Abstract:
A dual-input 18-pulse autotransformer rectifier unit for more electric aircraft AC-DC converter uses an autotransformer with a nine-phase output to condition AC power prior to DC rectifying the AC power.

Description:
TECHNICAL FIELD 
     The present disclosure is related generally to aircraft electric power systems, and particularly to a light weight dual-input nine-phase eighteen-pulse Autotransformer Rectifier Unit for a More Electric Aircraft AC-DC Converter. 
     BACKGROUND OF THE INVENTION 
     Modern aircraft include generators that generate power during flight and provide the generated power to onboard aircraft electric power systems. The generators utilize rotation of the aircraft engine to generate AC power using known power generation techniques. Power generated in this manner is typically 230V 400 Hz AC power. While the aircraft is on ground, aircraft engines can be turned off, the onboard generator ceases generating power, and the onboard electric system instead receives AC power from a ground cart. Power provided from the ground cart is typically 115V 400 Hz AC power. 
     While the power sources provide AC power, aircraft components often require DC power instead of AC power. AC-DC power conversion may be accomplished with a plurality of diode pairs, where each pair is connected to a different phase of the AC input, to provide a rectified DC output. However, this type of AC-DC conversion leads to substantial current harmonics that pollute the electric power generation and distribution system. To reduce current harmonics, multi-phase autotransformers are employed to increase the number of AC phases supplied to the rectifier unit. For example, in an 18-pulse passive AC-DC converter the autotransformer is used to transform the three-phase AC input, whose phases are spaced at 120°, into a system with nine phases spaced at 40°. This has the effect of reducing the harmonics associated with the AC-DC conversion. 
     SUMMARY OF THE INVENTION 
     Disclosed is a passive AC-DC converter having an autotransformer. The autotransformer has a plurality of first high voltage AC inputs, a plurality of second low voltage AC inputs, a winding topology having a plurality of windings corresponding to each of multiple phases, the plurality of windings configured such that the autotransformer generates an 18-pulse AC input current waveform, and a set of autotransformer outputs. The passive AC-DC converter further includes a bridge rectifier connected to the set of autotransformer outputs and a DC output from the bridge rectifier. 
     These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a nine-phase Autotransformer based AC-DC Converter in an aircraft electric power system context. 
         FIG. 2  illustrates dual-input nine-phase 40° phase shift autotransformer connected to a nine-phase rectifier unit. 
         FIG. 2(   b ) is an 18-step waveform closely approximating a desired sine-wave. 
         FIG. 3  illustrates autotransformer core and winding configuration. 
         FIG. 4A  illustrates 21-vector diagram representing the physical windings of a nine-phase autotransformer. 
         FIG. 4B  illustrates the 21-vector diagram of  FIG. 4A  with the added illustration of resultant vectors. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a power conversion system  110  for use in an aircraft electric power system. The power conversion system  110  includes a passive AC-DC converter  120  that accepts either a three-phase 230V AC input  140  from a 230V aircraft power source or a three-phase 115V AC input  150  from a 115V ground-cart power source and provides power to a DC load  165  via a DC+ connection  180 , a DC− connection  182 , and a 0V connection  181 . The passive AC-DC converter  120  includes an autotransformer  160  that accepts three phase AC inputs  162 ,  164  and provides a nine phase AC output  163  to a rectifier  170 . The DC outputs  180 ,  181 ,  182  are connected to the rectifier  170 . The particular power source  140 ,  150  connected to the passive AC-DC converter  120  is controlled by a set of break-before-make switches  130  on each input line  162 ,  164 . Each input  162 ,  164  is also filtered by a corresponding AC filter  190 ,  192 . 
       FIG. 2  illustrates an example passive AC-DC converter  200  that can be used as the AC-DC converter  120  of  FIG. 1 . Within the passive AC-DC converter  200 , an autotransformer  260  accepts either the three-phase voltage input  262 , or the three-phase voltage input  264 . The passive AC-DC converter  200  converts either input  262  or  264  to a nine-phase AC output  263 . The nine-phase AC output  263  is rectified to DC voltage by a rectifier  270 . The rectifier  270  provides a DC+ output  280  and a DC− output  282 . The DC+ and DC− outputs  280 ,  282  provide power to a power bus that distributes DC power to onboard aircraft components. The autotransformer  260  nine-phase output currents are square-wave, and the three-phase input current is an 18-step waveform  201  closely approximating a desired sine-wave (illustrated in  FIG. 2(   b ). The illustrated 18-step waveform  201  is presented for explanatory purposes and certain features, steps or other elements of the waveform are increased or decreased for illustrative effect. 
     In order to utilize a single passive AC-DC converter  200  for both the 230V AC power input  262  and the 115V AC power input  264  and maintain a constant DC output voltage, the autotransformer  260  further includes a 115V input to 230V step up when a 115V input  264  is connected. This step up ensures that a constant DC voltage output is provided from the DC outputs  280 ,  282  regardless of whether 115V AC source  264  or 230V AC source  262  is connected. 230V AC voltage is then stepped down by a ratio of γ, where 1≧γ≧0.5. DC output voltage  265  is proportional to step down ratio γ, and a designer can select step down ratio γ to meet a particular DC output voltage requirement. Thus, the autotransformer  260  nine-phase output  263  is γ×230V for either a 230V AC input  140  or a 115V AC input  150 . Since the nine-phase output  163 ,  263  is independent of the particular selected AC input  140 ,  150  (illustrated in  FIG. 1 ), the rectifier output  280  and  282  is also independent of the particular selected inputs  140 ,  150 . Furthermore, the kVA rating of the autotransformer  160 ,  200  is 
                 46.7   ⁢   %     γ     ,         
where γ is the autotransformer step down ratio and where 1≧γ≧0.5.
 
     Further referring to  FIG. 2 , the autotransformer AC output current waveform at pin  1  is pulsed, with high harmonic current contents. The output at pin  2  is also pulsed with a 40° phase shift relative to the output of pin  1 . The same applies to outputs of pins  3 ,  4 ,  5 ,  6 ,  7 ,  8 , and  9 . The autotransformer  260  synthesizes the nine output pulsed current waveforms into an 18-step current waveform that closely approximates a desired sinewave current at 230V AC input  262  A, B and C, and similarly at 115V AC input  264   a, b  and  c . Undesirable harmonic currents at nine-phase outputs  263  are cancelled through the autotransformer  260 . 
       FIG. 3  schematically illustrates the autotransformer  260 . The autotransformer  260  has a three-legged core  310 , with physical windings A 0 -A 6 , B 0 -B 6 , and C 0 -C 6  wound respectively about the three core legs,  320 A,  320 B, and  320 C. The 230V 3-phase AC inputs  262 A, B, C are located on each phase leg, as well as the 115V 3-phase AC inputs  264   a, b, c . Outputs  1 - 9  output nine-phase power from the autotransformer  260 . 
     With continued reference to  FIGS. 2 and 3 ,  FIG. 4A  illustrates a vector diagram  400  of the physical windings A 0 -A 6 , B 0 -B 6 , C 0 -C 6  of an example autotransformer  260  from the example AC-DC converter illustrated in  FIG. 2  with step down ratio γ=0.9.  FIG. 4B  illustrates the vector diagram of  FIG. 4A  with the added illustration of resultant vectors. The arrow length of each winding A 0 -A 6 , B 0 -B 6 , C 0 -C 6  is proportional to the number of winding turns, and the arrow points from the start of the winding to the finish of the same winding. The three 230V AC input voltage vectors are represented by drawing straight lines from A, B and C to triangle center o, to form A-o, B-o, and C-o. The three 115V AC input voltage vectors are represented by drawing straight lines from a, b and c to triangle center o, to form a-o, b-o, and c-o. Nine output voltage vectors are represented by drawing straight lines from points 1 through 9 to triangle center o, to form nine output voltage vectors  1 - o ,  2 - o ,  3 - o ,  4 - o ,  5 - o ,  6 - o ,  7 - o ,  8 - o , and  9 - o . The ratio of the length of vector  1 - o  to the length of vector A- o  is the autotransformer output voltage step down ratio γ from 230V, where γ=0.9. The ratio of the length of vector A- o  to the length of vector a- o  is 2, the fixed voltage ratio of 230V and 115V AC inputs. The phase angle from  1 - o  to  2 - o  is 40°, and the angle from  1 - o  to  9 - o  is also 40°. The phase angle from  4 - o  to  3 - o  is 40°, and the phase angle from  4 - o  to  5 - o  is 40°. Similarly, the phase angle from  7 - o  to  6 - o  is 40°, and the phase angle from  7 - o  to  8 - o  is 40°. As illustrated, all winding segments A 0 -A 6 , B 0 -B 6 , and C 0 -C 6  drawn as parallel to each other are on a shared phase leg of the autotransformer core  310  in  FIG. 3 . The phase legs  320 A, B, C of the three legged autotransformer core  310  receive windings A 0 -A 6 , B 0 -B 6 , and C 0 -C 6  respectively. 
     The nine AC power output connections  1 ,  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 , and  9  are illustrated as solid circles, and provide a nine-phase output to the rectifier  270 , illustrated in  FIG. 2 . The 230V AC inputs  262  illustrated in  FIG. 2  are connected to the autotransformer  260  at connection points A, B, and C. The 115V AC inputs  264  of  FIG. 2  are connected to the autotransformer  260  at connection points a, b, and c. Connection points a, b, and c connect the 115V input to the autotransformer  260 , such that the autotransformer  260  steps up the AC input  264  to an equivalent 230V AC before the desired step down, γ, resulting in a consistent nine-phase AC output voltage regardless of which of the two inputs  262 ,  264  is connected to its respective 230V AC or 115V AC source. 
     In addition to the winding polarity of the winding segments A 0 -A 6 , B 0 -B 6 , C 0 -C 6 , the number of winding turns of A 0 -A 6 , B 0 -B 6  and C 0 -C 6  is normalized to that of A 0 , B 0 , C 0 . It should be understood that the turns ratio between the various winding segments A 0 -A 6 , B 0 -B 6 , C 0 -C 6  is determined by the step down ratio γ. In the illustrated example of  FIGS. 4A and 4B , the autotransformer  260  steps down the 230V AC input, such that the AC output of the autotransformer  260  is γ=0.9 of the 230V AC input. To achieve this step down, the normalized number of turns are: A 0  is 1 turn, A 1  is 1.690 turns, A 2  is 4.668 turns, A 3  is 3.926 turns, A 4  is 3.433 turns, A 5  is 3.572 turns, and A 6  is 3.177 turns. The number of turns on each of the winding segments B 0 -B 6  and C 0 -C 6  are correspondingly identical to those of A 0 -A 6 . 
     While the particular example winding segments of  FIG. 4  are illustrated with a step down ratio of 0.9 (90%) (the nine AC output voltages are 90% of the AC input voltage at A, B and C), a worker of skill in the art would be able to adapt the vector diagram disclosed above to achieve a different step down ratio using the same principles as 0.9 step down example. By way of example, a person of skill in the art could utilize the above teachings to adjust the turns ratios to achieve varied step down ratios to achieve 1≧γ≧0.5 for step down application. 
     Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.