Abstract:
The invention is a method of tracking the overall maximum power point of a grounded bipolar photovoltaic array by tracking and regulating the voltage of the weaker of the two monopolar subarrays at any instant in time. The transfer between subarrays being tracked for the maximum power point is seamless when the voltage of one subarray becomes lower than the other. This tracking method insures stable operation and maximum power transfer under all balanced and unbalanced PV array conditions. This tracking algorithm would typically be part of a larger digital power converter control system.

Description:
BACKGROUND OF THE INVENTION 
     Photovoltaic (PV) sources have characteristic current verses voltage curves. At low currents, a PV source behaves like a non-ideal voltage source. At high currents, a PV source behaves like a non-ideal current source. At some point in between, maximum power can be delivered from a PV source. The maximum power of the PV source is a function of the level of solar radiation, typically specified in Watts per square meter and the temperature of the PV source, typically specified as the PV module cell temperature. Essentially all photovoltaic power converters dynamically track the composite maximum power point of a number of connected solar PV modules. The process is done with a perturb-and-observe algorithm wherein a PV voltage is set and regulated by adjusting the load on the PV source and the power is observed. The PV voltage is then adjusted slightly and the power is again observed, if the power increases as a result of the PV voltage change, the PV voltage is again adjusted in the same direction, if not the PV voltage is incremented in the opposite direction until the PV maximum power point is captured and the PV voltage is dithered about the PV maximum power point voltage. The tracking is dynamic in order to track rapid changes in radiation due to, for example, fast moving cloud cover. 
     When the maximum power point of a grounded bipolar PV array is tracked by regulating the pole-to-pole voltage of the array, there will be stability issues when the available power from the monopolar subarrays becomes imbalanced. With a grid-interactive inverter, if the positive subarray is partially shadowed and the negative subarray is in full sun, the voltage on the partially shadowed, positive array will become less than the positive peak voltage of the connected electrical grid, the positive DC inverter bus will collapse and the inverter will shut down. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention is a method of tracking the overall maximum power point of a grounded bipolar photovoltaic array by tracking and regulating the voltage of the weaker of the two monopolar subarrays at any instant in time. The transfer between subarrays being tracked for the maximum power point is seamless when the voltage of one subarray becomes lower than the other. This tracking method insures stable operation and maximum power transfer under all balanced and unbalanced PV array conditions. This tracking algorithm would typically be part of a larger digital power converter control system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an electrical schematic of a simple grid-interactive, bipolar photovoltaic to single-phase power conversion system intended to be used as a reference for  FIG. 2 . 
         FIG. 2  is a simplified schematic of an analog circuit illustrating the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a simplified electrical schematic of a grid-interactive, photovoltaic to single-phase power converter and power system. Power is converted by power converter  100  from solar photovoltaic sources  1  and  2 . Electric power grid  5  is a 60 Hz sinusoidal voltage source. Power converter  100  regulates sinusoidal current  21  in phase with electric power grid  5  voltage to achieve positive power transfer into electric power grid  5 . Photovoltaic sources  1  and  2  are collectively configured as a bipolar source having both positive and negative voltage potentials with respect to common system ground point  0 . Electric power grid  5  is also referenced to common system ground point  0 . In most applications where power is converted from a bipolar photovoltaic source, a three-phase power converter (inverter) will be used. For this example, a single-phase inverter is discussed for simplicity. 
     In  FIG. 1 , DC bus capacitors  3  and  4  are connected in parallel with photovoltaic sources  1  and  2 , respectively, to provide a low AC impedance for the half bridge semiconductor switching matrix configured from IGBT/diode combination  18  and IGBT/diode combination  19 . Control circuit  10  has four inputs, (i) positive DC voltage sense, +VDC SEN, which reads the voltage of photovoltaic source  1  with respect to system ground point  0  by way of a voltage divider configured from resistors  11  and  12 , (ii) negative DC voltage sense, −VDC SEN, which reads the voltage of photovoltaic source  2  with respect to system ground point  0  by way of a voltage divider configured from resistors  13  and  14 , (iii) AC line voltage sense, VAC SEN, which reads the voltage of electric power grid source  5  with respect to system ground point  0  by way of a voltage divider configured from resistors  15  and  16  and (iv) AC line current  21 , shown with a positive sense into electric power grid  5 , IAC SEN, is provided by current sensor  17 . The control circuit ground is shown as GND. Control circuit  10  has two outputs, T, a drive to switch top IGBT  18  on and off and B, a drive to switch bottom IGBT  19  on and off. The common point of this half bridge semiconductor switching matrix is connected through inductor  20 . Other semiconductor power switch types could be substituted for IGBTs  18  and  19 . The circuit in  FIG. 1  is shown for reference and is not considered novel. Typically, other capacitive and inductive line filter components will be included as secondary filter elements. Drives T and B are driven as complements, wherein both are never “on” at the same time. 
       FIG. 2  shows a simplified analog circuit equivalent of the invention control method. In almost all applications the regulation method according to the invention will be implemented with digital controls. The circuit shown in  FIG. 2  can all be considered within control circuit block  10  in  FIG. 1 .  FIG. 1  and  FIG. 2  have some common reference characters to purposefully link the two drawings. The  FIG. 2  discussion herein will refer back to  FIG. 1  when required to explain full functionality of the invention. 
     In  FIG. 2 , scaled positive monopole voltage with respect to ground +VDC SEN is applied to the input of buffer amplifier  31 . Buffer amplifier  31  has a gain of +1. Scaled negative monopole voltage with respect to ground −VDC SEN is applied to the input of inverting buffer amplifier  32 . Inverting buffer amplifier  32  has a gain of −1. The outputs of buffer amplifiers  31  and  32  are connected to the cathodes of precision rectifiers  33  and  34  respectively. The anodes of precision rectifiers  33  and  34 , a first terminal of pull-up resistor  35  and the non-inverting input of error amplifier  36  are connected together. A second terminal of pull-up resistor  35  is connected to positive control supply VCC. Error amplifier  36  is part of a servo loop which regulates  41 , the absolute value of the voltage of the lower of +VDC SEN and −VDC SEN, to PV voltage regulation reference value VPV REF. The output of error amp  36  is connected to the X input of multiplier  37 . The Y input of multiplier  37  is connected to SIN REF, a fixed amplitude sine wave synchronized with VAC SEN in  FIG. 1 . The output of multiplier circuit  37  is a variable amplitude sinewave  40 , the AC current reference, which is connected to the non-inverting terminal of error amplifier  38 . The inverting input of error amplifier  38  is connected to feedback signal IAC SEN. Error amplifier  38  regulates feedback current IAC SEN to match AC current reference  40 . The output of error amplifier  38  is connected to the non-inverting input of comparator  39 . The inverting input of comparator  39  is connected to triangle wave TRI. TRI runs at the pulse width modulation (PWM) frequency and is “beat” against the output of error amplifier  38  to provide the complementary comparator  39  outputs as PWM drive pulses for IGBT drives T and B as referenced in both  FIGS. 1 and 2 . 
     In  FIG. 2 , voltage regulation is accomplished with a perturb-and-observe algorithm wherein a VPV REF is set and the lower voltage subarray voltage is regulated by adjusting the load on the PV source by increasing or decreasing IAC REF  40  and by observing the overall inverter output power. VPV REF is then adjusted slightly and the overall inverter power is again observed, if the power increases as a result of the VPV REF change, the VPV REF is again adjusted in the same direction, if not VPV REF is incremented in the opposite direction until the PV source maximum power point is captured and the PV source voltage is dithered about the PV source maximum power point voltage. 
     The circuit function illustrated in  FIG. 2  is not novel with the exception of the arrangement of elements  31 ,  32 ,  33 ,  34  and  35  which provide a seamless transfer with respect to which subarray is being regulated to VPV REF according to which subarray has the lowest voltage at any point in time. Under normal operating conditions, the current through both subarrays, elements  1  and  2  in  FIG. 1 , will be the same. Therefore, the subarray with the lower voltage will be the weaker subarray or, restated, the subarray with the lower available power. 
     In practice, digital control will be used wherein the voltages of both PV monopoles will be signal conditioned and applied to analog-to-digital converter inputs. The two resultant digital numerical values will be compared and the subarray with the lower value at any point in time will be regulated to an internal numerical VPV REF value. The data from the subarray not being voltage regulated, at any point in time, will not be used to track the maximum power point but may be used to detect over and under voltage fault conditions.