  338  of second winding  328  is coupled to second common node  368 . A first terminal  342  of third winding  330  is coupled to third common node  370 . Each of first, second, and third windings  326 ,  328 , and  330  include a respective second terminal  336 ,  340 , and  344  that are coupled together at a common inductor node  364 . Inductor  332  is coupled between ground node  362  and inductor node  364 . In one embodiment, inductor node  364  is coupled to ground  306  via inductor  332 . In the alternative embodiment, inductor  332  is omitted, and each of first, second, and third winding second terminals  336 ,  340 , and  344  are coupled directly to ground  306 . 
     In the alternative embodiment, each of first, second, third, fourth, fifth, and sixth switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322  includes a respective transistor Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6 , and an associated diode  346 ,  348 ,  350 ,  352 ,  354 , and  356  that is coupled in parallel to respective transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6 . More specifically, in the alternative embodiment, transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5 , and Q 6  are insulated gate bipolar transistors (IGBT). Diodes  346 ,  348 ,  350 ,  352 ,  354  and/or  356  are configured to maintain a continuity of current from inductor  216  when transistors Q 1 , Q 2 , Q 3 , Q 4 , Q 5  and/or Q 6  are switched to an “off” state. In another embodiment, each of first, second, third, fourth, fifth, and sixth switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322  includes a different transistor type, or any other switching device that may operate as described herein. As described herein, device  300  facilitates centering bus  50  without requiring the use of a third harmonic suppression inductor, such as inductor  140  (shown in  FIG. 2 ). Rather, a design of device  300  facilitates centering bus  50  with inductor  332  and/or three leg interphase transformer  324  having a smaller inductance than inductor  140 . As such, inductor  332  and/or three leg interphase transformer  324  may be provided with at least one comparatively smaller core and/or winding, which facilitates reducing a weight of device  300 . 
     During operation, a first DC voltage Vp is applied to positive terminal  302  and a second DC voltage Vn is applied to negative terminal  304  via bus  50  such that Vp is more positive than Vn. First switching device  312  is switched by an external control device at a first duty cycle. Second switching device  314  is switched by an external control device at a second duty cycle. In the alternative embodiment, the first duty cycle is approximately 50%, and the second duty cycle is approximately 50%, but a turn-on of second switching device  314  is phase-delayed from a turn-on of first switching device  312  by approximately 180 degrees. Third switching device  316  is switched by an external control device at a third duty cycle. Fourth switching device  318  is switched by an external control device at a fourth duty cycle. In the alternative embodiment, the third duty cycle is approximately 50%, and the fourth duty cycle is approximately 50%, but a turn-on of fourth switching device  318  is phase-delayed from a turn-on of third switching device  316  by approximately 180 degrees. Moreover, the turn-on of third switching device  316  is phase-delayed from the turn-on of first switching device  312  by approximately 120 degrees, and the turn-on of fourth switching device  318  is phase-delayed from the turn-on of second switching device  314  by approximately 120 degrees. Fifth switching device  320  is switched by an external control device at a fifth duty cycle. Sixth switching device  322  is switched by an external control device at a sixth duty cycle. In the alternative embodiment, the fifth duty cycle is approximately 50%, and the sixth duty cycle is approximately 50%, but a turn-on of sixth switching device  322  is phase-delayed from a turn-on of fifth switching device  320  by approximately 180 degrees. Moreover, the turn-on of fifth switching device  320  is phase-delayed from the turn-on of first switching device  312  by approximately 240 degrees, and the turn-on of sixth switching device  322  is phase-delayed from the turn-on of second switching device  314  by approximately 240 degrees. In another embodiment, first, second, third, fourth, fifth, and/or sixth duty cycles are different than approximately 50%, and/or the switching of first, second, third, fourth, fifth, and sixth switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322  are phase-delayed by different amounts as required. In the alternative embodiment, first, second, third, fourth, fifth, and sixth switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322  are switched by one external control device. In another embodiment, first, second, third, fourth, fifth, and sixth switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322  are switched by a plurality of external control devices. 
     As such, first switching device  312  and second switching device  314  switch alternating times, in the same manner as described above in  FIG. 3 . In the same manner, third switching device  316  and fourth switching device  318  switch at alternating times, and fifth switching device  320  and sixth switching device  322  switch at alternating times. Accordingly, bus centering device  300  operates in a similar fashion as bus centering device  200  (shown in  FIG. 3 ), except that device  300  operates with three pairs of alternating switching devices, each pair being interleaved by 120 degrees from the previous pair. As such, a voltage at first common node  366  alternates between the first DC voltage Vp and the second DC voltage Vn in a substantially square wave pattern, and has an average voltage of that is approximately equal to (Vp+Vn)/2. Because an inductor generally enables a DC current to flow through the inductor substantially unimpeded, first winding  326  forces the voltage at first common node  366  to be approximately equal to the voltage at ground  306 . In the same manner, second winding  328  forces the voltage at second common node  368  to be approximately equal to the voltage at ground  306 , and third winding  330  forces the voltage at third common node  370  to be approximately equal to the voltage at ground  306 . As such, bus centering device  300  substantially centers Vp and Vn, and bus  50 , about ground  306 . More specifically, first DC voltage Vp is forced to substantially (Vp+Vn)/2 and the second DC voltage Vn is forced to substantially −(Vp+Vn)/2 with respect to ground  306 . 
     As a result of the interleaving of the first, second, third, fourth, fifth, and sixth duty cycles of switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322 , a ripple current is created at the common endpoint of first, second, and third winding second terminals  336 ,  340 , and  344 . This ripple current has a frequency that is substantially three times the switching frequency of bus centering device  300 . More specifically, the ripple current has a frequency that is substantially three times the switching frequency of first, second, third, fourth, fifth, and sixth switching devices  312 ,  314 ,  316 ,  318 ,  320 , and  322 . Moreover, as a result of the above described interleaving of the duty cycles, the ripple current has an amplitude that is substantially one third of the ripple current amplitude found in bus centering device  200 . As such, one of ordinary skill in the art will appreciate that bus centering device  300  facilitates use with higher power distribution systems. One of ordinary skill in the art will also appreciate that bus centering device  300  may be extended to include additional switching devices and/or transformer windings to accommodate higher electrical distribution system power levels. 
     The above-described embodiments facilitate providing an efficient and cost-effective method and apparatus for electrical bus centering. The described embodiments facilitate providing a lighter and more efficient device for centering an electrical power distribution system about ground. The bus centering device facilitates reducing third harmonic currents without using a third harmonic suppression inductor. The above described embodiments also facilitate enabling multiple power sources to be coupled in parallel. Moreover, the device facilitates reducing the magnitude of ripple currents within a power distribution system. 
     Exemplary embodiments of a method and apparatus for electrical bus centering are described above in detail. The method and apparatus are not limited to the specific embodiments described herein, but rather, components of the apparatus and/or steps of the method may be utilized independently and separately from other components and/or steps described herein. For example, the bus centering device may also be used in combination with other measuring systems and methods, and is not limited to practice with only aircraft power distribution systems as described herein. Rather, the exemplary embodiment can be implemented and utilized in connection with many other power system applications. 
     Although specific features of various embodiments of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.