Abstract:
A system and apparatus for generating power. In one embodiment, the apparatus comprises a power module for coupling to a DC power source via a DC bus, wherein the power module (i) converts a first power from the DC power source to a second power, and (ii) comprises a maximum power point tracking module unit for dynamically adjusting a load voltage of the DC power source; an AC bus; and a controller, physically separate from the power module and coupled to the power module via the AC bus, for operatively controlling the power module.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 12/121,616, entitled “Distributed Inverter and Intelligent Gateway” and filed May 15, 2008, which further claims the benefit of U.S. provisional patent application Ser. No. 60/938,663 filed May 17, 2007, both of which are herein incorporated by reference in their entirety. The present application is further related to co-pending U.S. patent application Ser. No. 12/121,578 entitled, “Photovoltaic AC Inverter Mount and Interconnect” and filed May 15, 2008, and U.S. patent application Ser. No. 12/121,580 entitled “Photovoltaic Module-Mounted AC Inverter” and filed May 15, 2008, both of which are hereby incorporated by this reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to power conversion including direct current (DC) to alternating current (AC) and, more particularly, to photovoltaic module output power conversion to AC. 
     2. Description of the Related Art 
     In coming years, distributed generation of electricity is likely to become a larger and larger part of the energy sourced to utility grids. Distributed sources of electrical energy such as solar photovoltaic modules, batteries, fuel cells and others generate direct current (DC) power, which must be converted to alternating current (AC) power for transmission and usage in residential and commercial settings. 
     Also, as distributed generation increases, the utility grid, commonly known as the “grid,” will be transformed to a still to be defined smart-grid that will support increased coordination between multiple generators and multiple loads. “Grid tied” photovoltaic systems are the most common form of solar electric systems today and they use a form of coordination called net-metering. 
     The larger number of photovoltaic installations are residential having an average capacity of about 2-3 kW. The bulk of new generating capacity is being installed in commercial buildings and utility scale installations. Residential systems commonly utilize single phase AC, while commercial systems most often use three phase AC. 
     Residential rooftops present a special challenge for the placement and interconnection of photovoltaic modules, due to the presence of gables, multiple roof angles, and other such obstructions. Such rooftops often do not expose a sufficiently large, commonly directed surface to the sun for photovoltaic modules to be positioned to harvest maximum power. Currently, conventional inverter-based interconnections are optimized to minimize IR (current times resistance) loss. This is referred to as string design. The inverters perform a function called maximum power point tracking (MPPT) on strings of PV modules. The MPPT process evaluates the PV module string output current-voltage curve on a continuous or sampled basis to determine the correct load voltage thus maximizing the string output power calculated as the string output voltage times current. Due to the nature of residential rooftops, the string design results in MPPT performance at the levels of the least power producing modules in the photovoltaic (PV) array. This degrades the AC power harvest from the entire array. 
     The use of microinverters in a one-to-one configuration with the PV modules removes the string design challenge, thereby enabling each PV module to produce current at its full capacity and truly permits MPPT at a per PV module level. The one-to-one arrangement of microinverters and PV modules is also referred to as AC PV modules in related art. 
     Commercial buildings and larger installations present slightly different challenges. In commercial buildings, large, commonly directed surfaces are generally available, but even then, obstructions, such as HVAC components must be dealt with as the components may block solar radiation. String design and MPPT also continue to be of concern. Additionally, since such installations often consist of thousands of PV modules, monitoring, operation and maintenance can be time consuming and expensive. 
     The use of AC PV modules for commercial installations simplifies string design, improves AC power harvest and provides the ability to remotely monitor the entire PV array on a module by module basis. A multiphase microinverter has the additional advantage of delivering substantially balanced multiphase AC power. 
     The benefits of microinverters have been documented in related art dating back almost three decades. Yet, the use of microinverters continues to be negligible due to their inferior reliability and efficiency as well as their high cost as compared to conventional inverters. 
     Typically, PV modules are placed in hostile outdoor environments in order to gain maximum exposure to solar radiation. Microinverters must be placed in proximity to the PV modules to realize their full benefits. Conventional inverters are typically placed in more benign environments, often indoors, e.g., on a protected wall or in a utility closet. 
     When microinverters are placed in proximity to PV modules, the hostile outdoor environment exacerbates the design challenge for achieving high reliability, high efficiency and low cost. Similarly, servicing and replacing microinverters on a rooftop is potentially more challenging and labor intensive than servicing and replacing centralized inverters. 
     The related art design approach for microinverters has been to implement them as miniaturized versions of conventional inverters, incorporating all the functions and components that were used in conventional inverters. Early versions of related art for microinverters utilized electrolytic capacitors, having a lifespan susceptible to degradation at high temperatures. Other versions of related art microinverters eliminate the electrolytic capacitor, thereby improving the lifespan. 
       FIG. 1  shows a simplified diagram of a related art grid tied photovoltaic system utilizing a conventional inverter. Referring to  FIG. 1 , PV modules  102  are mounted outdoors  110  for direct access to solar radiation and connected to a conventional inverter  105  using DC wiring  104 . Both inverter  105  and DC wiring  104  are located in a weather protected region  111  such as the interior of a structure. The inverter  105  output feeds local AC loads  106  through AC wiring  103 . The inverter&#39;s output is also tied for bi-directional flow of energy for net-metering to the utility grid  101  through exterior AC wiring  107 . 
       FIG. 2  shows a simplified diagram of a related art grid tied photovoltaic system including microinverters. Referring to  FIG. 2 , PV modules  202  are mounted outdoors  210  for direct access to solar radiation. Microinverters  203  are electrically coupled in one-to-one proximity to the PV modules  202  (typically under them) and convert individual PV module DC outputs to AC power which is then fed to AC wiring  204 . The AC wiring  204  feeds local loads  206  and the utility grid  201 . 
     A problem with the related art microinverters is that they either collocate all inverter functions including safety and code compliance within the microinverter or they do not address how these functions are to be implemented, thereby making the design for high reliability and long life difficult and expensive. The collocation may also require redevelopment and replacement of the microinverter when code compliance requirements change. 
     SUMMARY OF THE INVENTION 
     Accordingly, the invention is directed to a distributed inverter and intelligent gateway that substantially obviates one or more of the problems due to limitations and disadvantages of the related art. 
     An advantage of the invention is to provide a system and method of installation, operation, and maintenance of a power conversion system that is simple and safe. 
     Another advantage of the invention is to provide a high degree of reliability and a long lifetime to the microinverter. 
     Yet another advantage of the invention is to provide upgradeability of the system during the lifetime of the system. 
     Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The features of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings. 
     To achieve these and other advantages and in accordance with the purpose of the invention, an embodiment of the invention is directed towards a system for transforming energy. The system for transforming energy includes a plurality of photovoltaic modules. The plurality of microinverters is electrically coupled to the photovoltaic modules in a one-to-one relationship. The system also includes a gateway electrically coupled to the plurality of microinverters and features of the gateway are capable of being upgraded with at least one of hardware, software, and firmware. 
     In another aspect of the invention, an embodiment of the invention includes a gateway for use in a system for use in an energy generating system. The gateway includes an interface unit for interfacing with a plurality of microinverters and a utility grid. The gateway also includes a control unit electrically coupled to the interface unit for controlling at least one of safety functionality, synchronization functionality to synchronize the plurality of microinverters to the utility grid, and monitoring functionality to monitor the plurality of microinverters and the utility grid. The control unit is capable of being electrically coupled to an external monitor. In yet another aspect of the invention, an embodiment of the invention includes a photovoltaic system for transforming radiant energy into alternating current. The photovoltaic system includes a plurality of photovoltaic modules and a plurality of microinverters coupled to the photovoltaic modules in a one-to-one relationship. Each of the plurality of microinverters includes an inversion unit, a MPPT unit, a communications unit, a safety unit, an interface unit and a control unit. The inversion unit converts DC into AC and the MPPT unit optimizes power from the plurality photovoltaic modules. The communications unit provides communications to the gateway. The safety unit provides safety functions. In addition, a gateway is coupled to the plurality of microinverters. The gateway includes an interface unit and a control unit. The interface unit interfaces with a plurality of microinverters and a utility grid. The control unit is coupled to the interface unit for controlling at least one of safety functionality, synchronization to synchronize the plurality of microinverters to the utility grid and monitoring where the control unit is capable of being coupled to an external monitor. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       In the drawings: 
         FIG. 1  is a diagram of a related art grid tied photovoltaic system using a conventional centralized inverter; 
         FIG. 2  is a diagram of a related art grid tied photovoltaic system using conventional microinverters; 
         FIG. 3  is a diagram of a grid tied photovoltaic system using distributed converters including microinverters and a gateway according to an embodiment of the invention; 
         FIG. 4  is a block diagram of a distributed converter utilizing microinverters according to another embodiment of the invention; 
         FIG. 5  is a block diagram of a distributed converter utilizing string inverters according to another embodiment of the invention; 
         FIG. 6  is a block diagram of a distributed converter utilizing centralized inverters according to another embodiment of the invention; 
         FIG. 7  is a block diagram of a PV system utilizing microinverters according to another embodiment of the invention; 
         FIG. 8  is a block diagram of a PV system utilizing multiple string inverters according to another embodiment of the invention; and 
         FIG. 9  is a block diagram of a PV system utilizing centralized inverters according to another embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention include a novel approach whereby only those functions and components that are necessary to achieve the advantages of microinverters are placed in assemblies in proximity to the PV modules and other functions including, for example, system control, are located elsewhere. This separation of functions and components is termed partitioning. The control and coordination of the system is performed without additional wiring. Communications may occur via powerline, wired and/or wireless channels. Disabling the communications channel provides a way for turning the microinverters off thereby facilitating inverter or PV module replacement, maintenance or other desired tasks. Moreover, the partitioning provides for enhanced safety as compared to the related art. In the related art, in the presence of solar radiation, the PV module outputs are always enabled and are thus capable of electrocuting the installer. Accordingly, embodiments of the invention provide for safer and simpler installation and maintenance procedures. 
     In addition, the partitioning reduces the number and type of components placed in the PV module proximate assemblies that are subject to the hostile outdoors environment, such as temperature. Fewer components and simplified assembly also result in improved reliability and increased system lifetime. Cost is reduced by eliminating common system functions from the microinverter. In embodiments of the invention where these functions are realized with cheaper or less robust components located in a less hostile environment, the cost may also be further reduced. 
     In some embodiments, the partitioning may include physically locating programmable functions in an area distinct from the PV module assemblies, such as, in a gateway. Accordingly, the features located in the gateway are decoupled or partitioned from the PV module assemblies. The functions and components in the gateway may be upgraded over the lifetime of the system. Therefore, the development cycle of the gateway is decoupled from the development cycle of the microinverter assemblies coupled to the PV module assemblies, and thus several generations of gateways having different value added functions can be developed for use with the same microinverter assemblies. This leads to an upgradeable system at a significantly reduced cost as compared to the related art. 
     Reference will now be made in detail to embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
       FIG. 3  is a diagram of a grid tied photovoltaic system using distributed converters including microinverters and a gateway according to an embodiment of the invention. The grid tied photovoltaic system includes a plurality of PV modules  302  mounted outdoors  310  each coupled electrically and mechanically to microinverter assemblies  303 . In this embodiment, the gateway  305  is located in a weather protected environment  311 , e.g., indoors, for accessibility and protection from the elements. However, the gateway  305  could be located in an outdoor environment or other location. The gateway  305  receives AC current from the plurality of microinverters  303  via a first AC wiring  304 . The gateway  305  is connected to the grid  301  and to local loads  306  via a second AC wiring  307 . The loads  306  are thereby always connected to the grid  301 . The gateway  305  provides a point of measurement of the AC performance of the PV system and also the electrical behavior of the grid  301 . The gateway  305  also provides control and monitoring of the microinverters  303 . 
       FIG. 4  is a block diagram of a distributed converter according to another embodiment of the invention. Referring to  FIG. 4 , a system for transforming energy includes a first photovoltaic module  401  and a second photovoltaic module  411  coupled to a distributed inverter  410  with DC wiring  402 ,  412 . The distributed inverter  410  is functionally partitioned into a gateway  406  and a first microinverter  404  and a second microinverter  414 . The first PV module  401  connects to the first microinverter  404  via first DC wiring  402  and the second PV module  411  connects to the second microinverter  414  via second DC wiring  412 . Additional PV modules can be connected via independent DC wires to associated additional microinverters. 
     The AC output of all microinverters is connected in parallel via AC wiring  405  to the gateway  406 . The gateway  406  includes an interface unit  407  and a control unit  408 . The interface unit  407  is connected to the control unit  408  via interface wiring  409 . The interface unit connects to the utility grid via AC power wiring  420 . The control unit connects to an external monitor via data communications wiring  421  or via a wireless data communications channel. 
     The first and second microinverters  404 ,  414  include a power inversion unit, a maximum power point tracking (MPPT) unit, a communications unit and a safety unit. The power inversion unit converts DC power from the associated PV module to AC power. The AC power may be generated in single-phase or multi-phase form. The MPPT unit detects the output power of the associated PV module and adjusts the load voltage presented to the PV module in such a way as to maximize the power available from the PV module. The MPPT unit and functionality are described by Hussein K. H., Mutta I., Hoshino T. and Osakada M., in “Maximum photovoltaic power tracking: An algorithm for rapidly changing atmospheric conditions”, IEE Proceedings, Generation, Transmission and Distribution, Vol. 142, No. 1, January 1995, which is hereby incorporated by reference as if fully set forth herein. The communications unit provides bi-directional data communications between the microinverter  404 ,  414  and the gateway  406 . Data communications from the gateway  406  to the microinverter  404 ,  414  includes, for example, power conversion control, status requests, fail-safe shutdown operation and other relevant data operations. Data communications from the microinverter  404 ,  414  to the gateway  406  includes PV module DC output voltage and current data, microinverter AC output voltage and current data, microinverter operational status and other relevant data. 
     The safety unit establishes conditions to enable or disable current flow from the PV module  401 ,  411  to the microinverter  404 ,  414  and from the microinverter  404 ,  414  to the gateway  406 . One safety function of the safety unit is to establish that the utility grid voltage available at the AC wiring  405  is within specifications to allow the inverter to safely drive the grid. If the appropriate grid voltage specifications are met, then the safety unit enables both input and output current through the microinverter  404 ,  414 . This first function is also performed by the gateway  406  and the function is secondary within the microinverter  404 ,  414  as a backup fail-safe system in case of failure of the primary safety function of gateway  406 . 
     Another safety function of the safety unit is to test for a communications signal from the gateway  406  indicating that the grid is safe to drive with AC output current. If the gateway were disabled by any means, the communications signal emitted by the gateway  406  would be disabled. The safety unit detects this condition and immediately disables all current flow into and out of the microinverter  404 ,  414 . The communications signal from the gateway  406  is termed a “heartbeat” and provides a primary fail-safe mechanism for disabling PV system AC and DC power flow in the event of a grid failure, fire or other safety hazard. 
     Yet another safety function of the safety unit is to support removal and reattachment of the microinverter  404 ,  414  while the entire PV system is enabled. This is termed a hot-swap and requires that the microinverter  404 ,  414  shut down upon detection of the removal or reattachment of the microinverter to suppress any arcs or high voltages that may develop at the microinverter DC wiring  402 ,  412  or AC wiring  405  terminals. High voltages at the microinverter terminals are suppressed as a means to insure that maintenance personnel are not able to contact the exposed terminals while they are energized. The microinverter  404 ,  414  shuts down if either the AC output voltage at AC wiring  405  does not meet prescribed conditions, such as a disconnect or grid failure, or if the PV module DC input voltage at DC wiring  402 ,  412  appears to be disconnected. 
     The gateway  406  acts as master controller in the distributed inverter  410  by performing functions such as providing the above described heartbeat to the microinverters, monitoring their output, monitoring the grid and other related functions. The gateway  406  can turn off the heartbeat when, for example, the gateway detects a fault or unsafe conditions in the environment, maintenance is to be performed, an AC power failure occurs and/or other related operating conditions. Similarly, if the gateway  406  is physically absent, the heartbeat is thereby also absent, resulting in the disabling of the microinverters. 
     The communication between the gateway  406  and the microinverters  404 ,  414  may be performed over the AC wiring  405 , wirelessly, or by other suitable means such as independent wiring. Both communication over AC wiring and wireless communication have the advantage that no additional wiring is required beyond that to support the transfer of AC power in the system. In the case of communication over the AC wiring  405  a wireline modulator/demodulator sub-function is included within both the gateway  406  and the microinverters  404 ,  414  to perform the communication functions utilizing the AC wiring  405 . 
     In this embodiment, the gateway  406  functions are split into an interface unit  407  and a control unit  408 . The interface unit  407  includes a sensor unit, a communications unit and an isolation unit. The sensor unit provides a way to dynamically monitor grid conditions, for example to dynamically measure grid AC voltage, current, frequency, phase and other related grid signal characteristics. In addition, the communications unit provides means to send and receive data to and from connected microinverters  404 ,  414 . The isolation unit substantially prevents data communications between the microinverters  404 ,  414  and the gateway  406  from appearing at the grid wiring  420  or onto the grid. Similarly, noise and other signals, with the exception of the desired grid AC power voltage and current, are substantially prevented from appearing at the AC wiring  405  between the microinverters  404 ,  414  and the gateway  406 . 
     The control unit  408  provides monitoring and control functions to monitor the microinverters  404 ,  414 . Some of the control includes controlling at least safety functionality and synchronization functionality to synchronize the microinverters  404 ,  414  to the grid and monitoring functionality to monitor the microinverters  404 ,  414  and the utility grid. In addition, the control unit  408  provides grid synchronization, communications protocols, grid connection performance compliance and system monitoring. The control unit can be implemented with a computer or microcontroller. 
     In this embodiment, software and firmware run on the control unit  408  to implement functions such as generating the heartbeat, monitoring the microinverters, monitoring the grid and monitoring any AC loads. Using the previously described communications system the control unit  408  can address each microinverter  404 ,  414  individually, in subsets, or as an entire ensemble. 
     For example in one embodiment, the control unit  408  provides synchronization signals for matching the frequency and phase of the microinverter  404 ,  414  AC output to the grid AC power. The heartbeat fail-safe is also implemented in the control unit  408 . Other functions may include monitoring the health and productivity of each microinverter and each PV module. 
     The control unit  408  also performs grid related protocols such as detection of grid failure, anti-islanding detection, adjustment of parameters utilized in the anti-islanding detection and related functions. New grid related protocols will be defined to implement a future smart-grid in which the grid operator may enable, disable or modify the control system of a grid-connected PV system. The control unit  408  is constructed with flexible hardware, software and firmware to adapt to future grid-defined control and communications protocols, without requiring changes to the microinverters  404 ,  414 . 
     The gateway  406  as a whole can also be upgraded to support future system requirements by changing the hardware, software or firmware while utilizing the same microinverters  404 ,  414 . For the grid connection  420  of the gateway  406 , the control unit  408  can perform load management functions, as directed through the grid protocol or other external sources. Related art monitoring and display functions can also be implemented in the control unit  408 . 
     The wiring  409  carries all data between the interface unit  407  and the control unit  408 . External communications to the gateway  406  from an external monitor occurs via communications wiring  421 , or other communications means such as wireless communications. This is used to externally monitor and control operation of the gateway  406  and allows for system remote control via internet connection or other remote communications means. 
     By reducing the functions performed within the microinverter  404 ,  414  a reduction in the required microinverter complexity is achieved, thereby leading to simpler implementation of the microinverter and increased reliability, increased lifetime, and reduced cost as compared to the related art. In particular, the elimination from the microinverter of precision grid signal measurement requirements to support anti-islanding functionality for grid connection specification compliance as defined, for example, by IEEE standard IEEE-1547, eliminates considerable complexity, expense and lifetime limiting components from the microinverter. 
     IEEE standard IEEE-1547, and related standards used throughout the world, defined a narrow set of circumstances upon which the grid is driven by the inverter. If a break in the grid wiring occurs, an inverter could continue driving power into the un-connected branch of the grid. This region in which the primary grid generators no longer apply power is termed an island. The inverter is required by the IEEE-1547 standard to disable its output power under such conditions so as not to drive the island in the grid for both safety and technical reasons. This is known as anti-islanding. The gateway  406  assumes the primary role in detecting the defined islanding condition and communicates the associated inverter shut-down command to the microinverters  401 ,  411  to implement the anti-islanding function. Example grid conditions for an anti-islanding shutdown are a very high grid voltage, a low grid voltage, a high grid frequency, a low grid frequency or a significant variation in grid impedance. The grid is usually of low impedance, therefore the usual grid impedance variation is an increase when the island occurs. 
     The system of  FIG. 4  can also be used to convert power for primary or secondary power sources other than PV modules  401 ,  411  such as wind turbines, fuel cells, batteries, and other power sources. 
       FIG. 5  is a block diagram of a PV system according to another embodiment of the invention. A first PV module string  501  includes a first PV module  502 , a second PV module  503  and a third PV module  504  that are connected in series. A first output DC wire  505  and a second DC output wire  506  from the PV module string  501  are connected to a first string inverter  521 . A second PV module string  511  includes a fourth PV module  512 , a fifth PV module  513  and a sixth PV module  514  that are connected in series. A third output DC wire  515  and a fourth DC output wire  516  from the PV module string  511  are connected to a second string inverter  522 . The AC outputs of the first string inverter  521  and second string inverter  522  are connected in parallel via AC wiring  523  to the gateway  524 . The PV module strings  501 ,  511  may include any number of PV modules that are series connected. Any number of PV module strings may be used in conjunction with associated string inverters in this system. 
     PV module strings  501 ,  511  are not required to include equal numbers of PV modules  502 ,  503 ,  504 ,  512 ,  513 ,  514  as is the case in related art. This provides the benefit of a simple string design in which the string DC output voltages are not required to be matched between strings  501 ,  511 . Moreover, MPPT may be performed on a per string basis, so one string does not degrade the performance of another, as is the case with related art where equal length strings are connected in parallel. This arrangement has the potential to provide greater AC power harvest than conventional inverters, but less AC power harvest than microinverters. 
     A distributed inverter  520  is functionally partitioned into a gateway  524  and string inverters  521 ,  522 . The functions of the gateway  524  in this embodiment are the same as the functions of the gateway  406  as described herein. The gateway  524  includes an interface unit  525  and a control unit  526 , both of which are the same as described with reference to  FIG. 4  herein. Accordingly, the benefits of partitioning the gateway  524  from the multiple-string inverters  521 ,  522  in  FIG. 5  are substantially similar to the benefits of partitioning the gateway  406  from the microinverters  404 ,  414  in  FIG. 4 . For example, the benefits include using a heartbeat for safety and independent upgradeability of the gateway from that of the string inverters. In this embodiment, the multiple-string inverters  521 ,  522  are placed close to the gateway and away from the hostile outdoors environment. The AC output of all string inverters  521 ,  522  are connected to the gateway in parallel via AC power wiring  523 . The gateway  524  is connected to the grid via wiring  530 . External communications to the gateway  524  from an external monitor occurs via communications wiring  531 , or other communications means such as wireless communications. This is used to externally monitor and control operation of the gateway  524  and allows for system remote control via internet connection or other remote communications means. 
       FIG. 6  is a block diagram of a PV system according to another embodiment of the invention. A first PV module string  601  includes a first PV module  602 , a second PV module  603  and a third PV module  604  that are connected in series. A first output DC wire  605  and a second DC output wire  606  from the PV module string  601  are connected to the DC combiner  621 . A second PV module string  611  includes a fourth PV module  612 , a fifth PV module  613  and a sixth PV module  614  that are connected in series. A third output DC wire  615  and a fourth DC output wire  616  from the PV module string  611  are connected to the DC combiner  621 . 
     A first DC output  622  and a second DC output  623  from the DC combiner connects to the DC inputs of an inverter  624 . The AC outputs of the inverter  624  are connected in parallel to other inverters via AC wiring  625  to the gateway  626 . The PV module strings  601 ,  611  can include any number of PV modules that are connected in series. Any number of PV module strings can be used in conjunction with associated DC combiners. A plurality of inverters  624  may be used with outputs connected in parallel in the system. 
     In this embodiment, the PV module strings  601 ,  611  must be of equal length when connected to a common DC combiner  621 . This scheme is consistent with connection of conventional inverters as known to one of ordinary skill in the art. The AC output of the inverter  624  is connected to the gateway  626  in parallel via AC power wiring  625 . 
     In addition, the inverter  624  is similar to conventional inverters as known to one of ordinary skill in the art except that control functions are implemented in a gateway  626  rather than within the inverters. The functions of the gateway  626  in this embodiment are substantially similar to its functions in the embodiment shown in  FIG. 4 . The gateway  626  includes an interface unit  627  and a control unit  628 , both of which are the same as discussed with reference to  FIG. 4  herein. Accordingly, the development cycles of the inverters and controllers can be decoupled with improved system performance and the ability to upgrade without changing the inverters. The gateway  626  is connected to the grid via wiring  630 . External communications to the gateway  626  from an external monitor occurs via communications wiring  631 , or other communications means such as wireless communications. This is used to externally monitor and control operation of the gateway  626  and allows for system remote control via internet connection or other remote communications means. 
       FIG. 7  is a block diagram showing a PV system according to another embodiment of the invention. Referring to  FIG. 7 , a distributed inverter includes a gateway  712  based on the concepts embodied in  FIG. 4 . The gateway  712  includes an interface unit and a control unit, both of which are the same as discussed with reference to  FIG. 4  herein. 
     PV modules  701  are individually coupled to microinverters  702  on a first chain  703 , and PV modules  707  are individually coupled to microinverters  706  on a second chain  708 . The outputs of the microinverters  702  are connected in parallel to each other and, through AC wire  704 , to an AC circuit breaker  705 , and on to AC wiring  711 . The outputs of the microinverters  706  are connected in parallel to each other and, through AC wire  709 , to an AC circuit breaker  710  and on to AC wiring  711 . 
     The AC wiring  711  is connected to the gateway  712 , AC cutoff  713 , and to the grid through AC wiring  714 . Note that components and wiring  704 , wiring  711 , wiring  710 , AC cutoff  713  and wiring  714  are conventional AC electrical components and their selection and installation is consistent with the understanding of one of ordinary skill in the art. 
       FIG. 7  illustrates the scalability of installation using distributed inverters. The concept of hierarchical or replicated gateways suggests itself for much larger capacity systems, as does the concept of additional chains to the same gateway  712 . Without loss of generality, the output of the microinverters can be single-phase AC, split-phase AC or multi-phase AC. 
       FIG. 8  is a block diagram showing a PV system according to another embodiment of the invention. Referring to  FIG. 8 , a distributed inverter includes a gateway  816  and a multiple-string inverter, based on the concepts embodied in  FIG. 5 . The gateway  816  is the same as the gateway as described in  FIG. 5 . Accordingly, the upgradeability, safety features, and enhanced performance benefits may also be similar. A plurality of PV modules  801  are connected in a first series string and coupled to first string inverter  803  via DC wiring  802 . A plurality of PV modules  805  are connected in a second series string and coupled to second string inverter  807  via DC wiring  806 . A plurality of PV modules  809  are connected in a third series string and coupled to third string inverter  811  via DC wiring  810 . 
     The AC output of the first string inverter  803  is coupled through an AC circuit breaker  804  into AC wiring  814 . The AC output of the second string inverter  807  is coupled through an AC circuit breaker  808  into AC wiring  814 . The AC output of the third string inverter  811  is coupled through an AC circuit breaker  812  into AC wiring  814 . 
     The AC wiring  814  is connected to the gateway  816 , AC cutoff  815 , and to the grid through AC wiring  818 . Optionally, the inverters  803 ,  807 ,  811  and the AC circuit breakers  804 ,  808 ,  812  may be placed in a common enclosure  813  to simplify installation and protect the inverters and circuit breakers from environmental effects. The system may be expanded by increasing the number of PV modules in a string, strings, string inverters and AC circuit breakers. 
       FIG. 9  is a block diagram showing a PV system according to another embodiment of the invention. Referring to  FIG. 9 , a distributed converter including a gateway  917  and a multiple inverter, based on the concepts embodied in  FIG. 6 . The gateway  917  is the same as the gateway as described in  FIG. 6 . Accordingly, the upgradeability, safety features, and enhanced performance benefits may also be similar. A plurality of PV modules  901  are connected in a first series string and coupled to a first DC combiner  903  via DC wiring  902 . A plurality of PV modules  906  are connected in a second series string and coupled to a first DC combiner  903  via DC wiring  907 . A plurality of PV modules  908  are connected in a third series string and coupled to a second DC combiner  910  via DC wiring  909 . A plurality of PV modules  913  are connected in a fourth series string and coupled to a second DC combiner  910  via DC wiring  914 . 
     The DC output of the first DC combiner  903  is connected to the DC input of a first inverter  904 . The AC output of the first inverter  904  is coupled through an AC circuit breaker  905  into AC wiring  915 . The DC output of the second DC combiner  910  is connected to the DC input of a second inverter  911 . The AC output of the second inverter  911  is coupled through an AC circuit breaker  912  into AC wiring  915 . 
     The AC wiring  915  is connected to the gateway  917 , AC cutoff  916 , and to the grid through AC wiring  918 . Optionally, the inverters  904 ,  911 , the DC combiners  903 ,  910 , and the AC circuit breakers  905 ,  912  may be placed in a common enclosure (not shown) to simplify installation and to protect them from environmental effects. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.