Patent Publication Number: US-2023155412-A1

Title: System and method for intelligent power converter control of fuel cells and other auxiliary power sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/264,147, filed on Nov. 16, 2021. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to power converters, also known as uninterruptible power supplies (“UPSs”), and more particularly to systems and implementations of power converters which enable a power converter to communicate with, control or manage one or more auxiliary power sources, and/or to coordinate transmitting excess power back onto a power grid or back to an auxiliary power source, and/or to handle various transient step up and step down load conditions, as well as fuel cell startup and shutdown scenarios, and generally to optimize overall use and/or storage of available energy accessible by the power converter. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
     Power converters, often referred to as “uninterruptible power supplies” (“UPSs”), are now widely used as auxiliary power sources to support critical infrastructure loads in a wide variety of settings including, but not limited to, hospital and health care entities, financial institutions, manufacturing entities, governmental entities, and data center applications, just to name a few important applications. Power converters have become critically important in ensuring that momentary or short-term power outages occurring on a local power grid will not cause a shutdown of important electric power dependent equipment being powered from the power grid. The assignee of the present disclosure is a leader in state-of-the-art power converter systems which are presently being used widely throughout the world in a wide range of industries to support wide ranging critical applications. 
     In present day implementations, typically one or more auxiliary power sources are also used in connection with a power converter to provide auxiliary power to the power converter, or directly to a load, in the event of a power outage situation on an AC power grid. This is to address the possibility in which a local or widespread AC power grid outage extends beyond the run time capability, or the load supporting capability, of the power converter. In this instance such auxiliary power sources will take over supplying power to a load if/when the load requirements exceed those which the power converter is able to meet. Such additional auxiliary power sources may include, without limitation, diesel powered electric generators, long term batteries such as Lithium Ion batteries (“LIBs”) and others, and/or fuel cells such as solid oxide fuel cells (“SOFCs”) and proton exchange membrane fuel cells (“PEMFCs”, also known as polymer electrolyte membrane fuel cells). Still further, the use of intermediate term or short term rechargeable external supplemental battery systems, which are often of lithium ion construction and sometimes referred to as “short term LIBs,” are now being used to provide auxiliary power directly to a power converter, to further augment the output capacity of a power converter or to charge the internal batteries of the power converter. 
     While use of power converters in a great variety of applications has proliferated in recent years, little has been done to make use of a power converter in monitoring, communicating with and managing the various auxiliary power sources that are often available to provide power to a load that the power converter is associated with. More specifically, presently available commercial power backup and management systems that include one or more power converters have not addressed how to harness the capabilities of present-day power converters and the other available, longer duration auxiliary power sources in intelligent ways. For example, presently available power backup systems have not made use of the intelligent control capabilities inherent with present day power converters to monitor and manage the application of auxiliary power to a load, or to use the power converter to help monitor and manage the application of excess power available on a power grid to help charge the internal batteries of a power converter, or to manage excess power available on a short term basis from an auxiliary fuel cell, or to use the capacity of an available fuel cell to help manage transient load needs, or to manage load sharing between the power converter and other auxiliary power sources, or to manage fuel cell operating set points, or to help charge an external, supplemental battery system that is operatively associated with the power converter, or to accomplish other helpful operations to optimize overall system operation and intelligent use of the available power from a power grid together with different auxiliary power sources. 
     Accordingly, there remains a strong need for systems and methods that can use a power converter in highly intelligent ways to monitor and manage power being received from a power grid, as well as power being used, or readily available from, a wide variety of auxiliary power sources, in a manner which optimizes the use of all available power under a number of different operating scenarios and conditions. 
     SUMMARY 
     This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features. 
     In one aspect the present disclosure relates to a system for managing available electrical power use from a plurality of available power sources. The system may comprise an uninterruptible power supply (UPS) having a rectifier circuit, a DC/AC inverter circuit, a DC/DC charger circuit, an AC/AC bypass circuit, a main internal DC bus, and an electronic controller, the UPS providing an AC power output to a load. The system may also include an islanding logic subsystem configured to communicate with an AC power grid. The system may further include a fuel cell remote from the UPS and having an electronic controller for providing DC power to the UPS. The system may further include a supplemental battery subsystem (SBS) having an electronic controller and configured to provide supplemental DC power to the UPS. The system may further include a first external bus configured to couple an output of the SBS to the DC/DC converter of the UPS, and a second external bus configured to couple an output of the fuel cell to at least one of the main internal DC bus of the UPS or the first external bus, for providing auxiliary DC power from the fuel cell to the UPS. The system may further include a control signal bus configured to communicate control signals between the electronic controller of the UPS, the electronic controller of the fuel cell and the electronic controller of the SBS. The electronic controller of the UPS may be configured to communicate with the electronic controller of the SBS, the electronic controller of the fuel cell and the islanding logic subsystem via the control signal bus. The electronic controller of the UPS is thereby configured to enable the UPS to manage a use of available power from at least one of the AC power grid, the fuel cell or the SBS in accordance with a predetermined hierarchical supply power prioritization plan. 
     In another aspect the present disclosure relates to a method for managing available electrical power use from a plurality of available power sources. The method may comprise providing an uninterruptible power supply (UPS) having a main internal DC bus, and an electronic controller, the UPS providing an AC power output to a load. The method may further include using the UPS to communicate with an electronic controller of an islanding logic subsystem associated with an AC power grid, and further using the UPS to communicate with an electronic controller of a fuel cell remote from the UPS, wherein the fuel cell is configured to provide power to the UPS to at least assist the UPS in powering the load. The method may further include using the UPS to communicate with an electronic controller of a supplemental battery subsystem (SBS), where the SBS is configured to selectively provide supplemental DC power to the UPS. The electronic controller of the UPS may be configured to communicate with the electronic controller of the SBS, the electronic controller of the fuel cell and the electronic controller of the islanding logic subsystem to enable the UPS to manage a use of any available power from at least one of the AC power grid, the fuel cell or the SBS in accordance with a predetermined hierarchical supply power prioritization supply plan. 
     Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
       Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings. 
         FIG.  1    shows one configuration of a system in accordance with the present disclosure in which a power converter is configured to directly receive DC power from a fuel cell as well as DC power from a short term, high-rate, high cycle supplemental battery system (SBS), on a DC bus which couples the fuel cell and the SBS to the power converter, and where a control communication bus extends between the electronic controller of the power converter, the fuel cell and an islanding logic subsystem associated with the power grid; 
         FIG.  2    shows another configuration of the system in which the fuel cell and DC bus of  FIG.  1    are instead coupled to an internal DC bus of the power converter; 
         FIG.  3    shows the system of the  FIG.  1    configuration in which the power converter uses only power available from the power grid to fully support the load while a fuel cell is present but unavailable for use; 
         FIG.  4    shows the system of the  FIG.  3    configuration, but where both the power grid and the fuel cell are unavailable, and the power converter instead uses only power supplied by the SBS to fully support the load; 
         FIG.  5    shows the system of the  FIG.  1    configuration, and where the power converter controls a fuel cell startup operation by obtaining additional power needed for the startup operation from the power grid; 
         FIG.  6    shows the system of the  FIG.  5    configuration, but where the power grid is unavailable for use, and the power converter instead obtains the additional power needed for the fuel cell startup process entirely from the SBS; 
         FIG.  7    is a flowchart further highlighting various operations that may be performed by the power converter in carrying out a fuel cell startup process; 
         FIG.  8    shows the system as shown configured in  FIG.  1   , with the load being fully supported by the power grid and the fuel cell powered up and at temperature, but idle, ready for a power walk-in process to be commenced; 
         FIG.  9    shows the system of  FIG.  8    but with the load being fully supported by the SBS and the fuel cell powered up and at temperature, but idle, ready for a power walk-in process to be commenced; 
         FIG.  10    is a high level flowchart illustrating operations that the power converter electronic controller may perform in communicating with the fuel cell to carry out a power walk-in operation for the fuel cell. 
         FIG.  11    is a view of the system as shown in  FIG.  1    with the power converter of the system being used to control load sharing; 
         FIG.  12    shows the system of  FIG.  11   , but where the power converter is using power from both the power grid and from a fuel cell to cooperatively power the load; 
         FIG.  13    shows the system as configured in  FIG.  1   , and where the power grid is being used to support the load, and where the fuel cell is a PEMFC that is running at a steady state producing power that is not needed, and where the power converter exports the PEMFC&#39;s power back to the power grid; 
         FIG.  14    show the system of  FIG.  13   , but where the power converter instead exports the PEMFC&#39;s excess power capacity to an external mechanical load; 
         FIG.  15    shows the system of  FIG.  13    but where the power converter diverts the PEMFC&#39;s excess power to the SBS to charge the batteries of the SBS; 
         FIG.  16    is a flow chart showing various operations that the electronic controller of the power converter may perform in managing the scenario where the PEMFC is producing excess power which is not needed by the load; 
         FIG.  17    shows the system as configured in  FIG.  1   , and where the fuel cell is an SOFC and is working at a steady state producing a constant output power, but the output power exceeds the load requirement, and further, the power converter exports the excess power to the power grid or to the LIBs to reduce the power available to the load to the lesser needed amount; 
         FIG.  18    shows the system as configured in  FIG.  1   , and where the fuel cell is a PEMFC which is supplying the load when a transient step up load is encountered, and where the power grid is offline, and further showing how the power converter obtains the additional needed short term power for meeting the transient step up load condition from the SBS; 
         FIG.  19    shows the system of  FIG.  18    but where the fuel cell is a SOFC which is fully supplying the load when the transient step up load condition arises, and the power converter instead obtains the needed additional short term power from the power grid to meet increased transient step up load condition and fully power the load; 
         FIG.  20    shows the system of  FIG.  18    but where the fuel cell is a SOFC and is fully powering the load when a transient step up load condition arises, and the power grid is offline, and further showing how the power converter obtains the additional needed short term power from the SBS to fully meet the transient step up load condition and fully power the load; 
         FIG.  21    shows the system configured as presented in  FIG.  1   , but where the fuel cell is fully powering the load when a step down transient load condition occurs, and showing how the power converter diverts a portion of the fuel cell&#39;s output to the SBS to charge the batteries of the SBS; 
         FIG.  22    shows the system of  FIG.  21    but where the power converter addresses the step down transient load condition by exporting a portion of the fuel cell&#39;s output to the power grid; 
         FIG.  23    shows a flowchart to further illustrate various operations in greater detail that may be performed by the power converter&#39;s electronic controller in responding to the transient changes in load condition explained in connection with  FIGS.  18 - 22   ; 
         FIG.  24    shows the system configured as presented in  FIG.  1   , and how the power converter handles a fuel cell shutdown situation which requires an additional amount of power to be obtained and provided to the fuel cell to carry out the shutdown process, and how the power converter obtains the additional needed power from the power grid and diverts the additional needed power to the fuel cell; 
         FIG.  25    shows the system of  FIG.  24    but where the power converter instead obtains the additional needed power to carry out the fuel cell shutdown process from the SBS; and 
         FIG.  26    is a flowchart showing in greater detail various operations that the power converter may perform in prioritizing which ones of two or more available power sources to use in performing a fuel cell shutdown operation. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments will now be described more fully with reference to the accompanying drawings. 
     Referring to  FIG.  1   , one embodiment of a system  10  in accordance with the present disclosure is shown for using a power converter  12  (also known as an “uninterruptible power supply” or “UPS”) to intelligently manage the use of available power from a power grid  14 , an auxiliary power source shown in this example as a fuel cell  16 , and a high-rate, high cycle supplemental battery subsystem (“SBS”)  18  (often constructed as a lithium ion battery subsystem and often referred to in the industry simply as an “LIB” for short). The SBS  18  may have capacity to handle the needs of the load for a short term (e.g., an hour or longer) or for a longer intermediate term (e.g., 48 hours). In this embodiment the power converter  12  includes a rectifier circuit  12   a , an AC/AC bypass circuit  12   b , a DC/AC inverter circuit  12   c , a DC/DC charger circuit  12   d , an internal electronic controller  12   e , a main internal DC bus  12   f , a first external DC bus  12   g , and an AC power output bus  12   h . The first external DC bus  12   g  couples the DC/DC converter  12   d  of the power converter  12  to a DC input of the SBS  18 . A second external DC bus  20  couples a DC output of the fuel cell  16  to the first external DC bus  12   g . And while the auxiliary power source shown in  FIG.  1    is the fuel cell  16 , it will be appreciated that the present disclosure is not limited to use with only fuel cells, but could readily be modified for use with other types of auxiliary fuel sources, such as a diesel powered generator, a natural gas powered generator or a long term LIB. 
     The system  10  of  FIG.  1    also includes a control signal bus  22  which couples the internal electronic controller  12   e  of the power converter  12  with an electronic controller  16   a  of the fuel cell  16 . The control signal bus  22  also communicates with an islanding logic subsystem  24  associated with the power grid  14 . The islanding logic subsystem  24  may include a conventional subsystem well known in the industry for preventing the back flow of power into the power grid  14  under certain conditions, and will therefore not be described in detail here. The fuel cell  16  in one configuration may also include a DC/DC converter  16   b  which is coupled to its DC input port (not shown), and therefore communicates with the second external bus DC  20  to facilitate the transfer of DC power to and from the fuel cell  16  and other components of the system  10 , which will be explained more fully in the following paragraphs. The SBS  18  also includes an internal electronic controller  18   a  which may communicate with the electronic controller  12   e  of the power converter  12 , as well as with the electronic controller  16   a  of the fuel cell  16 , and possibly also with the islanding logic subsystem  24 . Accordingly, each of the electronic controllers  12   e ,  16   a  and  18   a , and the islanding logic subsystem  24 , may communicate information between them via the control signal bus  22 . The SBS  18  also includes a battery management subsystem  18   b  for helping to manage communications with the power converter  12  via a communications bus  18   c  which is in communications with the control signal bus  22 . 
       FIG.  2    shows the system  10  with a modification in which the second external DC bus  20  is instead directly coupled to the internal DC bus  12   f  of the power converter  12 . In this manner DC power can be transmitted directly between the internal bus  12   f  and the second external DC bus  20 . 
     The system  10  as shown in  FIGS.  1  and  2    enables the power converter  12  to control the interface between the SBS  18  and the fuel cell  16 , as well as the interfacing of these components with the load. The system  10  also enables the power converter  12  to manage operational set points of the fuel cell  16  to address transient loads that are sensed by the power converter  12  to more quickly and efficiently implement load sharing if/when needed with the power grid  14  and the SBS  18 . The power converter  12  is also able to control the diverting of excess power from the fuel cell  16  that may become available if the load suddenly drops while the load is being powered from the fuel cell  16 , for example to charge the internal batteries of the power converter  12  or to charge the batteries of the SBS  18 . The system  10  as shown in  FIGS.  1  and  2    also enables the power converter  12  to manage exporting power back onto to the power grid  14  (if this is permitted by the utility) if the fuel cell  16  is powering the load and a sudden drop in the load occurs, leaving a surplus of power available for a period of time. 
     It will also be appreciated that while the system  10  contemplates the use of a plurality of different types of auxiliary power sources, two types in particular are expected to be especially popular, with those being solid oxide fuel cells (“SOFCs”) and proton exchange membrane fuel cells (“PEMFCs”, also known as polymer electrolyte membrane fuel cells). If an SOFC is used as the fuel cell  16 , then more typically it will be used to provide main power to the load, and the power grid  14  will be used as a backup power supply. This is because of the physical makeup of an SOFC, which takes a significantly longer time to start up as well as to shut down (e.g., up to 24 hours in some cases). SOFCs also typically work at very high temperatures (e.g., 750/800° C.), so every time an SOFC switches on and off it may suffer thermal gradients that can, over time, be harmful for stack life. In addition, SOFCs often work with natural gas and have higher efficiencies than a PEMFC, which makes the SOFC a more suitable choice for use as a prime power source in most power supplying applications. Conversely, if a PEMFC is used, it will more typically be used as a backup power source, as it is capable of responding much more quickly than a SOFC. 
       FIGS.  3  and  4    further show how the system  10  operates when it is configured as shown in  FIG.  1   , with the second external DC bus  20  communicating directly with the first external DC bus  12   g . As shown in  FIG.  3   , with the fuel cell  16  powered off and with the load supported entirely by the power grid  14 , the power grid supplies power to the power converter  12 , which in turn provides an AC output signal at its output bus  12   h  to the load. 
       FIG.  4    shows the system  10  as configured in  FIG.  1    but with both the power grid  14  and the fuel cell  16  offline. In this operating scenario the SBS  18  may be controlled by the electronic controller  12   e  of the power converter  12  to provide power over the first external DC bus  12   g  to the DC/DC charging subsystem  12   d  of the power converter  12 . The power converter  12  uses this DC power to generate at the DC/AC inverter circuit  12   c  an AC output signal on its output  12   h  to power the load. Communications between the electronic controller  12   e  of the power converter  12  and the electronic controller  18   a  of the SBS  18  may be used to initiate and to stop the application of power from the SBS  18 . The transition from using SBS  18  supplied power to power grid  14  supplied power, or vice versa, may occur seamlessly through suitable control signals applied by the electronic controller  12   e  of the power converter  12  to the SBS electronic controller  18   a  and/or to the islanding logic subsystem  24 , in real time, as operating conditions change. 
     Referring now to  FIGS.  5  and  6   , the system  10  of  FIG.  1    is shown implementing two different control scenarios to carry out a startup operation of the fuel cell  16 . In  FIG.  5    power is available from the power grid  14 . In this scenario the system  10  may prioritize use of the power grid  14  to help provide the startup power necessary to bring the fuel cell  16  online. The electronic controller  12   e  of the power converter  12  controls internal switching subsystems within the power converter to route a portion of the AC output power back over a separate AC bus  26  to an AC power input (not shown) of the fuel cell  16 . This assumes that the fuel cell  16  in this instance is an AC powered fuel cell or at least has a provision for receiving an AC input signal. In this specific example, the power grid  14  is providing 500 kW through the power converter  12  to the load, and 50 kW is required to begin heating the stacks of the fuel cell  16  (which typically includes a plurality of stacks arranged together) to bring the fuel cell online. The electronic controller  12   e  may be preprogrammed with this information, or it may acquire the information from the fuel cell&#39;s electronic controller  16   a , but in either case it controls the application of the needed 50 kW to the fuel cell  16  to carry out the required heating of the fuel cell. Of course, this is assuming that this amount of power is beyond that presently required by the load. Alternatively, if the fuel cell  16  is directly connected to the power grid  14  and configured to receive AC power, then power (e.g., 230 VAC) may be provided directly from the power grid to the fuel cell  16  to heat the fuel cell. 
       FIG.  6    shows another operational scenario that can be handled by the system  10  while in its configuration shown in  FIG.  1   . In this example the power grid  14  is not available for AC power, and the SBS  18  is being used to power the load. The electronic controller  12   e  of the power converter  12  communicates with the electronic controller  18   a  of the SBS  18  to determine the available capacity of the SBS  18  and, provided sufficient capacity is available, draws the additional power needed to heat the stacks of the fuel cell  16 . In the example shown in  FIG.  6   , the load requires 500 kW, so the electronic controller  12   e  commands the power converter  12  to draw 550 kW, and then provides 50 kW to the fuel cell  16  via the AC bus connecting the output bus  12   h  to the AC input (not shown) of the fuel cell  16 . Once the fuel cell  16  is fully powered up, the electronic controller  12   e  will control the power converter  12  to stop drawing power from the SBS  18 , or to reduce its power draw to only what is needed to help power the load, assuming that the fuel cell  16  is not able to fully power the load by itself. 
     Referring now to  FIG.  7   , a high level flowchart  100  is shown of various operations that the electronic controller  12   e  of the power converter  12  of the system  10  may perform when the power converter is controlling the startup of the fuel cell  16 . The decision to supply power from the power grid  14  or the SBS  18  to the fuel cell  16  may be based on preprogrammed logic stored in a memory of the electronic controller  12   e . Initially the electronic controller  12   e  makes a check at operation  102  to determine if sufficient additional power from the power grid  14  is available to carry out the startup process. This check may involve the electronic controller  12   e  checking with an external subsystem (e.g., possibly the island logic subsystem) for the needed information. If the check at operation  102  indicates that grid power is available, then at operation  104  electronic controller  12   e  determines the grid power needed to start up the fuel cell  16 , and at operation  114  the electronic controller  12   e  begins supplying power from the power grid  14  to fully start up the fuel cell  16 . If the check at operation  102  indicates that grid power is not available, then at operation  106  the electronic controller  12   e  determines if the SBS  18  is presently supporting the load. If it is not, then the electronic controller  12   e  may determine that the SBS  18  has sufficient battery capacity to fully start up the fuel cell  16  by itself within the required startup time, as indicated at operation  108 . Then, at operation  114 , the electronic controller  12   e  of the power converter  12  begins supplying power from the SBS  18  to fully start up the fuel cell  16 . 
     With further reference to  FIG.  7   , if the check at operation  106  indicates that the SBS  18  is presently supporting the load, then a check is performed by the electronic controller  12   e  at operation  112  to determine if the SBS  18  has sufficient capacity to support the load while also providing the additional power needed to fully start up the fuel cell  16  within the required startup time. The electronic controller  12   e  may carry out this check by communicating with the electronic controller  18   a  of the SBS  18  via the communications bus  18   c  and the battery management subsystem  18   b . If the check at operation  112  produces a “Yes” answer, then the startup operation is carried out at operation  114  using the power from the SBS  18  to fully start up the fuel cell  16 . If the check at operation  112  produces a “No” answer, then at operation  110  the power converter  12  does not attempt to provide any power to the fuel cell  16  to begin the fuel cell startup process, and the startup operation is terminated. 
     Referring now to  FIGS.  8 - 10   , an operational scenario will be explained using the system  10  as configured in  FIG.  1    to carry out a walk-in process for the fuel cell  16 . Under this scenario, in both  FIGS.  8  and  9   , the fuel cell  16  startup is complete but the fuel cell  16  is idle (i.e., not supplying any power to the load), but supplying sufficient auxiliary power (e.g., 50 kW) to maintain its own balance of plant (“BOP”). The load, in this example 500 kW, is being supported entirely by the power grid  14  in  FIG.  8   . The power converter  12  operates in its traditional manner to receive the output from the power grid  14  and generates an AC output needed on its output bus  12   h  to fully power the load. However, in  FIG.  9   , the load is being fully supported instead by the SBS  18 . 
       FIG.  10    shows one example of a high level flowchart  200  showing various operations that the electronic controller  12   e  of the power converter  12  may use in carrying out the power walk-in process by which the fuel cell  16  will gradually assume the load within a predetermined power walk-in time. Initially at operation  202  the fuel cell  16  is fully powered up and “at temperature”, but idle and supplying only auxiliary power (e.g., to maintain its own BOP), as explained above with reference to  FIGS.  8  and  9   . 
     At operation  204  the electronic controller  12   e  of the power converter  12  determines the power set point value needed for the fuel cell to assume fully powering the load (e.g., 500 kW in the example of  FIGS.  8  and  9   ), and the electronic controller  12   e  then sends the needed power set point value to the electronic controller  16   a  of the fuel cell  16 . At operation  206  the power converter electronic controller  12   e  commands the electronic controller  16   a  of the fuel cell  16  to begin the power walk-in process. At operation  208  the electronic controller  12   e  makes a check to determine if the power walk-in process is complete (i.e., if the load is then fully supported by the fuel cell  16 ). If this check produces a “No” answer, then operation  206  is repeated to allow the power walk-in process to continue. If the check at operation  208  produces a “Yes” answer, then the power walk-in process is complete. At this point the fuel cell  16  is fully powering the load via an output provided by the power converter  12  at its output bus  12   h.    
     Referring now to  FIGS.  11  and  12   , an operational scenario will be explained where the system  10  as configured in  FIG.  1    manage load sharing among two or more available power sources. In this example the fuel cell  16  is up and running at a maximum, constant output power. However, the load requirements are equal to, or exceed, the output capacity of the fuel cell  16 . The power grid  14  and the SBS  18  are both available to provide additional power.  FIG.  11    shows a preferred control scheme when the fuel cell  16  is a PEMFC and a majority of the needed power for the load is available from the power grid  14 . The electronic controller  12   e  of the power converter  12  controls internal switching subsystems of the power converter to enable the output from the power grid  14  to be used to help generate a major portion of the required power for the load (in this example 500 kW) and also to enable an additional amount of supplemental power (in this example 100 kW) to be provided from the fuel cell  16  via the second external DC bus  20 . In this example, this leaves a small amount (e.g., 50 kW) still available as auxiliary power capacity from the fuel cell  16 . 
       FIG.  12    shows the system  10  with the fuel cell  16  being used as the primary power (the fuel cell  16  being a SOFC in this example), while the power grid  14  is being used as a supplemental power source to supply a minor portion (e.g., 100 kW) of the power needed for the load. Again, the electronic controller  12   e  of the power converter  12  controls the internal switching subsystems of the power converter to receive the required amount of power from both the power grid  14  and the fuel cell  16  (e.g., 600 kW total in this example) to power the load. In this example a small additional amount of auxiliary power capacity remains available from the fuel cell  16  (50 kW in this example). If the load requirements should increase further somewhat, the electronic controller  12   e  can communicate with the electronic controller  16   a  of the fuel cell  16  to command that additional power be provided from the fuel cell, potentially up to the maximum available power output of the fuel cell. Still further power capacity from the SBS  18 , which is not being used in this example, is available as well. 
     Referring now to  FIGS.  13 - 15   , the system  10  in its configuration of  FIG.  1    is shown, with the fuel cell  16  being a PEMFC in this example, to illustrate the operational scenario where the fuel cell is providing constant output power which is greater than that required to support the load. In other words, the fuel cell  16  has excess capacity that needs to be managed. The electronic controller  12   e  of the power converter  12  recognizes this situation and is able to communicate with other available power sources to facilitate the export of a portion of the fuel cell&#39;s  16  excess output to one or more of the other available power sources. In  FIG.  13    the power grid  14  is supplying the full needed power for the load (i.e., 500 kW in this example), while the fuel cell  14  is outputting a smaller amount of power (100 kW in this example), which is not needed for the load. The electronic controller  12   e  communicates with the SBS electronic controller  18   a  and with the islanding logic subsystem  24  and determines, based on information received from the SBS  18  and the power grid  14 , as well as using its preprogrammed priority rules, where the excess power from the fuel cell  16  should be exported to. Options are available to export the excess power to the SBS  18  to charge internal batteries of the SBS ( FIG.  15   ) or to the power grid  14  ( FIG.  13   ). Before attempting to use excess power to charge the batteries of the SBS  18 , the power converter  12  may calculate the state of charge of the batteries by obtaining information from the SBS on the real time state of charge of its batteries, analyzing the load, and making the needed calculations to determine how much additional charge capacity the batteries can absorb. 
       FIG.  13    shows that the electronic controller  12   e  has determined to send the excess power (e.g., 100 kW) back through the power converter  12  to the power grid  14 . This avoids the need to throttle down the fuel cell  16  and can add to the efficiency in the operation of the fuel cell, while making optimum use of the fuel cell&#39;s available excess power. 
       FIG.  14    shows another example of the system  10  handling the excess fuel cell  16  output situation, but in this example the electronic controller  12   e  exports the un-needed output (100 kW in this example) of the fuel cell  16  to an independent mechanical load “ML”. 
       FIG.  15    shows still another example of the system  10  handling the excess fuel cell  16  output situation, but in this example the electronic controller  12   e  of the power converter  12  determines, through communications with the electronic controller  18   a  of the SBS  18 , that the SBS can accept the full output of the fuel cell (100 kW in this example). The electronic controller  12   e  then communicates instructions to the electronic controller  16   a  of the fuel cell  16  to supply its full present output (100 kW in this example) to the SBS  18 . The full output of the fuel cell  16  then is used to charge the internal batteries of the SBS  18 . Simultaneously, the power converter  12  uses power from the power grid  14  to fully power the load. 
       FIG.  16    shows a flowchart  300  which further illustrates one example of the operations described above by which the power converter  12  manages load sharing among two or more available power sources and also decides which one of two or more available power sources the excess output of the PEMFC should be exported to. At operation  302  the power converter electronic controller  12   e  monitors the fuel cell  16  operation and the real time requirements of the load. At operation  304  the electronic controller  12   e  detects that the fuel cell  16  is producing more power than required by the load. This situation typically occurs when the load drops (e.g., from 600 kW to 500 kW) over a relatively short period of time. At operation  306  the electronic controller  12   e  makes a real time check if the now available excess power can be exported to the power grid  14 . In this example the power grid  14  is the preferred choice for exporting power to, provided it is available for such purpose at the time. If the check at operation  306  produces a “Yes” answer, then the electronic controller  12   e  controls its internal switching subsystems to route power from the fuel cell  16  through the power converter  12  back to the power grid  14 , as indicated at operation  310 , and monitoring of the fuel cell  16  operation continues at operation  302 . However, if the check at operation  306  produces a “No” answer, then the electronic controller  12   e  makes a real time check at operation  308  to determine if a mechanical load, known to the electronic controller  12   e  in advance to be available for use, has available capacity to receive the excess power from the fuel cell  16 . If this check at operation  308  produces a “Yes” answer, then the electronic controller  12   e  diverts the excess power from the fuel cell  16  to the mechanical load, as indicated at operation  314 , and monitoring of the fuel cell  16  operation continues at operation  302 . 
     Referring further to  FIG.  16   , if the check at operation  308  by the electronic controller  12   e  indicates that the mechanical load does not have the capacity to receive the excess power from the fuel cell  16 , then at operation  312  the electronic controller  12   e  instructs the fuel cell  16  to supply the excess portion of its output to the SBS  18  to charge the internal batteries of the SBS  18 . 
     Monitoring of the fuel cell  16  operation then continues at operation  302 . It will be appreciated that this hierarchical priority scheme enables the electronic controller  12   e  to use all of the available power sources, as well as all of the available mechanical load(s), if needed, to receive excess power from the fuel cell  16 . The electronic controller  12   e  further is able to prioritize which other power sources and/or mechanical load(s) may receive the excess power from the fuel cell  16  to make most efficient use of the excess power. 
     Referring now to  FIG.  17   , the system  10  is shown in its configuration as presented in  FIG.  1    to describe how excess power from the fuel cell  16  may be diverted when the steady state power of the fuel cell exceeds the requirements of the load, and when the fuel cell  16  is a SOFC. In this example the fuel cell  16  is being used as the primary power source for the load (e.g., supplying 500 kW in this example) and running at a steady state. However, the load requires less than the present steady state output of the fuel cell  16  (e.g., 400 kW in this example). The electronic controller  12   e  of the power converter  12  detects this condition through its real time monitoring of the load requirements and the output of the fuel cell  16 , and controls its internal switching subsystems to divert a portion of the fuel cell&#39;s steady state output (i.e., 100 kW in this example) back to the power grid  14 . In this scenario the electronic controller  12   e  knows, typically through a prior communication with the power grid  14  or through preprogrammed information, whether the power grid  14  is available for exporting power to. If the power grid  14  is not available to receive the exported power, the electronic controller  12   e  of the power converter  12  power may export the power to an external load bank (not shown) or to an additional bank of LIBs (e.g., extended life LIBs; not shown). 
     Referring now to  FIGS.  18 - 20   , the system  10  of  FIG.  1    is shown again to help illustrate another operational scenario where an additional transient load is experienced which requires a rapid step up in the power being provided to the load. In  FIG.  18   , the power grid  14  is unavailable and the fuel cell  16  is a PEMFC producing all of the power needed for the load (e.g., 450 kW in this example) before the transient load arises. The transient load requires an additional 50 kW. The electronic controller  12   e  of the power converter  12  detects this transient load in real time, determines in real time the immediate need for an additional 50 kW of power for the load, and queries the electronic controller  18   a  of the SBS  18  in real time to determine if the SBS  18  has the capacity to supply the needed power (i.e., 50 kW). If the SBS  18  can provide the needed power, then the electronic controller  12   e  controls the application of an additional 50 kW from the SBS  18  to the output bus  12   h , bringing the total power being supplied to the load up to the required amount to fully meet the transient load requirement (i.e., 500 kW total in this example). In practice these actions may occur within seconds or less, and more typically within milliseconds. The SBS  18  supplies the required amount of power until the PEMFC can provide the additional power to fully support the added load (i.e., in 30 seconds in this example). 
       FIG.  19    shows another example of the system  10  being used to handle the above-described transient step up load scenario, but where the fuel cell  16  is a SOFC providing all of the steady state power (e.g., 450 kW in this example) needed to supply the load before the transient step up load condition (e.g., requiring an additional 50 kW) arises. Since the SOFC is generally not well suited to respond quickly to transient conditions, the electronic controller  12   e  of the power converter  12  may be pre-programmed to immediately look to other available power sources to obtain the additional power needed to meet the transient load condition. In this example the electronic controller  12   e  instead uses the power grid  14  to provide the additional needed power (i.e., 50 kW) until the SOFC is able to provide the additional power required to meet the transient load requirements (e.g., in 5 minutes in this example). Power from the grid  14  is available virtually instantly, so the electronic controller  12   e  is able to obtain and provide this additional power to the load in real time, for example in just seconds or possibly even just milliseconds. In either event, the application of the additional power appears to happen seamlessly to the load. 
     Referring to  FIG.  20   , still another example of the system  10 , as configured in  FIG.  1   , is shown to describe the above discussed scenario where a transient increase in the load requirement requires a step up in power, but where the power grid  14  is unavailable but the SBS  18  is available to supply the additional needed power. In this instance the fuel cell  16  is a SOFC and is fully supplying the needed power for the load (e.g., 450 kW) before the need for additional power arises. The electronic controller  12   e  senses the transient load increase in real time and communicates with the electronic controller  18   a  of the SBS  18  to determine if the SBS  18  has sufficient capacity to supply the additional required power to fully power the load. If so, then the electronic controller  12  controls its internal switching subsystems to apply the additional needed power (e.g., 50 kW in this example), in real time, to the load to help fully power the load. 
       FIGS.  21  and  22    show two examples of the system  10 , as configured in  FIG.  1   , for handling the operational scenario where a transient step down in the power needed to power the load is experienced and needs to be handled by the system  10 .  FIGS.  21  and  22    show the system  10  with the fuel cell  16  being a SOFC. The electronic controller  12   e  of the power converter initially checks the state of charge of the batteries of the SBS  18 . If the state of charge is sufficiently low to enable the SBS to receive the excess power, then the electronic controller  12   e  controls the SBS  18  to receive the excess power that the fuel cell  16  provides until it drops from its steady state output (e.g., 500 kW in this example) to the new, lower value (e.g., 450 kW in this example). In the example, the time until the fuel cell  16  is able to supply the new, lower amount of power (e.g., 450 kW) is 5 minutes. It will be appreciated that the operational scenario in  FIG.  21    applies also to the PEMFC, but in this instance the fuel cell  16  is capable of reacting quickly to the step down in load (e.g., in 30 seconds). 
       FIG.  22    shows the system  10  configured as shown in  FIG.  1   , with the fuel cell  16  being a SOFC, and where the SBS  18  is not available to receive the excess power from the fuel cell during a transient step down scenario. In this instance the electronic controller  12   e  of the power converter  12  is aware of the unavailability of the SBS  18  to receive additional power, for example because the batteries of the SBS  18  are already fully charged. The electronic controller  12   e  instead controls its internal switching subsystems to channel the excess portion of the power from the fuel cell  16  (50 kW in this example) back to the power grid  14 . Again, the electronic controller  12   e  may check with the power grid  14  before exporting any excess power back to the power grid, or it may be preprogrammed to know when (i.e., various time periods) the power grid  14  will be available to receive power. The step down in power to the load appears seamless to the load. 
       FIG.  23    shows a high level flowchart  400  which describes in greater detail the various operations that may be performed by the system  10 , configured as shown in  FIGS.  19 - 22   , in carrying out the transient load step up and step down operations described above, when the SOFC is powering the load. Initially at operation  402  the electronic controller  12   e  of the power converter  12  monitors the fuel cell  16  operation and real time load requirements. At operation  404  the power converter  12  detects a transient change in the load. At operation  406  the power converter electronic controller  12   e  checks to determine if the transient change is a step up or step down load condition. If a step up load condition is detected, the electronic controller  12   e  determines the needed step up in power required to meet the transient load condition, as indicated at operation  418 . At operation  420  the electronic control  12   e  obtains the needed power from the power grid  14  or from the SBS  18  to provide the additional needed power to the load to supplement the power being provided by the fuel cell to fully power the load, as indicated at operation  420 . Operations  402  and  404  are then repeated. 
     If the check at operation  406  detects a step down load condition, the electronic controller  12   e  makes a check if the SBS  18  batteries can absorb the available excess power. If this check produces a “Yes” answer, then at operation  416  the electronic controller  12   e  diverts (i.e., exports) the excess power to the SBS  18 , and then operations  402  and  404  may be repeated. However, if the check at operation  408  indicates that the batteries of the SBS  18  are not able to absorb the excess power, then the electronic controller  12   e  makes a check at operation  410  to determine if the power grid  14  is available to absorb the excess power. If this check produces a “Yes” answer, then at operation  416  the power converter diverts the excess power to the power grid  14 , and operations  402  and  404  are repeated. If the check at operation  410  indicates that the power grid  14  is not available or otherwise able to receive the excess power, then at operation  412  the electronic controller  12   e  performs a check if an available external load bank (or an extended life LIB) is able to absorb the excess power. If this check produces a “Yes” answer, then the electronic controller  12   e  diverts the excess power to the external load bank, as indicated at operation  416 , and then operations  402  and  404  are repeated. However, If the check at operation  412  in  FIG.  23    indicates that the external load bank is not able to absorb the excess power, then the electronic controller  12   e  determines that a shutdown of the fuel cell  16  is required, as indicated at operation  414 , and subsequently takes the needed actions to command a shutdown of the fuel cell. 
       FIGS.  24  and  25    show how the system  10 , as configured in  FIG.  1   , is controlled during a fuel cell  16  shutdown operation. The operations described for this fuel cell  16  shutdown operation may apply whether the fuel cell is a SOFC or PEMFC, and the shutdown performed may be a controlled, gradual shutdown or an abrupt shutdown. If a controlled, gradual shutdown is needed for the fuel cell  16 , then the electronic controller  12   e  of the power converter  12  determines the amount of power needed to maintain the fuel cell  16  BOP for the specific fuel cell  16  being used. Alternatively, the electronic controller  12   e  may be preprogrammed with this information. In either case, the electronic controller  12   e  obtains the total amount of needed power from the power grid  14  to support the load as well as to enable the fuel cell  16  shutdown operation to be carried out while maintaining the BOP. In this example the load requires 500 kW, and the BOP will require 50 kW. Thus, the electronic controller  12   e  obtains 550 kW from the power grid  14  and uses 500 kW to power the load while sending 50 kW to the fuel cell  16  to maintain the BOP while carrying out the shutdown operation. 
       FIG.  25    shows the system  10  of  FIG.  24   , but where the power grid  14  is unavailable and the SBS  18  is providing the power to the load. In this example the electronic controller  12   e  checks with the electronic controller  18   a  of the SBS  18  to verify that sufficient extra power (i.e., 50 kW) is available beyond the present 500 kW that it is presently outputting to power the load, in order for the fuel cell  16  shutdown operation to be carried out while maintaining the BOP. If so, the electronic controller  12   e  obtains the additional 50 kW of power from the SBS  18  and controls its internal switching subsystems to route 50 kW of the now 550 kW power being produced to the fuel cell  16 . In both scenarios described in  FIGS.  24  and  25   , the shutdown process is seamless and transparent to the load and is carried out and controlled by the power converter  12 . 
       FIG.  26    shows a flowchart  500  illustrating in further detail one specific sequence of operations to be performed by the system  10  in carrying out the shutdown operations mentioned above in connection with the discussion of  FIGS.  24  and  25   . At operation  502  in  FIG.  26   , the electronic controller  12   e  of the power converter  12  initially makes a check to determine if the power grid  14  is online and able to support the load. If this check produces a “No” answer, then at operation  510  the electronic controller  12   e  makes a check to determine if the batteries of the SBS  18  are able to support the load (i.e., if the batteries have sufficient excess capacity to provide the needed auxiliary power). If this check produces a “No” answer, then the electronic controller  12   e  determines that the fuel cell  16  shutdown operation cannot be carried out, as indicated at operation  512 , and the shutdown operation is terminated. 
     With further reference to  FIG.  26   , if the check at operation  502  indicates that the power grid  14  is available to supply the auxiliary needed power for the shutdown operation, then the electronic controller  12   e  obtains the additional needed power from the power grid  14  (e.g., 50 kW in this example) and supplies the additional power to the fuel cell  16  to carry out the shutdown operation, as indicated at operation  506 , while maintaining the same level of power to the load (e.g., 500 kW in this example). At operation  508  the electronic controller  12   e  then makes a check to determine if the fuel cell is fully shutdown, and if this produces a “No” answer, then operations  506  and  508  are repeated. Finally, if the check at operation  510  indicates that the batteries of the SBS  18  are available to provide the needed additional power to carry out the fuel cell  16  shutdown operation, then the electronic controller  12   e  uses the batteries to carry out the subsequent shutdown operations  506 - 508 . 
     From  FIG.  26   , it will be appreciated that the electronic controller  12   e  of the power converter  12  carries out a hierarchical process to determine, in a predetermined hierarchical manner of checks, which of two or more available power sources are available for use, and which have sufficient additional capacity, to provide the additional needed power to carry out the fuel cell shutdown operation. This enables the power converter  12  to intelligently manage and carry out the shutdown operation while making the most efficient use of the power resources available to it at any given time. 
     The various embodiments described above all enable a power converter to manage virtually all the operations that may arise in powering a load, handling step up and step down transient load conditions, and handling startup and shutdown operations when a fuel cell is being used as a power source. Various hierarchical control schemes are disclosed by which the power converter can make intelligent choices as to which ones of several available power sources are prioritized for use to best handle different operating conditions that arise while powering a load and/or when using different power sources such as fuel cells and supplemental battery systems. A central advantage of the various embodiments described herein is that the power converter can implement a wide variety of different control operations without interrupting or affecting the power being provided to the load. As such, the various control scenarios discussed above can be carried out seamlessly with respect to the load. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure. 
     Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those who are skilled in the art. Numerous specific details are set forth such as examples of specific components, devices, and methods, to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms “a,” “an,” and “the” may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms “comprises,” “comprising,” “including,” and “having,” are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     When an element or layer is referred to as being “on,” “engaged to,” “connected to,” or “coupled to” another element or layer, it may be directly on, engaged, connected or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly engaged to,” “directly connected to,” or “directly coupled to” another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between” versus “directly between,” “adjacent” versus “directly adjacent,” etc.). As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer, or section. Terms such as “first,” “second,” and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments. 
     Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,” “lower,” “above,” “upper,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the example term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.