Power management in microgrids based on fuel cell systems

Abstract

To provide direct current (DC) power sources, such as fuel cell system based microgrids with more efficient use of fuel cell system capacity.SOLUTION: In a microgrid 300, excess DC current generated by a fuel cell stack 304 is provided to a current source inverter 302, and an AC current is output by the current source inverter 302 to a grid side bus 314. The AC current on the grid side bus 314 supports a load demand on a microgrid side bus 318 or is provided to a power grid. Further, transmission buses and electric conditioning and control devices, such as rectifiers 310, current source inverters 302, motors, generators, electric contactors, relays, and / or transfer switches 312 are used to utilize the AC current on the grid side bus 314 to provide an AC current to the microgrid side bus 318 to support the load demand on the microgrid side bus 318.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a direct current (DC) power source such as a microgrid based on a fuel cell system with more efficient use of the fuel cell system capacity.

Background Art

[0002] A common method of powering a microgrid system is a master / slave voltage source inverter relationship, where each voltage source inverter follows the command of a single master to maintain the output voltage, that is, all of the voltage source inverters deliver the same amount of voltage.

Summary of the Invention

[0003] According to one embodiment, a microgrid includes a plurality of direct current (DC) power sources, a plurality of voltage source inverters, a microgrid side bus configured to be electrically connected to a load, a plurality of current source inverters, a grid side bus, a switching switch configured to control a selective electrical connection of the grid side bus to an electrical utility power grid or the microgrid side bus, and a transmission bus electrically connected between the microgrid side bus and the grid side bus. Each DC terminal of each of the plurality of voltage source inverters is electrically connected to each of the plurality of DC power sources, each AC terminal of each of the plurality of voltage source inverters is electrically connected to the microgrid side bus, each DC terminal of each of the plurality of current source inverters is electrically connected to each of the plurality of DC power sources, and each AC terminal of each of the plurality of current source inverters is electrically connected to the grid side bus.

[0004] According to another embodiment, a method for operating a microgrid includes supplying electrical energy from each of a plurality of DC power sources to each of a plurality of voltage source inverters and each of a plurality of current source inverters, and having the plurality of voltage source inverters output voltages to the microgrid-side bus such that each of the plurality of voltage source inverters outputs substantially equal amounts of voltage to the microgrid-side bus, wherein the maximum output voltage of each of the plurality of voltage source inverters is based on the minimum generating capacity of one of the plurality of DC power sources, and having the plurality of current source inverters output a first current to the grid-side bus based on the amount of current generated by the plurality of DC power sources that exceeds the minimum generating capacity, and using the first current output to the grid-side bus to supply a second current to the microgrid-side bus. [Brief explanation of the drawing]

[0005] [Figure 1] Figure 1 is a perspective view of a fuel cell system according to various embodiments suitable for carrying out various models. [Figure 2] Figure 2 is a schematic side cross-sectional view of a hotbox according to various embodiments suitable for carrying out various models. [Figure 3] Figure 3 is a block diagram of the components of a microgrid based on a fuel cell system suitable for implementing various embodiments. [Figure 4A] Figure 4A is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 3, according to various embodiments. [Figure 4B] Figure 4B is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 3, according to various embodiments. [Figure 4C] Figure 4C is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 3, according to various embodiments. [Figure 5]Figure 5 is a block diagram of the components of a microgrid based on a fuel cell system suitable for implementing various embodiments. [Figure 6A] Figure 6A is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 5, according to various embodiments. [Figure 6B] Figure 6B is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 5, according to various embodiments. [Figure 6C] Figure 6C is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 5, according to various embodiments. [Figure 7] Figure 7 is a block diagram of the components of a microgrid based on a fuel cell system suitable for implementing various embodiments. [Figure 8A] Figure 8A is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 7, according to various embodiments. [Figure 8B] Figure 8B is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 7, according to various embodiments. [Figure 8C] Figure 8C is a process flow diagram of a method for managing the power of a microgrid based on the fuel cell system shown in Figure 7, according to various embodiments. [Modes for carrying out the invention]

[0006] Various embodiments will be described in detail with reference to the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings to refer to the same or similar parts. References to specific examples and embodiments are for illustrative purposes only and are not intended to limit the scope of the claims.

[0007] As used herein, the terms “DC power source” and “DC power supply” are interchangeable to refer to a generator capable of generating electricity from any source, such as fuel cells, combustion generators, photocells, concentrated solar systems, wind turbines, geothermal turbines, hydroelectric turbines, gas turbines, nuclear reactors, AC generators, induction generators, etc. The examples described herein with respect to fuel cells do not limit the scope of the claims and specification to such types of DC power sources. In some embodiments, the DC power source may be an AC generator combined with an AC / DC rectifier.

[0008] As used herein, the terms “storage system” and “energy storage system” refer to any form of energy storage that can be used interchangeably and converted into electricity, such as an electrical storage unit, a mechanical storage unit, an electromechanical storage unit, an electrochemical storage unit, or a thermal storage unit. Examples include batteries, capacitors, supercapacitors, flywheels, liquid reservoirs, gas reservoirs, and the like. In some embodiments, the energy storage system may include any combination of components, such as an electrical connector and / or an electrical energy regulator, configured to control the electrical energy output of the energy storage system in response to signals from a controller and / or an electrical energy bus.

[0009] As used herein, the terms “electrical energy” and “electrical energy output” refer to quantities of voltage, current, or power. The examples given herein with respect to voltage do not limit the claims and the scope of this specification to such types of electrical energy and electrical energy output.

[0010] The inventors have recognized that a weakness of the prior art master / slave control scheme is the difficulty in fully utilizing voltage source inverters with different or variable DC power capacities. Because each voltage source inverter follows the same single command, the DC power source with the weakest DC capacity tends to limit the maximum capacity of multiple inverters connected in parallel. As a result, any excess electrical energy generated by a DC power source that exceeds the electrical energy generated by the weakest DC power source in the microgrid is wasted.

[0011] Various embodiments include electrical circuits, electrical components, and methods for power management in a DC-powered microgrid that utilize both voltage source inverters and current source inverters for multiple DC power sources. The current source inverter can output excess electrical energy generated by DC power sources other than the weakest DC power source in the microgrid. Therefore, the excess electrical energy is not wasted and is supplied to the power grid and / or loads.

[0012] In one embodiment, the DC power source within the microgrid may include a fuel cell DC power source. A microgrid based on a fuel cell system may comprise a plurality of DC-to-AC voltage source inverters that electrically connect a plurality of fuel cell stacks (or a plurality of power modules each containing a plurality of columns of a fuel cell stack) of the fuel cell system in parallel to a microgrid bus; a plurality of DC-to-AC current source inverters that electrically connect the fuel cell stacks in parallel to the grid bus; and at least one electrical energy control device configured to control the electrical connection between the grid bus and the microgrid bus. A method for power management in a microgrid based on a fuel cell system may include controlling the current source inverters to output excess electrical energy generated by the fuel cell stacks to the grid bus, and controlling at least one electrical energy control device to electrically connect the grid bus and the microgrid bus and to supply electrical energy from the grid bus and the microgrid bus to support the load.

[0013] In microgrid applications, voltage is generated using inverters without connecting to the electric utility power grid. To achieve this, the microgrid system ensures that the voltage waveform remains constant by monitoring the output voltage and adjusting the power of the voltage source inverters in real time. One common method to achieve this goal is to monitor the output voltage with a single point sensor and supply control signals to a bank of voltage source inverters. This method employs a master / slave voltage source inverter relationship, where each voltage source inverter follows commands from a single master to maintain its output voltage, i.e., all voltage source inverters deliver the same amount of voltage. The strength of this method lies in its simplicity. Commands to the voltage source inverters are derived directly from the voltage signal in real time, and adjustments can be made to use any number of voltage source inverters to follow the commands, allowing for capacity expansion and contraction.

[0014] A weakness of this approach is that it is difficult to utilize the full DC electrical energy capacity from each DC power source available to each voltage source inverter during use. Since the master must supply a single command to all inverters, it is useful to consider the command as a percentage of full power. In a microgrid system supported by variable-capacity DC power sources, the available DC power is not fixed and is not always 100% of the intended rating. This means that if many DC power sources are attached to individual voltage source inverters connected in parallel, one of these DC power sources will be the weakest at any given time. Because each voltage source inverter follows the same single command, the weakest DC power source limits the maximum capacity of the multiple parallel voltage source inverters. If the master commands more power from the voltage source inverters, the weakest system will reach its limit first, resulting in a failure of the microgrid system to respond. Thus, the capacity of the entire bank of inverters is artificially limited by the DC power capacity of the weakest individual voltage source inverter. For N inverters, a loss of XkW from a single DC source results in a capacity loss of N×XkW for the microgrid system.

[0015] The embodiments described herein address the aforementioned weaknesses of microgrid systems. In a microgrid based on a fuel cell system, the fuel cell can supply a fixed amount of continuous DC electrical energy. A voltage source inverter can supply electrical energy to the microgrid based on the fuel cell system according to a single voltage control command, and a current source inverter can feed the remaining electrical energy to the grid during normal (i.e., steady-state, non-emergency) modes in which the grid is available. The voltage source inverter can supply the electrical energy required by the load, and the current source inverter can feed all the electrical energy beyond what is needed to support the load.

[0016] A current source inverter can directly measure the voltage from an external source (usually an electrical utility power grid) present on its terminals and push current from a DC power source in synchronization with the voltage waveform. These current source inverters (sometimes called grid tie or grid parallel inverters) can arbitrarily generate all the maximum output currents that an electrically connected DC power source can supply. The current source inverter enables detection of the DC electrical energy input from each DC power source and can determine when it reaches its capacity.

[0017] Embodiments provide a circuit that enables a current source inverter to send excess electrical energy generated by a DC power source (such as a fuel cell stack) to an electrical utility power grid and / or supply electrical energy from the grid side bus of a microgrid system to the microgrid side bus connected to the load of the microgrid system. The electrical energy can arbitrarily move from the grid side bus to the microgrid side bus, and thus up to 100% of the total DC electrical energy generated by a DC power source such as a fuel cell stack can be utilized.

[0018] FIG. 1 shows an example of a power generator including a modular fuel cell system fully described by U.S. Patent No. 8,440,362. This U.S. Patent is incorporated herein by reference with respect to the description of the modular fuel cell system. This modular system can include the modules and components described above and in U.S. Patent No. 9,190,693. This U.S. Patent is incorporated herein by reference with respect to the description of the modular fuel cell system. The modular design of the fuel cell system housing 10 provides for flexible system installation and operation.

[0019] The modular fuel cell system housing 10 includes a plurality of power module housings 12 (which house 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 housing can include from 2 to 30 power modules, such as any desired number of modules, for example, from 6 to 12 power modules. FIG. 1 shows a system housing 10 that includes six power modules (six modules stacked in a single row) on a common base 20, one fuel processing module, and one power conditioning module. Each module can include its own cabinet or housing. Alternatively, the power conditioning module and the fuel processing module can be combined into a single input / output module located within one cabinet or housing 14. For simplicity, each housing 12, 14, 16, 18 is hereinafter referred to as a "module".

[0020] A single row of power modules 12 is shown, but the system can include two or more rows of modules 12. For example, the system can include two rows of power modules stacked back-to-back.

[0021] Each power module 12 is configured to house one or more hot boxes 13. Each hot box houses one or more stacks or columns (not shown for clarity) of fuel cells, such as one or more stacks or columns of solid oxide fuel cells having a ceramic oxide electrolyte separated by a conductive interconnect plate. Other types of fuel cells, such as PEM, molten carbonate, phosphoric acid, etc., can also be used.

[0022] The modular fuel cell system housing 10 also includes one or more input or fuel processing modules 16. Each module 16 comprises a cabinet for housing components used for fuel pretreatment, such as a desulfurization bed. The fuel processing modules 16 can 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 can be provided in the same or separate cabinet. Different bed compositions tailored to specific fuels can be provided within each module. The processing module(s) 16 can process at least one of the fuels selected from natural gas, compressed natural gas, methane, propane, liquefied petroleum gas, gasoline, diesel, household heating oil, kerosene, JP-5, JP-8, aviation fuel, hydrogen, ammonia, ethanol, methanol, synthesis gas, biogas, biodiesel, and other suitable hydrocarbons or hydrogen-containing fuels supplied from a pipeline. A reformer 17 may be placed within the fuel processing module 16 if desired. Alternatively, if it is desirable to thermally integrate the reformer 17 with the fuel cell stack(s), a separate reformer 17 can be placed in each hot box 13 within each power module 12. Furthermore, if an internally reformed fuel cell is used, the external reformer 17 can be omitted entirely.

[0023] The modular fuel cell system enclosure 10 also includes one or more power adjustment modules 18. The power adjustment module 18 comprises components for converting DC power generated by the fuel cell stack to AC power, electrical connectors for AC power output to the grid, circuits for managing electrical transients, and a cabinet housing a system controller (e.g., a computer or a dedicated control logic device or circuit). The power adjustment module 18 can be designed to convert DC power from the fuel cell modules to different AC voltages and frequencies. Designs can be provided for 208V, 60Hz; 480V, 60Hz; 415V, 50Hz and other common voltages and frequencies.

[0024] The fuel processing module 16 and the power adjustment module 18 may be housed in a single input / output cabinet 14. If a single input / output cabinet 14 is provided, the modules 16 and 18 can be arranged vertically within the cabinet 14 (for example, with the components of the power adjustment module 18 above the desulfurization canister / floor of the fuel processing module 16) or side by side.

[0025] As shown in the exemplary embodiment of Figure 1, one input / output cabinet 14 is provided for one row of six power modules 12, and the six power modules 12 are arranged in a linear, side-by-side manner on one side of the input / output cabinet 14. The row of modules can be positioned, for example, adjacent to a building to which the system will supply power (for example, so that the back of the module cabinets faces the wall of the building). Although one row of power modules 12 is shown, the system may comprise two or more rows of modules 12. For example, as described above, the system may comprise two rows of power modules stacked back-to-back.

[0026] Each of the power module 12 and the input / output module 14 is provided with a door 30 (e.g., a hatch, access panel, etc.) that allows access to the internal components of the module (e.g., for maintenance, repair, replacement, etc.). According to one embodiment, the modules 12 and 14 are arranged in a linear arrangement with the door 30 on only one side of each cabinet, allowing consecutive rows of the system to be installed end-to-end. In this way, the size and capacity of the fuel cell housing 10 can be adjusted by additional modules 12 or 14 and base 20, while minimizing the need to rearrange the existing modules 12 and 14 and base 20. If desired, the door 30 to the module 14 may be located on the side of the cabinet rather than the front.

[0027] Figure 2 shows a plan view of a fuel cell system hotbox 13 comprising a fuel cell stack or column 40. The hotbox 13 is shown as comprising a fuel cell stack or column 40, however, the hotbox 13 may comprise two or more stacks or columns 40. The stacks or columns 40 may contain fuel cells 45 that are stacked on top of each other and electrically connected, with interconnects 50 located between the fuel cells 45. The first and last fuel cells 45 in the stack or column are positioned between their respective end plates 60 and interconnects 50. The end plates 60 are electrically connected to the electrical output of the fuel cell stack or column 40. The hotbox 13 may comprise other components such as fuel conduits, air conduits, seals, and electrical contacts, and can be incorporated into a fuel cell system that includes balance of plant components. The fuel cell 45 may be a solid oxide fuel cell comprising a ceramic electrolyte such as yttria-stabilized zirconia (YSZ) or scandia-stabilized zirconia (SSZ), an anode electrode such as nickel-YSZ, Ni-SSZ, or nickel-samaria-doped ceria (SDC) cermet, and a cathode electrode such as lanthanum strontium manganite (LSM). The interconnect 50 and / or end plate 60 may include any suitable gas-impermeable conductive material, such as a chromium-iron alloy, for example, an alloy containing 4 wt% to 6 wt% iron and the remainder chromium. The interconnect 50 electrically connects adjacent fuel cells 45 and provides channels for fuel and air to reach the fuel cells 45.

[0028] A fuel cell system, such as a modular fuel cell system housing 10, may include and / or be expanded by various support equipment. The support equipment may include various auxiliary equipment and systems to assist the operation of the fuel cell system. The support equipment may vary based on the constraints and / or characteristics of the site where the fuel cell system is installed. As a non-limiting example, the 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 the supply and / or exhaust fuel pressure within the fuel cell system, for example, a fuel blower or pump that supplies fuel to the fuel cell system, recirculates fuel / exhaust within the fuel cell system, and / or discharges fuel from the fuel cell system. Another type of fuel support equipment may be configured to process fuel for the fuel cell system, for example, a fuel preheater, an exhaust scrubber, etc. Other types of fuel support equipment may also be used. One type of air support equipment may be an air supply system configured to supply air to and / or discharge air from the fuel cell system, for example, a blower or fan that supplies air to and / or discharges air from the fuel cell cathode, anode tail gas oxidizer (ATO), air heat exchanger, CPOx reactor, etc. Other types of air support equipment may also be used. One type of ventilation support equipment may be an equipment configured to ventilate and / or circulate air in a part of the housing outside the hot box (for example, a part inside the modular fuel cell system housing 10 but outside the hot box 13 itself), for example, a ventilation fan that blows air from inside the housing 10 to outside the housing 10 to maintain an acceptable pressure in the housing 10. Other types of ventilation support equipment may also be used.

[0029] Figure 3 shows a microgrid 300 based on a fuel cell system suitable for implementing various embodiments. Referring to Figures 1 to 3, the microgrid 300 based on a fuel cell system may comprise a plurality of fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, a rectifier 310, a grid-side bus 314, a microgrid-side bus 318, transmission buses 316a, 316b, 316c, 316d, and a changeover switch 312. In some examples, the microgrid 300 based on a fuel cell system may also comprise storage modules 306a, 306b, 306c.

[0030] When used herein, each of the fuel cells 304a or 304b may include the cell stack or column 40 shown in Figure 2, or the power module 12 shown in Figure 1. In other words, the fuel cells 304a or 304b used below are single fuel cell power sources, but are not limited to a single fuel cell 45 comprising one electrolyte, one anode electrode, and one cathode electrode. Furthermore, although fuel cell microgrids 300 will be described later, it should be understood that fuel cells can be replaced with other DC power sources, such as photovoltaic power sources.

[0031] Fuel cells 304a and 304b can be electrically connected to the microgrid-side bus 318 by voltage source inverters 308a and 308b. Fuel cells 304a and 304b, voltage source inverters 308a and 308b, and the microgrid-side bus 318 can be electrically connected via transmission buses 316a and 316b. Fuel cells 304a and 304b, and consequently the voltage source inverters 308a and 308b, can be electrically connected in parallel to the microgrid-side bus 318. Fuel cells 304a and 304b can be electrically connected to the grid-side bus 314 by current source inverters 302a and 302b. Fuel cells 304a and 304b, current source inverters 302a and 302b, and the grid-side bus 314 can be electrically connected via transmission buses 317a and 317b. The fuel cells 304a and 304b, and consequently the current source inverters 302a and 302b, can be electrically connected in parallel to the grid-side bus 314. The grid-side bus 314 and the microgrid-side bus 318 can be selectively electrically connected to each other by the rectifier 310, the current source inverter 302c, and the transmission bus 316c. The rectifier 310 can be electrically connected to the grid-side bus 314 in parallel with the current source inverters 302a and 302b. The current source inverter 302c can be electrically connected to the microgrid-side bus 318 in parallel with the voltage source inverters 308a and 308b. The grid-side bus 314 and the microgrid-side bus 318 can also be selectively electrically connected to each other through the changeover switch 312 and the transmission bus 316d.

[0032] In some examples, the microgrid 300 based on the fuel cell system may also include storage modules 306a, 306b, and 306c. For example, the storage modules 306a, 306b, and 306c may include any form of energy storage that can be converted into electricity, such as an electrical storage unit, a mechanical storage unit, an electromechanical storage unit, an electrochemical storage unit, or a thermal storage unit. Examples include batteries, capacitors, supercapacitors, flywheels, liquid reservoirs, gas reservoirs, and the like. In some examples, the storage modules 306a, 306b, and 306c may include any combination of components such as electrical connectors and / or electrical energy regulators configured to control the input and output of electrical energy to the storage modules 306a, 306b, and 306c in response to signals from an electrical energy bus such as a controller 320 and / or transmission buses 316a, 316, and 316c. Storage modules 306a and 306b can be electrically connected to their respective fuel cells 304a and 304b and their respective voltage source inverters 308a and 308b via transmission buses 316a and 316b. Storage module 306c can be electrically connected in parallel to the rectifier 310 and current source inverter 302c via transmission bus 316c. Storage modules 306a, 306b, and 306c can also be electrically connected to the microgrid-side bus 318 via transmission buses 316a, 316b, and 316c.

[0033] A microgrid 300 based on a fuel cell system may include any number and combination of controllers 320 (e.g., a central processing unit (CPU), a microcontroller, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), or any other software-programmable processor) that are communicated to fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, a rectifier 310, a grid-side bus 314, a microgrid-side bus 318, transmission buses 316a, 316b, 316c, 316d, 317a, 317b, a changeover switch 312, and / or storage modules 306a, 306b, 306c. For example, one or more controllers 320 can be a component of a fuel cell system-based microgrid 300, communicating with and being integrated with fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, changeover switch 312, and / or storage modules 306a, 306b, 306c. As another example, one or more controllers 320 can be a component of a fuel cell system-based microgrid 300, communicating with and being integrated with fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, changeover switch 312, and / or storage modules 306a, 306b, 306c.

[0034] One or more controllers 320 can be configured to supply control signals to the fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, changeover switch 312, and / or storage modules 306a, 306b, 306c, and / or directly control their functions. One or more controllers 320 can be configured to receive signals from the voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, and / or changeover switch 312, which are configured to show the AC voltages in the grid-side bus 314, microgrid-side bus 318, and / or transmission buses 316a, 316b, 316c, 316d to one or more controllers 320. One or more controllers 320 can be configured to directly measure AC voltages in the grid-side bus 314, the microgrid-side bus 318, and / or transmission buses 316a, 316b, 316c, 316d, 317a, 317b in the voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, changeover switch 312, grid-side bus 314, microgrid-side bus 318, and / or transmission buses 316a, 316b, 316c, 316d, 317a, 317b.

[0035] The fuel cells 304a and 304b can be configured to supply DC electrical energy to the voltage source inverters 308a and 308b via transmission buses 316a and 316b. This DC electrical energy can be configured as an amount of DC voltage necessary to support the load demands of the load (i.e., the “load” in Figure 3) on which the microgrid 300 based on the fuel cell system is deployed. The amount of DC voltage output to the voltage source inverters 308a and 308b by the fuel cells 304a and 304b can be controlled by the controller 320.

[0036] The voltage source inverters 308a and 308b can be configured to receive DC voltage from fuel cells 304a and 304b, convert the DC voltage into AC electrical energy, and supply the AC electrical energy to the microgrid-side bus 318 via transmission buses 316a and 316b. This AC electrical energy can be configured as an amount of AC voltage necessary to support at least a portion of the load demand. The amount of AC voltage output to the microgrid-side bus 318 by the voltage source inverters 308a and 308b can be controlled to be the same amount of AC voltage for each of the voltage source inverters 308a and 308b. The amount of AC voltage output by the voltage source inverters 308a and 308b may be limited by the minimum DC voltage generation capacity of one of the fuel cells 304a and 304b. In other words, if fuel cells 304a and 304b have different DC voltage generation capacities, the lowest capacity limits the AC voltage output of the voltage source inverters 308a and 308b that are electrically connected to the fuel cells 304a and 304b with higher capacities. The amount of AC voltage output by the voltage source inverters 308a and 308b can be controlled by the controller 320.

[0037] The fuel cells 304a and 304b can be configured to supply DC electrical energy to the current source inverters 302a and 302b via the transmission buses 317a and 317b. This DC electrical energy can be configured as an amount of DC voltage generated by the fuel cells 304a and 304b that exceeds the amount used by the voltage source inverters 308a and 308b. For example, the amount of DC voltage used by the voltage source inverters 308a and 308b may be less than the total DC voltage generated by the fuel cells 304a and 304b if the equal distribution of the load demand is less than the minimum DC voltage generation capacity of the fuel cells 304a and 304b. In another example, the amount of DC voltage used by the voltage source inverters 308a and 308b may be less than the total DC voltage generated by at least one of the fuel cells 304a and 304b if the equal distribution of the load demand is greater than the minimum DC voltage generation capacity of one of the fuel cells 304a and 304b. The amount of DC voltage output to the current source inverters 302a and 302b by the fuel cells 304a and 304b can be controlled by the controller 320.

[0038] The current source inverters 302a and 302b can be configured to receive a DC voltage from the fuel cells 304a and 304b, convert the DC voltage into AC electrical energy, and supply the AC electrical energy to the grid-side bus 314 via the transmission buses 317a and 317b. This AC electrical energy can be configured as an AC current in an amount configured to follow a volt-watt curve. The amount of AC current output to the grid-side bus 314 by the current source inverters 302a and 302b can be controlled based on various electrical connections of the grid-side bus 314. For example, the grid-side bus 314 can be selectively electrically connected to the electric utility power grid (i.e., the "grid" in Figure 3) by a changeover switch 312, as further described herein. The current source inverters 302a and 302b can output AC current in accordance with a volt-watt curve based on the voltages in the grid-side bus 314, the current source inverters 302a and 302b, and / or the changeover switch 312. The AC current in the grid-side bus 314 can be used to send to the electric utility power grid and / or support loads, as further described below. In another example, the grid-side bus 314 can be selectively electrically connected to the microgrid-side bus 318 by a changeover switch 312 and a transmission bus 316d, as further described below. The current source inverters 302a and 302b can output AC current according to a volt-watt curve based on the voltage in the grid-side bus 314, the current source inverters 302a and 302b, the changeover switch 312, the transmission bus 316d, the microgrid-side bus 318, and / or the voltage source inverters 308a and 308b. The amount of AC current output by the current source inverters 302a and 302b can be controlled by the controller 320.

[0039] The changeover switch 312 can be configured to selectively electrically connect the grid-side bus 314 to either the electric utility power grid or the microgrid-side bus 318 via the transmission bus 316d. The changeover switch 312 can detect the availability of the electric utility power grid, for example, by sensing the voltage and / or current levels of the electric utility power grid. Depending on whether the electric utility power grid is available in normal operating mode, the changeover switch 312 can 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 316d. Depending on whether the electric utility power grid is unavailable in emergency operating mode, the changeover switch 312 can selectively electrically connect the grid-side bus 314 to the microgrid-side bus 318 via the transmission bus 316d and disconnect the grid-side bus 314 from the electric utility power grid. The changeover switch 312 can be controlled by the controller 320.

[0040] The rectifier 310 can be configured to draw AC current from the grid-side bus 314 via the transmission bus 316c, depending on whether more electrical energy is needed to support load demand when the changeover 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 308a, 308b may be insufficient to support load demand. To increase the amount of AC voltage supplied to the microgrid-side bus 318, the rectifier 310 can draw AC current (e.g., grid current and / or inversely converted fuel cell current) from the grid-side bus 314. The rectifier 310 can convert the AC current to DC current and supply the DC current to the current source inverter 302c via the transmission bus 316c. The remaining AC current in the grid-side bus 314 can be sent to the electric utility power grid. As the load demand is met by the voltage source inverters 308a and 308b, the rectifier 310 can be configured not to draw AC current from the grid-side bus 314, and can send the AC current in the grid-side bus 314 to the electric utility power grid. The amount of AC current drawn from the grid-side bus 314 by the rectifier 310, and the amount of DC current output to the current source inverter 302c, can be controlled by the controller 320.

[0041] The current source inverter 302c can be configured to receive DC current from the rectifier 310, convert the DC current into AC electrical energy, and supply the AC electrical energy to the microgrid-side bus 318 via the transmission bus 316c. This AC electrical energy can be configured as an amount of AC current configured to follow a volt-watt curve. The amount of AC current output to the microgrid-side bus 318 by the current source inverter 302c can be controlled based on load demand. For example, the current source inverter 302c can output AC current according to a volt-watt curve based on the voltage in the microgrid-side bus 318 and / or the voltage source inverters 308a, 308b, and the load demand. The AC current output by the current source inverter 302c may be sufficient to compensate for any shortage of AC voltage output from the voltage source inverters 308a, 308b to support the load demand. The amount of AC current output by the current source inverter 302c can be controlled by the controller 320.

[0042] During emergency operation mode, transmission bus 316d can electrically connect the grid-side bus 314 and the microgrid-side bus 318 when the changeover switch 312 selectively electrically connects buses 314 and 318. Current source inverters 302a and 302b can be configured to supply AC current to the grid-side bus 314 via transmission buses 317a and 317b, and to allow AC current to flow to the microgrid-side bus 318 via transmission bus 316d. The amount of AC current output to the grid-side bus 314 by current source inverters 302a and 302b can be controlled based on various electrical connections of the grid-side bus 314. The current source inverters 302a and 302b can output AC current according to a volt-watt curve based on the voltage in the grid-side bus 314, the current source inverters 302a and 302b, the changeover switch 312, the transmission bus 316d, the microgrid-side bus 318, and / or the voltage source inverters 308a and 308b. If the transmission bus 316d electrically connects the grid-side bus 314 and the microgrid-side bus 318, the flow of AC current from the grid-side bus 314 to the microgrid-side bus 318 can bypass at least the current source inverter 302c. The amount of AC current output by the current source inverters 302a and 302b can be controlled by the controller 320.

[0043] In some examples, storage modules 306a, 306b, and 306c can sink or source excess electrical energy when it is necessary to maintain a stable voltage for the microgrid based on the fuel cell system. For example, if the changeover switch 312 selectively electrically connects the grid-side bus 314 and the microgrid-side bus 318, storage modules 306a, 306b, and 306c can be used to supply additional electrical energy to the microgrid-side bus 318 or to receive excess electrical energy that is not needed in the microgrid-side bus 318. Storage modules 306a, 306b, and 306c can maintain a stable voltage for the microgrid based on the fuel cell system while the changeover switch 312 transitions from a state in which the microgrid based on the fuel cell system 300 is selectively electrically connected to the electric utility power grid to a state in which the microgrid based on the fuel cell system 300 is selectively electrically disconnected from the electric utility power grid.

[0044] Figures 4A to 4C are process flow diagrams of methods for power management of a fuel cell-based microgrid 300, shown in Figure 3, according to various embodiments. Referring to Figures 1 to 4C, the method 400 can be implemented using one or more controllers 320 configured to receive signals from any number or combination of fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316c, 316d, 317a, 317b, changeover switch 312, and / or storage modules 306a, 306b, 306c. Method 400 can be implemented using one or more controllers 320 configured to transmit control signals to any number and combination of voltage source inverters 308a, 308b, current source inverters 302a, 302b, 302c, rectifier 310, changeover switch 312, and / or storage modules 306a, 306b, 306c. To include alternative configurations that may be possible in various embodiments, the hardware that implements Method 400 will be referred to herein as “control device”. Any number and combination of blocks 402-448 can be implemented periodically, repeatedly, or continuously, and / or simultaneously with any other blocks 402-448.

[0045] Referring to Figure 4A, in block 402, the control unit can measure the voltage on the microgrid-side bus 318. If the grid-side bus 314 is disconnected from the microgrid-side bus 318 by the changeover switch 312, the voltage on the microgrid-side bus 318 can be measured by receiving a signal configured to show the voltage on the microgrid-side bus 318 from, for example, one of the voltage source inverters 308a, 308b, current source inverter 302c, and / or the changeover switch 312. The voltage on the microgrid-side bus 318 may also be measured by the control unit directly measuring the voltage on the microgrid-side bus 318 in one of the voltage source inverters 308a, 308b, current source inverter 302c, the microgrid-side bus 318, the transmission buses 316a, 316b, 316c, 316d, and / or the changeover switch 312. If the grid-side bus 314 is connected to the microgrid-side bus 318 by a changeover switch 312, the voltage on the microgrid-side bus 318 can be measured, as a further example, by receiving a signal from the grid-side bus 314 and / or one of the current source inverters 302a, 302b, configured to show the voltage on the microgrid-side bus 318 as shown in the control unit. The voltage on the microgrid-side bus 318 may also be measured by the control unit directly measuring the voltage on the microgrid-side bus 318 in the grid-side bus 314 and / or one of the current source inverters 302a, 302b.

[0046] In block 404, the control unit can control the voltage source inverters 308a and 308b. The control unit can control the AC voltage output to the microgrid-side bus 318 by the voltage source inverters 308a and 308b. For example, the control unit can signal the voltage source inverters 308a and 308b, or directly set the voltage source inverters 308a and 308b to a set point for AC voltage output. The control unit can control the voltage source inverters 308a and 308b to output the same amount of AC voltage to the microgrid-side bus 318. The control unit can control the voltage source inverters 308a and 308b to output AC voltage to the microgrid-side bus 318 based on the load demand of the load on which the fuel cell system-based microgrid 300 is deployed. The control device can further control the voltage source inverters 308a and 308b to limit the AC voltage output to the microgrid-side bus 318 based on the minimum electrical energy generation capacity of the fuel cells 304a and 304b. For example, if the equal distribution of load demand exceeds the minimum electrical energy generation capacity of the fuel cells 304a and 304b, the control device can limit the AC voltage output to the microgrid-side bus 318 by the voltage source inverters 308a and 308b to the amount that can be output by the voltage source inverters 308a and 308b, which receive DC voltage from the fuel cells 304a and 304b having the minimum electrical energy generation capacity.

[0047] In block 406, the control unit can control the voltage source inverters 308a and 308b to output a controlled amount of AC voltage to the microgrid-side bus 318. The controlled amount of AC voltage can be based on the control of the voltage source inverters 308a and 308b in block 404.

[0048] In block 408, the control unit can measure the voltage on the grid-side bus 314. The voltage on the grid-side bus 314 can be measured by receiving a signal from, for example, one of the current source inverters 302a, 302b, transmission buses 317a, 317b, rectifier 310, and / or changeover switch 312, which is configured to show the voltage on the grid-side bus 314 to the control unit. Alternatively, the voltage on the grid-side bus 314 may be measured by the control unit directly measuring the voltage on the grid-side bus 314 in one of the current source inverters 302a, 302b, rectifier 310, grid-side bus 314, transmission buses 317a, 317b, 316c, 316d, and / or changeover switch 312.

[0049] In block 410, the control unit can control the current source inverters 302a and 302b to output excess electrical energy to the grid-side bus 314. The current source inverters 302a and 302b can receive DC current generated by the fuel cells 304a and 304b that is not used by the voltage source inverters 308a and 308b to generate AC voltage for the microgrid-side bus 318. The control unit can control the AC current output to the microgrid-side bus 318 by the current source inverters 302a and 302b. For example, the control unit can signal the current source inverters 302a and 302b or directly set the current source inverters 302a and 302b to the AC current output setpoint. The control unit can control the current source inverters 302a and 302b to output AC current to the grid-side bus 314 based on the voltage and volt-watt curve in the grid-side bus 314.

[0050] In the decision block 412, the control device can determine whether or not the electric utility power grid is available. The control device can detect the availability of the electric utility power grid, for example, by detecting the voltage and / or current level of the electric utility power grid. The control device can detect the voltage and / or current level of the electric utility power grid, for example, by receiving a signal from any of the current source inverters 302a, 302b, rectifier 310, and / or changeover switch 312 that is configured to show the voltage and / or current level of the electric utility power grid to the control device. The voltage and / or current level of the electric utility power grid may also be measured by the control device directly measuring the voltage and / or current level of the electric utility power grid in any of the current source inverters 302a, 302b, rectifier 310, grid-side bus 314, transmission buses 317a, 317b, 316c, 316d, and / or changeover switch 312. The control unit can determine whether the electric utility power grid is available by comparing the voltage and / or current levels of the electric utility power grid with a grid availability threshold. If it is determined that the electric utility power grid is available (i.e., determination block 412 = "Yes"), the control unit can continue operating in normal operation mode in step "A" described later with respect to Figure 4B. If it is determined that the electric utility power grid is not available (i.e., determination block 412 = "No"), the control unit can operate in emergency mode in step "B" described later with respect to Figure 4C.

[0051] Referring to Figure 4B, in accordance with the determination that the electric utility power grid is available (i.e., determination block 412 = "Yes"), the control unit can determine in determination block 420 whether the microgrid 300 based on the fuel cell system is connected to the electric utility power grid. The control unit can determine the connection status of the microgrid 300 based on the fuel cell system to the electric utility power grid based on the state, position, etc. of the changeover switch 312. If the changeover switch 312 selectively electrically connects the grid-side bus 314 to the electric utility power grid, the microgrid 300 based on the fuel cell system can be connected to the electric utility power grid in normal operation mode. If the changeover switch 312 selectively electrically connects the grid-side bus 314 to the microgrid-side bus 318, the microgrid 300 based on the fuel cell system can be disconnected from the electric utility power grid in emergency operation mode.

[0052] In response to the determination that the microgrid 300 based on the fuel cell system is not connected to the electric utility power grid (i.e., determination block 420 = "No"), the control device may, in block 422, 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. The control device may control the changeover switch 312 to change its state or position in order 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.

[0053] In response to the determination that the microgrid 300 based on the fuel cell system is connected to the electric utility power grid (i.e., determination block 420 = "Yes"), or following block 422, the control unit may determine in determination block 424 whether the voltage at the microgrid-side bus 318 is sufficient to support the load. The control unit may measure the voltage at the microgrid-side bus 318, for example, as described herein with reference to block 402, or using the measured voltage at the microgrid-side bus 318 in block 402. The control unit may compare the measured voltage at the microgrid-side bus 318 with the load demand. The voltage at the microgrid-side bus 318 may be insufficient if the voltage at the microgrid-side bus 318 is insufficient to meet the load demand, and may be sufficient if the voltage at the microgrid-side bus 318 meets or exceeds the load demand.

[0054] In response to the determination that the voltage on the microgrid-side bus 318 is insufficient to support the load (i.e., determination block 424 = "No"), the control unit may, in block 426, control the rectifier 310 and the current source inverter 302c. The control unit may also control the rectifier 310 and the current source inverter 302c to supply electrical energy from the grid-side bus 314 to the microgrid-side bus 318. The electrical energy from the grid-side bus 314 may include AC currents output to the grid-side bus 318 by the current source inverters 302a, 302b, as described herein with reference to block 410. The control unit may control the rectifier 310 and the current source inverter 302c, for example, by signaling them to be controlled, or by directly setting the current output setpoints in the rectifier 310 and the current source inverter 302c. The current output setting point can be based on the voltage on the microgrid bus 318, as well as the amount of current required to meet the load demand based on the volt-watt curve.

[0055] In block 428, the control unit can control the rectifier 310 to draw AC current from the grid-side bus 314. Based on the control of the rectifier 310 in block 426, the control unit can control the rectifier 310 to draw a certain amount of AC current from the grid-side bus 314 and output a DC current. In block 430, the control unit can control the current source inverter 302c to output a controlled amount of AC current to the microgrid-side bus 318. Based on the control of the current source inverter 302c in block 426, the control unit can control the current source inverter 302c to convert a certain amount of DC current and output a controlled amount of AC current to the microgrid-side bus 318.

[0056] In response to the determination that the voltage on the microgrid-side bus 318 is sufficient to support the load (i.e., determination block 424 = "Yes"), or following block 430, the control unit may, in block 432, control the sending of excess electrical energy on the grid-side bus 314 to the electric utility power grid. The control unit may then continue to measure the voltage on the microgrid-side bus 318 in block 402.

[0057] Referring to Figure 4C, if it is determined that the electric utility power grid is unavailable (i.e., decision block 412 = "No"), the control device can determine in decision block 440 whether the microgrid 300 based on the fuel cell system is connected to the electric utility power grid. Based on the state, position, etc., of the changeover switch 312, the control device can determine the connection status of the microgrid 300 based on the fuel cell system to the electric utility power grid. If the changeover switch 312 selectively electrically connects the grid-side bus 314 to the electric utility power grid, the microgrid 300 based on the fuel cell system can be connected to the electric utility power grid. If the changeover switch 312 selectively electrically connects the grid-side bus 314 to the microgrid-side bus 318, the microgrid 300 based on the fuel cell system can be disconnected from the electric utility power grid.

[0058] In response to the determination that the microgrid 300 based on the fuel cell system is connected to the electric utility power grid (i.e., determination block 440 = "Yes"), the control unit may, in block 442, 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. The control unit may control the changeover switch 312 to change its state or position in order 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.

[0059] In response to the determination that the microgrid 300 based on the fuel cell system is not connected to the electric utility power grid (i.e., determination block 440 = "No"), or following block 442, the control unit may determine in determination block 444 whether the voltage at the microgrid-side bus 318 is sufficient to support the load. The control unit may measure the voltage at the microgrid-side bus 318, for example, as described herein with reference to block 402, or using the measured voltage at the microgrid-side bus 318 in block 402. The control unit may compare the measured voltage at the microgrid-side bus 318 with the load demand. The voltage at the microgrid-side bus 318 may be insufficient if the voltage at the microgrid-side bus 318 is insufficient to meet the load demand, and may be sufficient if the voltage at the microgrid-side bus 318 meets or exceeds the load demand.

[0060] In response to the determination that the voltage on the microgrid-side bus 318 is insufficient to support the load (i.e., determination block 444 = "No"), the control unit may, in block 446, control the current source inverters 302a and 302b. The control unit may control the current source inverters 302a and 302b to supply electrical energy from the fuel cells 304a and 304b to the microgrid-side bus 318 via the grid-side bus 314 and the transmission bus 316d. This electrical energy may include the AC current output to the grid-side bus 314 by the current source inverters 302a and 302b, as described herein with reference to block 410. The control unit may control the current source inverters 302a and 302b, for example, by transmitting signals to control them or by directly setting them to a set point for current output. The current output setting point can be based on the voltage on the microgrid bus 318, as well as the amount of current required to meet the load demand based on the volt-watt curve.

[0061] In block 448, the control unit can control the current source inverters 302a and 302b to output a controlled amount of AC current to the microgrid bus 318 via the grid bus 314. Based on the control of the current source inverters 302a and 302b in block 446, the control unit can convert a certain amount of DC current and control the current source inverters 302a and 302b to output a controlled amount of AC current to the microgrid bus 318 via the grid bus 314.

[0062] As long as it is determined that the voltage on the microgrid-side bus 318 is sufficient to support the load (i.e., determination block 444 = "Yes"), or following block 448, the control unit may continue measuring the voltage on the microgrid-side bus 318 in block 402.

[0063] Figure 5 shows a microgrid 500 based on a fuel cell system suitable for implementing various embodiments. Referring to Figures 1 to 5, the microgrid 500 based on a fuel cell system may comprise a plurality of fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 317a, 317b, and a changeover switch 312. In some examples, the microgrid 500 based on a fuel cell system may also comprise storage modules 306a, 306b, 306c. The fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 317a, 317b, changeover switch 312, and / or storage modules 306a, 306b, 306c are configured, constructed, electrically connected, and / or function as described herein with reference to Figures 3 to 4C, unless otherwise specified. The microgrid 500 based on the fuel cell system may also include a motor 502, a generator 504, and a transmission bus 316e instead of the rectifier 310, current source inverter 302c, and transmission bus 316c, respectively, as shown in Figure 3.

[0064] Motor 502 can be configured to draw AC current from grid-side bus 314 via transmission bus 316e when, in normal operating mode, the changeover switch 312 selectively electrically connects grid-side bus 314 to the electric utility power grid, more electrical energy is required to support the load demand. The AC voltage output to microgrid-side bus 318 by voltage source inverters 308a, 308b may be insufficient to support the load demand. To increase the amount of AC voltage supplied to microgrid-side bus 318, motor 502 can draw AC current from grid-side bus 314. Motor 502 can use the received AC current to drive itself. Motor 502 can operate at various speeds to drive the generator 504. The remaining AC current in grid-side bus 314 can be sent to the electric utility power grid. As the load demand is met by the voltage source inverters 308a and 308b, the motor 502 can be configured not to draw AC current from the grid-side bus 314, and the AC current in the grid-side bus 314 can be sent 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 the generator 504 operates and / or drives, can be controlled by the controller 320.

[0065] Motor 502 can drive a generator 504 using AC current drawn from the grid-side bus 314, which generates AC electrical energy and can supply it to the microgrid-side bus 318 via the transmission bus 316e. This AC electrical energy can be configured as an amount of AC current configured to follow a volt-watt curve. The amount of AC current output to the microgrid-side bus 318 by the generator 504 can be controlled based on load demand. For example, the generator 504 can output AC current according to a volt-watt curve based on the voltage in the microgrid-side bus 318 and / or the voltage source inverters 308a, 308b, and the load demand. The AC current output by the generator 504 may be sufficient to compensate for any shortage of AC voltage output from the voltage source inverters 308a, 308b to support the load demand. The amount of AC current output by the generator 504 can be controlled by the controller 320.

[0066] The storage module 306c can be electrically connected in parallel to the motor 502 and the generator 504. The storage module 306c can be electrically connected to the microgrid bus 318 via the transmission bus 316e.

[0067] In this embodiment, a large amount of short-circuit current may be available from the generator 504. Microgrid systems typically discharge far less short-circuit current than grid-hung systems, due to inverter technology. Therefore, it is advantageous for the generator 504 to act to discharge a large amount of short-circuit current in the event of a fault, thereby quickly clearing the protective devices.

[0068] Figures 6A to 6C are process flow diagrams of methods for power management of a fuel cell system-based microgrid 500, shown in Figure 5, according to various embodiments. Referring to Figures 1 to 6C, the method 600 can be implemented using one or more controllers 320 configured to receive signals from any number or combination of fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, motor 502, generator 504, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 316e, 317a, 317b, changeover switch 312, and / or storage modules 306a, 306b, 306c. Method 600 can be implemented using one or more controllers 320 configured to transmit control signals to any number and combination of voltage source inverters 308a, 308b, current source inverters 302a, 302b, motor 502, generator 504, changeover switch 312, and / or storage modules 306a, 306b, 306c. To encompass alternative configurations that may be possible in various embodiments, the hardware that implements Method 600 is referred to herein as “control device”. Any number and combination of blocks 402-448 and 602-606 can be implemented periodically, repeatedly, or continuously, and / or simultaneously with any other blocks 402-448 and 602-606. Blocks 402-448 can be described in part with respect to Method 400 as described herein with reference to Figures 4A-4C, and similarly, parts of Method 600 can be described with respect to blocks 402-448.

[0069] Referring to Figure 6A, in block 402, the control unit can measure the voltage on the microgrid-side bus 318. In block 404, the control unit can control the voltage source inverters 308a and 308b. In block 406, the control unit can control the voltage source inverters 308a and 308b to output a controlled amount of AC voltage to the microgrid-side bus 318. In block 408, the control unit can measure the voltage on the grid-side bus 314. In block 410, the control unit can control the current source inverters 302a and 302b to output excess electrical energy to the grid-side bus 314. In the determination block 412, the control unit can determine whether the electric utility power grid is available. If it is determined that the electric utility power grid is available (i.e., determination block 412 = "Yes"), the control unit can proceed to step "A" in Figure 6B. If it is determined that the electric utility power grid is unavailable (i.e., determination block 412 = "No"), the control unit may proceed to step "B" in Figure 6C.

[0070] Referring to Figure 6B, in accordance with the determination that the electric utility power grid is available (i.e., determination block 412 = "Yes"), the control unit may determine in determination block 420 whether the microgrid 500 based on the fuel cell system is connected to the electric utility power grid. In accordance with the determination that the microgrid 500 based on the fuel cell system is not connected to the electric utility power grid (i.e., determination block 420 = "No"), the control unit may in block 422 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 accordance with the determination that the microgrid 500 based on the fuel cell system is connected to the electric utility power grid (i.e., determination block 420 = "Yes"), or following block 422, the control unit may in determination block 424 determine whether the voltage on the microgrid-side bus 318 is sufficient to support the load.

[0071] In response to the determination that the voltage in the microgrid-side bus 318 is insufficient to support the load (i.e., determination block 424 = "No"), the control unit may, in block 602, control motor 502 to drive generator 504. The control unit may control motor 502 and generator 504 to supply electrical energy from grid-side bus 314 to the microgrid-side bus 318. The electrical energy from grid-side bus 314 may include AC current output to grid-side bus 314 by current source inverters 302a, 302b, as described above with reference to block 410. The control unit may control motor 502 and generator 504, for example, by signaling them to control motor 502 and generator 504, or by directly setting motor 502 and generator 504 to setpoints for operating speed and / or current output. The setpoints for operating speed and / or current output may be based on the voltage in the microgrid-side bus 318, as well as the amount of current required to meet the load demand based on a volt-watt curve.

[0072] In block 604, the control unit can control the motor 502 to draw AC current from the grid-side bus 314. Based on the control of the motor 502 in block 602, the control unit can control the motor 502 to draw a certain amount of AC current from the grid-side bus 314 and to operate at a certain speed. In block 606, the control unit can control the generator 504 to output a controlled amount of AC current to the microgrid-side bus 318. Based on the control of the generator in block 602, the control unit can control the generator 504 to operate at a certain speed and output a controlled amount of AC current to the microgrid-side bus 318.

[0073] In response to the determination that the voltage on the microgrid-side bus 318 is sufficient to support the load (i.e., determination block 424 = "Yes"), or following block 430, the control unit may, in block 432, control the sending of excess electrical energy on the grid-side bus 314 to the electric utility power grid. The control unit may then continue to measure the voltage on the microgrid-side bus 318 in block 402.

[0074] Referring to Figure 6C, if it is determined that the electric utility power grid is unavailable (i.e., decision block 412 = "No"), the control unit may determine in decision block 440 whether the microgrid 500 based on the fuel cell system is connected to the electric utility power grid. If it is determined that the microgrid 500 based on the fuel cell system is connected to the electric utility power grid (i.e., decision block 440 = "Yes"), the control unit may in block 442 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. If it is determined that the microgrid 500 based on the fuel cell system is not connected to the electric utility power grid (i.e., decision block 440 = "No"), or following block 442, the control unit may in decision block 444 determine whether the voltage on the microgrid-side bus 318 is sufficient to support the load. In response to the determination that the voltage on the microgrid-side bus 318 is insufficient to support the load (i.e., decision block 444 = "No"), the control unit may, in block 446, control the current source inverters 302a and 302b. In block 448, the control unit may control the current source inverters 302a and 302b to output a controlled amount of AC current to the microgrid-side bus 318 via the grid-side bus 314. In response to the determination that the voltage on the microgrid-side bus 318 is sufficient to support the load (i.e., decision block 444 = "Yes"), or following block 448, the control unit may continue to measure the voltage on the microgrid-side bus 318 in block 402.

[0075] Figure 7 shows a microgrid 700 based on a fuel cell system suitable for implementing various embodiments. Referring to Figures 1 to 7, the microgrid 700 based on a fuel cell system may comprise a plurality of fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 317a, 317b, and a changeover switch 312. In some examples, the microgrid 700 based on a fuel cell system may also comprise storage modules 306a, 306b. The fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 317a, 317b, changeover switch 312, and / or storage modules 306a, 306b are configured, constructed, electrically connected, and / or function as described herein with reference to Figures 3 to 6C, unless otherwise specified. The microgrid 700 based on the fuel cell system may also include a relay 702, an electrical contactor 704, and a transmission bus 316f instead of the motor 502, generator 504, and transmission bus 316e shown in Figure 5.

[0076] The current source inverters 302a and 302b can be configured to supply AC current to the grid-side bus 314 according to a volt-watt curve. The amount of AC current output to the grid-side bus 314 by the current source inverters 302a and 302b can be controlled based on various electrical connections of the grid-side bus 314. For example, in normal operating mode, the grid-side bus 314 can be selectively electrically connected to the electric utility power grid (i.e., the "grid" in Figure 7) by a changeover switch 312. The current source inverters 302a and 302b can output AC current according to a volt-watt curve based on the voltages in the grid-side bus 314, the microgrid-side bus 318, the current source inverters 302a and 302b, the voltage source inverters 308a and 308b, the relay 702, the electrical contactor 704, the transmission bus 316f, and / or the changeover switch 312. The AC current in the grid-side bus 314 can be used to support loads by being sent to the electric utility power grid and / or flowing to the microgrid-side bus 318. In another example, the grid-side bus 314 can be selectively electrically connected to the microgrid-side bus 318 by a changeover switch 312 and a transmission bus 316d in emergency operation mode. The current source inverters 302a and 302b can output AC current according to a volt-watt curve based on the voltage in the grid-side bus 314, the current source inverters 302a and 302b, the changeover switch 312, the transmission bus 316d, the microgrid-side bus 318, and / or the voltage source inverters 308a and 308b. The amount of AC current output by the current source inverters 302a and 302b can be controlled by the controller 320.

[0077] Relay 702 can be configured to detect the flow of current between the grid-side bus 314 and the microgrid-side bus 318 in the transmission bus 316f. When the AC current in the grid-side bus 314 supports a load, the AC current flows in the "forward" direction from the grid-side bus 314 to the microgrid-side bus 318. However, there are also cases where the AC current can flow in the "reverse" direction from the microgrid-side bus 318 to the grid-side bus 314. Electrical contactor 704 can be electronically controlled to allow or interrupt the flow of current in the transmission bus 316f. Electrical contactor 704 can be any form of electronically controlled contactor, such as a circuit breaker or a switch.

[0078] In response to the detection of reverse current in the transmission bus 316f, relay 702 can signal electrical contactor 704 to interrupt the reverse current in the transmission bus 316f. In some examples, relay 702 can directly signal electrical contactor 704 to interrupt the reverse current in the transmission bus 316f. In some examples, relay 702 can signal electrical contactor 704 to interrupt the reverse current in the transmission bus 316f via controller 320, by signaling controller 320 of the detection of reverse current in the transmission bus 316f, and controller 320 signaling electrical contactor 704 to interrupt the reverse current in the transmission bus 316f.

[0079] The positions or states of the changeover switch 312 and the electrical contactor 704 can be linked. For example, if the changeover switch 312 selectively electrically connects the grid-side bus 314 to the electric utility power grid in normal operation mode, the electrical contactor 704 can maintain the electrical connection between the grid-side bus 314 and the microgrid-side bus 318 on the transmission bus 316f, and vice versa. In another example, if the changeover switch 312 selectively electrically connects the grid-side bus 314 to the microgrid-side bus 318 in emergency operation mode, the electrical contactor 704 can disconnect the electrical connection between the grid-side bus 314 and the microgrid-side bus 318 on the transmission bus 316f, and vice versa.

[0080] Figures 8A to 8C are process flow diagrams of methods for power management of a fuel cell-based microgrid 700, shown in Figure 7, according to various embodiments. Referring to Figures 1 to 8C, the method 800 can be implemented using one or more controllers 320 configured to receive signals from any number or combination of fuel cells 304a, 304b, voltage source inverters 308a, 308b, current source inverters 302a, 302b, relays 702, electrical contactors 704, grid-side bus 314, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 316f, 317a, 317b, changeover switches 312, and / or storage modules 306a, 306b. Method 800 can be implemented using one or more controllers 320 and / or relays 704 configured to transmit control signals to any number and combination of voltage source inverters 308a, 308b, current source inverters 302a, 302b, electrical contactors 704, changeover switches 312, and / or storage modules 306a, 306b. To encompass alternative configurations that may be possible in various embodiments, the hardware implementing Method 800 is referred to herein as “control device”. Any number and combination of blocks 402-448 and 802-808 can be implemented periodically, repeatedly, or continuously, and / or simultaneously with any other blocks 402-448 and 802-808. Blocks 402-448 can be described in part with respect to Method 400 as described herein with reference to Figures 4A-4C, similar to how blocks 402-448 describe parts of Method 800.

[0081] Referring to Figure 8A, in block 802, the control unit can measure the voltage and / or current in the microgrid-side bus 318. If the grid-side bus 314 is connected to the microgrid-side bus 318 by an electrical contactor 704, the voltage and / or current in the microgrid-side bus 318 can be measured by receiving signals configured to show the voltage and / or current in the microgrid-side bus 318 from, for example, one of the voltage source inverters 308a, 308b, relay 702, electrical contactor 704, and / or changeover switch 312. The voltage and / or current in the microgrid-side bus 318 may also be measured by the control unit directly measuring the voltage and / or current in the microgrid-side bus 318 from one of the voltage source inverters 308a, 308b, relay 702, electrical contactor 704, microgrid-side bus 318, transmission buses 316a, 316b, 316d, 316f, and / or changeover switch 312. If the grid-side bus 314 is connected to the microgrid-side bus 318 by a changeover switch 312, the voltage and / or current in the microgrid-side bus 318 can be measured, as a further example, by receiving signals from either the grid-side bus 314 and / or the current source inverters 302a, 302b, configured to show the voltage and / or current in the microgrid-side bus 318 as shown in the control unit. The voltage and / or current in the microgrid-side bus 318 may also be measured by the control unit directly measuring the voltage in the microgrid-side bus 318 in either the grid-side bus 314 and / or the current source inverters 302a, 302b.

[0082] In block 804, the control unit can control the voltage source inverters 308a and 308b. The control unit can control the AC voltage output to the microgrid-side bus 318 by the voltage source inverters 308a and 308b. For example, the control unit can transmit a signal to the voltage source inverters 308a and 308b, or it can directly set the AC voltage output setpoint in the voltage source inverters 308a and 308b. The control unit can control the voltage source inverters 308a and 308b to output the same amount of AC voltage to the microgrid-side bus 318. The control unit can control the voltage source inverters 308a and 308b to output AC voltage to the microgrid-side bus 318 based on the load demand of the load to which the fuel cell system-based microgrid 700 is deployed. The control unit can control the voltage source inverters 308a and 308b to output AC voltage to the microgrid-side bus 318 so as to prevent current in the microgrid-side bus 318 from being sent to the electric utility power grid. The control device can further control the voltage source inverters 308a and 308b to limit the AC voltage output to the microgrid-side bus 318 based on the minimum electrical energy generation capacity of the fuel cells 304a and 304b. For example, if the equal distribution of load demand exceeds the minimum electrical energy generation capacity of the fuel cells 304a and 304b, the control device can limit the AC voltage output to the microgrid-side bus 318 by the voltage source inverters 308a and 308b to the amount that can be output by the voltage source inverters 308a and 308b, which receive DC voltage from the fuel cells 304a and 304b having the minimum electrical energy generation capacity.

[0083] In block 806, the control unit can control the voltage source inverters 308a and 308b to output a controlled amount of AC voltage to the microgrid bus 318. The controlled amount of AC voltage can be based on the control of the voltage source inverters 308a and 308b in block 804.

[0084] In block 408, the control unit can measure the voltage on the grid-side bus 314. In block 410, the control unit can control the current source inverters 302a and 302b to output excess electrical energy to the grid-side bus 314. In decision block 412, the control unit can determine whether or not the electric utility power grid is available. If it is determined that the electric utility power grid is available (i.e., decision block 412 = "Yes"), the control unit can proceed to step "A" in Figure 8B. If it is determined that the electric utility power grid is not available (i.e., decision block 412 = "No"), the control unit can proceed to step "B" in Figure 8C.

[0085] Referring to Figure 8B, if it is determined that the electric utility power grid is available (i.e., decision block 412 = "Yes"), the control unit can determine in decision block 420 whether the microgrid 700 based on the fuel cell system is connected to the electric utility power grid. If it is determined that the microgrid 700 based on the fuel cell system is not connected to the electric utility power grid (i.e., decision block 420 = "No"), the control unit can electrically disconnect the grid-side bus 314 from the microgrid-side bus 318 via the transmission bus 316f and electrically connect the grid-side bus 314 to the electric utility power grid in block 808. The control unit can control the changeover switch 312 to change its state or position in order to electrically connect the grid-side bus 314 to the electric utility power grid via the transmission bus 316d and electrically disconnect the grid-side bus 314 from the microgrid-side bus 318.

[0086] In accordance with the determination that the microgrid 700 based on the fuel cell system is connected to the electric utility power grid (i.e., determination block 420 = "Yes"), or following block 808, the control unit may determine in determination block 810 whether reverse power flow exists on the transmission bus 316f. In some examples, the control unit may detect reverse power flow on the transmission bus 316f by receiving a signal from relay 702 configured to indicate the presence of reverse power flow to the control unit. Reverse power flow may occur when the load electrical energy (e.g., power) demand is less than the total electrical energy (e.g., power) supplied to the microgrid-side bus 318 by the voltage source inverters 308a, 308b. Conversely, if the load electrical energy (e.g., power) demand is greater than the total electrical energy (e.g., power) supplied by the voltage source inverters 308a and 308b, no reverse power flow occurs in the transmission bus 316f because additional electrical energy (e.g., power) is drawn by the load from the current source inverters 302a and 302b and / or the electrical utility power grid.

[0087] In response to the determination that there is no reverse power flow in the transmission bus 316f (i.e., determination block 810 = "No"), the control device can control the current source inverters 302a and 302b in block 812. The control device can control the current source inverters 302a and 302b to supply electrical energy from the fuel cells 304a and 304b to the microgrid bus 318 via the grid bus 314 and the transmission bus 316f. This electrical energy may include the AC current output to the grid bus 314 by the current source inverters 302a and 302b, as described herein with reference to block 410. The control device can control the current source inverters 302a and 302b, for example, by transmitting signals to control them or by directly setting them to a set point for current output. The current output setting point can be based on the voltage on the microgrid bus 318, as well as the amount of current required to meet the load demand based on the volt-watt curve.

[0088] In block 814, the control device can control the electrical contactor 704 to close in order to electrically connect the grid-side bus 314 to the microgrid-side bus 318 via the transmission bus 316f. In other words, if the load power demand is less than the power output of the voltage source inverters 308a and 308b, the contactor 704 is closed to supply excess power from the current source inverters 302a and 302b to the load via the grid-side bus 314, the transmission bus 316f, and the microgrid-side bus 318 in order to meet the load power demand.

[0089] In response to the determination that reverse current exists in the transmission bus 316f (i.e., determination block 810 = "Yes"), the control device may control the electrical contactor 704 in block 816 to open in order to prevent reverse current in the transmission bus 316f. The control device may also control the electrical contactor 704 to change its state or position (i.e., open) in order to electrically disconnect the grid-side bus 314 from the microgrid-side bus 318 via the transmission bus 316f. In other words, if the load power demand is lower than the power output of the voltage source inverters 308a and 308b, the contactor 704 is opened to prevent reverse power flow from the microgrid bus 318 to the utility power grid.

[0090] Following block 814 or block 816, the control unit can, in block 432, control the sending of excess electrical energy from the grid-side bus 314 to the electric utility power grid. The control unit can then continue to measure the voltage on the microgrid-side bus 318 in block 802.

[0091] Referring to Figure 8C, if it is determined that the electric utility power grid is unavailable (i.e., decision block 412 = "No"), the control unit can determine in decision block 440 whether the microgrid 700 based on the fuel cell system is connected to the electric utility power grid. If it is determined that the microgrid 700 based on the fuel cell system is connected to the electric utility power grid (i.e., decision block 440 = "Yes"), the control unit can electrically connect the grid-side bus 314 to the microgrid-side bus 318 via the transmission bus 316d, and electrically disconnect the grid-side bus 314 from the electric utility power grid, in block 818.

[0092] The control device can control the changeover switch 312 to change its state or position in order to electrically connect the grid-side bus 314 to the microgrid-side bus 318 via the transmission bus 316d, and to electrically disconnect the grid-side bus 314 from the electric utility power grid. The control device can control the electrical contactor 704 to change its state or position (i.e., open) in order to electrically disconnect the grid-side bus 314 from the microgrid-side bus 318 via the transmission bus 316f.

[0093] In response to the determination that the microgrid 700 based on the fuel cell system is not connected to the electric utility power grid (i.e., determination block 440 = "No"), or following block 818, the control unit may determine in determination block 444 whether the voltage on the microgrid-side bus 318 is sufficient to support the load. In response to the determination that the voltage on the microgrid-side bus 318 is not sufficient to support the load (i.e., determination block 444 = "No"), the control unit may control the current source inverters 302a and 302b in block 446. In block 448, the control unit may control the current source inverters 302a and 302b to output a controlled amount of AC current to the microgrid-side bus 318 via the grid-side bus 314. As long as it is determined that the voltage on the microgrid-side bus 318 is sufficient to support the load (i.e., determination block 444 = "Yes"), or following block 448, the control unit may continue measuring the voltage on the microgrid-side bus 318 in block 802.

[0094] The above-described descriptions of the embodiments of the disclosure are provided to enable those skilled in the art to practice or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may apply to other embodiments without departing from the scope of the invention. Accordingly, the invention is not intended to be limited to the embodiments shown herein, but rather to be given the broadest scope consistent with the principles and novel features disclosed herein.

[0095] The above-described method 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 those skilled in the art will understand, the order of the steps in the above embodiments can be performed in any order. Furthermore, terms such as “thereafter,” “then,” and “next” are not intended to limit the order of the steps. These terms are used only to guide the reader throughout the description of the method.

[0096] One or more figures are used to illustrate exemplary embodiments. The use of the figures is not intended to limit the order of operations performed. The above description of exemplary embodiments is presented for illustrative and explanatory purposes only. It is not intended to be exhaustive or limiting with respect to the exact form disclosed, and modifications and variations may be possible in light of the above teachings or obtained from the practice of the disclosed embodiments. The scope of the invention is intended to be defined by the claims and equivalents attached herein.

[0097] Control devices and control elements including the controller 320 described herein can be implemented using a computing device (such as a computer) which includes a programmable processor, memory, and other components in which instructions for performing a specific function are programmed, or can be implemented within a processor designed to perform a specific function. The processor can be any programmable microprocessor, microcomputer, or one or more multiprocessor chips that can be configured to perform a variety of functions, including the functions of the various embodiments described herein, by software instructions (applications). In some computing devices, multiple processors may be provided. Typically, software applications can be stored in internal memory before being accessed and loaded into the processor. In some computing devices, the processor may have sufficient internal memory to store application software instructions.

[0098] Various exemplary logic blocks, modules, circuits, and algorithmic steps described in connection with the embodiments disclosed herein can be implemented as electronic hardware, computer software, or a combination of both. To clearly illustrate this hardware and software compatibility, various exemplary components, blocks, modules, circuits, and steps have been outlined above in terms of their functionality. Whether such functionality is implemented as hardware or software depends on the specific application and the design constraints imposed on the overall system. Those skilled in the art can implement the described functionality in various ways for specific applications, but such decisions should not be construed as resulting in a departure from the scope of the invention.

[0099] Hardware used to implement the various exemplary logic, logic blocks, modules, and circuits described in relation to the embodiments disclosed herein may be implemented or run using a control device that may be or may include a general-purpose processor, digital signal processor (DSP), application-specific integrated circuit (ASIC), field-programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware component, or any combination thereof, designed to perform the functions described herein. The general-purpose processor may be a microprocessor, but in alternative forms, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor may also be implemented as a combination of computing devices, for example, a DSP and a microprocessor, multiple microprocessors, one or more microprocessors working in conjunction with a DSP core, or any other combination of such configurations. Alternatively, some blocks or methods may be implemented by circuit sections specific to a given function.

[0100] The above-described embodiments of the disclosure are provided to enable a person skilled in the art to implement or use any of the embodiments described. Various modifications to these embodiments will be readily apparent to a person skilled in the art, and the general principles defined herein may apply to other embodiments without departing from the scope of this disclosure. Accordingly, the claims are not intended to be limited to the embodiments shown herein, but are given the broadest scope consistent with the language of the claims and the principles and novel features disclosed herein.

Claims

1. Multiple DC power supplies, Multiple voltage source inverters, A microgrid-side bus configured to be electrically connected to the load, Multiple current source inverters, Grid-side bus and A changeover switch configured to control the selective electrical connection of the grid-side bus to an electrical utility power grid or the microgrid-side bus, The system comprises a transmission bus electrically connected between the microgrid bus and the grid bus, Each DC terminal of the plurality of voltage source inverters is electrically connected to each DC power supply of the plurality of DC power supplies. Each of the AC terminals of the plurality of voltage source inverters is electrically connected to the microgrid bus. The DC terminal of each of the plurality of current source inverters is electrically connected to the DC power supply of each of the plurality of DC power supplies. Each of the AC terminals of the plurality of current source inverters is electrically connected to the grid-side bus. The plurality of current source inverters are configured to output as AC current to the grid-side bus any electrical energy generated by the plurality of DC power sources that exceeds the amount used by the plurality of voltage source inverters. Microgrid.

2. The microgrid according to claim 1, comprising a plurality of DC power sources and a plurality of fuel cell stacks.

3. The plurality of voltage source inverters are configured to output approximately equal amounts of AC voltage to the microgrid bus, The maximum output AC voltage of each of the aforementioned multiple voltage source inverters is determined based on the minimum generating capacity of any of the aforementioned multiple fuel cell stacks. The microgrid according to claim 2, wherein the plurality of current source inverters are configured to output an AC current to the grid-side bus from a DC current generated by any of the plurality of fuel cell stacks that exceeds the minimum generation capacity.

4. A rectifier electrically connected to the grid-side bus via the transmission bus, An additional current source inverter is located on the transmission bus, electrically connected to the rectifier at the DC terminal and electrically connected to the microgrid bus at the AC terminal, The microgrid according to claim 1, further comprising the following:

5. The microgrid according to claim 4, wherein the rectifier and the additional current source inverter are configured to use a first AC current from the grid-side bus to supply a second AC current to the microgrid-side bus when the changeover switch selectively electrically connects the grid-side bus to the electric utility power grid.

6. A motor electrically connected to the grid-side bus via the transmission bus, A generator that is electrically connected to the motor, driven by the motor, and electrically connected to the microgrid bus via the transmission bus, The microgrid according to claim 1, further comprising the following:

7. The microgrid according to claim 6, wherein the motor and the generator are configured to use a first AC current from the grid-side bus to supply a second AC current to the microgrid-side bus when the changeover switch selectively electrically connects the grid-side bus to the electric utility power grid.

8. An electrical contactor configured to selectively electrically complete the circuit between the grid-side bus and the microgrid-side bus along the transmission bus, and to selectively electrically disconnect the circuit between the grid-side bus and the microgrid-side bus along the transmission bus, A control device is electrically connected to the transmission bus and configured to detect the flow of current and to transmit a signal to the electrical contactor to complete or interrupt the circuit in response to the detection of the flow of current from the microgrid bus to the grid bus. The microgrid according to claim 1, further comprising the following:

9. The changeover switch and the electrical contactor are interlocked so that, in response to the unavailability of the electrical utility power grid, the changeover switch selectively electrically connects the grid-side bus to the microgrid-side bus, the electrical contactor selectively electrically disconnects the circuit between the grid-side bus and the microgrid-side bus along the transmission bus, and in response to the flow of reverse current from the microgrid-side bus to the grid-side bus, the electrical contactor selectively electrically disconnects the circuit between the grid-side bus and the microgrid-side bus along the transmission bus, the changeover switch selectively electrically connects the grid-side bus to the microgrid-side bus, according to claim 8.

10. The changeover switch is further configured to selectively electrically connect the grid-side bus to the microgrid-side bus and selectively electrically disconnect the grid-side bus from the electric utility power grid, depending on whether the electric utility power grid is unavailable. The microgrid according to claim 1, wherein the plurality of current source inverters are configured to output AC current to the microgrid bus via the grid bus when the changeover switch selectively electrically connects the grid bus to the microgrid bus.

11. To supply electrical energy from each of multiple DC power sources to each of multiple voltage source inverters and each of multiple current source inverters, The plurality of voltage source inverters output voltage to the microgrid bus such that each of the plurality of voltage source inverters outputs approximately equal amounts of voltage to the microgrid bus, wherein the maximum output voltage of each of the plurality of voltage source inverters is based on the minimum generating capacity of one of the plurality of DC power supplies. Based on the amount of current generated by the multiple DC power supplies that exceeds the minimum generation capacity, the multiple current source inverters output a first current to the grid-side bus. Using the first current output to the grid-side bus, a second current is supplied to the microgrid-side bus. A method for operating a microgrid, including [the following].

12. To determine whether the voltage meets the load demand, In accordance with the determination that the voltage does not meet the load demand, The rectifier draws the first current from the grid-side bus, The rectifier outputs a third current to an additional current source inverter, The additional current source inverter outputs the second current to the microgrid bus, The method according to claim 11, further comprising:

13. To determine whether the electric utility power grid is available, In response to the determination that the said electric utility power grid is available, the grid-side bus is selectively electrically connected to the said electric utility power grid, and in response to the grid-side bus being selectively electrically connected to the said electric utility power grid, the first current is drawn from the grid-side bus and the second current is output to the microgrid-side bus. The method according to claim 12, further comprising:

14. Using the first current output to the grid-side bus to supply the second current to the microgrid-side bus is, To determine whether the voltage meets the load demand, In accordance with the determination that the voltage does not meet the load demand, The first current is drawn from the grid-side bus to drive the motor, The aforementioned motor is used to drive the generator, Outputting the second current from the generator to the microgrid bus, The method according to claim 11, including the method described in claim 11.

15. To determine whether the electric utility power grid is available, In response to the determination that the said electric utility power grid is available, the grid-side bus is selectively electrically connected to the said electric utility power grid, and in response to the grid-side bus being selectively electrically connected to the said electric utility power grid, the first current is drawn from the grid-side bus and the second current is output to the microgrid-side bus. The method according to claim 14, further comprising:

16. Using the first current output to the grid-side bus to supply the second current to the microgrid-side bus is, To determine whether the voltage meets the load demand, In accordance with the determination that the voltage does not meet the load demand, the grid-side bus is electrically connected to the microgrid-side bus, and the contactor is closed to form a circuit between the grid-side bus and the microgrid-side bus. The method according to claim 11, including the method described in claim 11.

17. To detect the reverse current flow from the microgrid bus to the grid bus, In response to the detection of the reverse current flow, the contactor is opened to electrically disconnect the circuit between the grid-side bus and the microgrid-side bus. The method according to claim 16, further comprising:

18. The method according to claim 17, further comprising electrically disconnecting the grid-side bus from the electric utility power grid and electrically connecting the grid-side bus to the microgrid-side bus in response to opening the contactor.

19. To determine whether the electric utility power grid is available, In accordance with the fact that the aforementioned electric utility power grid is unavailable, The aforementioned grid-side bus is electrically disconnected from the aforementioned electric utility power grid, The grid-side bus is electrically connected to the microgrid-side bus, The method according to claim 11, further comprising:

20. The method according to claim 11, wherein the plurality of DC power sources include a fuel cell power source.

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