Abstract:
A method for determining continuity in a PV panel string by calculating a discharge ratio includes operatively coupling a PV panel to a PV panel string; measuring a first voltage between points of coupling of the PV panel to the PV panel string; disconnecting the PV panel from the PV panel string; waiting for a discharge period to expire; measuring a second voltage at an expiration of the discharge period; calculating a discharge ratio of the second voltage to the first voltage; and comparing the discharge ratio to a predetermined threshold ratio. An apparatus for determining continuity in a PV panel string includes a discharge resistance serially connected in the PV panel string; a capacitance parallel connected to the discharge resistance; a voltage sensor parallel connected to the discharge resistance; and a first switch parallel connected to the discharge resistance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Patent Application Ser. No. 62/025,366 filed on Jul. 16, 2014, the complete disclosure of which, in its entirety, is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The embodiments herein generally relate to photovoltaic (PV) solar panels, and more particularly to techniques to ensure safe interaction with PV solar panels. 
         [0004]    2. Description of the Related Art 
         [0005]    PV solar panels are an important source of electrical power. Large, megawatt arrays with PV panels numbering in the tens of thousands are increasingly common. A typical PV panel is organized as a series connection of individual PV cells. A common configuration is 72 PV cells per panel. A typical PV cell operating voltage under full illumination is approximately 0.7 V. An illuminated PV panel with 72 direct current (DC) PV cells will therefore have an output voltage of approximately 50 volts DC. PV panels are typically serially connected to form a panel “string”. In a DC PV panel system, the output of the PV panel string could connect to a central inverter which converts the DC power of the PV panels into Alternating Current (AC) power suitable for the electrical grid. Typically, there are between five and twenty PV panels in a PV panel string producing a combined voltage of several hundred volts. 
         [0006]    PV panels produce power whenever they are illuminated. As described above, the voltages on a PV panel string could reach hazardous levels of hundreds of volts. These voltages could be a safety hazard during PV panel installation and maintenance. If the PV panels are mounted to a roof or integrated into building structures, these voltages can also represent a hazard during emergency operations such as fire fighting since the PV panels will continue to generate voltage even when the PV installation is disconnected from the electrical grid. Accordingly, there remains a need for a technique to allow for safer interaction with PV solar panels. 
       SUMMARY 
       [0007]    In view of the foregoing, an embodiment herein provides a method for determining continuity in a PV panel string by calculating a discharge ratio, the method comprising operatively coupling a PV panel to a PV panel string; measuring a first voltage between points of coupling of the PV panel to the PV panel string; disconnecting the PV panel from the PV panel string; waiting for a discharge period to expire; measuring a second voltage at an expiration of the discharge period; calculating a discharge ratio of the second voltage to the first voltage; and comparing the discharge ratio to a predetermined threshold ratio. The operatively coupling the PV panel to the PV panel string may comprise coupling a panel interface device (PID) that is operatively connected to the PV panel to the PV panel string, wherein the PID permits disconnection of the PV panel from the PV panel string, and wherein the PID permits reconnection of the PV panel to the PV panel string. 
         [0008]    The PID may comprise a discharge resistance connected between points of coupling of the PID to the PV panel string. The PID may disconnect the PV panel from the PV panel string upon any of an open circuit in the PV panel string, a high resistance condition in the PV panel string, and a current below a predetermined threshold value in the PV panel string. The PID may comprise a switch; a controller that controls operation of the switch; the discharge resistance operatively connected to the point of coupling; an output capacitance operatively connected to the point of coupling; and a voltage sensor operatively connected to the point of coupling. 
         [0009]    The PID may allow the PV panel to be bypassed when the PV panel cannot match a string current in the PV panel string. The PID may comprise a “buck” type DC to DC converter. The PID may convert a DC panel voltage at a first level to a DC output voltage at a second level. The predetermined threshold ratio may be a minimum possible discharge ratio that would occur over the discharge period if the PV panel string is continuous and coupled to an input capacitance of an inverter operatively connected to the PV panel string. 
         [0010]    The predetermined threshold ratio may be given by the formula: 
         [0000]    
       
         
           
             
               R 
               = 
               
                  
                 
                   
                     - 
                     
                       T 
                       
                         DIS 
                         / 
                       
                     
                   
                    
                   
                     MR 
                     PID 
                   
                    
                   
                     C 
                     INV 
                   
                 
               
             
             , 
           
         
       
     
         [0000]    wherein: CINV is an input capacitance of an inverter operatively connected to the PV panel string, RPID is a discharge resistance, TDIS is a discharge period, and M is a number of PIDs operatively coupled to the PV panel string. 
         [0011]    When the discharge ratio is greater than the predetermined threshold ratio, the method may further comprise operatively coupling the PV panel permanently to the PV panel string. When the discharge ratio is less than the predetermined threshold ratio, the PV panel remains disconnected from the PV panel string. The method may further comprise the PID performing a plurality of discharge ratio measurements. The method may further comprise operatively coupling the PV panel permanently to the PV panel string when a minimum number of consecutive discharge ratio measurements are all greater than a predetermined threshold ratio. The method may further comprise the PID disconnecting the PV panel from the PV panel string during a waiting period between performing the plurality of discharge ratio measurements. 
         [0012]    The length of the waiting period may comprise a random value. The method may further comprise the PID operatively coupling the PV panel to the PV panel string during a waiting period between performing the plurality of discharge ratio measurements after a minimum number of consecutive discharge ratio measurements have all been greater than a predetermined threshold ratio. The method may further comprise gradually increasing a duty cycle of the switch to limit a charging current to charge an input capacitance of an inverter operatively connected to the PV panel string. The method may further comprise gradually increasing a duty cycle of the “buck” type DC to DC converter to limit a charging current to charge an input capacitance of an inverter operatively connected to the PV panel string. 
         [0013]    Another embodiment provides an apparatus for determining continuity in a PV panel string comprising a discharge resistance serially connected in the PV panel string; a capacitance parallel connected to the discharge resistance; a voltage sensor parallel connected to the discharge resistance; and a first switch parallel connected to the discharge resistance. The apparatus may further comprise a PV panel input terminal and a second switch operatively coupling between the PV panel input terminal and the panel string. The apparatus may further comprise a PID operatively connected to the PV panel string and a PV panel, wherein the PID may comprise a “buck” type DC to DC converter. The second switch may comprise a diode. The apparatus may further comprise a second switch serially connected to the discharge resistance and the PV panel string. 
         [0014]    These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
           [0016]      FIG. 1A  illustrates a block diagram of an example grid tied PV installation according to an embodiment herein; 
           [0017]      FIG. 1B  illustrates a block diagram of another example grid tied PV installation according to an embodiment herein; 
           [0018]      FIG. 2A  illustrates a block diagram of still another example grid tied PV installation according to an embodiment herein; 
           [0019]      FIG. 2B  illustrates a block diagram of yet another example grid tied PV installation according to an embodiment herein; 
           [0020]      FIG. 3A  is a schematic diagram of a panel interface device (PID) according to an embodiment herein; 
           [0021]      FIG. 3B  a schematic diagram of another PID according to an embodiment herein; 
           [0022]      FIG. 4A  illustrates a block diagram of a PV panel string operatively connected to a string inverter according to an embodiment herein; 
           [0023]      FIG. 4B  illustrates a block diagram of another PV panel string operatively connected to a string inverter according to an embodiment herein; 
           [0024]      FIGS. 5A and 5B  are flow diagrams illustrating PV panel string continuity monitoring methods performed by a PID according to an embodiment herein; 
           [0025]      FIG. 6A  illustrates a schematic diagram of an equivalent circuit of a PID connected to a PV panel string and a string inverter according to an embodiment herein; 
           [0026]      FIG. 6B  illustrates a schematic diagram of an approximate equivalent circuit of a PID connected to a PV panel string and a string inverter according to an embodiment herein; 
           [0027]      FIG. 7  illustrates a block diagram of a PID controller according to an embodiment herein; and 
           [0028]      FIG. 8  is a schematic diagram of another panel interface device (PID) according to an embodiment herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein. 
         [0030]    The embodiments herein provide a PV panel string continuity detection method. Referring now to the drawings, and more particularly to  FIGS. 1 through 8  where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
         [0031]      FIG. 1A  is a block diagram of an example grid tied PV installation  100 . The example installation  100  includes PV panel string  160  operatively connected to the input of inverter  120 . String inverter  120  converts the DC power of PV panel string  160  to AC power. PV panel string  160  comprises a series connection of PV panels  111   1 , . . .  111   N . The AC output of PV panel string inverter  120  operatively connects to electrical grid  150  through grid disconnect switch  140 .  FIG. 1A  is an example only and other arrangements are possible in accordance with the embodiments herein. For example there could be multiple PV panel strings arranged in parallel and connected to inverter  120 . 
         [0032]      FIG. 1B , with respect to  FIG. 1A , is a block diagram of another example grid tied PV installation  101 . In example installation  101 , there is DC disconnect switch  141  between PV panel string  160  and the input of inverter  120 . In other embodiments additional disconnect means may be used between the PV panel string  160  and the string inverter  120  and/or between the string inverter  120  and the grid  150 , including fuses for example. Additional disconnect means may provide for easier maintenance of the inverter  120 . For example, in other embodiments there could be multiple PV panel strings with individual DC disconnect switches. 
         [0033]    Again, power production by PV panels under illumination can represent a potential safety hazard. It could therefore be useful to have the PV panels in a PV installation isolate themselves from their PV panel string and not output power whenever the PV installation is disconnected from the grid. 
         [0034]    A PV installation could disconnect from the grid for any of a number of reasons. These could include a manual disconnect for maintenance purposes and/or during an emergency such as a fire. Also, a PV installation could instead automatically disconnect due to an electrical fault on the grid. A PV installation could also be disconnected from the grid prior to its commissioning. 
         [0035]    It could be useful if PV panels isolated themselves from their PV panel string in the event of an open circuit condition in their PV panel string. This might be caused by, for example: a physical break in the panel string; removal of one or more PV panels from the PV panel string for maintenance, repair, or replacement; theft of a panel, disconnection of the PV panel string from an inverter for inverter repair or replacement by opening of a disconnect switch; a fault in the inverter; and/or during initial PV panel installation before all PV panels are installed in a panel string. 
         [0036]      FIG. 2A , with reference to  FIGS. 1A and 1B , is a block diagram of another example grid tied PV installation  200  according to an embodiment herein. PV installation  200  comprises PV panel string  261  and inverter  120 . PV panel string  261  comprises PV panels  111   1 , . . .  111   N  and PIDs  201   1 , . . .  201   N . The DC outputs of PV panels  111   1 , . . .  111   N  respectively and operatively connect to the inputs of PIDs  201   1 , . . .  201   N . PIDs  201   1 , . . .  201   N  could perform a disconnect function and disconnect their respective PV panels  111   1 , . . .  111   N  from PV panel string  261  in the event of an open circuit or high resistance condition in PV panel string  261 . In one embodiment, PIDs  201   1  . . .  201   N  monitor the current in PV panel string  261  and disconnect their respective PV panels  111   1  . . .  111   N  when the PV panel string current drops below a predetermined threshold value. 
         [0037]    An open circuit condition could be caused by, for example, removal of one or more PV panels  111   1 , . . .  111   N , a physical break in the PV panel string  261 , the opening of a switch (not shown) in the PV panel string  261 , the blowing of a PV panel string fuse (not shown), and/or a fault in inverter  120 . 
         [0038]    A high resistance condition could be caused by PV panel string inverter  120  ceasing operation. PV panel string inverter  120  could cease operation due to, for example, manual disconnection of the PV installation  200  from electrical grid  150  and/or anti-islanding of inverter  120 . Anti-islanding refers to the automatic disconnection of a PV installation  200  from the grid  150  when the PV installation  200  detects that the main grid generator (not shown) is no longer present. Anti-islanding prevents the creation of “power islands” on parts of the grid  150  during a power failure. If the example PV installation  200  disconnects from grid  150  by, for example, opening of grid disconnect switch  140 , then the input of string inverter  120  will become high resistance and the string current will fall substantially to zero. 
         [0039]      FIG. 2B , with reference to  FIGS. 1A through 2A , is a block diagram of another example grid tied PV installation  210  according to an embodiment herein. In this example installation  210 , PV panel strings  211   1 ,  211   2 , . . .  211   N  are connected in parallel to the inputs of inverter  120 . Each PV panel string  211   1 ,  211   2 , . . .  211   N  comprises a series connection of PV panels and respective PIDs as shown in  FIG. 2A  for example. 
         [0040]      FIG. 3A , with reference to  FIGS. 1A through 2B , is a block diagram of one embodiment of a PID  301  according to an embodiment herein. PID  301  comprises controller  310 , switch  320 , series switch  330 , current sensor  340 , diode  350 , discharge resistance  354 , output capacitance  356 , voltage sensor  341 , and output terminal pair  382 ,  384 . The input of PID  301  operatively connects to PV panel  311 . PID output terminal pair  382 ,  384  operatively connect to the PV panel string (not shown in  FIG. 3A ). The output terminal pair  382 ,  384  comprises the points of coupling between the PID  301  and the PV panel string (not shown in  FIG. 3A ). 
         [0041]    Switches  320 ,  330  could be implemented using any of a variety of means, including but not limited to: power Metal Oxide Semiconductor Field Effect Transistors (MOSFETs), Insulated Gate Bipolar Transistors (IGBTs), thyristors, and/or relays, for example. Diode  350  is connected in parallel with switch  320 . Controller  310  controls the operation of switches  320 ,  330 . Controller  310  draws its power from PV panel  311  and receives current and voltage measurements from current sensor  340  and voltage sensor  341 , respectively. Current sensor  340  monitors the string current. Voltage sensor  341  monitors the output voltage of PID  301 . 
         [0042]    Switch  320  allows panel  311  to be bypassed in the event that it cannot match the string current. Panel  311  might not be able to match the string current if, for example, it were heavily shaded. Bypass of the panel could prevent harmful reverse bias voltages being developed across panel  311 . Diode  350  allows panel  311  to be bypassed in the event that PV panel  311  cannot supply sufficient power to operate controller  310 . If PV panel  311  was defective or heavily shaded, for example, it might be unable to supply sufficient power to operate controller  310 . In this situation controller  310  might be unable to supply sufficient drive voltage to keep switch  320  closed. String current could, however, still bypass the panel by flowing through diode  311 . 
         [0043]    In normal operation, with PV panel  311  illuminated and producing a DC voltage, switch  320  is open and switch  330  is closed. PV panel  311  is in series with the other panels in the PV panel string (not shown in  FIG. 3A ) and contributes to the string voltage and current. 
         [0044]      FIG. 3B , with reference to  FIGS. 1A through 3A , is a block diagram of another embodiment of a PID according to an embodiment herein. PID  302  incorporates DC power optimizer functionality in addition to disconnect functionality. 
         [0045]    A DC power optimizer uses a DC to DC converter to maximize the power output of a PV panel  311 . A DC power optimizer also matches its current output to the string current. Although such terms as optimizer, optimize, maximize, and the like are used herein, these terms are not intended to infer absolute optimality or maxima. For instance, power optimization functionality may improve performance, but might not necessarily achieve theoretical maximum or optimal power production or output. 
         [0046]    PID  302 , in the example shown in  FIG. 3B , comprises controller  312 , switches  322 ,  332 , inductor  360 , output capacitance  370 , current sensor  342 , diode  352 , discharge resistance  372 , voltage sensor  343 , and output terminals  385 ,  386 . The input of PID  302  operatively connects to PV panel  311 . Output terminal pair  385 ,  386  operatively connect to the PV panel string (not shown in  FIG. 3B ). The output terminal pair  385 ,  386  comprises the points of coupling between the PID  302  and the PV panel string (not shown in  FIG. 3B ). In one example embodiment, discharge resistance  372  has a value of 15 k-ohms, output capacitance  370  has a value of 10 μF, inductor  360  has a value of 5.6 μH, current sensor  342  is a 2 milli-ohm resistor, and the MPP voltage of panel  311  is 33 V and its MPP current is 10 A. However, the embodiments herein are not restricted to these example values. 
         [0047]    PID  302  is of a “buck” type DC to DC converter configuration. A buck type DC to DC converter converts a DC input voltage at one level to a DC output voltage at another, lower level. Other types of DC to DC converter topologies are possible in accordance with the embodiments herein. 
         [0048]    When PV panel  311  is illuminated and there is string current flowing, PID  302  performs a voltage conversion and power optimization operation. The operation is controlled by controller  312 . Switches  322 ,  332  are switched with a frequency “f” and operate in a complementary fashion, such that when one switch is open (ON) the other switch will be closed (OFF). 
         [0049]    When switch  332  is closed and switch  322  is open, current from PV panel  311  flows into inductor  360 , storing energy therein. When switch  332  is opened and switch  322  is closed, the voltage across the inductor  360  reverses and it sources current into output capacitance  370  and the panel string. The duty cycle of PID  302  (D BUCK ) is defined as the ratio of the ON time of switch  332  to the switching period T and is normally expressed as a percentage. The duty cycle could range from 0 to 100%. For example, if switch  332  is ON for 70% of the switching period then the duty cycle is 70%. The relationship of output voltage (V BUCK ) of PID  302  to its input voltage (V IN ) depends on the duty cycle and is given by the equation: 
         [0000]    
       
      
       V 
       BUCK 
       =D 
       BUCK 
       V 
       IN  
      
     
         [0050]    V BUCK  is defined as the voltage across terminal pair  385 ,  386  and V IN  is defined as the voltage of PV panel  311 . The relationship of the output current (I OUT ) of PID  302  to its input current (I IN ) also depends on the duty cycle and is given by the equation: 
         [0000]    
       
      
       I 
       OUT 
       =I 
       IN 
       /D 
       BUCK  
      
     
         [0051]    Switches  322 ,  332  could be implemented using any of a variety of means, including but not limited to: power MOSFETs, IGBTs, thyristors, and/or relays, for example. Diode  352  is in parallel with switch  322 . Diode  352  allows string current to flow in the event that PV panel  311  cannot supply sufficient power to operate controller  312  and keep switch  322  closed. 
         [0052]    Disconnect Operations 
         [0053]    Referring to  FIG. 2A , in the event of the occurrence of a PV panel string disconnect condition, PIDs  201 , . . .  201   N  disconnect their respective PV panels  111   1 , . . .  111   N  from PV panel string  261 . A PV panel string disconnect condition could be an open circuit or high resistance condition on PV panel string  261 . 
         [0054]    An open circuit or high resistance condition in a PV panel string could result in the loss or substantial reduction in the PV panel string current. In PID  301  of  FIG. 3A , a loss or reduction in PV panel string current could be detected by controller  310  using current sensor  340 . Similarly, in PID  302  of  FIG. 3B  a loss or reduction in PV panel string current could be detected by controller  312  using current sensor  342 . In one embodiment, a PV panel string current of less than 200 mA is interpreted as a PV panel string disconnect condition and triggers a disconnect of PV panel  311  by PID  301 ,  302 . In PID  301  of  FIG. 3A , controller  310  opens switch  330 , disconnecting PV panel  311  from the PV panel string (not shown in  FIG. 3A ) and interrupting the flow of power. Similarly, in PID  302  of  FIG. 3B , controller  312  opens switch  332 , disconnecting PV panel  311  from the PV panel string (not shown in  FIG. 3B ). 
         [0055]    Reconnect Operations 
         [0056]    After a PID has isolated its PV panels from the panel string, it could check to determine whether the disconnect condition has been resolved and whether its PV panel can be reconnected to the PV panel string to provide power. A variety of reconnect techniques are possible such as the ones described in U.S. patent application Ser. No. 13/840,162 entitled “INTELLIGENT SAFETY DISCONNECT SWITCHING” and in U.S. patent application Ser. No. 14/073,473 entitled “STRING CONTINUITY MONITORING” the complete disclosures of which, in their entireties, are herein incorporated by reference. 
         [0057]    Referring to  FIG. 2A , after PID  201   1 , . . .  201   N  has disconnected PV panel  111   1 , . . .  111   N  from PV panel string  261  due to a disconnect condition, PID  201   1 , . . .  201   N  could check that the PV panel string has continuity (is physically continuous and connected to the input of inverter  140 ). The input capacitance of a string inverter  120  varies by manufacturer and power capacity but is normally large and could be in the range of approximately 50 μF to 5 mF. 
         [0058]    In some cases, after a string inverter has stopped converting power it cannot immediately resume converting power. There could be a mandatory start-up period after the inverter&#39;s grid connection has been restored or the inverter&#39;s input voltage has returned to a valid operating value before the inverter can start to draw DC power at its input. A string inverter could be unable to start operation and draw DC current from the PV panel string until this start-up period expires. The start-up period could be on the order of approximately five minutes. Therefore, even if a PV panel string has physical continuity and is connected to the input of its string inverter there could be a high resistance in the string and no DC current until the inverter&#39;s start-up period expires. 
         [0059]    In addition, a string inverter could have a minimum input voltage requirement below which it will not operate and draw DC current at its input. Therefore, it could be beneficial for a PID to keep its panel connected to its PV panel string despite there being high resistance in the string and the absence of DC current for a time on the order of at least the string inverter&#39;s start-up period. For example, after PID  201   1 , . . .  201   N  has reconnected PV panel  111   1 , . . .  111   N  to PV panel string  261  it could be beneficial for it to remain connected to allow string inverter  120  sufficient time to start even if there is no PV panel string current flowing. However, if all of PIDs  201   1 , . . .  201   N  have reconnected their respective PV panels  111   1 , . . .  111   N  to PV panel string  261  the PV panel string voltage at the input of string inverter  120  could be several hundred volts. If the disconnect condition occurred from a physical break in PV panel string  261  by, for example, removal of a panel, disconnection or severing of a cable, etc., serious injury or damage could result. Accordingly, the embodiments herein provide a technique to continually monitor the continuity of a PV panel string (e.g., verify that a PV panel string is still physically continuous and connected to the inverter&#39;s input) while waiting for the string inverter to start, which could improve the safety of a PV panel array. 
         [0060]      FIG. 4A , with reference to  FIGS. 1A through 3B , is a block diagram of a PV panel string  401  operatively connected to a string inverter  482  according to an embodiment herein. Although the input capacitance of inverter  482  is represented by capacitance  480  in  FIG. 4A  it should be understood that it is a part of inverter  482  and is not separate from it. In one embodiment, capacitance  480  has a value of 100 μF. PV panel string  401  comprises PV panels  311 ,  411   1 , . . .  411   N  and PIDs  301 ,  401   1 , . . .  401   N . PIDs  301 ,  401   1 , . . .  401   N  are serially connected. PIDs  401   1 , . . .  401   N  could have a similar configuration to PID  301 . In one embodiment PIDs  401   1 , . . .  401   N  are identical to PID  301  and have a discharge resistance and an output capacitance at their output terminals of the same values as discharge resistance  354  and output capacitance  356  of PID  301 . 
         [0061]      FIG. 4B  with reference to  FIGS. 1A through 4A , is a block diagram of a PV panel string  402  operatively connected to a string inverter  482  according to an embodiment herein. Although the input capacitance of inverter  482  is represented by capacitance  480  in  FIG. 4B  it should be understood that it is a part of inverter  482  and is not separate from it. In one embodiment capacitance  480  has a value of 100 μF. Panel string  402  comprises PV panels  311 ,  411   1 , . . .  411   N  and PIDs  302 ,  402   1 , . . .  402   N . PIDs  302 ,  402   1 , . . .  402   N  are serially connected. PIDs  402   1 , . . .  402   N  could have a similar configuration to PID  302 . In one embodiment, PIDs  401   1 , . . .  401   N  are identical to PID  302  and have a discharge resistance and capacitance at their output terminals of the same values as discharge resistance  372  and output capacitance  370  of PID  302 . 
         [0062]      FIG. 5A , with reference to  FIGS. 1A through 4B , is a flow diagram of a string continuity monitoring method performed by a PID that has disconnected from a panel string. At step  502 , the PID determines if the string current is less than a minimum value (I MIN ). In one embodiment, referring to  FIG. 4A , controller  310  receives a current measurement from current sensor  340  and compares the measured value to a minimum value. In one example embodiment the minimum value is 350 mA. In another embodiment, referring to  FIG. 4B , controller  312  receives a current measurement from current sensor  342  and compares the measured value to a minimum value. If the string current is greater or equal to the minimum value (YES at step  502 ), then the string is determined to have continuity, the PV panel is operatively coupled to the PV panel string permanently at  503  and the continuity monitoring method ends at step  504 . Permanently in this context means that the PV panel remains operatively coupled to the PV panel string until the next PV panel string disconnect condition occurs. If the string current is less than I MIN  (NO at step  502 ), then the PID operatively couples the PV panel to the PV panel string at step  506 . In one embodiment, referring to  FIG. 4A  controller  310  of PID  301  closes switch  330  and output capacitance  356  is charged to the output voltage of PV panel  311  (V PV ) by panel  311 . In one example embodiment, V PV  is 33 V. If there is continuity in string  401  and string  401  is connected to inverter  482 , then capacitance  480  of inverter  482  will also be incrementally charged by a voltage of V PV . Remaining PIDs  401   1 , . . .  401   N  in string  401  could have capacitances and discharge resistances connected at their outputs however their capacitances will not be charged since they are shunted by their discharge resistances. Switch  330  of PID  301  could have a very low ON resistance (milli-ohms) to reduce power losses during normal operation of the PID. The charging current to charge input capacitance  480  could therefore be large and could cause damage. 
         [0063]    In one embodiment, the charging current is limited by gradually increasing the duty cycle of switch  330 . Switch  330  is switched to repetitively close and open with a period of T SW , a switch ON time of T PULSE  and a duty cycle of D. The duty cycle is the ratio of T PULSE /T SW . The duty cycle is gradually increased to control the current through the switch. In one example embodiment, the duty cycle is increased from 0 to 100% over 50 milli-seconds. 
         [0064]    In another embodiment, referring to  FIG. 4B , controller  312  of PID  302  switches switch  332  with a switching frequency “f BUCK ” and a duty cycle D BUCK  to produce a voltage V BUCK =D BUCK V PV . In one example embodiment the switching frequency is 200 kHz. Output capacitance  370  is charged to \T our  by PID  302 . If there is continuity in string  402  and string  402  is connected to inverter  482  then capacitance  480  of inverter  482  will also be incrementally charged by a voltage of V BUCK . Again, to prevent too large a charging current the duty cycle of switch  332  could be gradually increased. In one example embodiment the duty cycle increases from 0 to 75% over a time of 50 milli-seconds. 
         [0065]    Again with reference to  FIG. 5A , at step  508  the voltage at the output of the PID (V 1 ) is measured. In one embodiment controller  310 ,  312  measure the output voltage of PID  301 ,  302  using voltage sensor  341 ,  343 , respectively. At step  510  the PID disconnects the PV panel from the panel string. In one embodiment controller  310 ,  312  of PID  301 ,  302  opens switch  330 ,  332 , respectively. At step  512  the PID waits for a discharge period of duration T DIS  during which the PID output voltage could decrease. In one example embodiment T DIS  is 100 milli-seconds. In one embodiment, discharge resistance  354 ,  372  begins to discharges output capacitance  356 ,  370  in PID  301 ,  302 . If there is continuity in string  401 ,  402  then input capacitance  480  of inverter  482  will support the voltage across output capacitance  356 ,  370  during the discharge period and cause it to decrease at a lower rate than if there was no continuity in the string. 
         [0066]    At step  514  the voltage at the output of the PID after the discharge period (V 2 ) is measured. In one embodiment, controller  310 ,  312  measures the output voltage of PID  301 ,  302  using voltage sensor  341 ,  343 , respectively. The value of V 2  depends on the string continuity. If there is no string continuity, then V 2  is given by the formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     2 
                   
                   = 
                   
                     
                       V 
                       1 
                     
                      
                     
                        
                       
                         
                           - 
                           
                             T 
                             
                               DIS 
                               / 
                             
                           
                         
                          
                         
                           R 
                           PID 
                         
                          
                         
                           C 
                           PID 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C PLD  is the PID output capacitance and R PID  is the PID output resistance. For example, in PID  301 ,  302  C PLD  is the value of capacitance  356 ,  370  and R PID  is the value of discharge resistance  354 ,  372 . If the string does have continuity and is connected to a string inverter then V 2  is given by the formula: 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     2 
                   
                   = 
                   
                     
                       V 
                       1 
                     
                      
                     
                        
                       
                         
                           - 
                           
                             T 
                             
                               DIS 
                               / 
                             
                           
                         
                          
                         
                           MR 
                           PID 
                         
                          
                         
                           C 
                           INV 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where C INV  is the string inverter input capacitance and M is the number of PIDs in the string and all PIDs have an identical output resistance of R PID . For example, in string  401 ,  402  C INV  is the value of capacitance  480 , the number of PIDs is equal to N+1 and R PID , is the value of resistance  354 ,  372 . The above equation is an approximation and is valid when C INV &gt;&gt;C PID . 
         [0067]    At step  516  the ratio of V 2  to V 1  is compared to a threshold value (R). The value of R could represent the minimum possible discharge ratio that would occur if the string is continuous and connected to the inverter input capacitance and could be calculated using equations (1) and (2). In one embodiment R is given by the equation: 
         [0000]    
       
         
           
             
               
                 
                   R 
                   = 
                   
                      
                     
                       
                         - 
                         
                           T 
                           
                             DIS 
                             / 
                           
                         
                       
                        
                       
                         MR 
                         PID 
                       
                        
                       
                         C 
                         INV 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    In one example embodiment R is equal to 0.23. 
         [0068]    If the ratio of V 2  to V 1  is greater than R (YES at step  516 ) then the string is determined to have continuity and the PV panel is operatively coupled to the string at step  518 . At step  520  the PID waits for a time T WAIT . After a time of T WAIT  the string current is again evaluated at step  502 . T WAIT  could represent a safe time interval between string continuity checks. In one example embodiment T WAIT  is a random value between 3 and 8 seconds. If the ratio of V 1  to V 2  is not greater than R at (NO at step  516 ) then the string is deemed to not have continuity and the PID disconnects from the string at step  522 . 
         [0069]    More elaborate string continuity monitoring methods based on this type of discharge ratio measurements are also possible. For example, it could be beneficial for a PID to perform a minimum number of discharge ratio measurements and only couple to the string if a minimum number of consecutive discharge ratio measurements are all greater than the threshold value R (indicating string continuity). This could prevent a single erroneous discharge ratio measurement causing a PV panel to be operatively coupled permanently to and powering a PV panel string which did not have continuity. It could be beneficial to not have the PV panel operatively coupled to the PV panel string during the waiting periods between the minimum required number of consecutive discharge ratio measurements. It could be beneficial for the PV panel to be operatively coupled to the PV panel string over the waiting period between discharge ratio measurements only after a minimum number of consecutive discharge ratio measurements have all been greater than the threshold value R, showing continuity in the string and the presence of the inverter.  FIG. 5B  is a flow diagram of this more elaborate string continuity monitoring method. At  531  a discharge counter (COUNT) is initialized to one. At  532  the PID determines if the string current is less than a minimum value (I MIN ). If the string current is greater or equal to the minimum value (YES at  532 ) then the string is determined to have continuity and normal operation resumes at  534 . If the string current is not greater than or equal to I MIN  (NO at  532 ) then the PID operatively couples the PV panel to the PV panel string at  536 . At  538  the voltage at the output of the PID (V 1 ) is measured. At  540  the PID disconnects the PV panel from the panel string. At  542  the PID waits for a discharge period of duration T DIS . At  544  the voltage at the output of the PID (V 2 ) is measured. At  546  the ratio of V 2  to V 1  (the discharge ratio) is compared to a discharge ratio threshold (R). If the ratio of V 2  to V 1  is greater than R (YES at  546 ) then the discharge count is compared to a minimum required number of consecutive discharge ratio measurements above the discharge threshold (MIN) at  548 . If the discharge count is less than the minimum required number of consecutive discharge measurements above the discharge threshold (YES at  548 ) then the discharge count is incremented at  550  and the PID waits for waiting period T WAIT  at  552 . The PID is not connected to the string during this waiting period so that in the event of an erroneous discharge ratio measurement the string is not powered and safety could be increased. However, since the PV panel is not operatively coupled to the PV panel string the inverter cannot start operation during this waiting period and is unlikely to start before the minimum required number of consecutive discharge ratio measurements above the discharge threshold have been achieved. At the end of the waiting period the string current is again compared to the minimum current (I MIN ) at  532 . 
         [0070]    If the discharge count is not less than MIN (NO at  548 ) then a regular string continuity monitoring operation begins at  554 . A regular string continuity monitoring operation could be as described in  FIG. 5A . It could involve repetitive discharge ratio measurements with the PV panel operatively coupled to the inverter between discharge measurements which could allow the inverter to start operation. In one embodiment the required minimum number of consecutive discharge measurements above the discharge threshold is five, the waiting period between discharge ratio measurements is a random value between 10 and 20 seconds, the discharge period is 100 mS and the discharge ratio threshold is 0.23. 
         [0071]      FIG. 6A , with reference to  FIGS. 1A through 5B , is a schematic diagram of an equivalent circuit of a PID connected to a string and a string inverter. The output resistance and capacitance of the PID is represented by resistance  672  and capacitor  670 . The inverter capacitance is represented by capacitance  680  and the output resistance and capacitance of the “N” other PIDs in the string are represented by resistors  692   1 , . . .  692   N  of identical value and capacitances  690   1 , . . .  690   N  of identical value.  FIG. 6B , with reference to  FIGS. 1A through 6A , is a schematic diagram of an approximate equivalent circuit of a PID connected to a string and a string inverter.  FIG. 6B  is appropriate when the inverter capacitance is more than ten times the PID output capacitance. 
         [0072]      FIG. 7 , with reference to  FIGS. 1A through 6 , is a block diagram of one embodiment of a PID controller  700  according to an embodiment herein. Controller  700  could be used for controller  310 ,  312  in accordance with the embodiments herein. Controller  700  comprises driver  710 , memory  720 , clock  730 , voltage regulator  740 , central processing unit (CPU)  750 , user interface (UI)  760 , and control and data bus  770 . Voltage regulator  740  converts the variable PV panel output voltage to a constant controller supply voltage in an embodiment. Driver  710  supplies switch drive signals to switches  320 ,  330 ,  322 ,  332  to control their respective opening and closing. Firmware for the operation of the controller  700  is stored in memory  720 . In one embodiment, memory  720  comprises non-volatile memory such as Flash, Electrically Erasable Programmable Read Only Memory (EEPROM), EPROM, ROM, etc. The firmware is executed on CPU  750 . Clock  730  controls the internal timing of the operation of the controller  700 . UI  760  indicates the status of a PID to a user. Control and data bus  770  interconnects these components of the controller  700  with each other as shown, in one embodiment herein. 
         [0073]    It could be that power dissipation from the flow of current in discharge resistances  356 ,  357  of PID  301 ,  302  during normal operation (when the PID is not performing a discharge ratio string continuity monitoring operation) is unacceptably high in some applications. In those applications it could be beneficial to place a switch in series with the discharge resistance to reduce power dissipation. 
         [0074]      FIG. 8 , with reference to  FIGS. 1A through 7 , is a schematic diagram of an example PID with reduced power dissipation. PID  801  comprises controller  812 , switches  822 ,  832 , inductor  860 , output capacitance  870 , current sensor  842 , diode  852 , discharge resistance  872 , discharge switch  802 , voltage sensor  843 , and output terminals  884 ,  886 . The input of PID  801  operatively connects to PV panel  811 . Output terminal pair  884 ,  886  operatively connects to the PV panel string (not shown in  FIG. 8 ). The output terminal pair  884 ,  886  comprises the points of coupling between the PID  801  and the PV panel string (not shown in  FIG. 8 ). In some circumstances, one may not want to take the power penalty associated with continuous power dissipation through discharge resistance  872  in the PID  801 , which is why switch  802  is included in PID  801 . In one example embodiment, discharge resistance  872  has a value of 15 k-ohms, output capacitance  870  has a value of 10 μF, inductor  860  has a value of 5.6 μH, current sensor  842  is a 2 milli-ohm resistor, and the MPP voltage of panel  811  is 33 V and its MPP current is 10 A. However, the embodiments herein are not restricted to these example values. 
         [0075]    PID  801  is of a “buck” type DC to DC converter configuration and performs voltage conversion and MPP tracking similarly to PID  302  of  FIG. 3B . In normal operation discharge switch  802  is open, no current flows in discharge resistance  872  and there is no power dissipated in discharge resistor  872 . During a discharge ratio string continuity monitoring operation controller  812  closes switch  802 . When the discharge ratio string continuity monitoring operation finishes controller opens switch  802 . For example, referring to  FIG. 5A  switch  802  could be closed at  506  and opened at  504 . Referring to  FIG. 5B , switch  802  could be closed at  536 . 
         [0076]    In accordance with the embodiments herein, utilizing a discharge resistance (e.g., discharge resistance  354 ,  372 ,  872 ) is uncommon in power applications/configurations due to the potential degradation of performance as a result of power loss caused by the discharge resistance. Accordingly, the embodiments herein are utilizing the discharge resistance  354 ,  372 ,  872  in order to perform a discharge measurement, which is contrary to industry practices. 
         [0077]    The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. For example, it should also be appreciated that the embodiments disclosed herein are not necessarily restricted to single PV panel string implementations. Accordingly, multiple PV panel strings could be connected in parallel to the same inverter, as shown in  FIG. 2B . Moreover, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the description herein.