Abstract:
A system and method of monitoring a photovoltaic (PV) installation includes providing a string of multiple panel interface device enabled PV panels; operatively connecting an inverter to the string; operatively connecting at least one PV panel to the string; discharging an input capacitance of the inverter; comparing a current in the string to a predetermined current threshold value; and controlling connection of the at least one PV panel to the string based on the comparing of the current in the string to the predetermined current threshold value. A panel interface device may be used to discharge the input capacitance of the inverter.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. patent application Ser. No. 13/840,162 filed on Mar. 15, 2013, 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 connected in series 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 AC power suitable for the electrical grid. Typically, there are between five and twenty PV panels in a 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 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 of monitoring a PV installation, the method comprising providing at least one PV panel reconnected to a power system after a previous disconnection, wherein the power system does not draw current from the at least one PV panel; determining whether a maintain connection condition is satisfied to maintain connection of the at least one PV panel to the power system; and controlling the connection of the at least one PV panel to the power system based on whether the maintain connection condition is satisfied. The controlling the connection of the at least one PV panel to the power system may comprise automatically resuming the connection of the at least one PV panel to the power system responsive to determining that the maintain connection condition is satisfied. The controlling the connection of the at least one PV panel to the power system may comprise automatically disconnecting the at least one PV panel from the power system responsive to determining that the maintain connection condition is not satisfied. The maintain connection condition may comprise determining whether there is a minimum magnitude of a negative current in the power system. The negative current may comprise a discharge current of an inverter input capacitance, and wherein the determining whether the maintain connection condition is satisfied comprises determining whether there is at least a minimum magnitude of current flow in the power system. The method may further comprise toggling at least one switch that is operatively connected to the at least one PV panel to control an amplitude of the negative current. The method may further comprise providing a string of PV panels; and using a panel interface device (PID) to operatively disconnect the at least one PV panel from the string. 
         [0008]    Another embodiment provides a method of monitoring a PV installation, the method comprising providing a string of multiple panel interface device (PID) enabled PV panels; operatively connecting an inverter to the string; operatively connecting at least one PV panel to the string; discharging an input capacitance of the inverter; comparing a current in the string to a predetermined current threshold value; and controlling connection of the at least one PV panel to the string based on the comparing of the current in the string to the predetermined current threshold value. The method may further comprise using a PID to discharge the input capacitance of the inverter. The method may further comprise waiting for a predetermined period of time prior to the discharging of the input capacitance. The method may further comprise checking for a negative DC current in the string during the waiting process. The method may further comprise immediately maintaining connection of the at least one PV panel to the string when the negative DC current in the string is detected during the waiting process. The method may further comprise maintaining connection of the at least one PV panel to the string when the current in the string is less than the predetermined current threshold value. The method may further comprise disconnecting the at least one PV panel to the string when the current in the string is greater than or equal to the predetermined current threshold value. The method may further comprise toggling at least one switch that is operatively connected to the at least one PV panel to control a rise of the current. 
         [0009]    Another embodiment provides a PV system comprising at least one PV panel; a power system operatively connected to the at least one PV panel, wherein the power system does not draw current from the at least one PV panel; at least one switch that controls maintaining connection of the at least one PV panel to the power system; and a controller operatively connected to the at least one switch, wherein the controller determines whether a maintain connection condition is satisfied to maintain connection of the at least one PV panel to a power system, and supplies switch drive signals to the at least one switch for the maintaining connection of the at least one PV panel to the power system based on whether the maintain connection condition is satisfied. The controller may supply the switch drive signals to the at least one switch to automatically resume the connection of the at least one PV panel to the power system responsive to determining that the maintain connection condition is satisfied. The controller may supply the switch drive signals to the at least one switch to automatically disconnect the at least one PV panel from the power system responsive to determining that the maintain connection condition is not satisfied. The PV system may further comprise a current sensor operatively connected to the controller, wherein the maintain connection condition comprises using the current sensor to determine whether there is a minimum magnitude of a negative current in the power system. The PV system may further comprise a PID operatively connected to the at least one PV panel; and an inverter operatively connected to the PID, wherein the negative current comprises a discharge current of an input capacitance of the inverter, and wherein the current sensor determines whether there is at least a minimum magnitude of current flow in the power system. The controller may toggle the at least one switch to control an amplitude of the negative current. 
         [0010]    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 
         [0011]    The embodiments herein will be better understood from the following detailed description with reference to the drawings, in which: 
           [0012]      FIG. 1A  is a block diagram of an example grid tied PV installation; 
           [0013]      FIG. 1B  is a block diagram of another example grid tied PV installation; 
           [0014]      FIG. 2A  is a block diagram of a further example grid tied PV installation according to an embodiment herein; 
           [0015]      FIG. 2B  is a block diagram of another example grid tied PV installation according to an embodiment herein; 
           [0016]      FIG. 3A  is a block diagram of one embodiment of a panel interface device (PID) according to an embodiment herein; 
           [0017]      FIG. 3B  is a block diagram of another example embodiment of a panel interface device (PID) according to an embodiment herein; 
           [0018]      FIG. 4A  is a block diagram of an embodiment of a panel string according to an embodiment herein; 
           [0019]      FIG. 4B  is a block diagram of another embodiment of a panel string according to an embodiment herein; 
           [0020]      FIG. 5  is a flow diagram of an example reconnect operation according to an embodiment herein; and 
           [0021]      FIG. 6  is a block diagram of an embodiment of a controller according to an embodiment herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    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. 
         [0023]    The embodiments herein provide a technique for safer interaction with PV solar panels. Referring now to the drawings, and more particularly to  FIGS. 1A through 6 , where similar reference characters denote corresponding features consistently throughout the figures, there are shown preferred embodiments. 
         [0024]      FIG. 1A  is a block diagram of an example grid tied PV installation  100 . The example installation  100  includes panel “string”  160  operatively connected to the input of inverter  120  (e.g., a power inverter, etc.). String  160  comprises a series connection of PV panels  111   1 , . . .  111   N . The output of 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. 
         [0025]      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 an additional disconnect switch  141  between string  160  and the input of inverter  120 . In other embodiments additional disconnect means may be used between the string  160  and the inverter  120  and/or between the inverter  120  and the grid  150 , including fuses for example. Additional disconnect means may provide for easier maintenance of the inverter  120 . 
         [0026]    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 string and not output power whenever the PV installation is disconnected from the grid. 
         [0027]    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. 
         [0028]    It could be useful if PV panels isolated themselves from their string in the event of an open circuit condition in their string. This might be caused by, for example: a physical break in the string; removal of one or more PV panels from the string for maintenance, repair, or replacement; disconnection of the 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 string. 
         [0029]      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 string  261  and inverter  120 . String  261  comprises PV panels  111   1 , . . .  111   N  and panel interface devices (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  perform a safety disconnect function and disconnect their respective PV panels  111   1 , . . .  111   N  (where i can be in the range for 1 to N) from the string in the event of an open circuit or high resistance condition on the string. In one embodiment PIDs  201   1  . . .  201   N  monitor the current in string  261  and disconnect their respective PV panels  111   1  . . .  111   N  when the string current drops below a predetermined threshold value. 
         [0030]    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 string  261 , the opening of a switch (not shown) in the string  261 , the blowing of a string fuse (not shown) and/or a fault in inverter  120 . 
         [0031]    A high resistance condition could be caused by inverter  120  ceasing operation. 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 inverter  120  will become high resistance and the string current will fall substantially to zero. 
         [0032]      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  includes a string of PV panels with respective PIDs as shown in  FIG. 2A  for example. The example shown in  FIG. 2A  includes only one PV panel string  261 , and the example shown in  FIG. 2B  includes multiple PV panel strings  211   1 ,  211   2 , . . .  211   N . 
         [0033]      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 , shunt switch  320 , series switch  330 , current sensor  340 , diode  350 , and output terminals  382 ,  384 . The input of PID  301  operatively connects to PV panel  311 . PID output terminals  382 ,  384  operatively connect to the PV panel string (not shown in  FIG. 3A ). 
         [0034]    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 . Diode  350  allows string current to flow in the event that PV panel  311  cannot supply sufficient power to operate controller  310 . PV panel  311  could be unable to power controller  310  if it was defective or heavily shaded, for example. In this situation controller  310  would be unable to supply drive voltage to switch  320  and keep it closed, however current can still flow through diode  350  to bypass the panel  311 . Under conditions where the controller  310  has sufficient power to drive the switch  320 , the closed switch dissipates less power than the diode  350 . 
         [0035]    Current sensor  340  monitors the string current. Controller  310  draws its power from PV panel  311  and receives a current measurement from current sensor  340 . Controller  310  controls the operation of switches  320 ,  330 . 
         [0036]    In normal operation, with PV panel  311  illuminated and producing a DC voltage, the shunt switch  320  is open and series switch  330  is closed. PV panel  311  is in series with the other panels in the panel string (not shown in  FIG. 3A ) and contributes to the string voltage and current. 
         [0037]      FIG. 3B , with reference to  FIGS. 1A through 3A , is a block diagram of another embodiment of a PID  302  according to an embodiment herein. PID  302  incorporates DC power optimizer functionality in addition to safety disconnect functionality. A DC power optimizer uses a DC to DC converter to maximize the energy 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. 
         [0038]    PID  302 , in the example shown in  FIG. 3B , comprises controller  312 , switches  322 ,  332 , inductor  360 , capacitor  370 , current sensor  342 , diode  352 , and output terminals  384 ,  386 . The input of PID  302  operatively connects to PV panel  311 . Output terminals  382 ,  384  operatively connect to the PV panel string (not shown in  FIG. 3B ). 
         [0039]    PID  302  is of a “buck” type DC to DC converter design and 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. 
         [0040]    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). 
         [0041]    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 capacitor  370  and the string. The duty cycle “D” of PID  302  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 OUT ) of PID  302  to its input voltage (V IN ) depends on the duty cycle and is given by the equation: 
         [0000]      V OUT =DV IN    
         [0042]    V OUT  is defined as the voltage across terminals  384 ,  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 
             
           
         
       
     
         [0043]    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  352  is in parallel with switch  320 . Diode  352  allows string current to flow in the event that PV panel  311  cannot supply sufficient power to operate controller  310  and keep switch  322  closed. 
         [0044]    Disconnect Operations 
         [0045]    Referring to  FIG. 2A , in the event of an open circuit or high resistance condition in string  261 , PIDs  201   1 ,  201   i , . . .  201   N  disconnect their respective PV panels  111   1 ,  111   i , . . .  111   N  from the string. PIDs  201   1 ,  201   i , . . .  201   N  could also disconnect their respective PV panels  111   1 ,  111   i , . . .  111   N  from string  261  in the event of an arc fault in the string  261 . An arc fault is large and undesired current flow between a PV string and ground or between different conductors in the string and may occur when the insulation of the string wiring or string connectors fails or when the string is severed. Arc faults can lead to electrification of the PV panel mounting system, serious damage to equipment, fire, and/or injuries to personnel. 
         [0046]    An open circuit or high resistance condition in a PV panel string results in the loss or substantial reduction in string current. In PID  301  of  FIG. 3A , a loss or reduction in string current is detected by controller  310  using current sensor  340 . In one embodiment, a string current of less than 200 mA triggers a disconnect of the PV panel  311 . Here, controller  310  opens series switch  330 , disconnecting PV panel  311  from the string (not shown in  FIG. 3A ). With series switch  330  open, maintenance and emergency workers are safely protected from the output of PV panel  311 . Controller  310  could also close shunt switch  320 , thereby maintaining the electrical continuity of the string at the point of connection of PID  301  to the string. In another embodiment controller  310  could keep switch  320  open. This could prevent damage to PID  301  in the event that it were to reconnect to an inverter with a fully charged input. In another embodiment switch  320  could be kept open and an auxiliary switch (not shown) in parallel with switch  320  is held closed and maintains the electrical continuity of the string. The auxiliary switch could have a sufficiently large “ON” resistance to prevent damage to PID  301  in the event that it were to reconnect to an inverter with a fully charged input. In one embodiment its “ON” resistance is approximately 10 ohms. 
         [0047]    In another embodiment, controller  310  monitors current sensor  340  for the presence of an arc fault in the string. On detection of an arc fault, controller  310  disconnects PV panel  311  from the string by opening series switch  330 . With switch  330  open, the PV panel  311  is disconnected from the PID  301 , and the output voltage of the PV panel  311  does not contribute to the arc fault. 
         [0048]    Arc fault detection could be in accordance with any of various known techniques. Some arc fault detection methods involve spectral analysis of the string current or voltage for a characteristic arc “signature”. In one embodiment, controller  310  contains a Digital Signal Processor (DSP) (not shown) to facilitate the spectral analysis of the string current sensed by current sensor  340 . 
         [0049]    In PID  302  of  FIG. 3B , the loss or reduction in string current from an open circuit or high resistance condition in the string (not shown in  FIG. 3B ) is detected by controller  312  using current sensor  342 . In one embodiment, a string current of less than 200 mA triggers a disconnect of the PV panel  311 . Here, controller  312  disconnects panel  311  from the string by holding switch  332  open. Accordingly, with switch  332  held open, PV panel  311  is disconnected from the string. 
         [0050]    Controller  312  could also close shunt switch  322 , thereby maintaining the electrical continuity of the string at the point of connection of PID  302  to the string. In another embodiment controller  312  could keep switch  322  open. This could prevent damage to PID  302  in the event that it were to reconnect to an inverter with a fully charged input. In another embodiment switch  322  could be kept open and an auxiliary switch (not shown) in parallel with switch  322  is held closed and maintains the electrical continuity of the string. The auxiliary switch could have a sufficiently large “ON” resistance to prevent damage to PID  302  in the event that it were to reconnect to an inverter with a fully charged input. In one embodiment the “ON” resistance is approximately 10 ohms. 
         [0051]    In another embodiment, controller  312  monitors current sensor  342  for the presence of an arc fault in the string. On detection of an arc fault, controller  312  disconnects PV panel  311  from the string by opening switch  332 . With switch  332  open, output voltage of the PV panel  311  does not contribute to the arc fault. 
         [0052]    Reconnect Operations 
         [0053]    After a PID has isolated its PV panels from the string, it could check to determine whether the disconnect condition has been resolved and whether its PV panel can be safely reconnected to the string. 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” the complete disclosure of which, in its entirety, is herein incorporated by reference. 
         [0054]    Reconnect 
         [0055]    Again referring to  FIG. 2A , after PID  201   i  (where i can be in the range for 1 to N) has disconnected PV panel  111   i  from string  261  due to, for example, a loss of string current or arc fault, etc., PID  201   i  checks for the presence of inverter  120  connected to the string  261 . In one embodiment, PID  201   i  reconnects PV panel  111   i  to string  261  when it senses the input capacitance of inverter  120 . In this embodiment, PV panel  111   i  is reconnected to string  261  even in the absence of a string current. The input capacitance of a PV string inverter  120  varies by manufacturer but is normally large and could be in the range of approximately 100 uF to 5 mF. 
         [0056]    In many cases, after an inverter has stopped converting power it cannot immediately resume converting power. There is often a mandatory waiting period after the grid connection has been restored or the input voltage has returned to a valid operating value before the inverter can restart. The inverter cannot start operation and draw DC current from the string until this waiting period expires. The waiting period could be on the order of approximately five minutes in some embodiments. Therefore, after PID  201   i  has reconnected its PV panel  111   i  to string  261  it should remain connected to allow the inverter  120  sufficient time to start operation even if there is no string current flowing. After all PIDs  201   1 , . . .  201   N  have reconnected their respective PV panels  111   1 , . . .  111   N  to string  261  the string voltage at the input of the inverter  120  could be several hundred volts. If a break is created in the string  261  by, for example, someone removing a panel or disconnecting a cable, etc., serious injury could result. Accordingly, the embodiments herein provide a technique to monitor the continuity of a string (e.g., verify that a string is still physically continuous and connected to the inverter input) while waiting for the inverter to start, which substantially improves the safety of a PV panel array. 
         [0057]      FIG. 4A , with reference to  FIGS. 1A through 3B , is a block diagram of a PV panel string  401  operatively connected to an inverter  482  according to an embodiment herein. Inverter  482  comprises capacitor  480  which represents the input capacitance of the inverter  482 . The remaining inverter components have not been shown for clarity. String  401  comprises PV panels  311   1 , . . .  311   N  and PIDs  301   1 , . . .  301   N . PIDs  301   1 , . . .  301   N  are connected in series. 
         [0058]    When PID  301   i  (where i can be in the range for 1 to N) reconnects its illuminated PV panel  311   i  to the string  401  by closing switch  330  and opening switch  320 , a positive current will flow as inverter capacitor  480  is charged by the illuminated PV panel  311   i  so long as there is string continuity. Capacitor  480  will incrementally charge by the output voltage of PV panel  311   i . 
         [0059]    Furthermore, the charged inverter input capacitor  480  can be usefully employed to monitor string continuity. In one embodiment of a string continuity monitoring method, after capacitor  480  is incrementally charged by an amount equal to the output voltage of panel  311   i  by a reconnect operation, controller  310  opens switch  330  and closes switch  320 . So long as there is string continuity, capacitor  480  will discharge through closed switch  320  and the remaining PV panels  311   1 , . . .  311   N  in the string  401 , thereby causing a negative string current. The negative current is sensed by controller  310  through current sensor  340 . A negative current amplitude above a predetermined threshold value verifies string continuity and that it is still safe to have PV panel  311   i  remain connected to the string  401 . In one embodiment a negative current amplitude threshold value of approximately −300 mA is used. Controller  310  then opens switch  320  and closes switch  330  to maintain the connection of PV panel  311   i  to string  401 . This also recharges capacitor  480 . In one embodiment, controller  310  opens switch  330  and toggles switch  320  open and closed to control the rise in negative string current and prevents the string current from rising faster than the response time of PID  301 . In one embodiment the switch is toggled open and closed for approximately 10 mS with a frequency of 100 kHz. 
         [0060]      FIG. 4B , with reference to  FIGS. 1A through 4A , is a another block diagram of a PV panel string  401  operatively connected to an input of an inverter  482  according to an embodiment herein. Inverter  482  comprises capacitor  480  which represents the input capacitance of the inverter  482 . The remaining inverter components have not been shown for clarity. String  401  comprises PV panels  311   1 , . . .  311   N  and PIDs  302   1 , . . .  302   N . PIDs  302   1 , . . .  302   N  are connected in series. 
         [0061]    When PID  302 , (where i can be in the range for 1 to N) reconnects its respective illuminated PV panel  311   i  to the string  401 , the capacitor  480  will incrementally charge by an amount V OUT , which is the output voltage of PID  302   i . In one embodiment, V OUT  is the open circuit voltage of PV panel  311   i . 
         [0062]    In an embodiment of a string continuity monitoring method, after capacitor  480  is charged by a reconnect operation, controller  312  opens switch  332  and toggles switch  322  open and closed. In one embodiment the toggle frequency is in the range of 100 kHz and switch  322  is toggled for a duration of 10 mS. So long as there is string continuity, capacitor  480  will discharge through switch  320  and the remaining PV panels  311   1 , . . .  311   N  in the string  401  causing a negative string current. In one embodiment the width of the toggle “ON” pulse is varied to control the rise in string current and prevent it rising faster than the PID&#39;s response. The negative current is sensed by controller  312  through current sensor  342 . A negative current amplitude above a predetermined threshold value verifies string continuity and that it is still safe to have panel  311   i  remain connected to the string  401 . In one embodiment, a negative current amplitude threshold value of approximately −300 mA is used. PID  302  then resumes outputting voltage V OUT  to the string  401  to maintain the connection of PV panel  311   i  to string  401 . This also recharges capacitor  480 . Diode  352  allows string current to flow in the event that PV panel  311   i  cannot supply sufficient power to operate controller  312 . 
         [0063]      FIG. 5 , with reference to  FIGS. 1 through 4B , is a flowchart illustrating an exemplary string continuity monitoring method  500  for a string of PID enabled PV panels (e.g., PV panels  311   1 , . . .  311   N ) according to an embodiment herein. At step  502 , the PV panel (e.g., PV panel  311   i ) is reconnected to the string (e.g., string  401 ) by a reconnect operation. At step  504 , there is a waiting period of length T WAIT . T WAIT  could be any suitable period of time. In one example, the waiting period T WAIT  could be in the range of approximately 1 to 10 seconds. However, the embodiments herein are not restricted to a particular waiting period or range. After the waiting period expires, a PID discharge operation is performed at step  506  whereby the PID (e.g., PID  302   i ) attempts to discharge the input capacitance of the string inverter (e.g., inverter  482 ). The discharge operation could include a temporary disconnection of the PV panel (e.g., PV panel  311   i ) from the string (e.g., string  401 ) while checking for negative current as described next, however it is not considered to be permanently “disconnected state”. At step  508 , the string current (I STRING ) is measured and compared to a negative current threshold value I MIN . In one example embodiment, I MIN , is approximately −300 mA. However, the embodiments herein are not restricted to a particular threshold value or range of values. If I STRING  is less than I MIN  (Yes) as detected by a sensor (e.g., current sensor  342 ), then continuity of the string (e.g., string  401 ) is confirmed and the panel connection to the string is maintained at step  510 . A discharge current is conventionally negative in this method. If however in step  508  I STRING  is greater than or equal to I MIN  (No), then continuity of the string (e.g., string  401 ) is not confirmed and the PV panel (e.g., PV panel  311   i ) is disconnected from the string (e.g., string  401 ) at step  512  and enters a “disconnected state” until the continuity of the string (e.g., string  401 ) can be confirmed in a subsequent process. 
         [0064]    Although not shown in  FIG. 5  a separate check for a positive DC string current could be simultaneously running during method  500 . If a positive DC string current is detected by a sensor (e.g., current sensor  342 ) it could indicate that the inverter (e.g., inverter  482 ) has/had started and could cause method  500  to terminate. 
         [0065]    Although not shown in  FIG. 5  a separate check for a negative DC string current could be continuously running during wait operation  504 . A negative current could be produced by the discharge operation of another PID  311   j  (where j≠i) on string  401 , for example. If a negative current is detected during wait operation  504 , then the PV panel (e.g. PV panel  311   i ) connection could immediately be maintained at step  510 . 
         [0066]      FIG. 6 , with reference to  FIGS. 1A through 5 , is a block diagram of one embodiment of a PID controller  600  according to an embodiment herein. Controller  600  could be used for controller  310 ,  312  in accordance with the embodiments herein. Controller  600  comprises driver  610 , memory  620 , clock  630 , voltage regulator  640 , central processing unit (CPU)  650 , user interface (UI)  660 , and control and data bus  670 . Voltage regulator  640  converts the variable PV panel output voltage to a constant controller supply voltage in an embodiment. Driver  610  supplies switch drive signals to switches  320 ,  330 ,  322 ,  332  to control their respective opening and closing and toggling. Firmware for the operation of the controller  600  is stored in memory  620 . In one embodiment, memory  620  comprises non-volatile memory such as Flash, Electrically Erasable Programmable Read Only Memory (EEPROM), EPROM, ROM, etc. The firmware is executed on CPU  650 . Clock  630  controls the internal timing of the operation of the controller  600 . UI  660  indicates the status of a PID to a user. Control and data bus  670  interconnects these components of the controller  600  with each other as shown, in one embodiment herein. 
         [0067]    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 appended claims.