Abstract:
Circuitry and a method for maximizing power from multiple serially coupled DC power sources. Electrodes provide for serial coupling of multiple DC power sources, capacitive circuitry couples to each one of the electrodes, switching circuitry couples among various ones of the electrodes, and inductive circuitry couples among various others of the electrodes and the switching circuitry.

Description:
RELATED APPLICATIONS 
     This patent application is a continuation of U.S. patent application Ser. No. 12/796,489, filed Jun. 8, 2010, which claims priority from U.S. provisional patent application 61/185,264, filed Jun. 9, 2009, and entitled “Solar Array Inverter with Per-Panel Maximum Power Point Tracking”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to circuits and methods for harvesting power from energy conversion circuits, and in particular to such circuits and methods for maximizing power harvested from serially coupled DC power sources. 
     BACKGROUND 
     As is well known in the art, central inverter based energy conversion installations use groups of DC power sources connected in series, sometimes called strings. Examples include biomass, chemical, fuel cells and energy storage batteries. However, long strings of such sources have inherent deficiencies. For example, in the case of solar panels, if panels are shaded or obscured, they generate less photocurrent. Panel currents also may be unbalanced for other reasons. Because solar panels in a series string are constrained to all conduct the same current, the weakest one will limit the power harvest from the remaining solar panels, thereby reducing the overall efficiency of the array. For similar reasons, solar panels in a string should be given the same orientation and be of identical type and size. This is not always possible or desirable for aesthetic or other architectural reasons. 
     Another well-known approach is the microinverter. Microinverters convert solar energy from a single solar panel for feeding to the commercial AC power grid. Although MPPT is done on a per panel basis, thereby maximizing energy harvesting, cost effectiveness of microinverter installations are low due to higher cost per watt and lower efficiency of the electrical conversion. 
     SUMMARY 
     In accordance with the presently claimed invention, circuitry and a method are provided for maximizing power from multiple serially coupled DC power sources. Electrodes provide for serial coupling of multiple DC power sources, capacitive circuitry couples to each one of the electrodes, switching circuitry couples among various ones of the electrodes, and inductive circuitry couples among various others of the electrodes and the switching circuitry. 
     In accordance with one embodiment of the presently claimed invention, circuitry for maximizing power from multiple DC power sources includes: a plurality of electrodes, including a plurality of intermediate electrodes between first and last electrodes, for serially coupling a plurality of DC power sources; capacitive circuitry coupled to each one of the plurality of electrodes; first switching circuitry coupled to at least one of the first and last electrodes and at least one of the plurality of intermediate electrodes; first inductive circuitry coupled between a first portion of the plurality of intermediate electrodes and the first switching circuitry; second switching circuitry coupled to at least one of the first and last electrodes; and second inductive circuitry coupled between a second portion of the plurality of intermediate electrodes and the second switching circuitry. 
     In accordance with another embodiment of the presently claimed invention, a method for maximizing power from multiple DC power sources includes: serially coupling a plurality of DC power sources with a plurality of electrodes, including a plurality of intermediate electrodes between first and last electrodes; capacitively coupling to each one of the plurality of electrodes; switchably and inductively coupling at least one of the first and last electrodes and a first portion of the plurality of intermediate electrodes; and switchably and inductively coupling at least one of the first and last electrodes and a second portion of the plurality of intermediate electrodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a power balancer in accordance with one embodiment. 
         FIG. 2  illustrates operation of the power balancer of  FIG. 1 . 
         FIG. 3  is an exemplary signal timing diagram for the switch control signals used in the power balancer of  FIG. 1 . 
         FIG. 4  illustrates full and zoomed graphs of current-voltage and power-voltage characteristic curves for a typical solar panel. 
         FIG. 5  is a schematic diagram of a power balancer module implementation with multiple power balancers connected in series in accordance with another embodiment. 
         FIG. 5   a  is a schematic diagram of a power balancer module implementation with multiple power balancers connected in series in accordance with another embodiment. 
         FIG. 5   b  is a block diagram of an exemplary embodiment of the control unit of  FIG. 5   a.    
         FIG. 5   c  is a flowchart of exemplary embodiments of processes running in the control unit of  FIG. 5   b.    
         FIG. 6  is a block diagram of four power balancer modules coupled to provide a DC current to be converted into AC current by a DC/AC converter in accordance with another embodiment. 
         FIG. 6   a  is a block diagram of a distributed inverter system architecture in accordance with another embodiment. 
         FIG. 7  is a schematic diagram of a DC/AC converter for a 3-phase power grid in accordance with another embodiment. 
         FIG. 7   a  is a schematic diagram of a DC/AC converter for a single-phase power grid in accordance with another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
     Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, absent a clear indication otherwise, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. Moreover, to the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. Thus, for example, one or more of the functional blocks (e.g., processors, memories, etc.) may be implemented in a single piece of hardware (e.g., a general purpose signal processor, random access memory, hard disk drive, etc.). Similarly, any programs described may be standalone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, etc. 
     As discussed in more detail below, the presently claimed invention maximizes harvesting of power from serially coupled DC power sources. As a means for presenting an example application, the discussion that follows is based on an example of conversion of solar energy into AC current to be fed into commercial power grids, with increased conversion efficiency and cost effectiveness of the conversion. Higher solar energy harvesting is achieved using MPPT for each solar panel. Based on published statistical data, such per-module MPPT contributes to energy harvest gains of 5-25% for typical solar installations. Maintenance of solar installations additionally benefits from continuous monitoring of the performance of each photovoltaic module. A further advantage is an absence of high DC voltage wiring on a roof. 
     Referring to  FIG. 1 , an important feature of the presently claimed invention is a power balancer. As shown for this exemplary embodiment, two solar panels PV 1   100 , PV 2   101  are connected in series. If they have identical characteristics, a MPPT converter will find a point at which both panels deliver maximum power. If they are not identical, the weaker panel will limit the power harvested from the stronger one. For example, if one panel PV 1  is shaded and produces one ampere less current than the other panel PV 2 , the power balancer allows a MPPT converter to harvest more power from both panels, e.g., by compensating for the mismatch between the panels. 
     In accordance with one embodiment, the power balancer includes capacitors C 1   102 , C 2   103 , an inductor L 1   104 , switches SW 1   105 , SW 2   106  (e.g., metal oxide semiconductor field effect transistors (MOSFETs)), and a control unit  107 , interconnected substantially as shown. If the switches  105 ,  106  are operated with a 50% duty cycle, the power balancer will maintain substantially equal voltages on the panels PV 1 , PV 2  independent from the currents produced by the panels PV 1 , PV 2 . 
     Suppose panel PV 1  produces current I 1  and panel PV 2  produces I 2 =I 1 +I delta . If the power balancer is not used, a MPPT converter would receive total current I 1  and I delta  would not be recovered, or harvested. The purpose of the power balancer is to harvest this additional current I delta  from panel PV 2  and deliver the extra power associated with it to the MPPT. 
     Referring to  FIG. 2 , operation of the power balancer can be compared to that of a classic boost converter. During the interval that switch SW 2   200  is closed, switch SW 1   203  is open and the current difference I 2 −I 1 =I delta  is conducted via PATH 1   202  and energy accumulates in the magnetic field of the inductor L 1   201 . During the interval that switch SW 1   203  is closed, switch SW 2   200  is open and current is conducted via PATH 2   205  and the energy previously stored in the inductor L 1   201  is delivered as additional current (having an average magnitude I added  related to the difference current I delta  of the solar panels PV 1 , PV 2 , and ideally equal to I delta *V PV2   mpp /(V PV1   mpp +V PV2   mpp ) to the MPPT converter  204 . Thus the power balancer converts only the unbalanced portion of energy  206  but does not affect the balanced portion of the energy associated with current I 1 , which goes directly to the MPPT converter  204 . Accordingly, the efficiency of energy conversion in the power balancer is substantially proportional to the magnitude of the unbalanced portion of energy  206 . The capacitance values of the capacitors C 1   204 , C 2   203  are not critical, but should be selected based on criteria including switching frequency of the switches SW 1   105 , SW 2   106  and maximum allowed panel mismatch. Also, the capacitance values should be sufficiently high to ensure that the voltage ripple across them does not cause the voltages across the solar panels PV 1 , PV 2  to deviate too much from the maximum power point (MPP) voltages of the panels PV 1 , PV 2 . This will maintain close to maximum energy harvesting from the solar panels. For a typical crystalline silicon solar panel, 99.5% energy harvest will be achieved if voltage ripples are about 5% (peak-to-peak) from panel MPP voltages. 
     Referring to  FIG. 4 , further improvement in power harvesting can be obtained by optimizing voltages individually for each panel. Graph  400  illustrates current-voltage and power-voltage characteristic curves for a typical solar panel. For example, the “PV 1  I-V curve” corresponds to panel PV 1   100 , which is shaded, while the “PV 2  I-V curve” corresponds to panel PV 2   101 , which is not shaded. Zoomed graph  401  shows that maximum power for panel PV 2  is reached at a slightly higher voltage than for panel PV 1 , e.g., V PV2   mpp    402  and V PV1   mpp    403 , respectively. This optimization is performed by a secondary (e.g., slower) optimization loop in the control unit  107  ( FIG. 1 ) based on measurements of the voltages V 1    108 , V 2    109  and currents I 1    110 , I 2    111  (discussed in more detail below). 
     Referring to  FIG. 3 , the control unit  107  sets the duty cycle of the control signals for opening and closing the switches SW 1 , SW 2  as follows: T 1 /T 2 =V PV2   mpp /V PV1   mpp , where V PV1   mpp  and V PV2   mpp  are optimal voltages on panels PV 1  and PV 2 , respectively, and T 1  and T 2  are pulse widths of the control signals SW 1    300 , SW 2    301 , respectively. 
     Referring to  FIG. 5 , in accordance with another embodiment, a similar technique can be applied to maximize the power harvested from more than two solar panels, e.g., in a multi-balancer power harvest maximizer module implementation having four solar panels PV 1   500 , PV 2   501 , PV 3   502 , PV 4   503  connected in series. Three power balancers are used to balance the four panels, which are arranged as two balanced pairs. Power balancing of each pair of solar panels PV 1 +PV 2 , PV 3 +PV 4 , operates as described above. The third power balancer balances the two pairs of solar panels PV 1 +PV 2 , PV 3 +PV 4 . As a result, all four panels get balanced and provide power for the MPPT converter  504 . The MPPT converter  504  includes an inductor L 4   505 , a switch SW 7   506  (e.g., a MOSFET), a diode D 1   507 , and a capacitor C 5   508 , interconnected substantially as shown, which operate as an efficient boost converter. 
     The control unit  509  implements four optimization loops. The fastest loop tracks the MPP for all four balanced panels by controlling the duty cycle of the switch SW 7   506  of the MPPT converter  504 . Two slower optimization loops (preferably identical) optimize the individual solar panels within the solar panel pairs. Solar panel pair PV 1 +PV 2  is controlled by the switching duty cycles of their switches SW 1   510 , SW 2   511 , while solar panel pair PV 3 +PV 4  is controlled by the switching duty cycles of their switches SW 3   512 , SW 4   513 . The slowest optimization loop balances the solar panel pairs PV 1 +PV 2 , PV 3 +PV 4  as a group by controlling the switching duty cycles of its switches SW 5   514 , SW 6   515 . 
     Referring to  FIG. 5   a , in accordance with another embodiment, a power balancer for multiple solar panels can implemented for an odd number of solar panels. For example, five solar panels can be balanced by four power balancer circuits, arranged in a way that reduces the maximum voltage applied to any single power balancer. In this implementation, a single power balancer circuit is used to balance each electrically adjacent pair in the series-connected solar panels (as discussed above), where each solar panel pair is balanced by interleaving power balancer circuits as shown. Operation of the interleaved power balancers is such that the optimized voltage ratios are maintained among all solar panels in the series. The first power balancer  502   a  which includes two switches SW 1 , SW 2  and maintains a ratio of voltages on its associated solar panels PV 1 , PV 2  substantially equal to the inverse ratio of the duty cycle of the states of the switches SW 1 , SW 2 . Similarly, the second power balancer  504   a  defines the voltage ratio between its associated solar panels PV 2 , PV 3 , and so forth for all panels in the chain. Thus the five solar panels PV 1 , PV 2 , PV 3 , PV 4 , PV 5  have their respective voltage ratios defined by four power balancer circuits  502   a,    504   a,    505   a,    506   a,  and provide power to the MPPT converter  507   a  as discussed above. In the general case of this implementation, N solar panels connected in series can be balanced by N−1 power balancer circuits. 
     Referring to  FIG. 5   b , in accordance with one embodiment, the control units  107 ,  509   a,    508   a  of  FIGS. 1 ,  5  and  5   a  can be implemented substantially as shown. Voltage and current sensing of the solar panel voltages and currents, respectively, are performed using transducers  501   b,    502   b,    503   b,    504   b  provide analog voltages proportional to the solar panel voltages and currents for conversion by an analog-to-digital converter (ADC)  505   b.  The MPPT feedback voltage is also digitized by the ADC  505   b.  The ADC  505   b  supplies these converted signals as digitized measurement data to a microprocessor  507   b,  which, in concert with its associated data memory  506   b  (e.g., random access memory (RAM)) and program memory  508   b  (e.g., read only memory (ROM)), performs computational algorithms (discussed in more detail below). The results are written to switch pulse forming logic  509   b,  e.g., implemented using a field programmable gate array (FPGA). Output pulses from the switch pulse forming logic  509   b  is converted to appropriate levels by signal drivers  510   b  (e.g., MOSFET drivers) for driving the power balancer and MPPT converter switches, as discussed above. 
     Referring to  FIG. 5   c , in accordance with exemplary embodiments, operation of the control units, e.g., as depicted in  FIG. 5   b , can proceed as shown. Following initialization  501   c  of the MPPT converter control loop, the switch control pulse width is increased  502   c  by a small increment of dTm, after which the sum of the power for all solar panels is calculated  503   c,  according to the formula
 
 P   current   =V 1* I 1+ V 2* I 2+. . . + Vn*In  
         where: Vn=voltage from solar panel N   In=current from solar panel N       

     This newly computed power P current  is compared  504   c  with the previously stored power P previous . If the current power P current  is P higher than the previous power P previous , the stored value for power is updated  506   c  with the current value, and the loop continues. Otherwise, the sign of the switch control pulse increment dTm is reversed  505   c,  and the loop continues. 
     Similarly, a slower power balancer control loop is initialized  507   c,  followed by a change  508   c  in the balancer switch timing. The timing relationship between two switches in a power balancer ( FIG. 3 ) is altered by increasing the duty cycle T 1  of one switch control signal SW 1  by an amount dTn and decreasing the duty cycle T 2  of the other switch control signal SW 2  by the same amount dTn. Power is measured  509   c  as before, but only for those solar panels attached to the power balancer. This value P current  is compared  510   c  with the previously stored power P previous . If the current power P current  is higher than the previous power P previous , the stored value for power is updated  512   c  with the current value, and the loop continues  513   c  for the next power balancer. Otherwise, the sign of the switch control pulse increment dTn is reversed  511   c,  and the loop continues, as discussed above. 
     Referring to  FIG. 6 , the output of a MPPT converter acts as a current source, thereby allowing several MPPT converters to be connected in parallel, e.g., four parallel-connected MPPT converters M 1   600 , M 2   601 , M 3   602 , M 4   603  (identified as “Power Harvest Maximizer Modules”). The DC current generated by them is converted into AC current by a DC/AC converter  604 . 
     Referring to  FIG. 6   a , the stand-alone nature of the power harvest maximizer module makes it practical to separate this function physically from the grid-connected DC-to-AC inverter function. This allows a distributed inverter system architecture, as shown. In this implementation, two or more solar panels  601   a  are connected to and optimized by one or more power harvest maximizer modules  602   a.  Each power harvest maximizer module serves one or more solar panels and includes one MPPT converter and an appropriate number of balancers for the number of solar panels connected. Two or more power harvest maximizer modules are then connected to the DC-to-3-phase AC converter  603   a.  A monitoring and control gateway interface  605   a  provides a means for monitoring of the system via power line communications and provides a communication bridge to allow local and remote interaction with the power system for status monitoring and control. 
     Referring to  FIG. 7 , shows an implementation of a DC/AC converter for providing power to a 3-phase power grid. The DC current goes into a chopper  700 , which produces a high frequency (e.g., &gt;20 KHz) voltage for a step-up transformer T 1   701 . The galvanically isolated output of the transformer Ti is rectified with a rectifier  702  to produce a DC voltage across a filter capacitor C 1   703 . Six switches SW 1   704 , SW 2   705 , SW 3   706 , SW 4   707 , SW 5   708 , SW 6   709  (e.g., MOSFETs or insulated gate bipolar transistors (IGBTs)) produce a 3-phase AC voltage which is filtered by a filter circuit  710  for delivery to a commercial 3-phase power grid. 
     A control unit  711  monitors the 3-phase grid voltage V 3-phase  and current I 3-phase , and generates control pulses for the switches SW 1 , SW 2 , SW 3 , SW 4 , SW 5 , SW 6  to deliver energy to the grid with proper phases. If galvanic isolation is not required for standards compliance, the DC input voltage V inp  can be applied directly to the filter capacitor  703 , thereby eliminating the need for the chopper  700 , transformer T 1   701  and rectifier  702 . 
     The above architecture advantageously maximizes efficiency of DC/AC power conversion by keeping the conversion ratio V rect /V inp  constant. As a result, the input DC voltage V inp  tracks the grid voltage. The rectified DC voltage V rect  across the switches SW 1 , SW 2 , SW 3 , SW 4 , SW 5 , SW 6  is kept at the minimal level required to provide an undistorted output waveform for the current grid voltage. In this architecture, matching of varying solar energy and varying grid voltage is performed in one place, i.e., the MPPT converter  504  ( FIG. 5 ). 
     Referring to  FIG. 7   a , in accordance with another embodiment, the power harvest maximizer can be used in single phase power inverters. As discussed above, the solar panels provide a continuous flow of energy. Because energy cannot be continuously supplied to the single phase grid, an energy storage buffer is required. Such energy storage can be implemented as a capacitor  701   a,  as shown. The sum of the voltage across the series-connected, balanced solar panels applied to the MPPT boost converter results in a high power harvest maximizer module output voltage. Because energy stored in the capacitor is proportional to the square of the applied voltage, this results in a corresponding reduction in the physical size of the capacitive storage element compared to existing single solar panel inverters. Voltage ripple present across the capacitor  701   a  can be isolated from the solar panels by an appropriate power harvest maximizer module control algorithm, further reducing the size of the capacitor  701   a.    
     Based upon the foregoing discussion, in accordance with the presently claimed invention, it can be seen that: a power balancer is provided that improves energy harvesting from serially coupled DC power sources such as solar panels by compensating mismatches between the solar panels; such a power balancer can be used to balance more than two solar panels; the MPPT converter, which acts as a current source, allows multiple converters to be connected in parallel for summing their output currents; a highly efficient architecture is provided for a DC/AC power converter that compensates for varying solar energy levels and varying grid voltages in a single place in the energy conversion chain; and multi-loop control algorithms are provided for optimizing system performance. 
     Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.