Abstract:
A local power generation system generates a substantially DC voltage at an inverter input, which is modulated to generate a resulting output AC power signal to a load. The inverter input voltage may be obtained from an engine generator, providing an AC power signal that is rectified. An energy storage device helps maintain the DC voltage at the inverter input when load power draw increases or during engine startup or acceleration, for example, until the engine accommodates the increased power demand. The system may also be used in an uninterruptible power supply (UPS) application, in which the load draws power from a utility-provided AC power source until a fault condition appears. When the fault condition appears, the load switches its power draw from the utility-provided AC power source to the inverter output. The energy storage device is charged through a bidirectional DC-to-DC converter and through an inverter that operates in a rectifier mode to rectify a utility-provided AC electrical power signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This patent application is related to David J. Koenig&#39;s co-pending and commonly-assigned U.S. patent application Ser. No. 09/999,788 entitled “GENERATOR WITH DC BOOST FOR UNINTERRUPTIBLE POWER SUPPLY SYSTEM OR FOR ENHANCED LOAD PICKUP,” which was filed on Oct. 26, 2001, and which is incorporated herein by reference in its entirety, including its description of such a system using an energy storage device for providing a DC boost. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This document relates generally to providing electrical power from a fuel-powered generator and particularly, but not by way of limitation, to a generator with DC boost and a split bus bidirectional DC-to-DC converter for an uninterruptible power supply system or for enhanced load pickup.  
         BACKGROUND  
         [0003]    Both businesses and households rely on electrical equipment powered from one-phase, two-phase, three-phase, or other suitable utility-provided alternating-current (AC) power sources. However, commercial power reliability may not suffice for certain applications, for example, for computer facilities, hospitals, banking systems, or industrial motor loads. Therefore, a backup—or even an independent—local power source may be desirable to supplement or substitute for a utility-provided AC power source.  
           [0004]    One type of a local power source is a static system, which typically uses an inverter to generate the load&#39;s AC power from a direct current (DC) battery bank. Providing power from such a static system for an extended period of time, however, may require a large and costly bank of batteries. Another type of local power source is a rotary system, which typically uses a gasoline or diesel engine to rotate the shaft of an AC generator to produce an AC load current for an extended period of time. In such a system, a providing a stable output voltage signal typically requires a constant rotational shaft velocity. However, load-switching, motor-starting, or other load variations may perturb shaft velocity and, in turn, may perturb the stability of the output voltage signal. A mechanical flywheel storing kinetic energy may assist in maintaining a constant shaft velocity may be maintained by storing kinetic energy, such as in a mechanical flywheel. However, this, provides a bulky, costly, and inefficient solution. For these and other reasons, the present inventors have recognized a need for an improved backup and/or substitute local AC power source. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]    In the drawings, which are offered by way of example, and not by way of limitation, and which are not necessarily drawn to scale, like numerals describe substantially similar components throughout the several views. Like numerals having different letter suffixes represent different instances of substantially similar components.  
         [0006]    [0006]FIG. 1 is a schematic/block diagram illustrating generally one example of a local power generation system that includes, among other things, an enhanced immunity to drawn power variations by the load.  
         [0007]    [0007]FIG. 2 is a schematic diagram illustrating generally portions of the local power generation system of FIG. 1 in more detail. 
     
    
     DETAILED DESCRIPTION  
       [0008]    The following detailed description refers to the accompanying drawings which form a part hereof. These drawings show, by way of illustration, specific embodiments of practicing the invention. This document describes these embodiments in sufficient detail to enable those skilled in the art to practice the invention. One should understand that the embodiments may be combined, other embodiments may be utilized, or structural, logical and/or electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents.  
         [0009]    [0009]FIG. 1 is a schematic/block diagram illustrating generally one example of a local power generation system  100  that includes, among other things, an enhanced immunity to drawn power variations by the load  102 . In this example, the load  102  normally receives AC power from an electrical utility source, such as through a utility input bus  104  and an AC controller circuit  106 , which outputs AC power to the load  102 . The system  100  also includes an AC power generator  108 , a rectifier circuit  110 , an energy storage device  112 , and an inverter circuit  114 , which operate to provide an alternate (e.g., supplementary or substitute) source of AC power to the load  102 , such as when the load  102  cannot obtain sufficient AC power from the electrical utility provider. The system also includes an energy storage device  116  and a bidirectional DC-to-DC converter circuit  118 , which assist in maintaining an adequate DC voltage stored on the energy storage device  112  at the input of the inverter  114  (such as, for example, during the startup of the generator  108 ). In one example, energy stored in the energy storage device  116  is received from the utility, such as through the AC controller circuit  106 , through the inverter  114  (operating in “reverse” in a rectifier mode), and through the bidirectional DC-to-DC converter  118 , as discussed further below.  
         [0010]    In one example, the generator  108  is a variable-speed generator powered by, for example, a gasoline engine, a diesel engine, a reciprocating internal combustion engine, a gas turbine, a steam turbine, a Sterling engine, or a rotary engine. The generator  108  provides a multi-phase AC generator output coupled, at the bus  120 , to an input of the rectifier  110 . The rectifier  110  converts the AC input signal at the bus  120  to a rectified approximately DC output signal. This approximately DC output signal is provided at a rectifier output coupled, at the bus  122 , to the energy storage device  112 , which, in turn, is located at the input bus  124  of the inverter  114 . The inverter  114  converts the DC signal at its input bus  124  to an AC signal provided at an inverter output, which is coupled, at the bus  128 , to the load  102 .  
         [0011]    In this example, the energy storage device  116  includes at least one capacitor to store electrical energy. In one example, the capacitor is an electrochemical capacitor cell (also referred to as an “ultracapacitor” or a “supercapacitor”). The electrochemical capacitor includes a dielectric layer that forms naturally in its electrolyte when an voltage is applied. Because the dielectric may form in a very thin double layer on the surface of the capacitor&#39;s electrodes, such an example of an electrochemical capacitor is sometimes referred to as a double layer capacitor (DLC). Although referred to in the art and herein as an electrochemical capacitor, the charge storage typically occurs electrostatically. Other examples of the energy storage device  116  include a rechargeable battery or any other suitable device for storing energy in any form and providing an electrical energy output at the bus  132 .  
         [0012]    In the example of FIG. 1, because the operating voltage needed at the inverter input at the bus  124  may differ from that obtained at the bus  132  from the energy storage device  116 , a switched-mode or other bidirectional converter between these buses performs a DC-to-DC voltage conversion, if needed. In one example, in which the inverter  114  delivers a 60 Hz, 139V rus (line-to-neutral)/240V rms (line-to-line) magnitude three-phase AC signal at the bus  128  to the load  102 , an inverter DC input voltage of about 400V is needed at bus  124 . In this example, the energy storage device  116  is an electrochemical capacitor storing a DC voltage, provided at the bus  132 , that is approximately between 105V and 210V. Therefore, in this example, the bidirectional DC-to-DC converter  118  includes a “step-up” or “boost” mode to translate the voltage at the bus  132  upward to the about 400V needed at the bus  124  to operate the inverter  114 .  
         [0013]    During normal utility operation, the load  102  receives power from the utility through the AC controller  106 . Moreover, during such operation, energy obtained from the utility through the AC controller circuit  106  is stored upon the energy storage device  116  by operating the inverter  114  in a “reverse” or “rectifier” mode in which the inverter  114  operates as a rectifier. Such utility-provided energy passed through the inverter  114 , operating in the rectifier mode, is translated downward in voltage (i.e., from a higher approximately DC voltage at the bus  130  to a lower approximately DC voltage at the bus  132 ) by the bidirectional converter  118 .  
         [0014]    In steady-state operation during a utility power fault, while the power drawn by the load  102  remains stable, the generator  108  provides such power through the rectifier  110  and the inverter  114 . However, during start-up of the generator  108 , or when the power drawn by the load  102  increases faster than the generator  108  can accelerate to accommodate the increase (referred to as “surge power”), the bidirectional DC-to-DC converter  118  transfers at least a portion of the energy in the energy storage device  116  to the energy storage device  112  at the inverter input, at the bus  124 , to maintain a sufficient DC voltage there. Because the response of the bidirectional converter  118  is faster than the acceleration response of the generator  108 , the system  100  provides improved load pickup when a motor in the load  102  is turned on, when other loads are switched into parallel with the load  102 , or when the power drawn by the load  102  is otherwise abruptly increased. The energy storage device  116  is sized to provide enough energy storage capacity to maintain the DC voltage at the bus  124  until the speed of the generator  108  increases sufficiently to maintain the needed DC voltage at the bus  124 .  
         [0015]    [0015]FIG. 2 is a schematic diagram illustrating generally, by way of example, but not by way of limitation, portions of the system  100  in more detail. In this example, at the utility bus  104 , the system  100  receives a 240V rms (line-to-line) magnitude three phase electrical power signal from the utility provider. The bus  104  includes three separate lines  104 A-C for receiving the respective phases of the three phase electrical power signal. The lines  104 A-C are received at respective input terminals  200 A-C of the AC controller  106 . In this example, the AC controller  106  includes, for each phase, back-to-back thyristors (also referred to as semiconductor-controlled rectifiers, or “SCRs”)  202 A-F, for example, such as included in part number SKUT85/12ES from Semikron International, of Nürnburg, Germany. A gate-control timing circuit  203  is connected to the gate terminals of each of the SCRs  202 A-F, such as for controlling their turn-on to provide power factor correction or other desired power conditioning for the electrical power delivered at the three phase output terminals  204 A-C of the AC controller  106 . The output terminals  204 A-C are connected to the respective lines  128 A-C of the three phase bus  128  delivering the conditioned electrical power to the load  102 . The AC controller  106  also provides isolation between the load bus  128  and the utility bus  104 , such as during a utility power fault and for a short time period after the utility comes back online (e.g., until the power signal on the load bus  128  is synchronized to the power signal delivered by the utility bus  104 ). This avoids overvoltages upon the load bus  128  and/or backflow of electrical power onto the utility line  104  and the associated utility power grid upon reconnection between the utility bus  104  and the load bus  128 .  
         [0016]    In operation as an uninterruptible power supply (UPS) during a utility power fault, load power is received (in a steady-state) from the generator  108 , which has outputs connected to the generator bus  120 . The generator bus  120  includes three separate lines  120 A-C for receiving the respective phases of the three phase electrical power signal output from the generator  108 . The generator bus  120  also includes an additional neutral line  120 D, to which the line voltages on the lines  120 A-C are referenced, and to which various other nodes in the system  100  must be referenced. In one example, the neutral line  120 D is referred to as a “floating neutral line”  120 D because it is not electrically connected in common with an actual earth ground.  
         [0017]    In the example of FIG. 2, the generator output lines  120 A-C are received at respective input terminals  206 A-C of the rectifier  110 . In this example, the rectifier  110  includes a contactor circuit  208 . The contactor  208  includes respective mechanical or other switches  210 A-C for contacting the respective generator output lines  120 A-C. Through the respective series-connections of these switches  210 A-C, the generator output lines  120 A-C are connected to respective input terminals  212 A-C of a diode-rectifier circuit  214 . One example of a suitable diode-rectifier circuit  214  is sold as part number SKD160/16, manufactured by Semikron International of Nürnburg, Germany. In this example, the rectifier circuit  214  includes unidirectional current flow devices, such as diodes  216 A-F. In this example, the input terminal  212 A is connected to the anode of the diode  216 A and the cathode of the diode  216 D. The input terminal  212 B is connected to the anode of the diode  216 B and the cathode of the diode  216 E. The input terminal  212 C is connected to the anode of the diode  216 C and the cathode of the diode  216 F. The cathodes of the diodes  216 A-C are commonly connected to a positive (with respect to the floating neutral node  12 D) voltage output terminal  218 A of the rectifier circuit  110 . The anodes of the diodes  216 D-F are commonly connected to a negative (with respect to the floating neutral node  120 D) voltage output terminal  218 B of the rectifier circuit  110 . The positive voltage output terminal  218 A is connected to a line  122 A of the bus  122 ; the negative voltage output terminal  218 B is connected to a line  122 B of the bus  122 . In operation, the rectifier circuit  110  converts the three phase AC electrical signal at the generator output nodes  120 A-C to an approximately DC voltage across the nodes  122 A-B that is stored upon the energy storage device  112  located at the input bus  124  of the inverter  114 .  
         [0018]    The energy storage device  112  includes, in this example, four 4700 microFarad capacitors  220 A-D, which are referenced to the floating neutral line  120 D. The capacitors  220 A-B are in parallel with each other, and are located between a positive (approximately) DC voltage node  122 A and the floating neutral node  120 D. The capacitors  220 C-D are in parallel with each other, and are located between a negative (approximately DC voltage) node  122 B and the floating neutral node  120 D. The input bus  124  to the inverter  114  includes the positive DC voltage node  122 A, the negative DC voltage node  122 B, and the floating neutral node  120 D, which are received at respective input terminals  222 A-C of the inverter  114 .  
         [0019]    The switched-mode inverter  114  includes switching circuits  224 A-C, each of which is connected between the positive DC voltage node  122 A and the negative DC voltage node  122 B. Each switching circuit  224 A-C is also inductively coupled, by respective inductors  226 A-C, to one of the three-phase lines  128 A-C of the load bus  128  to the load  102 . Each of the three-phase lines  128 A-C of the load bus  128  is also capacitively coupled, by a respective one of capacitors  228 A-C, to the floating neutral reference node  120 D.  
         [0020]    Each switching circuit  224 A-C includes a respective switching device, such as an NPN insulated gate bipolar transistor (IGBT)  230 A-C having a collector that is coupled to the positive DC voltage node  122 A, and having an emitter that is coupled to a respective one of the inductors  226 A-C, which, in turn is series-coupled to a respective line of the load bus  128 . Moreover, each switching circuit  224 A-C also includes a respective switching device, such as an NPN IGBT  232 A-C having an emitter that is coupled to the negative DC voltage node  122 B, and having a collector that is coupled to an emitter of a respective one of the IGBTs  230 A-C, and to a respective one of the inductors  226 A-C that is series coupled to the load bus  128 . Each switching circuit  224 A-C also includes a unidirectional current device, such as a respective diode  234 A-C having a cathode connected to a collector of a respective IGBT  230 A-C, and having an anode connected to an emitter of the respective IGBT  230 A-C. Similarly, each switching circuit  224 A-C further includes a unidirectional current device, such as a respective diode  236 A-C having a cathode connected to a collector of a respective IGBT  232 A-C, and having an anode connected to an emitter of the respective IGBT  232 A-C. One example of a switching circuit  224  is available as part number FF200R12KE3ENG, from Eupec, Inc., of Warstein, Germany. In an “inverter mode” of operation, the timing of the switching circuits  224 A-C is controlled for switching the IGBTs  230 A-C and  232 A-C to convert the DC voltages at nodes  122 A and  122 B to a three phase AC signal delivered to the load  102  by the lines  128 A-C of the load bus  128 A-C.  
         [0021]    In the example of FIG. 2, the inverter  114  also includes three 1.0 microFarad capacitors  238 A-C connected between the positive DC voltage node  122 A and the negative DC voltage node  122 B. The capacitors  238 A-C increase the immunity of the positive DC voltage node  122 A and the negative DC voltage node  122 B to transients caused by high-frequency (e.g., at a frequency of about 17 kHz) switching of the switching circuits  224 A-C in the inverter mode of operation.  
         [0022]    In the example of FIG. 2, the inverter  114  also includes a dynamic breaking circuit  240 . In this example, the dynamic breaking circuit  240  includes an IGBT  242  having an emitter connected to the negative DC voltage node  122 B and a collector that is coupled to an anode of a unidirectional current device, such as the diode  244 , which has its cathode connected to the positive DC voltage node  122 A. A diode  246  includes an anode coupled to the emitter of the IGBT  242  and a cathode that is coupled to the collector of the IGBT  242  and to the anode of the diode  244 . A power dissipation resistor is coupled between the positive DC voltage node  122 A and the commonly connected anode of the diode  244 , collector of the IGBT  242 , and the cathode of the diode  246 . In one example, the dynamic breaking circuit  240  includes part number BSM200GAL120DLC, available from Eupec, Inc. of Warstein, Germany. In operation, the dynamic breaking circuit  240  is controlled by a voltage sensor at the bus  124 . When the DC voltage between node  222 A and node  222 B exceeds 900V, then the IGBT  242  turns on to dissipate power in the resistor  248 . This protects the capacitors  220  from overvoltages and, in turn, protects against overvoltages at the load bus  128  that might otherwise result, for example, when the load power draw decreases abruptly.  
         [0023]    The inverter  114  also includes a “rectifier mode” in which the 240V rms (line-to-line) three phase AC electrical energy provided by the utility through the AC controller  106  is rectified by the inverter  114  to provide the approximately 400V DC voltage between the positive DC voltage node  122 A and the negative DC voltage node  122 B. During such rectifier mode, the IGBTs  230 A-C and  232 A-C are not switching; they are instead turned off. During this rectification mode, electrical energy flows “backward” through the inverter  114  from the inverter output at load bus lines  128 A-C to the inverter input terminals  222 A-C. In this mode, the diodes  234 A-C and  236 A-C provide such rectification. In one example, the rectifier mode of operating the inverter  114  is used during charging of the energy storage device  116 .  
         [0024]    In the example of FIG. 2, the energy storage device  116  includes terminals  250 A-C. The terminal  250 A is connected to a positive (relative to the neutral line  120 D) DC voltage line  132 A of the lower voltage DC bus  132 . The terminal  250 B is connected to a negative (relative to the neutral line  120 D) DC voltage line  132 B of the lower voltage DC bus  132 . The terminal  250 C is connected to the neutral line  132 C of lower voltage DC bus  132 , which is electrically in common with the neutral line  120 D from the generator  108 , which is referred to as a “floating neutral” line because it is not in common with or referenced to an earth ground. In this example, the energy storage device  116  includes ultracapacitors providing a total of 32 Farads of capacitance for storing electrical energy. Of this capacity, 16 Farads is provided by 42 parallel ultracapacitor cells connected between terminals  250 A and  250 C, and another 16 Farads is provided by another 42 parallel ultracapacitor cells connected between terminals  250 B and  250 C.  
         [0025]    The example of FIG. 2 also includes a bidirectional converter  118 , which is connected in series between energy storage devices  116  and  112 , that is, between lower voltage DC bus  132  and higher voltage DC bus  130 . In this example, the bidirectional converter  118  has first terminals  252 A-C that are respectively connected to the positive DC voltage line  132 A, the negative DC voltage line  132 B, and the neutral DC voltage line  132 C (which is electrically in common with the floating neutral line  120 D). In this example, the bidirectional converter  118  also includes second terminals  254 A-C that are respectively connected to the positive DC voltage line  130 A, the negative DC voltage line  130 B, and the neutral DC voltage line  130 C of the higher voltage DC bus  130 .  
         [0026]    In the example of FIG. 2, the bidirectional converter  118  includes: two approximately 60 microHenry inductors  256 A-B; series switching devices such as IGBTs  258 A-B; shunt switching devices such as IGBTs  260 A-B; shunt capacitors  262 A-B (associated with the first terminals  252 A-C); shunt capacitors  264 A-B (associated with the second terminals  254 A-C); diodes  266 A-B (respectively associated with IGBTs  258 A-B), and diodes  268 A-B (respectively associated with IGBTs  260 A-B). In one example, a shunt switching device  260  is available in a module together with a series switching device  258 , such as part number FF300R12KS4ENG available from Eupec, Inc. of Warstein, Germany.  
         [0027]    At the lower voltage DC bus  132 , the shunt capacitor  262 A is about 1500 microFarads and is connected between the positive DC voltage line  132 A and the neutral DC voltage line  132 C, which is electrically in common with the neutral line  120 D output from the generator  108 . The shunt capacitor  262 B is about 1500 microFarads and is connected between the negative DC voltage line  132 B and the neutral DC voltage line  132 C, which is electrically in common with the neutral line  120 D output from the generator  108 .  
         [0028]    At the higher voltage DC bus  130 , the shunt capacitor  264 A is about 4 microFarads and is connected between the positive DC voltage line  130 A and the neutral DC voltage line  130 C, which is electrically in common with the neutral line  120 D output from the generator  108 . The shunt capacitor  264 B is about 4 microFarads and is connected between the negative DC voltage line  130 B and the neutral DC voltage line  130 C, which is electrically in common with the neutral line  120 D output from the generator  108 .  
         [0029]    The inductor  256 A is connected in series between the positive DC voltage line  132 A of the lower voltage bus  132  and a node  270 A, to which a collector of the NPN IGBT  260 A is coupled, and to which an emitter of the NPN IGBT  258 A is also coupled. Similarly, the inductor  256 B is connected in series between the negative DC voltage line  132 B of the lower voltage bus  132  and a node  270 B, to which an emitter of the NPN IGBT  260 B is coupled, and to which a collector of the NPN IGBT  258 B is also coupled. The emitter of the IGBT  260 A and the collector of the IGBT  260 B are connected in common to the neutral line  120 D output from the generator  108 . The collector of the IGBT  258 A is connected to the positive DC voltage line  130 A of the higher voltage DC bus  130 . The emitter of the IGBT  258 B is connected to the negative DC voltage line  130 B of the lower voltage DC bus  130 .  
         [0030]    An anode of the diode  268 A is connected to the emitter of the IGBT  260 A (and to the neutral line  120 D output from the generator  108 ) and a cathode of the diode  268 A is connected to the collector of the IGBT  260 A (at node  270 A). An anode of the diode  268 B is connected to the emitter of the IGBT  260 B (at node  270 B) and a cathode of the diode  268 B is connected to the collector of the IGBT  260 B (and to the neutral line  120 D output from the generator  108 ). An anode of the diode  266 A is connected to the emitter of the IGBT  258 A (at node  270 A) and a cathode of the diode  266 A is connected to the collector of the IGBT  258 A at the positive DC voltage line  130 A of the higher voltage DC bus  130 . An anode of the diode  266 B is connected to the emitter of the IGBT  258 B (at the negative DC voltage line  130 B of the higher voltage DC bus  130 ) and a cathode of the diode  266 B is connected to the collector of the IGBT  258 B (at node  270 B).  
         [0031]    In operation, during a boost mode, energy is transferred from the energy storage device  116  (at the lower DC voltage bus  132 ) to the energy storage device  112  (at the higher DC voltage bus  130 ). This supports the input voltage at the input bus  124  of the inverter  114  (the lines of the input bus  124  are electrically connected in common to corresponding lines of the higher DC voltage bus  130 ). Such boost mode energy transfer is useful, for example, during startup of the generator  108 , or during a sudden increase in power draw by the load  128  that cannot be accommodated fast enough by accelerating the generator  108 .  
         [0032]    During the boost mode, the bidirectional converter  118  operates as a switched mode boost converter. The gate terminals of the IGBTs  260 A-B are driven by a high frequency switching control signal that switches IGBTs  260 A-B on and off, such as at a frequency of about 10 kHz. The IGBTs  258 A-B are not switched, but rather, are turned off during the boost mode. However, in the boost mode, power is conducted across the IGBTs  258 A-B through their respective parallel diodes  266 A-B. In the boost mode of operation, current drawn through the inductors  256 A-B produces respective voltages across such inductors that is additive to the voltage across the ultracapacitors of the energy storage device  116 . This permits the bidirectional converter  118  to transform the lower voltage (e.g., about 200V) energy stored in the energy storage device  116  at the lower DC voltage bus  132  to into the higher voltage (e.g., about 400V) stored upon the energy storage device  112  at the higher DC voltage bus  130 , such as where such a higher voltage is needed to ensure proper operation of the inverter  114 . A feedback control loop turns off the switching of the IGBTs  260 A-B when the desired voltage (e.g., about 400V) is obtained across the energy storage device  112  (until such voltage falls below the desired value, at which time such switching is resumed).  
         [0033]    During a charging mode, energy is transferred from the energy storage device  112  (at the higher DC voltage bus  130 ) to the energy storage device  116  (at the lower DC voltage bus  132 ). In one example, such energy obtained from the energy storage device  112  is provided by the utility through the inverter  114  (operating in a rectifier mode, i.e., using the diodes across its switching devices rather than gating such switching devices at a high frequency to perform the DC-to-AC inverter function). In an alternative example, such energy obtained from the energy storage device  112  is provided by the generator  108  through the rectifier  110 .  
         [0034]    During the charging mode, the bidirectional converter  118  operates as a switched-mode buck converter. The gate terminals of the IGBTs  258 A-B are driven by a high frequency control signal that switches the IGBTs  258 A-B on and off, such as at a frequency of about 25 kHz. The IGBTs  260 A-B are not so switched, but rather, are turned off during the charging mode. However, in the charging mode, current continuity through the inductors  256 A-B is preserved by current conduction across the IGBTs  260 A-B through their respective parallel diodes  268 A-B. A feedback control loop turns off the switching of the IGBTs  258 A-B when the desired voltage (e.g., about 200V) is obtained across the energy storage device  116  (until such voltage falls below the desired value, at which time such switching is resumed).  
         [0035]    Some of the discussion in this document discuss load pickup and UPS applications in terms of using a generator  108  such as a variable-speed engine generator. However, the system  100  is not so limited. Both the load pickup and UPS techniques discussed above similarly apply to a system  100  in which the generator  108  is a substantially constant speed engine generator. In one such example, the constant speed generator  108  is sized to run at a speed that provides sufficient output power to meet the maximum power draw requirements of the load  102 . However, if the load  102  draws surge power beyond that being provided by such a constant speed generator, the system  100  can assist in momentarily providing such surge power. This, in turn, assists in maintaining a constant engine generator speed while accommodating the increased power drawn by the load  102 . Similarly, in the UPS applications discussed herein, the generator  108  may be a variable-speed generator or a substantially constant speed generator.  
         [0036]    It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-discussed embodiments may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.