Patent Publication Number: US-11658482-B2

Title: Distributed power harvesting systems using DC power sources

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 13/308,517, filed Nov. 30, 2011, and titled “Distributed Power Harvesting Systems Using DC Power Sources,” which is a continuation of U.S. patent application Ser. No. 11/950,271, filed Dec. 4, 2007 (issued as U.S. Pat. No. 9,088,178 on Jul. 21, 2015), and titled “Distributed Power Harvesting Systems Using DC Power Sources,” which claims priority to U.S. Provisional Patent Applications, Ser. No. 60/868,851, filed Dec. 6, 2006, and titled “Distributed Solar Array Monitoring, Management and Maintenance,” Ser. No. 60/868,893, filed Dec. 6, 2006, and titled “Distributed Power Harvesting System for Distributed Power Sources,” 60/868,962, filed Dec. 7, 2006, and titled “System, Method and Apparatus for Chemically Independent Battery,” Ser. No. 60/908,095, filed Mar. 26, 2007, and titled “System and Method for Power Harvesting from Distributed Power Sources,” and Ser. No. 60/916,815, filed May 9, 2007, and titled “Harvesting Power From Direct Current Power Sources,” the entire content of which is incorporated herein by reference. Further, this Application is related to ordinary U.S. patent application Ser. No. 11/950,224 (issued as U.S. Pat. No. 7,900,361 on Mar. 8, 2011) titled “Current Bypass for Distributed Power Harvesting Systems Using DC Power Sources,” “Monitoring of Distributed Power Harvesting Systems Using DC Power Sources,” “Removable Component Cartridge for Increasing Reliability in Power Harvesting Systems,” “Battery Power Delivery Module,” and “A Method for Distributed Power Harvesting Using DC Power Sources” that are filed in at the U.S. Patent and Trademark Office on Dec. 4, 2007 and incorporates the entire content of these applications by this reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The field of the invention relates generally to power production from distributed DC power sources, and more particularly to management of distributed DC power sources in series installations. 
     2. Related Arts 
     The recent increased interest in renewable energy has led to increased research in systems for distributed generation of energy, such as photovoltaic cells (PV), fuel cells, batteries (e.g., for hybrid cars), etc. Various topologies have been proposed for connecting these power sources to the load, taking into consideration various parameters, such as voltage/current requirements, operating conditions, reliability, safety, costs, etc. For example, most of these sources provide low voltage output (normally a few volts for one cell, or a few tens of volts for serially connected cells), so that many of them need to be connected serially to achieve the required operating voltage. Conversely, a serial connection may fail to provide the required current, so that several strings of serial connections may need to be connected in parallel to provide the required current. 
     It is also known that power generation from each of these sources depends on manufacturing, operating, and environmental conditions. For example, various inconsistencies in manufacturing may cause two identical sources to provide different output characteristics. Similarly, two identical sources may react differently to operating and/or environmental conditions, such as load, temperature, etc. In practical installations, different source may also experience different environmental conditions, e.g., in solar power installations some panels may be exposed to full sun, while others be shaded, thereby delivering different power output. In a multiple-battery installation, some of the batteries may age differently, thereby delivering different power output. While these problems and the solutions provided by the subject invention are applicable to any distributed power system, the following discussion turns to solar energy so as to provide better understanding by way of a concrete example. 
     A conventional installation of solar power system  10  is illustrated in  FIG.  1   . Since the voltage provided by each individual solar panel  101  is low, several panels are connected in series to form a string of panels  103 . For a large installation, when higher current is required, several strings  103  may be connected in parallel to form the overall system  10 . The solar panels are mounted outdoors, and their leads are connected to a maximum power point tracking (MPPT) module  107  and then to an inverter  104 . The MPPT  107  is typically implemented as part of the inverter  104 . The harvested power from the DC sources is delivered to the inverter  104 , which converts the fluctuating direct-current (DC) into alternating-current (AC) having a desired voltage and frequency, which is usually 110V or 220V at 60 Hz, or 220V at 50 Hz (It is interesting to note the even in the US many inverters produce 220V, which is then split into two 110V feeds in the electric box). The AC current from the inverter  104  may then be used for operating electric appliances or fed to the power grid. Alternatively, if the installation is not tied to the grid, the power extracted from the inverter may be directed to a conversion and charge/discharge circuit to store the excess power created as charge in batteries. In case of a battery-tied application, the inversion stage might be skipped altogether, and the DC output of the MPPT stage  107  may be fed into the charge/discharge circuit. 
     As noted above, each solar panel  101  supplies relatively very low voltage and current. The problem facing the solar array designer is to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage requires very high currents, which cause large conduction losses on the order of the second power of the current (I 2 ). Furthermore, a power inverter, such as the inverter  104 , which is used to convert DC current to AC current, is most efficient when its input voltage is slightly higher than its output RMS voltage multiplied by the square root of 2. Hence, in many applications, the power sources, such as the solar panels  101 , are combined in order to reach the correct voltage or current. The most common method connects the power sources in series in order to reach the desirable voltage and in parallel in order to reach the desirable current, as shown in  FIG.  1   . A large number of the panels  101  are connected into a string  103  and the strings  103  are connected in parallel to the power inverter  104 . The panels  101  are connected in series in order to reach the minimal voltage required for the inverter. Multiple strings  103  are connected in parallel into an array to supply higher current, so as to enable higher power output. 
     While this configuration is advantageous in terms of cost and architecture simplicity, several drawbacks have been identified in the literature for such architecture. One recognized drawback is inefficiencies cause by non-optimal power draw from each individual panel, as explained below. As explained above, the output of the DC power sources is influenced by many conditions. Therefore, to maximize the power draw from each source, one needs to draw the combination of voltage and current that provides the peak power for the currently prevailing conditions. As conditions change, the combination of voltage and current draw (i.e., the input impedance, Ω=V/I) may need to be changed as well. 
       FIG.  2    illustrates one serial string of DC sources, e.g., solar panels  201   a - 201   d , connected to MPPT circuit  207  and inverter  204 . The current versus voltage (IV) characteristics plotted ( 210   a - 210   d ) to the left of each DC source  201 . For each DC source  201 , the current decreases as the output voltage increases. At some voltage value the current goes to zero, and in some applications may assume a negative value, meaning that the source becomes a sink. Bypass diodes are used to prevent the source from becoming a sink. The power output of each source  201 , which is equal to the product of current and voltage (P=I*V), varies depending on the voltage drawn from the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It is desirable to operate a power generating cell at this maximum power point. The purpose of the MPPT is to find this point and operate the system at this point so as to draw the maximum power from the sources. 
     In a typical, conventional solar panel array, different algorithms and techniques are used to optimize the integrated power output of the system  10  using the MPPT module  107 . The MPPT module  107  receives the current extracted from all of the solar panels together and tracks the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. The MPPT module  107  maintains a current that yields the maximum average power from the overall system  10 . 
     Maximum power point tracking techniques are reviewed in: “Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques” by T. Esram &amp; P. L. Chapman, IEEE Transactions on Energy Conversion (Accepted for future publication, Volume PP, Issue 99, 2006 Page(s):1-1, Digital Object Identifier 10.1109/TEC.2006.874230), the entire content of which is incorporated herein by this reference. 
     However, since the sources  201   a - 201   d  are connected in series to a single MPPT  207 , the MPPT must select a single point, which would be somewhat of an average of the MPP of the serially connected sources. In practice, it is very likely that the MPPT would operate at an I-V point that is optimum to only a few or none of the sources. In the example of  FIG.  2   , the selected point is the maximum power point for source  201   b , but is off the maximum power point for sources  201   a ,  201   c  and  201   d . Consequently, the arrangement is not operated at best achievable efficiency. 
     Turning back to the example of a solar system  10  of  FIG.  1   , fixing a predetermined constant output voltage from the strings  103  may cause the solar panels to supply lower output power than otherwise possible. Further, each string carries a single current that is passed through all of the solar panels along the string. If the solar panels are mismatched due to manufacturing differences, aging or if they malfunction or are placed under different shading conditions, the current, voltage and power output of each panel will be different. Forcing a single current through all of the panels of the string causes the individual panels to work at a non-optimal power point and can also cause panels which are highly mismatched to generate “hot spots” due to the high current flowing through them. Due to these and other drawbacks of conventional centralized methods, the solar panels have to be matched properly. In some cases external diodes are used to bypass the panels that are highly mismatched. In conventional multiple string configurations all strings have to be composed of exactly the same number of solar panels and the panels are selected of the same model and must be install at exactly the same spatial orientation, being exposed to the same sunlight conditions at all times. This is difficult to achieve and can be very costly. 
     Various different topologies have been proposed in order to overcome the above deficiencies of the serial installation. For example, some have proposed to have inverters coupled to each DC source, and connect all of the inverters in parallel. Others have proposed to have DC/DC converter connected to each DC source, and to connect all of the converters serially or in parallel to a central inverter. Among the DC/DC converters proposed for use with the DC sources are boost converter, buck converter, buck-boost converter, or a Cuk converter. It has also been proposed to incorporate MPPT into each DC power source, e.g., into each solar panel, and connect the panels serially. 
     For further discussion of the above issues relating to distributed power sources and solar panels, the reader is highly encouraged to review the following literature, which may or may not be prior art.
     Cascade DC-DC Converter Connection of Photovoltaic Modules, G. R. Walker and P. C. Sernia, Power Electronics Specialists Conference, 2002. (PESC02), Vol. 1 IEEE, Cairns, Australia, pp. 24-29.   Topology for Decentralized Solar Energy Inverters with a Low Voltage AC-Bus, Bjorn Lindgren.   Integrated Photovoltaic Maximum Power Point Tracking Converter, Johan H. R. Enslin et al., IEEE Transactions on Industrial Electronics, Vol. 44, No. 6, December 1997.   A New Distributed Converter Interface for PV Panels, R. Alonso et al., 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain.   Intelligent PV Module for Grid-Connected PV Systems, Eduardo Roman, et al., IEEE Transactions on Industrial Electronics, Vol. 53, No. 4, August 2006. Also in Spanish patent application ES2249147.   A Modular Fuel Cell, Modular DC-DC Converter Concept for High Performance and Enhanced Reliability, L. Palma and P. Enjeti, Power Electronics Specialists Conference, 2007, PESC 2007, IEEE Volume, Issue, 17-21 Jun. 2007 Page(s):2633-2638. Digital Object Identifier 10.1109/PESC.2007.4342432.   Experimental Results of Intelligent PV Module for Grid-Connected PV Systems, R. Alonso et al., Twentyfirst European Photovoltaic Solar Energy Conference, Proceedings of the International Conference held in Dresden, Germany, 4-8 Sep. 2006.   Cascaded DC-DC Converter Connection of Photovoltaic Modules, G. R. Walker and P. C. Semia, IEEE Transactions on Power Electronics, Vol. 19, No. 4, July 2004.   Cost Effectiveness of Shadow Tolerant Photovoltaic Systems, Quaschning, V.; Piske, R.; Hanitsch, R., Euronsun 96, Freiburg, Sep. 16-19, 1996.   Evaluation Test results of a New Distributed MPPT Converter, R. Orduz and M. A. Egido, 22nd European Photovoltaic Solar Energy Conference, 3-7 Sep. 2007, Milan, Italy.   Energy Integrated Management System for PV Applications, S. Uriarte et al., 20th European Photovoltaic Solar Energy Conference, 6-10 Jun. 2005, Barcelona, Spain.   U.S. Published Application 2006/0185727   

     As noted in some of the above cited works, integrating inverters into the individual cells has many drawbacks, including high costs, low safety (especially in solar installations), and reliability. Therefore, serial connection is still preferred, especially for solar panel installations. The proposals for including DC-DC converters and MPPT into the individual sources, and then connect their outputs serially to an inverter are attractive. However, incorporating MPPT into each panel is still problematic in serial application, as each MPPT would attempt to drive its source at different current, while in a serial connection the same current must flow through all of the panels. Moreover, it is unclear what type of DC-DC converter would provide the best results and how to incorporate an MPPT into such an arrangement. Therefore, solutions are still needed for an effective topology for connecting multiple DC power sources to the load, i.e., power grid, power storage bank, etc. 
     As already mentioned above, various environmental and operational conditions impact the power output of DC power sources. In the case of solar panels, solar radiance, ambient temperature, and shading, whether from near objects such as trees or far objects such as clouds, impact the power extracted from each solar panel. Depending on the number and type of panels used, the extracted power may vary widely in the voltage and current. Owners and even professional installers find it difficult to verify the correct operation of the solar system. With time, many other factors, such as aging, dust and dirt collection and module degradation affect the performance of the solar array. 
     The sensitivity of photovoltaic panels to external conditions is even more profound when concentrated photovoltaics (CPV) are used. In such installations, the sun radiation is concentrated by use of lenses or mirrors onto small cells. These cells may be much more efficient than typical PV cells and use a technology knows as double- or triple-junction, in which a number of p-n junctions are constructed one on top of the other—each junction converts light from a certain part of the spectrum and allows the rest to pass-through to the next junction. Thus, these cells are much more efficient (with peak efficiencies of over 40%). Since these cells are expensive, they are usually used in CPV applications which call for smaller cells. However, the power output of CPV installations now depends upon fluctuations in the intensity of different parts of the spectrum of the sun (and not only the total intensity), and imperfections or distortions in the lenses or mirrors used. Thus, having a single MPPT for many panels will lead to significant power loss, and great benefits are realized from using a panel- (or cell-) level MPPT as described in aspects of the present invention. 
     Another field in which traditional photovoltaic installations face many problems is the developing market of building-integrated photovoltaics (BIPV). In BIPV installations, the panels are integrated into buildings during construction—either as roof panels or as structural or additional elements in the walls and windows. Thus, BIPV installations suffer greatly from local partial shading due to the existence of other structural elements in the vicinity of the panels. Moreover, the panels are naturally positioned on many different facets of the building, and therefore the lighting conditions each panel experiences may vary greatly. Since in traditional solutions the panels are stringed together to a joint MPPT, much power is lost. A solution that could harvest more power would obviously be very beneficial in installations of this type. 
     Yet another problem with traditional installations is the poor energy utilization in cases of low sun-light. Most inverters require a certain minimal voltage (typically between 150V to 350V) in order to start functioning. If there is low light, the aggregated voltage from the panels may not reach this minimal value, and the power is thus lost. A solution that could boost the voltage of panels suffering from low light, would therefore allow for the produced energy to be harvested. 
     During installation of a solar array according to the conventional configurations  10 , the installer can verify the correctness of the installation and performance of the solar array by using test equipment to check the current-voltage characteristics of each panel, each string and the entire array. In practice, however, individual panels and strings are generally either not tested at all or tested only prior to connection. This happens because current measurement is done by either a series connection to the solar array or a series resistor in the array which is typically not convenient. Instead, only high-level pass/fail testing of the overall installation is performed. 
     After the initial testing of the installation, the solar array is connected to inverter  104  which optionally includes a monitoring module which monitors performance of the entire array. The performance information gathered from monitoring within the inverter  104  includes integrated power output of the array and the power production rate, but the information lacks any fine details about the functioning of individual solar panels. Therefore, the performance information provided by monitoring at the inverter  104  is usually not sufficient to understand if power loss is due to environmental conditions, from malfunctions or from poor installation or maintenance of the solar array. Furthermore, integrated information does not pinpoint which of solar panels  101  is responsible for a detected power loss. 
     In view of the above, a newly proposed topology for connecting multiple DC power sources to the load should also lend itself to easy testing and operational verification during and after installation. 
     SUMMARY 
     The following summary of the invention is provided in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention, and as such it is not intended to particularly identify key or critical elements of the invention, or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     Aspects of the invention provide a topology for distributed DC power sources serially connected to a central power supplier, e.g., a single inverter or a single converter. Aspects of the invention provide system and a method for monitoring of individual DC power sources in a distributed power harvesting installation and adjusting the current and voltage from each DC power source to maximize power output from each DC power source. 
     According to aspects of the invention, a distributed power harvesting system comprising: a plurality of DC power sources; a plurality of converters, each of the converters comprising: input terminals coupled to a respective DC power source; output terminals coupled in series to the other converters, thereby forming a serial string; a circuit loop setting the voltage and current at the input terminals of the converter according to predetermined criteria; and, a power conversion portion for converting the power received at the input terminals to an output power at the output terminals; and a power supplier coupled to the serial string, the power supplier comprising a control part maintaining the input to the power supplier at a predetermined value. The control part may maintain the input voltage to the power supplier at a predetermined value. The control part may maintain the input current to the power supplier at a predetermined value. The power supplier may comprise a DC/AC inverter. The power supplier may comprise a battery charger. The circuit loop may comprise an MPPT part setting the voltage and current at the input terminals of the converter to maximum power point of the respective DC power source. The power conversion portion may comprise: a buck converter; a boost converter; a controller selectively activating either the buck converter or the boost converter in response to the MPPT part and current or voltage at the output terminals. An inductor may be shared by the buck converter and the boost converter, and the controller comprises a pulse-width modulation portion. The control part may comprise a shunt regulator coupled in parallel with the power supplier and regulating the input voltage to a preselected constant input voltage. The system may further comprise one or more additional serial strings coupled to the power supplier. The system may further comprise: a plurality of current sensors; and a plurality of voltage sensors; wherein each of the current sensors and each of the voltage sensors is coupled between a respective converter and DC power source and providing current information and voltage information to the MPPT part. Each of the plurality of DC power sources may comprise a solar panel or a building integrated solar panel. At least one of the plurality of DC power sources may comprise a fuel cell. At least one of the plurality of DC power sources may comprise a battery. Each of the plurality of converters may further comprise a safety module limiting the output to a preset safe value until a predetermined event has occurred. The predetermined event may comprise one of a load above a preset threshold is applied to the converter or a release signal has been detected. Each of the converters may further comprise a plurality of switching devices, each of the switching devices forming a current bypass to at least one DC power source. The solar panel may comprise a plurality of cell strings, each cell string comprising serially connected solar cells and a switching device coupled to bypass the serially connected solar cells. The switching device may comprise a transistor. Each of the converters may further comprise a monitoring module monitoring and transmitting status related data, the status related data comprising at least one of: input current to the converter, input voltage to the converter, temperature of the power source, input power to the converter, and available illumination. 
     According to an aspect of the invention, a method for harvesting power from a distributed power system having a plurality of DC power sources and a plurality of DC power converters is provided, the method comprising: coupling each of the power sources to a respective DC power converter; coupling the power converters in series, to thereby form at least one serial string; coupling the serial string to a power delivery device; fixing one of input voltage or input current to the power delivery device to a predetermined value, thereby forcing current flowing through the serial string to vary according to power provided by the power sources; and controlling power output from each power source individually and individually varying the input voltage and current to each respective converter according to a predetermined criteria. Fixing one of the input voltage or input current may comprise fixing to a predetermined constant value. Coupling the serial string to a power delivery device may comprise coupling the serial string to a DC/AC inverter and fixing the input voltage to the inverter. Monitoring power output may comprise tracking maximum power point of the power source, and individually varying the input voltage and current comprises setting the input voltage and current so as to draw maximum power from each power source. The method may further comprise individually converting the input voltage and current of each converter to output power at current level dictated by the current flowing through the serial string and at a floating voltage. The method may further comprise individually converting the input voltage and current of each converter to output power at current level dictated by the current flowing through the serial string and at a floating voltage. The method may further comprise monitoring load on each converter individually and limiting power output from each converter to a preset safe level until the load reaches a preset value. The method may further comprise monitoring power output of at least one of the power source and DC power converter and directing current to a bypass when the power output exhibits predetermined characteristics. The method may further comprise individually operating each power converter to monitor and report power related data, the power related data comprising at least one of: input current to the converter, input voltage to the converter, temperature of the power source, input power to the converter, and available illumination. 
     According to aspects of the invention, a solar power installation is provided, comprising: a DC/AC inverter comprising means for maintaining the input voltage or current to the inverter at a predetermined value; a plurality of serial strings coupled in parallel to the DC/AC inverter, each of the serial string comprising: a plurality of solar panels; a plurality of converters, each of the converters comprising: input terminals coupled to a respective solar panel; output terminals coupled in series to the other converters, thereby forming one serial string; an MPPT part setting the voltage and current at the input terminals of the converter according to maximum power point of the respective solar panel; and, a power conversion portion for converting the power received at the input terminals to an output power at the output terminals. The predetermined value may comprise a constant value. The power conversion portion may convert the power received at the input terminals to output power having current substantially equal to the total power provided by the plurality of solar panels in the serial string divided by the predetermined constant voltage at the input of the inverter. The power conversion portion may comprise a power conversion controller controlling pulse width modulation of the power conversion portion so as to output power having current substantially equal to the total power provided by the plurality of solar panels in the serial string divided by the predetermined constant voltage at the input of the inverter. Each of the power conversion portion may comprise: a buck converter; a boost converter; a pulse width modulator; and, a digital controller controlling the pulse width modulator to selectively operate either the buck converter or the boost converter. Each of the serial strings may further comprise: a plurality of current sensors, each measuring current output of one solar panel and sending measured current signal to a respective digital controller; and a plurality of voltage sensors, each measuring voltage output of one solar panel and sending measured voltage signal to a respective digital controller; wherein each digital controller adjusts current and voltage draw to obtain maximum available power. The solar power installation may further comprise a safety module limiting the output voltage to a preset safe value as long as no load above a preset threshold is applied to the converter. The solar power installation of claim  30 , wherein each of the solar panels comprises a plurality of cell strings, each cell string comprising serially connected solar cells and a switching device coupled to bypass the serially connected solar cells. The switching device may comprise a transistor. Each of the converters may further comprise a monitoring module monitoring and transmitting power related data, the power related data comprising at least one of: input current to the converter, input voltage to the converter, temperature of the power source, spatial orientation of the power source, and available illumination. 
     According to aspects of the invention, a method for improving the reliability of components within the load in a distributed power system having a plurality of DC power sources coupled to a central load is provided, comprising: coupling the DC power sources to the central load; maintaining the input to the central load to a fixed predetermined voltage, the voltage being a safe operating voltage for the components within the load; varying the current input to the central load according to the power drawn from the DC power sources. The central load may comprise a DC/AC inverter, and the step of maintaining the input comprises maintaining the input voltage to the inverter. Coupling the DC power sources may comprise coupling each of a plurality of solar panels to a respective converter from a plurality of converters, and coupling all of the converters to the inverter. The method may further comprise operating each converter to boost the voltage obtained from a respective solar panel as soon as the respective solar panel starts to output electrical energy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIG.  1    illustrates a conventional centralized power harvesting system using DC power sources. 
         FIG.  2    illustrates current versus voltage characteristic curves for one serial string of DC sources. 
         FIG.  3    illustrates a distributed power harvesting system, according to aspects of the invention, using DC power sources. 
         FIGS.  4 A and  4 B  illustrate the operation of the system of  FIG.  3    under different conditions, according to aspects of the invention. 
         FIG.  4 C  illustrates an embodiment of the invention wherein the inverter controls the input current. 
         FIG.  5    illustrates a distributed power harvesting system, according to other aspects of the invention, using DC power sources. 
         FIG.  6    illustrates an exemplary DC-to-DC converter according to aspects of the invention. 
         FIG.  7    illustrates a power converter, according to aspects of the invention including control features of the aspects of the invention. 
         FIG.  8    illustrates an arrangement of a solar panel according to the prior art. 
         FIG.  9    illustrates an arrangement according to an embodiment of the invention for reducing the power loss in solar strings. 
         FIG.  10    illustrates another arrangement according to an embodiment of the invention for reducing the power loss in solar strings. 
         FIG.  11    illustrates an arrangement according to an embodiment of the invention for bypassing a solar string. 
         FIGS.  12 A- 12 D  illustrate aspects of the present invention incorporated from U.S. application Ser. No. 60/908,095. 
     
    
    
     DETAILED DESCRIPTION 
     The topology provided by the subject invention solves many of the problems associated with, and has many advantages over, the prior art topologies. For example, the inventive topology enables serially connecting mismatched power sources, such as mismatched solar panels, panel of different models and power ratings, and even panels from different manufacturers and semiconductor materials. It allows serial connection of sources operating under different conditions, such as, e.g., solar panels exposed to different light or temperature conditions. It also enables installations of serially connected panels at different orientations or different sections of the roof or structure. This and other features and advantages will become apparent from the following detailed description. 
     Aspects of the present invention provide a system and method for combining power from multiple DC power sources into a single power supply. According to aspects of the present invention, each DC power source is associated with a DC-DC power converter. Modules formed by coupling the DC power sources to their associated converters are coupled in series to provide a string of modules. The string of modules is then coupled to an inverter having its input voltage fixed. A maximum power point control loop in each converter harvests the maximum power from each DC power source and transfers this power as output from the power converter. For each converter, substantially all the input power is converted to the output power, such that the conversion efficiency may be 90% or higher in some situations. Further, the controlling is performed by fixing the input current or input voltage of the converter to the maximum power point and allowing output voltage of the converter to vary. For each power source, one or more sensors perform the monitoring of the input power level to the associated converter. In some aspects of the invention, a microcontroller may perform the maximum power point tracking and control in each converter by using pulse width modulation to adjust the duty cycle used for transferring power from the input to the output. 
     One aspect of the present invention provides a greater degree of fault tolerance, maintenance and serviceability by monitoring, logging and/or communicating the performance of each solar panel. In one aspect of the invention, the microcontroller that is used for maximum power point tracking, may also be used to perform the monitoring, logging and communication functions. These functions allow for quick and easy troubleshooting during installation, thereby significantly reducing installation time. These functions are also beneficial for quick detection of problems during maintenance work. Aspects of the present invention allow easy location, repair, or replacement of failed solar panels. When repair or replacement is not feasible, bypass features of the current invention provide increased reliability. 
     In one aspect, the present invention relates to arrays of solar cells where the power from the cells is combined. Each converter may be attached to a single solar cell, or a plurality of cell connected in series, in parallel, or both, e.g., parallel connection of strings of serially connected cells. In one embodiment each converter is attached to one panel of photovoltaic strings. However, while applicable in the context of solar power technology, the aspects of the present invention may be used in any distributed power network using DC power sources. For example, they may be used in batteries with numerous cells or hybrid vehicles with multiple fuel cells on board. The DC power sources may be solar cells, solar panels, electrical fuel cells, electrical batteries, and the like. Further, although the discussion below relates to combining power from an array of DC power sources into a source of AC voltage, the aspects of the present invention may also apply to combining power from DC sources into another DC voltage. 
       FIG.  3    illustrates a distributed power harvesting configuration  30 , according to an embodiment of the present invention. Configuration  30  enables connection of multiple power sources, for example solar panels  301   a - 301   d , to a single power supply. In one aspect of the invention, the series string of all of the solar panels may be coupled to an inverter  304 . In another aspect of the invention, several serially connected strings of solar panels may be connected to a single inverter  304 . The inverter  304  may be replaced by other elements, such as, e.g., a charging regulator for charging a battery bank. 
     In configuration  30 , each solar panel  301   a - 301   d  is connected to a separate power converter circuit  305   a - 305   d . One solar panel together with its associated power converter circuit forms a module, e.g., module  302 . Each converter  305   a - 305   d  adapts optimally to the power characteristics of the connected solar panel  301   a - 301   d  and transfers the power efficiently from converter input to converter output. The converters  305   a - 305   d  can be buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters  305   a - 305   d  may also contain a number of component converters, for example a serial connection of a buck and a boost converter. 
     Each converter  305   a - 305   d  includes a control loop that receives a feedback signal, not from the converter&#39;s output current or voltage, but rather from the converter&#39;s input coming from the solar panel  301 . An example of such a control loop is a maximum power point tracking (MPPT) loop. The MPPT loop in the converter locks the input voltage and current from each solar panel  301   a - 301   d  to its optimal power point. 
     Conventional DC-to-DC converters may have a wide input voltage range at their input and an output voltage that is predetermined and fixed. In these conventional DC-to-DC voltage converters, a controller within the converter monitors the current or voltage at the input, and the voltage at the output. The controller determines the appropriate pulse width modulation (PWM) duty cycle to fix the output voltage to the predetermined value by increasing the duty cycle if the output voltage drops. Accordingly, the conventional converter includes a feedback loop that closes on the output voltage and uses the output voltage to further adjust and fine tune the output voltage from the converter. As a result of changing the output voltage, the current extracted from the input is also varied. 
     In the converters  305   a - 305   d , according to aspects of the present invention, a controller within the converter  305  monitors the voltage and current at the converter input and determines the PWM in such a way that maximum power is extracted from the attached panel  301   a - 301   d . The controller of the converter  305  dynamically tracks the maximum power point at the converter input. In the aspects of the present invention, the feedback loop is closed on the input power in order to track maximum input power rather than closing the feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters. 
     As a result of having a separate MPPT circuit in each converter  305   a - 305   d , and consequently for each solar panel  301   a - 301   d , each string  303  in the embodiment shown in  FIG.  3    may have a different number or different brand of panels  301   a - 301   d  connected in series. The circuit of  FIG.  3    continuously performs MPPT on the output of each solar panel  301   a - 301   d  to react to changes in temperature, solar radiance, shading or other performance factors that impact that particular solar panel  301   a - 301   d . As a result, the MPPT circuit within the converters  305   a - 305   d  harvests the maximum possible power from each panel  301   a - 301   d  and transfers this power as output regardless of the parameters impacting the other solar panels. 
     As such, the aspects of the invention shown in  FIG.  3    continuously track and maintain the input current and the input voltage to each converter at the maximum power point of the DC power source providing the input current and the input voltage to the converter. The maximum power of the DC power source that is input to the converter is also output from the converter. The converter output power may be at a current and voltage different from the converter input current and voltage. The output current and voltage from the converter are responsive to requirements of the series connected portion of the circuit. 
     In one aspect of the invention, the outputs of converters  305   a - 305   d  are series connected into a single DC output that forms the input to the load or power supplier, in this example, inverter  304 . The inverter  304  converts the series connected DC output of the converters into an AC power supply. The load, in this case inverter  304 , regulates the voltage at the load&#39;s input. That is, in this example, an independent control loop  320  holds the input voltage at a set value, say 400 volts. Consequently, the inverter&#39;s input current is dictated by the available power, and this is the current that flows through all serially connected DC sources. On the other hand, while the output of the DC-DC converters must be at the inverter&#39;s current input, the current and voltage input to the converter is independently controlled using the MPPT. 
     In the prior art, the input voltage to the load was allowed to vary according to the available power. For example, when a lot of sunshine is available in a solar installation, the voltage input to the inverter can vary even up to 1000 volts. Consequently, as sunshine illumination varies, the voltage varies with it, and the electrical components in the inverter (or other power supplier or load) are exposed to varying voltage. This tends to degrade the performance of the components and ultimately causes them to fail. On the other hand, by fixing the voltage or current to the input of the load or power supplier, here the inverter, the electrical components are always exposed to the same voltage or current and therefore would have extended service life. For example, the components of the load (e.g., capacitors, switches and coil of the inverter) may be selected so that at the fixed input voltage or current they operate at, say, 60% of their rating. This would improve the reliability and prolong the service life of the component, which is critical for avoiding loss of service in applications such as solar power systems. 
       FIGS.  4 A and  4 B  illustrate the operation of the system of  FIG.  3    under different conditions, according to aspects of the invention. The exemplary configuration  40  is similar to configuration  30  of  FIG.  3   . In the example shown, ten DC power sources  401 / 1  through  401 / 10  are connected to ten power converters  405 / 1  through  405 / 10 , respectively. The modules formed by the DC power sources and their corresponding converters are coupled together in series to form a string  403 . In one aspect of the invention, the series-connected converters  405  are coupled to a DC-to-AC inverter  404 . 
     The DC power sources may be solar panels and the example is discussed with respect to solar panels as one illustrative case. Each solar panel  401  may have a different power output due to manufacturing tolerances, shading, or other factors. For the purpose of the present example, an ideal case is illustrated in  FIG.  4 A , where efficiency of the DC-to-DC conversion is assumed to be 100% and the panels  401  are assumed to be identical. In some aspects of the invention, efficiencies of the converters may be quite high and range at about 95%-99%. So, the assumption of 100% efficiency is not unreasonable for illustration purposes. Moreover, according to embodiments of the subject invention, each of the DC-DC converters is constructed as a power converter, i.e., it transfers to its output the entire power it receives in its input with very low losses. 
     Power output of each solar panel  401  is maintained at the maximum power point for the panel by a control loop within the corresponding power converter  405 . In the example shown in  FIG.  4 A , all of the panels are exposed to full sun illumination and each solar panel  401  provides 200 W of power. Consequently, the MPPT loop will draw current and voltage level that will transfer the entire 200 W from the panel to its associated converter. That is, the current and voltage dictated by the MPPT form the input current I in  and input voltage V in  to the converter. The output voltage is dictated by the constant voltage set at the inverter  404 , as will be explained below. The output current I out  would then be the total power, i.e., 200 W, divided by the output voltage V out . 
     As noted above, according to a feature of the invention, the input voltage to inverter  404  is controlled by the inverter (in this example, kept constant), by way of control loop  420 . For the purpose of this example, assume the input voltage is kept as 400V (ideal value for inverting to 220 VAC). Since we assume that there are ten serially connected power converters, each providing 200 W, we can see that the input current to the inverter  404  is 2000 W/400V=5 A. Thus, the current flowing through each of the converters  401 / 1 - 401 / 10  must be 5 A. This means that in this idealized example each of the converters provides an output voltage of 200 W/5 A=40V. Now, assume that the MPPT for each panel (assuming perfect matching panels) dictates VMPP=32V. This means that the input voltage to the inverter would be 32V, and the input current would be 200 W/32V=6.25 A. 
     We now turn to another example, wherein the system is still maintained at an ideal mode (i.e., perfectly matching DC sources and entire power is transferred to the inverter), but the environmental conditions are not ideal. For example, one DC source is overheating, is malfunctioning, or, as in the example of  FIG.  4 B , the ninth solar panel  401 / 9  is shaded and consequently produces only 40 W of power. Since we keep all other conditions as in the example of  FIG.  4 A , the other nine solar panels  401  are unshaded and still produce 200 W of power. The power converter  405 / 9  includes MPPT to maintain the solar panel  501 / 9  operating at the maximum power point, which is now lowered due to the shading. 
     The total power available from the string is now 9×200 W+40 W=1840 W. Since the input to the inverter is still maintained at 400V, the input current to the inverter will now be 1840 W/40V=4.6 A. This means that the output of all of the power converters  405 / 1 - 405 / 10  in the string must be at 4.6 A. Therefore, for the nine unshaded panels, the converters will output 200 W/4.6 A=43.5V. On the other hand, the converter  405 / 9  attached to the shaded panel  401 / 9  will output 40 W/4.6 A=8.7V. Checking the math, the input to the inverter can be obtained by adding nine converters providing 43.5V and one converter providing 8.7V, i.e., (9×43.5V)+8.7V=400V. 
     The output of the nine non-shaded panels would still be controlled by the MPPT as in  FIG.  4 A , thereby standing at 32V and 6.25 A. On the other hand, since the nines panel  401 / 9  is shaded, lets assume its MPPT dropped to 28V. Consequently, the output current of the ninth panel is 40 W/28V=1.43 A. As can be seen by this example, all of the panels are operated at their maximum power point, regardless of operating conditions. As shown by the example of  FIG.  4 B , even if the output of one DC source drops dramatically, the system still maintains relatively high power output by fixing the voltage input to the inverter, and controlling the input to the converters independently so as to draw power from the DC source at the MPP. 
     As can be appreciated, the benefit of the topology illustrated in  FIGS.  4 A and  4 B  are numerous. For example, the output characteristics of the serially connected DC sources, such as solar panels, need not match. Consequently, the serial string may utilize panels from different manufacturers or panels installed on different parts of the roofs (i.e., at different spatial orientation). Moreover, if several strings are connected in parallel, it is not necessary that the strings match, rather each string may have different panels or different number of panels. This topology also enhances reliability by alleviating the hot spot problem. That is, as shown in  FIG.  4 A  the output of the shaded panel  401 / 9  is 1.43 A, while the current at the output of the unshaded panels is 6.25 A. This discrepancy in current when the components are series connected causes a large current being forced through the shaded panel that may cause overheating and malfunction at this component. However, by the inventive topology wherein the input voltage is set independently, and the power draw from each panel to its converter is set independently according to the panels MPP at each point in time, the current at each panel is independent on the current draw from the serially connected converters. 
     It is easily realized that since the power is optimized independently for each panel, panels could be installed in different facets and directions in BIPV installations. Thus, the problem of low power utilization in building-integrated installations is solved, and more installations may now be profitable. 
     The described system could also easily solve the problem of energy harvesting in low light conditions. Even small amounts of light are enough to make the converters  405  operational, and they then start transferring power to the inverter. If small amounts of power are available, there will be a low current flow—but the voltage will be high enough for the inverter to function, and the power will indeed be harvested. 
     According to aspects of the invention, the inverter  404  includes a control loop  420  to maintain an optimal voltage at the input of inverter  404 . In the example of  FIG.  4 B , the input voltage to inverter  404  is maintained at 400V by the control loop  420 . The converters  405  are transferring substantially all of the available power from the solar panels to the input of the inverter  404 . As a result, the input current to the inverter  404  is dependent only on the power provided by the solar panels and the regulated set, i.e., constant, voltage at the inverter input. 
     The conventional inverter  104 , shown in  FIG.  1    and  FIG.  3 A , is required to have a very wide input voltage to accommodate for changing conditions, for example a change in luminance, temperature and aging of the solar array. This is in contrast to the inverter  404  that is designed according to aspects of the present invention. The inverter  404  does not require a wide input voltage and is therefore simpler to design and more reliable. This higher reliability is achieved, among other factors, by the fact that there are no voltage spikes at the input to the inverter and thus the components of the inverter experience lower electrical stress and may last longer. 
     When the inverter  404  is a part of the circuit, the power from the panels is transferred to a load that may be connected to the inverter. To enable the inverter  404  to work at its optimal input voltage, any excess power produced by the solar array, and not used by the load, is dissipated. Excess power may be handled by selling the excess power to the utility company if such an option is available. For off-grid solar arrays, the excess power may be stored in batteries. Yet another option is to connect a number of adjacent houses together to form a micro-grid and to allow load-balancing of power between the houses. If the excess power available from the solar array is not stored or sold, then another mechanism may be provided to dissipate excess power. 
     The features and benefits explained with respect to  FIGS.  4 A and  4 B  stem, at least partially, from having the inverter dictates the voltage provided at its input. Conversely, a design can be implemented wherein the inverter dictates the current at its input. Such an arrangement is illustrated in  FIG.  4 C .  FIG.  4 C  illustrates an embodiment of the invention wherein the inverter controls the input current. Power output of each solar panel  401  is maintained at the maximum power point for the panel by a control loop within the corresponding power converter  405 . In the example shown in  FIG.  4 C , all of the panels are exposed to full sun illumination and each solar panel  401  provides 200 W of power. Consequently, the MPPT loop will draw current and voltage level that will transfer the entire 200 W from the panel to its associated converter. That is, the current and voltage dictated by the MPPT form the input current I in  and input voltage V in  to the converter. The output voltage is dictated by the constant current set at the inverter  404 , as will be explained below. The output voltage V out  would then be the total power, i.e., 200 W, divided by the output current I out . 
     As noted above, according to a feature of the invention, the input current to inverter  404  is dictated by the inverter by way of control loop  420 . For the purpose of this example, assume the input current is kept as 5 A. Since we assume that there are ten serially connected power converters, each providing 200 W, we can see that the input voltage to the inverter  404  is 2000 W/5 A=400V. Thus, the current flowing through each of the converters  401 / 1 - 401 / 10  must be 5 A. This means that in this idealized example each of the converters provides an output voltage of 200 W/5 A=40V. Now, assume that the MPPT for each panel (assuming perfect matching panels) dictates VMPP=32V. This means that the input voltage to the inverter would be 32V, and the input current would be 200 W/32V=6.25 A. 
     Consequently, similar advantages have been achieved by having the inverter control the current, rather than the voltage. However, unlike the prior art, changes in the output of the panels will not cause in changes in the current flowing to the inverter, as that is dictated by the inverter itself. Therefore, if the inverter is designed to keep the current or the voltage constant, then regardless of the operation of the panels, the current or voltage to the inverter will remain constant. 
       FIG.  5    illustrates a distributed power harvesting system, according to other aspects of the invention, using DC power sources.  FIG.  5    illustrates multiple strings  503  coupled together in parallel. Each of the strings is a series connection of multiple modules and each of the modules includes a DC power source  501  that is coupled to a converter  505 . The DC power source may be a solar panel. The output of the parallel connection of the strings  503  is connected, again in parallel, to a shunt regulator  506  and a load controller  504 . The load controller  504  may be an inverter as with the embodiments of  FIGS.  4 A and  4 B . Shunt regulators automatically maintain a constant voltage across its terminals. The shunt regulator  506  is configured to dissipate excess power to maintain the input voltage at the input to the inverter  504  at a regulated level and prevent the inverter input voltage from increasing. The current which flows through shunt regulator  506  complements the current drawn by inverter  504  in order to ensure that the input voltage of the inverter is maintained at a constant level, for example at 400 V. 
     By fixing the inverter input voltage, the inverter input current is varied according to the available power draw. This current is divided between the strings  503  of the series connected converters. When each converter includes a controller loop maintaining the converter input voltage at the maximum power point of the associated DC power source, the output power of the converter is determined. The converter power and the converter output current together determine the converter output voltage. The converter output voltage is used by a power conversion circuit in the converter for stepping up or stepping down the converter input voltage to obtain the converter output voltage from the input voltage as determined by the MPPT. 
       FIG.  6    illustrates an exemplary DC-to-DC converter  605  according to aspects of the invention. DC-to-DC converters are conventionally used to either step down or step up a varied or constant DC voltage input to a higher or a lower constant voltage output, depending on the requirements of the circuit. However, in the embodiment of  FIG.  6    the DC-DC converter is used as a power converter, i.e., transferring the input power to output power, the input voltage varying according to the MPPT, while the output current being dictated by the constant input voltage to the inverter. That is, the input voltage and current may vary at any time and the output voltage and current may vary at any time, depending on the operating condition of the DC power sources. 
     The converter  605  is connected to a corresponding DC power source  601  at input terminals  614  and  616 . The converted power of the DC power source  601  is output to the circuit through output terminals  610 ,  612 . Between the input terminals  614 ,  616  and the output terminals  610 ,  612 , the remainder of the converter circuit is located that includes input and output capacitors  620 ,  640 , backflow prevention diodes  622 ,  642  and a power conversion circuit including a controller  606  and an inductor  608 . 
     The inputs  616  and  614  are separated by a capacitor  620  which acts as an open to a DC voltage. The outputs  610  and  612  are also separated by a capacitor  640  that also acts an open to DC output voltage. These capacitors are DC-blocking or AC-coupling capacitors that short when faced with alternating current of a frequency for which they are selected. Capacitor  640  coupled between the outputs  610 ,  612  and also operates as a part of the power conversion circuit discussed below. 
     Diode  642  is coupled between the outputs  610  and  612  with a polarity such that current may not backflow into the converter  605  from the positive lead of the output  612 . Diode  622  is coupled between the positive output lead  612  through inductor  608  which acts a short for DC current and the negative input lead  614  with such polarity to prevent a current from the output  612  to backflow into the solar panel  601 . 
     The DC power sources  601  may be solar panels. A potential difference exists between the wires  614  and  616  due to the electron-hole pairs produced in the solar cells of panel  601 . The converter  605  maintains maximum power output by extracting current from the solar panel  601  at its peak power point by continuously monitoring the current and voltage provided by the panel and using a maximum power point tracking algorithm. The controller  606  includes an MPPT circuit or algorithm for performing the peak power tracking. Peak power tracking and pulse width modulation, PWM, are performed together to achieve the desired input voltage and current. The MPPT in the controller  606  may be any conventional MPPT, such as, e.g., perturb and observe (P&amp;O), incremental conductance, etc. However, notably the MPPT is performed on the panel directly, i.e., at the input to the converter, rather than at the output of the converter. The generated power is then transferred to the output terminals  610  and  612 . The outputs of multiple converters  605  may be connected in series, such that the positive lead  612  of one converter  605  is connected to the negative lead  610  of the next converter  605 . 
     In  FIG.  6   , the converter  605  is shown as a buck plus boost converter. The term “buck plus boost” as used herein is a buck converter directly followed by a boost converter as shown in  FIG.  6   , which may also appear in the literature as “cascaded buck-boost converter.” If the voltage is to be lowered, the boost portion is substantially shorted. If the voltage is to be raised, the buck portion is substantially shorted. The term “buck plus boost” differs from buck/boost topology which is a classic topology that may be used when voltage is to be raised or lowered. The efficiency of “buck/boost” topology is inherently lower than a buck or a boost. Additionally, for given requirements, a buck-boost converter will need bigger passive components then a buck plus boost converter in order to function. Therefore, the buck plus boost topology of  FIG.  6    has a higher efficiency than the buck/boost topology. However, the circuit of  FIG.  6    continuously decides whether it is bucking or boosting. In some situations when the desired output voltage is similar to the input voltage, then both the buck and boost portions may be operational. 
     The controller  606  may include a pulse width modulator, PWM, or a digital pulse width modulator, DPWM, to be used with the buck and boost converter circuits. The controller  606  controls both the buck converter and the boost converter and determines whether a buck or a boost operation is to be performed. In some circumstances both the buck and boost portions may operate together. That is, as explained with respect to the embodiments of  FIGS.  4 A and  4 B , the input voltage and current are selected independently of the selection of output current and voltage. Moreover, the selection of either input or output values may change at any given moment depending on the operation of the DC power sources. Therefore, in the embodiment of  FIG.  6    the converter is constructed so that at any given time a selected value of input voltage and current may be up converted or down converted depending on the output requirement. 
     In one implementation, an integrated circuit (IC)  604  may be used that incorporates some of the functionality of converter  605 . IC  604  is optionally a single ASIC able to withstand harsh temperature extremes present in outdoor solar installations. ASIC  604  may be designed for a high mean time between failures (MTBF) of more than 25 years. However, a discrete solution using multiple integrated circuits may also be used in a similar manner. In the exemplary embodiment shown in  FIG.  6   , the buck plus boost portion of the converter  605  is implemented as the IC  604 . Practical considerations may lead to other segmentations of the system. For example, in one aspect of the invention, the IC  604  may include two ICs, one analog IC which handles the high currents and voltages in the system, and one simple low-voltage digital IC which includes the control logic. The analog IC may be implemented using power FETs which may alternatively be implemented in discrete components, FET drivers, A/Ds, and the like. The digital IC may form the controller  606 . 
     In the exemplary circuit shown, the buck converter includes the input capacitor  620 , transistors  628  and  630  a diode  622  positioned in parallel to transistor  628 , and an inductor  608 . The transistors  628 ,  630  each have a parasitic body diode  624 ,  626 . In the exemplary circuit shown, the boost converter includes the inductor  608 , which is shared with the buck converter, transistors  648  and  650  a diode  642  positioned in parallel to transistor  650 , and the output capacitor  640 . The transistors  648 ,  650  each have a parasitic body diode  644 ,  646 . 
     As shown in  FIG.  1   , adding electronic elements in the series arrangement may reduce the reliability of the system, because if one electrical component breaks it may affect the entire system. Specifically, if a failure in one of the serially connected elements causes an open circuit in the failed element, current ceases to flow through the entire series, thereby causing the entire system to stop function. Aspects of the present invention provide a converter circuit where electrical elements of the circuit have one or more bypass routes associated with them that carry the current in case of the electrical element fails. For example, each switching transistor of either the buck or the boost portion of the converter has its own bypass. Upon failure of any of the switching transistors, that element of the circuit is bypassed. Also, upon inductor failure, the current bypasses the failed inductor through the parasitic diodes of the transistor used in the boost converter. 
       FIG.  7    illustrates a power converter, according to aspects of the invention.  FIG.  7    highlights, among others, a monitoring and control functionality of a DC-to-DC converter  705 , according to embodiments of the present invention. A DC voltage source  701  is also shown in the figure. Portions of a simplified buck and boost converter circuit are shown for the converter  705 . The portions shown include the switching transistors  728 ,  730 ,  748  and  750  and the common inductor  708 . Each of the switching transistors is controlled by a power conversion controller  706 . 
     The power conversion controller  706  includes the pulse-width modulation (PWM) circuit  733 , and a digital control machine  730  including a protection portion  737 . The power conversion controller  706  is coupled to microcontroller  790 , which includes an MPPT module  719 , and may also optionally include a communication module  709 , a monitoring and logging module  711 , and a protection module  735 . 
     A current sensor  703  may be coupled between the DC power source  701  and the converter  705 , and output of the current sensor  703  may be provided to the digital control machine  730  through an associated analog to digital converter  723 . A voltage sensor  704  may be coupled between the DC power source  701  and the converter  705  and output of the voltage sensor  704  may be provided to the digital control machine  730  through an associated analog to digital converter  724 . The current sensor  703  and the voltage sensor  704  are used to monitor current and voltage output from the DC power source, e.g., the solar panel  701 . The measured current and voltage are provided to the digital control machine  730  and are used to maintain the converter input power at the maximum power point. 
     The PWM circuit  733  controls the switching transistors of the buck and boost portions of the converter circuit. The PWM circuit may be a digital pulse-width modulation (DPWM) circuit. Outputs of the converter  705  taken at the inductor  708  and at the switching transistor  750  are provided to the digital control machine  730  through analog to digital converters  741 ,  742 , so as to control the PWM circuit  733 . 
     A random access memory (RAM) module  715  and a non-volatile random access memory (NVRAM) module  713  may be located outside the microcontroller  790  but coupled to the microcontroller  790 . A temperature sensor  779  and one or more external sensor interfaces  707  may be coupled to the microcontroller  790 . The temperature sensor  779  may be used to measure the temperature of the DC power source  701 . A physical interface  717  may be coupled to the microcontroller  790  and used to convert data from the microcontroller into a standard communication protocol and physical layer. An internal power supply unit  739  may be included in the converter  705 . 
     In various aspects of the invention, the current sensor  703  may be implemented by various techniques used to measure current. In one aspect of the invention, the current measurement module  703  is implemented using a very low value resistor. The voltage across the resistor will be proportional to the current flowing through the resistor. In another aspect of the invention, the current measurement module  703  is implemented using current probes which use the Hall Effect to measure the current through a conductor without adding a series resistor. After translating the current to voltage, the data may be passed through a low pass filter and then digitized. The analog to digital converter associated with the current sensor  703  is shown as the A/D converter  723  in  FIG.  7   . Aliasing effect in the resulting digital data may be avoided by selecting an appropriate resolution and sample rate for the analog to digital converter. If the current sensing technique does not require a series connection, then the current sensor  703  may be connected to the DC power source  701  in parallel. 
     In one aspect of the invention, the voltage sensor  704  uses simple parallel voltage measurement techniques in order to measure the voltage output of the solar panel. The analog voltage is passed through a low pass filter in order to minimize aliasing. The data is then digitized using an analog to digital converter. The analog to digital converter associated with the voltage sensor  704  are shown as the A/D converter  724  in  FIG.  7   . The A/D converter  724  has sufficient resolution to generate an adequately sampled digital signal from the analog voltage measured at the DC power source  701  that may be a solar panel. 
     The current and voltage data collected for tracking the maximum power point at the converter input may be used for monitoring purposes also. An analog to digital converter with sufficient resolution may correctly evaluate the panel voltage and current. However, to evaluate the state of the panel, even low sample rates may be sufficient. A low-pass filter makes it possible for low sample rates to be sufficient for evaluating the state of the panel. The current and voltage date may be provided to the monitoring and logging module  711  for analysis. 
     The temperature sensor  779  enables the system to use temperature data in the analysis process. The temperature is indicative of some types of failures and problems. Furthermore, in the case that the power source is a solar panel, the panel temperature is a factor in power output production. 
     The one or more optional external sensor interfaces  707  enable connecting various external sensors to the converter  705 . External sensors are optionally used to enhance analysis of the state of the solar panel  701 , or a string or an array formed by connecting the solar panels  701 . Examples of external sensors include ambient temperature sensors, solar radiance sensors, and sensors from neighboring panels. External sensors may be integrated into the converter  705  instead of being attached externally. 
     In one aspect of the invention, the information acquired from the current and voltage sensors  703 ,  704  and the optional temperature and external sensors  705 ,  707  may be transmitted to a central analysis station for monitoring, control, and analysis using the communications interface  709 . The central analysis station is not shown in the figure. The communication interface  709  connects a microcontroller  790  to a communication bus. The communication bus can be implemented in several ways. In one aspect of the invention, the communication bus is implemented using an off-the-shelf communication bus such as Ethernet or RS422. Other methods such as wireless communications or power line communications, which could be implemented on the power line connecting the panels, may also be used. If bidirectional communication is used, the central analysis station may request the data collected by the microcontroller  790 . Alternatively or in addition, the information acquired from sensors  703 ,  704 ,  705 ,  707  is logged locally using the monitoring and logging module  711  in local memory such as the RAM  715  or the NVRAM  713 . 
     Analysis of the information from sensors  703 ,  704 ,  705 ,  707  enables detection and location of many types of failures associated with power loss in solar arrays. Smart analysis can also be used to suggest corrective measures such as cleaning or replacing a specific portion of the solar array. Analysis of sensor information can also detect power losses caused by environmental conditions or installation mistakes and prevent costly and difficult solar array testing. 
     Consequently, in one aspect of the invention, the microcontroller  790  simultaneously maintains the maximum power point of input power to the converter  705  from the attached DC power source or solar panel  701  based on the MPPT algorithm in the MPPT module  719  and manages the process of gathering the information from sensors  703 ,  704 ,  705 ,  707 . The collected information may be stored in the local memory  713 ,  715  and transmitted to an external central analysis station. In one aspect of the invention, the microcontroller  790  uses previously defined parameters stored in the NVRAM  713  in order to operate. The information stored in the NVRAM  713  may include information about the converter  705  such as serial number, the type of communication bus used, the status update rate and the ID of the central analysis station. This information may be added to the parameters collected by the sensors before transmission. 
     The converters  705  may be installed during the installation of the solar array or retrofitted to existing installations. In both cases, the converters  705  may be connected to a panel junction connection box or to cables connecting the panels  701 . The converters may be integrated into the panel or the junction box. Each converter  705  may be provided with the connectors and cabling to enable easy installation and connection to solar panels  701  and panel cables. 
     In one aspect of the invention, the physical interface  717  is used to convert to a standard communication protocol and physical layer so that during installation and maintenance, the converter  705  may be connected to one of various data terminals, such as a computer or PDA. Analysis may then be implemented as software which will be run on a standard computer, an embedded platform or a proprietary device. 
     The installation process of the converters  705  includes connecting each converter  705  to a solar panel  701 . One or more of the sensors  703 ,  704 ,  705 ,  707  may be used to ensure that the solar panel  701  and the converter  705  are properly coupled together. During installation, parameters such as serial number, physical location and the array connection topology may be stored in the NVRAM  713 . These parameters may be used by analysis software to detect future problems in solar panels  701  and arrays. 
     When the DC power sources  701  are solar panels, one of the problems facing installers of photovoltaic solar panel arrays is safety. The solar panels  701  are connected in series during the day when there is sunlight. Therefore, at the final stages of installation, when several solar panels  701  are connected in series, the voltage across a string of panels may reach dangerous levels. Voltages as high as 600V are common in domestic installations. Thus, the installer faces a danger of electrocution. The converters  705  that are connected to the panels  701  may use built-in functionality to prevent such a danger. For example, the converters  705  may include circuitry or hardware of software safety module that limits the output voltage to a safe level until a predetermined minimum load is detected. Only after detecting this predetermined load, the microcontroller  790  ramps up the output voltage from the converter  705 . 
     Another method of providing a safety mechanism is to use communications between the converters  705  and the associated inverter for the string or array of panels. This communication, that may be for example a power line communication, may provide a handshake before any significant or potentially dangerous power level is made available. Thus, the converters  705  would wait for an analog or digital release signal (i.e., be slavedly controlled) from the inverter in the associated array before transferring power to inverter. 
     The above methodology for monitoring, control and analysis of the DC power sources  701  may be implemented on solar panels or on strings or arrays of solar panels or for other power sources such as batteries and fuel cells. 
       FIG.  8    illustrates an arrangement of a solar panel according to the prior art. In  FIG.  8   , solar panel  800  comprises solar cells  805 , which are grouped into serially connected strings  810 . The strings  810  are connected together in series. For each string  810 , a bypass diode  820  is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective diode  820  instead of having the cells enter a negative voltage region, which will lead to power dissipation across them and may cause them to burn. However, when current flows through the diodes, they dissipate energy. For example, if a current of 5 A flows through a conventional diode having 0.7 volt cut-in voltage, the loss is 3.5 W. In practice the loss may easily amount to 10 W. 
       FIG.  9    illustrates an arrangement according to an embodiment of the invention for reducing the power loss in solar strings. In  FIG.  9   , the solar panel  900  is made of solar cells  905 , which are grouped into serially connected strings  910 . The strings  910  are connected together in series. For each string  910 , a bypass diode  920  is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective diode  920 . Additionally, one switching device, such as FET or IGBT (insulated gate bipolar transistor),  925  is connected in a by-pass configuration so as to bypass the respective diode. Once it is sensed that current is flowing via one diode  920  (or once the voltage across string  910  is sensed to be negative), its respective switching device  925  is activated. This directs the current through the switching device, so that the loss of energy is drastically reduced. The sensing can be done by, for example, sensing the voltage across the string or the current across the diode. 
       FIG.  10    illustrates another arrangement according to an embodiment of the invention for reducing the power loss in solar strings. In  FIG.  10   , the solar panel  1000  is made of solar cells  1005 , which are grouped into serially connected strings  1010 . The strings  1010  are connected together in parallel. For each string  1010 , a bypass switching device  1025 , such as FET or IGBT, is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective switching device  1025 . Once it is sensed that a string  1010  enters reverse bias (whether due to poor lighting or malfunction), the respective switching device  1025  is turned on so that current is flowing via its respective switching device  1025 . The sensing can be done by, for example, sensing the voltage or current of the string. 
       FIG.  11    illustrates an arrangement according to an embodiment of the invention for bypassing a solar string. That is,  FIG.  11    illustrates how a converter, such as, for example, the converter of  FIG.  6   , may be utilized to trigger the bypass of the solar string and/or a diode coupled across a solar string. In  FIG.  11   , the solar panel  1100  is made of solar cells  1105 , which are grouped into serially connected strings  1110 . The strings  1110  are connected together in parallel. For each string  1110 , a bypass diode  1120  is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective diode  1120 . However, as explained with respect to  FIG.  10   , the diodes may be eliminated. Additionally, one switching device, such as FET or IGBT,  1125  is connected in a by-pass configuration so as to bypass the respective string  1110  and/or diode  1120 . Once it is sensed that a solar string enters reverse bias, its respective switching device  1125  is activated by the controller  906 . This directs the current through the switching device  1125 , so that the loss of energy is drastically reduced. The sensing can be done by, for example, sensing the voltage across the string or the current across the diode, as explained with respect to elements  703  and  704  of  FIG.  7   . 
       FIGS.  12 A- 12 D  and the following excerpts are incorporated from U.S. Provisional Application 60/908,095:
         a. Safety Measures: One of the problems facing installers of PV systems is safety. Since all panels are connected in series and work is done during the day when there is sunlight, at the final stages of installation— when many panels are connected in series— the voltage across the panels might reach dangerous levels (voltages as high as 600V are common in domestic installations). Thus, the installer faces a real danger of electrocution.   b. In order to prevent such a risk in our proposed solution, the modules connected to the panels may use built-in functionality to prevent such danger. For example, the modules may limit the output voltage to a low (and thus, safe) value as long as it does not detect current drawn from the inverter. Only after detecting such power requirement, it would ramp-up the output voltage.   c. Another way to provide such a safety measure would be to use the communication ability between the modules and the inverter (e.g. power line communication) to provide a handshake which will be required before any significant (read— potentially harmful) amount of power is transmitted over the line. Thus, the modules would wait for a predetermined message from the inverter before transferring power.   d. Inverter: The distributed power harvesting specification describes, in addition to the power converting modules, the use of a novel inverter which includes a shunt regulator to dissipate any excess power that may be produced by the PV panels (or any other DC sources). It may be noted, that in a case where there is usage of all power produces bv the array, also a standard inverter may be used successfully. This is the case, for example, where any excess power may be sold back to the utility company and send to the grid. Note that in this case the MPPT functionality of the inverter is not necessary.   e. Furthermore, measures can be taken in the modules to enable use with standard inverter. For example, the module might monitor the voltage at its output, and in case it notices the voltage rises above a predetermined level, stops transferring some of the power from the PV panel to its output. Thus, only the amount of power needed at the input of the inverter is sent, and all excess power is dissipated across the solar panels.   f. The present invention converts the input power of all power sources to its output. In cases where not all power is needed by the load, the excess power can be used to charge batteries in off grid applications. In grid connected application the excess power can be sold back to the power utility company. In cases where both options are not available a shunt regulator is used to dissipate the excess power and ensure that the output voltage does not rise above the determined threshold.   g. To enable the inverter to work at its optimal input voltage the excess power must be dissipated. This can be achieved by selling the excess power to the utility company if possible. Another possible option is to store the excess energy in batteries. This is especially useful in off grid solar arrays. The shunt regulator is configured to dissipate excess power if the power is not stored or soled. This is achieved by allowing current to flow through the shunt regulator once the voltage increases over the inyerters maximum input voltage. The current which flows through the shunt regulator will always complement the inyerters current. This will ensure that the input voltage of the inverter is constant.   h. The MPPT module is an up/down DC-DC converter with a control loop closed on the input power level. Usually the control loop has medium bandwidth and can track power changes in the array relatively fast. The control loop has certain tracking parameters that are changed at low bandwidth to optimally adapt for slow environmental changes (such as temperature, cell degradation, etc.). Since the control loop monitors the power input, the output voltage of the converter is variable and dependent of the power level transferred through the module and the output load (i.e., the current through all the modules output). The entire system&#39;s feedback loop is closed through the shared output current (the inyerters input current). This allows for a fixed voltage at the inyerters input. For example, suppose a 20 100W panels installation. Should we require a fixed 400V at the inverter&#39;s input, the inverter will serve as a current source with current that generates a 400V input voltage (Total power is 2000W. Total current is 2000/400=5A. Each module&#39;s output voltage is 100W/5A=20V).   i. Example 1: An electronic system for maximizing electric power, comprising: a. a direct current source, b. a voltage conyerting electronic module connected to said direct current source, c. said module containing means for maximizing the power output of said current source, d. said module containing output terminals, whereby said system extracts maximum peak power from said direct current source and produces direct current through said output terminals.   j. Example 2: The system of example 1 wherein said direct current source is selected from the group consisting of a photovoltaic cell and a plurality of connected photovoltaic cells.   k. Example 3: The system of example 1 wherein said direct current source is selected from the group consisting of a battery and a plurality of connected batteries.   l. Example 4: The system of example 1 wherein said direct current source is selected from the group consisting of a fuel cell and a plurality of connected fuel cells.   m. Example 5: A plurality of systems described in example 1, wherein said systems are connected in series.   n. Example 6: An installation, comprising: a. the serially connected systems of example 5, b. an inverter, said inyerter comprising of: i: direct current input terminals, ii: alternating current output terminals, iii: said input terminals connected to means of converting direct current to alternating current, said alternating current connected to said output terminals, c. said serially connected systems are connected to said inverters input terminals, d. said inyerters output terminals connected to an alternating current load, whereby said installation utilizes said direct current sources to produce alternating current.   o. Example 7: The installation of example 6, wherein said inverter has a maximum peak power tracking unit.   p. Example 8: The installation of example 6, wherein said inverter has a means of dissipating power not needed by said alternating current load.   q. Example 9: The installation of example 8, wherein said means of dissipating power is a shunt regulator.   r. Example 10: The system of example 1 wherein said module further contains safety means for prevention of electrocution.   s. Example 11: A plurality of systems described in example 10, wherein said systems are connected in series.   t. Example 12: The system of example 1 wherein said module further contains means for bypassing said module in case an event selected from the group consisting of a failure in said module and a failure in said direct current source.   u. Example 13: The system of example 12, wherein said voltage converting module uses a buck converter and a boost converter.   v. Example 14: The system of example 12, wherein said voltage converting module uses a push-pull converter.   w. Example 15: The system of example 12, wherein said voltage converting module uses a flyback converter.   x. Example 16: The system of example 1 wherein said module is comprised of an application specific integrated circuit, and discrete electronic and magnetic components.   y. Example 17: The system of example 1 wherein said module is comprised of a plurality of application specific integrated circuits, and discrete electronic and magnetic components.   z. Example 18: The system of example 1 wherein said module uses a single direct current conversion providing maximum peak power harvesting from said direct current source, whereby said modules could be connected in series to provide overall maximum power harvesting.       

     The following excerpts are incorporated from U.S. Provisional Application 60/916,815, with reference designators updated to refer to the numbering in the pending figures.
         a. The term “substantially” in the context of “substantially all input power is converted to output power” refers to high power conversion efficiency greater than ninety per cent   b. The term “microcontroller” as used herein refers to a means of controlling operation of a circuit or algorithm, whether by use of central processing unit (CPU), a digital signal processing (DSP) unit, a state machine either based on discrete components, an FPGA an integrated circuit (1C), or an analog circuit.   c. Converter 605 includes a control mechanism and PW.M controller 606, which controls a buck converter or a boost converter. Either the buck or boost converter could be used at any given time, at the discretion of the controller. If buck conversion is used, transistor 650 is left constantly short and transistor 648 is left constantly disconnected, effectively bypassing the boost converter. Similarly, if boost conversion is used, transistor 630 is left constantly short and 628 is left constantly disconnected, effectively bypassing the buck converter.   d. One of the problems facing installers of photovoltaic solar panel arrays is safety. Since solar panels 101 are connected in series during the day when there is sunlight, at the final stages of installation— when many panels 101 are connected in series— the voltage across panels 101 may reach dangerous levels. Voltages as high as 600V are common in domestic installations. Thus, the installer faces a real danger of electrocution. In order to prevent such a risk, modules 405 connected to panels 101 may use built-in functionality to prevent such a danger. For example, modules 101 may limit the output voltage to a low (and thus safe) level until a predetermined minimum load is detected. Only after detecting this predetermined power requirement, does microcontroller 790 ramp-up output voltage.   e. Another way to provide such a safety mechanism is to use communications between modules 405 and inverter 404 (e.g., power line communication) to provide a handshake which is required before any significant or potentially dangerous power level is available. Thus, modules 205 would wait for an analog or digital signal from inverter 404 before transferring power to inverter 404.   f. Example 1: A system for combining power from a plurality of direct-current electrical power sources, the system comprising: (a) a plurality of electrical power converters, wherein said power sources are connected respectively as inputs to said electrical power converters, wherein each said electrical power converter converts input power to output power by monitoring and controlling said input power at a maximum power level; wherein respective outputs of said electrical power converters are series connected into at least one series-connected direct-current output; and (b) an inverter which inverts said at least one series-connected direct-current output into an alternating-current output, said inverter controlling voltage of said at least one series-connected direct-current output at a previously-determined voltage by varying the amount of current drawn from said at least one series-connected direct-current output   g. Example 2: The system, according to example 1, wherein all components of said electrical power converters have a current bypass path on failure, whereby upon failure of one component of at least one of said electrical power converters and said at least one electrical power converter becoming a failed electrical power converter, current from all other said electrical power converters flows through said failed electrical power converter.   h. Example 3: The system, according to example 1, whereby for each said electrical power converter, substantially all said input power is converted to said output power, and said controlling is performed by allowing output voltage to vary.   i. Example 4: The system, according to example 3, further comprising: (c) a microcontroller which performs said controlling by adjusting duty cycle using pulse width modulation.   j. Example 5: The system, according to example 1, further comprising: (c) a shunt regulator electrically connected between said at least one series-connected direct-current output and said inverter, said shunt regulator configured to dissipate any electrical power in excess of electrical power required by a load connected to said alternating-current output.   k. Example 6: The system, according to example 1, wherein the direct-current electrical power sources are selected from the group consisting of: solar cells, solar panels, electrical fuel cells and electrical batteries.   l. Example 7: The system, according to example 1, further including for each said power source at least one sensor for performing said monitoring and said controlling of said input power, said at least one sensor selected from the group of sensors consisting of: a current sensor which senses current from said power source, a voltage sensor which senses voltage of said power source, a temperature sensor which senses temperature of said power source, a luminance sensor, a current sensor of the module output, and a voltage sensor of the module output.   m. Example 8: The system, according to example 1, wherein said at least one series-connected direct-current output is a plurality of series-connected direct-current outputs connected in parallel to said inverter.   n. Example 9: The system, according to example 7, further comprising: (c) a microcontroller which performs said monitoring and controlling of said input power wherein said at least one sensor is operatively connected to said microcontroller.   o. Example 10: The system, according to example 9, further comprising: (d) a memory for logging at least one datum resulting from said at least one sensor.   p. Example 11: The system, according to example 9, further comprising: (d) a communications interface for transferring at least one datum resulting from said at least one sensor to a central monitoring facility.   q. Example 12: The system, according to example 1, further comprising: (c) a safety mechanism attached to at least one of said electrical power converters which limits said output power when said inverter is not drawing substantial current.   r. Example 13: A method for combining power from a plurality of direct-current electrical power sources, the method comprising the steps of: (a) connecting the power sources respectively as inputs to a plurality of electrical power converters; (b) for each of said electrical power converters, conyerting input power to output power by monitoring and controlling said input power at a maximum power level; (c) connecting in series respective outputs of said electrical power converters into at least one series-connected direct-current output; and (d) inverting said at least one series-connected direct-current output into an alternating-current output, by controlling voltage of said at least one series-connected direct-current output at a previously-determined minimal voltage by varying the amount of current drawn from said at least one series-connected direct-current output.   s. Example 14: The method, according to example 13, whereby for each said electrical power converter, substantially all said input power is converted to said output power, and said controlling is performed by allowing output voltage to vary.   t. Example 15: The method, according to example 13, wherein all components of said electrical power converters have a current bypass path on failure, wherebv upon failure of one component of at least one of said electrical power converters and said at least one electrical power converter becoming a failed electrical power converter, current from all other said electrical power converters flows through said failed electrical power converter.   u. Example 16:A direct-current (DC)-to-DC electrical power converter which converts input power from a power source to output power by monitoring and controlling said input power at a maximum power level of said power source; wherein all components of said electrical power converter have a current bypass path on failure, whereby upon failure of one component of said electrical power converter wherein said electrical power converter becomes a failed electrical power converter, substantially all current from an external current source flows through said failed electrical power converter despite said failure.   v. Example 17: An electronic system for maximizing electric power, comprising: (a) a direct current source; (b) a power converting electronic module connected to said direct current source; and (c) said module including: (i) means for maximizing the power output of said current source; (ii) output terminals; whereby the system maximizes power from said direct current source and outputs direct current through said output terminals.   w. Example 18: The electronic system, according to example 17, wherein said module includes a direct current power converter selected from the group consisting of buck and boost converters.   x. Example 19: The electronic system, according to example 17, further comprising: (d) a series connection to another said electronic system, thereby producing at least one series-connected direct-current output.   y. Example 20: The electronic system, according to example 19, further comprising: (e) a means for controlling voltage of said at least one series-connected direct-current output at a previously determined minimal voltage by varying the amount of current drawn from said at least one series-connected direct-current output.       

     The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.