Abstract:
A method and apparatus is disclosed for intelligently inverting DC power from DC sources such as photovoltaic (PV) solar modules to single-phase or three-phase AC power to supply power for off-grid applications. A number of regular or redundant off-grid Mini-Inverters with one, two, three, or multiple input channels in a mixed variety can easily connect to one, two, three, or multiple DC power sources such as solar PV modules, invert the DC power to AC power, and daisy chain together to generate and supply AC power to electrical devices that are not connected to the power grid including motors, pumps, fans, lights, appliances, and homes.

Description:
INVENTION 
     This application claims priority to U.S. Provisional Application No. 61/495,540 filed on Jun. 10, 2011, which is herein incorporated by reference. 
    
    
     The subject of this patent relates to direct current (DC) to alternating current (AC) power inverters that invert DC power from single or multiple DC power sources to single-phase or three-phase AC power; where the DC power sources include but are not limited to photovoltaic (PV) solar modules, fuel cells, batteries, and other DC power generators. More particularly, this patent relates to a method and apparatus that can intelligently invert DC power generated by single or multiple solar modules to single-phase or three-phase AC power to supply power to electrical devices including but not limited to motors, pumps, fans, lights, appliances, and homes that are not connected to the electrical power grid. 
     In the U.S. patent application Ser. No. 12/837,162, the entirety of which is hereby incorporated by reference, we described the novel smart and scalable power inverters and the unique scalable design so that the DC to AC power inversion system can include as few as one inverter and one DC source, up to a selected number of inverters and multiple DC sources. A number of smart single-input, dual-input, triple-input, quad-input, and multiple-input power inverters in a mixed variety can easily connect to single, dual, triple, quad, and multiple DC power sources, invert the DC power to AC power, and daisy chain together to generate a total power, which is equal to the summation of the AC power supplied by each smart and scalable power inverter. 
     In the U.S. patent application No. 61/442,991, the entirety of which is hereby incorporated by reference, we described the scalable and redundant Mini-Inverters that have double, triple, or quadruple redundant capabilities so that the Mini-Inverters can work in a harsh environment for a prolonged period of time. A number of regular, redundant, triple redundant, or quadruple redundant Mini-Inverters with one, two, three, or multiple input channels in a mixed variety can easily connect to one, two, three, or multiple DC power sources such as solar PV modules, invert the DC power to AC power, and daisy chain together to generate AC power to feed the power grid. 
     In this patent, we expand the invention by introducing the Smart and Scalable Off-Grid Mini-Inverters that not only have the key scalable and redundant features as described in U.S. patent applications Ser. No. 12/837,162 and No. 61/442,991, but can also supply power to electrical devices that are not connected to the power grid including motors, pumps, fans, lights, appliances, and homes. What is more, we are introducing a novel concept: Grid Flexibility. That is, the same family of the smart and scalable power inverters can be designed to include both Grid-tie and Off-grid Mini-Inverters. 
    
    
     
       In the accompanying drawing: 
         FIG. 1  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 2-channel off-grid AC Master Mini-Inverter inverts the DC power from 2 DC sources to single-phase AC power to supply electricity to the AC load. 
         FIG. 2  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 2-channel AC Master Mini-Inverter and one or more 2-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from 2 DC sources to single-phase AC power to supply electricity to the AC load. 
         FIG. 3  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 2-channel AC Master redundant Mini-Inverter and one or more 2-channel off-grid redundant Mini-Inverters daisy chain, each of which inverts the DC power from 2 DC sources to three-phase AC power to supply electricity to the AC load. 
         FIG. 4  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 1-channel AC Master Mini-Inverter and one or more 1-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from one DC source to single-phase AC power to supply electricity to the AC load. 
         FIG. 5  is a block diagram illustrating a smart and scalable off-grid solar power system where one 4-channel AC Master Mini-Inverter and one or more 4-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from 4 solar panels to single-phase AC power to supply electricity to the AC load. 
         FIG. 6  is a block diagram illustrating a smart and scalable off-grid solar power system where one 6-channel AC Master Mini-Inverter and one or more 6-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from 6 solar panels to three-phase AC power to supply electricity to the AC load. 
         FIG. 7  is a block diagram illustrating a smart and scalable off-grid redundant solar power system where one 6-channel AC Master redundant Mini-Inverter and one or more 6-channel off-grid redundant Mini-Inverters daisy chain, each of which inverts the DC power from 6 solar panels to three-phase AC power to supply electricity to the AC load. 
         FIG. 8  is a block diagram illustrating a smart and scalable off-grid solar power system where one 4-channel off-grid redundant Mini-Inverter, one 6-channel AC Master Mini-Inverter, one 8-channel off-grid Mini-Inverter, and a number of off-grid Mini-Inverters in a mixed variety daisy chain to generate single-phase AC power to supply electricity to the AC load. 
         FIG. 9  is a block diagram illustrating a smart and scalable off-grid solar power system where one 1-channel AC Master triple redundant Mini-Inverter, one 5-channel off-grid Mini-Inverter, one 8-channel off-grid Mini-Inverter, and a number of Mini-Inverters in a mixed variety daisy chain to generate three-phase AC power to supply electricity to the AC load. 
         FIG. 10  is a block diagram illustrating a 2-channel off-grid AC Master Mini-Inverter that inverts the DC power from two DC sources to single-phase AC power. 
         FIG. 11  is a block diagram illustrating an m-channel off-grid AC Master Mini-Inverter that inverts the DC power from m DC sources to single-phase AC power. 
         FIG. 12  is a block diagram illustrating an m-channel off-grid regular Mini-Inverter that inverts the DC power from m DC sources to three-phase AC power. 
         FIG. 13  is a block diagram illustrating an m-channel off-grid AC Master Redundant Mini-Inverter that inverts the DC power from m DC sources to single-phase AC power. 
         FIG. 14  is a flow chart describing the main software program running in the digital microcontroller of a smart and scalable off-grid Mini-Inverter, which includes Control &amp; Management tasks, Redundancy tasks, and Communication tasks. 
         FIG. 15  is a flow chart describing the Generation and Synchronization Subroutine, which is invoked by the Power Generation Mechanism of  FIG. 14 , running in the digital microcontroller of a smart and scalable off-grid Mini-Inverter that can be an off-grid AC Master Mini-Inverter or a regular off-grid Mini-Inverter according to this invention. 
     
    
    
     The term “mechanism” is used herein to represent hardware, software, or any combination thereof. The term “solar module” or “solar panel” refers to photovoltaic (PV) solar modules. The term “AC load” is used herein to represent one or more single-phase or three-phase electrical devices including but not limited to motors, pumps, fans, lights, appliances, and homes. The term “AC Master” is used herein to represent a special off-grid Mini-Inverter in a solar power generation system to generate AC power for off-grid applications. An AC Master has the responsibility to be the “leading inverter” to generate AC power to an off-grid powerline to allow the other off-grid Mini-Inverters also connected to the same AC powerline to synchronize the AC power being produced. 
     Throughout this document, m=1, 2, 3, . . . , as an integer, which is used to indicate the number of the DC input ports of a Mini-Inverter. The term “input channel” refers to the DC input port of the Mini-Inverter. Then, an m-channel Mini-Inverter means that the Mini-Inverter has m input channels or m DC input ports. 
     Throughout this document, n=1, 2, 3, . . . , as an integer, which is used to indicate the number of Mini-Inverters that daisy chain in the same power inversion system. 
     Throughout this document, if a power inversion system or a power inverter is used to generate single-phase AC, it can also be applied to three-phase AC without departing from the spirit or scope of our invention. If a power inversion system or a power inverter is used to generate three-phase AC, it can also be applied to single-phase AC without departing from the spirit or scope of our invention. 
     Without losing generality, all numerical values given in this patent are examples. Other values can be used without departing from the spirit or scope of our invention. 
     DESCRIPTION 
       FIG. 1  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 2-channel off-grid AC Master Mini-Inverter inverts the DC power from 2 DC sources to single-phase AC power to supply electricity to the AC load. The off-grid AC Master Mini-Inverter  1  comprises an AC power input port  3 , an AC power output port  4 , and two DC input channels  2 . Each DC power source  5  such as a solar module comprises a DC power connector  6  connecting to a DC input channel  2  of the Mini-Inverter via a DC power cable  7 . The Mini-Inverter&#39;s AC output port  4  is connected to the AC load  9  via the single-phase AC powerline  8 . 
     In a scalable off-grid power inversion system where one or multiple scalable off-grid Mini-Inverters are connected through AC cables, there must exist one and only one AC Master Mini-Inverter according to this invention. If someone connects two or more AC Masters together in the same system, all the AC Masters other than the first one will simply not turn on and will send an error signal. 
     On the other hand, if there is only one Mini-Inverter in a scalable off-grid power inversion system, the inverter has to be an AC Master Mini-Inverter. The AC Master performs the following functions: (1) Checks the impedance of the AC powerline to determine if the connected AC load is within certain specifications; (2) Initially energizes the AC powerline that has no power running to it; (3) Continually delivers AC power to the AC powerline to allow the other off-grid Mini-Inverters also connected on the same powerline to synchronize the AC power being produced; and (4) Continually checks and determines whether the AC load is too large or too small for the power generation system to handle. If it is too large or too small, turns the power off and triggers an error signal. When this happens, all other daisy chained off-grid Mini-Inverters will turn off automatically and immediately. 
       FIG. 2  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 2-channel AC Master Mini-Inverter and one or more 2-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from 2 DC sources to single-phase AC power to supply electricity to the AC load. The system comprises n 2-channel off-grid Mini-Inverters  10  and two times more of the DC power sources  18 , so the total number of DC power sources is 2×n (2 times n). One and only one of the n 2-channel off-grid Mini-Inverters is an AC. Master. In  FIG. 2 , the AC Master  11  is located on the left and is labeled Mini-Inverter  1 . 
     Each off-grid Mini-Inverter  10 , including the AC Master Mini-Inverter  11 , comprises an AC power input port  14 , an AC power output port  16 , and two DC input channels  12 . Each DC power source such as a solar module comprises a DC power connector  20  connecting to a DC input channel  12  of its corresponding Mini-Inverter via a DC power cable  22 . All Mini-Inverters  10  daisy chain, where the first Mini-Inverter&#39;s AC input port  15  is left open, and the last Mini-Inverter&#39;s AC output port  17  is connected to the AC load  28  via the single-phase AC powerline  26 . 
     Throughout this document, n=1, 2, 3, . . . , as an integer, which is used to indicate the number of Mini-Inverters that daisy chain in the same power inversion system. Based on the number of input channels of all the Mini-Inverters in the same power inversion system, there could be a limit to the actual number of Mini-Inverters that can daisy chain. This is because the total generated AC power cannot exceed the limit of the connected AC load within a specification. Otherwise, the “Open Load” situation will occur causing the Mini-Inverters to shut down based on their open-load protection mechanism. 
     Without losing generality, let us say n=5 as an example. That means, five 2-channel scalable off-grid Mini-Inverters can daisy chain, where the AC output port of a Mini-Inverter connects to the AC input port of the next Mini-Inverter, and so on. The first Mini-Inverter&#39;s AC input port is left open, and the last Mini-Inverter&#39;s AC output port is connected to the AC load to supply electricity to the load. This method greatly simplifies the wiring job when installing a PV solar power system. 
     Although we say the Mini-Inverters daisy chain, where the AC output port of each Mini-Inverter is connected to the AC input port of the next Mini-Inverter, the actual connection of the inverters is pass-through. That means, the generated AC power from each Mini-Inverter is added in parallel onto the AC powerline. In this scheme, a defective or low-producing Mini-Inverter will not interfere with other Mini-Inverters that are able to generate usable AC power. Unless the AC powerline is broken, all the healthy Mini-Inverters on the AC powerline will continue to work. For the same reason, the actual location of the AC Master connected on the AC powerline is not important. It can be located at any position in the daisy chain. 
       FIG. 3  is a block diagram illustrating a smart and scalable off-grid power inversion system where one 2-channel AC Master redundant Mini-Inverter and one or more 2-channel off-grid redundant Mini-Inverters daisy chain, each of which inverts the DC power from 2 DC sources to three-phase AC power to supply electricity to the AC load. The system comprises n 2-channel redundant Mini-Inverters  30  and two times more of the DC power sources  38 , so the total number of DC power sources is 2×n (2 times n). Each off-grid Mini-Inverter, including the AC Master redundant Mini-Inverter  31 , comprises an AC power input port  34 , an AC power output port  36 , and two redundant DC input channels  32 . Each DC power source such as a solar module comprises a DC power connector  40  connecting to a redundant DC input channel  32  of its corresponding Mini-Inverter via a DC power cable  42 . All Mini-Inverters  30  daisy chain, where the first Mini-Inverter&#39;s AC input port  35  is left open, and the last Mini-Inverter&#39;s AC output port  37  is connected to a three-phase AC load  48  via the three-phase AC powerline  46 . 
       FIG. 4  is a block diagram illustrating a smart and scalable off-grid power inversion system where one I-channel AC Master Mini-Inverter and one or more 1-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from one DC source to single-phase AC power to supply electricity to the AC load. The system comprises n 1-channel Mini-Inverters  50  and the same number of DC power sources or solar panels  58 . Each Mini-Inverter, including the AC Master Mini-Inverter  51 , comprises an AC power input port  54 , an AC power output port  56 , and one DC input channel  52 . Each DC power source such as a solar panel comprises a DC power connector  60  connecting to a DC input channel  52  of its corresponding Mini-Inverter via a DC power cable  62 . All Mini-Inverters  50  daisy chain, where the first Mini-Inverter&#39;s AC input port  55  is left open, and the last Mini-Inverter&#39;s AC output port  57  is connected to the AC load  68  via the single-phase AC powerline  66 . 
     Please note that a single channel or 1-channel off-grid Mini-Inverter may look like a solar power Microinverter. However, the off-grid design of the Mini-Inverter as well as the AC Master Mini-Inverter are novel. The details will be described in  FIGS. 10 to 15 . 
     In  FIGS. 5 to 9 , we will use solar panels as DC sources as an example. This does not exclude other forms of DC sources such as fuel cells, batteries, etc. 
       FIG. 5  is a block diagram illustrating a smart and scalable off-grid solar power system where one 4-channel AC Master Mini-Inverter and one or more 4-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from 4 solar panels to single-phase AC power to supply electricity to the AC load. The system comprises n 4-channel Mini-Inverters  70  and four times more of the DC power sources or solar panels  78 , so the total number of DC power sources or solar panels is 4×n (4 times n). Each Mini-Inverter, including the AC Master Mini-Inverter  71 , comprises an AC power input port  74 , an AC power output port  76 , and four DC input channels  72 . Each DC power source such as a solar panel comprises a DC power connector  80  connecting to a DC input channel  72  of its corresponding Mini-Inverter via a DC power cable  82 . All Mini-Inverters  70  daisy chain, where the first Mini-Inverter&#39;s AC input port  75  is left open, and the last Mini-Inverter&#39;s AC output port  77  is connected to the AC load  88  via the single-phase AC powerline  86 . 
       FIG. 6  is a block diagram illustrating a smart and scalable off-grid solar power system where one 6-channel AC Master Mini-Inverter and one or more 6-channel off-grid Mini-Inverters daisy chain, each of which inverts the DC power from 6 solar panels to three-phase AC power to supply electricity to the AC load. The system comprises n 6-channel Mini-Inverters  90 ,  92 , . . . ,  94  and six times more of the DC power sources or solar panels, so the total number of DC power sources or solar panels is 6×n (6 times n). Each Mini-Inverter comprises an AC power input port, an AC power output port, and six DC input channels. Each solar panel is connected to a DC input channel of its corresponding Mini-Inverter. 
     With a systematic approach, we will form each Mini-Inverter and its connected solar panels into groups. In Group  1 , the 6-channel off-grid Mini-Inverter  90  is connected to Solar Panels  11 ,  12 ,  13 ,  14 ,  15 , and  16 . In Group  2 , the 6-channel off-grid Mini-Inverter  92  is connected to Solar Panels  21 ,  22 ,  23 ,  24 ,  25 , and  26 . In Group n, the 6-channel AC Master Mini-Inverter  94  is connected to Solar Panels n 1 , n 2 , n 3 , n 4 , n 5 , and n 6 . There could be more groups in between Group  2  and Group n. The actual number will be based on the size of the solar power system as well as the number of the Mini-Inverters that can daisy chain without violating the power limit of the AC load  98 . All Mini-Inverters from Group  1  to Group n daisy chain, where the AC input port of the first Mini-Inverter  94  is left open, and the AC output port of the last Mini-Inverter  90  is connected to the AC load  98  via the three-phase AC powerline  96 . 
       FIG. 7  is a block diagram illustrating a smart and scalable off-grid redundant solar power system where one 6-channel AC Master redundant Mini-Inverter and one or more 6-channel off-grid redundant Mini-Inverters daisy chain, each of which inverts the DC power from 6 solar panels to three-phase AC power to supply electricity to the AC load. In Group  1 , the 6-channel AC Master redundant Mini-Inverter  100  is connected to Solar Panels  11 ,  12 ,  13 ,  14 ,  15 , and  16 . In Group  2 , the 6-channel off-grid redundant Mini-Inverter  102  is connected to Solar Panels  21 ,  22 ,  23 ,  24 ,  25 , and  26 . In Group n, the 6-channel off-grid redundant Mini-Inverter  104  is connected to Solar Panels n 1 , n 2 , n 3 , n 4 , n 5 , and n 6 . There could be more groups in between Group  2  and Group n. The actual number will be based on the size of the solar power system as well as the number of the Mini-Inverters that can daisy chain without violating the power limit of the AC load  108 . All Mini-Inverters from Group  1  to Group n daisy chain, where the AC input port of the first Mini-Inverter  104  is left open, and the AC output port of the last Mini-Inverter  100  is connected to the AC load  108  via the three-phase AC powerline  106 . 
       FIG. 8  is a block diagram illustrating a smart and scalable off-grid solar power system where one 4-channel off-grid redundant Mini-Inverter, one 6-channel AC Master Mini-Inverter, one 8-channel off-grid Mini-Inverter, and a number of off-grid Mini-Inverters in a mixed variety daisy chain to generate single-phase AC power to supply electricity to the AC load. In Group I, the 4-channel off-grid redundant Mini-Inverter  110  is connected to Solar Panels  11 ,  12 ,  13 , and  14 . In Group  2 , the 6-channel AC Master Mini-Inverter  112  is connected to Solar Panels  21 .  22 ,  23 ,  24 ,  25 , and  26 . In Group n, the 8-channel off-grid Mini-Inverter  114  is connected to Solar Panels n 1 , n 2 , n 3 , n 4 , n 5 , n 6 , n 7 , and n 8 . Then, all Mini-Inverters from Group  1  to Group n daisy chain, where the AC input port of the first Mini-Inverter  114  is left open, and the AC output port of the last Mini-Inverter  110  is connected to the AC load  118  via the AC powerline  116 . 
     In a smart and scalable off-grid solar power system, one off-grid AC Master Mini-Inverter or one off-grid AC Master redundant Mini-Inverter as well as multiple off-grid Mini-Inverters and off-grid redundant Mini-Inverters can work together according to this invention. As described above, a mixed variety of off-grid Mini-Inverters and off-grid redundant Mini-Inverters can daisy chain through their AC input and output ports. This is a powerful and user-friendly design which provides scalability and can significantly reduce the total cost of solar power systems. On the other hand, since there must exist one and only one AC Master Mini-Inverter in a scalable off-grid solar power system according to this invention, it is a good idea to design the AC Master Mini-Inverter with redundant capabilities to assure that it can work in a harsh environment for a prolonged period of time. 
       FIG. 9  is a block diagram illustrating a smart and scalable off-grid solar power system where one 1-channel AC Master triple redundant Mini-Inverter, one 5-channel off-grid Mini-Inverter, one 8-channel off-grid Mini-Inverter, and a number of Mini-Inverters in a mixed variety daisy chain to generate three-phase AC power to supply electricity to the AC load. In Group  1 , the 1-channel AC Master triple redundant Mini-Inverter  120  is connected to the Solar Panel  11 . In Group  2 , the 5-channel off-grid Mini-Inverter  122  is connected to Solar Panels  21 ,  22 ,  23 ,  24 , and  25 . In Group n, the 8-channel off-grid Mini-Inverter  124  is connected to Solar Panels n 1 , n 2 , n 3 , n 4 , n 5 , n 6 , n 7 , and n 8 . Then; all Mini-Inverters from Group  1  to Group n daisy chain, where the AC input port of the first Mini-Inverter  124  is left open, and the AC output port of the last Mini-Inverter  120  is connected to a three-phase AC load  128  via the three-phase AC powerline  116 . 
       FIG. 10  is a block diagram illustrating a 2-channel of AC Master Mini-Inverter that inverts the DC power from two DC sources to single-phase AC power. The AC Master Mini-Inverter comprises 2 DC-DC boost converters  133 ,  134 , a DC power combiner  136 , a DC-AC inverter  138 , a load interlace circuit  140 , an internal AC powerline  142 , a load detector  144 , a digital microcontroller  146 , a line sensing circuit  148 , an interface circuit for powerline communications  150 , a powerline communications Modem  152 , a DC power supply  154 , and an external AC powerline  156 . The power from DC sources  131 ,  132  is delivered to the corresponding DC-DC boost converters  133 ,  134 , respectively. The DC power is then combined in the DC power combiner  136 . The total combined DC power is inverted to AC power within a user specified voltage range such as 120 VAC+/−10% or 240 VAC+/−10% by the DC-AC inverter  138 . The generated AC power is sent to the AC load through the load interface circuit  140 , internal AC powerline  142 , load detector  144 , and external AC powerline  156 . A line sensing circuit  148  connected to the AC powerline  142  is used to detect if there is AC power on the powerline prior to the startup of the AC Master Mini-Inverter. The line sensing circuit  148  is also used for monitoring the load on the AC powerline for over voltage, under voltage, over current, or under current conditions so that the total AC output voltage can be regulated to protect the Mini-Inverters in the power generation system as well as the AC load. 
     The DC-DC boost converters that can be used in this embodiment are any of a number of well known converters described in the “Power Electronics Handbook” edited by Muhammad H. Rashid, published by Academic Press in 2007, including Buck Converter, Boost Converter, Buck-Boost Converter, Super-Lift Luo Converter, and Cascade Boost Converter. The DC-AC inverters that can be used in this embodiment are any of a number of well known DC-AC inverters described in the same book including Half-Bridge Inverter, Full-Bridge Inverter, Bipolar PWM Inverter, Unipolar PWM Inverter, and Sinusoidal PWM Inverter. The DC combiners used in this embodiment can be designed with a circuit that allow the output from all DC-DC boost converters to connect in parallel so that all DC currents will be added together. The Powerline Modem that can be used in this embodiment can be any of a number of commercially available integrated circuits capable of providing 2-way digital communications through a powerline. Other modules discussed in this embodiment including load interface, solid state switch, line sensing circuit, powerline interface circuit, and DC power supply can be implemented using one or more known combinations of conventional electronic components such as resisters, capacitors, inductors, solid-state switches, transformers, diodes, transistors, operational amplifiers, and ceramic filters, etc. 
     The load detector  144  as well as the ones to be described in  FIGS. 11 and 13  arc electronic circuits that can detect the impedance of the connected AC load. If no AC power is detected on the powerline, the Load Detector  144  checks the impedance of the AC powerline to determine if the connected AC load is within certain specifications. The Load Detector in this embodiment can be designed using standard LRC meter impedance measurement circuits and mechanism such as those described in the book, “The measurement of Lumped Parameter Impedance: A Metrology Guide” published by University of Michigan Library in January 1974. 
     A powerline communications Modem  152 , which is isolated by an interface circuit  150 , is used to establish a 2-way digital signal communication between the digital microcontroller  146  and the outside world through the AC powerline. 
     The external AC powerline  156  as well as the ones to be described in  FIGS. 11 ,  12 , and  13  is connected to an off-grid AC load. 
     The digital microcontroller  146  as well as the ones to be described in  FIGS. 11 ,  12 , and  13  arc small computers on a single integrated circuit (IC) or a set of ICs that consists of a central processing unit (CPU) combined with functions and peripherals including a crystal oscillator, timers, watchdog, serial and analog I/Os, memory modules, pulse-width-modulation (PWM) generators, and user software programs. A 32-bit high-performance floating-point microcontroller is selected for this application. 
     For a regular off-grid Mini-Inverter discussed in this embodiment, the digital microcontroller is used to perform a number of tasks including (i) monitoring the DC boost voltage from each DC-DC boost converter, (ii) controlling the DC-DC boost converters, (iii) performing maximum power point tracking (MPPT) for each DC source, (iv) performing DC-AC inversion and AC power synchronization, (v) monitoring AC current and voltage for generated power amount and status, (vi) performing powerline communications, (vii) performing logic controls such as AC powerline switching and isolation. Redundant Mini-Inverters also perform redundancy functions, which has been described in the U.S. patent application No. 61/442,991. 
     For an AC Master Mini-Inverter discussed in this embodiment, the digital microcontroller performs the tasks including (i) monitoring the DC boost voltage from each DC-DC boost converter, (ii) controlling the DC-DC boost converters, (iii) performing maximum power point tracking (MPPT) for each DC source, (iv) performing DC-AC inversion, (v) monitoring AC current and voltage for generated power amount and status, (vi) performing powerline communications, (vii) checking the impedance of the AC powerline to determine if the connected AC load is within certain specifications, (viii) initially energizing the AC powerline that has no power running to it, (ix) continually delivering AC power to the AC powerline to allow the other off-grid Mini-Inverters also connected on the same powerline to synchronize the AC power being produced, (x) continually checking and determining whether the AC load is too large or too small for the power generation system to handle, and (xi) turning the power off and triggering an error signal if the load is too large or too small. 
     Model-Free Adaptive (MFA) control software embedded in the digital microcontroller  146  as well as the ones to be described in  FIGS. 11 ,  12 , and  13  can be used to control the DC-DC boost converter to achieve better performance and higher conversion efficiency. MFA optimizers running inside the digital microcontroller can be used to provide maximum power point tracking (MPPT) to allow the Mini-Inverter to achieve optimal power production. The MFA control and optimization technologies have been described in U.S. Pat. Nos. 6,055,524, 6,556,980, 6,360,131, 6,684,115, 6,684,112, 7,016,743, 7,142,626, 7,152,052, 7,415,446, related international patents, and other pending patents. 
     The DC power combiner  136  as well as the ones to be described in  FIGS. 11 ,  12 , and  13  provides adequate power to the DC power supply  154  as well as the ones to be described in  FIGS. 11 ,  12 , and  13 , which supply DC power to the electronic components of the Mini-Inverter. 
       FIG. 11  is a block diagram illustrating an m-channel off-grid AC Master Mini-Inverter that inverts the DC power from m DC sources to single-phase AC power. The Mini-Inverter comprises m DC-DC boost converters  162 ,  163 , . . . ,  164 , a DC power combiner  166 , a DC-AC inverter  168 , a load interface circuit  170 , an internal AC powerline  172 , a load detector  174 , a digital microcontroller  176 , a line sensing circuit  178 , an interface circuit for powerline communications  180 , a powerline communications Modem  182 , a DC power supply  184 , and an external AC powerline  186 . The power from DC sources  158 ,  159 , . . . ,  160  is delivered to the corresponding DC-DC boost converters  162 ,  163 , . . . ,  164 , respectively. The DC power is then combined in the DC power combiner  166 . The total combined DC power is inverted to AC power within a user specified voltage range such as 120 VAC+/−10% or 240 VAC+/−10% by the DC-AC inverter  168 . The generated AC power is sent to the AC load through the load interface circuit  170 , internal AC powerline  172 , load detector  174 , and external AC powerline  186 . A line sensing circuit  178  connected to the AC powerline  172  is used to detect if there is AC power on the powerline prior to the startup of the AC Master Mini-Inverter. The line sensing circuit  178  is also used for monitoring the load on the AC powerline for over voltage, under voltage, over current, or under current conditions so that the total AC output voltage can be regulated to protect the Mini-Inverters in the power generation system as well as the AC load. A powerline communications Modem  182 , which is isolated by an interface circuit  180 , is used to establish a 2-way digital signal communication between the digital microcontroller  176  and the outside world through the AC powerline. 
       FIG. 12  is a block diagram illustrating an m-channel off-grid regular Mini-Inverter that inverts the DC power from m DC sources to three-phase AC power. The. Mini-Inverter comprises m DC-DC boost converters  192 ,  193 , . . . ,  194 , a DC power combiner  196 , a DC-AC inverter  198 , a load interface circuit  200 , an internal AC powerline  202 , a solid-state switch circuit  204 , a digital microcontroller  206 , a line sensing circuit  208 , an interface circuit for powerline communications  210 , a powerline communications Modem  212 , a DC power supply  214 , and an external AC powerline  216 . The power from DC sources  188 ,  189 , . . . ,  190  is delivered to the corresponding DC-DC boost converters  192 ,  193 , . . . ,  194 , respectively. The DC power is then combined in the DC power combiner  196 . The total combined DC power is inverted to AC power within a user specified voltage range such as 120 VAC+/−10% or 240 VAC+/−10% by the DC-AC inverter  198 . The generated AC power is sent to the AC load through the load interface circuit  200 , internal AC powerline  202 , solid-state switch  204 , and external AC powerline  216 . A line sensing circuit  208  connected to the AC powerline  202  is used to detect the phase and zero-crossing point of the AC signal on the AC power line. The phase and zero-crossing point signals are sent to the digital microcontroller  206  for AC power synchronization to assure that the power inverter provides high quality synchronized power to the AC load. The solid-state switch mechanism  204  can be used to automatically disconnect the connections between the Internal AC Powerline  202  and External AC Powerline  216 . 
       FIG. 13  is a block diagram illustrating an m-channel off-grid AC Master Redundant Mini-Inverter that inverts the DC power from m DC sources to single-phase AC power. The Mini-Inverter comprises m DC input channel selectors  222 , m main DC-DC boost converters  224 , m backup DC-DC boost converters  226 , a DC power combiner  228 , a DC-AC inverter  229 , a load interface circuit  230 , an internal AC powerline  232 , a load detector  234 , a digital microcontroller  236 , a line sensing circuit  238 , an interface circuit for powerline communications  240 , a powerline communications Modem  242 , a DC power supply  244 , and an external AC powerline  246 . 
     For each of the in input channels, the power from the DC source  220  is delivered to either the main DC-DC boost converter  224  or the backup DC-DC boost converter  226  through the DC input channel selector  222 . Based on the command from the digital microcontroller  236 , each DC input channel selector  222  can direct the DC power to the selected DC-DC boost converter. The DC power from the in main converters  224  and from the in backup converters  226  is then combined in the DC power combiner  228 . For each of the m input channels, the main converter will supply power to the DC power combiner if it is working and the backup converter produces zero power. If the main converter fails, its corresponding backup converter will be automatically switched to supply power to the DC power combiner. 
     The total combined DC power from the DC power combiner  228  is then inverted to AC power within a user specified voltage range such as 120 VAC+/−10% or 240 VAC+/−10% by the DC-AC inverter  229 . The generated AC power is sent to the AC load through the load interface circuit  230 , internal AC powerline  232 , load detector  234 , and external AC powerline  246 . A line sensing circuit  238  connected to the AC powerline  232  is used to detect if there is AC power on the powerline prior to the startup of the AC Master Mini-Inverter. The line sensing circuit  238  is also used for monitoring the load on the AC powerline for over voltage, under voltage, over current, or under current conditions so that the total AC output voltage can be regulated to protect the Mini-Inverters in the power generation system as well as the AC load. A powerline communications Modem  242 , which is isolated by an interface circuit  240 , is used to establish a 2-way digital signal communication between the digital microcontroller  236  and the outside world through the AC powerline. 
       FIG. 14  is a flow chart describing the main software program running in the digital microcontroller of a smart and scalable off-grid Mini-Inverter, which includes Control &amp; Management tasks, Redundancy tasks, and Communication tasks. At Block  300 , initialization is taking place in the microcontroller device level, peripheral level, system level, and for the interrupt service routine and analog and digital control routines. More specifically, initialization will include but is not limited to setting up registers, I/Os, and timers and enabling interrupts for the interrupt service routine. At the end, it will set Task=1. 
     In the main program, there are three major tasks. Task  1  is related to the control and management of the Mini-Inverter. Task  2  is related to the redundancy of the Mini-Inverter. Task  3  is related to the communications of the Mini-Inverter to the outside world through the powerline Modem. After initialization, the main program enters the main loop entry point  302  and then goes to Block  304 . 
     At Block  304 , the program checks to see if Task  1  is scheduled to run. If the answer is Yes, the program will execute the functions in Block  306  to (i) turn on/off the power generation mechanism based on the conditions of the DC power source(s), the Mini-Inverter, and the AC powerline, (ii) calculate power statistics such as the amount of power generated during a certain period of time, and (iii) perform system diagnosis. Then, it sets Task=2 and returns to Block  302 , which is the entry of the main loop. 
     When the program continues, it will go through Block  304 , and reach Block  308 . At Block  308 , the program checks to see if Task  2  is scheduled to run. If the answer is Yes, the program will execute the functions in Block  310  to run the redundancy routine for each input channel that the Mini-Inverter has. Then, it sets Task=3 and returns to Block  302 . 
     When the program further continues, it will go through Block  304  and  308 , and reach Block  312 . At Block  312 , the program checks to see if Task  3  is scheduled to run. If the answer is Yes, the program will execute the functions in Block  314  to (i) set the unit address for the Mini-Inverter, and (ii) respond to queries from data gathering or acquisition devices to report the power statistics. Then, it sets Task=1 and returns to Block  302 . The main program runs continuously based on a preset loop rate to execute the scheduled tasks. At any time an interrupt is triggered, the digital microcontroller immediately processes the pending interrupt service routine. 
     The key components, functions, and steps in the interrupt service routine embedded in the digital microcontroller are described in the U.S. patent application Ser. No. 12/837,162. 
       FIG. 15  is a flow chart describing the Generation and Synchronization Subroutine, which is invoked by the Power Generation Mechanism at Block  306 , running in the digital microcontroller of a smart and scalable off-grid Mini-Inverter that can be an off-grid AC Master Mini-Inverter or a regular off-grid Mini-Inverter according to this invention. At Block  316 , the subroutine checks to see if the Mini-Inverter is an AC Master. If the answer is Yes, it will go to Block  318  to further check if the AC Master is generating power. If it is, it means the AC Master is working and it will go to Block  326 . If it is Not, the subroutine will go to Block  320  to further check if AC is present on the AC power line. If the answer is Yes, the subroutine will go to Block  324  and send an error signal through the Status LEDs (light-emitting diodes) of the Mini-Inverter to indicate that the off-grid AC Master Mini-Inverter is connected to a power line that has AC power. People may have mistakenly connected the off-grid AC Master Mini-Inverter to the AC grid. Since the AC Master Mini-Inverter has detected this abnormal condition, it will not even start-up to avoid potential problems. The subroutine will exit and then return at the next program cycle. 
     At Block  320 , if the AC Master Mini-Inverter did not see that AC is present, it will go to Block  322  to do the AC load impedance check. If the AC load does not pass the impedance requirement tests based on certain specifications, the subroutine will go to Block  324  and send an error signal through the Status LEDs to indicate that the AC Load has some problems and needs to be checked out. The subroutine will exit and then return at the next program cycle. 
     At Block  322 , if the AC load passes the impedance requirement tests based on certain specifications, the subroutine will go to Block  326 , where the Mini-Inverter will generate digital Sinewave signals based on the internal clock. As discussed, the AC Master&#39;s main job is to produce a lead Sinewave signal for the rest of the off-grid Mini-Inverters to follow. 
     At Block  316 , if the inverter is not an AC Master, it means it is a regular off-grid Mini-Inverter, the subroutine will proceed to Block  328  to see if AC is present. For a regular off-grid Mini-Inverter, it works just like a grid-tie Mini-Inverter. It is connected to an AC powerline where there must exist a leading AC signal before the Mini-Inverter can start to generate power, since its generated AC power needs to be synchronized with the leading AC signal. Therefore, at Block  328 , if the answer is No, it means there is no AC on the powerline. In this case, the subroutine will exit and then return at the next program cycle. 
     At Block  328 , if the answer is Yes, it means there is a leading AC signal. The subroutine will go to Block  330 . At Block  330 , the subroutine gets the AC Zero-Crossing Time of the AC signal. At Block  332 , it synchronizes the internal clock with the AC Zero-Crossing Time. At Block  334 , it gets the present AC Phase; At Block  336 , it generates digital Sinewave signal based on AC Zero-Crossing Time and Phases. Then, the program exits the Generation and Synchronization Subroutine. At this time, the Mini-Inverter is enabled to generate AC power. 
     To summarize, this patent introduces a novel concept: Grid Flexibility. That is, the same family of the smart and scalable power inverters can be designed to include both Grid-tie and Off-grid Mini-Inverters. The Smart and Scalable Off-grid Mini-Inverters described in this patent offer a cost effective clean energy solution for a broad range of off-grid applications and can play an important role in our mission to build a world that is run on clean energies.