Abstract:
Systems and methods for supplying power to a load include a static switch between a primary power source and a power conditioner associated with a secondary power source, and maintenance switches between the primary and secondary power sources and a load. A controller is operable to actuate the switches. The static switch is operable to conduct power from the primary power source to a capacitor associated with the power conditioner. Current supplied from the primary power source includes portions at a fundamental frequency and a harmonic frequency. The secondary power source or the capacitor, or both, can be used to supply reactive power having a current equal and opposite that of the harmonic portion such that substantially all of the current provided to the load by the primary power source is at the fundamental frequency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to the U.S. Provisional Application for Patent having the Application Ser. No. 61/833,288, filed Jun. 10, 2013, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Embodiments usable within the scope of the present disclosure relate, generally, to uninterruptible power systems and supplies, and more specifically, to devices, systems, and methods for controlling the quality of power delivered by an interruptible power system, e.g., during normal and fault conditions. 
       BACKGROUND 
       [0003]    A basic function of an uninterruptible power system (“UPS”) is to ensure continued delivery of power to loads under a variety of primary power fault conditions and disturbances. With reference to the block diagram of  FIG. 1 , for example, a UPS  100  may comprise a first input  102  for receiving energy from a primary power source  103 , such as an AC utility source delivered from a power grid; a second input  104  for receiving energy from a second (e.g., backup) power source  105 , such as a battery or an AC generator; and an output  106  for delivering energy to loads  112 . In some embodiments the second power source  105  may be included within the UPS  100 . Under “normal” operating conditions (e.g., conditions under which the primary power source is within defined, acceptable, operating limits of voltage and frequency), power for loads  112  may be derived from the primary power source  103 . Otherwise, power may be derived from the backup power source  105 . 
         [0004]    Increasing use of alternative energy sources is contributing to degradation in the quality of the power delivered by the AC power grid. Compared to conventional large-scale AC power generation facilities, alternative power sources are more likely to exhibit power interruptions and power quality issues, thereby contributing, in aggregate, to a variety of power line disturbances, such as, e.g., power sags, power surges, undervoltage or overvoltage conditions, transients associated with source switching on the utility line, utility line noise, frequency variations, harmonic distortion, line brownouts and line dropouts. Contemporary loads, however, and particularly electronic loads, may require an uninterrupted flow of high quality AC power. Regulatory requirements may also limit the harmonic content and/or power factor of equipment connected to utility lines. The extent to which a UPS can reduce or eliminate the effects of line disturbances on the quality of the AC power which it delivers, as well as control the harmonic content and power factor reflected back to the utility source, may be important factors in evaluation of UPS performance. 
         [0005]    Various UPS configurations are known. One configuration, referred to herein as a double-conversion UPS, is illustrated in the block diagram of  FIG. 2 . The double-conversion UPS  100 A may, e.g., receive primary power from a three-phase AC utility source  103  and receive backup power from a bank of storage batteries  105 A. A rectifier-charger circuit  114  converts the three-phase AC input into DC; an inverter circuit  116  converts the DC back into a three-phase AC output for delivery to loads  112 . A controller  118  may monitor various system parameters and control the rectifier-charger circuit  114  and the inverter circuit  116  as a means of providing uninterrupted power flow to the loads  112 ; the controller may also control the inverter  116  to control the quality of the power delivered to the loads as a means of reducing or eliminating the effects of line disturbances and/or controlling power factor reflected back to the utility line. 
         [0006]    Another UPS configuration, referred to herein as a line-interactive UPS, is shown in  FIG. 3 . The line interactive UPS  100 B may, e.g., receive primary power from a three-phase AC utility source  103  and receive backup power from a backup AC generator  105 B. The backup AC generator may, e.g., be a flywheel motor/generator of the kind described in U.S. Pat. No. 5,932,935, which is incorporated herein in its entirety by reference. Each phase of the line-interactive UPS  100 B can include a static AC switch  122  and a backup power conditioner  130 . With reference to  FIG. 4 , a static AC switch  122  can include a pair of back-to-back SCRs  161 ,  162 . The backup power conditioner can include a flywheel converter  128 , a storage capacitor  126 , a utility converter  124  and an output filter (indicated by inductor  134 ). A controller  120  monitors the various inputs and outputs and controls the static AC switch  122  and the backup power conditioner  130  to provide uninterrupted power flow to the loads  112  and compensate for line disturbances. Operation of a line-interactive converter is described in detail in  Operation and Performance of a Flywheel - Based Uninterruptible Power Supply  ( UPS )  System , White Paper #108, published by Active Power Inc., Austin, Tex., 78758, USA (found at http://www.activepower.com/documents/white_papers/), which is incorporated by reference herein in its entirety. Under “normal” operating conditions, the static AC switch  122  is ON and three-phase power is delivered from the AC utility source  103  to the loads via the output three-phase bus  136 ; the controller  120  may also regulate the magnitude of the output three-phase bus voltage by controlling the flow of reactive power between the power conditioner  130  and the bus  136 . 
         [0007]    Other known UPS topologies include, but are not limited to, Delta Conversion UPS, Rotary UPS and Hybrid UPS. Known backup energy sources include, but are not limited to, batteries, flywheel motor-generators, compressed air, fuel cells and fossil fuel powered motor-generator sets. 
         [0008]    As shown in  FIGS. 2 and 3 , a UPS can include a bypass circuit  140 , which can include, e.g., a static AC switch  122  such as the type shown in  FIG. 4 . When enabled, the bypass circuit  140  provides an essentially direct connection between the primary power source and the loads. 
         [0009]    Conversion efficiency during normal operation is a recognized UPS performance factor, because higher conversion efficiency translates into reduced power loss and lower utility costs. Because the double-conversion UPS configuration processes utility power in each of two cascaded stages, its operating efficiency under normal operating conditions may be lower when compared, e.g., to a line interactive UPS, in which normal power flow is through a static AC switch. To improve normal operating efficiency, a double-conversion UPS may, under normal operating conditions, enable its bypass circuit  140 , thereby allowing power to flow directly from the AC utility source  103  to the loads  112  and avoiding some of the losses associated with cascade power processing. This “eco-mode” of operation may improve normal conversion efficiency to a level comparable to the efficiency of a line-interactive converter; in doing so, however, some or all of the advantages provided by the double-conversion topology may be lost. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a block diagram of an uninterruptible power system (“UPS”). 
           [0011]      FIG. 2  shows a block diagram of a double-conversion UPS. 
           [0012]      FIG. 3  shows a block diagram of a line-interactive UPS. 
           [0013]      FIG. 4  shows a partial schematic of a static AC switch. 
           [0014]      FIG. 5  shows an embodiment of a UPS usable within the scope of the present disclosure. 
           [0015]      FIG. 6  shows a secondary source comprising an ultracapacitor. 
           [0016]      FIG. 7  shows a secondary source comprising a flywheel motor/generator and a battery. 
           [0017]      FIG. 8  shows a secondary source comprising an ultracapacitor and a battery. 
           [0018]      FIG. 9  shows a secondary source comprising two or more energy sources. 
           [0019]      FIG. 10  shows an embodiment of a UPS usable within the scope of the present disclosure. 
           [0020]      FIG. 11  shows a partial schematic of an embodiment of a UPS usable within the scope of the present disclosure comprising a line inductor. 
       
    
    
       [0021]    Like reference numbers in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0022]      FIG. 5  depicts an embodiment of a UPS  200  usable within the scope of the present disclosure. The UPS  200  may, e.g., receive primary power from a primary AC power source  203  (e.g., a three-phase AC utility source; an AC generator; a fuel cell; and/or a wind turbine) and receive backup power from one or more secondary sources. One exemplary type of secondary source  205 , shown in  FIG. 5 , can include a backup AC motor/generator  206 , such as a flywheel motor/generator of the kind described in U.S. Pat. No. 5,932,935, incorporated by reference above, and a backup power conditioner  230 . In an embodiment, the backup power conditioner can include an AC-to-DC flywheel converter  128 , a DC bus  127 , a DC storage capacitor  126  connected across the bus, and a DC-to-AC utility converter  124 . The UPS  200  can include a bypass static switch  222 , a first maintenance switch  202 A and a second maintenance switch  202 B. In an embodiment, the bypass static switch  222  can be of the type shown in  FIG. 4 . The maintenance bypass switches can include contactors and/or static switches, such as the type shown in  FIG. 4 . A controller  220  can be used to monitor system conditions (e.g., voltages, currents, frequency) and control the static AC switch  222 , the maintenance switches  202 A,  202 B, the backup power conditioner  230  and/or the backup AC motor/generator  205 , to control the flow of energy between and among the primary power source  203 , the secondary source  205  and system loads  212 , in order to provide an uninterrupted flow of high quality power to the loads  212 . In various embodiments, monitoring and power conversion can be performed at frequencies (e.g. 6 KHz, 50 KHz) that are much higher than the nominal frequency of the utility source  203  (e.g., 50 Hz, 60 Hz), enabling the system to detect and respond to disturbances within a fraction of a line cycle. A line filter (indicated by inductor  234 ) can provide smoothing of the switched waveform delivered by backup conditioner  230 . In an embodiment, the controller  220  can include a Harmonic Controller  226 , discussed in more detail below. 
         [0023]    Startup of the system  200  can be accomplished by closing maintenance bypass switch  202 A, while the second maintenance switch  202 B is open, thereby connecting the primary AC source  203  to, and disconnecting the bypass static switch  222  and the power conditioner  230  from, the loads  212 . Controller  220  phase-controls the bypass static switch  222 , and controls the backup power conditioner  230  and the motor/generator  205 , to control a transfer of energy from the primary AC source  203  to the motor/generator  206 . When the motor/generator stores sufficient energy, and the storage capacitor  126  is charged to a pre-determined nominal DC voltage, the controller turns the bypass static switch  222  fully ON. Subsequently, the controller turns the second maintenance switch  202 B ON and the first maintenance switch  202 A OFF in an overlapped, controlled, transfer, thereby connecting both the bypass static switch  222  and the output of the backup power conditioner  230  to the loads  212  via three-phase bus  236 . 
         [0024]    Under normal operating conditions, the static AC switch  222  is ON and the primary AC source  203  is effectively connected in parallel with the secondary source  205 . Current delivered by the primary AC source, I 1 , would thereby be the sum of the current delivered to the secondary source, I 2 , and the current delivered to the load, I L : 
         [0000]      I 1 =I 2 +I L   (1)
 
         [0000]    In a typical installation, the current drawn by the load will not be a pure sinusoid at the fundamental frequency. Rather, the load current I L  may be composed of two components: 
         [0000]      I L =I f +I h   (2)
 
         [0000]    where I f  is a component at the fundamental frequency, f, of the power source  203  and I h  is the sum of all of the components at harmonics of the fundamental frequency. 
         [0025]    The harmonic controller  226  can be configured to control the harmonic content of the power delivered from the primary AC power source  203 . In one example, the controller  220  may be configured to control the secondary source  205  so that I 2 =−I h , thereby causing I 1  to equal I f  and eliminating harmonic components from the primary source current I 1 . In this configuration, the secondary source  205  can supply all of the reactive harmonic currents I h  and the primary power source  203  can deliver all of the real and reactive load current at the fundamental frequency. The harmonic controller  226  may alternatively be configured to perform power factor correction: i.e., control the secondary source  205  to deliver both the reactive power at the fundamental frequency and the reactive power associated with the harmonics. For such a configuration, the secondary source could supply all of the reactive load current and the primary power source would only deliver the real power required by the load. In each configuration described above, the secondary source  205  delivers reactive power only. 
         [0026]    In an embodiment, under normal operating conditions the bus capacitor  126  can supply substantially all of the reactive load current as well as transient currents that do not cause the DC bus  127  voltage to decline below a pre-determined level. The flywheel can be controlled to supply power that cannot be supplied by the capacitor (e.g., during abnormal conditions), up to the total real and reactive power required by the loads  212 . 
         [0027]    Another configuration of a secondary source, illustrated in  FIG. 6 , can include a bank of ultracapacitors  227 , a DC-DC converter  129  (e.g., a boost converter), a bus capacitor  126 , and a DC-to-AC utility converter  124 . The ultracapacitors may be configured to store energy comparable to the energy stored in a flywheel (e.g. sufficient energy to operate loads  212  for a period of time, such as several minutes). Under normal operating conditions, the bus capacitor  126  can supply substantially all of the reactive load current as well as transient currents that do not cause the bus voltage to decline below a pre-determined level. Under abnormal conditions, the ultracapacitors can supply power that cannot be supplied by the bus capacitor, up to the total real and reactive power required by the loads  212 . 
         [0028]    Conventional systems may include a bank of batteries (e.g., storage batteries  105 A, shown in  FIG. 2 ) to provide backup power and to supply reactive and transient currents. Battery lifetime, however, is diminished by exposure to transient currents and discharge events. This is not the case for the secondary sources shown in  FIGS. 5 and 6 . Use of a flywheel and bus capacitor, and/or of the ultracapacitor and bus capacitor, may therefore provide for improved system reliability and reduced system maintenance. 
         [0029]      FIGS. 7 and 8  depict embodiments of secondary power sources usable within the scope of the present disclosure. In  FIG. 7 , the depicted system includes an AC motor/generator  206 , such as a flywheel motor/generator of the kind described in U.S. Pat. No. 5,932,935, incorporated by reference above, and a battery bank  207 . Power from the flywheel motor/generator  206  can be delivered to the DC bus  127  by means of AC-DC flywheel converter  128 ; power from the battery bank  207  can be delivered to the DC bus by means of DC-DC converter  129 . 
         [0030]    In  FIG. 8 , the depicted system includes a bank of ultracapactors  127  and a battery bank  207 . Power from the ultracapacitor bank can be delivered to the DC bus  127  by means of DC-DC converter  129 A; power from the battery bank  207  can be delivered to the DC bus by DC-DC converter  129 B. Under normal operating conditions the bus capacitor  126  can supply substantially all of the reactive load current as well as transient currents that do not cause the bus voltage to decline below a pre-determined level. The flywheel motor/generator  206  ( FIG. 7 ) or the ultracapacitor  127  ( FIG. 8 ) may be controlled to supply power that cannot be supplied by the bus capacitor (e.g., during abnormal conditions), up to the total real and reactive power required by the loads  212 . When the flywheel or ultracapacitor can no longer supply the power demanded by the load, the battery bank  207  can be controlled to supply load power, up to the total real and reactive power required by the loads  212 . The secondary sources of  FIGS. 7 and 8  may be configured so that relatively frequent short-term disturbances are managed by the combination of the bus capacitor and the flywheel or ultracapacitor, while the battery bank  207  is only used to deliver power in the event of a fault in the AC utility source  203  that exceeds the duration for which the flywheel and/or ultracapacitor is able to supply backup power. By using the batteries in this manner, backup time may be extended and battery life improved relative to systems in which the batteries are the principal power conditioning source. While  FIGS. 7 and 8  depict discrete embodiments in which a flywheel and/or ultracapacitor are used as secondary power sources, it should be understood that in various embodiments, other types of secondary power sources could be used, and in still other embodiments, multiple secondary power sources could be used. 
         [0031]      FIG. 9  depicts an embodiment of a secondary power source  205  that includes two or more forms of energy storage  327 A,  327 B . . .  327 N, with corresponding converters  328 A,  328 B . . .  328 N, connected to a common DC bus  127 . The bus can include a storage capacitor  126 , as previously described (not shown in  FIG. 9 ). The energy storages  327 A,  327 B . . .  327 N can be selected to provide a desired combination of response speed, backup time and reliability characteristics. For example, a secondary power source  205  could include a first energy source  327 A capable of handling frequent charge-discharge cycles (e.g., a flywheel AC generator and/or an ultracapacitor) and a second energy source  328 B with relatively high energy density and/or economy for managing longer duration faults in the primary AC source (e.g., lead-acid batteries, lithium-ion batteries, fuel cells, and/or fossil fuel or compressed air electrical generators). 
         [0032]    In the system depicted in  FIG. 5 , transferring load power from the secondary source  205  back to the primary source  203  can be accomplished by turning on bypass static switch  222 , thereby exposing the primary AC source to a potentially large step change in load. Some primary AC sources (e.g., a motor-generator set) may not be able to supply a significant step in load power.  FIG. 10  shows an embodiment of a system  300  that is configured to enable a gradual transition from the secondary source  205  to a primary AC source  303 . As illustrated in  FIG. 10 , the primary source can include one or more types of AC sources  303 A,  303 B . . .  303 N, such as, e.g., the AC grid, a motor generator set, a fuel cell, a wind turbine, etc. 
         [0033]    In comparison to the system  200  of  FIG. 5 , the system  300  of  FIG. 10  includes a line static switch  223  and a line inductor  235 . The line static switch  223 , which in an embodiment, may be configured as shown in  FIG. 4 , can be phase controlled by controller  220 . In the system of  FIG. 10 , controller  220  controls the transfer of load from the secondary source  205  to the primary AC source  303  by phase controlling the line static switch  223  to gradually increase the AC current I 3 , while simultaneously controlling the secondary source to provide a corresponding gradual reduction in the current supplied by the secondary source  205 . Controlling current in this manner can enable maintenance of the power quality and total power delivery to the loads  212 , and the transfer of load to the primary AC source  303  in a manner that is within the capability of the source. Although secondary source  205  is shown in  FIG. 10  to be identical to the secondary source  205  of  FIG. 5 , it is understood that it any type of secondary source, as described above, can be included in any of the depicted systems. 
         [0034]    In various embodiments, some or all of the functional characteristics of a controller may be configured to be programmable by a user, thereby enabling a user to match system operating characteristics to a particular load or set of loads. A user may, for example, program the system to perform power factor correction only when the controller determines that load power factor is a predetermined value (e.g., load power factor is below 0.97). When power factor correction is required, the secondary source can be controlled to supply reactive currents, with corresponding power losses owing to flow of reactive currents in non-ideal circuit elements. When power factor correction is not required, however, the secondary source can be controlled to be in a standby mode, and losses may be reduced. Programming of other characteristics, such as, e.g., the magnitude and duration of transients that require correction, the normal AC voltage range over which no backup power is required, and others, may enable a user to optimize system performance and efficiency in an operation. 
         [0035]    In various embodiments, a controller  220  and harmonic controller  226 , usable within the scope of the present disclosure, can include various types of equipment. For example, some or all of a controller may be implemented as hardware and/or as software code and/or logical instructions that are processed by a computer, a microprocessor, a digital signal processor or other means, or a combination thereof. The logical processes, such as those illustrated in  FIG. 7 , may run concurrently or sequentially with respect to each other or with respect to other processes, such as measurement processes, UPS output voltage regulation processes and related calculations. A controller may be implemented in mixed-signal circuitry; in circuitry that includes mixed-signal circuitry and/or a microprocessor and/or digital signal processor core and/or a field-programmable-gate-array (FPGA) and/or an application-specific integrated circuit (ASIC); or in circuitry that includes a combination of mixed-signal circuitry and a separate microprocessor, digital signal processor, FPGA or ASIC. Such controllers can be implemented as an integrated circuit or a hybrid device. Additional functions can also be associated with the controller. 
         [0036]    It will be understood that various modifications may be made to the inventions described herein without departing from the spirit and scope of the invention. For example, embodied systems could include one or more additional primary or secondary power sources (e.g. a motor-generator set; fuel cell; wind turbine) to supply load power for relatively long periods of time should both the primary and secondary sources be unable to do so. Some system configurations can include a line inductor  248  connected in series with the bypass static switch  222 , as illustrated in the partial schematic in  FIG. 11 ; addition of the inductor may enable the controller  220  to perform voltage regulation, in addition to other functions described herein, and as described in the  Operation and Performance of a Flywheel - Based Uninterruptible Power Supply  ( UPS )  System , incorporated by reference above.