Patent Publication Number: US-10778007-B2

Title: UPS systems and methods using coordinated static switch and inverter operation for generator walk-in

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 14/283,744, filed May 21, 2014 entitled UPS SYSTEMS AND METHODS USING COORDINATED STATIC SWITCH AND INVERTER OPERATION FOR GENERATOR WALK-IN, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The inventive subject matter relates to power conversion apparatus and methods and, more particularly, to uninterruptible power supply (UPS) apparatus and methods. 
     UPS systems are commonly used in installations such as data centers, medical centers and industrial facilities. UPS systems may be used in such installations, for example, to provide backup power to maintain operation in event of failure of the primary utility supply. 
     UPS systems may have any of a number of different types of architectures. For example, AC UPS systems may have an on-line or double conversion architecture including a rectifier configured to be coupled to an AC power source and an inverter coupled to the rectifier by a DC bus and configured to provide AC power to a load. A battery or other DC source may be coupled to the DC source, which may provide backup power in the event of failure of the AC source. Standby AC UPS systems may include an inverter that is configured to be coupled to a load by a transfer switch that switches a load between the inverter and an AC source. AC UPS system may have other architectures, such as line interactive and delta conversion architectures. 
     Large data centers have proliferated with the advent of web services and cloud computing. Some newer large data centers occupy millions of square feet and house hundreds of thousands of servers. Typically powered by the local grid, these centers may include backup power supply systems including UPSs and diesel or gas powered backup engine-generator sets to support continued operation when utility power is lost. 
     In a double conversion UPS, rectifier and inverter controls may be significantly decoupled because of the presence of energy storage on the DC link between the rectifier and inverter. The rectifier can be used to address source compatibility issues (e.g., voltage droop, harmonics, power factor, distortion, etc.), while the inverter can be used to protect the load (e.g., voltage, distortion, load regulation, etc.). 
     When an engine-generator set is used in a data center or similar application, the load applied to the engine-generator set is often gradually increased to avoid transient overloading of the engine-generator set and tripping its protection mechanisms. A double conversion UPS can be used to gradually increase loading of an engine-generator set (to “walk in” the engine-generator set) by using the battery and inverter to provide power to the load while the rectifier gradually increases loading of the engine-generator set. Such generator walk-in techniques may be unavailable, however, in UPS systems that do not have a double conversion architecture. 
     SUMMARY 
     Some embodiments of the inventive subject matter provide an uninterruptible power supply (UPS) system including an AC output, an inverter coupled to the AC output and configured to provide power at the AC output, and a switch configured to selectively couple a generator (e.g., an engine-generator set) to the AC output. The system further includes a control circuit configured to variably modulate the switch to gradually increase control power flow from the generator to the AC output while causing the inverter to concurrent provide power to the AC output. 
     In some embodiments, the switch includes a static switch, e.g., a static switch that includes at least one silicon controlled rectifier (SCR). The control circuit may be configured to control a conduction interval of the static switch to control power flow from the generator to the AC output. In some embodiments, the control circuit may be configured to change the conduction interval in discrete steps over respective discrete time intervals to control the power flow from the generator to the AC output. In some embodiments, the control circuit may be configured to vary the conduction interval responsive to a signal indicating a performance margin of the generator. 
     In some embodiments, the control circuit may be configured to operate the inverter to provide reactive power compensation as power transfer from the generator to the AC output gradually increases. In further embodiments, the control circuit may be configured to operate the inverter to provide harmonic compensation as power transfer from the generator to the AC output gradually increases. 
     Further embodiments of the inventive subject matter provide a system including a utility power source, an engine-generator set, a static switch configured to couple the engine-generator set to a load, and an inverter having an output configured to be coupled to the load. The system further includes a control circuit operatively coupled to the engine-generator set, the static switch, and the inverter and configured to detect a failure of the utility power source, to activate the engine-generator set and to variably modulate the static switch to gradually increase control power flow from the engine-generator set to the load while causing the inverter to concurrently provide power to the load. The static switch may include at least one SCR and the control circuit may be configured to control a firing angle of the at least one SCR to control power flow from the generator and the load. For example, in some embodiments, the control circuit may be configured to change a conduction interval of the at least one SCR in discrete steps over respective discrete time intervals to control the power flow from the generator to the AC output. In some embodiments, the control circuit may be configured to vary a conduction interval of the SCR responsive to a signal indicating a performance margin of the generator. The control circuit may be further configured to operate the inverter to provide reactive power compensation and/or harmonic compensation. 
     Methods according to some embodiments of the inventive subject matter include providing AC power to a load from an AC power source. A loss of the AC power source is detected, and the load is responsively transitioned to an inverter of a UPS. A generator is activated responsive to detected loss of the AC power source. A switch configured to couple the generator to the load is variably modulated to gradually increase control power flow from the generator to the load while the inverter concurrently provides power to the load. 
     The switch may include a static switch including at least one SCR and variably modulating the switch may comprise controlling a conduction interval of the at least one SCR to gradually increase control power flow between the generator and the AC output. Controlling the conduction interval of the at least one SCR may include changing the conduction interval in discrete steps over respective discrete time intervals. In some embodiments, controlling the conduction interval of the at least one SCR may include varying the conduction interval responsive to a signal indicating a performance margin of the generator. The inverter may also be used to provide reactive power compensation and/or harmonic compensation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating a UPS system according to some embodiments. 
         FIG. 2  is a schematic diagram illustrating a UPS system coupled to a utility source and an engine-generator set according to further embodiments. 
         FIG. 3  is a flowchart illustrating operations of a UPS system of  FIG. 2  according to further embodiments. 
         FIG. 4  is a schematic diagram illustrating a UPS system coupled to a utility source and engine-generator set according to further embodiments. 
         FIG. 5  is a flowchart illustrating operations of a UPS system of  FIG. 2  according to further embodiments. 
         FIG. 6  is a schematic diagram illustrating a three-phase UPS system according to some embodiments. 
         FIGS. 7-10  are flowcharts illustrating exemplary operations of UPS systems according to further embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Specific exemplary embodiments of the inventive subject matter now will be described with reference to the accompanying drawings. This inventive subject matter may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive subject matter to those skilled in the art. In the drawings, like numbers refer to like elements. It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive subject matter. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless expressly stated otherwise. It will be further understood that the terms “includes,” “comprises,” “including” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this inventive subject matter belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Some embodiments of the inventive subject matter arise from a realization that in applications such as data center uninterruptible power supply (UPS) systems, a UPS system having no regulator and inverter DC link to implement conventional generator walk-in, i.e., a “rectifierless” UPS, may operate an inverter and a static switch in a coordinated manner to provide generator walk-in. According to some embodiments, the generator and inverter may be coupled in parallel to the load and generator walk-in may be achieved by using phase control (e.g., firing angle control) of a waveform derived from the generator in conjunction with phase/frequency control of the inverter, e.g., power transfer from the generator is gradually increased as the inverter performs complementary operations to maintain a desired input (voltage and/or current) to the load. As used herein, “gradual” increase in power transfer from a generator to a load refers to a general trend to increase power transfer to the load from the generator. It will be understood that a “gradual” increase in power transfer may include periods in which the power transfer remains constant and/or decreases (e.g., to preserve generator stability as described below) within a general trend of increasing power transfer. In some embodiments, such a gradual increase in power transfer may be achieved by using a conduction interval of the static switch to limit the amount of power drawn from the generator to prevent tripping of protective devices or other events that may accompany an overly large step increase in generator load. Inverter phasing/frequency may be used to maintain the load as the conduction interval is increased. The inverter can also be used for power factor correction and/or harmonic compensation. 
       FIG. 1  illustrates a system  100  according to some embodiments of the inventive subject matter. The system  100  includes a switch  110 , which is configured to couple and decouple a generator  10  to and from a load  20 . The system further includes an inverter  120  having an output coupled to the load  20 . A control circuit  130  is configured to variably modulate the switch  110  to gradually increase power transfer from the generator  10  to the load  20  while concurrently causing the inverter  120  to maintain the load by providing additional power. For example, in embodiments described below, a conduction interval of the switch  110  may be incrementally increased to gradually load the generator  10 . The inverter  120  may also be operated to control power factor and/or harmonics. 
     It will be appreciated that the system  100  may be implemented in a number of different ways. For example, the switch  110 , the inverter  120 , and the control circuit  130  may be incorporated in one or more units (e.g., in a UPS unit) or may be distributed. The switch  110  may be a semiconductor switch, such as a switch utilizing silicon controlled rectifiers (SCRs), insulated gate bipolar transistors (IGBTs), power MOSFETs or other power semiconductor devices. The inverter  120  may employ a half-bridge or other configuration. The control circuit  130  may be implemented using any of a variety of different analog and/or digital circuits, including data processing devices, such as a microcontroller, and may be integrated with the inverter  120  and/or the switch  110  (e.g., in an integrated UPS assembly) or distributed among multiple such components. The generator  10  may be engine-generator set, such as a diesel- or gas-powered engine-generator set. 
       FIG. 2  illustrates a system  200  according to further embodiments. A utility source  12  and an engine-generator set  14  are coupled to inputs of a transfer switch  210 . An output of the transfer switch  210  is coupled to a static switch  220 , which has an output coupled to a load  20  in parallel with an inverter  230 . The inverter  230  is coupled to a DC power source  240 , which may include, for example, a battery and DC/DC converter for interfacing the battery to the inverter  230 . A control circuit  250  is operatively coupled to the inverter  230 , the static switch  220 , the transfer switch  210  and/or the engine-generator set  14  to allow control and/or monitoring thereof. 
     Operations of the system  200  according to some embodiments of the inventive subject matter are illustrated in  FIG. 3 . The transfer switch  210  is configured to selectively couple the utility source  12  and the engine-generator set  14  to the static switch  220  based on a status of the utility source  12 . Under normal conditions, the load  20  is powered from the utility source  12  (block  310 ). If the utility source  12  fails, the control circuit  250  may cause the static switch  220  to open, cause the inverter  230  to maintain the load  20  using power from the DC power source  240 , activate the engine-generator set  14 , and cause the transfer switch  210  to couple the engine-generator set  14  to the input of the static switch  220  (blocks  320 ,  330 ). Activation of the engine-generator set  14  and operation of the transfer switch  210  may be coordinated in various ways, e.g., the transfer switch may  210  may be transitioned before and/or concurrent with activation of the engine-generator set  14  or may be delayed until the engine-generator set  14  reaches a predetermined operating state after activation. Once the engine-generator set  14  is on line and its output available at the output of the transfer switch  210 , the control circuit  250  may then variably modulate the static switch  220  (e.g., by varying a conduction interval thereof) while operating the inverter  230  as described above to gradually walk-in the engine-generator set  14  (block  340 ). During this process, the inverter  230  may be used to maintain the load and provide reactive power and/or harmonic compensation as described above. Once the engine-generator set  14  is fully able to carry the load  20 , the inverter  230  may be deactivated (blocks  350 ,  360 ). In some embodiments, however, the inverter  230  may remain active and be used to continue to provide reactive power compensation, harmonic compensation and/or other power conditioning. The inverter  230  may also be operated to facilitate power flow from the engine-generator set  14  to the DC source  240  for battery charging. 
       FIG. 4  illustrates a system  400  according to further embodiments with a different configuration from the system  200  of  FIG. 2 . A utility source  12  and an engine-generator set  14  are coupled to inputs of respective first and second static switches  410 ,  420  having outputs configured to be coupled in common to a load  20 , in parallel with an inverter  430 . The inverter  430  is coupled to a DC power source  440  (e.g., a battery and DC/DC converter). A control circuit  450  is operatively coupled to the inverter  430 , the first and second static switches  410 ,  420  and/or the engine-generator set  14  to allow control and/or monitoring thereof. 
     Operations of the system  400  according to some embodiments of the inventive subject matter are illustrated in  FIG. 5 . Under normal conditions, the load  20  is powered from the utility source  12  (block  510 ). If the utility source  12  fails, the control circuit  450  may cause the first static switch  410  to open, cause the inverter  430  to maintain the load  20  using power from the DC power source  440 , and activate the engine-generator set  14  (blocks  520 ,  530 ). Once the engine-generator set  14  is activated, the control circuit  450  may then operate the second static switch  420  and the inverter  430  as described above to gradually walk-in the engine-generator set  14  (block  540 ). During this process, the inverter  430  may be used to maintain the load and provide reactive power and/or harmonic compensation as described above. Once the engine-generator set  14  is able to carry the load  20 , the inverter  430  may be deactivated (blocks  550 ,  560 ). In some embodiments, the inverter  430  may remain active and continue to provide reactive power, harmonic compensation and/or other power conditioning while the engine-generator set  14  serves the load. The inverter  430  may also be operated to facilitate power flow from the engine-generator set  14  to the DC source  440  for battery charging. 
       FIG. 6  illustrates an exemplary implementation of a three-phase UPS system  600  according to some embodiments of the inventive subject matter. The system  600  has an AC input  601  configured to be coupled to an engine-generator set  14  (e.g., directly or via one or more intermediate components, such as a transfer switch) and an AC output configured to be coupled to a load  20 . The system  600  includes an inverter circuit  630  including a plurality of half-bridge circuits  632  coupled to a DC/DC converter  642  that generates a DC voltage vDC from a battery voltage vB generated by a battery  644 . Respective inductors  634  couple the half-bridge circuits  632  to the AC output  602 . The system  600  further includes a plurality of SCR-based static switches  622  coupled to the AC input  601 . Respective inductors  624  couple the static switches  622  to the AC output  602 . A control circuit  650  is configured to control the static switches  622  and the half-bridge circuits  632  responsive to various inputs, such as an AC voltage vAC produced by the engine-generator set  14 , a DC voltage vB produced by the battery, a DC voltage vDC produced by the DC/DC converter, a load current il, a load voltage vl, a bypass current ibyp through the static switches  622 , and an inverter current iinv at the output of the inverter  630 . The control circuit  650  may also operate response to other information, such as information pertaining to the engine-generator set  14 . Such information may include, for example, information relating to the generator&#39;s power delivery capability, stability, and the like. The control circuit may also operate responsive to information pertaining to external switches (e.g., transfer switches) that couple the AC input  601  to various power sources. 
     According to some embodiments, the control circuit  650  may operate the static switches  622  and the half-bridge circuits  632  to walk-in the engine-generator set  14  while maintaining a desired voltage and current at AC output  602 . For example, the control circuit  650  may gradually increase a conduction (“on”) interval tc of the static switches  622 , while using the inverter  630  to maintain the desired current and voltage at the AC output  602 . 
     The walk-in may occur in any of a number of different ways. For example, as illustrated in  FIG. 7 , the conduction interval tc of the SCRs of the static switches  622  may be increased in discrete steps over a series of discrete time intervals by changing the firing angles of the SCRs. The variance of the conduction interval tc may also be limited subject to indications of desirable generator performance. 
     Referring to  FIG. 7 , the conduction interval tc of the static switches  622  may be incrementally increased (block  710 ) by incrementally changing the firing angles of SCRs of the static switches  622 , and the inverter  630  responsively adjusted to maintained the load (block  720 ). After a predetermined time interval has elapsed without indication of an undesirable generator state, the conduction interval tc may again be incrementally increased (blocks  730 ,  710 ). An undesirable generator state may be indicated by, for example, waveform degradation and/or signaling from the control system of the engine-generator set  14  (e.g., the engine-generator set  14  may generate a signal in response to an unacceptably high error in its speed control). If an undesirable generator state is detected, the static switch conduction interval tc may be reduced (blocks  730 ,  750 ). If the predetermined time interval has not yet elapsed and the generator state is acceptable, the static switch conduction interval tc may be maintained until the predetermined time interval has elapsed (blocks  730 ,  740 ). 
     The static switch conduction interval tc may also be controlled responsive to measure of a performance margin of the engine-generator set  14 . For example, a control circuit of the engine-generator set  14  may be configured to signal a degree of available performance margin, e.g., a signal derived from an error signal of a speed control loop of the engine-generator set  14 , and the control circuit  650  may control the static switch conduction interval tc responsive to this measure. This may allow for faster walk-in of the engine-generator set  14  in comparison to the process illustrated in  FIG. 7 . 
     Referring to  FIG. 8 , the static switch conduction interval is increased concurrent with adjusting inverter operation to maintain the load (blocks  810 ,  820 ). As long as sufficient margin for generator operation is indicated, the conduction interval is increased and inverter responsively adjusted (blocks  830 ,  810 ). If an insufficient generator margin is indicated, however, the conduction interval may be maintained or reduced as indicated by the generator margin (blocks  830 ,  840 ). It will be appreciated that the static switch conduction interval and the inverter operation may be continuously adjusted using, for example, proportional feedback control loops. 
     According to various embodiments, a static switch and inverter may be used in a number of different ways to provide walk-in as described above. For example, in some embodiments, a UPS system may alternate a load between the generator and the inverter during non-overlapping time intervals. In further embodiments, the inverter may remain active while the generator is coupled to the load, but may be used in a mode in which it acts as a current source to provide real and reactive power and/or compensate for harmonics. 
     Referring to  FIG. 9 , in response the loss of a primary (e.g., utility) source, the inverter is used to provide power to the load (block  905 ). When the system determines that transfer to a generator is appropriate (e.g., after the generator has been started and brought up to a nominal operating condition), the inverter is synchronized to the generator&#39;s output and a value x, corresponding to an amount of a third of a cycle (for a three-phase system) of the generator AC waveform for which the load is to be coupled to the generator by a static switch, is initialized (blocks  910 ,  915 ,  920 ). The inverter is turned off at point during the generator voltage waveform corresponding to a third of a cycle minus the value x (block  925 ). The inverter may, for example, be placed in a standby mode in which its power switching devices are disabled (e.g., placed in a high impedance state), while the inverter&#39;s control circuitry remains active and continues to be synchronized to the generator waveform. After a delay suitable to ensure the inverter has commutated off, the static switch is triggered to couple the generator to the load (block  930 ). The value x is incremented and, if still less than a third of a cycle of the generator waveform, the static switch is turned off (e.g., allowed to commutate off at the next zero crossing of the generator voltage) (blocks  935 ,  950 ). The inverter is then turned on at the state of a new third of a cycle and provides power to the load for a reduced period of time (blocks  955 ,  925 ). If the value x equals or exceeds the length of a third of a cycle, the static switch is kept on to allow the generator to continue to power the load (blocks  940 ,  945 ). The inverter may remain deactivated at this point or may be selectively used for power conditioning (e.g., harmonic suppression) or other purposes. The amount Δx by which the value x is incremented may be a fixed or otherwise predetermined value and/or may be a value that is adjusted based on monitored system parameters (e.g., voltage, frequency, generator speed loop error, etc.). 
     In further embodiments, when walk-in of the generator commences, an inverter may be switched from a mode in which it solely controls the voltage/frequency at the load to a parallel operation mode. Referring to  FIG. 10 , in response the loss of a primary (e.g., utility) source, the inverter is used to provide power to the load (block  1010 ). When the system determines that transfer to a generator is appropriate (e.g., after the generator has been started and brought up to a nominal operating condition), the inverter is transitioned to a paralleled operation mode in which the inverter may provide real or reactive power to the load to reduce loading of the generator and/or to provide harmonic compensation to produce a desirable waveform at the load, e.g., limit frequency deviation and/or voltage droop (blocks  1020 ,  1030 ). While the inverter is operating in this mode, the static switch is modulated to gradually increase the “on” time in which the generator is coupled to the load. The on time of the static switch may be increased in a predetermined manner and/or may be adjusted based on monitored system parameters. 
     It will be appreciated that the system and operations illustrated in  FIGS. 6-10  are provided for purposes of illustration, and that embodiments of the inventive subject matter are not limited thereto. For example, although the system of  FIG. 6  uses an SCR-based static switch, other types of switches may be used and controlled in the same or different manner. For example, a transistor-based switch may be controlled using pulse-width modulation or other techniques, rather than the firing angle control of waveform-commutated SCRs described above. Such transistor-based switching devices may also be used for power factor control and other functions. It will be further appreciated that the inventive subject matter may be power distribution system such as the configurations shown in  FIGS. 2 and 4 , as well in other power distribution system arrangements. 
     In the drawings and specification, there have been disclosed exemplary embodiments of the inventive subject matter. Although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the inventive subject matter being defined by the following claims.