Microgrid including dual-mode microgrid inverter and method for load management

Abstract

To provide a microgrid and an operation method therefor that during an island mode, provide power to a critical load via a system power line, an ATS, and a critical load line such that the power is not provided to a non-critical load.SOLUTION: A microgrid includes a power system 40 for outputting system power and an automatic transfer switch (ATS) 330. The ATS includes a normal terminal N that is electrically connected to a grid power line 316 for receiving grid power from a power utility 201, an emergency terminal E that is electrically connected to a system power line 312 for receiving system power from the power system, a load terminal L that is electrically connected to a critical load line 314 for providing power to a critical load 23, a bypass line 322 electrically connected to the system power line and the critical load line so as to bypass the ATS, and a circuit breaker 320 for controlling a power flow through the bypass line.SELECTED DRAWING: Figure 3A

Description

[Technical Field]

[0001] The present disclosure is directed generally to microgrids, and more particularly to microgrids including fuel cell systems and dual-mode inverters, and methods of load management utilizing microgrids. [Background technology]

[0002] A microgrid is a group of interconnected loads and distributed energy resources within a well-defined electrical boundary that functions as a single controllable entity relative to the main power grid. For example, a microgrid may include local generators, loads, load management controllers, power grid isolation switches, and switch controllers.

[0003] The microgrid can operate in grid mode, where it is electrically connected to the main power grid, or in island mode, where the microgrid is disconnected from the main power grid and the microgrid is powered only by local generators. In grid mode, the power grid can act as a buffer (slack bus) to supply or absorb the power difference between local generation and the loads in the microgrid. Summary of the Invention

[0004] According to various embodiments, a microgrid includes a power system configured to output system power and an automatic transfer switch (ATS). The ATS includes a normal terminal electrically connected to a grid power line configured to receive grid power from an electric utility, an emergency terminal electrically connected to the system power line configured to receive system power from the electric utility, and a load terminal electrically connected to a critical load line configured to supply power to a critical load. The microgrid also includes a bypass line electrically connected to the system power line and the critical load line to bypass the ATS, and a circuit breaker configured to control power flow through the bypass line.

[0005] According to various embodiments, a method includes sequentially operating a microgrid in a grid parallel mode and an islanded mode. During the grid parallel mode, the method includes supplying grid power to non-critical loads via grid power lines electrically connected to the non-critical loads and to critical loads via the grid power lines, critical load lines electrically connected to the critical loads, and an automatic transfer switch (ATS) electrically connected to the grid power lines and the critical load lines, and supplying system power generated by the power system to the critical loads via the system power lines, the critical load lines, and a bypass line electrically connected to the system power lines and the critical load lines in parallel with the ATS. During the islanded mode, the method includes supplying system power to the critical loads via the system power lines, the ATS, and the critical load lines such that system power is not supplied to the non-critical loads. [Brief explanation of the drawings]

[0006] [Figure 1A] FIG. 1A is a schematic diagram illustrating power flow through the electrical architecture of a fuel cell system during system startup, according to an embodiment of the present disclosure. [Figure 1B] FIG. 1B is a schematic diagram illustrating power flow through the electrical architecture of a fuel cell system during steady-state operation, according to an embodiment of the present disclosure. [Figure 2A] FIG. 2A is a schematic diagram illustrating the electrical architecture of a fuel cell system according to another embodiment of the present disclosure. [Figure 2B] FIG. 2B is a schematic diagram illustrating the electrical architecture of a fuel cell system according to another embodiment of the present disclosure. [Figure 3A] FIG. 3A is a schematic diagram of a microgrid according to various embodiments of the present disclosure. [Figure 3B] FIG. 3B is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. [Figure 3C] FIG. 3C is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. [Figure 3D] FIG. 3D is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. [Figure 3E] FIG. 3E is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. [Figure 3F] FIG. 3F is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. [Figure 3G] FIG. 3G is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. [Figure 3H] FIG. 3H is a schematic diagram illustrating power flow through the microgrid of FIG. 3A as the microgrid transitions between grid parallel mode and islanded mode. DETAILED DESCRIPTION OF THE INVENTION

[0007] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the disclosed apparatus and method and, together with the general description above and the detailed description below, serve to explain features of the invention.

[0008] Various embodiments are described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers are used throughout the drawings to refer to the same or like parts. References to specific embodiments and implementations are for illustrative purposes only and are not intended to limit the scope of the invention or the claims. It should also be understood that the embodiments shown in the figures are not mutually exclusive. Features shown in one embodiment (e.g., one figure) may be included in other embodiments (e.g., other figures).

[0009] Fuel cell systems are a type of distributed power source that can be used to power one or more loads, such as the essential and non-essential loads of a microgrid. When such a microgrid is connected to a main power grid, power in excess of that required by the microgrid loads can be supplied to the power grid. Additionally, power from the power grid can be used to run balance-of-plant (BOP) components of the fuel cell system during start-up.

[0010] For example, fuel cell power generation systems, such as solid oxide fuel cell (SOFC)-based distributed power generation systems, can have relatively high operating temperatures during steady-state power generation mode, e.g., temperatures of 750°C or higher for SOFCs. Therefore, during system startup, such systems typically receive power from an external power source, such as a connected utility or other external AC generator, to power BOP components (e.g., fans, blowers, switches, etc.) and / or parasitic loads. This external startup power can power the BOP components and other parasitic loads by converting the externally supplied AC voltage to a usable DC voltage through a converter.

[0011] 1A schematically illustrates the power flow through the electrical architecture of a fuel cell system 10 during system startup, and FIG. 1B schematically illustrates the power flow through the electrical architecture of a fuel cell system during steady-state operation, according to a comparative example of the present disclosure. As will be apparent to those skilled in the art, references to "connection" of components described below shall mean "electrical connection" unless otherwise specified.

[0012] 1A and 1B, a system 10 may include an inverter module 200 and at least one power module 100 including fuel cells 110. The fuel cells 110 may be arranged in one or more stacks or columns. The fuel cells 110 may include multiple solid oxide fuel cells having a ceramic oxide electrolyte separated by conductive interconnect plates. Other fuel cell types, such as proton exchange membrane (PEM), molten carbonate, and phosphoric acid, may also be used. The power module 100 may be connected to the inverter module 200 by a start bus and a fuel cell (e.g., steady-state) bus, as described below. In some embodiments, the system 10 may include multiple inverter modules 200 connected to multiple power modules 100, respectively.

[0013] The system 10 may include a controller 50 configured to control the operation of the inverter module 200 and / or the power module 100. The controller 50 may be located within or operably connected to the inverter module 200 or the power module 100. The controller 50 may include a memory and / or a central processing unit configured to store operating software. In some embodiments, the controller 50 may provide an interface to allow an operator to control the system 10.

[0014] The controller 50 may be implemented using a computing device (such as a computer) that includes a processor, memory, and other components that are programmed with instructions to perform certain functions, or may be implemented in a processor designed to perform certain functions. The processor may be any programmable microprocessor, microcomputer, or multiprocessor chip, or chip that can be configured with software instructions (applications) to perform various functions, including those of the various embodiments described herein. In some computing devices, multiple processors may be provided. Typically, software applications are stored in internal memory before being accessed and loaded into the processor. In some computing devices, the processor may include sufficient internal memory to store application software instructions. The processor may include a general-purpose processor, a digital signal processor (DSP), an application-specific integrated circuit (ASIC), a field-programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor; alternatively, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a combination of multiple microprocessors, a combination of one or more microprocessors in combination with a DSP core, or any other such configuration.

[0015] The inverter module 200 may include a steady-state transformer 202 and a start-up transformer 204 connected to an external power source, such as a power utility 201 (e.g., a power grid or an external generator), by an external power bus 226. The steady-state transformer 202 may be connected to an AC / DC inverter 212. The inverter 212 may be connected to a steady-state diode 210 connected to a first balance-of-plant load BOP1. The start-up transformer 204 may be connected to a rectifier 206. The rectifier 206 may be connected via a start-up bus 222 to a start-up diode 208 connected to the load BOP1.

[0016] The power module 100 may include a DC / DC converter 218, a start-up diode 214, and a steady-state diode 216. The DC / DC converter 218 may be electrically connected to the inverter 212 and the fuel cell 110 of the power module 100. The DC / DC converter 218 may also be connected to the steady-state diode 216. The diodes 214, 216 may be connected to a second balance-of-plant load BOP2. The start-up diode 214 may also be electrically connected to the rectifier 206.

[0017] 1A , during start-up of the system 10, the fuel cells 110 may not be able to generate a sufficient amount of power to power the BOP loads before they reach a steady-state operating temperature (e.g., at least 750° C., such as 750° C. to 900° C. for SOFCs). Therefore, during start-up, power from the utility is supplied to the loads BOP1 and BOP2 via a start-up bus 222. In particular, power flows through the start-up bus 222, through the start-up transformer 204, the rectifier 206, the start-up diodes 208 and 214, and then to the BOP loads BOP1 and BOP2. The inverter 212, the converter 218, and the steady-state bus 220 are not used during system start-up.

[0018] 1B , once the fuel cell reaches the desired steady-state operating temperature (e.g., 750° C. or higher), steady-state operation of the system 10 begins, with power being supplied from the fuel cell 110 to the loads BOP1 and BOP2. Specifically, power flows from the fuel cell 110 via the steady-state bus 220, through the DC / DC converter 218, the inverter 212, and the steady-state transformer 202 to the utility and / or any local loads. Power can also be diverted through the steady-state diodes 210 and 216 to the loads BOP1 and BOP2, if desired. Thus, the rectifier 206 and the start-up transformer 204 are not used. Steady-state mode operation continues whenever the fuel cell 110 is hot enough to generate power, which accounts for substantially the entire operating life of the fuel cell. This includes periods when the transformers 202, 204 are disconnected from the utility 201 (eg, the power grid) and the steady state transformer 202 supplies power from the fuel cell 110 to the local load "L" rather than to the power grid.

[0019] Thus, the transformer 204, rectifier 206, fuses (not shown), and wiring used for power conversion during startup are unused during steady-state operation, which represents the majority of the life cycle of the system 10. This architecture therefore increases the cost of the system 10 and reduces the energy density of the system 10. The rectifier 206 can also generate harmonic currents during startup.

[0020] 2A is a schematic diagram illustrating the electrical architecture of another comparative embodiment of the present disclosure, a fuel cell system 20. The electrical architecture of system 20 is similar to the electrical architecture of system 10 shown in FIGS. 1A and 1B, and therefore only the differences between the two will be described in detail.

[0021] Referring to FIG. 2A , system 20 includes power module 100A and inverter module 200A electrically connected by interconnection bus 224. Inverter module 200A includes bidirectional inverter 240 electrically connected to interconnection bus 224. During system startup, bidirectional inverter 240 can convert AC power supplied from the utility into DC power suitable for driving loads BOP1 and BOP2, and during steady-state operation, can convert DC power from fuel cell 110 into AC power that can be supplied to the utility. In other words, interconnection bus 224 can be used to transfer power during both steady-state operation and startup operation. Therefore, the startup bus 222, startup transformer 204, rectifier 206, and startup diodes 208 and 214 of system 10 of FIGS. 1A and 1B can be omitted from system 20 without losing startup functionality.

[0022] FIG. 2B is a schematic diagram illustrating the electrical architecture of a fuel cell system 30 according to various embodiments of the present disclosure. The electrical architecture of system 30 is similar to the electrical architecture of system 10 shown in FIGS. 1A and 1B, and only the differences between the two will be described in detail. Referring to FIG. 2B, system 30 includes a power module 100 and an inverter module 200B, which are electrically connected by a steady-state bus 220 and a start-up bus 222. While one power module 100 is shown, system 30 may include multiple power modules 100 electrically connected to inverter module 200B. System 30 may also include a controller 50 configured to control the steady-state and start-up operation of system 30.

[0023] Inverter module 200B may include a first bidirectional inverter 240, a second bidirectional inverter 242, and at least one unidirectional third inverter 244 (e.g., 1 to 10 unidirectional inverters), which may be electrically connected to a power utility (e.g., a power source external to system 30) and a system transformer 202 by an external power bus 226. Inverter module 200B includes a single system transformer 202 that operates in both startup and steady-state modes, instead of the two transformers 202, 204 shown in FIGS. 1A and 1B. Inverter module 200B may also include a startup diode 208 and a steady-state diode 210, which are electrically connected to first balance-of-plant load BOP1. Inverter module 200B may also include a fault diode 230 and an optional auxiliary diode 232.

[0024] The power module 100 may include a startup diode 214 electrically connected to a startup bus 222 and a steady-state diode 216 electrically connected to a steady-state bus 220. Both diodes 214, 216 control current flow to the second BOP load BOP2.

[0025] During startup mode, the external power bus 226 can sequentially supply power from the utility 201 to the system transformer 202 and the first inverter 240. The transformer 202 can operate to change the voltage of the utility power, and the first inverter 240 can operate to convert AC utility power (i.e., alternating current) to DC power (i.e., direct current). The startup bus 222 can then supply the utility power to the first BOP load BOP1 via startup diode 208. Thus, the first inverter 240 can operate as a rectifier, eliminating the separate rectifier 206 and startup transformer 204 shown in FIG. 1A and reducing the cost of the system 30. Thus, the system 30 can eliminate an additional transformer (other than the single system transformer 202) and rectifier. The startup bus 222 can also supply the utility 201 power to the second BOP load BOP2 via startup diode 214. The fault diode 230 prevents current from flowing through the steady-state bus 220 to the fuel cell 110 during startup mode. The second inverter 242 can remain off to prevent power from flowing through the second inverter 242 to the steady-state bus 220. Furthermore, because the third inverter 244 is unidirectional, the third inverter 244 can also prevent power from being supplied to the steady-state bus 220 during startup. Thus, during system startup, the inverter module 200B can be configured to supply utility power to both loads BOP1, BOP2 via the startup bus 222.

[0026] Microgrid Inverters and Load Management

[0027] The formation of microgrid islands due to power grid faults is typically detected via standard protective relays, such as SEL751 or Beckwith relays, which monitor the voltage (V) and frequency (F) of the power grid. Aside from monitoring V and F, these protective relays also monitor the power trailing in either direction between the power grid and the microgrid to detect islanding and open the power grid isolation device. However, for microgrids designed to export excess power from excess generation within the microgrid to the utility grid, the reverse power method of islanding detection does not work. Therefore, expensive UL-certified active anti-islanding devices may be required for all generators in the microgrid.

[0028] Microgrids typically include a load management controller that operates to ensure that the total load on the microgrid is always less than the total available generation. If sufficient energy storage buffers are available within the microgrid, the microgrid controller can detect excess demand by monitoring the stored output power and shed the load accordingly. However, if sufficient energy storage is not available, after a grid fault, the microgrid load management controller must act to shed the load before local generation can begin supporting the local microgrid load. Load shedding can involve several challenges and several single points of failure.

[0029] Figure 3A is a schematic diagram of a microgrid 400 in accordance with various embodiments of the present disclosure. Figures 3B-3H are schematic diagrams illustrating power flow through the microgrid 400 as the microgrid 400 transitions between grid mode and islanded mode.

[0030] 3A , a microgrid 400 may include a power system 40, a power distribution system (PDS) 310, a circuit breaker 320, and an automatic transfer switch (ATS) 330. The PDS 310 may include additional conventional circuit breakers. The circuit breaker 320 may be a wraparound circuit breaker, which will be described in more detail below. The power system 40 may include at least one power module 100 and an inverter module 300. In one embodiment, the power system 40 may include multiple power modules 100 and multiple inverter modules 300 connected to the same PDS 310. The power module 100 may include a fuel cell stack, such as a SOFC stack.

[0031] The ATS 330 can include a normal terminal N, an emergency terminal E, a load terminal L, and an optional floating intermediate terminal or position M. The ATS 330 can be switched between a normal position (where the ATS 330 electrically connects the normal terminal N to the load terminal L), an open position (where the ATS 330 does not electrically connect the load terminal L to the normal terminal N or the emergency terminal E), and an emergency position (where the ATS 330 connects the emergency terminal E to the load terminal L). Alternatively, the ATS 330 can be a two-position ATS rather than a three-position ATS. The ATS 330 can be a contactor-based, circuit breaker-based, or molded case switch-based ATS 330 depending on system requirements.

[0032] The power module 100, the inverter module 300, and the PDS 310 may be electrically connected to an emergency terminal E of the ATS 330 by a system power line 312 (e.g., a local power line). The load terminal L may be connected by a critical load line 314 to a critical load panel 22 including one or more critical loads 23. The normal source terminal N may be connected to the utility power system 201 by a grid power line 316. The grid power line 316 may be electrically connected to a main panel (i.e., main load switchboard or panel) 24 including one or more non-critical loads 25.

[0033] The bypass line 322 may electrically connect the system power line 312 and the critical load line 314 while bypassing the ATS 330. The circuit breaker 320 may be disposed on the bypass line 322 and configured to control the flow of power through the bypass line 322. The circuit breaker 320 may be an electrically actuated circuit breaker, contactor, relay, or semiconductor switch depending on the current rating of the ATS 330. In some embodiments, the microgrid 400 may include an uninterruptible power supply (UPS) 326 and an intervening relay 324 for actuating the circuit breaker 320.

[0034] 1A-2B , inverter module 300 may be similar to any of inverter modules 200, 200A, or 200B. For example, inverter module 300 may include controller 50, a transformer 202 electrically connected to a system power line 312, at least one inverter, such as bidirectional first and second inverters 240, 242, electrically connected to transformer 202, a fault diode 230 electrically connected to first inverter 240, a first balance-of-plant (BOP) load BOP1, a steady-state bus 220 electrically connecting first and second inverters 240, 242 to first and second BOP loads BOP2 of power module 100, and a start-up bus 222 electrically connecting first and second inverters 240, 242 to first and second BOP loads BOP1 and BOP2. Alternatively, the at least one inverter may include one or more unidirectional inverters.

[0035] The controller 50 of the inverter module 300 may be configured to monitor the connection status of the circuit breaker 320 via the data connection path “CB Status,” the connection status of the ATS 330 via the data connection path “ATS Status,” and the status of the grid power (e.g., the voltage and / or frequency of the grid power). The data connections may be wired or wireless. For example, the inverter module 300 may be electrically connected and / or configured to monitor the current flow through the circuit breaker 320 and / or the ATS 330. The inverter module 300 may also be configured to control the power output to the system power lines 312 and the operation of the circuit breaker 320 (e.g., via the intervening relay 324 operated by the UPS 326) based on the status of the power grid 201, the connection status of the circuit breaker 320, and / or the connection status of the ATS 330.

[0036] 3A , when the microgrid 400 is operating in the grid parallel mode, the ATS 330 may be set to a normal position to electrically connect the grid power line 316 to the critical load line 314. Specifically, the ATS 330 may connect the normal terminal N to the load terminal L. The circuit breaker 320 may be closed such that the bypass line 322 electrically connects the system power line 312 to the critical load line 314. The DC power generated by the power module 100 may be output as AC power from the inverter module 300 at the grid mode voltage and may be supplied to the critical load 23 via the system power line 312, the bypass line 322, and the critical load line 314. Excess generated power not required by the critical load 23 may be supplied to the non-critical load 25 and / or the power grid 201 via the ATS 330 and the grid power line 316. For example, the voltage output by inverter module 300 may be synchronized with the voltage requirements of critical loads 23 and / or may be synchronized with the voltage of power grid 201. If the power demand of critical loads 23 exceeds the power output of power module 100, additional power may be supplied from power grid 201 through ATS 330 to critical loads 23 to meet the load demand of critical loads 23. If the power demand of non-critical loads 25 exceeds the power output of power module 100, additional power may be supplied from power grid 201 through grid power lines 316 to non-critical loads 25 to meet the load demand of non-critical loads 25.

[0037] The inverter module 300 may monitor the grid power on the load side of the ATS 330 to determine whether the grid power is compatible for interconnection with the power system 40. For example, the inverter module 300 may continuously or periodically monitor the grid power to determine whether it complies with the power utility, IEEE 1547, and / or UL 1741 standards. The inverter module 300 may continuously or periodically monitor the current flow through the circuit breaker 320 and / or the ATS 330 to monitor the connection status. Additionally, the ATS 330 may also continuously or periodically monitor the status of the power grid 201.

[0038] 3B , if inverter module 300 detects a grid power mismatch condition not detected by ATS 330, inverter module 300 stops exporting power to system power lines 312 and power system 40 operates in a no-load state while ATS 330 remains in its normal position supplying grid power from power grid 201 to critical loads 23. Inverter module 300 may continue to monitor grid power through closed circuit breaker 320, and if inverter module 300 determines that the grid power is in compliance with the electric utility, IEEE 1547, and / or UL 1741 standards, inverter module 300 may resume supplying fuel cell system power to system power lines 312.

[0039] 3C-3E, during the island transfer mode of the microgrid 400, if the controller 50 (which monitors the status of the inverter module 300 and / or the ATS 330) determines that the grid power is incompatible with the interconnection—for example, if grid power from the power grid 201 fails or is interrupted—the inverter module 300 stops outputting power to the system power lines 312, and the power system 40 operates in a no-load state. As shown in FIG. 3C, the ATS 330 remains in its normal position, and the circuit breaker 320 remains closed. Thus, the inverter module 300 acts as a primary anti-islanding device, in accordance with utility, IEEE 1547, and / or UL 1741 limits, and immediately shuts down (e.g., disconnects from the system power lines at the PDS 310) if the power grid (e.g., utility) 201 fails.

[0040] Referring to FIG. 3D , if the controller 50 of the inverter module 300 detects (via path ATS Status) that grid power from the power grid 201 is not available at the load terminal L of the ATS 330 and determines (via path CB Status) that the circuit breaker 320 is not faulty, the inverter module 300 outputs a drive signal (via the third data path of FIG. 3D ) to cause the intervening relay 324 to open the circuit breaker 320 using power from the UPS 326.

[0041] Next, the controller 50 of the inverter module 300 can confirm that the circuit breaker 320 is open by reading the auxiliary contacts of the circuit breaker 320 via the data path CB status. After confirming the open circuit breaker 320, the power system 40 initiates island mode, the PDS 310 is activated, and the inverter module 300 outputs an island mode voltage, such as 480V AC voltage, to the system power lines 312 via the PDS 310. If the ATS 330 detects the island voltage, then after the expiration of a first delay period, the ATS 330 switches to the middle position M.

[0042] 3E , if the ATS 330 continues to detect the island voltage, the controller 50 controls the microgrid 400 to initiate island mode operation. In particular, the controller 50 controls the ATS 330 (via the path ATS status) to switch the ATS 330 to its emergency position after the expiration of the second delay period, electrically connecting the system power line 312 to the critical load line 314 by connecting the emergency terminal E and the load terminal L. Thus, in island mode, the island mode voltage can be supplied from the power module 100 to the critical load 23 of the critical load panel 22 via the system power line 312, the ATS 330, and the critical load line 314. Thus, power from the power system 40 is supplied to the critical load 23 and not to the non-critical load 25. The controller 50 of the inverter module 300 can constantly or periodically monitor the connection status of the ATS 330 during island mode operation.

[0043] If the ATS 330 detects that grid power has been restored (i.e., grid power is detected at the ATS 330 by the controller 50 via the data path ATS status) while the power system 40 is in island mode, the microgrid 400 can initiate grid parallel transfer mode, as shown in FIGS. 3F-3H. In particular, as shown in FIG. 3F, the controller can switch the ATS 330 to its intermediate position after the expiration of the third delay period so that power is not supplied to the critical loads 23. If the controller 50 of the inverter module 300 determines that the ATS 330 is in intermediate position M, the power system 40 can stop island mode operation (e.g., the PDS 310 is open and no power flows from the inverter module 300 to the system power lines 312) and initiate transfer mode operation. During transfer mode operation, the power system 40 can reduce regenerative loads, such as motor loads, and stop outputting power to the system power lines 312. The ATS 330 may remain at the intermediate position M for a fourth delay period.

[0044] 3G , after the expiration of the fourth delay period, the controller 50 can switch the ATS 330 to normal terminal N to electrically connect the grid power line 316 to the load terminal L to supply grid power to the critical load 23. The inverter module 300 can stop all power output to the system power line 312 during and / or after the fourth delay period. Thus, the non-critical load 25 and the critical load 23 are supplied with grid power from the power grid 201 via lines 316 and 314 and the ATS 330.

[0045] 3H, if the controller 50 of the inverter module 300 subsequently determines that the grid power is compatible with the interconnection, the controller 50 of the inverter module 300 sends a signal to close the circuit breaker 320. Specifically, if the controller 50 of the inverter module 300 detects that there is no voltage applied to the system power line 312, the ATS 330 electrically connects the normal terminal N to the load terminal L, grid power is available through the ATS 330, and there is no fault in the circuit breaker 320, the controller 50 of the inverter module 300 closes the circuit breaker 320.

[0046] When the controller 50 of the inverter module 300 detects that the ATS 330 is in the normal position, the circuit breaker 320 is closed, and the grid power is compatible with the interconnection, as shown in FIG. 3B , the controller 50 of the inverter module 300 outputs the grid mode voltage to the system power line 312 via the PDS 310, and controls the power system 40 such that grid parallel mode operation of the power system 40 resumes.

[0047] Embodiments of the present invention provide a hardware configuration and control method for reliably supporting critical loads in a microgrid while allowing power to be exported to the grid when the grid is available. Specifically, without requiring real-time load management when switching between the grid parallel mode and the island mode, one or more inverters can operate in the grid parallel mode and the island mode, first offsetting power to the critical loads when the grid is available, then exporting excess power to non-critical loads, and then exporting it to the grid.

[0048] In one embodiment, the critical loads, non-critical loads, and the power grid are electrically connected through a standard ATS 330 and a wraparound circuit breaker 320 that functions as an operable switch. The wraparound circuit breaker 320 is controlled to isolate the power grid and non-critical loads from the microgrid with the critical loads or to provide excess power to the non-critical loads and the power grid. Using the circuit breaker 320 as an operable switch allows the removal of load shedding controls while maintaining voltage ride-through.

[0049] The structures and arrangements shown in the various embodiments are illustrative only. While only a few embodiments are described in detail in this disclosure, many modifications are possible (e.g., changes in the size, dimensions, structure, shape, and proportions of various elements, parameter values, mounting arrangements, material use, color, orientation, etc.) without substantially departing from the novel teachings and advantages of the subject matter described herein. Some elements shown as integrally formed may be composed of multiple parts or elements, the location of elements may be reversed or changed, and the nature, number, or location of individual elements may be changed or modified. The order or sequence of any process, logic algorithm, or method step may be modified or re-sequenced in accordance with alternative embodiments. Other substitutions, modifications, changes, and omissions may be made in the design, operating conditions, and arrangements of the various embodiments without departing from the scope of the present disclosure. Any one or more features of any embodiment may be used in any combination with any one or more other features of one or more other embodiments. The specification and examples are intended to be considered exemplary only, with the true scope being indicated by the following claims and their equivalents.

Claims

1. a power system configured to output a system power; An automatic transfer switch (ATS), comprising: a common terminal electrically connected to a grid power line configured to receive grid power from an electric utility; an emergency terminal electrically connected to a system power line configured to receive system power from the power system; a load terminal electrically connected to a critical load line configured to supply power to the critical load; the automatic transfer switch (ATS) comprising: a bypass line electrically connected to the system power line and the critical load line so as to bypass the ATS; a circuit breaker configured to control power flow through the bypass line; Including, The power system includes: a power module including a fuel cell stack; an inverter module including a controller configured to control the system power output to the system power line and the operation of the circuit breaker based on at least one of a connection state of the ATS, a connection state of the circuit breaker, or compatibility of the grid power interconnection; Including, microgrids.

2. When the microgrid is operating in a grid parallel mode, the microgrid: supplying the grid power to the critical load via the grid power line, the ATS, and the critical load line; closing the circuit breaker; Providing the system power to the critical loads through the bypass line The microgrid of claim 1 , configured as follows:

3. 3. The microgrid of claim 2, wherein when the microgrid is operating in the grid parallel mode, the microgrid is configured to supply system power to non-critical loads electrically connected to one of the grid power lines or the power utility in excess of power requirements of the critical loads.

4. When the microgrid is operating in islanded mode, the microgrid: Opening the circuit breaker; Providing the system power to the critical loads via the ATS The microgrid of claim 1 , configured as follows:

5. The ATS is When the controller determines that the grid power is suitable for interconnection, electrically connect the grid power line to the critical load line; If the controller determines that the grid power is not compatible for interconnection and the power system is supplying the system power to the system power line, electrically connect the system power line to the critical load line. The microgrid of claim 1 configured as follows:

6. 6. The microgrid of claim 5, wherein the ATS is configured to electrically disconnect the load terminal from the normal terminal and the emergency terminal for a delay period before electrically connecting the emergency terminal to the load terminal.

7. 7. The microgrid of claim 6, wherein the ATS is configured to electrically reconnect the load terminal to the normal terminal when the controller determines that grid power compatible for interconnection is restored to the grid power line.

8. The microgrid of claim 1 , wherein the controller is configured to open the circuit breaker if the controller determines that the grid power is incompatible with interconnection.

9. The microgrid of claim 8 , wherein the controller is configured to close the circuit breaker when the controller determines that the grid power is compatible for interconnection.

10. a power system configured to output a system power; An automatic transfer switch (ATS), comprising: a common terminal electrically connected to a grid power line configured to receive grid power from an electric utility; an emergency terminal electrically connected to a system power line configured to receive system power from the power system; a load terminal electrically connected to a critical load line configured to supply power to the critical load; the automatic transfer switch (ATS) comprising: a bypass line electrically connected to the system power line and the critical load line so as to bypass the ATS; a circuit breaker configured to control power flow through the bypass line; Including, When the microgrid is operating in a grid parallel mode, the microgrid: supplying the grid power to the critical load via the grid power line, the ATS, and the critical load line; closing the circuit breaker; Providing the system power to the critical loads through the bypass line The microgrid is configured as follows:

11. 11. The microgrid of claim 10, wherein when the microgrid is operating in the grid parallel mode, the microgrid is configured to supply system power to non-critical loads electrically connected to one of the grid power lines or the power utility in excess of power requirements of the critical loads.

12. When the microgrid is operating in islanded mode, the microgrid: Opening the circuit breaker; Providing the system power to the critical loads via the ATS The microgrid of claim 10 configured as follows:

13. The ATS is When the microgrid is operating in the grid parallel mode, When the controller of the power system determines that the grid power is suitable for interconnection, electrically connect the grid power line to the critical load line; If the controller determines that the grid power is not compatible for interconnection and the power system is supplying the system power to the system power line, electrically connect the system power line to the critical load line. The microgrid of claim 10 configured as follows:

14. When the controller determines that the grid power is incompatible with interconnection and the power system is supplying the system power to the system power line, and electrically connects the system power line to the critical load line, 14. The microgrid of claim 13, wherein the ATS is configured to electrically disconnect the load terminal from the normal terminal and the emergency terminal for a delay period before electrically connecting the emergency terminal to the load terminal.

15. When the emergency terminal is electrically connected to the load terminal, 15. The microgrid of claim 14, wherein the ATS is configured to electrically reconnect the load terminal to the normal terminal when the controller determines that grid power compatible for interconnection is restored to the grid power lines.

16. When the microgrid is operating in the grid parallel mode, The microgrid of claim 10 , wherein the power system controller is configured to open the circuit breaker if the controller determines that the grid power is incompatible with interconnection.

17. When the controller determines that the grid power is not compatible with interconnection and opens the circuit breaker, 17. The microgrid of claim 16, wherein the controller is configured to close the circuit breaker when the controller determines that the grid power is compatible for interconnection.

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