Power management in fuel cell system based microgrids

Abstract

Excess DC current generated by the fuel cell stack can be supplied to a current source inverter, and AC current can be output to the grid-side bus by the current source inverter. The AC current on the grid-side bus can be used to support the load demand of the microgrid-side bus or supplied to the power grid. For example, various transmission buses and electrical conditioning and control devices, such as rectifiers, current source inverters, motors, generators, electric contactors, relays, and / or transmission switches, can be configured to use the AC current on the grid-side bus to supply AC current to the microgrid-side bus to support the load demand on the microgrid-side bus.

Description

Technology Field

[0001] The present disclosure relates to a direct current (DC) power source, such as a fuel cell system-based microgrid that utilizes the fuel cell system capacity more efficiently. Background Technology

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

[0003] According to an embodiment, the microgrid comprises: a plurality of direct current (DC) power sources, a plurality of voltage source inverters—each DC end of each of the plurality of voltage source inverters is electrically connected to each DC power source of the plurality of DC power sources—a microgrid-side bus—each AC end of each of the plurality of voltage source inverters is electrically connected to the microgrid-side bus, and the microgrid-side bus is electrically connected to a load—a plurality of current source inverters—each DC end of each of the plurality of current source inverters is electrically connected to each DC power source of the plurality of DC power sources—a grid-side bus—each AC end of the plurality of current source inverters is electrically connected to the grid-side bus—a transfer switch configured to control the selective electrical connection of the grid-side bus to an electric utility power grid or the microgrid-side bus, and a transfer bus electrically connected between the microgrid-side bus and the grid-side bus.

[0004] According to another embodiment, a method for operating a microgrid comprises: providing 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; outputting a voltage by the plurality of voltage source inverters to the microgrid-side bus such that each of the plurality of voltage source inverters outputs nearly the same amount of voltage to the microgrid-side bus - the maximum output of the voltage of each of the plurality of voltage source inverters is based on the minimum generation capacity of one of the plurality of DC power sources -, outputting a first current by the plurality of current source inverters to the grid-side bus based on the amount of current generated by the plurality of DC power sources exceeding the minimum generation capacity; and using the first current output to the grid-side bus to provide a second current to the microgrid-side bus. Brief explanation of the drawing

[0005] FIG. 1 is a perspective view of a fuel cell system according to various embodiments suitable for implementing various embodiments. FIG. 2 is a schematic side cross-sectional view of a high-temperature box according to various embodiments suitable for implementing various embodiments. FIG. 3 is a block diagram of the components of a fuel cell system-based microgrid suitable for implementing various embodiments. FIGS. 4a to 4c are process flow diagrams of a power management method for a fuel cell system-based microgrid illustrated in FIG. 3 according to various embodiments. FIG. 5 is a block diagram of the components of a fuel cell system-based microgrid suitable for implementing various embodiments. FIGS. 6a to 6c are process flow diagrams of a power management method for a fuel cell system-based microgrid illustrated in FIG. 5 according to various embodiments. FIG. 7 is a block diagram of the components of a fuel cell system-based microgrid suitable for implementing various embodiments. FIGS. 8a to 8c are process flow diagrams of a power management method for a fuel cell system-based microgrid illustrated in FIG. 7 according to various embodiments. Specific details for implementing the invention

[0006] Various embodiments will be described in detail with reference to the accompanying drawings. Where possible, the same reference numerals will be used throughout the drawings to refer to identical or similar parts. References to specific embodiments and implementations 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 used interchangeably to refer to a generator capable of generating power from any source, such as a fuel cell, a combustion generator, a photovoltaic cell, a concentrated solar system, a wind turbine, a geothermal turbine, a hydroelectric turbine, a gas turbine, a nuclear reactor, an AC generator, an induction generator, etc. The embodiments described herein with respect to fuel cells do not limit the scope of the claims and detailed description of the invention to this type of DC power source. In some embodiments, the DC power source may be an AC generator coupled with an AC / DC rectifier.

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

[0009] As used herein, the terms “electrical energy” and “electrical energy output” refer to a quantity of voltage, current, or power. Examples described herein with respect to voltage do not limit the scope of claims and descriptions regarding these types of electrical energy and electrical energy output.

[0010] The inventors recognized that a weakness of the master / slave control system of the prior art is the difficulty in fully utilizing voltage source inverters with different or variable DC power capacities. Since 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. This wastes excess electrical energy generated by DC power sources that exceed the electrical energy generated by the weakest DC power source in the microgrid.

[0011] Various embodiments include electrical circuits, electrical components, and methods for power management in a microgrid-based DC power source utilizing both voltage and current source inverters for a plurality of DC power sources. The current source inverter can output surplus electrical energy generated from DC power sources other than the weakest DC power source in the microgrid. Thus, the surplus electrical energy is not wasted and is supplied to the power grid and / or load.

[0012] In one embodiment, the DC power of the microgrid may include a fuel cell DC power. A fuel cell system-based microgrid may include: a plurality of DC-to-AC power inverters (or a plurality of columns of a plurality of power modules and / or fuel cell stacks, each comprising a plurality of columns) electrically connecting a plurality of fuel cell stacks of the fuel cell system to a microgrid bus in parallel; a plurality of DC-to-AC current source inverters electrically connecting the fuel cell stacks to a grid bus in parallel; and at least one electrical energy control device configured to control the electrical connection between the grid bus and the microgrid bus. A power management method in a fuel cell system-based microgrid may include: controlling a current source inverter to output surplus electrical energy generated from a fuel cell stack to a grid bus; controlling at least one electrical energy control device to electrically connect the microgrid bus and the grid bus; and providing electrical energy from the grid bus and the microgrid bus to support a load.

[0013] In microgrid applications, inverters are used to generate voltage without being connected to the electric utility power grid. To achieve this, the microgrid system monitors the output voltage and adjusts the power of the voltage source inverters in real time to ensure the voltage waveform remains constant. One common method to achieve this goal is to use a single-point sensor to monitor the output voltage and provide control signals to a bank of voltage source inverters. This method utilizes a master / slave voltage source inverter relationship, where each inverter follows the commands of a single master to maintain the output voltage. In other words, they all output the same amount of voltage. The strength of this approach lies in its simplicity. Commands for the voltage source inverters are derived directly from the voltage signal in real time, and through tuning, they can follow commands that allow for capacity expansion using any number of voltage source inverters.

[0014] The weakness of this approach is the difficulty in utilizing the total DC electrical energy capacity from each available DC power source for all voltage source inverters in use. Since the master must provide a single command to all inverters, it is useful to consider the command as a percentage of the total power. In the case of microgrid systems with variable-capacity DC power sources that support this, 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 connected to individual voltage source inverters linked in parallel, one of those DC power sources will be the weakest at some point. Because each voltage source inverter follows the same single command, the weakest DC power source limits the maximum capacity of the multiple voltage source inverters in parallel. As the master commands increasingly more power from the voltage source inverters, the weakest system reaches its limit first, causing the microgrid system's response to fail. Therefore, the capacity of the entire inverter bank is artificially limited by the DC power capacity of the weakest individual voltage source inverter. If there are N inverters, and X kW of loss occurs from a single DC power source, N*X kW of capacity loss occurs in the microgrid system.

[0015] The embodiments described herein address the aforementioned weaknesses of microgrid systems. In a fuel cell system-based microgrid, the fuel cell can provide a fixed amount of continuous DC electrical energy. A voltage source inverter can provide electrical energy to the fuel cell system-based microgrid according to a single voltage control command, and a current source inverter can output the remaining electrical energy to the grid during normal (i.e., steady state, not emergency) mode when the grid is available. The voltage source inverter can supply the electrical energy required by the load, and the current source inverter can output electrical energy in excess of what is required to support the load.

[0016] A current source inverter can directly measure the voltage present at the terminals of an external source (typically a power grid) and push current from a DC source in synchronization with the voltage waveform. Such a current source inverter (also called a grid-connected or grid-parallel inverter) can generate any output current that can be provided by an electrically connected DC source. The current source inverter may be allowed to detect the input of DC electrical energy from each DC source and determine when its capacity is reached.

[0017] The embodiments provide a circuit that allows a current source inverter to send / dispatch surplus electrical energy generated by a DC source (such as a fuel cell stack) to an electric utility power grid or to provide electrical energy from a grid-side bus of a microgrid system to a load-connected microgrid-side bus of a microgrid system. Since electrical energy can be moved arbitrarily from the grid-side bus to the microgrid-side bus, it may be possible to use up to 100% of all DC electrical energy produced from a DC source, such as a fuel cell stack.

[0018] FIG. 1 illustrates an embodiment of a power generator comprising a modular fuel cell system more fully described in U.S. Patent No. 8,440,362, which is incorporated herein by reference for the description of a modular fuel cell system. The modular system may include modules and components described in U.S. Patent Publication No. 9,190,693, which is incorporated herein by reference for the description of a modular fuel cell system, in addition to those described above. The modular design of the fuel cell enclosure (10) provides flexible system installation and operation.

[0019] A modular fuel cell system enclosure (10) comprises a plurality of power module housings (12) (containing fuel cell power module components), one or more fuel input (i.e., fuel processing) module housings (16), and one or more power regulation (i.e., electrical output) module housings (18). For example, the system enclosure may include any desired number of modules, such as 2 to 30 power modules, such as 6 to 12 power modules. FIG. 1 illustrates a system enclosure (10) comprising 6 power modules (6 modules in a row arranged side by side), one fuel processing module, and one power regulation module on a common base (20). Each module may include its own cabinet or housing. Alternatively, the power regulation and fuel processing modules may be combined into a single input / output module located in a single cabinet or housing (14). For simplification, each housing (12, 14, 16, 18) will be referred to as a "module" below.

[0020] Although a single row of power modules (12) is shown, the system may include more than one row of modules (12). For example, the system may include two rows of power modules stacked back to back.

[0021] Each power module (12) is configured to house one or more high-temperature boxes (13). Each high-temperature box comprises one or more stacks or columns of fuel cells (not shown for clarity), such as one or more stacks or columns of solid oxide fuel cells having ceramic oxide electrolytes separated by conductive interconnection plates. Other fuel cell types, such as PEM, molten carbonate, phosphoric acid, etc., may also be used.

[0022] The modular fuel cell system enclosure (10) also includes one or more input or fuel processing modules (16). The module (16) includes a cabinet containing components used for the pre-processing of fuel, such as a desulfurizer bed. The fuel processing module (16) may be designed to process different types of fuel. For example, a diesel fuel processing module, a natural gas fuel processing module, and an ethanol fuel processing module may be provided in the same or separate cabinets. Different bed compositions tailored to specific fuels may be provided in each module. The processing module(s) (16) may process at least one of the fuels selected from natural gas, compressed natural gas, methane, propane, liquefied petroleum gas, gasoline, diesel, domestic heating oil, kerosene, JP-5, JP-8, aviation fuel, hydrogen, ammonia, ethanol, methanol, synthesis gas, biogas, biodiesel, and other suitable hydrocarbons or fuels containing hydrogen. If desired, the reformer (17) may be located in the fuel processing module (16). Alternatively, if it is desirable to thermally integrate the reformer (17) with the fuel cell stack(s), the individual reformer (17) may be located in each high-temperature box (13) within each power module (12). Additionally, if an internal reforming fuel cell is used, the external reformer (17) may be completely omitted.

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

[0024] The fuel processing module (16) and the power control module (18) may be housed in a single input / output cabinet (14). If a single input / output cabinet (14) is provided, the modules (16, 18) may be positioned vertically (e.g., the fuel processing module (16) and the power control module (18) components on the desulfurizer canister / bed) or side by side in the cabinet (14).

[0025] As illustrated in the exemplary embodiment of FIG. 1, one input / output cabinet (14) is provided in one row of six power modules (12), which are arranged linearly side by side on one side of the input / output modules (14). For example, the row of modules may be located adjacent to a building to which the system supplies power (e.g., so that the rear of the cabinet of the modules faces the building wall). Although one row of power modules (12) is illustrated, the system may include one or more rows of modules (12). For example, as mentioned above, the system may include two rows of power modules stacked back to back.

[0026] Each of the power module (12) and the input / output module (14) includes a door (30) (e.g., a hatch, an access panel, etc.) that allows the internal components of the module to be accessed (e.g., for maintenance, repair, or replacement). According to one embodiment, the modules (12, 14) are arranged in a linear array having a door (30) on only one side of each cabinet, so that a continuous row of the system can be installed adjacent to each other at the ends. In this way, the size and capacity of the fuel cell enclosure (10) can be adjusted with additional modules (12 or 14) and bases (20) with minimal rearrangement required for the existing modules (12 and 14) and bases (20). If desired, the door (30) for the module (14) may be on the side rather than the front of the cabinet.

[0027] FIG. 2 illustrates a top view of a fuel cell system hotbox (13) comprising a fuel cell stack or heat (40). The hotbox (13) is illustrated as comprising a fuel cell stack or heat (40). However, the hotbox (13) may comprise two or more stacks or heats (40). A stack or heat (40) may comprise electrically connected fuel cells (45) stacked together, and an interconnection (50) is disposed between the fuel cells (45). The first and last fuel cells (45) of a stack or heat are disposed between their respective end plates (60) and interconnection (50). The end plates (60) are electrically connected to the electrical output of the fuel cell stack or heat (40). The hotbox (13) may comprise other components, such as fuel conduits, air conduits, seals, electrical contacts, etc., and may be integrated within a fuel cell system comprising a balance of power plant components. The fuel cell (45) comprises 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 interconnects (50) and / or end plates (60) may comprise any suitable gas impermeable and electrically conductive material, such as a chromium-iron alloy, such as an alloy containing 4 to 6 weight percent iron and balanced chromium. The interconnection unit (50) electrically connects adjacent fuel cells (45) and provides a channel for fuel and air to reach the fuel cells (45).

[0028] A fuel cell system, such as a modular fuel cell system enclosure (10), may include and / or be reinforced by various parts of support equipment. Support equipment may include various auxiliary equipment and systems to support the operation of the fuel cell system. Support equipment may vary based on the constraints and / or characteristics of the site where the fuel cell system is installed. As a non-limiting embodiment, support equipment may include fuel support equipment, aviation support equipment, and / or ventilation support equipment. One type of fuel support equipment may include equipment configured to control exhaust fuel pressure and / or supply in the fuel cell system, such as a fuel blower or pump configured to supply fuel to the fuel cell system, recirculate fuel / exhaust, and / or discharge exhaust fuel. Another type of fuel support equipment may be configured to process fuel for the fuel cell system, such as 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 configured to provide air within the fuel cell system and / or provide exhaust air from the fuel cell system, such as a blower or fan that provides air and / or exhaust air to 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 include equipment configured to ventilate and / or circulate air from or inside a housing outside the hot box (e.g., parts within the modular fuel cell system enclosure (10) but outside the hot box (13) itself), such as a ventilation fan (80) for blowing air from within or outside the enclosure (10) to maintain an acceptable enclosure (10) pressure.Other types of ventilation support equipment may also be used.

[0029] FIG. 3 illustrates a fuel cell system-based microgrid (300) suitable for implementing various embodiments. Referring to FIGS. 1 through 3, the fuel cell system-based microgrid (300) includes 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 transfer switch (312). In some embodiments, the fuel cell system-based microgrid (300) may also include storage modules (306a, 306b, 306c).

[0030] As used herein, each fuel cell (304a or 304b) may comprise a cell stack or column (40) shown in FIG. 2 or a power module (12) shown in FIG. 1. That is, the fuel cell (304a or 304b) used below is a single fuel cell power source and is not limited to being a single fuel cell (45) comprising one electrolyte, one anode electrode and one cathode electrode. Also, while the fuel cell microgrid (300) is described below, it should be understood that the fuel cell can be replaced with another DC power source, such as a photovoltaic power source, for example.

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

[0032] In some embodiments, the fuel cell system-based microgrid (300) may also include storage modules (306a, 306b, 306c). For example, the storage modules (306a, 306b, 306c) may include any form of energy storage that can be converted into power, such as electrical storage, mechanical storage, electromechanical storage, electrochemical storage, thermal storage, etc. Embodiments may include batteries, capacitors, supercapacitors, flywheels, liquid storage, gas storage, etc. In some embodiments, the storage modules (306a, 306b, 306c) may include a combination of any components configured to control the electrical energy input and output of the storage modules (306a, 306b, 306c), such as an electrical connection device and / or an electrical energy conditioning device, in response to a signal from an electrical energy bus, such as a controller (320) and / or a transmission bus (316a, 316, 316c). The storage modules (306a, 306b) may be electrically connected to each fuel cell (304a, 304b) and each voltage source inverter (306a, 306b) via the transmission bus (316a, 316b). The storage module (306c) may be electrically connected in parallel to a rectifier (310) and a current source inverter (302c) via the transmission bus (316c). The storage modules (306a, 306b, 306c) can also be electrically connected to the microgrid side bus (318) by the transmission bus (316a, 316b, 316c).

[0033] A fuel cell system-based microgrid (300) 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 communically connected to fuel cells (304a, 304b), a voltage source inverter (308a, 308b), a current source inverter (302a, 302b, 302c), a rectifier (310), a grid-side bus (314), a microgrid-side bus (318), a transmission bus (316a, 316b, 316c, 316d, 317a, 317b), a switching switch (312), and / or storage modules (306a, 306b, 306c). For example, one or more controllers (320) may be components of a fuel cell system-based microgrid (300) that are externally connected and communicately connected to: a fuel cell (304a, 304b), a voltage source inverter (308a, 308b), a current source inverter (302a, 302b, 302c), a rectifier (310), a switching switch (312) and / or a storage module (306a, 306b, 306c). In another example, one or more controllers (320) may be components of a fuel cell system-based microgrid (300) that are communicatably connected to and integrally formed with a fuel cell (304a, 304b), a voltage source inverter (308a, 308b), a current source inverter (302a, 302b), 302c, a rectifier (310), a switching switch (312) and / or a storage module (306a, 306b, 306c).

[0034] One or more controllers (320) may be configured to directly control the functions of the fuel cell (304a, 304b), voltage source inverter (308a, 308b), current source inverter (302a, 302b, 302c), rectifier (310), switch (312), and / or storage module (306a, 306b, 306c) and / or provide control signals. One or more controllers (320) may be configured to receive a signal configured to display to one or more controllers (320) an AC voltage on a grid-side bus (314), a microgrid-side bus (318), and / or a transmission bus (316a, 316b, 316c, 316d) from a voltage source inverter (308a, 308b), a current source inverter (302a, 302b, 302c), a rectifier (310), and / or a switching switch (312). One or more controllers (320) may be configured to directly measure AC voltage on the grid-side bus (314), microgrid-side bus (318), and / or transmission bus (316a, 316b, 316c, 317a, 317b), in the voltage source inverter (308a, 308b), current source inverter (302a, 302b, 302c), rectifier (310), transmission bus (316a, 316b, 316c, 316d), switch (312), grid-side bus (314), microgrid-side bus (318) and / or transmission bus (316a, 316b, 316c, 316d).

[0035] Fuel cells (304a, 304b) may be configured to provide DC electrical energy to voltage source inverters (308a, 308b) via transmission buses (316a, 316b). The DC electrical energy may consist of an amount of DC voltage required to support the load demand of a load (i.e., the “load” of FIG. 3), to which a fuel cell system-based microgrid (300) is deployed for this purpose. The amount of DC voltage output by the fuel cells (304a, 304b) to the voltage source inverters (308a, 308b) may be controlled by a controller (320).

[0036] The voltage source inverters (308a, 308b) may be configured to receive DC voltage from the fuel cells (304a, 304b), convert the DC voltage into AC electrical energy, and provide the AC electrical energy to the microgrid-side bus (318) via the transmission bus (316a, 316b). The AC electrical energy may consist of an amount of AC voltage required 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, 308b) may be controlled to be equal to the amount of AC voltage for each voltage source inverter (308a, 308b). The amount of AC voltage output by the voltage source inverters (308a, 308b) may be limited by the lowest DC voltage generating capacity of one of the fuel cells (304a, 304b). That is, when the fuel cells (304a, 304b) have different capacities for generating DC voltage, the lowest capacity limits the AC voltage output of the voltage source inverter (308a, 308b) electrically connected to the fuel cell (304a, 304b) with the larger capacity. The amount of AC voltage output by the voltage source inverter (308a, 308b) can be controlled by the controller (320).

[0037] Fuel cells (304a, 304b) may be configured to provide DC electrical energy to current source inverters (302a, 302b) via transmission buses (317a, 317b). The DC electrical energy may consist of an amount of DC voltage generated by the fuel cells (304a, 304b) that exceeds what is used by the voltage source inverters (308a, 308b). For example, the amount of DC voltage used by the voltage source inverters (308a, 308b) may be less than all DC voltage generated by the fuel cells (304a, 304b) when an equal share of the load demand is less than the minimum DC voltage generating capacity of the fuel cells (304a, 304b). In another embodiment, the amount of DC voltage used by the voltage source inverter (308a, 308b) may be less than all DC power generated by at least one of the fuel cells (304a, 304b) when an equal share of the load demand is greater than the lowest DC voltage generating capacity of one of the fuel cells (304a, 304b). The amount of DC voltage output by the fuel cells (304a, 304b) to the current source inverter (302a, 302b) may be controlled by the controller (320).

[0038] Current source inverters (302a, 302b) may be configured to receive a DC voltage from a fuel cell (304a, 304b), convert the DC voltage into AC electrical energy, and provide the AC electrical energy to a grid-side bus (314) via a transmission bus (317a, 317b). The AC electrical energy may consist of an amount of AC current 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, 302b) may be controlled based on various electrical connections of the grid-side bus (314). As further described herein, the grid-side bus (314) may be optionally electrically connected to an electric utility power grid (i.e., the "grid" of FIG. 3) by a switching switch (312). The current source inverters (302a, 302b) can output AC current following a volt-watt curve based on the voltage at the grid-side bus (314), the current source inverters (302a, 302b), and / or the switching switch (312). The AC current on the grid-side bus (314) can be sent to the electric utility power grid and / or used to support loads, as described further below. In another embodiment, the grid-side bus (314) can be optionally electrically connected to the microgrid-side bus (318) by the switching switch (312) and the transfer bus (316d), as described further below. The current source inverter (302a, 302b) can output an AC current following a volt-watt curve based on the voltage at the grid-side bus (314), the current source inverter (302a, 302b), the switching switch (312), the transmission bus (316d), the microgrid-side bus (318), and / or the voltage source inverter (308a, 308b). The amount of AC current output by the current source inverter (302a, 302b) can be controlled by the controller (320).

[0039] The switching switch (312) may be configured to selectively electrically connect the grid-side bus (314) to the power grid or microgrid-side bus (318) via the transmission bus (316d). The switching switch (312) may detect the availability of the electric utility power grid, for example, by detecting the voltage and / or current levels of the electric utility power grid. In response to the electric utility power grid being available in normal operating mode, the switching switch (312) may selectively electrically connect the grid-side bus (314) to the electric utility power grid and disconnect the grid-side bus (314) from the transmission bus (316d). In response to the electric utility power grid being unavailable in emergency operating mode, the switching switch (312) may 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 switching switch (312) can be controlled by the controller (320).

[0040] The rectifier (310) may be configured to draw AC current from the grid-side bus (314) via the transmission bus (316c) in response to the demand for more electrical energy to support load demand when the switching switch (312) optionally 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 provided to the microgrid-side bus (318), the rectifier (310) may draw AC current (e.g., grid current and / or inverted fuel cell current) from the grid-side bus (314). The rectifier (310) may convert the AC current into DC current and provide the DC current to the current source inverter (302c) via the transmission bus (316c). Any AC current remaining in the grid-side bus (314) can be sent to the electric utility power grid. In response to load demands met by the voltage source inverters (308a, 308b), the rectifier (310) can be configured not to draw AC current from the grid-side bus (314), and the AC current from the grid-side bus (314) can be sent 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) may be configured to receive DC current from the rectifier (310), convert the DC current into AC electrical energy, and provide the AC electrical energy to the microgrid-side bus (318) via the transmission bus (316c). The AC electrical energy may consist of 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) may be controlled based on load demand. For example, the current source inverter (302c) may output AC current following a volt-watt curve based on the voltage and load demand at the microgrid-side bus (318) and / or the voltage source inverter (308a, 308b). The AC current output by the current source inverter (302c) may be an amount sufficient to supplement the shortfall in AC voltage output of the voltage source inverter (308a, 308b) to support load demand. The amount of alternating current output by the current source inverter (302c) can be controlled by the controller (320).

[0042] The transmission bus (316d) can electrically connect the grid-side bus (314) and the microgrid-side bus (318) when the switching switch (312) selectively electrically connects the buses (314, 318) during the emergency operation mode. The current source inverters (302a, 302b) can be configured to provide AC current to the grid-side bus (314) through the transmission buses (317a, 317b), and the AC current can flow to the microgrid-side bus (318) through the transmission bus (316d). The amount of AC current output to the grid-side bus (314) by the current source inverters (302a, 302b) can be controlled based on various electrical connections of the grid-side bus (314). The current source inverter (302a, 302b) can output an AC current following a watt curve based on the voltage at the grid-side bus (314), current source inverter (302a, 302b), switching switch (312), transmission bus (316d), microgrid-side bus (318) and / or voltage source inverter (308a, 308b). When the transmission bus (316d) electrically connects the grid-side bus (314) and the microgrid-side bus (318), the flow of alternating 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 inverter (302a, 302b) can be controlled by the controller (320).

[0043] In some embodiments, the storage modules (306a, 306b, 306c) may sink excess electrical energy or source additional electrical energy when necessary to maintain a stable fuel cell system-based microgrid voltage. For example, when the switching switch (312) selectively electrically connects the grid-side bus (314) and the microgrid-side bus (318), the storage modules (306a, 306b, 306c) may be used to provide additional electrical energy to the microgrid-side bus (318) or to receive surplus electrical energy that is not needed on the microgrid-side bus (318). The storage modules (306a, 306b, 306c) can maintain the fuel cell system-based microgrid voltage during the switching of the switching switch (312) by selectively electrically connecting the fuel cell system-based microgrid (300) to the electric utility power grid and selectively electrically disconnecting the fuel cell system-based microgrid (300) from the electric utility power grid.

[0044] FIGS. 4a through 4c are process flow diagrams of a fuel cell system-based microgrid power management method for a fuel cell system-based microgrid (300) illustrated in FIG. 3 according to various embodiments. Referring to FIGS. 1 through 4c, the method (400) may 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), rectifiers (310), grid-side buses (314), microgrid-side buses (318), transmission buses (316a, 316b, 316c, 316d, 317a, 317b), switching switches (312), and / or storage modules (306a, 306b, 306c). The method (400) may 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), rectifiers (310), switching switches (312) and / or storage modules (306a, 306b, 306c). To include alternative configurations possible in various embodiments, the hardware implementing the method (400) is referred to herein as a "control device." Any number and combination of blocks (402-448) may be implemented periodically, repeatedly, or sequentially and / or simultaneously with any other blocks (402-448).

[0045] Referring to FIG. 4a, in block (402), the control device can measure the voltage at the microgrid side bus (318). When the grid side bus (314) is disconnected from the microgrid side bus (318) by the switching switch (312), the voltage at the microgrid side bus (318) can be measured by receiving signals configured to display the voltage at the microgrid side bus (318) to the control device from any of the voltage source inverters (308a, 308b), the current source inverter (302c), and / or the switching switch (312). The voltage at the microgrid side bus (318) can be measured by directly measuring the voltage at the microgrid side bus (318) by a control device at any of the voltage source inverters (308a, 308b), current source inverters (302c), microgrid side bus (318), transmission buses (316a, 316b, 316c, 316d), and / or switching switches (312). When the grid side bus (314) is connected to the microgrid side bus (318) by switching switches (312), the voltage at the microgrid side bus (318) can be additionally measured by receiving signals configured to display the voltage at the microgrid side bus (318) to the control device, for example, from any of the grid side bus (314) and / or current source inverters (302a, 302b). The voltage at the microgrid side bus (318) can be measured by directly measuring the voltage at the microgrid side bus (318) by a control device, additionally at any of the grid side bus (314) and / or current source inverters (302a, 302b).

[0046] In block (404), the control device can control the voltage source inverters (308a, 308b). The control device can control the AC voltage that the voltage source inverters (308a, 308b) output to the microgrid side bus (318). For example, the control device can send a signal to the voltage source inverters (308a, 308b) or directly set the voltage source inverters (308a, 308b) as a set point for the AC voltage output. The control device can control the voltage source inverters (308a, 308b) to output the same amount of AC voltage to the microgrid side bus (318). The control device can control the voltage source inverters (308a, 308b) to output AC voltage to the microgrid side bus (318) based on the load demand for the load to which the fuel cell system-based microgrid (300) is placed. The control device may further control the voltage source inverters (308a, 308b) to limit the output AC voltage to the microgrid side bus (318) based on the lowest electrical energy generation capacity among the fuel cells (304a, 304b). For example, if an equal portion of the load demand exceeds the lowest electrical energy generation capacity among the fuel cells (304a, 304b), the control device may limit the AC voltage output to the microgrid side bus (318) by the voltage source inverters (308a, 308b) to an amount that can be output by the voltage source inverters (308a, 308b) that receive DC voltage from the fuel cell (304a, 304b) with the lowest electrical energy generation capacity.

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

[0048] In block (408), the control device can measure the voltage at the grid-side bus (314). The voltage at the grid-side bus (314) can be measured by receiving a signal configured to indicate the voltage at the grid-side bus (314) to the control device, for example, from any of the current source inverters (302a, 302b), transmission buses (317a, 317b), rectifier (310) and / or switching switch (312). The voltage at the grid-side bus (314) can be measured by the control device directly measuring the voltage at the grid-side bus (314) from any of the current source inverters (302a, 302b), rectifier (310), grid-side bus (314), transmission buses (317a, 317b, 316c, 316d) and / or switching switch (312).

[0049] In block (410), the control device can control the current source inverters (302a, 302b) to output surplus electrical energy to the grid-side bus (314). The current source inverters (302a, 302b) can receive DC current generated by the fuel cell (304a) and not used by the voltage source inverters (308a, 308b) to generate AC voltage for the microgrid-side bus (318). The control device can control the AC current output to the microgrid-side bus (318) by the current source inverters (302a, 302b). For example, the control device can send a signal to the current source inverters (302a, 302b) or directly set the current source inverters (302a, 302b) as a setpoint for AC current output. The control device can control the current source inverters (302a, 302b) to output AC current to the grid-side bus (314) based on the voltage and volt-watt curve at the grid-side bus (314).

[0050] In the determination block (412), the control device can determine whether 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 levels of the electric utility power grid. The control device can detect the voltage and / or current levels of the electric utility power grid by receiving signals configured to indicate the voltage and / or current levels of the electric utility power grid to the control device, for example, from any of the current source inverters (302a, 302b), rectifiers, and / or switching switches (312). The voltage and / or current levels of the electric utility power grid can be measured by the control device directly measuring the voltage and / or current levels of the electric utility power grid at any of the current source inverters (302a, 302b), rectifier (310) grid-side bus (314), transmission buses (317a, 317b, 316c, 316d), and / or switching switches (312). The control device may 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. In response to the determination that the electric utility power grid is available (i.e., the determination block (412) is "yes"), the control device may continue to operate in a normal operating mode in step "A" as described below in relation to FIG. 4b. In response to the determination that the electric utility power grid is not available (i.e., the determination block (412) is "no"), the control device may operate in an emergency mode in step "B" as described below in relation to FIG. 4c.

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

[0052] In response to a decision that the fuel cell system-based microgrid (300) is not connected to the electric utility power grid (i.e., decision 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 a switching switch (312) to change the 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] Depending on the determination that the fuel cell system-based microgrid (300) is connected to the electric utility power grid (i.e., determination block (420) = "Yes") or block (422), the control device may determine whether the voltage at the microgrid-side bus (318) is sufficient to support the load in the determination block (424). The control device may measure the voltage at the microgrid-side bus (318) by using a measurement of the voltage at the microgrid-side bus (318) in block (402), for example, as described in the specification with reference to block (402). The control device may compare the voltage measurement at the microgrid-side bus (318) with the load demand. The voltage at the microgrid-side bus (318) may be insufficient when the voltage at the microgrid-side bus (318) does not meet the load demand, and may be sufficient when the voltage at the microgrid-side bus (318) meets or exceeds the load demand.

[0054] In response to a determination that the voltage at the microgrid-side bus (318) is insufficient to support the load (i.e., determination block (424) = "No"), the control device may control the rectifier (310) and the current source inverter (302c) in block (426). The control device may control the rectifier (310) and the current source inverter (302c) to provide 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 current output to the grid-side bus (318) by the current source inverters (302a, 302b) as described herein with reference to block (410). The control device can control the rectifier (310) and the current source inverter (302c) by, for example, sending a signal to control the rectifier (310) and the current source, or by directly setting a set point for the current output in the rectifier (310) and the current source inverter (302c). The set point for the current output may be based on the amount of current required to satisfy the load demand based on the volt-watt curve, in addition to the voltage at the microgrid-side bus (318).

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

[0056] In response to a decision that the voltage at the microgrid-side bus (318) is sufficient to support the load (i.e., decision block (424) = "Yes") or following block (430), the control device can control the export of excess electrical energy from the grid-side bus (314) to the electric utility power grid in block (432). The control device can continue to measure the voltage at the microgrid-side bus (318) in block (402).

[0057] Referring to FIG. 4c, in response to a determination that the electric utility power grid is not available (i.e., determination block (412) = "No"), the control device may determine in the determination block (440) whether the fuel cell system-based microgrid (300) is connected to the electric utility power grid. The control device may determine the connection status of the fuel cell system-based microgrid (300) to the electric utility power grid based on the state, position, etc. of the switching switch (312). If the switching switch (312) selectively electrically connects the grid-side bus (314) to the electric utility power grid, the fuel cell system-based microgrid (300) may be connected to the electric utility power grid. If the switching switch (312) selectively electrically connects the grid-side bus (314) and the microgrid-side bus (318), the fuel cell system-based microgrid (300) may be disconnected from the electric utility power grid.

[0058] In response to a determination that the fuel cell system-based microgrid (300) is connected to the electric utility power grid (i.e., determination block (440) = "Yes"), the control device may electrically connect the grid-side bus (314) to the microgrid-side bus (318) in block (442) and electrically disconnect the grid-side bus (314) from the electric utility power grid. The control device may control a switching switch (312) to change the state or position 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] Depending on the determination that the fuel cell system-based microgrid (300) is not connected to the electric utility power grid (i.e., determination block (440) = "No") or block (442), the control device may determine in determination block (444) whether the voltage at the microgrid-side bus (318) is sufficient to support the load. The control device may measure the voltage at the microgrid-side bus (318) by using a measurement of the voltage at the microgrid-side bus (318) of block (402), for example, as described herein with reference to block (402). The control device may compare the measurement of the voltage at the microgrid-side bus (318) with the load demand. The voltage at the microgrid-side bus (318) may be insufficient when the voltage at the microgrid-side bus (318) does not meet the load demand, and may be sufficient when the voltage at the microgrid-side bus (318) meets or exceeds the load demand.

[0060] In response to a determination that the voltage at the microgrid-side bus (318) is not sufficient to support the load (i.e., determination block (444) = "No"), the control device may control the current source inverters (302a, 302b) in block (446). The control device may control the current source inverters (302a, 302b) to provide electrical energy from the fuel cells (304a, 304b) to the microgrid-side bus (318) via the grid-side bus (314) and the transmission bus (316d). The electrical energy may include AC current output to the grid-side bus (318) by the current source inverters (302a, 302b) as described herein with reference to block (410). The current source inverter (302a, 302b) can be controlled by giving a signal to control the current source inverter (302a, 302b) or by directly setting the current source inverter (302a, 302b) as a set point for current output. The set point for current output can be based on the amount of current required to satisfy load demand based on the volt-watt curve, in addition to the voltage at the microgrid side bus (318).

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

[0062] After the determination that the voltage of the microgrid-side bus (318) is sufficient to support the load (i.e., determination block (444) = "Yes") or block (448), the control device can continue to measure the voltage of the microgrid-side bus (318) in block (402).

[0063] FIG. 5 illustrates a fuel cell system-based microgrid (500) suitable for implementing various embodiments. Referring to FIGS. 1 through 5, the fuel cell system-based microgrid (500) may include: a plurality of fuel cells (304a, 304b), voltage source inverters (308a, 308b), current source inverters (302a, 302b), a grid-side bus (314), a microgrid-side bus (318), transmission buses (316a, 316b, 316d, 317a, 317b), and a switching switch (312). In some embodiments, the fuel cell system-based microgrid (500) may also include storage modules (306a, 306b, 306c). Fuel cells (304a, 304b), voltage source inverters (308a, 308b), current source inverters (302a, 302b), grid-side buses (314), microgrid-side buses (318), transmission buses (316a, 316b, 316d, 317a, 317b), switching switches (312) and / or storage modules (306a, 306b, 306c) may be configured, structured, electrically connected, and / or function as described herein with reference to FIG. 3 through 4c unless otherwise noted. The fuel cell system-based microgrid (500) may include a motor (502), a generator (504), and a transmission bus (316e) instead of the respective rectifier (310), current source inverter (302c), and transmission bus (316c) shown in FIG. 3.

[0064] The motor (502) may be configured to draw AC current from the grid-side bus (314) via the transmission bus (316e) in response to a demand for more electrical energy to support load demand when the switching switch (312) selectively electrically connects the grid-side bus (314) to the electric utility power grid in normal operating mode. The AC voltage output to the microgrid-side bus (318) by the voltage source inverter (308a, 308b) may be insufficient to support load demand. To increase the amount of AC voltage provided to the microgrid-side bus (318), the motor (502) may draw AC current from the grid-side bus (314). The motor (502) may drive the motor (502) using the received AC current. The motor (502) may operate at various speeds to drive the generator (504). Any AC current remaining in the grid-side bus (314) can be sent to the electric utility power grid. In response to load demand met by the voltage source inverters (308a, 308b), the motor (502) can be configured not to draw AC current from the grid-side bus (314), and the AC current from the grid-side bus (314) can be sent to the electric utility power grid. The amount of AC current drawn from the grid-side bus (314) by the motor (502) and the speed at which the generator (504) operates and / or drives can be controlled by the controller (320).

[0065] The motor (502) can drive the generator (504) using alternating current drawn from the grid-side bus (314), and the generator (504) can generate alternating current electrical energy and provide alternating current electrical energy to the microgrid-side bus (318) via the transmission bus (316e). The AC electrical energy may consist of 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) may be controlled based on load demand. For example, the generator (504) may output AC current following a volt-watt curve based on the voltage and load demand at the microgrid-side bus (318) and / or the voltage source inverter (308a, 308b). The amount of AC current output by the generator (504) may be sufficient to supplement the shortfall in AC voltage output of the voltage source inverter (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 side bus (318) by the transmission bus (316e).

[0067] In this embodiment, a large amount of short-circuit current may be available from the generator (504). Microgrid systems generally source much less short-circuit current than grid-connected systems due to inverter technology. Therefore, the generator (504) is advantageous in that it sources a large amount of short-circuit current in the event of a fault to quickly remove the protection device.

[0068] FIGS. 6a through 6c are process flow diagrams of a fuel cell system-based microgrid power management method for a fuel cell system-based microgrid (500) illustrated in FIG. 5 according to various embodiments. Referring to FIGS. 1 through 6c, the method (600) may 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), motors (502), generators (504), grid-side buses (314), microgrid-side buses (318), transmission buses (316a, 316b), 316d, 316e, 317a, 317b, transmission switches (312), and / or storage modules (306a, 306b, 306c). The method (600) may 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), motors (502), generators (504), switching switches (312), and / or storage modules (306a, 306b, 306c). To include alternative configurations possible in various embodiments, the hardware implementing the method (600) is referred to herein as a “control device.” Any number and combination of blocks (402-448 and 602-606) may be implemented periodically, repeatedly, or sequentially and / or simultaneously with any other blocks (402-448 and 602-606). Blocks (402-448) may describe a part of the method (600) in a manner similar to blocks (402-448) described in this specification for the method (400) with reference to FIG. 4a through 4c.

[0069] Referring to FIG. 6a, in block (402), the control device can measure the voltage on the microgrid-side bus (318). In block (404), the control device can control the voltage source inverters (308a, 308b). In block (406), the control device can control the voltage source inverters (308a, 308b) to output a controlled amount of AC voltage to the microgrid-side bus (318). In block (408), the control device can measure the voltage on the grid-side bus (314). In block (410), the control device can control the current source inverters (302a, 302b) to output surplus electric energy to the grid-side bus (314). In the determination block (412), the control device can determine whether the electric utility power grid is available. In response to a determination that the electric utility power grid is available (i.e., determination block (412) = "Yes"), the control device may continue step "A" of FIG. 6b. In response to a determination that the electric utility power grid is not available (i.e., determination block (412) = "No"), the control device may continue step "B" of FIG. 6c.

[0070] Referring to FIG. 6b, in response to a determination that the electric utility power grid is available (i.e., determination block (412) = "Yes"), the control device can determine whether the fuel cell system-based microgrid (500) is connected to the electric utility power grid in the determination block (420). In response to a determination that the fuel cell system-based microgrid (500) is not connected to the electric utility power grid (i.e., determination block (420) = "No"), the control device can electrically disconnect the grid-side bus (314) from the microgrid-side bus (318) in the block (422) and electrically connect the grid-side bus (314) to the electric utility power grid. In response to a decision that the fuel cell system-based microgrid (500) is connected to the electric utility power grid (i.e., decision block (420) = "Yes"), or following block (422), the control device can determine in decision block (424) whether the voltage on the microgrid-side bus (318) is sufficient to support the load.

[0071] In response to a determination that the voltage of the microgrid-side bus (318) is insufficient to support the load (i.e., determination block (424) = "No"), the control device may control the motor (502) to drive the generator (504) in block (602). The control device may control the motor (502) and the generator (504) to provide 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 current output to the grid-side bus (318) by the current source inverters (302a, 302b) as described above with reference to block (410). The control device may control the motor (502) and the generator (504) by, for example, sending a signal to control the motor (502) and the generator (504), or by directly setting the motor (502) and the generator (504) to a set point for operating speed and / or current output. The set point for the operating speed and / or current output can be based on the amount of current required to meet load demand based on the volt-watt curve, in addition to the power at the microgrid side bus (318).

[0072] In block (604), the control device can control the motor (502) to draw AC current from the grid-side bus (314). The control device can control the motor (502) to operate at a specific speed based on the control of the motor (502) in block (602) by drawing an amount of AC current from the grid-side bus (314). In block (606), the control device can control the generator (504) to output a controlled amount of AC current to the microgrid-side bus (318). The control device can control the generator (504) so ​​that it operates at a specific speed and outputs a controlled amount of AC current to the microgrid-side bus (318) based on the control of the generator in block (602).

[0073] After a decision that the voltage at the microgrid-side bus (318) is sufficient to support the load (i.e., decision block (424) = "Yes") or block (430), the control device can control the outflow of surplus electrical energy from the grid-side bus (314) to the electric utility power grid in block (432). The control device can continue to measure the voltage at the microgrid-side bus (318) in block (402).

[0074] Referring to FIG. 6c, in response to a determination that the electric utility power grid is not available (i.e., determination block (412) = "No"), the control device may determine in determination block (440) whether the fuel cell system-based microgrid (500) is connected to the electric utility power grid. In response to a determination that the fuel cell system-based microgrid (500) is connected to the electric utility power grid (i.e., determination block (440) = "Yes"), the control device may connect the grid-side bus (314) to the microgrid-side bus (318) in block (442) and electrically disconnect the grid-side bus (314) from the electric utility power grid. In response to a determination that the fuel cell system-based microgrid (500) is not connected to the electric utility power grid (decision block (440) = "No") or following block (442), the control device may determine in block (444) whether the voltage at the microgrid-side bus (318) is sufficient to support the load. In response to a determination that the voltage at the microgrid-side bus (318) is insufficient to support the load (i.e., determination block (444) = "No"), the control device may control the current source inverters (302a, 302b) in block (446). In block (448), the control device may control the current source inverters (302a, 302b) to output a controlled amount of AC current to the microgrid-side bus (318) through the grid-side bus (314). In response to a determination that the voltage at the microgrid-side bus (318) is sufficient to support the load (i.e., determination block (444) = "Yes"), following block (448), the control device may continue to measure the voltage at the microgrid-side bus (318) in block (402).

[0075] FIG. 7 illustrates a fuel cell system-based microgrid (700) suitable for implementing various embodiments. Referring to FIGS. 1 through 7, the fuel cell system-based microgrid (700) may include: a plurality of fuel cells (304a, 304b), voltage source inverters (308a, 308b), current source inverters (302a, 302b), a grid-side bus (314), a microgrid-side bus (318), transmission buses (316a, 316b, 316d, 317a, 317b), and a switching switch (312). In some embodiments, the fuel cell system-based microgrid (700) may also include storage modules (306a, 306b, 306c). Fuel cells (304a, 304b), voltage source inverters (308a, 308b), current source inverters (302a, 302b), grid-side buses (314), microgrid-side buses (318), transmission buses (316a, 316b, 316d, 317a, 317b), switching switches (312) and / or storage modules (306a, 306b, 306c) may be configured, structured, electrically connected and / or function as described herein with reference to FIG. 3 through 6c unless otherwise noted. A fuel cell system-based microgrid (700) may include a relay (702), an electric contactor (704), and a transmission bus (316f) instead of the motor (502), generator (504), and transmission bus (316e) shown in FIG. 5.

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

[0077] The relay (702) may be configured to detect current flow between the grid-side bus (314) and the microgrid-side bus (318) on the transmission bus (316f). When the AC current of the grid-side bus (314) supports the load, the AC current flows from the grid-side bus (314) to the microgrid-side bus (314) in a "forward flow." However, there are cases where the AC current may flow from the microgrid-side bus (314) to the grid-side bus (314) in a "reverse flow." The electric contactor (704) may be electronically controlled to allow or block current flow on the transmission bus (316f). The electric contactor (704) may be any type of electronically controlled contactor, such as a circuit breaker, a switch, etc.

[0078] In response to the detection of backflow on the transmission bus (316f), the relay (702) may send a signal to the electric contactor (704) to block the backflow on the transmission bus (316f). In some embodiments, the relay (702) may send a signal directly to the electric contactor (704) to block the reverse flow on the transmission bus (316f). In some embodiments, the relay (702) may signal the detection of backflow on the transmission bus (316f) to the controller (320) to send a signal to the electric contactor (704) to block the backflow on the transmission bus (316f) through the controller (320), and the controller (320) sends a signal to the electric contactor (704) to block the backflow on the transmission bus (316f).

[0079] The position or state of the switching switch (312) and the electric contactor (704) may be linked. For example, when the switching switch (312) selectively electrically connects the grid-side bus (314) to the electric utility power grid, in normal operation mode, the electric contactor (704) may maintain an electrical connection between the grid-side bus (314) and the microgrid-side bus (318) on the transmission bus (316f) and vice versa. In another embodiment, when the switching switch (312) selectively electrically connects the grid-side bus (314) and the microgrid-side bus (318) in emergency operation mode, the electric contactor (704) may 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] FIGS. 8a through 8c are process flow diagrams of a fuel cell system-based microgrid power management method for a fuel cell system-based microgrid (700) illustrated in FIG. 7 according to various embodiments. Referring to FIGS. 1 through 8c, the method (800) may 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), electric contactors (704), grid-side buses (314), microgrid-side buses (318), transmission buses (316a, 316b, 316d, 316f, 317a, 317b), switching switches (312), and / or storage modules (306a, 306b). The method (800) may 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), electric contactors (704), switching switches (312) and / or storage modules (306a, 306b). To include alternative configurations possible in various embodiments, the hardware implementing the method (800) is referred to herein as a "control unit." Any number and combination of blocks (402-448 and 802-808) may be implemented periodically, repeatedly, or sequentially and / or simultaneously with any other blocks (402-448 and 802-808). Blocks (402-448) may describe a part of the method (800) in a manner similar to blocks (402-448) described in this specification for the method (400) with reference to FIG. 4a through 4c.

[0081] Referring to FIG. 8a, in block (802), the control device can measure voltage and / or current at the microgrid side bus (318). When the grid side bus (314) is connected to the microgrid side bus (318) by an electric contactor (704), the voltage and / or current of the microgrid side bus (318) can be measured by receiving signals configured to display the voltage and / or current at the microgrid side bus (318) to the control device, for example, from a voltage source inverter (308a, 308b), a relay (702), an electric contactor (704) and / or a transmission switch (312). The voltage and / or current of the microgrid side bus (318) may be measured by a control device at any one of the voltage source inverters (308a, 308b), relays (702), electric contactors (704), microgrid side buses (318), transmission buses (316a, 316b, 316d, 316f), and / or switching switches (312) by directly measuring the voltage and / or current of the microgrid side bus (318). When the grid side bus (314) is connected to the microgrid side bus (318) by switching switches (312), the voltage and / or current in the microgrid side bus (318) may be measured, as an additional example, by receiving a signal configured to display the voltage and / or current in the microgrid side bus (318) from the grid side bus (314) and / or current source inverters (302a, 302b) to the control device. The voltage and / or current at the microgrid side bus (318) can be measured by additionally controlling the voltage at the microgrid side bus (318) directly by a control device at either the grid side bus (314) and / or the current source inverter (302a, 302b).

[0082] In block (804), the control device can control the voltage source inverters (308a, 308b). The control device can control the AC voltage output to the microgrid side bus (318) by the voltage source inverters (308a, 308b). For example, the control device can signal the voltage source inverters (308a, 308b) or directly set the voltage source inverters (308a, 308b) as a set point for AC voltage output. The control device can control the voltage source inverters (308a, 308b) to output the same amount of AC voltage to the microgrid side bus (318). The control device can control the voltage source inverters (308a, 308b) to output AC voltage to the microgrid side bus (318) based on the load demand for the load to which the fuel cell system-based microgrid (700) is deployed. The control device can control the voltage source inverters (308a, 308b) to output an AC voltage to the microgrid side bus (318) to prevent current on the microgrid side bus (318) from leaking into the electric utility power grid. The control device can further control the voltage source inverters (308a, 308b) to limit the output AC voltage to the microgrid side bus (318) based on the lowest electric energy generation capacity among the fuel cells (304a, 304b). For example, if an equal portion of the load demand exceeds the lowest electrical energy generation capacity among the fuel cells (304a, 304b), the control device may limit the output AC voltage to the microgrid side bus (318) by the voltage source inverter (308a, 308b) to an amount that can be output by the voltage source inverter (308a, 308b) receiving the DC voltage from the fuel cell (304a, 304b) having the lowest electrical energy generation capacity.

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

[0084] In block (408), the control device can measure the voltage on the grid-side bus (314). In block (410), the control device can control the current source inverters (302a, 302b) to output excess electrical energy to the grid-side bus (314). In the determination block (412), the control device can determine whether the electric utility power grid is available. In response to the determination that the electric utility power grid is available (i.e., determination block (412) = "Yes"), the control device can continue to step "A" of FIG. 8b. In response to the determination that the electric utility power grid is not available (i.e., determination block (412) = "No"), the control device can continue to step "B" of FIG. 8c.

[0085] Referring to FIG. 8b, in response to a determination that the electric utility power grid is available (i.e., determination block (412) = "Yes"), the control device can determine in determination block (420) whether the fuel cell system-based microgrid (700) is connected to the electric utility power grid. In response to a determination that the fuel cell system-based microgrid (700) is not connected to the electric utility power grid (i.e., determination block (420) = "No"), the control device can electrically disconnect the grid-side bus (314) from the microgrid-side bus (318) via the transmission bus (316f) in block (808) and electrically connect the grid-side bus (314) to the electric utility power grid. The control device can control the transmission switch (312) to change the state or position to electrically connect the grid-side bus (314) to the electric utility power grid and electrically disconnect the grid-side bus (314) from the microgrid-side bus (318) via the transmission bus (316d).

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

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

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

[0089] In response to a determination that there is a reverse flow in the transmission bus (316f) (i.e., determination block (810) = "Yes"), the control device may control the electric contactor (704) in block (816) to open so as to prevent reverse flow on the transmission bus (316f). The control device may control the electric contactor (704) to change its state or position (i.e., open) to electrically isolate the grid-side bus (314) from the microgrid-side bus (318) through the transmission bus (316f). In other words, when the load power demand is lower than the power output of the voltage source inverter (308a, 308b), the contactor (704) is opened to prevent reverse power flow from the microgrid bus (318) to the utility power grid.

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

[0091] Referring to FIG. 8c, in response to a determination that the electric utility power grid is not available (i.e., determination block (412) = "No"), the control device can determine whether the fuel cell system-based microgrid (700) is connected to the electric utility power grid in the determination block (440). In response to a determination that the fuel cell system-based microgrid (700) is connected to the electric utility power grid (i.e., determination block (440) = "Yes"), the control device can electrically connect the grid-side bus (314) to the microgrid-side bus (318) via the transmission bus (316d) in block (818) and electrically disconnect the grid-side bus (314) from the electric utility power grid.

[0092] The control device can control a switching switch (312) to change its state or position 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. The control device can control an electric contactor (704) to change its state or position (i.e., open) to electrically disconnect the grid-side bus (314) from the microgrid-side bus (318) via the transmission bus (316f).

[0093] In response to a determination that the fuel cell system-based microgrid (700) is not connected to the electric utility power grid (i.e., determination block (440) = "No") or according to block (818), the control device may determine whether the microgrid-side bus (318) is sufficient to support the load in determination block (444). In response to a determination that the voltage of the microgrid-side bus (318) is not sufficient to support the load (i.e., determination block (444) = "No"), the control device may control the current source inverters (302a, 302b) in block (446). In block (448), the control device may control the current source inverters (302a, 302b) to output a controlled amount of AC current to the microgrid-side bus (318) through the grid-side bus (314). In response to a decision that the voltage at the microgrid-side bus (318) is sufficient to support the load (i.e., decision block (444) = "Yes") or after block (448), the control device may continue to measure the voltage at the microgrid-side bus (318) in block (802).

[0094] The foregoing description of the disclosed embodiments is provided to enable those skilled in the art to practice or use the invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments without departing from the scope of the invention. Accordingly, the invention is not intended to be limited to the embodiments presented herein, and the invention should be given the broadest scope consistent with the principles and novel features disclosed herein.

[0095] The description of the method and drawings described above are provided as exemplary embodiments and are not intended to imply or require that the steps of the various embodiments be performed in the order presented. As will be recognized by those skilled in the art, the steps of the described embodiments may be performed in any order. Additionally, words such as “thereafter,” “then,” and “next” are not intended to limit the order of the steps, and are simply used to guide the reader through the description of the method.

[0096] One or more drawings have been used to illustrate exemplary embodiments. The use of the drawings does not imply any limitation in the order of operations performed. The foregoing description of exemplary embodiments is provided for the purposes of explanation and example. It is not intended to be limiting or complete with respect to the exact form disclosed, and modifications and variations may be possible in light of the foregoing teachings or may be obtained while practicing the disclosed embodiments. The scope of the invention is intended to be defined by the claims and equivalents thereof appended below.

[0097] The control element, including the connected controller (320) and the control device described herein, may be implemented in a processor designed to perform a specific function, or may be implemented using a computing device (e.g., a computer) that includes other components, memory, and a programmable processor programmed by instructions to perform a specific function. The processor may be any programmable microprocessor, microcomputer, or a plurality of processor chips or chips that may be configured by software instructions (applications) to perform various functions, including the functions of the various embodiments disclosed herein. In some computing devices, a plurality of processors may be provided. Typically, software applications may be stored in internal memory before being loaded and accessed by the processor. In some computing devices, the processor may include sufficient internal memory to store application software instructions.

[0098] Various exemplary logic blocks, modules, circuits, and algorithm steps described in conjunction with the embodiments disclosed herein may be implemented in electronic hardware, computer software, or a combination thereof. To clearly exemplify such interchangeability of hardware and software, various exemplary components, blocks, modules, circuits, and steps have been described above in general terms in relation to their functions. Whether such functionality is implemented in hardware or software depends on the design constraints imposed on the overall system and the specific application. Those skilled in the art may implement certain functionality in various ways for each specific application, but such a decision regarding implementation should not be interpreted as causing a departure from the scope of the invention.

[0099] Hardware used to implement the various exemplary logic, logic blocks, modules, and circuits described in connection with the embodiments disclosed herein may be implemented or performed by a general-purpose processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA), or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof, or a control device that may include such components, designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but alternatively, the processor may be any conventional processor, controller, microcontroller, or state machine. The processor may also be implemented by a combination of computing devices, e.g., a combination of a DSP and a microprocessor, multiple microprocessors, one or more microprocessors associated with a DSP core, or any other such configuration. Alternatively, some blocks or methods may be performed by circuits specific to a given function.

[0100] The foregoing description of the disclosed embodiments is provided to enable those skilled in the art to manufacture or use any of the disclosed embodiments. Various modifications to these embodiments will be obvious to those skilled in the art, and general principles defined herein may be applied to other embodiments without departing from the scope of the disclosure. Accordingly, the claims are not intended to limit the embodiments shown herein, but are intended to be consistent with the language of the claims and to be consistent with the novel features and principles disclosed herein.

Claims

Claim 1 A microgrid comprising: a plurality of direct current (DC) power sources; a plurality of voltage source inverters, wherein the DC end of each of the plurality of voltage source inverters is electrically connected to each of the DC power sources among the plurality of DC power sources; a microgrid-side bus, wherein the AC end of each of the plurality of voltage source inverters is electrically connected to the microgrid-side bus, and the microgrid-side bus is configured to be electrically connected to a load; a plurality of current source inverters, wherein the DC end of each of the plurality of current source inverters is electrically connected to each of the DC power sources among the plurality of DC power sources; a grid-side bus, wherein the AC end of each of the plurality of current source inverters is electrically connected to the grid-side bus; a transfer switch configured to control the selective electrical connection of the grid-side bus to an electric utility power grid or the microgrid-side bus; and a transmission bus electrically connected between the microgrid-side bus and the grid-side bus. Claim 2 In claim 1, the plurality of direct current (DC) power sources comprises a plurality of fuel cell stacks, a microgrid. Claim 3 A microgrid according to paragraph 2, wherein the plurality of voltage source inverters are configured to output the same amount of AC voltage to the microgrid-side bus, the maximum output of each of the plurality of voltage source inverters is based on the lowest power generation capacity among the plurality of fuel cell stacks, and the plurality of current source inverters are configured to output AC current to the grid-side bus from a DC current generated in excess of the lowest power generation capacity among the plurality of fuel cell stacks. Claim 4 A microgrid according to claim 1, further comprising: a rectifier electrically connected to the grid-side bus via the transmission bus; and an additional current source inverter located on the transmission bus, electrically connected to the rectifier at the DC end, and electrically connected to the microgrid-side bus at the AC end. Claim 5 In paragraph 4, the rectifier and additional current source inverter are configured to provide a second AC current to the microgrid-side bus using a first AC current from the grid-side bus when the switching switch selectively electrically connects the grid-side bus to the electric utility power grid. Claim 6 A microgrid according to claim 1, further comprising: a motor electrically connected to the grid-side bus via the transmission bus; and a generator electrically connected to the motor and driven by the motor, and electrically connected to the microgrid-side bus via the transmission bus. Claim 7 In paragraph 6, the motor and generator are configured to provide a second AC current to the microgrid-side bus using a first AC current from the grid-side bus when the switching switch selectively electrically connects the grid-side bus to the electric utility power grid. Claim 8 A microgrid according to claim 1, further comprising: an electric contactor configured to selectively electrically complete a circuit along the transmission bus between the grid-side bus and the microgrid-side bus, and configured to selectively electrically disconnect the circuit along the transmission bus between the grid-side bus and the microgrid-side bus; and a control device electrically connected to the transmission bus, configured to detect current flow, and to send a signal to the electric contactor to complete or disconnect the circuit in response to the detection of current flow from the microgrid-side bus to the grid-side bus. Claim 9 In paragraph 8, the switching switch and the electric contactor are interlocked such that, in response to the electric utility power grid being unavailable, the switching switch selectively electrically connects the grid-side bus to the microgrid-side bus, the electric contactor selectively electrically disconnects the circuit along the transmission bus between the grid-side bus and the microgrid-side bus, and in response to the flow of reverse current from the microgrid-side bus to the grid-side bus, the electric contactor selectively electrically disconnects the circuit along the transmission bus between the grid-side bus and the microgrid-side bus, the switching switch selectively electrically connects the grid-side bus to the microgrid-side bus. Claim 10 A microgrid according to claim 1, wherein the switching switch is also 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 in response to the electric utility power grid being unavailable; and the plurality of current source inverters are configured to output AC current to the microgrid-side bus through the grid-side bus when the switching switch selectively electrically connects the grid-side bus to the microgrid-side bus. Claim 11 A method of operating a microgrid, comprising: providing electrical energy from each of a plurality of DC power sources to each voltage source inverter among a plurality of voltage source inverters and each current source inverter among a plurality of current source inverters; outputting voltage to a microgrid-side bus by the plurality of voltage source inverters such that each of the plurality of voltage source inverters outputs the same amount of voltage to the microgrid-side bus - wherein the maximum output of each of the plurality of voltage source inverters is based on the minimum generation capacity of one of the plurality of DC power sources -; outputting a first current to a grid-side bus by the plurality of current source inverters based on the amount of current generated by the plurality of DC power sources in excess of the minimum generation capacity; and using the first current output to the grid-side bus to provide a second current to the microgrid-side bus. Claim 12 A method of operating a microgrid according to claim 11, further comprising: a step of determining whether the voltage satisfies the load demand; and in response to the determination that the voltage does not satisfies the load demand: a step of drawing the first current from the grid-side bus by a rectifier; a step of outputting the third current from the rectifier to an additional current source inverter; and a step of outputting the second current from the additional current source inverter to the microgrid-side bus. Claim 13 A method of operation of a microgrid according to claim 12, further comprising: a step of determining whether an electric utility power grid is available; and a step of selectively electrically connecting the grid-side bus to the electric utility power grid in response to the determination that the electric utility power grid is available; wherein the step of drawing the first current from the grid-side bus and the step of outputting the second current to the microgrid-side bus occur in response to the grid-side bus being selectively electrically connected to the electric utility power grid. Claim 14 A method of operating a microgrid according to claim 11, wherein the step of using a first current output to the grid-side bus to provide a second current to the grid-side bus comprises: determining whether the voltage meets the load demand; and in response to the determination that the voltage does not meet the load demand: drawing the first current from the grid-side bus to drive a motor; driving a generator using the motor; and outputting a second current from the generator to the microgrid-side bus. Claim 15 A method of operation of a microgrid according to claim 14, further comprising: a step of determining whether an electric utility power grid is available; a step of selectively electrically connecting the grid-side bus to the electric utility power grid in response to the determination that the electric utility power grid is available; and a step of drawing the first current from the grid-side bus and a step of outputting the second current to the microgrid-side bus, wherein the steps occur in response to the grid-side bus being selectively electrically connected to the electric utility power grid. Claim 16 A method of operating a microgrid according to claim 11, wherein the step of using a first current output to the grid-side bus to provide a second current to the grid-side bus comprises: determining whether the voltage meets the load demand; and in response to the determination that the voltage does not meet the load demand, closing a contactor to electrically connect the grid-side bus to the microgrid-side bus and form a circuit between the grid-side bus and the microgrid-side bus. Claim 17 A method of operating a microgrid according to claim 16, further comprising: a step of detecting a reverse current flow from the microgrid-side bus to the grid-side bus; and a step of opening a contactor to electrically cut off the circuit between the grid-side bus and the microgrid-side bus in response to detecting the reverse current flow. Claim 18 A method of operating a microgrid according to claim 17, further comprising the step of 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 the step of opening the contactor. Claim 19 A method of operating a microgrid according to claim 11, further comprising: a step of determining whether an electric utility power grid is available; and in response to the electric utility power grid not being available: a step of electrically disconnecting the grid-side bus from the electric utility power grid; and a step of electrically connecting the grid-side bus to the microgrid-side bus. Claim 20 In claim 11, a method of operating a microgrid in which the plurality of DC power sources include fuel cell power sources.

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