Uninterruptible power supply system with shelving power restoration

Abstract

An uninterruptible power supply (UPS) system with shelf power restoration has a plurality of UPS modules, wherein one or more of the UPS modules can be used to provide shelf power to a restored power bus. When a UPS module is used to provide shelf power to a restored power bus, an AC / AC converter associated with the UPS module provides AC power that is synchronized with the AC power provided to the restored power bus by each of the other AC / AC converters providing AC power. In this manner, all AC / AC converters providing AC power to the restored power bus have the same voltage, the same frequency, and are in phase.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Provisional Application No. 62 / 926,594, filed on October 28, 2019. The entire disclosure of the above application is incorporated herein by reference. Technical Field

[0003] The present disclosure relates to uninterruptible power supply systems having multiple uninterruptible power supply modules, and more particularly to shelved power restoration in such systems. Background Art

[0004] This section provides background information related to the present disclosure which is not necessarily prior art.As used herein, "UPS" is "Uninterruptible Power Supply".

[0005] A common UPS system configuration has multiple UPS modules, usually several, each of which has a load associated with it, and the UPS module provides power to the load. Such UPS systems are usually used in data centers. Such UPS systems usually have unused capacity or shelved capacity that cannot be immediately used to deliver to the load. This may be due to errors or insufficient information during design, unexpected changes in system configuration, or even intentional underutilization to allow future development. Unutilized power can be a relatively small percentage of the capacity of any single UPS module included in a large UPS system. However, the underutilized capacity of each of the UPS modules can be easily sufficient to power any one of the system loads when combined. In any case, if a UPS module fails or needs to be shut down for maintenance, it is desirable to have this total extra capacity available to power the load. This extra capacity is usually referred to as "shelved capacity" or "shelved power", and will be referred to as "shelved power" in this article.

[0006] One prior art method of utilizing stranded power is to restore stranded power by connecting the AC power output of one or more UPS modules to a common AC power bus (referred to herein as a "restored power bus") using a static switch (e.g., solid-state switch, relay, remote circuit breaker). This restored power bus is then used as a standby or backup power source. In the event of a UPS module failure or shutdown, the load associated with the UPS module is switched away from the AC power output of the UPS module UPS and connected to the restored power bus through a static transfer switch or circuit breaker.

[0007] Figure 1 1 is a simplified block diagram of an example prior art UPS system 100 with such shelved power restoration. The UPS system 100 includes a plurality of UPS modules 102. Figure 1In the example of , the UPS system 100 has five UPS modules 102 - UPS modules 102 (a) to 102 (e). Each UPS module 102 (a) to 102 (e) has a load 104 associated therewith, and the particular UPS module 102 (a) to 102 (e) provides power to the load. Each UPS module 102 (a) to 102 (e) also has a static transfer switch 106 associated therewith. Each UPS module 102 (a) to 102 (e) has an AC power output 108 coupled to an AC power input 110 of the associated static transfer switch 106. Each static transfer switch 106 has an AC power output 112 coupled to the associated load 104. Each static transfer switch 106 has a second AC power input 114 coupled to a restored power bus 116.

[0008] One or more of the UPS modules 102(a)-102(e) may be used to provide shelf power to the restored power bus 116. The AC power output 108 of each UPS module 102(a)-102(e) that may be used to provide shelf power to the restored power bus 116 is coupled to the AC power input 118 of an associated static switch 120. The AC power output 122 of each static switch 120 is coupled to the restored power bus 116.

[0009] It should be understood that there may be UPS modules 102 that are not used to provide shelf power to the restored power bus 116. In this regard, each UPS module 102(a)-102(e) that is not used to provide shelf power to the restored power bus 116 will not have an associated static switch 120 and therefore will not be coupled to the power sharing control bus 124. Figure 1 The UPS module 102(e) is shown as an example.

[0010] It will be appreciated that all UPS modules 102(a) to 102(e) that are available to provide restored shelved power to the restored power bus 116 are effectively connected together through the static switches 120 and the restored power bus 116 when their associated static switches 120 are closed. This imposes certain constraints on the system design. For example, it is required that these UPS modules 102(a) to 102(e) must each have a power sharing control system that is connected between all other modules and can be connected to the restored power bus 116. This is achieved by coupling the power sharing control system of each UPS module 102(a) to 102(e) to a separate control bus that is connected at Figure 1102(a) to 102(e) are representatively shown as a power sharing control bus 124. It is also required that all of these UPS modules 102(a) to 102(e) are synchronized with respect to the voltage, frequency, and phase relationship of their AC output voltage when connected to the restored power bus. In this regard, the AC power provided by the restored power bus has a nominal voltage and frequency, such as 480VAC at 60Hz. When a UPS module 102(a) to 102(e) is connected to the restored power bus, it must provide AC power at its output at the nominal voltage and frequency. The provided AC power must also be in phase with the AC power provided by the other UPS modules 102(a) to 102(e) connected to the restored power bus. Finally, all of these UPS modules 102 must operate in the same mode (i.e., double conversion, economy mode, bypass, etc.) when connected to the restored power bus.

[0011] These design constraints may create many undesirable limitations. On the one hand, the power sharing system must be pre-installed, or the UPS system must be shut down to install the power sharing system before shelved power recovery can be enabled. All UPS modules 102 (a) -102 (e) that can be used to provide shelved power to the restored power bus must have the same power rating (KVA size) to enable power sharing to work properly. If any of the UPS modules connected to the restored power bus is converted to an operating mode different from the other UPS modules connected to the restored power bus, the UPS module must be disconnected from the restored power bus and will no longer be available to contribute shelved power to the restored power bus. Therefore, it is desirable to have a UPS system with shelved power recovery that is not affected by the above constraints. Summary of the invention

[0012] This section provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.

[0013] In one aspect, the present disclosure relates to an uninterruptible power supply system that communicates with a restored power bus. The system may include a plurality of uninterruptible power supply (UPS) modules, wherein one or more of the UPS modules may be used to provide shelved power to the restored power bus. Each UPS module may be used to provide shelved power to the restored power bus, and has a static transfer switch, a load, and an AC / AC converter associated therewith. The AC / AC converter receives a first AC power output from the UPS at its AC input, and generates a second AC power output at its AC output, which is applied to the restored power bus. The UPS has a first AC power output coupled to a first AC power input of the static transfer switch, and a third AC power output associated with the static transfer switch is coupled to the load. The second AC power input of the static transfer switch is coupled to the restored power bus. When any one of the UPS modules is used to provide shelved power to the restored power bus, its associated AC / AC converter provides a second AC power output at its associated AC power output. The second AC power output is synchronized with other second power outputs of the second power output provided to the restored power bus by each of the other AC / AC converters providing AC power to the restored power bus. In this way, the second AC power outputs of all AC / AC converters provided to the restored power bus have at least one of the same voltage, the same frequency, or the same phase.

[0014] In another aspect, the present disclosure relates to an uninterruptible power supply system that communicates with a restored power bus. The system may include a plurality of uninterruptible power supply (UPS) modules, wherein one or more of the UPS modules may be used to provide shelved power to the restored power bus. Each UPS module may be used to provide shelved power to the restored power bus, and has a static transfer switch associated therewith, the static transfer switch having a first AC power input and a second AC power input and an AC output. Each UPS module also has an associated load and an associated AC / AC converter having an active rectifier front end and an inverter back end. The active rectifier front end has an input for receiving a first AC power output from the UPS, and the inverter back end generates a second AC power output applied to the restored power bus. The UPS also has a first AC power output coupled to the first AC power input of the static transfer switch, and the second AC power input of the static transfer switch is coupled to the restored power bus, wherein the AC output of the static transfer switch is coupled to the load. When any one of the UPS modules is used to provide shelved power to the restored power bus, its associated AC / AC converter provides a second AC power output at its associated AC power output. The second AC power output is synchronized with the other second power outputs of the second power output provided to the restored power bus by each of the other AC / AC converters providing AC power to the restored power bus. In this way, the second AC power outputs of all AC / AC converters provided to the restored power bus have the same voltage, the same frequency and the same phase.

[0015] In yet another aspect, the present disclosure relates to a method for forming an uninterruptible power supply (UPS) system that communicates with a restored power bus. The method includes using a plurality of UPS modules to provide a first AC power output to each of the first inputs of an associated static transfer switch and the inputs of an associated AC / AC converter. The method also includes using an AC / AC converter to each provide a second AC power output to the restored power bus. The method also includes: using a static transfer switch to receive power from the restored power bus at its second input, and using each of the static transfer switches to provide a third power output to a load associated therewith. The load associated therewith is also associated with a specific one of the UPS. The method also includes: controlling all AC / AC converters so that all second AC power outputs applied to the restored power bus are synchronized in at least one aspect of voltage, frequency, or phase.

[0016] Further areas of applicability will become apparent from the description provided herein.The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure.

[0018] Figure 1 is a simplified block diagram of a prior art UPS system with shelved power restoration;

[0019] Figure 2 is a simplified block diagram of a UPS system with shelved power restoration according to an aspect of the present disclosure; and

[0020] Figure 3 According to one aspect of the present disclosure, a method for controlling Figure 2 Flow chart of control logic for an AC / AC converter of a UPS system to provide shelved power restoration. DETAILED DESCRIPTION

[0021] Example embodiments will now be described more fully with reference to the accompanying drawings. Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.

[0022] Figure 2 1 is a simplified block diagram of a UPS system 200 with shelved power restoration according to one aspect of the present disclosure. Due to the differences discussed below, the UPS system 200 is identical to the UPS system 100, and the components common to the UPS system 200 and the UPS system 100 are described in detail below. Figure 2 Medium Figure 1 The same reference numerals used in the drawings are used to identify the same.

[0023] In the UPS system 200, the UPS modules 102(a) to 102(e) that can provide shelved power to the restored power bus 116 have associated AC / AC converters 202 instead of associated static switches 120. The AC power output of each of these UPS modules 102(a) to 102(e) is coupled to the AC power input 204 of the associated AC / AC converter 202, and the AC power output 206 of each AC / AC converter 202 is coupled to the restored power bus 116. Since the UPS system 200 does not require a power sharing control system, Figure 2 The UPS system 200 is not shown having a power sharing control bus 124 .

[0024] The AC / AC converter 202 illustratively has an active rectifier front end 201 with an AC power input 204 and an inverter back end 203 with an AC power output 206. Thus, the AC / AC converter 202 has the same topology as a double conversion uninterruptible power supply, but without a backup DC power supply or bypass. The active rectifier front end 201 of each AC / AC converter is self-synchronized to the output power provided by the associated UPS module 102 in a known manner in the same manner as the active rectifier of a double conversion UPS is self-synchronized to the input power. Thus, there is no need and in one aspect there is no synchronization signal from the UPS module 102 to the associated AC / AC converter 202 to synchronize the active rectifier front end 201 of the AC / AC converter 202 to the associated UPS module 102 (a) to 102 (e) or any other type of control signal from the UPS module 102 to the associated AC / AC converter 202. This is particularly advantageous when retrofitting a UPS system to add shelved power restoration, because the AC / AC converter 202 operates independently of the UPS 102 and can be installed as a standalone unit without any control interconnection with the UPS 102(a) to 102(e). The AC / AC converter can be connected to the output of any UPS module 102 without requiring any modifications to the UPS modules 102(a) to 102(e). The UPS modules 102(a) to 102(e) then treat their associated AC / AC converter 102 simply as another load.

[0025] The outputs 206 of the AC / AC converters 202 are synchronized with each other so that the AC power at the AC power outputs of the AC / AC converters 202 all have a nominal voltage and a nominal frequency within industry-accepted tolerances, such as a voltage not exceeding + / -1% (RMS) and a frequency not exceeding + / -0.1%, at which the AC power will be provided on the restored power bus 116, and the AC power at the AC power outputs of the AC / AC converters 202 are all in phase. In this regard, each AC / AC converter 202 includes a controller 208, which is configured to control the AC / AC converter 202 so that the AC power at the AC power output 206 of the AC / AC converter 202 has a nominal voltage, a nominal frequency, and is in phase with the AC power at the AC power outputs of the other AC / AC converters 202 that are providing AC power to the restored power bus 116.

[0026] exist Figure 2In the example shown, the AC / AC converters 202 each have a synchronization input 210 coupled to a common synchronization reference 212. By way of example and not limitation, the common synchronization reference 212 provides an AC signal (analog or digital), referred to herein as an AC reference signal, whose frequency is a nominal frequency. For example, if the nominal output frequency is 60 Hz, the frequency of the AC reference signal is 60 Hz, and if the nominal frequency is 50 Hz, the frequency of the AC reference signal is 50 Hz. By way of example and not limitation, the controller 208 uses the AC reference signal as a phase reference. The controller 208 is configured to control its respective AC / AC converter 202 so that the phase of the AC output power at the AC power output 206 is synchronized to the phase of the AC reference signal, so that the AC output power of the AC / AC converter 202 at the AC power output 206 is all in phase. Then, the controller 208 is also configured to control its respective AC / AC converter 202 so that they all have a nominal voltage and frequency, thereby having the same voltage and frequency. It should be appreciated that controller 208 controls its respective AC / AC converter using conventional control techniques, such as by controlling parameters of a pulse width modulated signal that causes the power switching semiconductors of AC / AC converter 202 to switch.

[0027] It should be understood that the common synchronization reference 212 may be separate from the AC / AC converter 202, or may be implemented in one of the controllers 208. In the latter example, the controller 208 of one of the AC / AC converters acts as a synchronization master and issues a synchronization signal, which may also be an AC reference signal (analog or digital), to the controllers 208 of each of the other AC / AC converters.

[0028] Each controller 208 monitors the AC power outputted by the UPS module 102 associated with the AC / AC converter 202 having the controller 208. For example, each controller 208 has inputs 214, 216 coupled to a voltage sensor 218 and a current sensor 220, respectively, which are coupled to the AC power output 108 of the associated UPS module 102. The power ratings of the associated UPS modules 102(A)-102(e) are preprogrammed into the controller 208 of the associated AC / AC converter 202. As understood in the art, the power rating of a UPS module is the maximum power that the UPS module is rated to provide.

[0029] In a variation, the controller 208 may receive input, for example, from its associated UPS module 102 (a) to 102 (e) or other device, which informs the controller 208 in real time what the current maximum available power capacity of the associated UPS module 102 (a) to 102 (e) is. This can be used for controlled load separation. For example, if the associated UPS module 102 (a) is in a backup power mode - powered by a backup DC power source (e.g., a battery or generator) of the UPS module 102 (a), the maximum available power capacity of the UPS module 102 (a) can be reduced and transmitted to the controller 208 of its associated AC / AC converter 202. The controller 208 then, for example, reduces the amount of power that the AC / AC converter 202 recovers from the associated UPS module 102 (a).

[0030] Figure 3 2 is a flow chart of a control routine of illustrative control logic implemented in each controller 208 for controlling the operation of the AC / AC converter 202. At operation 300, the control routine monitors the AC power output by a particular UPS module 102(a)-102(e) associated with that AC / AC converter 202. At operation 302, the control routine determines whether the associated UPS module 102(a)-102(e) can be used to provide shelf power to the restored power bus 116. If it cannot be used to provide shelf power to the restored power bus 116, the control routine branches back to 300. For example, the associated UPS module 102(a)-102(e) may be offline and therefore unable to provide shelf power to the restored power bus 116, or the load 104 associated with that particular UPS module 102(a)-102(e) may be drawing the maximum power that that particular UPS module 102(a)-102(e) is rated to provide.

[0031] If the control routine determines at 302 that the associated UPS module 102(a)-102(e) is available to provide shelf power to the restored power bus 116, then at operation 304 the control routine determines the shelf power available from the associated UPS module 102(a)-102(e) by subtracting the amount of AC power being output by the associated UPS module 102(a)-102(e) from the power rating of the associated UPS module 102(a)-102(e) preprogrammed in the controller 208. At operation 306, the control routine synchronizes the AC power at the AC power output 206 of that AC / AC converter 202 to a nominal voltage, nominal frequency, and nominal phase so that it will have the same voltage, frequency, and phase as the AC power at the AC power output 206 of the other AC / AC converters 202 providing power to the restored power bus, and the control routine controls the AC / AC converter 202 to provide some or all of the shelf power to the restored power bus 116. The control routine then branches back to 300 .

[0032] In one aspect, the controller 208 is configured to control the AC / AC converter 202 to limit the power that the AC / AC converter can draw from the associated UPS module 102(a) to 102(e) so as not to exceed the power rating of the associated UPS module 102(a) to 102(e). In one aspect, the controller 208 is configured to control the AC / AC converter 202 so that the maximum power that the AC / AC converter can draw from the associated UPS module 102(a) to 102(e) is less than the amount of available shelf power. By way of example and not limitation, the controller 208 is configured so that the maximum power that its associated AC / AC converter 202 can draw from its associated UPS module 102(a) to 102(e) is ten percent less than the amount of shelf power that the controller 208 determines is available. It should be understood that this setting may be a calibratable setting.

[0033] In a variation, the controllable circuit breaker 222 (at Figure 2The controllable circuit breakers 222 are shown in phantom in the figure, and are installed between the restored power bus 116 and the second AC power input 114 of the static transfer switch 106, which is associated with the UPS module 102 of the UPS modules 102 (a) to 102 (e) that can be connected to the restored power bus 116. The controllable circuit breakers 222 are controlled so that they prevent any load 104 from connecting to the restored power bus 116 that cannot be supported by the AC power available on the restored power bus 116, which is the total unused power or stranded power of the UPS modules 102 (a) to 102 (e) that provide AC power to the restored power bus 116 via their associated AC / AC converters 202. The controllable circuit breakers 222 can be controlled, for example, by the controller 208 or by a separate controller (not shown). By way of example and not limitation, controllable circuit breaker 222 may be a remotely controlled circuit breaker, a static AC switch, or a contactor.In one aspect of this variation, controllers 208 of AC / AC converters 202 communicate with each other, for example wirelessly or via a communication bus (not shown).

[0034] It should be understood that the logic for the aforementioned control of the AC / AC converter 202 by its respective controller 208 can be implemented illustratively in hardware logic, software logic, or a combination of hardware logic and software logic. In this regard, each controller 208 can be or can include any of a digital signal processor (DSP), a microprocessor, a microcontroller, or other programmable device programmed with software that implements the above-described method. It should be understood that, alternatively, the controller 208 is or includes other logic devices, such as a field programmable gate array (FPGA), a complex programmable logic device (CPLD), or an application-specific integrated circuit (ASIC). When stating that the controller 208 performs a function or is configured to perform a function, it should be understood that the controller 208 is configured to do so using appropriate logic (e.g., in software, a logic device, or a combination thereof).

[0035] The terms used herein are only for the purpose of describing specific example embodiments, and are not intended to be restrictive. As used herein, the singular forms "one", "an" and "the" may also be intended to include plural forms unless the context clearly indicates otherwise. The terms "include", "comprise", "contain", and "have" are inclusive, and therefore specify the existence of stated features, integers, steps, operations, elements and / or parts, but do not exclude the existence or addition of one or more other features, integers, steps, operations, elements, parts and / or their groups. Unless specifically identified as the order of execution, the method steps, processes and operations described herein should not be interpreted as necessarily requiring them to be performed in the specific order discussed or shown. It should also be understood that additional steps or alternative steps may be adopted.

[0036] For the purpose of illustration and description, the foregoing description of the embodiments has been provided. It is not intended to be exhaustive or to limit the present disclosure. The individual elements or features of a particular embodiment are generally not limited to the particular embodiment, but are interchangeable and can be used in the selected embodiment where applicable, even if not specifically shown or described. The individual elements or features of a particular embodiment may also vary in many ways. Such variations should not be considered as departing from the present disclosure, and all such modifications are intended to be included within the scope of the present disclosure.

Claims

1. An uninterruptible power supply (UPS) system in communication with a restored power bus, the system comprising: a plurality of uninterruptible power supply (UPS) modules, one or more of the UPS modules being operable to provide shelved power to the restored power bus; Each of the UPS modules operable to provide shelved power to the restored power bus is associated with: Static transfer switch; load; and an AC / AC converter that receives a first AC power output from the UPS module at an AC input of the AC / AC converter and generates a second AC power output at an AC output of the AC / AC converter that is applied to the restored power bus; The UPS module has the first AC power output coupled to the first AC power input of the static transfer switch, and a third AC power output associated with the static transfer switch is coupled to the load; and a second AC power input of the static transfer switch coupled to the restored power bus; and When any one of the UPS modules is used to provide standby power to the restored power bus, the AC / AC converter associated with the UPS module provides the second AC power output at its associated AC output, and the second AC power output is synchronized with other second AC power outputs among a plurality of second AC power outputs provided to the restored power bus by each of the other AC / AC converters providing AC power to the restored power bus, so that the second AC power outputs of all the AC / AC converters provided to the restored power bus have at least one of the same voltage, the same frequency or the same phase.

2. The UPS system according to claim 1, wherein: The second AC power outputs of all of the AC / AC converters that provide AC power to the restored power bus have the same voltage, the same frequency, and the same phase.

3. The UPS system according to claim 1, wherein: The AC / AC converter has an active rectifier front end for receiving the first AC power output from the UPS module and an inverter back end for generating the second AC power output.

4. The UPS system according to claim 3, wherein: Each of the AC / AC converters operates independently of the UPS module associated with the AC / AC converter, wherein its associated active rectifier front end self-synchronizes to its associated first AC power output of the UPS module associated with the AC / AC converter without requiring any synchronization control signal from the UPS module associated with the AC / AC converter.

5. The UPS system according to claim 3, wherein: Each of the AC / AC converters has an associated controller having a synchronization input coupled to a common synchronization reference, the controller of each of the AC / AC converters being configured to synchronize the second AC power output based on the common synchronization reference, the second AC power output being provided at the AC output of its associated AC / AC converter.

6. The UPS system according to claim 5, wherein: The controller of each of the AC / AC converters providing power to the restored power bus is configured to determine an amount of power to provide to the restored power bus based on a power rating of the UPS module associated with its AC / AC converter and an amount of AC power output by its associated UPS module.

7. The UPS system according to claim 6, wherein: The controller of each of the AC / AC converters providing power to the restored power bus is configured to determine an amount of power to provide to the restored power bus based on an input indicating a current maximum available power capacity of the UPS module associated with its associated AC / AC converter and an amount of AC power output by the associated UPS module through the first AC power output.

8. The UPS system according to claim 5, wherein: The system further includes a separate circuit breaker for each of the static transfer switches; and Wherein, each of the individual circuit breakers is coupled between the restored power bus and an associated one of the controllers to control whether the UPS module associated with the circuit breaker is able to couple the load associated with its associated UPS module to the restored power bus.

9. An uninterruptible power supply (UPS) system in communication with a restored power bus, the system comprising: a plurality of uninterruptible power supply (UPS) modules, one or more of the UPS modules being operable to provide shelved power to the restored power bus; Each of the UPS modules operable to provide shelved power to the restored power bus is associated with: a static transfer switch having a first AC power input and a second AC power input and an AC output; load; and AC / AC converter with active rectifier front end and inverter back end; The active rectifier front end has an input for receiving a first AC power output from the UPS module, and the inverter back end generates a second AC power output that is applied to the restored power bus; The UPS module also has the first AC power output coupled to the first AC power input of the static transfer switch, and the second AC power input of the static transfer switch is coupled to the restored power bus, and the AC output of the static transfer switch is coupled to the load; and When any one of the UPS modules is used to provide standby power to the restored power bus, the AC / AC converter associated with the UPS module provides the second AC power output at its associated AC output, and the second AC power output is synchronized with other second AC power outputs among a plurality of second AC power outputs provided to the restored power bus by each of the other AC / AC converters providing AC power to the restored power bus, so that the second AC power outputs of all the AC / AC converters provided to the restored power bus have the same voltage, the same frequency and the same phase.

10. The UPS system according to claim 9, wherein: Each of the AC / AC converters operates independently of the UPS module associated with the AC / AC converter, wherein its associated active rectifier front end is self-synchronized to the first AC power output associated with the UPS module associated with the AC / AC converter without requiring any synchronization control signal from the UPS module associated with the AC / AC converter.

11. The UPS system according to claim 10, wherein: Each of the AC / AC converters has an associated controller for synchronizing the AC / AC converter associated with the controller with a synchronization signal.

12. The UPS system according to claim 11, wherein: Each of the controllers has a synchronization input coupled to a common synchronization reference, which generates the synchronization signal.

13. The UPS system according to claim 12, wherein: The controller of each of the AC / AC converters is configured to synchronize the second AC power output provided by the AC / AC converter associated with the controller to the restored power bus based on the common synchronization reference.

14. The UPS system according to claim 13, wherein: The controller of each of the AC / AC converters providing power to the restored power bus is configured to determine an amount of power to provide to the restored power bus based on a power rating of the UPS module associated with its AC / AC converter and an amount of AC power output by its associated UPS module.

15. The UPS system according to claim 11, wherein: The system also includes a separate circuit breaker for each of the static transfer switches.

16. The UPS system according to claim 15, wherein: Each of the individual circuit breakers is coupled between the restored power bus and an associated one of the controllers to control whether the UPS module associated with the circuit breaker is able to couple the load associated with its associated UPS module to the restored power bus.

17. A method for forming an uninterruptible power supply (UPS) system in communication with a restored power bus, the method comprising: using a plurality of UPS modules to each provide a first AC power output to a first input of an associated static transfer switch and an input of an associated AC / AC converter; providing a second AC power output to each of the restored power buses using the AC / AC converters; receiving power from the recovered power bus at a second input of the static transfer switch using the static transfer switch; providing a third power output using each of the static transfer switches to a load associated with the static transfer switch, wherein the load associated with the static transfer switch is also associated with a particular one of the plurality of UPS modules; and All of the AC / AC converters are also controlled such that all of the second AC power outputs applied to the restored power bus are synchronized in at least one of voltage, frequency or phase.

18. The method of claim 17, further controlling the AC / AC converter so that all of the second AC power outputs applied to the restored power bus are synchronized in each of voltage, frequency, and phase.

19. The method according to claim 17, wherein: Using an AC / AC converter includes using an AC / AC converter having an active rectifier front end to receive the first AC power output from the UPS module associated with the AC / AC converter.

20. The method according to claim 19, wherein: Using an AC / AC converter includes using an AC / AC converter having an inverter back end for generating the second AC power output applied to the restored power bus.

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