Abstract:
This disclosure includes systems and methods for managing the interaction between inverter-based DC and other power systems. In one embodiment, a 3-phase isolation transformer is fluxed to create a 3-phase rotating field from the output of a source inverter. An inductive filter turns that output into three sine waves. A secondary inverter regenerates the system, sometimes after the isolation transformer is fluxed, and by advancing or retarding the secondary inverter&#39;s phase, current (and, thus, the DC voltage and power direction) is controlled. In another embodiment, an inverter is supplied by a DC source. The inverter is controlled to match its output voltage, current, and phase to a live AC grid, then the two are connected. The inverter frequency is then driven to advance the phase of the inverter in relation to the grid. Alternatively, the inverter voltage is then driven at a level greater than that of the grid.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is a nonprovisional of, and claims priority to, U.S. Provisional Application No. 61/313,778, filed Mar. 14, 2010, with title “Rapid-Transfer Controller for Supplemental Power Generators,”. The entire disclosure in that application is incorporated herein by reference as if fully set forth. 
    
    
     FIELD 
     The present disclosure relates to electric power conversion systems. More specifically, the present invention relates to managing the interaction between inverter-based DC systems and other power systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a first embodiment of substantially bumpless transfer grid synchronization. 
         FIG. 2  is a state diagram for the embodiment of  FIG. 1 . 
         FIG. 3  is a schematic drawing illustrating the control software functionality for the inverter in the embodiment of  FIG. 1 . 
         FIG. 4  is a schematic diagram of a second embodiment showing a battery-to-common DC bus, DC-to-DC converter. 
         FIG. 5  is a schematic diagram of a third embodiment showing a photovoltaic-to-common DC bus, DC-to-DC converter. 
         FIG. 6  is a schematic diagram of a fourth embodiment showing a battery-to-battery DC-to-DC converter. 
         FIG. 7  is a block diagram of a fifth embodiment of substantially bumpless transfer grid synchronization. 
     
    
    
     SUMMARY 
     In one implementation, a DC-to-AC converter system manages intentional islanding of the connection between an electric utility grid, an electrical load, and an alternate AC power source; and the alternate AC power source is capable of coordinating reconnection to the grid in a process of substantially bumpless transfer grid synchronization. The alternate AC power source, the grid synchronization inverter, is supplied by inputs from a common DC bus attached to the grid synchronization inverter, such as a direct or converted DC input from a battery or photovoltaic supply, a DC generator with a diode rectifier, or a rectified AC input from a generator or windmill, for example. When supplemented with appropriate input power sources (e.g., battery or capacitor, and/or a generator set) this configuration will allow maintenance of a critical load should the grid be disrupted, operate in an island mode by dropping out a grid contactor, and then will synchronize and provide a substantially bumpless reconnection to the grid when power is restored. 
     In another implementation, a DC-to-DC power converter system manages transforming and isolating power with off-the-shelf inverter technology, using components of a DC-to-AC converter (as described in the first implementation above), coupled with an AC to DC converter, via an inductive filter. The second implementation relates a new and inventive field of technology where DC power needs to be (1) converted to a different DC voltage or to a higher potential; (2) isolated, for the purpose of electrical isolation or for safety; or (3) stored, as in a battery, capacitor, ultracapacitor, or other form of DC storage. 
     DESCRIPTION 
     For the purpose of promoting an understanding of the principles of the present invention, reference will now be made to the embodiments illustrated in the drawings, and specific language will be used to describe the same. It will, nevertheless, be understood that no limitation of the scope of the invention is thereby intended; any alterations and further modifications of the described or illustrated embodiments and any further applications of the principles of the invention as illustrated therein are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     First Embodiment 
     A feature of successful micro-grids is the ability to maintain power to a critical load, disconnect from the utility grid if utility grid power goes away, and reconnect to the grid when power returns without disrupting operation of the critical load.  FIG. 1  illustrates the general structure of a first embodiment, system  100 , which uses an arbitrary number of DC and AC power sources to supply power to inverter subsystem  106 . Inverter subsystem  106  uses a series of contactors and sensors to connect to utility grid  110  and to provide uninterruptable power to critical load  120 , as described in detail herein. As shown in  FIG. 1 , inverter subsystem  106  uses power from common DC bus  112 , which is fed by an arbitrary number of DC sources  190   a ,  190   b , . . . ,  190   m , each of which may employ a DC-to-DC converter  195   a ,  195   b , . . . ,  195   m , as well as an arbitrary number of AC sources  180   a ,  180   b , . . . ,  180   n  in combination with rectifiers  185   a ,  185   b , . . . ,  185   n . Each AC input  180   x  may include an AC genset or windmill connected to rectifier  185   x , for example, and rectifier  185   x  may consist of a full-wave bridge rectifying circuit or a dedicated inverter working as an active rectifier. Each DC power input consists of a DC source  190   x  in the form of a photovoltaic (PV) panel array, a battery, an ultra-capacitor, or another form of DC storage or supply. Each DC source  190   x  connects to common DC bus  112  through a DC-to-DC Converter  195   x  if required to obtain isolation, power conversion, and/or bidirectional power flow. Also, a diode rectifier  195   x  could be used in place of a DC-to-DC converter  195   x , for example on a large DC generator. Site controller  165  controls and coordinates power flow and storage on the common DC bus  112  power inputs, or can provide additional AC power back to critical load  120 , normal loads  122 , or even utility grid  110 . 
     State Diagram 
     The site controller  165  has four operational hardware states, which are presented in system  200  as the hardware state diagram shown in  FIG. 2 . They are (1) off mode  210 , wherein both the M1 contactor  125  and 52U contactor  105  are open; (2) grid only mode  230 , wherein the M1 contactor  125  is open and 52U contactor  105  is closed; (3) grid isolated mode  240  or islanding (per IEEE 1547), wherein the M1 contactor  125  is closed and 52U contactor  105  is open; and (4) grid parallel mode  250 , wherein both the M1 contactor  125  and 52U contactor  105  are closed. 
     The site controller  165  operates in grid parallel mode  250  when the inverter subsystem  106  is connected to the grid  110  (i.e., both M1 contactor  125  and 52U contactor  105  are closed). In grid parallel mode  250 , inverter subsystem  106  is synchronized to the utility grid  110 , so the grid (in conjunction with the current limit of inverter subsystem  106 ) effectively controls the frequency of inverter subsystem  106 . The current limit of inverter subsystem  106  will fold back the frequency (or phase advance) of inverter subsystem  106  to maintain the current limit of power output to utility grid  110 . The site controller  165  regulates the amount of power supplied by the various AC sources  180   x  and DC sources  190   x , which supply output to the load. The power flow is controlled by site controller  165  and may be directed to charge or discharge the DC storage devices and thus import or export power based on the estimated or immediate cost and availability of power. Also, inverter subsystem  106  provides power factor correction to utility grid  110  anytime it is connected to utility grid  110  in grid parallel mode  250 . In this mode, the critical load  120  has both a primary and a secondary power source working simultaneously. 
     In grid isolated mode  240 , also known as “islanding,” inverter system  106  opens 52U contactor  105  and maintains M1 contactor  125  closed, thus supplying all power to the critical load  120 . Inverter subsystem  106  controls the AC frequency to critical load  120  while in grid isolated mode  240 . Islanding typically would be employed in the case of loss of utility grid  110  power and, when the grid  110  was restored, inverter system  106  would then reconnect to the grid in a current-limited, phase-synchronized bumpless fashion. 
     In grid only mode  230 , the 52U contactor  105  is closed and M1 contactor  125  to inverter subsystem  106  is open. This control state is used in the case of photovoltaic sources at night, wind generators when there is no wind, when the business case for running the generator is more expensive than buying power from the utility grid  110 , or when a generator is down, for example. 
     Software Schematic 
       FIG. 3  illustrates the general process flow of the inverter control software. Software system flow  300  uses several state inputs and settings to control the drive  310  of inverter subsystem  106 . In this situation, as an example, as the system is controlling drive  310 &#39;s frequency, it collects grid feedback  312  through signal conditioning circuitry  314  and collects inverter output feedback  316  through signal conditioning circuit  318 . Phase comparator  320  outputs a signal corresponding to the phase difference between the grid signal and the inverter output. Frequency control circuit  324  uses the output of phase comparator  320  and a first frequency set point  326  to generate a frequency control feedback signal. If according to signal  305  the grid is not connected, multiplexer  328  passes the frequency control output signal  324  to drive  310 . If according to signal  305  the grid is connected, multiplexer  328  passes second frequency set point  330  to drive  310  as its control signal. 
     Voltage comparator  322  outputs a signal that indicates the relative voltage levels of the conditioned grid feedback and the conditioned inverter output feedback. Reactive current feedback  332  and power factor target value  334  are considered by power factor control circuitry  336  to yield a voltage adjustment signal effective to move the inverter&#39;s power factor toward unity. If signal  305  indicates that the grid is connected, decision block  338  selects the output of power factor control circuit  336  and passes it to voltage control logic  340 . If signal  305  indicates that the grid is not connected, decision block  338  selects the output of voltage comparator  322  and passes it to voltage control logic  340 . Voltage control logic  340  combines that information with voltage set point  342  and outputs a voltage control signal for drive  310 . Drive  310  takes the appropriate input depending on its mode (if in frequency-control mode, the signal selected by decision block  328 ; or if in voltage-control mode, the output of voltage control logic  340 ) and adjusts its performance accordingly. 
     Second Embodiment 
     Generally, one form of the present system is a Battery-to-Common DC Bus Converter system. On a battery, the system provides isolation, which can help prevent a catastrophic ARC flash, and also provides bi-directional energy management, regardless of voltage potential (certain laws of physics apply in regard to the hardware specific to the application). The inverters in these systems are coupled using a three-phase LCL filter or other inductive filter. By processing three-phase power rather than single-phase power in the conversion, both current and voltage ripple are significantly reduced, allowing for higher efficiencies. (By some estimates, single-phase voltage ripple requires approximately six times as much filtration as three-phase ripple.) 
     Turning to system  400  in  FIG. 4 , one DC-to-DC converter system  495  works with a lithium ion automotive battery having a voltage range of ˜250 VDC to ˜390 VDC. With this particular system, the battery source inverter  451  produces an AC three-phase rotating electrical field at 176 VAC. This three-phase rotating electrical field (similar to that provided to a motor winding) powers the generating electrical field (here, the secondary of the LCL filter  455 ) with a ratio of 208/480 VAC and generates 406 VAC (574 VDC peak) to a three-phase bus inverter  452 , which in turn can boost the voltage to 625 to 780 VDC in order to power an isolated and boosted common DC bus  412 . Voltage gain, current (or power) regulation, and isolation are all features of the DC-to-DC converter system  495 . In the case of a micro grid, the DC-to-DC converter system  495  provides both voltage gain and isolation. In typical operation, the battery source inverter  451  threshold detector (drive comparator circuit) senses that there is sufficient voltage to flux the three-phase transformer of LCL filter  455 . If there is sufficient power, the battery source inverter  451  will run to the specified operating frequency (typically 60 Hz) or to the transformer fluxing frequency of LCL filter  455 . The transformer fluxing voltage is generally set about 5 to 10% below the value of the common DC bus  412  voltage divided by the square root of two (176=˜250/1.414). Once the battery source inverter  451  (or primary drive) is at the fluxing frequency, the “enable” signal is transferred to the secondary (synchronizing, or regenerative) bus inverter  452 . The bus inverter  452  then synchronizes to the LCL filter secondary coils (battery source inverter  451  frequency) by the means of an internal inverter synchronizer for 60 Hz (or, in the illustrated embodiment, any frequency from 0 to 100 Hz) or to a secondary analog or digital synchronizer (not shown, see  FIG. 6 ) for target frequencies of 400 Hz or greater. Once the synchronization process is “phased,” additional voltage gain beyond the square root of two can be attained on the isolated DC bus of secondary bus inverter  452 . At this point the secondary bus inverter  452  can control power flow by voltage set point, current set point, motoring current, regenerative current, and symmetrical current, which are set so as not to stall or over-current the source battery inverter  451 , nor saturate the transformer of LCL filter  455 . 
     Third Embodiment 
     A third embodiment of the present system shown in  FIG. 5 , is a photovoltaic to common DC bus, DC-to-DC converter  595  application, where the photovoltaic source panel  590  operates below 600 VDC (typical output range of 400 to 600 VDC). In order to make quality 480 VAC power for a substantially bumpless transfer grid synchronization, DC-to-AC converter, the photovoltaic source  590  output is up-converted to a voltage above 650 VDC, for example, in applications such as a substantially bumpless transfer grid synchronization system, or with a micro grid. In this exemplary PV implementation, a typical unloaded circuit would be slightly under 600 VDC on a sunny day. Under load, that voltage needs to be pulled down towards 400 VDC to move power. As that voltage is pulled down towards 400 VDC with this particular system, the PV source inverter  551  produces an AC three-phase rotating electrical field at 176 VAC as in the previous discussion. This three-phase rotating electrical field (similar to that provided to a motor winding) powers the generating electrical field (here, the secondary of the LCL filter  555 ) with a ratio of 208/480 VAC and generates 406 VAC (574 VDC peak) to a three-phase bus inverter  552 , which in turn can boost the voltage to 625 to 780 VDC, in order to power an isolated and boosted common DC bus  512 . Voltage gain, current (or power) regulation, and isolation are all features of this photovoltaic to common DC bus, DC-to-DC converter system  595 , as described above. 
     Fourth Embodiment 
     A fourth embodiment of the present system is a battery-to-battery, DC-to-DC converter  695  application, where the synchronization is done external to the inverters at frequencies from 100 to 400 Hz and higher (see  FIG. 6 ). A secondary (analog or digital) synchronizer  653  conditions and senses voltage, frequency, and phase coming from source battery inverter  651 , and enables the secondary synchronizing inverter  652  to synchronize as described previously, with application for frequencies above 100 Hz. 
     Additional forms of the present system could substitute items as follows:
         In place of an LCL filter  655 , some embodiments use a 1:1 isolation transformer.   In place of the LCL filter  655 , some embodiments use a transformer of any ratio to attain additional current or voltage gain.   In the LCL filter  655 , a reactor is usually installed on the high voltage/low current side for cost reasons.   In some embodiments, the system can be synchronized external to the secondary synchronizing inverter  652 , so that 400 Hz or other higher frequencies can be used for lowering the weight of the LCL filter.   Another form of sensing and filtering circuitry could be used in place of the secondary synchronizer  653 , such as transformer or op-amp filters.       

     Fifth Embodiment 
       FIG. 7  illustrates a fifth embodiment, which includes a single AC genset as a local power supply. System  700  connects to utility grid  710  through site disconnect switch  715 . Load-following CT  742  detects the current at that point in the circuit and provides the detected value to site controller  765 . This segment of the circuit provides power to normal loads  722 , generally only under power from grid  710 . Voltage sensor  741  provides an output voltage sense signal to inverter  750  in inverter subsystem  706 , and to site controller  765 . Contactor  705  (also known as “52U”) controllably interrupts the flow of power from inverter subsystem  706  to the normal loads  722  and utility grid  710 , providing an auxiliary contactor status output both to inverter  750  and through the auxiliary switch  745  of M1 contactor  725  to site controller  765 . 
     On the inverter side of 52U contactor  705 , sensor  743  provides a voltage sense signal to site controller  765 , and critical electrical load  720  connects to the AC bus to receive power. Protecting the loads from faults in inverter subsystem  706  and genset subsystem  703 , circuit breaker  737  is connected in series between 52U contactor  705  and controllable M1 contactor  725  to LCL-filter  730 , such as a delta-wye transformer (for example and not as a limitation). Sensor  744  provides a voltage sense signal to inverter  750  for uses that are discussed elsewhere herein. AC power from genset subsystem  703  is provided through circuit breaker  739 , which is rectified to DC by rectifier  785 , connected to common DC bus  712 , and transformed to AC power by inverter  750 , which provides it to LCL-filter  730  for powering the loads  720  and  722 . 
     Genset subsystem  703  provides power to the inverter subsystem  706 , such as from a local fuel source. In this example system, genset  780  is controlled by site controller  765  to generate power through generator breaker  735  to inverter subsystem  706 . Genset  780  in this example is a generator powered by natural gas, biogas, or other fuel source. Current transformer  746  measures the current being output by genset  780  for site controller  765 , and sensor  748  provides a generator output voltage sense signal to site controller  765  as well. Site controller  765  controls the operation and output of genset  780  as will be understood by those skilled in the art. 
     With additional reference to  FIG. 7 , the site controller  765  operates in grid parallel mode  250  when the inverter subsystem  706  is connected to the grid  710  (i.e., both M1 contactor  725  and 52U contactor  705  are closed). In grid parallel mode  250 , inverter  750  is synchronized to the utility grid  710  and the grid effectively controls the inverter  750  frequency, while the site controller  765  regulates the throttle of genset  780  to control its output to the load. For the system  700  to operate optimally in grid parallel mode  250 , the site controller  765  preferably regulates its output to control the speed of genset  780 . 
     In grid isolated mode  240 , the site controller  765  regulates both the output voltage and frequency of genset  780 . Inverter  750  regulates its own output voltage and frequency, and its output current in proportion to a signal from site controller  765 . 
     Any change in the load  720  results in a change in the common DC bus  712  voltage of the inverter  750 . The genset subsystem  703  adjusts its output voltage to maintain the common DC bus  712  to the inverter subsystem  706 . The inverter subsystem  706  limits its output current to prevent a bus under-voltage condition caused by the limitations (maximum KW output or ramp rate, for example) of the components in the system. These limits are preferably configurable parameters of site controller  765 . 
     In the grid parallel mode  250  of inverter  750 , the site controller  765  adjusts the speed/frequency of the genset  780  in response to the load demand. Any changes in frequency result in a change in the analog “load command” signal sent to the inverter  750 . The inverter  750  then regulates its output current to maintain the desired frequency of the genset  780 . 
     When the system is operating in grid parallel mode  250 , genset subsystem  703  provides AC output power through closed generator breaker  735  to inverter subsystem  706 . The rectifier  785  of inverter subsystem  706  converts the AC power to DC and connects to common DC bus  712 , connected to inverter  750 , which converts power to AC and provides output power to critical load  720  and, if 52U contactor  705  is closed, to normal loads  722  and even to utility grid  710 . In this example, however, system  700  is installed in a situation where loads  720  and  722  are already connected to a utility grid  710 , and genset subsystem  703  and inverter subsystem  706  are not yet energized (so M1 contactor  725  is open, and 52U contactor  705  is closed). The system  700  then transitions through a “normal start” state transition process. The (human) operator inserts a key into the inverter subsystem  706  and switches it on, presses an “automatic on” control and site controller  765  begins operation of the genset subsystem  703 , which then starts up. The site controller  765  notes that the mains (52U) contactor  705  is closed, but M1 contactor  725  is open, so it closes generator breaker  735  and enters grid isolated mode  240  of operation. Inverter  750  then wakes up, senses the mains voltage (using sensor  741 ), and synchronizes to the grid as discussed herein. When inverter  750  is synchronized, it senses the closed state of 52U contactor  705  and its synchronism to the grid  710 , closes M1 contactor  725 , and enters grid parallel mode  250 . Site controller  765  senses the closing of 52U contactor  705  and also switches to its grid parallel mode  250  of operation. 
     One of the well known problems with providing alternative power sources to grid-connected loads is handling a failure of the grid. Prior to the failure, we presume that the system  700  is operating in normal grid parallel mode  250  as described above. Prior to the loss of grid power, genset  780  is operational, generator breaker  735  and circuit breaker  739  are closed, inverter  750  is on and synchronizing in grid parallel mode  250 , and M1 contactor  725 , circuit breaker  737 , and 52U contactor  705  are all closed. When the voltage provided by utility grid  710  drops below an acceptable level, 52U contactor  705  opens. Site controller  765  detects this opening (through the auxiliary status output of 52U contactor  705 ) and changes from grid parallel mode  250  to grid isolated mode  240 . With both subsystems in grid isolated mode  240  and 52U contactor  705  open, the genset subsystem  703  and inverter subsystem  706  provide power to critical load  720  while effectively disconnected from utility grid  710 . 
     In such situations, the grid typically recovers after a period of time and resumes making power available. At this point, site controller  765  is operating in grid isolated mode  240 , converting the output of genset  780  into AC to power critical load  720 . 52U contactor  705  is open, so normal loads  722  are powered by the grid, while critical load  720  is powered by inverter subsystem  706 . 
     When utility grid  710  resumes providing power, inverter  750  detects the voltage (through sensor  741 ) and synchronizes its output voltage, frequency, and phase to the supply from utility grid  710 . When inverter  750  and the site controller  765  detects that synchronization has been achieved, 52U contactor  705  is commanded to close. Then, when inverter  750  senses that 52U contactor  705  has closed and that its synchronization with utility grid  710  has been maintained, it switches from grid isolated mode  240  to grid parallel mode  250 . Likewise, when site controller  765  senses that 52U contactor  705  and M1 contactor  725  are closed, it also switches from grid isolated mode  240  to grid parallel mode  250 . Because these modes of operation are substantially the same except for the method of controlling changes in the output of genset  780  and inverter  750 , these mode changes have no instantaneous effect upon the output of inverter  750 , and have little or no effect on critical load  720 . At this point, then, the system has returned to normal, grid parallel mode  250 . 
     It is also possible for genset  780  or inverter  750  to fail, and system  700  is able to maintain uninterrupted power from utility grid  710  to critical load  720  even in these conditions. For example, if genset  780  fails, inverter  750  would detect the under-voltage on the common DC bus  712 , and open M1 contactor  725 . Similarly, if inverter  750  fails, it opens M1 contactor  725 , which site controller  765  senses and switches itself from grid parallel mode  250  to grid only mode  230 . When the fault in inverter  750  is corrected, system  700  then resynchronizes with the grid and closes M1 contactor  725 . 
     Some situations may require a “black start” from off mode  210  in which neither system  700  nor utility grid  710  is powering loads  720  and  722 . This situation begins with 52U contactor  705  and M1 contactor  725  open, as is genset breaker  735 . The system operator keys the system on and initiates automatic operation and “run-with-load” operation. The engine in genset  780  starts up. Site controller  765  senses the absence of mains voltage from utility grid  710  and the open state of 52U contactor  705  and M1 contactor  725 , so it closes generator breaker  735  and enters grid isolated mode  240 . Sensing the output of genset  780 , inverter  750  wakes up. It senses no mains voltage from utility grid  710  and the open state of 52U contactor  705 , generates an internally maintained sine-wave output at an appropriate frequency and voltage, closes M1 contactor  725 , and enters grid isolated mode. At this point, system  700  is islanding and powering critical load  720  independently of utility grid  710 . 
     During each transition in which the inverter subsystem  706  output is being synchronized to the utility grid  710  in this embodiment, system synchronization occurs through voltage match, frequency match, and phase match using a unique combination of signal conditioning boards (double Butterworth design), inverter hardware, and drive programming. The system is configured with contactors on both the grid side (52U) and the auxiliary supply side (M1) of the critical load  720 , whereas many similar designs use shunt trip breakers. Signal conditioning boards monitor voltage on the line/grid side of the 52U contactor  705  and the inverter side of the M1 contactor  725 , and function to match voltage, phase, and frequency during synchronization. The signal conditioning board is set up as a low-pass filter to remove PWM frequencies typically at 3 KHz, and to pass the 50 to 60 Hz signal. Alternatively, sensing and filtering circuitry might be comprised of transformers or op-amp filters. The inverter technology used in this electrical control configuration is implemented using a modular PWM based IGBT inverter, a programming module, and regenerative hardware with an IEEE 519 PWM filter; and can produce a variable output frequency of, for example, either 50 or 60 Hz. The LCL harmonic filter  730  in this embodiment is an output inductor coupled with a three-phase capacitor and uses an output transformer (for example and not as a limitation), to complete the LCL circuit, as typically applied to regenerative AC drive systems and dynamometers. 
     To match voltage in this embodiment, the DC bus minimum control limit is maintained above the commanded output AC voltage peak produced by the inverter  750 . Transformers are used if necessary to satisfy voltage-matching requirements if the generated DC voltage supplied to the inverter does not exceed the AC sine peak with sufficient voltage potential to meet IEEE 519. Typically, in this example, one would set a commanded output voltage of 420 VAC for the motor nameplate (rated motor voltage drive parameter) to provide sufficient headroom between the DC bus voltage (such as 650 VDC for a 460-volt inverter) and the peak voltage of the simulated 60 Hz AC output from inverter  750  (such as 600 VDC for a 420-volt rated output). Voltage matching is accomplished by detecting output line voltage (in this case, transformed output from LCL filter  730 ) at the M1 contactor  725  by the signal conditioning circuit detecting voltage sense  744 , and applying PID loop feedback on the inverter output (in this case, 420 VAC voltage output to the transformer of LCL filter  730 ). 
     In the case of the present 460 VAC system, the inverter  750  is connected to the grid using a delta-wye transformer with an approximate value of plus/minus ten percent (+/−10%) taps, connecting the grid to the wye side of the transformer, and connecting the inverter to the minus ten percent taps on the delta side of the transformer in LCL filter  730 . This specified transformer configuration provides a voltage boost to the inverter output to compensate for the limited inverter output voltage, as described herein (i.e., to provide the necessary headroom (50 to 60 VDC) thus assuring the DC bus is higher in voltage than the peak of the AC line (i.e., PWM carrier frequency) generated by inverter  750 ). The same effect could be obtained in other implementations of this example design by increasing the generator output capacity to maintain the desired differential between the common DC bus  712  minimum and the inverter  750  output voltage peak, or using a standard 380/460 delta-wye transformer and attaching the inverter  750  to the 380-volt delta side. Additionally, this configuration would not require a boost transformer to meet local requirements in other jurisdictions and could substitute a line reactor in the place of the transformer as part of the LCL filter  730 . Still other designs will occur to those skilled in the art. 
     In the case of a phase loss or complete power interruption, the inverter subsystem  706  electrical control configuration equipped with a combination of battery power and a genset  780  provides for a substantially bumpless transfer from the grid. Here, the M1 contactor  725  is maintained in the closed position, and the battery DC source  790  provides instantaneous ride-through while the genset  780  starts up. Alternatively, with a genset running 24/7, no battery would be needed as described in the  FIG. 7  example above. Ideally, the inverter  750  provides a current-limit drive status, with a microsecond current-loop update rate. In this example, the inverter  750  current-limit drive status parameter provides a deterministic heartbeat for monitoring grid presence, with an update rate that meets UL 1741 requirements, thus providing anti-islanding protection based on current loop update trends within the inverter. The 52U contactor  705  opens when the grid failure is detected and the electrical control configuration operates in an “islanding mode,” or grid isolated mode  240 , waiting for the grid to be restored. 
     Upon return of grid power, the signal conditioning circuit for the grid side of 52U contactor  705  detects voltage and frequency for all three phases of utility grid  710 . The inverter  750  control loops coupled with the signal conditioner  753  conditioning board matches voltage and frequency for two phases, and uses the third phase in a comparator circuit to determine whether the inverter  750  is in phase with the utility grid  710 , or 180 degrees out of phase. Once voltage and phases are within specification, the phase-lock comparator circuit provides a digital input to the inverter, allowing the 52U contactor  705  to close. The phase-lock loop control can be located on the signal conditioner  753 , an external analog signal conditioning board, or can be a digital signal internal to inverter  750  using the drive coprocessor, for example. When 52U contactor  705  and M1 contactor  725  close, ideally a bit shift in the drive operating program brings in a fixed offset, which commands the inverter  750  output frequency faster than the utility grid  710  frequency, and thus permits the current limit drive parameter to control the inverter  750  output. Once the 52U contactor  705  is closed, the inverter shifts modes from grid isolated mode  240  to grid parallel mode  250  and is locked in phase with the grid to complete substantially bumpless transfer grid synchronization. Finally, the power factor control loop is enabled, gradually adjusting inverter output until unity power factor is achieved. 
     As an example and not as a limitation, the current-controlled solution used in the present substantially bumpless transfer grid synchronization system distinguishes this approach from other synchronizing techniques, which typically use voltage control. Current-limited, phase-synchronized, substantially bumpless transfer operation is achieved in this embodiment by limiting the in-rush current while setting the motoring current limit to approximately 10% above the critical load, allowing the 52U contactor  705  to close without causing a cascading failure of over-current check devices. One skilled in the art of electronic controls will appreciate the plethora of other control systems beyond this example that will accomplish the scope and spirit of the complete invention contained herein. 
     Alternative Implementations 
     Now as an example and not as a limitation, DC-to-DC conversion also has application for time shifting the availability of grid power from off-usage night time hours to peak demand time frames by storing DC potential in batteries during the evenings and returning it to the grid during daytime peak demand hours. Also, using solar- and wind-generated power during peak demand times to reduce dependency on conventional power supply sources is a rapidly growing field today, Typically, these renewable sources could be configured by one having ordinary skill in the art to wake up and synchronize to the utility grid using the substantially bumpless transfer grid synchronization technology described herein. 
     In some embodiments, an electrical power system serves as an alternate AC source to supply the electrical needs of, for example, a home. In this example, extra power supplied by the system but not required for the home may be sold back to the electric utility. In this same example, a combined heat and power (“CHP”) system provides additional heat for the home. The control systems described herein provide uninterrupted power for the loads in the home, including a substantially bumpless transfer of supply from the grid to the local source(s) in the event of grid failure. (That is, there is no delay between failure of the grid and the effective supply of energy from local sources to the load.) 
     All publications, prior applications, and other documents cited herein are hereby incorporated by reference in their entirety as if each had been individually incorporated by reference and fully set forth. While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.