Abstract:
Systems, methods and devices for power generation systems are described. In particular, embodiments of the invention relate to the architecture of power conditioning systems for use with fuel cells and methods used therein. More particularly, embodiments of the present invention relate to methods and systems usable to reduce ripple currents in fuel cells.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to the field of power conversion systems. More particularly, embodiments of the present invention employ various architectures of power conversion systems and various methods to convert power. Still more particularly, embodiments of the present invention relate to power conversion systems for power generated by fuel cells. 
     Fuel cell systems are currently being used in a variety of power supply applications. If an application requires a particular voltage or current, fuel cells are combined into units called “stacks” in which the fuel cells are electrically connected in series to meet the requirements. A generalization of a fuel cell stack is the so-called fuel cell segment or column, which can contain one or more fuel cell stacks. In certain applications, many fuel cell segments may be required for higher power due to the limitation of current carrying capability of the fuel cells. For reliable operation, individual stack current can be controlled, to improve fuel utilization. 
     Fuel cells generate power that is converted in a fuel cell power conversion system, also known as a power conditioning system. A power conversion system is a system that alters the characteristics of power produced by a source in some way. For the case of fuel cells, which generate DC (direct current) power, this can mean the conversion of the DC power to different voltage and/or current levels, the conversion to AC (alternating current) power with a particular RMS (root mean squared) voltage, the generation of three-phase AC power, or all of the above. Typically, a change in the voltage level of a DC source can be accomplished using a DC/DC (direct current/direct current) converter, whereas the change from DC to AC is accomplished using a DC/AC (direct current/alternating current) converter or inverter. 
     The expected increase in fuel cell use in the future, in terms of both volume and number of applications, requires that the design and construction of fuel cell power systems be made as efficient as possible. To facilitate the design and manufacturing of fuel cell power control systems, it is advantageous to allow design flexibility while minimizing the number of components required to produce such an architecture. 
     In particular, it is of interest in power generation systems using fuel cells to reduce the effect of ripple currents through the fuel cells. Ripple currents are AC (alternating current) components of the total current function of the fuel cell. Ripple currents can be produce by a variety of components in a fuel cell system. Ripple currents can cause inefficient power generation by fuel cells and can damage a fuel cell. Thus, there is a need to reduce the effect of ripple currents in fuel cell systems. 
     SUMMARY OF THE INVENTION 
     As discussed hereinafter, one embodiment of the invention employs a power conversion system that is constructed with a two-bus approach and center-tapped neutral line. Such an embodiment is used to facilitate a modular approach and control of power drawn from the fuel cell stack by the power conversion system. Furthermore, it is advantageous in some embodiments to have a fuel cell power conversion system where the number of individually wired stack columns in a fuel cell hot box is evenly divisible by 3 and 2 (meaning evenly divisible by 6) in order to achieve an optimum power electronics architecture. 
     One exemplary embodiment relates to a fuel cell circuit, comprising: a first fuel cell segment having positive and negative terminals; a second fuel cell segment having positive and negative terminals; a neutral line; and wherein the negative terminal of the first fuel cell segment is electrically connected to both the positive terminal of the second fuel cell segment and the neutral line. 
     Another embodiment relates to a power conversion module, comprising: a plurality of DC/DC converter branches, each branch comprising a DC/DC converter; and a plurality of series connections comprising two fuel cell segments; wherein each DC/DC converter branch is connected to at most one of the series connections comprising two fuel cell segments; and wherein the total number of DC/DC converter branches is an integer multiple of three. 
     Yet another embodiment relates to a method for converting DC to AC, comprising accepting a first output of a first series connection comprising two fuel cell segments at an input of a first DC/DC converter; accepting a second output of a first series connection comprising two fuel cell segments at an input of a second DC/DC converter; accepting a first output of the first DC/DC converter at a first input of a first inverter; accepting a first output of the second DC/DC converter at a second input of the first inverter; and generating a first AC output from the first and second inputs of the first inverter relative to a neutral line connected to a reference potential. 
     Yet another embodiment relates to a method for reducing a ripple current in a fuel cell system, comprising supplying the positive output of a fuel cell segment to a first DC/DC converter; supplying the negative output of the fuel cell segment to a second DC/DC converter; wherein an output of the first DC/DC converter and an output of the second DC/DC converter are electrically connected; supplying the positive output of the fuel cell segment to a third DC/DC converter; supplying the negative output of the fuel cell segment to a fourth DC/DC converter; and wherein an output of the third DC/DC converter and an output of the fourth DC/DC converter are electrically connected. 
     Still another embodiment relates to an architecture for a fuel cell power conditioning system, comprising: a fuel cell segment arranged to have a positive terminal and a negative terminal; a first DC/DC converter, an input of which is connected to the positive terminal of the fuel cell segment; a second DC/DC converter, an input of which is connected to the negative terminal of the fuel cell segment; wherein an output of the first DC/DC converter is electrically connected with an output of the second DC/DC converter; and further comprising a third DC/DC converter, an input of which is connected to the positive terminal of the fuel cell segment; a fourth DC/DC converter, an input of which is connected to the negative terminal of the fuel cell segment; wherein an output of the third DC/DC converter is electrically connected with an output of the fourth DC/DC converter; further comprising a DC/AC converter comprising a first input electrically connected to the output of the first DC/DC converter and a second input electrically connected to the output of the third DC/DC converter; wherein the DC/AC converter produces a three-phase current output from the first and second inputs. 
     Another embodiment of the invention relates to a power generation system, comprising first, second and third DC/AC converters; first through ninth DC/DC Converters; and first through third fuel cell segments; wherein an input to the first DC/AC converter is electrically connected to an output of the first DC/DC converter, an output of the second DC/DC converter and an output of the third DC/DC converter; wherein the first fuel cell segment comprises positive terminal, a negative terminal and a middle node, and an input to the first DC/DC converter is connected to the positive terminal of the first fuel cell segment; wherein the second fuel cell segment comprises positive terminal, a negative terminal and a middle node, and an input to the second DC/DC converter is electrically connected to the middle node of the second fuel cell segment; wherein the third fuel cell segment comprises positive terminal, a negative terminal and a middle node, and an input to the third DC/DC converter is electrically connected to the positive terminal of the third fuel cell segment; wherein an input to the second DC/AC converter is electrically connected to an output of the fourth DC/DC converter, an output of the fifth DC/DC converter and an output of the sixth DC/DC converter; wherein an input to the third DC/AC converter is electrically connected to an output of the seventh DC/DC converter, an output of the eighth DC/DC converter and an output of the ninth DC/DC converter; wherein an input to the fourth DC/DC converter is electrically connected to the middle node of the first fuel cell segment; wherein an input to the fifth DC/DC converter is electrically connected to the positive terminal of the second fuel cell segment; wherein an input to the sixth DC/DC converter is electrically connected to the positive terminal of the third fuel cell segment; wherein an input to the seventh DC/DC converter is electrically connected to the middle node of the first fuel cell segment; wherein an input to the eighth DC/DC converter is electrically connected to the middle node of the second fuel cell segment; and wherein an input to the ninth DC/DC converter is electrically connected to the positive terminal of the third fuel cell segment. 
     Yet another embodiment of the invention relates to a system comprising at least one DC/DC converter and at least one fuel cell, wherein the system is configured to operate such that the fuel cell ripple current has an RMS amplitude of less than 5% of the DC fuel cell current. 
     A further embodiment of the invention relates to a system comprising at least one DC/DC converter and at least one fuel cell, wherein the system is configured to operate such that the fuel cell ripple current has an RMS amplitude of less than 1% of the DC fuel cell current. 
     Another embodiment of the invention relates to a system comprising at least one DC/DC converter and at least one fuel cell, wherein the system is configured to operate such that the fuel cell ripple current has produced by the DC/DC converter is not observable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block cell circuit diagram illustrating a fuel cell power conversion system architecture embodiment. 
         FIG. 2  is a block cell circuit diagram illustrating a second fuel cell power conversion system architecture embodiment. 
         FIG. 3  is a block cell circuit diagram illustrating a third fuel cell power conversion system architecture embodiment. 
         FIG. 4  shows instantaneous fuel cell current as a function of time. 
         FIG. 5  shows is a simulated graph of instantaneous power on the +ve and −ve busses. 
         FIG. 6  illustrates a proposed system architecture for part of a fuel cell power conditioning system. 
         FIG. 7  shows is a simulated graph of instantaneous fuel cell current as a function of time. 
         FIG. 8  illustrates a proposed system architecture for part of a fuel cell power conditioning system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a fuel cell power conversion system architecture embodiment  100  with two parallel busses is shown. The architecture  100  is illustrated from fuel cells to three-phase power output, and may be referred to as a type of power conversion module, where “module” is a general term referring to a group of system components. The system  100  comprises two fuel cell segments  102  and  104  containing one or more fuel cells and having respective connections  106  and  108  to DC/DC converters  110  and  112 . DC/DC converters  110  and  112  are similar converters. They are usually boost converters. DC/DC converters  110  and  112  have respective output busses  114  and  116 , referred to hereinafter as the +ve and −ve busses, respectively. Each circuit branch having a DC/DC converter and extending, for example, from connection  106  through DC/DC converter  110  and ending with bus  114  may be referred to as a DC/DC converter branch. If the fuel cell segments are included, the terminology “stack column” or “segment column” may be used. 
     The fuel cell segments  102  and  104  are connected in series at node  132 , which is also connected to neutral line  122 . Node  132  highlights that possibility that fuel cells, stacks and segments of the herein described embodiments can be individually wired, which allows a system designer to advantageously connected mid-cell, mid-stack and mid-segment potentials to a system bus. Here, “individually wired” means that the fuel cell or stack end plates which normally comprise the terminals of a fuel cell or stack are not electrically connected by face to face contact with other end plates, but are rather electrically connected by a conductor, such as a wire. 
     The output of DC/DC converter  110  is connected to +ve bus  114 , which is connected over a voltage drop of +ve by capacitor  118  to node  134 , which is connected to and at the same potential as neutral line  122 . The output of DC/DC converter  112  is similarly connected to −ve bus  116 , which is connected over a voltage drop of −ve by capacitor  120  to node  134 . Capacitors  118  and  120  also serve to smooth AC components of the signals carried by +ve bus  114  and −ve bus  116 , known as “ripple currents”. 
     The +ve bus  114  and −ve bus  116  serve as inputs to DC/AC converter or inverter  124 , which generates three-phase outputs  126 ,  128  and  130 . The three-phase outputs have a desired RMS (root mean-squared) potential over neutral line  122  and 120 degree phase offsets relative to one another. In the embodiment  100 , all three phases are generated by DC/AC converter or inverter  124 , although this is not strictly necessary. Neutral  122  is already efficiently provided by the dual bus architecture. 
     The split fuel cell connection as shown with fuel cell segments  102  and  104  provides high efficiency for a power conversion system with its dual bus architecture. The split bus provides a neutral and facilitates easy paralleling of converters. The efficiency of the architecture derives from the fact that two fuel cell segments  102  and  104  are used, which facilitates the construction of counterpoised +ve and −ve busses  114  and  116  respectively and the easy generation of a neutral line  122  between the two. It will be apparent that the dual bus architecture may be multiplied and extended to systems involving integral multiples of fuel cell stack systems arranged according to the configuration of  FIG. 1 . 
     Referring now to  FIG. 2 , a second embodiment in the form of a fuel cell power conversion system architecture  200  is shown. The architecture  200  may also be referred to as a power conversion module. The embodiment of  FIG. 2  comprises three pairs of fuel cell segments. Fuel cell segments  202  and  204  are paired in an arrangement similar to that described with reference to  FIG. 1 . Fuel cell segments  206  and  208 , as well as fuel cell segments  210  and  212  are also similarly paired. As an example, fuel cell segment  202  is connected with its negative terminal to node  226 , while fuel cell segment  204  is connected with its positive terminal to node  226 . Fuel cell segments  206  and  210  are also connected with their negative terminals to nodes  228  and  230  respectively. Nodes  228  and  230  are connected to nodes  226  and  254  as well as neutral line  232 , and are at the same electrical potential. Fuel cell segments  208  and  212  are connected with their positive terminals to nodes  228  and  230  respectively. 
     Each fuel cell segment  202 ,  206  and  210  has an output bus  214 ,  216  or  218  respectively. The output busses feed fuel cell segment output to a DC/DC converter  234 ,  236  or  238 , respectively. The DC/DC converters  234 ,  236  or  238  are generally boost converters and serve to bring the +ve output bus  264  to a voltage level with respect to neutral line  232  appropriate for the power system application. The negative terminals of fuel cell segments  204 ,  208  and  212  are also connected to output busses  220 ,  222  and  224  respectively, leading to DC/DC converters  244 ,  242  and  240  respectively. These DC/DC converters serve the same function as DC/DC converters  234 ,  236  and  238 , albeit with an opposite polarity, bringing the voltage on −ve bus  266  to a suitably negative level with respect to neutral line  232 . 
     The +ve bus  264  is joined at node  246 , placing the DC/DC converters in a parallel circuit architecture. A voltage drop occurs at capacitor  250  to neutral line  232  at node  254 . Capacitor  252  also serves to smooth out ripple currents produced by DC/DC converters  234 ,  236  and  238 . Similarly, −ve bus  266  is joined at node  248 , placing DC/DC converters  240 ,  242  and  244  in a parallel circuit architecture. A voltage increase occurs from node  248  over capacitor  252  to neutral line  232  at node  254 . Capacitor  252  also serves to smooth out ripple currents from DC/DC converters  240 ,  242  and  244 . 
     The +ve bus  264  and −ve bus  266  are connected as inputs to AC/DC converter or inverter  256 . AC/DC converter or inverter  256  takes the two inputs with a DC voltage difference of +2ve and creates a three phase output made up of phase A  258 , phase B  260  and phase C  262 . Each of the three phases is an AC signal with a 120 degree phase offset to each other phase and an RMS voltage with respect to neutral line  232  that is appropriate for the power system application. 
     The fuel cell power conversion system architecture  200  presents advantages in terms of modular design of the fuel system architecture. The architecture has adopted the dual bus architecture of the system of  FIG. 1 , with a neutral line derived from the middle node of the two fuel cell segment components. This architecture is multiplied by three in parallel for the embodiment of  FIG. 2 . That is, there are six parallel branches for DC/DC conversion, each containing a fuel cell segment pair in series with a neutral derived from the middle point of the pair. Thus, the fuel cell power conversion system of the embodiment of  FIG. 2  are designed using integral multiples of six fuel cell segments per AC/DC converter or inverter. An integral multiple of 3 fuel cell segment pairs will make the power distribution along each phase easier and with minimum number of DC/DC converters.  FIG. 3  shows a fuel cell power conversion system architecture  300  with output transformers for a four-wire (with neutral) system. The architecture shows a fuel cell power conversion system implementation with a single DC bus approach. 
     The system  300  comprises three parallel circuit branches  302 ,  304  and  306 , each having a fuel cell segment pair  308  and  310 ,  312  and  314 , or  316  and  318 , respectively. Each fuel cell segment pair is connected in series so that their respective voltage increases are summed. 
     The outputs of fuel cell segments  308 ,  312  and  316  are connected to busses  320 ,  322  and  324  respectively, which lead to the inputs of DC/DC converters  326 ,  328  and  330 , respectively. The negative terminal of the series connected fuel cell segments  308 ,  312  and  316  are connected to reference busses  332 ,  334  and  336  respectively, which are each connected to a reference potential such as ground. It is clear that a reference potential can be any relatively steady or convenient potential relative to non-reference lines. 
     The outputs of each DC/DC converter  326 ,  328  and  330  are each connected to +ve busses  338 ,  340  and  342  respectively. The +ve busses  338 ,  340  or  342  are connected to a capacitors  344 ,  346  and  348 , respectively, which are in turn connected to reference busses  332 ,  334  and  336  respectively, to produce a voltage drop of +ve over each capacitor. The capacitors  344 ,  346  and  348  also serve to smooth ripple currents produced by DC/DC converters  326 ,  328  and  330  respectively. 
     The +ve busses  338 ,  340  and  342  and reference busses  332 ,  334  and  336  serve as inputs to DC/AC converters or inverters  350 ,  352  and  354  respectively. In contrast to the embodiments of  FIGS. 1 and 2 , each phase of the three-phase current output has its own DC/AC converter or inverter. The output of each DC/AC converter  350 ,  352  and  354  produces a respective phase signal  356 ,  358  or  360  respectively and a neutral line  362 ,  364  or  366  respectively. The use of three separate input branches for DC/AC inverter is particularly advantageous for the development of three-phase AC current, as each phase has its own neutral, positive and negative busses, that is, each of the three phases runs from its own DC bus. 
     The three phase outputs  356 ,  358  or  360  are transformed to an application RMS voltage by transformers  368 ,  370  and  372  respectively. The neutral line outputs from the transformers are connected at node  382  to generate a single neutral line. 
     The Figures also show in exemplary fashion methods that may be used for power generation and conditioning. For example,  FIG. 1  demonstrates a method that may be used generally to convert the output of a direct current source to alternating current. As a first step, a first output  106  of a first series connection of two fuel cell segments (segments  102  and  104 ) is accepted at an input of a first DC/DC converter  110 . A second output  108  of a first series connection of two fuel cell segments (segments  102  and  104 ) is accepted at an input of a second DC/DC converter  112 . A first output  114  of the first DC/DC converter  110  is accepted at a first input  114  of a first inverter (i.e., DC/AC converter)  124 . A first output  116  of the second DC/DC converter  112  is accepted at a second input  116  of the first inverter  124 . A first AC output  126  is generated from the first and second inputs  114  and  116  of the first inverter  124  relative to a neutral line  122  connected to a reference potential. 
     Such a method may be expanded as shown in  FIG. 2 , wherein three fuel cell segment pairs are shown (including segments  202  and  204 ;  206  and  208 ; and  210  and  212  respectively), the outputs of each of which are accepted at the inputs of two DC/DC converters ( 234  and  244 ;  236  and  242  and  238  and  240  respectively) per fuel cell segment pair. The outputs of the DC/DC converters are electrically connected. The outputs of upper DC/DC converters  234 ,  236  and  238  are accepted at a first input of inverter  256 , while the outputs of lower DC/DC converters  240 ,  242  and  244  are accepted at a second input of inverter  256 . From these inputs, inverter  256  generates three phase outputs  258 ,  260  and  262  relative to neutral line  232 . 
     Such a method may be also be expanded as shown in  FIG. 3  to encompass multiple fuel cell stack segment pairs with corresponding DC/DC converters using a single inverter. Additionally, such methods may be used in the system of  FIG. 3 , wherein the first outputs  320 ,  322  and  324  respectively of multiple fuel cell stack segments (fuel cells  308  and  310 ;  312  and  314 ; and  316  and  318  respectively) are accepted at the inputs of DC/DC converters  326 ,  328  and  330  respectively. The outputs of the DC/DC converters and the second outputs of the fuel cell stack segments are in turn accepted at respective inverters (DC/AC converters)  350 ,  352  and  354 . The inverters  350 ,  352  and  354  produce three phases  356 ,  358  and  360 . 
     Another embodiment of the invention is explained in reference to  FIG. 4 . DC/DC converters, although their purpose is to change the voltage with respect to ground of DC power sources, can add non-DC components to the DC source. In addition, inverters such as the inverter  124  shown in  FIG. 1  can add their own higher-frequency components to AC output. Such non-DC components are known in the art as “ripple currents”. Ripple currents are detrimental both to the fuel cell stack, where they increase the RMS current draw of a fuel cell stack and lower fuel cell efficiency, as well as to the ultimate application, where the presence of ripple currents can decrease the efficiency of power usage. The ripple current through the fuel cell stack should be reduced as much as possible, as it decreases the fuel utilization and can starve the fuel cells for want of fuel. 
       FIG. 4  shows a simulated example of ripple currents as they would be produced through an exemplary system as shown in  FIG. 1 .  FIG. 4  is a graph  400  that shows fuel cell current as a function of time. Graph  400  has a Y-axis  402  which represents fuel cell current in dimensionless units, an X-axis  404  which represents the passage of time in seconds, and a current versus time function  406 . The current versus time function  406  has an oscillating component and a constant (DC) component as indicated by  408 , around which the oscillating component oscillates. The oscillating component can be referred to as the ripple current. As seen from  FIG. 4 , the amplitude ripple current oscillation is approximately 30% of the value of the constant component. 
     Again with respect to the architecture shown in  FIG. 1 ,  FIG. 5  shows the simulated instantaneous power carried by the positive and negative DC busses  114  and  116  of  FIG. 1 .  FIG. 5  is a graph  500 , which in turn has a Y-axis  502  representing instantaneous power in dimensionless units and an X-axis  504  representing the passage of time in seconds. The graph  500  in  FIG. 5  also has a negative DC Bus power function  506 , a positive DC bus power function  508 , and an average power function  510 , which represents the average of the instantaneous power functions  506  and  508 . It can be seen that the power functions  506  and  508  are approximately harmonic functions that are 180 degrees out of phase for 3-phase balanced power, resulting in an average function  510  that is approximately zero. 
       FIG. 6  illustrates a partial power conditioning system architecture  600  that reduces ripple currents.  FIG. 6  has a fuel cell or fuel cell stacks  602  and  604 , connected in series over nodes  606  and  607 . As mentioned previously, the various combinations of fuel cells and fuel cell stacks can be referred to generically as fuel cell segments. The positive terminal output of fuel cell or fuel cell stack  602  is connected to the input of DC/DC converter  608  as well as DC/DC converter  612 . The negative terminal output of fuel cell or fuel cell stack  604  is connected to DC/DC converter  610 , as well as DC/DC converter  614 . Node  606  is connected to the inputs of DC/DC converters  608  and  612  and node  607  is connected to the inputs of DC/DC converters  614  and  610 . Outputs of DC/DC converters  608  and  614  are electrically connected to +ve bus  620  at node  616 . Outputs of DC/DC converters  612  and  610  are electrically connected to −ve bus  622  at node  618 . Second outputs of DC/DC converters  608 ,  612 ,  614  and  610  are connected to neutral line  624  at node  623 . Bus  620  is connected over capacitor  624  to neutral line  632 , which itself is connected over capacitor  626  to bus  622 . 
     The electrical connection of the outputs of DC/DC converters  608  and  614  can be accomplished by, for example, providing that both DC/DC converters convert their respective input voltage to the same output voltage (i-ve). Similarly, the connection of the outputs of DC/DC converters  610  and  612  can be accomplished by, for example, providing that both DC/DC converters convert their respective input voltage to the same output voltage (−ve). In other words, the DC/DC converters have different conversion ratios. This is preferably done using isolated DC/DC converters. 
     Since the power ripple currents generated by DC/DC converters  608  and  614  are approximately 180 degrees out of phase as shown in  FIG. 5 , a substantial cancellation of these ripple currents can be effected by using the architecture of  FIG. 6 . A similar cancellation can be effected by connecting the outputs of DC/DC converters  610  and  612 . 
       FIG. 7  shows the simulated effect on fuel cell current for fuel cells used in the architecture shown in  FIG. 6 .  FIG. 7  is a graph  700  having a Y-axis  702  that represents fuel cell current in dimensionless units, an X-axis  704  representing the passage of time in seconds, and a fuel cell current function  704 . It can be seen that the effect of the ripple current, previously shown in  FIG. 4  to be almost 30% of the constant component of the current function, is largely eliminated. As shown by the simulated results in  FIG. 4 , the ripple current cancellation techniques as shown herein can result in a ripple current RMS (root mean squared) amplitude that is less than 5% of the constant average (DC) fuel cell current. As shown in  FIG. 7 , ripple current cancellation techniques as shown herein can result in a ripple current RMS amplitude that is less than 1% of the constant average (DC) fuel cell current. 
       FIG. 8  shows a partial power conditioning system architecture according to another embodiment of the invention.  FIG. 8  shows three fuel cell or fuel cell stack pairs  802  and  804 ,  806  and  808  and  810  and  812 . Each of these pairs is connected in series over a respective middle node  814 ,  816  or  818 . The positive output of fuel cell or fuel cell stack  802  is connected to the input of DC/DC converter  820 . The middle node  814  is connected to the inputs of DC/DC converters  828  and  830 . The positive output of fuel cell or fuel cell stack  806  is connected to the output of DC/DC converter  826 , while the middle node  816  is connected to the inputs of DC/DC converters  828  and  830 . The positive output of fuel cell or fuel cell stack  810  is connected to the inputs of DC/DC converters  832 ,  834  and  836 . 
     The +ve bus  838  is connected to the output of DC/DC converters  820 ,  830  and  834  at node  839 . Similarly, the outputs of DC/DC converters  822 ,  826  and  836  are connected to the +ve bus  842  at node  844 , and the outputs of DC/DC converters  824 ,  828  and  832  are connect to +ve bus  846  at node  848 . These connections allow the substantial cancellation of ripple currents generated by these converters. 
     The +ve busses  838 ,  842  and  846  are each respectively coupled to a neutral bus  852 ,  856  or  860  over capacitors  850 ,  854  and  858 , respectively. Each +ve bus  838 ,  842  and  846  and neutral lines  852 ,  856  and  860  serve as inputs to DC/AC converters or inverters  862 ,  864  or  866 . These inverters generate three phases on lines  868 ,  876  and  884 , relative to neutral lines  870 ,  878  and  886  respectively. Transformers  872 ,  880  and  888  convert the AC phases to appropriate voltage levels for the application in question. The system thus produces the three phase voltage on lines  872 ,  882  and  888  relative to neutral line  882 , while significantly reducing deleterious ripple currents. 
     The foregoing embodiments of the invention are intended to be illustrative in nature and not limiting. It will be clear to a person of skill in the art that various modifications may be made without parting from the spirit and scope of the present invention, which should be defined only by the following claims.