Patent Publication Number: US-2023146218-A1

Title: Power management in fuel cell system based microgrids

Description:
FIELD 
     The present disclosure is directed to direct current (DC) power sources, such as fuel cell system based microgrids with more efficient use of fuel cell system capacity. 
     BACKGROUND 
     A common method of powering microgrid systems is a Master/Slave voltage source inverter relationship where each voltage source inverter follows a single Master&#39;s command to maintain output voltage, meaning that they all export the same amount of voltage. 
     SUMMARY 
     According to an embodiment, a microgrid comprises a plurality of direct current (DC) power sources, a plurality of voltage source inverters, wherein a DC end of each of the plurality of voltage source inverters is electrically connected to a respective DC power source of the plurality of DC power sources, a microgrid side bus, wherein an alternating current (AC) end of each of the plurality of voltage source inverters is electrically connected to the microgrid side bus, and the microgrid side bus is configured to be electrically connected to a load, a plurality of current source inverters, wherein a DC end of each of the plurality of current source inverters is electrically connected to a respective DC power source of the plurality of DC power sources, a grid side bus, wherein an AC end of each of the plurality of current source inverters is electrically connected to the grid side bus, a transfer switch configured to control a selective electrical connection of the grid side bus to an electric utility power grid or to the microgrid side bus, and a transmission bus electrically connected between the microgrid side bus and the grid side bus. 
     According to another embodiment, a method of operating a microgrid comprises providing electric energy from each of a plurality of DC power sources to a respective one of a plurality of voltage source inverters and to a respective one of a plurality of current source inverters, outputting a voltage by the plurality of voltage source inverters to the microgrid side bus such that each of the plurality of voltage source inverters outputs approximately equal amounts of voltage to the microgrid side bus, wherein a maximum output of voltage of each of the plurality of voltage source inverters is based on a lowest generation capacity of one of the plurality of DC power sources, outputting a first current by the plurality of current source inverters to a grid side bus based on an amount of current generated by the plurality of DC power sources in excess of the lowest generation capacity, and using the first current output to the grid side bus to provide a second current to the microgrid side bus. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view of a fuel cell system according to various embodiments suitable for implementing various embodiments. 
         FIG.  2    is a schematic side cross-sectional view of a hot box according to various embodiments suitable for implementing various embodiments. 
         FIG.  3    is a component block diagram of a fuel cell system based microgrid suitable for implementing various embodiments. 
         FIGS.  4 A- 4 C  are process flow diagrams of a method of power management for the fuel cell system based microgrid illustrated in  FIG.  3    according to various embodiments. 
         FIG.  5    is a component block diagram of a fuel cell system based microgrid suitable for implementing various embodiments. 
         FIGS.  6 A- 6 C  are process flow diagrams of a method of power management for the fuel cell system based microgrid illustrated in  FIG.  5    according to various embodiments. 
         FIG.  7    is a component block diagram of a fuel cell system based microgrid suitable for implementing various embodiments. 
         FIGS.  8 A- 8 C  are process flow diagrams of a method of power management for the fuel cell system based microgrid illustrated in  FIG.  7    according to various embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the claims. 
     As used herein, the terms “DC power source” and “DC power supply” are used interchangeably to refer to a generator capable of generating electric power from any source, such as a fuel cell, a combustion generator, a photovoltaic cell, a concentrated solar system, a wind turbine, a geothermal turbine, a hydroelectric turbine, a gas turbine, a nuclear reactor, an alternator, an induction generator, etc. Examples herein described in terms of fuel cells do not limit the scope of the claims and descriptions to such types of DC power sources. In some embodiments, a DC power source may be an AC generator in combination with an AC/DC rectifier. 
     As used herein, the term “storage system” and “energy storage system” are used interchangeably to refer to any form of energy storage that may be converted to electric power, such as electrical storage, mechanical storage, electromechanical storage, electrochemical storage, thermal storage, etc. Examples may include a battery, a capacitor, a supercapacitor, a flywheel, a liquid reservoir, a gas reservoir, etc. In some embodiments, the energy storage system may include any combination of components configured to control electric energy output of the energy storage system, such as an electric connection device and/or an electric energy conditioning device, in response to a signal from a controller and/or an electric energy bus. 
     As used herein, the terms “electric energy” and “electric energy output” are referred to amounts of electric voltage, current, or power. Examples herein described in terms of voltage do not limit the scope of the claims and descriptions to such types of electric energy and electric energy output. 
     The present inventors realized that a weakness of the prior art Master/Slave control scheme is that voltage source inverters of different or variable direct current (DC) power capacity are difficult to completely utilize. Since each voltage source voltage source inverter follows the same single command, the DC power source with the weakest DC capacity tends to limit the ultimate capacity of multiple inverters connected in parallel. This wastes the excess electric energy generated by the DC power sources which is in excess of the electric energy generated by the weakest DC power source in a microgrid. 
     Various embodiments include electrical circuits, electrical components, and methods for power management in DC power source based microgrids which utilizes both voltage and current source inverters for a plurality of DC power sources. The current source inverters may output excess electric energy generated by the DC power sources other than the weakest DC power source in the microgrid. Therefore, the excess electric energy is not wasted and is provided to a power grid and/or to a load. 
     In one embodiment, the DC power sources in a microgrid may comprise fuel cell DC power sources. A fuel cell system based microgrid may include multiple direct current (DC) to alternating current (AC) voltage source inverters electrically connecting multiple fuel cell stacks, (or multiple columns of fuel cell stacks and/or multiple power modules each containing plural columns) of a fuel cell system to a microgrid bus in parallel, multiple DC to AC current source inverters electrically connecting the fuel cell stacks to a grid bus in parallel, and at least one electric energy control device configured to control electrical connection between the grid bus and the microgrid bus. Methods for power management in fuel cell system based microgrids may include controlling the current source inverters to output excess electric energy generated by the fuel cell stacks to the grid bus, and controlling the at least one electric energy control device to electrically connect the grid bus and the microgrid bus and provide electric energy from the grid bus and the microgrid bus to support a load. 
     For microgrid applications, inverters are used to form voltage without any connection to an electric utility power grid. In order to accomplish this, a microgrid system monitors the output voltage and adjust the power of its voltage source inverters in real-time to ensure the voltage waveform remains constant. One common method to accomplish this goal is to have a single point sensor monitoring the output voltage, and providing a control signal to a bank of voltage source inverters. This method employs a Master/Slave voltage source inverter relationship where each voltage source inverter follows a single Master&#39;s command to maintain output voltage, meaning that they all export the same amount of voltage. The strength of this approach is in its simplicity. The command to the voltage source inverters is derived directly from the voltage signal in real-time, and with tuning any number of voltage source inverters can be used to follow the command allowing for scaling in capacity. 
     A weakness in this approach is a difficulty in utilizing the full DC electric energy capacity from each DC power source available to every voltage source inverter in use. Because the Master must supply a single command which is given to all inverters, it is useful to think of the command as a percentage of full power. For microgrid systems with variable capacity DC power sources backing them, the DC power available is not fixed and not always 100% of the intended rating. This means that in cases where many DC power sources are attached to individual voltage source inverters connected in parallel, one of those DC power sources will be weakest at any point in time. Since each voltage source inverter follows the same single command, the weakest DC power source limits the ultimate capacity of multiple voltage source inverters in parallel. As the Master commands more and more power from the voltage source inverters, the weakest system will reach its limit first, and that will cause the response of the microgrid system to fail. As such, the capacity of the entire bank of inverters is artificially limited by its weakest individual voltage source inverter&#39;s DC power source capacity. In cases with N inverters, a loss of X kW from a single DC source will cause an N*X kW loss of capability to the microgrid system. 
     Embodiments described herein address the foregoing weaknesses of microgrid systems. In a fuel cell system based microgrid, the fuel cells may provide a fixed amount of continuous DC electric energy. Voltage source inverters may provide the electric energy to the fuel cell system based microgrid following a single voltage control command and current source inverters may export the remaining electric energy to the grid during a normal (i.e., steady-state non-emergency) mode when the grid is available. The voltage source inverters may supply electric energy demanded from the load, and the current source inverters may export whatever electric energy is in excess of what is required to support the load. 
     The current source inverters may directly measure a voltage present on their terminals from an external source (typically the electric utility power grid) and push current from a DC power source in sync with the voltage waveform. These current source inverters (sometimes called grid-tie or grid-parallel inverters) may arbitrarily generate output current up to whatever electrically connected DC power sources can provide. The current source inverter may be allowed to sense the DC electric energy input from the respective DC power source to determine when that capacity is reached. 
     Embodiments provide circuit which permits the current source inverters to export excess electric energy produced by the DC power sources (such as fuel cell stacks) to the electric utility power grid and/or to provide electric energy from a grid side bus of the microgrid system to a load connected microgrid side bus of microgrid system. Electric energy may be moved from the grid side bus to the microgrid side bus arbitrarily, and therefore up to 100% utilization of all DC electric energy produced by the DC power sources, such as fuel cell stacks, may become possible. 
       FIG.  1    illustrates an example of one electrical power generator which comprises modular fuel cell system that is more fully described in U.S. Pat. No. 8,440,362, incorporated herein by reference for descriptions of the modular fuel cell system. The modular system may contain modules and components described above as well as in U.S. Pat. No. 9,190,693, which is incorporated herein by reference for descriptions of the modular fuel cell system. The modular design of the fuel cell system enclosure  10  provides flexible system installation and operation. 
     The modular fuel cell system enclosure  10  includes a plurality of power module housings  12  (containing a fuel cell power module components), one or more fuel input (i.e., fuel processing) module housings  16 , and one or more power conditioning (i.e., electrical output) module housings  18 . For example, the system enclosure may include any desired number of modules, such as 2-30 power modules, for example 6-12 power modules.  FIG.  1    illustrates a system enclosure  10  containing six power modules (one row of six modules stacked side to side), one fuel processing module, and one power conditioning module, on a common base  20 . Each module may comprise its own cabinet or housing. Alternatively, the power conditioning and fuel processing modules may be combined into a single input/output module located in one cabinet or housing  14 . For brevity, each housing  12 ,  14 ,  16 ,  18  will be referred to as “module” below. 
     While one row of power modules  12  is shown, the system may comprise more than one row of modules  12 . For example, the system may comprise two rows of power modules stacked back to back. 
     Each power module  12  is configured to house one or more hot boxes  13 . Each hot box contains one or more stacks or columns of fuel cells (not shown for clarity), such as one or more stacks or columns of solid oxide fuel cells having a ceramic oxide electrolyte separated by conductive interconnect plates. Other fuel cell types, such as PEM, molten carbonate, phosphoric acid, etc. may also be used. 
     The modular fuel cell system enclosure  10  also contains one or more input or fuel processing modules  16 . This module  16  includes a cabinet which contains the components used for pre-processing of fuel, such as desulfurizer beds. The fuel processing modules  16  may be designed to process different types of fuel. For example, a diesel fuel processing module, a natural gas fuel processing module, and an ethanol fuel processing module may be provided in the same or in separate cabinets. A different bed composition tailored for a particular fuel may be provided in each module. The processing module(s)  16  may processes at least one of the following fuels selected from natural gas provided from a pipeline, compressed natural gas, methane, propane, liquid petroleum gas, gasoline, diesel, home heating oil, kerosene, JP-5, JP-8, aviation fuel, hydrogen, ammonia, ethanol, methanol, syn-gas, bio-gas, bio-diesel and other suitable hydrocarbon or hydrogen containing fuels. If desired, a reformer  17  may be located in the fuel processing module  16 . Alternatively, if it is desirable to thermally integrate the reformer  17  with the fuel cell stack(s), then a separate reformer  17  may be located in each hot box  13  in a respective power module  12 . Furthermore, if internally reforming fuel cells are used, then an external reformer  17  may be omitted entirely. 
     The modular fuel cell system enclosure  10  also contains one or more power conditioning modules  18 . The power conditioning module  18  includes a cabinet which contains the components for converting the fuel cell stack generated DC power to AC power, electrical connectors for AC power output to the grid, circuits for managing electrical transients, a system controller (e.g., a computer or dedicated control logic device or circuit). The power conditioning module  18  may be designed to convert DC power from the fuel cell modules to different AC voltages and frequencies. Designs for 208V, 60 Hz; 480V, 60 Hz; 415V, 50 Hz and other common voltages and frequencies may be provided. 
     The fuel processing module  16  and the power conditioning module  18  may be housed in one input/output cabinet  14 . If a single input/output cabinet  14  is provided, then modules  16  and  18  may be located vertically (e.g., power conditioning module  18  components above the fuel processing module  16  desulfurizer canisters/beds) or side by side in the cabinet  14 . 
     As shown in an example embodiment in  FIG.  1   , one input/output cabinet  14  is provided for one row of six power modules  12 , which are arranged linearly side to side on one side of the input/output module  14 . The row of modules may be positioned, for example, adjacent to a building for which the system provides power (e.g., with the backs of the cabinets of the modules facing the building wall). While one row of power modules  12  is shown, the system may include more than one row of modules  12 . For example, as noted above, the system may include two rows of power modules stacked back to back. 
     Each of the power modules  12  and input/output modules  14  include a door  30  (e.g., hatch, access panel, etc.) to allow the internal components of the module to be accessed (e.g., for maintenance, repair, replacement, etc.). According to one embodiment, the modules  12  and  14  are arranged in a linear array that has doors  30  only on one face of each cabinet, allowing a continuous row of systems to be installed abutted against each other at the ends. In this way, the size and capacity of the fuel cell enclosure  10  can be adjusted with additional modules  12  or  14  and bases  20  with minimal rearranging needed for existing modules  12  and  14  and bases  20 . If desired, the door  30  to module  14  may be on the side rather than on the front of the cabinet. 
       FIG.  2    illustrates a plan view of a fuel cell system hotbox  13  including a fuel cell stack or column  40 . The hotbox  13  is shown to include the fuel cell stack or column  40 . However, the hotbox  13  may include two or more of the stacks or columns  40 . The stack or column  40  may include the electrically connected fuel cells  45  stacked on one another, with the interconnects  50  disposed between the fuel cells  45 . The first and last fuel cells  45  in the stack or column are disposed between a respective end plate  60  and interconnect  50 . The end plates  60  are electrically connected to electrical outputs of the fuel cell stack or column  40 . The hotbox  13  may include other components, such as fuel conduits, air conduits, seals, electrical contacts, etc., and may be incorporated into a fuel cell system including balance of plant components. The fuel cells  45  may be solid oxide fuel cells containing a ceramic electrolyte, such as yttria stabilized zirconia (YSZ) or scandia stabilized zirconia (SSZ), an anode electrode, such as a nickel-YSZ, a Ni-SSZ or a nickel-samaria doped ceria (SDC) cermet, and a cathode electrode, such as lanthanum strontium manganite (LSM)). The interconnects  50  and/or end plates  60  may comprise any suitable gas impermeable and electrically conductive material, such as a chromium-iron alloy, such as an alloy containing 4 to 6 wt % iron and balance chromium. The interconnects  50  electrically connect adjacent fuel cells  45  and provide channels for fuel and air to reach the fuel cells  45 . 
     Fuel cell systems, such as modular fuel cell system enclosure  10 , may include and/or be augmented by various pieces of support equipment. Support equipment may include various auxiliary equipment and systems to support the operation of the fuel cell system. Support equipment may vary based on constraints and/or features at a site where the fuel cell system is installed. As non-limiting examples, support equipment may include, fuel support equipment, air support equipment, and/or ventilation support equipment. One type of fuel support equipment may include equipment configured to control supply and/or exhaust fuel pressure in the fuel cell system, such as a fuel blower or pump to supply fuel to, recycle fuel/exhaust in, and/or exhaust fuel from the fuel cell system. Another type of fuel support equipment may be configured to process fuel for the fuel cell system, such as a fuel pre-heater, exhaust scrubber, etc. Other types of fuel support equipment may also be used. One type of air support equipment may be air supply equipment configured to provide air into the fuel cell system and/or exhaust air from the fuel cell system, such as blowers or fans to provide air to and/or exhaust air from a fuel cell cathode, an anode tail gas oxidizer (ATO), an air heat exchanger, a CPOx reactor, etc. Other types of air support equipment may also be used. One type of ventilation support equipment may include equipment configured to ventilate from and/or circulate air in portions of housings external of the hot box (e.g., portions within modular fuel cell system enclosure  10  but external of the hot box  13  itself), such as a ventilation fan to blow air from within the enclosure  10  out of the enclosure  10  to maintain an acceptable enclosure  10  pressure. Other types of ventilation support equipment may also be used. 
       FIG.  3    illustrates a fuel cell system based microgrid  300  suitable for implementing various embodiments. With reference to  FIGS.  1 - 3   , the fuel cell system based microgrid  300  may include multiple fuel cells  304   a ,  304   b , voltage source inverters  308   a ,  308   b , current source inverters  302   a ,  302   b ,  302   c , a rectifier  310 , a grid side bus  314 , a microgrid side bus  318 , transmission buses  316   a ,  316   b ,  316   c ,  316   d , and a transfer switch  312 . In some examples, the fuel cell system based microgrid  300  may also include storage modules  306   a ,  306   b ,  306   c.    
     As used herein, each of the fuel cells  304   a  or  304   b  may comprise a cell stack or column  40  shown in  FIG.  2    or a power module  12  shown in  FIG.  1   . In other words, a fuel cell  304   a  or  304   b  as used below is a single fuel cell power source and is not limited to being a single fuel cell  45  containing one electrolyte, one anode electrode and one cathode electrode. Furthermore, while a fuel cell microgrid  300  is described below, it should be understood that the fuel cells may be replaced with other DC power sources, such as photovoltaic power sources for example. 
     The fuel cells  304   a ,  304   b  may be electrically connected to the microgrid side bus  318  by the voltage source inverters  308   a ,  308   b . The fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , and the microgrid side bus  318  may be electrically connected via the transmission buses  316   a ,  316   b . The fuel cells  304   a ,  304  and thus the voltage source inverters  308   a ,  308   b  may be electrically connected to the microgrid side bus  318  in parallel. The fuel cells  304   a ,  304   b  may be electrically connected to the grid side bus  314  by the current source inverters  302   a ,  302   b . The fuel cells  304   a ,  304   b , the current source inverters  302   a ,  302   b , and the grid side bus  314  may be electrically connected via the transmission buses  317   a ,  317   b . The fuel cells  304   a ,  304   b  and thus the current source inverters  302   a ,  302   b  may be electrically connected to the grid side bus  314  in parallel. The grid side bus  314  and the microgrid side bus  318  may be selectively electrically connected to each other by the rectifier  310 , the current source inverter  302   c , and the transmission bus  316   c . The rectifier  310  may be electrically connected to the grid side bus  314  in parallel with the current source inverters  302   a ,  302   b . The current source inverter  302   c  may be electrically connected to the microgrid side bus  318  in parallel with the voltage source inverters  302   a ,  302   b . The grid side bus  314  and the microgrid side bus  318  may also be selectively electrically connected to each other through a transfer switch  312  and the transmission bus  316   d.    
     In some examples, the fuel cell system based microgrid  300  may also include storage modules  306   a ,  306   b ,  306   c . For example, the storage modules  306   a ,  306   b ,  306   c  may include any form of energy storage that may be converted to electric power, such as electrical storage, mechanical storage, electromechanical storage, electrochemical storage, thermal storage, etc. Examples may include a battery, a capacitor, a supercapacitor, a flywheel, a liquid reservoir, a gas reservoir, etc. In some examples, the storage modules  306   a ,  306   b ,  306   c  may include any combination of components configured to control electric energy input and output of the storage modules  306   a ,  306   b ,  306   c , such as an electric connection device and/or an electric energy conditioning device, in response to a signal from a controller  320  and/or an electric energy bus, such as transmission bus  316   a ,  316 ,  316   c . The storage modules  306   a ,  306   b  may be electrically connected to the respective fuel cells  304   a ,  304   b  and the respective voltage source inverters  306   a ,  306   b  via the transmission buses  316   a ,  316   b . The storage module  306   c  may be electrically connected in parallel to the rectifier  310  and the current source inverter  302   c  via the transmission bus  316   c . The storage modules  306   a ,  306   b ,  306   c  may also be electrically connected to microgrid side bus  318  by the transmission buses  316   a ,  316   b ,  316   c.    
     The fuel cell system based microgrid  300  may include any number and combination of controllers  320  (e.g., central processing unit (CPU), microcontroller, field programmable gate array (FPGA), application specific integrated circuit (ASIC), or any other software programmable processor) communicatively connected to the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the grid side bus  314 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   c ,  316   d ,  317   a ,  317   b , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . For example, one or more controllers  320  may be components of the fuel cell system based microgrid  300  communicatively connected to and external to the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . For another example, one or more controllers  320  may be components of the fuel cell system based microgrid  300  communicatively connected to, and integral to the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c.    
     The one or more controllers  320  may be configured to provide control signals to and/or directly control functions of the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . The one or more controllers  320  may be configured to receive signals configured to indicate to the one or more controllers  320  an AC voltage on the grid side bus  314 , the microgrid side bus  318 , and/or the transmission buses  316   a ,  316   b ,  316   c ,  316   d  from the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , and/or the transfer switch  312 . The one or more controllers  320  may be configured to directly measure an AC voltage on the grid side bus  314 , the microgrid side bus  318 , and/or the transmission buses  316   a ,  316   b ,  316   c ,  316   d ,  317   a ,  317   b  at the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the transfer switch  312 , the grid side bus  314 , the microgrid side bus  318 , and/or the transmission buses  316   a ,  316   b ,  316   c ,  316   d.    
     The fuel cells  304   a ,  304   b  may be configured to provide DC electric energy to the voltage source inverters  308   a ,  308   b  via the transmission buses  316   a ,  316   b . The DC electric energy may be configured as an amount of DC voltage needed to support a load demand of a load (i.e., “load” in  FIG.  3   ) for which the fuel cell system based microgrid  300  is deployed. The amount of DC voltage output by the fuel cells  304   a ,  304   b  to the voltage source inverters  308   a ,  308   b  may be controlled by a controller  320 . 
     The voltage source inverters  308   a ,  308   b  may be configured receive the DC voltage from the fuel cells  304   a ,  304   b , to convert the DC voltage to AC electric energy, and to provide the AC electric energy to the microgrid side bus  318  via the transmission buses  316   a ,  316   b . The AC electric energy may be configured as an amount of AC voltage needed to support at least part of a load demand. The amount of AC voltage output by the voltage source inverters  308   a ,  308   b  to the microgrid side bus  318  may be controlled to be a same amount of AC voltage for each of the voltage source inverters  308   a ,  308   b . The amount of AC voltage output by the voltage source inverters  308   a ,  308   b  may be limited by a lowest DC voltage generation capacity of one of the fuel cells  304   a ,  304   b . In other words, when the fuel cells  304   a ,  304   b  have different capacities for generating DC voltage, the lowest capacity limits the output of AC voltage of the voltage source inverters  308   a ,  308   b  electrically connected to the fuel cells  304   a ,  304   b  with higher capacity. The amount of AC voltage output by the voltage source inverters  308   a ,  308   b  may be controlled by a controller  320 . 
     The fuel cells  304   a ,  304   b  may be configured to provide DC electric energy to the current source inverters  302   a ,  302   b  via the transmission buses  317   a ,  317   b . The DC electric energy may be configured as an amount of DC voltage generated by the fuel cells  304   a ,  304   b  in excess of what is used by the voltage source inverters  308   a ,  308   b . For example, the amount of DC voltage used by the voltage source inverters  308   a ,  308   b  may be less than all of the DC voltage generated by the fuel cells  304   a ,  304   b  when an equal share of the load demand is less than the lowest DC voltage generation capacity of the fuel cells  304   a ,  304   b . In another example, the amount of DC voltage used by the voltage source inverters  308   a ,  308   b  may be less than all of the DC voltage generated by at least one of the fuel cells  304   a ,  304   b  when an equal share of the load demand is greater than the lowest DC voltage generation capacity of one of the fuel cells  304   a ,  304   b . The amount of DC voltage output by the fuel cells  304   a ,  304   b  to the current source inverters  302   a ,  302   b  may be controlled by a controller  320 . 
     The current source inverters  302   a ,  302   b  may be configured receive the DC voltage from the fuel cells  304   a ,  304   b , to convert the DC voltage to AC electric energy, and to provide the AC electric energy to the grid side bus  314  via the transmission bus  317   a ,  317   b . The AC electric energy may be configured as an amount of AC current configured to follow a volt-watt curve. The amount of AC current output by the current source inverters  302   a ,  302   b  to the grid side bus  314  may be controlled based on various electrical connections of the grid side bus  314 . For example, the grid side bus  314  may be selectively electrically connected to an electric utility power grid (i.e., “grid” in  FIG.  3   ) by the transfer switch  312 , as described further herein. The current source inverters  302   a ,  302   b  may output AC current following a volt-watt curve based on a voltage at the grid side bus  314 , the current source inverters  302   a ,  302   b , and/or the transfer switch  312 . The AC current on the grid side bus  314  may be exported to the electric utility power grid and/or used to support the load, as described further below. In another example, the grid side bus  314  may be selectively electrically connected to the microgrid side bus  318  by the transfer switch  312  and the transmission bus  316   d , as described further below. The current source inverters  302   a ,  302   b  may output AC current following a volt-watt curve based on a voltage at the grid side bus  314 , the current source inverters  302   a ,  302   b , the transfer switch  312 , the transmission bus  316   d , the microgrid side bus  318 , and/or the voltage source inverters  308   a ,  308   b . The amount of AC current output by the current source inverters  302   a ,  302   b  may be controlled by a controller  320 . 
     The transfer switch  312  may be configured to selectively electrically connect the grid side bus  314  to the electric utility power grid or to the microgrid side bus  318  via the transmission bus  316   d . The transfer switch  312  may detect availability of the electric utility power grid, for example, by detecting a voltage and/or current level of the electric utility power grid. In response to the electric utility power grid being available in a normal operating mode, the transfer switch  312  may selectively electrically connect the grid side bus  314  to the electric utility power grid and disconnect the grid side bus  314  from the transmission bus  316   d . In response to the electric utility power grid being unavailable in an emergency operating mode, the transfer switch  312  may selectively electrically connect the grid side bus  314  to the microgrid side bus  318  via the transmission bus  316   d  and disconnect the grid side bus  314  from the electric utility power grid. The transfer switch  312  may be controlled by a controller  320 . 
     The rectifier  310  may be configured to draw AC current from the grid side bus  314  via transmission bus  316   c  in response to a need for more electric energy to support the load demand when the transfer switch  312  selectively electrically connects the grid side bus  314  to the electric utility power grid. The AC voltage output to the microgrid side bus  318  by the voltage source inverters  308   a ,  308   b  may be insufficient to support the load demand. To increase the amount of AC voltage provided to the microgrid side bus  318 , the rectifier  310  may draw AC current (e.g., grid current and/or inverted fuel cell current) from the grid side bus  314 . The rectifier  310  may convert the AC current to a DC current and provide the DC current to the current source inverter  302   c  via transmission bus  316   c . Any remaining AC current at the grid side bus  314  may be exported to the electric utility power grid. In response to the load demand being satisfied by the voltage source inverters  308   a ,  308   b , the rectifier  310  may be configured to not draw AC current from the grid side bus  314 , and the AC current at the grid side bus  314  may be exported to the electric utility power grid. The amount of AC current drawn by the rectifier  310  from the grid side bus  314  and the amount of DC current output to the current source inverter  302   c  may be controlled by a controller  320 . 
     The current source inverter  302   c  may be configured to receive DC current from the rectifier  310  to convert the DC current to AC electric energy, and to provide the AC electric energy to the microgrid side bus  318  via the transmission bus  316   c . The AC electric energy may be configured as an amount of AC current configured to follow a volt-watt curve. The amount of AC current output by the current source inverter  302   c  to the microgrid side bus  318  may be controlled based on the load demand. For example, the current source inverter  302   c  may output AC current following a volt-watt curve based on a voltage at the microgrid side bus  318  and/or the voltage source inverters  308   a ,  308   b , and the load demand. The AC current output by the current source inverter  302   c  may be an amount sufficient to supplement the shortfall of the output of AC voltage of the voltage source inverters  308   a ,  308   b  to support the load demand. The amount of AC current output by the current source inverter  302   c  may be controlled by a controller  320 . 
     The transmission bus  316   d  may electrically connect the grid side bus  314  and the microgrid side bus  318  when the transfer switch  312  selectively electrically connects the buses  314 ,  318  during the emergency operating mode. The current source inverters  302   a ,  302   b  may be configured to provide the AC current to the grid side bus  314  via the transmission bus  317   a ,  317   b , and the AC current may flow to the microgrid side bus  318  via transmission bus  316   d . The amount of AC current output by the current source inverters  302   a ,  302   b  to the grid side bus  314  may be controlled based on various electrical connections of the grid side bus  314 . The current source inverters  302   a ,  302   b  may output AC current following a volt-watt curve based on a voltage at the grid side bus  314 , the current source inverters  302   a ,  302   b , the transfer switch  312 , the transmission bus  316   d , the microgrid side bus  318 , and/or the voltage source inverters  308   a ,  308   b . When the transmission bus  316   d  electrically connects the grid side bus  314  and the microgrid side bus  318 , the flow of the AC current from the grid side bus  314  to the microgrid side bus  318  may bypass at least the current source inverter  302   c . The amount of AC current output by the current source inverters  302   a ,  302   b  may be controlled by a controller  320 . 
     In some examples, the storage modules  306   a ,  306   b ,  306   c  may sink excess electric energy or source extra electric energy when needed to keep the fuel cell system based microgrid voltage stable. For example, when the transfer switch  312  selectively electrically connects the grid side bus  314  and the microgrid side bus  318 , the storage modules  306   a ,  306   b ,  306   c  may be used to provide additional electric energy to the microgrid side bus  318  or receive excess electric energy not needed at the microgrid side bus  318 . The storage modules  306   a ,  306   b ,  306   c  may keep the fuel cell system based microgrid voltage stable during the transition of the transfer switch  312  from selectively electrically connecting the fuel cell system based microgrid  300  to the electric utility power grid to selectively electrically disconnecting the fuel cell system based microgrid  300  from the electric utility power grid. 
       FIGS.  4 A- 4 C  are process flow diagrams of a method for fuel cell system based microgrid power management for the fuel cell system based microgrid  300  illustrated in  FIG.  3    according to various embodiments. With reference to  FIGS.  1 - 4 C , the method  400  may be implemented using one or more controllers  320  configured to receive signals from any number or combination of the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the grid side bus  314 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   c ,  316   d ,  37   a ,  317   b , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . The method  400  may be implemented using the one or more controllers  320  configured to send control signals to any number and combination of the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b ,  302   c , the rectifier  310 , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . In order to encompass the alternative configurations enabled in various embodiments, the hardware implementing the method  400  is referred to herein as a “control device.” Any number and combination of blocks  402 - 448  may be implemented periodically, repeatedly, or continuously, and/or concurrently with any other of blocks  402 - 448 . 
     Referring to  FIG.  4 A , in block  402 , the control device may measure a voltage at the microgrid side bus  318 . When the grid side bus  314  is disconnected from the microgrid side bus  318  by the transfer switch  312 , the voltage at the microgrid side bus  318  may be measured by receiving signals configured to indicate to the control device the voltage at the microgrid side bus  318  from, for example, any of the voltage source inverters  308   a ,  308   b , the current source inverter  302   c , and/or the transfer switch  312 . The voltage at the microgrid side bus  318  may be measured by directly measuring the voltage at the microgrid side bus  318  by the control device at any of the voltage source inverters  308   a ,  308   b , the current source inverter  302   c , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   c ,  316   d , and/or the transfer switch  312 . When the grid side bus  314  is connected to the microgrid side bus  318  by the transfer switch  312 , the voltage at the microgrid side bus  318  may be measured by receiving signals configured to indicate to the control device the voltage at the microgrid side bus  318  from, for additional example, any of the grid side bus  314  and/or the current source inverters  302   a ,  302   b . The voltage at the microgrid side bus  318  may be measured by directly measuring the voltage at the microgrid side bus  318  by the control device additionally at any of the grid side bus  314  and/or the current source inverters  302   a ,  302   b.    
     In block  404 , the control device may control the voltage source inverters  308   a ,  308   b . The control device may control the AC voltage output by the voltage source inverters  308   a ,  308   b  to the microgrid side bus  318 . For example, the control device may signal to the voltage source inverters  308   a ,  308   b  or directly set the voltage source inverters  308   a ,  308   b  to a set point for the AC voltage output. The control device may control the voltage source inverters  308   a ,  308   b  to output the same amount of AC voltage to the microgrid side bus  318 . The control device may control the voltage source inverters  308   a ,  308   b  to output AC voltage to the microgrid side bus  318  based on a load demand for a load for which the fuel cell system based microgrid  300  is deployed. The control device may additionally control the voltage source inverters  308   a ,  308   b  to limit the output AC voltage to the microgrid side bus  318  based on a lowest electric energy generation capacity from among the fuel cells  304   a ,  304   b . For example, when an equal share of the load demand exceeds the lowest electric energy generation capacity from among the fuel cells  304   a ,  304   b , the control device may limit the output AC voltage to the microgrid side bus  318  by the voltage source inverters  308   a ,  308   b  to the amount that may be output by the voltage source inverters  308   a ,  308   b  receiving DC voltage from the fuel cell  304   a ,  304   b  with the lowest electric energy generation capacity. 
     In block  406 , the control device may control the voltage source inverters  308   a ,  308   b  to output a controlled amount of AC voltage to the microgrid side bus  318 . The controlled amount of AC voltage may be based on the control the voltage source inverters  308   a ,  308   b  in block  404 . 
     In block  408 , the control device may measure a voltage at the grid side bus  314 . The voltage at the grid side bus  314  may be measured by receiving signals configured to indicate to the control device the voltage at the grid side bus  314  from, for example, any of the current source inverters  302   a ,  302   b , the transmission buses  317   a ,  317   b , the rectifier  310 , and/or the transfer switch  312 . The voltage at the grid side bus  314  may be measured by directly measuring the voltage at the grid side bus  314  by the control device at any of the current source inverters  302   a ,  302   b , the rectifier  310 , the grid side bus  314 , the transmission buses  317   a ,  317   b ,  316   c ,  316   d , and/or the transfer switch  312 . 
     In block  410 , the control device may control the current source inverters  302   a ,  302   b  to output excess electric energy to the grid side bus  314 . The current source inverters  302   a ,  302   b  may receive the DC current generated by the fuel cells  304   a ,  304   b  and not used by the voltage source inverters  308   a ,  308   b  to generate AC voltage for the microgrid side bus  318 . The control device may control the AC current output by the current source inverters  302   a ,  302   b  to the microgrid side bus  318 . For example, the control device may signal to the current source inverters  302   a ,  302   b  or directly set the current source inverters  302   a ,  302   b  to a set point for the AC current output. The control device may control the current source inverters  302   a ,  302   b  to output AC current to the grid side bus  314  based on a voltage at the grid side bus  314  and a volt-watt curve. 
     In determination block  412 , the control device may determine whether the electric utility power grid is available. The control device may detect availability of the electric utility power grid, for example, by detecting a voltage and/or current level of the electric utility power grid. The control device may detect the voltage and/or the current level of the electric utility power grid by receiving signals configured to indicate to the control device the voltage and/or the current level of the electric utility power grid from, for example, any of the current source inverters  302   a ,  302   b , the rectifier  310 , and/or the transfer switch  312 . The voltage and/or the current level of the electric utility power grid may be measured by directly measuring the voltage and/or the current level of the electric utility power grid by the control device at any of the current source inverters  302   a ,  302   b , the rectifier  310 , the grid side bus  314 , the transmission buses  317   a ,  317   b ,  316   c ,  316   d , and/or the transfer switch  312 . The control device may determine whether the electric utility power grid is available by comparing the voltage and/or the current level of the electric utility power grid to a grid availability threshold. In response to determining that the electric utility power grid is available (i.e., determination block  412 =“Yes”), the control device may continue to operate in the normal operating mode in steps “A” described below with respect to  FIG.  4 B . In response to determining that the electric utility power grid is not available (i.e., determination block  412 =“No”), the control device may operate in the emergency mode in steps “B” described below with respect to  FIG.  4 C . 
     Referring to  FIG.  4 B , in response to determining that the electric utility power grid is available (i.e., determination block  412 =“Yes”), the control device may determine whether the fuel cell system based microgrid  300  is connected to the electric utility power grid in determination block  420 . The control device may determine the connection status to the fuel cell system based microgrid  300  to the electric utility power grid based on a state, position, etc. of the transfer switch  312 . When the transfer switch  312  selectively electrically connects the grid side bus  314  to the electric utility power grid, the fuel cell system based microgrid  300  may be connected to the electric utility power grid in the normal operating mode. When the transfer switch  312  selectively electrically connects the grid side bus  314  to the microgrid side bus  318 , the fuel cell system based microgrid  300  may be disconnected from the electric utility power grid in the emergency operating mode. 
     In response to determining that the fuel cell system based microgrid  300  is not connected to the electric utility power grid (i.e., determination block  420 =“No”), the control device may electrically disconnect the grid side bus  314  from the microgrid side bus  318  and electrically connect the grid side bus  314  to the electric utility power grid in block  422 . The control device may control the transfer switch  312  to change states or positions to electrically disconnect the grid side bus  314  from the microgrid side bus  318  and electrically connect the grid side bus  314  to the electric utility power grid. 
     In response to determining that the fuel cell system based microgrid  300  is connected to the electric utility power grid (i.e., determination block  420 =“Yes”) or following block  422 , the control device may determine whether the voltage at the microgrid side bus  318  is sufficient to support the load in determination block  424 . The control device may measure the voltage at the microgrid side bus  318 , for example, as described herein with reference to block  402  or using the measurement of the voltage at the microgrid side bus  318  of block  402 . The control device may compare the measurement of the voltage at the microgrid side bus  318  to the load demand. The voltage at the microgrid side bus  318  may be insufficient when the voltage at the microgrid side bus  318  falls short of the load demand, and sufficient when the voltage at the microgrid side bus  318  meets or exceeds the load demand. 
     In response to determining that the voltage at the microgrid side bus  318  is not sufficient to support the load (i.e., determination block  424 =“No”), the control device may control the rectifier  310  and current source inverter  302   c  in block  426 . The control device may control the control the rectifier  310  and current source inverter  302   c  to provide electric energy from the grid side bus  314  to the microgrid side bus  318 . The electric energy from the grid side bus  314  may include AC current output to the grid side bus  318  by the current source inverter  302   a ,  302   b  as described herein with reference to block  410 . The control device may control the rectifier  310  and the current source inverter  302   c , for example, by signaling to control the rectifier  310  and the current source inverter  302   c  or directly setting at the rectifier  310  and the current source inverter  302   c  set points for current output. The set points for current output may be based on an amount of current needed, in addition to the voltage at the microgrid side bus  318 , to satisfy the load demand based on a volt-watt curve. 
     In block  428 , the control device may control the rectifier  310  to draw AC current from the grid side bus  314 . The control device may control the rectifier  310  to draw an amount of AC current from the grid side bus  314  and output DC current based on the control of the rectifier  310  in block  426 . In block  430 , the control device may control the current source inverter  302   c  to output a controlled amount of AC current to the microgrid side bus  318 . The control device may control the current source inverter  302   c  to convert an amount of the DC current and output the controlled amount of AC current to the microgrid side bus  318  based on the control of the current source inverter  302   c  in block  426 . 
     In response to determining that the voltage at the microgrid side bus  318  is sufficient to support the load (i.e., determination block  424 =“Yes”) or following block  430 , the control device may control export of excess electric energy at the grid side bus  314  to the electric utility power grid in block  432 . The control device may continue to measure the voltage at the microgrid side bus  318  in block  402 . 
     Referring to  FIG.  4 C , in response to determining that the electric utility power grid is not available (i.e., determination block  412 =“No”), the control device may determine whether the fuel cell system based microgrid  300  is connected to the electric utility power grid in determination block  440 . The control device may determine the connection status to the fuel cell system based microgrid  300  to the electric utility power grid based on a state, position, etc. of the transfer switch  312 . When the transfer switch  312  selectively electrically connects the grid side bus  314  to the electric utility power grid, the fuel cell system based microgrid  300  may be connected to the electric utility power grid. When the transfer switch  312  selectively electrically connects the grid side bus  314  to the microgrid side bus  318 , the fuel cell system based microgrid  300  may be disconnected from the electric utility power grid. 
     In response to determining that the fuel cell system based microgrid  300  is connected to the electric utility power grid (i.e., determination block  440 =“Yes”), the control device may electrically connect the grid side bus  314  to the microgrid side bus  318  and electrically disconnect the grid side bus  314  from the electric utility power grid in block  442 . The control device may control the transfer switch  312  to change states or positions to electrically connect the grid side bus  314  to the microgrid side bus  318  and electrically disconnect the grid side bus  314  from the electric utility power grid. 
     In response to determining that the fuel cell system based microgrid  300  is not connected to the electric utility power grid (i.e., determination block  440 =“No”) or following block  442 , the control device may determine whether the voltage at the microgrid side bus  318  is sufficient to support the load in determination block  444 . The control device may measure the voltage at the microgrid side bus  318 , for example, as described herein with reference to block  402  or using the measurement of the voltage at the microgrid side bus  318  of block  402 . The control device may compare the measurement of the voltage at the microgrid side bus  318  to the load demand. The voltage at the microgrid side bus  318  may be insufficient when the voltage at the microgrid side bus  318  falls short of the load demand, and sufficient when the voltage at the microgrid side bus  318  meets or exceeds the load demand. 
     In response to determining that the voltage at the microgrid side bus  318  is not sufficient to support the load (i.e., determination block  444 =“No”), the control device may control the current source inverters  302   a ,  302   b  in block  446 . The control device may control the control the current source inverters  302   a ,  302   b  to provide electric energy from the fuel cells  304   a ,  304   b  to the microgrid side bus  318  via the grid side bus  314  and the transmission bus  316   d . The electric energy may include AC current output to the grid side bus  318  by the current source inverters  302   a ,  302   b  as described herein with reference to block  410 . The control device may control the current source inverters  302   a ,  302   b , for example, by signaling to control the current source inverters  302   a ,  302   b  or directly setting the current source inverters  302   a ,  302   b  to set points for current output. The set points for current output may be based on an amount of current needed, in addition to the voltage at the microgrid side bus  318 , to satisfy the load demand based on a volt-watt curve. 
     In block  448 , the control device may control the current source inverters  302   a ,  302   b  to output a controlled amount of AC current to the microgrid side bus  318  via the grid side bus  314 . The control device may control the current source inverters  302   a ,  302   b  to convert an amount of the DC current and output the controlled amount of AC current to the microgrid side bus  318  via the grid side bus  314  based on the control of the current source inverters  302   a ,  302   b  in block  446 . 
     In response to determining that the voltage at the microgrid side bus  318  is sufficient to support the load (i.e., determination block  444 =“Yes”) or following block  448 , the control device may continue to measure the voltage at the microgrid side bus  318  in block  402 . 
       FIG.  5    illustrates a fuel cell system based microgrid  500  suitable for implementing various embodiments. With reference to  FIGS.  1 - 5   , the fuel cell system based microgrid  500  may include the multiple fuel cells  304   a ,  304   b , voltage source inverters  308   a ,  308   b , current source inverters  302   a ,  302   b , a grid side bus  314 , a microgrid side bus  318 , transmission buses  316   a ,  316   b ,  316   d ,  317   a ,  317   b  and a transfer switch  312 . In some examples, the fuel cell system based microgrid  500  may also include storage modules  306   a ,  306   b ,  306   c . The fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b , the grid side bus  314 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   d ,  317   a ,  317   b , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c  may be configured, structured, electrically connected, and/or function as described herein with reference to  FIGS.  3 - 4 C  unless otherwise stated. The fuel cell system based microgrid  500  may also include a motor  502 , a generator  504 , and a transmission bus  316   e  instead of the respective rectifier  310 , current source inverter  302   c  and the transmission bus  316   c  shown in  FIG.  3   . 
     The motor  502  may be configured to draw AC current from the grid side bus  314  via the transmission bus  316   e  in response to a need for more electric energy to support the load demand when the transfer switch  312  selectively electrically connects the grid side bus  314  to the electric utility power grid in the normal operating mode. The AC voltage output to the microgrid side bus  318  by the voltage source inverters  308   a ,  308   b  may be insufficient to support the load demand. To increase the amount of AC voltage provided to the microgrid side bus  318 , the motor  502  may draw AC current from the grid side bus  314 . The motor  502  may use the received AC current to drive the motor  502 . The motor  502  may operate at various speeds to drive the generator  504 . Any remaining AC current at the grid side bus  314  may be exported to the electric utility power grid. In response to the load demand being satisfied by the voltage source inverters  308   a ,  308   b , the motor  502  may be configured to not draw AC current from the grid side bus  314 , and the AC current at the grid side bus  314  may be exported to the electric utility power grid. The amount of AC current drawn by the motor  502  from the grid side bus  314  and the speed at which to operate and/or drive the generator  504  may be controlled by a controller  320 . 
     The motor  502  may drive the generator  504  using the AC current drawn from the grid side bus  314 , and the generator  504  may generate AC electric energy and provide the AC electric energy to the microgrid side bus  318  via the transmission bus  316   e . The AC electric energy may be configured as an amount of AC current configured to follow a volt-watt curve. The amount of AC current output by the generator  504  to the microgrid side bus  318  may be controlled based on the load demand. For example, the generator  504  may output AC current following a volt-watt curve based on a voltage at the microgrid side bus  318  and/or the voltage source inverters  308   a ,  308   b , and the load demand. The AC current output by the generator  504  may be an amount sufficient to supplement the shortfall of the output of AC voltage of the voltage source inverters  308   a ,  308   b  to support the load demand. The amount of AC current output by the generator  504  may be controlled by a controller  320 . 
     The storage module  306   c  may be electrically connected in parallel to the motor  502  and the generator  504 . The storage module  306   c  may be electrically connected to microgrid side bus  318  by the transmission bus  316   e.    
     In this embodiment, a large amount of the short-circuit current may be available from the generator  504 . Microgrid systems generally source far less short circuit current than grid-tied systems due to the inverter technology. Thus, the generator  504  advantageously acts to source a large amount of short circuit current in the event of a fault, quickly clearing the protective device. 
       FIGS.  6 A- 6 C  are process flow diagrams of a method for fuel cell system based microgrid power management for the fuel cell system based microgrid  500  illustrated in  FIG.  5    according to various embodiments. With reference to  FIGS.  1 - 6 C , the method  600  may be implemented using one or more controllers  320  configured to receive signals from any number or combination of the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b , the motor  502 , the generator  504 , the grid side bus  314 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   d ,  316   e ,  317   a ,  317   b , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . The method  600  may be implemented using the one or more controllers  320  configured to send control signals to any number and combination of the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b , the motor  502 , the generator  504 , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c . In order to encompass the alternative configurations enabled in various embodiments, the hardware implementing the method  600  is referred to herein as a “control device.” Any number and combination of blocks  402 - 448  and  602 - 606  may be implemented periodically, repeatedly, or continuously, and/or concurrently with any other of blocks  402 - 448  and  602 - 606 . The blocks  402 - 448  may describe portions of the method  600  in a manner similar to the blocks  402 - 448  as described herein for the method  400  with reference to  FIGS.  4 A- 4 C . 
     Referring to  FIG.  6 A , in block  402 , the control device may measure a voltage at the microgrid side bus  318 . In block  404 , the control device may control the voltage source inverters  308   a ,  308   b . In block  406 , the control device may control the voltage source inverters  308   a ,  308   b  to output a controlled amount of AC voltage to the microgrid side bus  318 . In block  408 , the control device may measure a voltage at the grid side bus  314 . In block  410 , the control device may control the current source inverters  302   a ,  302   b  to output excess electric energy to the grid side bus  314 . In determination block  412 , the control device may determine whether the electric utility power grid is available. In response to determining that the electric utility power grid is available (i.e., determination block  412 =“Yes”), the control device may continue to steps “A” in  FIG.  6 B . In response to determining that the electric utility power grid is not available (i.e., determination block  412 =“No”), the control device may continue to steps “B” in  FIG.  6 C . 
     Referring to  FIG.  6 B , in response to determining that the electric utility power grid is available (i.e., determination block  412 =“Yes”), the control device may determine whether the fuel cell system based microgrid  500  is connected to the electric utility power grid in determination block  420 . In response to determining that the fuel cell system based microgrid  500  is not connected to the electric utility power grid (i.e., determination block  420 =“No”), the control device may electrically disconnect the grid side bus  314  from the microgrid side bus  318  and electrically connect the grid side bus  314  to the electric utility power grid in block  422 . In response to determining that the fuel cell system based microgrid  500  is connected to the electric utility power grid (i.e., determination block  420 =“Yes”) or following block  422 , the control device may determine whether the voltage at the microgrid side bus  318  is sufficient to support the load in determination block  424 . 
     In response to determining that the voltage at the microgrid side bus  318  is not sufficient to support the load (i.e., determination block  424 =“No”), the control device may control the motor  502  to drive the generator  504  in block  602 . The control device may control the control the motor  502  and the generator  504  to provide electric energy from the grid side bus  314  to the microgrid side bus  318 . The electric energy from the grid side bus  314  may include AC current output to the grid side bus  318  by the current source inverter  302   a ,  302   b  as described above with reference to block  410 . The control device may control the motor  502  and the generator  504 , for example, by signaling to control the motor  502  and the generator  504  or directly setting the motor  502  and the generator  504  to set points for operating speed and/or current output. The set points for operating speed and/or current output may be based on an amount of current needed, in addition to the voltage at the microgrid side bus  318 , to satisfy the load demand based on a volt-watt curve. 
     In block  604 , the control device may control the motor  502  to draw AC current from the grid side bus  314 . The control device may control the motor  502  to draw an amount of AC current from the grid side bus  314  to operate at a certain speed based on the control of the motor  502  in block  602 . In block  606 , the control device may control the generator  504  to output a controlled amount of AC current to the microgrid side bus  318 . The control device may control the generator  504  to operate at a certain speed and output the controlled amount of AC current to the microgrid side bus  318  based on the control of the generator in block  602 . 
     In response to determining that the voltage at the microgrid side bus  318  is sufficient to support the load (i.e., determination block  424 =“Yes”) or following block  430 , the control device may control export of excess electric energy at the grid side bus  314  to the electric utility power grid in block  432 . The control device may continue to measure the voltage at the microgrid side bus  318  in block  402 . 
     Referring to  FIG.  6 C , in response to determining that the electric utility power grid is not available (i.e., determination block  412 =“No”), the control device may determine whether the fuel cell system based microgrid  500  is connected to the electric utility power grid in determination block  440 . In response to determining that the fuel cell system based microgrid  500  is connected to the electric utility power grid (i.e., determination block  440 =“Yes”), the control device may electrically connect the grid side bus  314  to the microgrid side bus  318  and electrically disconnect the grid side bus  314  from the electric utility power grid in block  442 . In response to determining that the fuel cell system based microgrid  500  is not connected to the electric utility power grid (i.e., determination block  440 =“No”) or following block  442 , the control device may determine whether the voltage at the microgrid side bus  318  is sufficient to support the load in determination block  444 . In response to determining that the voltage at the microgrid side bus  318  is not sufficient to support the load (i.e., determination block  444 =“No”), the control device may control the current source inverters  302   a ,  302   b  in block  446 . In block  448 , the control device may control the current source inverters  302   a ,  302   b  to output a controlled amount of AC current to the microgrid side bus  318  via the grid side bus  314 . In response to determining that the voltage at the microgrid side bus  318  is sufficient to support the load (i.e., determination block  444 =“Yes”) or following block  448 , the control device may continue to measure the voltage at the microgrid side bus  318  in block  402 . 
       FIG.  7    illustrates a fuel cell system based microgrid  700  suitable for implementing various embodiments. With reference to  FIGS.  1 - 7   , the fuel cell system based microgrid  700  may include the multiple fuel cells  304   a ,  304   b , voltage source inverters  308   a ,  308   b , current source inverters  302   a ,  302   b , a grid side bus  314 , a microgrid side bus  318 , transmission buses  316   a ,  316   b ,  316   d ,  317   a ,  317   b , and a transfer switch  312 . In some examples, the fuel cell system based microgrid  700  may also include storage modules  306   a ,  306   b ,  306   c . The fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b , the grid side bus  314 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   d ,  317   a ,  317   b , the transfer switch  312 , and/or the storage modules  306   a ,  306   b ,  306   c  may be configured, structured, electrically connected, and/or function as described herein with reference to  FIGS.  3 - 6 C  unless otherwise stated. The fuel cell system based microgrid  700  may also include a relay  702 , an electric contactor  704 , and a transmission bus  316   f  instead of the motor  502 , the generator  504 , and the transmission bus  316   e  shown in  FIG.  5   . 
     The current source inverters  302   a ,  302   b  may be configured to provide the AC current to the grid side bus  314  following a volt-watt curve. The amount of AC current output by the current source inverters  302   a ,  302   b  to the grid side bus  314  may be controlled based on various electrical connections of the grid side bus  314 . For example, the grid side bus  314  may be selectively electrically connected to an electric utility power grid (i.e., “grid” in  FIG.  7   ) by the transfer switch  312  in the normal operating mode. The current source inverters  302   a ,  302   b  may output AC current following a volt-watt curve based on a voltage at the grid side bus  314 , the microgrid side bus  318 , the current source inverters  302   a ,  302   b , the voltage source inverters  308   a ,  308   b , the relay  702 , the electric contactor  704 , the transmission bus  316   f , and/or the transfer switch  312 . The AC current on the grid side bus  314  may be exported to the electric utility power grid and/or used to support the load by flowing to the microgrid side bus  318 . In another example, the grid side bus  314  may be selectively electrically connected to the microgrid side bus  318  by the transfer switch  312  and the transmission bus  316   d  in the emergency operating mode. The current source inverters  302   a ,  302   b  may output AC current following a volt-watt curve based on a voltage at the grid side bus  314 , the current source inverters  302   a ,  302   b , the transfer switch  312 , the transmission bus  316   d , the microgrid side bus  318 , and/or the voltage source inverters  308   a ,  308   b . The amount of AC current output by the current source inverters  302   a ,  302   b  may be controlled by a controller  320 . 
     The relay  702  may be configured to detect current flow between the grid side bus  314  and the microgrid side bus  318  at the transmission bus  316   f . When the AC current at the grid side bus  314  supports the load, AC current flows from the grid side bus  314  to the microgrid side bus  314  in a “forward flow”. However, there are instances in which AC current may flow from the microgrid side bus  314  to the grid side bus  314  in a “reverse flow”. The electric contactor  704  may be electronically controlled to allow or interrupt current flow on the transmission bus  316   f . The electric contactor  704  may be any form of electronically controlled contactor, such as a circuit breaker, switch, etc. 
     In response to detecting the reverse flow on the transmission bus  316   f , the relay  702  may signal to the electric contactor  704  to interrupt the reverse flow on the transmission bus  316   f . In some examples, the relay  702  may signal directly to the electric contactor  704  to interrupt the reverse flow on the transmission bus  316   f . In some examples, the relay  702  may signal to the electric contactor  704  to interrupt the reverse flow on the transmission bus  316   f  via the controller  320 , by signaling detection of the reverse flow on the transmission bus  316   f  to the controller  320 , and the controller  320  signaling to the electric contactor  704  to interrupt the reverse flow on the transmission bus  316   f.    
     The transfer switch  312  and the electric contactor  704  positions or states may be interlocked. For example, when the transfer switch  312  selectively electrically connects the grid side bus  314  to the electric utility power grid, in the normal operating mode, the electric contactor  704  may maintain an electrical connection between the grid side bus  314  and the microgrid side bus  318  on the transmission bus  316   f , and vice vera. In another example, when the transfer switch  312  selectively electrically connects the grid side bus  314  to the microgrid side bus  318  in the emergency operating mode, the electric contactor  704  may interrupt an electrical connection between the grid side bus  314  and the microgrid side bus  318  on the transmission bus  316   f , and vice vera. 
       FIGS.  8 A- 8 C  are process flow diagrams of a method for fuel cell system based microgrid power management for the fuel cell system based microgrid  700  illustrated in  FIG.  7    according to various embodiments. With reference to  FIGS.  1 - 8 C , the method  800  may be implemented using one or more controllers  320  configured to receive signals from any number or combination of the fuel cells  304   a ,  304   b , the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b , the relay  702 , the electric contactor  704 , the grid side bus  314 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   d ,  316   f ,  317   a ,  317   b , the transfer switch  312 , and/or the storage modules  306   a ,  306   b . The method  800  may be implemented using the one or more controllers  320  and/or the relay  704  configured to send control signals to any number and combination of the voltage source inverters  308   a ,  308   b , the current source inverters  302   a ,  302   b , the electrical contactor  704 , the transfer switch  312 , and/or the storage modules  306   a ,  306   b . In order to encompass the alternative configurations enabled in various embodiments, the hardware implementing the method  800  is referred to herein as a “control device.” Any number and combination of blocks  402 - 448  and  802 - 808  may be implemented periodically, repeatedly, or continuously, and/or concurrently with any other of blocks  402 - 448  and  802 - 808 . The blocks  402 - 448  may describe portions of the method  800  in a manner similar to the blocks  402 - 448  as described herein for the method  400  with reference to  FIGS.  4 A- 4 C . 
     Referring to  FIG.  8 A , in block  802 , the control device may measure a voltage and/or a current at the microgrid side bus  318 . When the grid side bus  314  is connected to the microgrid side bus  318  by the electric contactor  704 , the voltage and/or current at the microgrid side bus  318  may be measured by receiving signals configured to indicate to the control device the voltage and/or current at the microgrid side bus  318  from, for example, any of the voltage source inverters  308   a ,  308   b , the relay  702 , the electric contactor  704 , and/or the transfer switch  312 . The voltage and/or current at the microgrid side bus  318  may be measured by directly measuring the voltage and/or current at the microgrid side bus  318  by the control device at any of the voltage source inverters  308   a ,  308   b , the relay  702 , the electric contactor  704 , the microgrid side bus  318 , the transmission buses  316   a ,  316   b ,  316   d ,  316   f , and/or the transfer switch  312 . When the grid side bus  314  is connected to the microgrid side bus  318  by the transfer switch  312 , the voltage and/or current at the microgrid side bus  318  may be measured by receiving signals configured to indicate to the control device the voltage and/or current at the microgrid side bus  318  from, for additional example, any of the grid side bus  314  and/or the current source inverters  302   a ,  302   b . The voltage and/or current at the microgrid side bus  318  may be measured by directly measuring the voltage at the microgrid side bus  318  by the control device additionally at any of the grid side bus  314  and/or the current source inverters  302   a ,  302   b.    
     In block  804 , the control device may control the voltage source inverters  308   a ,  308   b . The control device may control the AC voltage output by the voltage source inverters  308   a ,  308   b  to the microgrid side bus  318 . For example, the control device may signal to the voltage source inverters  308   a ,  308   b  or directly set at the voltage source inverters  308   a ,  308   b  a set point for the AC voltage output. The control device may control the voltage source inverters  308   a ,  308   b  to output the same amount of AC voltage to the microgrid side bus  318 . The control device may control the voltage source inverters  308   a ,  308   b  to output AC voltage to the microgrid side bus  318  based on a load demand for a load for which the fuel cell system based microgrid  700  is deployed. The control device may control the voltage source inverters  308   a ,  308   b  to output AC voltage to the microgrid side bus  318  to prevent export of the current on the microgrid side bus  318  to the electric utility power grid. The control device may additionally control the voltage source inverters  308   a ,  308   b  to limit the output AC voltage to the microgrid side bus  318  based on a lowest electric energy generation capacity from among the fuel cells  304   a ,  304   b . For example, when an equal share of the load demand exceeds the lowest electric energy generation capacity from among the fuel cells  304   a ,  304   b , the control device may limit the output AC voltage to the microgrid side bus  318  by the voltage source inverters  308   a ,  308   b  to the amount that may be output by the voltage source inverters  308   a ,  308   b  receiving DC voltage from the fuel cell  304   a ,  304   b  with the lowest electric energy generation capacity. 
     In block  806 , the control device may control the voltage source inverters  308   a ,  308   b  to output a controlled amount of AC voltage to the microgrid side bus  318 . The controlled amount of AC voltage may be based on the control the voltage source inverters  308   a ,  308   b  in block  804 . 
     In block  408 , the control device may measure a voltage at the grid side bus  314 . In block  410 , the control device may control the current source inverters  302   a ,  302   b  to output excess electric energy to the grid side bus  314 . In determination block  412 , the control device may determine whether the electric utility power grid is available. In response to determining that the electric utility power grid is available (i.e., determination block  412 =“Yes”), the control device may continue to steps “A” in  FIG.  8 B . In response to determining that the electric utility power grid is not available (i.e., determination block  412 =“No”), the control device may continue to steps “B” in  FIG.  8 C . 
     Referring to  FIG.  8 B , in response to determining that the electric utility power grid is available (i.e., determination block  412 =“Yes”), the control device may determine whether the fuel cell system based microgrid  700  is connected to the electric utility power grid in determination block  420 . In response to determining that the fuel cell system based microgrid  700  is not connected to the electric utility power grid (i.e., determination block  420 =“No”), the control device may electrically disconnect the grid side bus  314  from the microgrid side bus  318  via the transmission bus  316   f  and electrically connect the grid side bus  314  to the electric utility power grid in block  808 . The control device may control the transfer switch  312  to change states or positions to electrically connect the grid side bus  314  to the electric utility power grid and to electrically disconnect the grid side bus  314  from the microgrid side bus  318  via the transmission bus  316   d.    
     In response to determining that the fuel cell system based microgrid  700  is connected to the electric utility power grid (i.e., determination block  420 =“Yes”) or following block  808 , the control device may determine whether there is reverse power flow on the transmission bus  316   f  in determination block  810 . In some examples, the control device may detect reverse power flow on the transmission bus  316   f  by receiving a signal from a relay  702  configured to indicate to the control device the presence of the reverse flow. Reverse power flow may occur is the load electric energy (e.g., power) demand is less than the total electric energy (e.g., power) provided by the voltage source inverters  308   a ,  308   b  to the microgrid side bus  318 . In contrast, if the load electric energy (e.g., power) demand is greater than the total electric energy (e.g., power) provided by the voltage source inverters  308   a ,  308   b , then no reverse power flow occurs on the transmission bus  316   f  because additional electric energy (e.g., power) is drawn by the load from the current source inverters  302   a ,  302   b  and/or from the electric utility power grid. 
     In response to determining that there is no reverse power flow on the transmission bus  316   f  (i.e., determination block  810 =“No”), the control device may control the current source inverters  302   a ,  302   b  in block  812 . The control device may control the control the current source inverters  302   a ,  302   b  to provide electric energy from the fuel cells  304   a ,  304   b  to the microgrid side bus  318  via the grid side bus  314  and the transmission bus  316   f . The electric energy may include AC current output to the grid side bus  318  by the current source inverters  302   a ,  302   b  as described herein with reference to block  410 . The control device may control the current source inverters  302   a ,  302   b , for example, by signaling to control the current source inverters  302   a ,  302   b  or directly setting the current source inverters  302   a ,  302   b  to set points for current output. The set points for current output may be based on an amount of current needed, in addition to the voltage at the microgrid side bus  318 , to satisfy the load demand based on a volt-watt curve. 
     In block  814 , the control device may control the electrical contactor  704  to close to electrically connect the grid side bus  314  to the microgrid side bus  318  via the transmission bus  316   f . In other words, if the load power demand is below the power output of the voltage source inverters  308   a ,  308   b , then the contactor  704  is closed to provide the excess power from the current source inverters  302   a ,  302   b  to the load via the grid side bus  314 , the transmission bus  316   f  and the microgrid side bus  318  to satisfy the load power demand. 
     In response to determining that there is reverse flow on the transmission bus  316   f  (i.e., determination block  810 =“Yes”), the control device may control the electric contactor  704  to open to prevent the reverse flow on the transmission bus  316   f  in block  816 . The control device may control the electrical contactor  704  to change states or positions (i.e., to open) to electrically disconnect the grid side bus  314  from the microgrid side bus  318  via the transmission bus  316   f . In other words, if the load power demand is lower than the power output of the voltage source inverters  308   a ,  308   b , then the contactor  704  is opened to prevent reverse power flow from the microgrid bus  318  to the utility electric power grid. 
     Following block  814  or block  816 , the control device may control export of excess electric energy at the grid side bus  314  to the electric utility power grid in block  432 . The control device may continue to measure the voltage at the microgrid side bus  318  in block  802 . 
     Referring to  FIG.  8 C , in response to determining that the electric utility power grid is not available (i.e., determination block  412 =“No”), the control device may determine whether the fuel cell system based microgrid  700  is connected to the electric utility power grid in determination block  440 . In response to determining that the fuel cell system based microgrid  700  is connected to the electric utility power grid (i.e., determination block  440 =“Yes”), the control device may electrically connect the grid side bus  314  to the microgrid side bus  318  via transmission bus  316   d  and electrically disconnect the grid side bus  314  from the electric utility power grid in block  818 . 
     The control device may control the transfer switch  312  to change states or positions to electrically connect the grid side bus  314  to the microgrid side bus  318  via transmission bus  316   d  and electrically disconnect the electrically connect the grid side bus  314  from the electric utility power grid. The control device may control the electrical contactor  704  to change states or positions (i.e., to open) to electrically disconnect the grid side bus  314  from the microgrid side bus  318  via the transmission bus  316   f.    
     In response to determining that the fuel cell system based microgrid  700  is not connected to the electric utility power grid (i.e., determination block  440 =“No”) or following block  818 , the control device may determine whether the voltage at the microgrid side bus  318  is sufficient to support the load in determination block  444 . In response to determining that the voltage at the microgrid side bus  318  is not sufficient to support the load (i.e., determination block  444 =“No”), the control device may control the current source inverters  302   a ,  302   b  in block  446 . In block  448 , the control device may control the current source inverters  302   a ,  302   b  to output a controlled amount of AC current to the microgrid side bus  318  via the grid side bus  314 . In response to determining that the voltage at the microgrid side bus  318  is sufficient to support the load (i.e., determination block  444 =“Yes”) or following block  448 , the control device may continue to measure the voltage at the microgrid side bus  318  in block  802 . 
     The preceding description of the disclosed aspects is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects without departing from the scope of the invention. Thus, the present invention is not intended to be limited to the aspects shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. 
     The foregoing method descriptions and diagrams are provided merely as illustrative examples and are not intended to require or imply that the steps of the various embodiments must be performed in the order presented. As will be appreciated by one of skill in the art the order of steps in the foregoing embodiments may be performed in any order. Further, words such as “thereafter,” “then,” “next,” etc. are not intended to limit the order of the steps; these words are simply used to guide the reader through the description of the methods. 
     One or more diagrams have been used to describe exemplary embodiments. The use of diagrams is not meant to be limiting with respect to the order of operations performed. The foregoing description of exemplary embodiments has been presented for purposes of illustration and of description. It is not intended to be exhaustive or limiting with respect to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the disclosed embodiments. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. 
     Control elements, including the control device as well as controllers  320  described herein, may be implemented using computing devices (such as computer) that include programmable processors, memory and other components that have been programmed with instructions to perform specific functions or may be implemented in processors designed to perform the specified functions. A processor may be any programmable microprocessor, microcomputer or multiple processor chip or chips that can be configured by software instructions (applications) to perform a variety of functions, including the functions of the various embodiments described herein. In some computing devices, multiple processors may be provided. Typically, software applications may be stored in the internal memory before they are accessed and loaded into the processor. In some computing devices, the processor may include internal memory sufficient to store the application software instructions. 
     The various illustrative logical blocks, modules, circuits, and algorithm steps described in connection with the embodiments disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. To clearly illustrate this interchangeability of hardware and software, various illustrative components, blocks, modules, circuits, and steps have been described above generally in terms of their functionality. Whether such functionality is implemented as hardware or software depends upon the particular application and design constraints imposed on the overall system. Skilled artisans may implement the described functionality in varying ways for each particular application, but such implementation decisions should not be interpreted as causing a departure from the scope of the present invention. 
     The hardware used to implement the various illustrative logics, logical blocks, modules, and circuits described in connection with the aspects disclosed herein may be implemented or performed with a control device that may be or include a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but, in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. Alternatively, some blocks or methods may be performed by circuitry that is specific to a given function. 
     Fuel cell systems of the embodiments of the present disclosure are designed to reduce greenhouse gas emissions and have a positive impact on the climate. 
     The preceding description of the disclosed embodiments is provided to enable any person skilled in the art to make or use any of the described embodiments. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the scope of the disclosure. Thus, the claims are not intended to be limited to the embodiments shown herein but are to be accorded the widest scope consistent with the claim language and the principles and novel features disclosed herein.