Patent Publication Number: US-11031905-B2

Title: Intelligent safety disconnect switching

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 13/840,162 filed on Mar. 15, 2013, the contents which are incorporated in their entirety herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to the field of PhotoVoltaic (PV) solar panels and in particular to methods and apparatus for their safety. 
     BACKGROUND 
     Photovoltaic (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 illumination is about 0.7 V. An illuminated PV panel with 72 Direct Current (DC) PV cells will therefore have an output voltage of about 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 string voltage of several hundred volts. 
     PV panels produce power whenever they are illuminated. As described above, string voltages could reach hazardous levels of hundreds of volts. This voltage represents a safety hazard during PV panel installation and maintenance. If the PV panels are mounted to a roof or integrated into building structures, then 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. 
     SUMMARY 
     According to an aspect of the present disclosure, a method involves: detecting, in a switching device that is connected between a PV panel and a PV panel string, a low current condition or an arc fault condition in the PV panel string; and automatically controlling the switching device to disconnect the PV panel from the PV panel string, responsive to detection of the low current condition or the arc fault condition. 
     The detecting may involve independently detecting the low current condition or the arc fault condition at each of multiple switching devices connected between multiple PV panels, including the PV panel, that are connected to the PV panel string. The automatically controlling could then involve independently automatically controlling each of the switching devices to disconnect each PV panel from the PV panel string responsive to detection of the low current condition or the arc fault condition at each PV panel. 
     In an embodiment, the detecting involves spectral analysis of current or voltage for an arc signature to detect an arc fault condition. 
     A method may also include waiting for a randomly chosen wait time after detecting the low current condition or the arc fault condition before automatically disconnecting the PV panel from the PV panel string. 
     In another embodiment, the method involves receiving, at the PV panel, a disconnect command; and automatically disconnecting the PV panel from the PV panel string responsive to the disconnect command. 
     A method may involve determining whether an inverter to which the PV panel string is connected, is connected to an electrical grid; and sending the disconnect command to the PV panel where the inverter is not connected to the electrical grid. 
     The determining may involve determining whether an output voltage of the inverter is below a specified minimum voltage, in which case the sending may involve sending the disconnect command to the PV panel where the inverter output voltage is below the specified minimum voltage. 
     A PV panel disconnect switching arrangement according to another aspect of the present disclosure includes switches to control connection of the PV panel to a PV panel string and bypass of the PV panel on disconnection of the PV panel from the PV panel string; and a controller operatively coupled to the switches, to detect a low current condition or an arc fault condition in the PV panel string while the PV panel is connected to the PV panel string, to automatically disconnect the PV panel from the PV panel string responsive to detection of the low current condition or the arc fault condition. 
     The controller may be configured to detect the low current condition or the arc fault condition by independently detecting the low current condition or the arc fault condition at each of a plurality of PV panels, including the PV panel, that are connected to the PV panel string, and to automatically disconnect the PV panel from the PV panel string by independently automatically disconnecting each PV panel from the PV panel string responsive to detection of the low current condition or the arc fault condition at each PV panel. 
     The controller may be configured to detect the arc fault condition by spectral analysis of current or voltage for an arc signature. 
     In an embodiment, the controller is configured to wait for a randomly chosen wait time after the low current condition or the arc fault condition is detected before automatically disconnecting the PV panel from the PV panel string. 
     The PV panel disconnect switching arrangement may also include a communication module, operatively coupled to the controller, to receive a disconnect command, and the controller could be further configured to automatically disconnect the PV panel from the PV panel string responsive to the disconnect command. 
     The communication module may enable communication with a grid sensor that is configured to determine whether an inverter to which the PV panel string is connected, is connected to an electrical grid, and to send the disconnect command to the PV panel disconnect switching arrangement where the inverter is not connected to the electrical grid. 
     The grid sensor could be configured to determine whether the inverter is connected to the electrical grid by determining whether an output voltage of the inverter is below a specified minimum voltage, and to send the disconnect command to the PV panel where the inverter output voltage is below the specified minimum voltage. 
     According to a further aspect, a power system includes: a plurality of PV panels coupled together in a PV panel string; respective PV panel disconnect switching arrangements at the PV panels and respectively coupled to the PV panels; an inverter coupled to the PV panel string; and a grid sensor to determine whether the inverter is connected to an electrical grid, and to send a disconnect command to the PV panel disconnect switching arrangements where the inverter is not connected to the electrical grid. Each of the PV panel disconnect switching arrangements includes: switches to control connection of the PV panel to the PV panel string and bypass of the PV panel on disconnection of the PV panel from the PV panel string; a communication module, operatively coupled to the controller, to receive the disconnect command; and a controller operatively coupled to the switches, to detect a low current condition or an arc fault condition in the PV panel string while the PV panel is connected to the PV panel string, and to automatically disconnect the PV panel from the PV panel string responsive to the disconnect command or detection of the low current condition or the arc fault condition. 
     The grid sensor could be configured to determine whether the inverter is connected to the electrical grid by determining whether an output voltage of the inverter is below a specified minimum voltage, and to send the disconnect command to the PV panel where the inverter output voltage is below the specified minimum voltage. 
     In an embodiment, the grid sensor includes: a voltage sensor to monitor the inverter output voltage; and a communication module that enables communication with the PV panel disconnect switching arrangements. 
     The grid sensor could include a User Interface (UI) to indicate a present or absent grid state based on whether the inverter is connected to the electrical grid. 
     The controller of each of the PV panel disconnect switching arrangements could include a random number generator. For example, the controller of each of the PV panel disconnect switching arrangements could be configured to wait for a respective randomly chosen wait time after the low current condition or the arc fault condition is detected before automatically disconnecting, from the PV panel string, the PV panel to which the PV panel disconnect switching arrangement is coupled. 
     According to another aspect of the disclosure a method involves: determining whether a reconnect condition, for reconnecting a PV panel to a power system from which the PV panel is disconnected, is satisfied; automatically reconnecting the PV panel to the power system responsive to determining that the reconnect condition is satisfied; determining whether a power system operating condition is satisfied on reconnection of the PV panel; automatically disconnecting the PV panel from the power system responsive to determining that the power system operating condition is not satisfied on reconnection of the PV panel. 
     In an embodiment, the reconnect condition includes a time condition, and the determining whether a reconnect condition is satisfied involves determining whether a time period has elapsed after disconnection of the PV panel from the power system. 
     The power system operating condition could include a current flow condition, in which case the determining whether a power system operating condition is satisfied might involve determining whether there is at least a minimum magnitude of current flow in the power system. 
     The determining whether there is at least a minimum magnitude of current flow in the power system could involve measuring current flow in the power system over a time during which the PV panel remains reconnected on reconnection to the power system. 
     The power system operating condition could be a current pulse condition, with the determining whether a power system operating condition is satisfied involving determining whether a current pulse is detected at the PV panel on reconnection of the PV panel to the power system. 
     Where the reconnect condition is a connected PV panel limit condition, the determining whether a reconnect condition is satisfied involves determining whether a predetermined number of other PV panels are reconnected to the power system within a predetermined period of time. 
     Determining whether a predetermined number of other PV panels are reconnected to the power system within a predetermined period of time could involve detecting, at the PV panel, current pulses generated in the power system on reconnection of the other PV panels to the power system during the predetermined period of time. 
     The determining whether a predetermined number of other PV panels are reconnected to the power system within a predetermined period of time could instead involve detecting, at the PV panel, signals sent by other PV panels on reconnection of the other PV panels to the power system during the predetermined period of time. 
     The method could also involve broadcasting a signal from the PV panel on connection of the PV panel to the power system. 
     In an embodiment, the reconnect condition is a connected PV panel limit condition, and the determining whether a reconnect condition is satisfied involves determining whether a predetermined minimum number of PV panels including the PV panel are ready to reconnect to the power system. The determining whether the predetermined minimum of PV panels are ready to reconnect to the power system could involve detecting, at the PV panel, current pulses generated in the power system on reconnection of the other PV panels to the power system, or detecting, at the PV panel, signals sent by other PV panels on reconnection of the other PV panels to the power system during the predetermined period of time. 
     The reconnect condition could also include a current pulse condition. The determining whether a reconnect condition is satisfied could then involve: reconnecting the PV panel to the power system; determining whether a current pulse is detected at the PV panel on reconnection of the PV panel to the power system; disconnecting the PV panel from the power system on detection of a current pulse or expiry of a pulse detection time period; determining that the reconnect condition is satisfied where a current pulse is detected at the PV panel during the pulse detection time period and the predetermined minimum of PV panels including the PV panel are ready to reconnect to the power system. 
     The determining whether the predetermined minimum of PV panels are ready to reconnect to the power system could involve detecting, at the PV panel, signals sent by other PV panels on reconnection of the other PV panels to the power system during the predetermined period of time, in which case the determining whether a reconnect condition is satisfied could involve: reconnecting the PV panel to the power system; determining whether a current pulse is detected at the PV panel on reconnection of the PV panel to the power system; broadcasting a signal from the PV panel on detection of a current pulse during a pulse detection time period; disconnecting the PV panel from the power system on detection of a current pulse or expiry of the pulse detection time period; determining that the reconnect condition is satisfied where a current pulse is detected at the PV panel during the pulse detection time period and the predetermined minimum of PV panels including the PV panel are ready to reconnect to the power system. 
     In an embodiment, the reconnecting involves reconnecting the predetermined minimum number of PV panels to the power system on determining that the predetermined minimum of PV panels are ready to reconnect to the power system. 
     The reconnect condition could be a numeric value condition, with the determining whether a reconnect condition is satisfied involving comparing a random number associated with the PV panel to a predefined value. 
     The reconnect condition could be receipt of a connect command from a grid sensor coupled to the power system, in which case the determining whether a reconnect condition is satisfied involves determining whether the connect command is received. 
     A PV panel disconnect switching arrangement includes: switches to control connection of the PV panel to a power system and bypass of the PV panel on disconnection of the PV panel from the power system; a controller operatively coupled to the switches, to determine whether a reconnect condition for reconnecting the PV panel to the power system is satisfied, to automatically reconnect the PV panel to the power system responsive to determining that the reconnect condition is satisfied, to determine whether a power system operating condition is satisfied on reconnection of the PV panel, to automatically disconnect the PV panel from the power system responsive to determining that the power system operating condition is not satisfied on reconnection of the PV panel. 
     The controller could be configured to determine that the reconnect condition is satisfied where one or more of: a time period has elapsed after disconnection of the PV panel from the power system; a predetermined number of other PV panels are reconnected to the power system within a predetermined period of time; a predetermined minimum number of PV panels including the PV panel are ready to reconnect to the power system; a random number associated with the PV panel has a predetermined relationship to a predefined value; a connect command is received from a grid sensor coupled to the power system. 
     In an embodiment, the controller is configured to determine that the reconnect condition is satisfied where one or more of: there is at least a minimum magnitude of current flow in the power system; a current pulse is detected at the PV panel on reconnection of the PV panel to the power system. 
     According to another aspect, a method involves: detecting, at a PV panel that is connected to a power system, a low current condition or an arc fault condition in the power system; automatically disconnecting the PV panel from the power system responsive to detection of the low current condition or the arc fault condition. 
     The detecting could involve independently detecting the low current condition or the arc fault condition at each of a plurality of PV panels, including the PV panel, that are connected to the power system, and the automatically disconnecting could involve independently automatically disconnecting each PV panel from the power system responsive to detection of the low current condition or the arc fault condition at each PV panel. 
     A PV panel disconnect switching arrangement includes: switches to control connection of the PV panel to a power system and bypass of the PV panel on disconnection of the PV panel from the power system; a controller operatively coupled to the switches, to detect a low current condition or an arc fault condition in the power system while the PV panel is connected to the power system, to automatically disconnect the PV panel from the power system responsive to detection of the low current condition or the arc fault condition. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a block diagram of an example grid tied PV installation. 
         FIG. 1B  is a block diagram of another example grid tied PV installation. 
         FIG. 2A  is a block diagram of a further example grid tied PV installation. 
         FIG. 2B  is a block diagram of another example grid tied PV installation. 
         FIG. 2C  is a schematic diagram of one embodiment of an Intelligent Safety Disconnect Switch (ISDS). 
         FIG. 3A  is a block diagram of one embodiment of a controller. 
         FIG. 3B  is a block diagram of another embodiment of a controller. 
         FIG. 4  is a flow diagram of an example reconnect operation. 
         FIG. 5  is a flow diagram of another embodiment of a reconnect operation. 
         FIG. 6A  is a flow diagram of an example reconnect operation using current pulses. 
         FIG. 6B  is a flow diagram of a further example reconnect operation using current pulses. 
         FIG. 6C  is a flow diagram of another example reconnect operation using current pulses. 
         FIG. 7  is block diagram of another embodiment of a controller. 
         FIG. 8  is a block diagram of an example PV installation with a grid sensor. 
         FIG. 9  is a block diagram of one embodiment of a grid sensor. 
         FIG. 10A  is a flow diagram of an example method, involving gradual reconnect with communication. 
         FIG. 10B  is a flow diagram of another example method, involving simultaneous reconnect with communication. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  is a block diagram of an example grid tied PV installation. The example installation  100  includes a panel string  160  connected to the DC inputs of a central inverter  120 . String  160  includes individual PV panels  111 ,  112 ,  113 . The output of inverter  120  connects to electrical grid  150  through grid disconnect switch  140 .  FIG. 1A  is an example only and other arrangements are possible. 
       FIG. 1B , for instance, is a block diagram of another example grid tied PV installation. In this example installation  101 , there is an additional disconnect switch  141  between string  160  and the inputs of the inverter  120 . In  FIGS. 1A and 1B , other 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. 
     As noted above, power production by PV panels under illumination can represent a potential safety hazard. It could therefore be useful to have PV panels in a PV installation isolate themselves and not output power whenever the PV installation was disconnected from the grid. 
     A PV installation may 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. A PV installation might also or instead automatically disconnect due to an electrical fault on the grid. A PV installation will also be disconnected from the grid prior to its commissioning. 
     It might also be useful if PV panels isolated themselves in the event of an open circuit in a PV panel string. This might be caused by, for example: a physical break in the string; removal of one or more PV panels in the string for maintenance, repair, or replacement; disconnection of the string from a central inverter for inverter repair or replacement; a fault in the inverter; and/or during initial PV panel installation before all PV panels are installed in a string. 
       FIG. 2A  is a block diagram of a further example grid tied PV installation  200 . The DC outputs of PV panels  111 ,  112 ,  113  respectively connect to the inputs of Intelligent Safety Disconnect Switches (ISDSs)  201 ,  202 ,  203 . The ISDSs  201 ,  202 ,  203  disconnect their respective PV panels  111 ,  112 ,  113  from the string in the event of an open circuit or high resistance condition on the string. An open circuit condition could be caused by, for example, removal of one or more PV panels, a physical break in the string, the opening of a switch in the string, the blowing of a string fuse and/or a fault in inverter  120 . A high resistance condition could be caused by, for example, disconnection of the PV installation from the electrical grid  150  and/or anti-islanding of the inverter  120 . Anti-islanding refers to the automatic disconnection of a PV installation from the grid  150  when the PV installation detects that the main grid generator 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 the 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. 
       FIG. 2B  is a block diagram of another example grid tied PV installation. In this example installation  210 , PV panel strings  211 ,  212 ,  213  are connected in parallel to the inputs of inverter  120 . Each PV panel string  211 ,  212 ,  213  includes a string of PV panels with respective ISDSs as shown in  FIG. 2A  for example. The example shown in  FIG. 2A  includes only one PV panel string, and the example shown in  FIG. 2B  includes multiple PV panel strings. The diodes  214 ,  215 ,  216  are provided in the example installation  210  to avoid “reverse” power flow from one PV panel string  211 ,  212 ,  213  to another PV panel string in which all PV panels are bypassed. Without the diodes  214 ,  215 ,  216 , bypassing of all PV panels in any one string  211 ,  212 ,  213  would effectively shunt all of the other strings. 
       FIG. 2C  is a schematic diagram of one embodiment of an ISDS. In the embodiment shown, ISDS  201  includes a controller  210 , shunt switch  220 , series switch  230 , current sensor  240  such as an ammeter, diode  250 , and voltage sensor  270  such as a voltmeter, interconnected as shown. The other ISDSs  202 ,  203  in  FIG. 2A  may have a similar structure. 
     Switches  220  and  230  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  250  is in parallel with switch  220 . Diode  250  allows string current to flow in the event that PV panel  111  cannot supply sufficient power to operate controller  210 . In this situation controller  210  would be unable to supply drive voltage to switch  220  and keep it closed, however current can still flow through diode  250  to bypass the panel  111 . Under conditions where the controller  210  has sufficient power to drive the switch  220 , the closed switch dissipates less power than the diode  250 . 
     Current sensor  240  monitors the string current. Controller  210  draws its power from PV panel  111  and receives a current measurement from current sensor  240 . Controller  210  controls the operation of switches  220  and  230 . In some embodiments current sensor  240  is a part of controller  210 . 
     The voltage sensor  270  is an optional component, connected to the output of PV panel  111  in some embodiments for monitoring the PV panel output voltage. 
       FIG. 3A  is a block diagram of one embodiment of a controller  210 . Voltage regulator  340  converts variable PV panel output voltage to a constant controller supply voltage in an embodiment. Driver  310  supplies drive signals to switches  230  and  220  to control their opening and closing. Firmware for the operation of the example controller  210  is stored in memory  320 . In one embodiment, memory  320  is non-volatile memory such as Flash, Electrically Erasable Programmable Read Only Memory (EEPROM), EPROM, ROM. The firmware is executed on Central Processing Unit (CPU)  350 . Clock  330  controls the internal timing of the controller operation. User Interface (UI)  360  indicates the status of the ISDS to a user. Control and data bus  370  interconnects these components with each other as shown, in an embodiment. 
     In one embodiment, the UI  560  includes a Light Emitting Diode (LED) that indicates the state of the ISDS (connected or disconnected). It may indicate the ISDS state by its illumination state (on or off) or by its color (green or red), for example. In one embodiment, it indicates the detection of an arc fault. In one embodiment, UI  360  contains a manual switch. In one embodiment, the manual switch is a reset switch which initiates a manual reconnect of the ISDS. In one embodiment, the manual switch initiates a manual reconnect of the ISDS after an arc fault. In one embodiment, UI  360  allows for the programming of timing parameters such as a reconnect delay after a disconnect and/or a wait or a delay time. In one embodiment, the manual switch is a test switch which initiates testing of an arc detection function. 
       FIG. 3B  is a block diagram of another embodiment of a controller. In this embodiment controller  210  contains a random number generator (RNG)  351 . The RNG could be usefully employed to generate random delays and wait times for some operations. 
     Disconnect Operations 
     Referring again to  FIG. 2C , when PV panel  111  is illuminated and there is string current flowing, shunt switch  220  of the example ISDS  201  is open and series switch  230  is closed. The PV panel  111  is in series with the other panels in the panel string and contributes to the string voltage and current. 
     In the event of an open circuit or high resistance condition in the string, the string current falls to substantially zero. This loss of current is detected by controller  210  using current sensor  240 . Controller  210  opens series switch  230  and disconnects the PV panel  111  from the string. Controller  210  also closes shunt switch  220 , thereby maintaining the electrical continuity of the string at the PV panel  111 &#39;s point of connection. With series switch  230  open, maintenance and emergency workers are safely protected from the output of the PV panel  111 . 
     In another embodiment, controller  210  monitors current sensor  240  for the presence of an arc fault in the string. An arc fault is large and undesired current flow between the 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 accidentally severed. Arc faults can lead to electrification of a PV panel mounting system, serious damage to equipment, fire, and/or injuries to personnel. In one embodiment, controller  210  monitors current sensor  210  for the presence of an arc fault current. On detection of an arc fault, controller  210  disconnects the PV panel  111  from the string by opening series switch  230  and closing shunt switch  220 . With switch  220  closed the PV panel is bypassed and the output voltage of the PV panel does not contribute to the arc fault. 
     Arc fault detection could be in accordance with any of various techniques. Some arc fault detection methods involve spectral analysis of the string current or voltage for an arc “signature”. In one embodiment, the controller  210  contains a Digital Signal Processor (DSP) to facilitate the spectral analysis. 
     In some situations the simultaneous disconnection of all panels in a string and the closing of their shunt switches may create undesirably large currents due to the discharge of the inverter&#39;s input capacitance through the low resistance closed shunt switches. In one embodiment ISDSs  201 ,  202 ,  203  each wait for a randomly chosen wait time after sensing a disconnect condition such as an arc fault or a loss of string current before disconnecting respective PV panels  111 ,  112 ,  113  from the string. This ensures a gradual disconnection of panels  111 ,  112 ,  113  and a smaller peak discharge current. 
     Reconnect Operations 
     After an ISDS has isolated its PV panel from the string, it could check to determine whether the PV panel can be safely reconnected to the string, illustratively by checking for DC continuity in the panel string circuit. 
     In one embodiment of a reconnect operation, the controller  210  monitors the string current using current sensor  240 . If the DC value of the string current I STRING  is equal to or above a minimum DC current value I MIN  then this is interpreted as electrical continuity in the string and the example ISDS  201  reconnects the PV panel  111  to the string by opening shunt switch  220  and closing series switch  230 . If I STRING  is below I MIN  then the example ISDS  201  keeps the PV panel  111  disconnected. 
     In another embodiment of a reconnect operation after detection of an arc fault the example ISDS  201  reconnects the panel  111  to the string when it receives a reset signal through user interface  360 . The reset signal need not be a locally generated reset signal, and could instead be sent from a remote source. 
     In another embodiment of a reconnect operation, the example ISDS  201  performs an electrical continuity check on the string. The controller  210  applies a test voltage from the PV panel  111  to the string and receives a measurement of the resulting string current from the current sensor  240 . For safety, it might be desirable to limit the test voltage magnitude and its duration on the string. 
       FIG. 4  is a flow diagram of an example reconnect operation or method  400 , with electrical continuity checking. After an ISDS has isolated its PV panel, controller  210  ( FIG. 2C ) waits for time interval “T START ” at  402  and then opens shunt switch  220  and closes series switch  230  at  404 , which applies the PV panel  111  voltage to the string. In one embodiment, T START  is the inverter minimum anti-islanding wait time of the inverter  120  ( FIG. 2A ), as mandated by an electrical grid authority, for example. The controller  210  ( FIG. 2C ) measures the string current I STRING  resulting from the PV panel  111  voltage at  406 , using current sensor  240 . If the DC value of I STRING  is below a minimum DC current value I MIN  as determined at  408 , then this is interpreted as an absence of electrical continuity in the string. Controller  210  then re-isolates the PV panel  111  by opening series switch  230  and closing shunt switch  220  at  412 . The controller  210  waits for a time period T WAIT  at  414  and then repeats the cycle. 
     If, however, the measured string current I STRING  is equal to or above the minimum DC value I MIN  as determined at  408 , then controller  210  ends this cycle or control loop at  410  and maintains series switch  230  closed and shunt switch  220  open. In one embodiment, the remaining ISDSs in the string monitor the string current and also reconnect to the string once they sense the string current. 
     In one embodiment T START  and T WAIT  are identical values and are stored in the controller memory  320  ( FIGS. 3A, 3B ). 
     Single Probabilistic Reconnect 
     In another embodiment, T START  is a random value generated by the controller  210  and is between an upper bound (T MAX ) and a lower bound (T MIN ). In some embodiments, T MIN  may correspond to a minimum anti-islanding waiting time. In some embodiments, T MAX  might be equal to the sum of T MIN  and T WAIT . In this embodiment, all ISDSs in a string could have identical values of T WAIT . The randomized start times in this embodiment avoid the situation of multiple switches attempting to reconnect to the string simultaneously, which might create undesirably large voltages on the string. In this embodiment, the ISDS with the smallest value of “T START ” is the first ISDS to attempt a reconnect followed by the ISDS with the next smallest value of T START  and so on in sequence according to values of T START  until all ISDSs have attempted a reconnect. The cycle repeats with a period of T WAIT . This is referred to herein as a Single Probabilistic Reconnect. 
     Single Deterministic Reconnect 
     In another embodiment, the T START  values of the N ISDSs for a string of N PV panels are all different and are programmed into an ISDS memory such as the controller memory  320  ( FIGS. 3A, 3B ). In one embodiment, the start time of the i-th ISDS (T START,i ) is chosen according to the formula: 
     
       
         
           
             
               T 
               
                 START 
                 , 
                 i 
               
             
             = 
             
               
                 T 
                 
                   START 
                   , 
                   1 
                 
               
               + 
               
                 
                   ( 
                   
                     i 
                     - 
                     1 
                   
                   ) 
                 
                 ⨯ 
                 
                   
                     T 
                     WAIT 
                   
                   N 
                 
               
             
           
         
       
     
     In this embodiment, reconnect attempts are made at regular intervals of T WAIT /N beginning at a time T START,1  after disconnect. This is referred to herein as a Single Deterministic Reconnect. 
     Multiple Deterministic Reconnect 
     In some inverter designs, the voltage of a single PV panel might not be sufficiently large to cause an inverter to operate after the PV installation has been reconnected to the grid. An inverter normally requires a minimum input voltage V START  to begin operation. If the voltage of a single PV panel is less than V START  when its ISDS attempts a reconnect, then the inverter might not operate. The ISDS might then disconnect since the detected string current could be below I MIN  due to inverter impedance. For some inverter designs, multiple ISDSs attempt to reconnect simultaneously to generate a sufficient inverter input voltage such that the inverter will operate and string current will flow. 
     Thus, in another embodiment, groups of ISDSs in a string are assigned identical values of T START  such that all ISDSs in the group will attempt to reconnect simultaneously. This is referred to herein as a Multiple Deterministic Reconnect. The number of ISDSs in the group is chosen to meet the start-up voltage of the inverter. For example, if a minimum of three PV panels in a string would generate a string voltage sufficient for an inverter to operate, then a minimum of three ISDSs are assigned identical values of T START . The starting time for the j-th group could be assigned according to the formula below, where M is the number of groups 
               T     START   ,   j       =       T     START   ,   1       +       (     j   -   1     )     ⨯       T   WAIT     M               
Multiple Probabilistic Reconnect
 
     In some situations the clocks of the ISDSs might not be synchronized. For example, a PV panel might not be able to generate enough power to keep the controller  210  of the example ISDS  201  ( FIG. 2C ) working during the night time. In the morning, when the PV panels begin to generate sufficient power to operate their controllers, the clocks  330  ( FIGS. 3A, 3B ) of the different controllers in a string of PV panels might not be synchronized, since each PV panel could power up its controller at a slightly different time. In this circumstance, it could be difficult to control the ISDSs such that a sufficient number of PV panels to supply the inverter start-up voltage are connected to the string simultaneously. 
       FIG. 5  is a flow diagram of another embodiment of a reconnect operation or method  500 . This embodiment is referred to herein as a Multiple Probabilistic Reconnect and could be used in the case of unsynchronized controller clocks, for example. In this embodiment all ISDS controllers wait for a time T START  at  502  after they have disconnected or have been powered up. In some embodiments, T START  corresponds to the minimum anti-islanding wait time mandated by the electrical grid authority. 
     After T START  has expired at  502 , a controller waits a further time T WAIT  at  504  and then connects its PV panel to the string by opening its shunt switch  220  ( FIG. 2C ) and closing its series switch  230  at  506 . It maintains these switch positions for a time T MEAS  during which I STRING  is measured at  508 . If the DC value of the string current I STRING  is less than I MIN  as determined at  510  in this example, then the controller disconnects the PV panel at  512  by opening series switch  230  and closing shunt switch  220 . The ISDS then waits a further time T MAX −T WAIT  at  514  and the cycle repeats. If the DC value of the string current I STRING  is equal to or above I MIN  as determined at  510  then controller  210  ends the reconnect operation at  516  with series switch  230  closed, shunt switch  220  open and the PV panel connected. 
     In this embodiment, the value of T WAIT  is a random value between zero and an upper bound T MAX . Values of T MAX  and T MEAS  could be chosen such that the probability of multiple ISDSs and their respective panels being simultaneously connected to the string and generating the inverter start-up input voltage is sufficiently high to meet an operational objective, for example. 
     For a single cycle of period T MAX  the probability “P” of “n” ISDSs connecting simultaneously to the string is 
     
       
         
           
             P 
             = 
             
               1 
               - 
               
                 exp 
                 [ 
                 
                   - 
                   
                     
                       ( 
                       
                         
                           N 
                           STRING 
                         
                         n 
                       
                       ) 
                     
                     
                       
                         ( 
                         
                           
                             T 
                             MAX 
                           
                           
                             T 
                             MEAS 
                           
                         
                         ) 
                       
                       
                         n 
                         - 
                         1 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
         
         
           
             where N STRING  is the number of ISDSs in the string. For a string of 20 PV panels in which 4 PV panels are to connect in order to start the inverter, a T MEAS  of 1 second (s) and a T MAX  of 10s yields a probability of 99% that four PV panels will be connected simultaneously. 
           
         
       
    
     A DC string current will flow once the minimum number of PV panels have connected if there is continuity in the string circuit. The measured DC string current I STRING  would then be above the minimum DC value I MIN  as determined at  510 , and controller  210  exits the cycle or control loop at  516  and maintains series switch  230  closed and shunt switch  220  open. The remaining ISDSs in the string then sense the string current and connect their PV panels. 
     Current Pulse Reconnect 
     In another embodiment of the reconnect operation the input impedance of the inverter is usefully employed. The input impedance of an inverter is substantially capacitive when applied voltage is below the start voltage. When a PV panel reconnects to the string, a current pulse will flow as this input capacitance is charged by the PV panel as long as the string is connected to the inverter input and there are no breaks in the string. In one embodiment a PV panel reconnects long enough to charge the inverter capacitance to substantially the panel voltage. In another embodiment a PV panel reconnects only long enough to partially charge the capacitance to a fraction of the panel voltage. 
     The current pulse from charging of the inverter input capacitance can be usefully employed to verify that a string is physically continuous and connected to the inverter input, and/or as a communication means between ISDSs on a string. Thus, in one embodiment of the reconnect operation, controller  210  in the example ISDS  201  in  FIG. 2C  connects PV panel  111  to the string and monitors its current sensor  240  for a characteristic current pulse from the charging of the inverter capacitance to verify that the string is continuous and connected to the inverter  120 . The ISDS controller  210  may also monitors the string for current pulses generated by other ISDSs, as they connect their respective PV panels, to track the number of connected ISDSs. This is referred to herein as Current Pulse Reconnect. 
       FIG. 6A  is a flow diagram of an example reconnect operation or method  632  using current pulses. In this embodiment, after a disconnect or after power-up in the morning, for example, an ISDS chooses a wait time T WAIT  at  634 . The ISDS waits until the expiration of T WAIT  as determined at  636 . In one embodiment, T WAIT  is generated randomly by the ISDS. This may reduce the likelihood of two ISDSs connecting simultaneously and generating overlapping pulses. In this embodiment controller  210  in the example ISDS  201  in  FIG. 2C  connects PV panel  111  to the string at  638  by opening bypass switch  220  and closing series switch  230  and monitors its current sensor  240  for a characteristic current pulse from the charging of the inverter capacitance at  640 . If no pulse is detected at  640  the ISDS disconnects the PV panel at  642  by opening series switch  230  and closing shunt switch  220  and returns to its waiting state  636 . If a pulse is detected at  640  the ISDS waits for a maximum time of T MAX  for the DC string current to meet or exceed a minimum value I MIN . If I STRING  is greater than or equal to I MIN  as determined at  644  the reconnect operation ends at  648 . If I STRING  has not met or exceeded I MIN  by the time T MAX  expires, as determined at  646 , the ISDS disconnects the PV panel at  649  and returns to the wait state at  636 . 
     Gradual Reconnect with Current Pulses 
       FIG. 6B  is a flow diagram of an example reconnect operation or method  600  using current pulses. In this embodiment, after a disconnect or after power-up in the morning, for example, an ISDS first sets its pulse count (PC) and timer (T) to zero at  602  and then waits for a time T START . In one embodiment the ISDS measures the PV panel voltage prior to beginning a reconnect operation and will not commence a reconnect operation until the PV panel voltage is above a threshold value. This may ensure that a current pulse of sufficient magnitude for detection can be produced. In one embodiment, T START  is generated randomly by the ISDS. This may reduce the likelihood of two ISDSs connecting simultaneously and generating overlapping pulses. In one embodiment, T START  is always chosen to be long enough such that, if the ISDS is the first ISDS to power up in the morning, the remaining ISDSs in the string will have also powered up before T START  expires. 
     During the waiting period T START  the ISDS monitors the string at  604  for a current pulse from any other ISDS on the string reconnecting. Any of various pulse detection methods could be used. In one embodiment, the output of the current sensor  240  ( FIG. 2C ) is digitized by an analog to digital converter in the controller  210  and compared to a stored pulse profile. 
     If no pulse has been detected at  604  when the T START  waiting period expires (as determined at  606 ), then the ISDS reconnects its PV panel to the string at  608  by opening bypass switch  220  and closing series switch  230  and monitors for the current pulse generated from its own reconnection. If no pulse is detected at  610 , it means that there is no physical continuity in the string, the string is not connected to the inverter, or the ISDS did not generate a pulse with the correct characteristics to be detected. In all cases the ISDS disconnects at  612  by closing bypass switch  220  and opening series switch  230 . If an ISDS cannot generate a pulse with the correct characteristics to be detected by its own controller, then it is unlikely that the pulse can be detected by other ISDSs on the string. The ISDS should therefore disconnect. The correct characteristics of a pulse to be detected may include, for example, a current magnitude, a current rise time, a rate of current increase, a rate of current decrease and/or a pulse width. 
     If a pulse is detected at  610 , then the ISDS remains connected and becomes the first connected ISDS. The first connected ISDS will then wait for a time T MAX  for the DC string current I STRING  to meet or exceed I MIN . If the DC string current meets or exceeds I MIN  as determined at  626  the operation terminates at  628 . If the time exceeds T MAX  as determined at  622  then the ISDS disconnects at  630  by closing bypass switch  220  and opening series switch  230  and the operation restarts at  602 . 
     If a pulse is detected at  604  during the T START  waiting period, then the pulse counter is set to a value of one, the timer is reset to zero and a random second wait time (T PULSE ) is generated, at  614 . The ISDS monitors the string for additional current pulses over the second waiting period T PULSE . In this embodiment T PULSE  is a random value between zero and T MAX . T MAX  is chosen such that the probability of two switches connecting simultaneously and generating overlapping current pulses before the minimum number of switches has connected is low. The generation of overlapping current pulses might cause a miscounting of the number of connected switches. If a current pulse has a width of W and the operation terminates once the number of connected switches reaches S MIN  then the probability “P” of two overlapping pulses occurring over the time period T MAX  is given by the formula 
     
       
         
           
             P 
             = 
             
               1 
               - 
               
                 exp 
                 [ 
                 
                   - 
                   
                     
                       S 
                       MIN 
                       2 
                     
                     
                       2 
                       ⁢ 
                       
                         ( 
                         
                           
                             T 
                             MAX 
                           
                           W 
                         
                         ) 
                       
                     
                   
                 
                 ] 
               
             
           
         
       
     
     For example, if S MIN  is 4, W is 1 millisecond and T MAX  is 1 second then P=0.8%. 
     The pulse counter is incremented at  618  when an additional current pulse is detected at  616  within the time period T PULSE , and the incremented pulse counter is compared at  620  to the minimum number of connected switches (S MIN ). If the pulse counter has reached the minimum value as determined at  620 , then the minimum number of ISDSs and PV panels are now connected to the string and the ISDS should not yet attempt to connect. The ISDS therefore waits at  622  until the timer reaches T MAX  or the string current I STRING  reaches the minimum value I MIN . During T MAX , if the string current I STRING  meets or exceeds I MIN  as determined at  626 , then the ISDS may connect (since I STRING  is now at or above I MIN ) and the operation terminates at  628 . 
     If the minimum pulse count has not been exceeded at the end of the T PULSE  waiting period, which is detected at  624 , then the ISDS connects to the string at  608 , and as described above monitors for a current pulse generated from its connection to the string, at  610 . If no pulse is detected, then the ISDS disconnects at  612 . If a pulse is detected then the ISDS remains connected and will then wait for a time T MAX  for the DC string current I STRING  to meet or exceed I MIN . If the DC string current meets or exceeds I MIN  as determined at  626  the operation terminates at  628 . If the time exceeds T MAX  as determined at  622  then the ISDS disconnects at  630  and the operation restarts at  602 . 
     In this embodiment, there is a minimum number of connected switches (S MIN ). The string voltage gradually increases as more ISDSs connect. This embodiment is referred herein to as Gradual Reconnect with Current Pulses. S MIN  corresponds to the minimum number of PV panels to produce the start-up voltage for the inverter in an embodiment. In this way, string voltage is kept to the minimum value to start the inverter, until the inverter starts up. In this embodiment ISDSs connect to the string one by one. 
     Although not shown in  FIG. 6B , all ISDSs continue to monitor the string current for a minimum value of DC current indicative of the start-up of the inverter during the entire operation. An ISDS will end the operation and connect to the string if it detects a DC string current greater than the minimum value. 
     Simultaneous Reconnect with Pulses 
     Some inverters could have minimum starting voltages that are hazardous. In this situation, and in order to reduce hazards to personnel, the amount of time that the string is energized during a reconnect attempt could be minimized. 
     In another embodiment of the reconnect operation, the PV panels that supply the inverter start-up voltage reconnect to the string simultaneously rather than one by one as in the Gradual Reconnect with Current Pulses embodiment. This embodiment is referred to herein as Simultaneous Reconnect with Pulses. In this embodiment, an ISDS momentarily connects to the string to determine whether the string is physically continuous and connected to the inverter. If physical continuity is established, then the status of the ISDS is set to “READY”, indicating that it is ready to participate in a reconnect operation. When a sufficient number of ISDSs are “READY”, the “READY” ISDSs all connect to the string substantially simultaneously. The connected ISDSs measure the DC string current. If the current is greater than the minimum value, then the ISDSs remain connected. Otherwise, they all disconnect and the reconnect operation terminates. In this embodiment, the duration of any high string voltage can be minimized to only the time it takes to establish a DC current. 
       FIG. 6C  is a flow diagram of another example reconnect operation using current pulses, which represents an embodiment of the Simultaneous Reconnect with Pulses. 
     In the example reconnect operation or method  650 , after a disconnect or after power-up an ISDS first sets its pulse count (PC) and timer (T) to zero and sets its status to “Not READY” at  652  and chooses a waiting time T WAIT . In one embodiment T WAIT  is generated randomly by the ISDS between zero and a maximum value T MAX . In one embodiment, the ISDS may also measure the PV panel voltage prior to beginning a reconnect operation and not commence a reconnect operation until the PV panel voltage is above a threshold value which ensures that a current pulse of measurable magnitude can be produced. 
     In the embodiment shown in  FIG. 6C , the ISDS waits for a time T WAIT . During the waiting period T WAIT  the ISDS monitors the string for a current pulse from any other ISDS on the string reconnecting. If a pulse is detected at  654 , then the ISDS increments the pulse counter at  656 . 
     Once the T WAIT  waiting period expires, as detected at  658 , the ISDS momentarily connects to the string at  660  by opening bypass switch  220  and closing series switch  230  and looks for the current pulse generated from its connection to the string, at  662 . If a pulse is detected, then it means that the ISDS is connected to the inverter output. The ISDS&#39;s status is changed to READY and the pulse counter is incremented at  664 . The ISDS then disconnects at  666  by closing bypass switch  220  and opening series switch  230 . In the example ISDS shown in  FIG. 2C , the disconnect involves opening series switch  230  and closing shunt switch  220 . The closing of shunt switch  220  will discharge the capacitance of the inverter input. This allows the inverter input capacitance to be charged by the next ISDS connecting to the string and a pulse to be generated. 
     The ISDS next checks the state of the pulse counter at  668 . If the pulse counter is equal to the S MIN  then the minimum number of ISDSs (S MIN ) to provide the inverter start-up voltage are ready to connect, then the ISDS connects to the string at  670  by opening bypass switch  220  and closing series switch  230 . The DC component of the string current is then measured and evaluated at  672 . If the current is below the minimum value (I MIN ), then the ISDS disconnects at  674  by closing bypass switch  220  and opening series switch  230  and the operation restarts at  652 . If the DC string current is equal to or greater than I MIN  it remains connected and exits the reconnect process at  676 . 
     If the pulse counter is less than S MIN  as determined at  668 , then this indicates there are not enough ISDSs ready to reconnect. The ISDS will continue to listen for pulses in the string at  678 . If additional pulses are detected, then the pulse counter is incremented at  680  and re-evaluated at  668 . 
     As in the previous embodiment, the generation of overlapping pulses could result in a miscount of the number of ISDSs that are ready to connect. T MAX  might therefore be chosen such that the probability of two switches connecting simultaneously and generating overlapping current pulses before the minimum number of switches are READY is low. 
     Simultaneous Probabilistic Reconnect 
     In another embodiment, after a first pulse is detected, the remaining ISDSs are connected to the string by selection of a random number. This method is referred to herein as Simultaneous Probabilistic Reconnect. In this method, after a first pulse is detected in the T START  period, the remaining ISDSs each generate a random number between 0 and 1. If the ISDS&#39;s random number is less than or equal to a predefined value “p”, then that ISDS will connect to the string. If the random number is greater than p, then it does not connect. The probability P(n) that “n” out of the remaining N−1 unconnected ISDSs will connect is given by the binomial distribution 
     
       
         
           
             
               P 
               ⁡ 
               
                 ( 
                 n 
                 ) 
               
             
             = 
             
               
                 ( 
                 
                   
                     
                       
                         N 
                         - 
                         1 
                       
                     
                   
                   
                     
                       n 
                     
                   
                 
                 ) 
               
               ⁢ 
               
                 
                   
                     p 
                     n 
                   
                   ⁡ 
                   
                     ( 
                     
                       1 
                       - 
                       p 
                     
                     ) 
                   
                 
                 
                   N 
                   - 
                   n 
                   - 
                   1 
                 
               
             
           
         
       
     
     The probability that n or more ISDSs will connect is given by the cumulative distribution function 
     
       
         
           
             
               P 
               ⁡ 
               
                 ( 
                 n 
                 ) 
               
             
             = 
             
               
                 ∑ 
                 
                   k 
                   = 
                   n 
                 
                 
                   N 
                   - 
                   1 
                 
               
               ⁢ 
               
                 
                   ( 
                   
                     
                       
                         
                           N 
                           - 
                           1 
                         
                       
                     
                     
                       
                         k 
                       
                     
                   
                   ) 
                 
                 ⁢ 
                 
                   
                     
                       p 
                       k 
                     
                     ⁡ 
                     
                       ( 
                       
                         1 
                         - 
                         p 
                       
                       ) 
                     
                   
                   
                     N 
                     - 
                     k 
                     - 
                     1 
                   
                 
               
             
           
         
       
     
     Thus, a value of “p” can be chosen to provide for a desired probability P(n) of connection of at least “n” ISDSs. 
     Communication 
       FIG. 7  is block diagram of another embodiment of a controller. In this embodiment, controller  710  contains communication means or module  760 . In one embodiment, communication means or module  760  is wireless communication means such as a Wi-Fi or Bluetooth module. In one embodiment, communication means or module  760  supports mesh style communication in which signals pass through multiple ISDSs between origin and destination. In another embodiment, communication means or module  760  is a wired communication means such as a Powerline communications module, and communication is through the string wiring. In another embodiment, communication is through dedicated wiring such as optical fiber or twisted pair. 
     In one embodiment, the reconnect operation is controlled through the communication means or module  760 . In this embodiment, a reconnect command could be received by the example ISDS controller  710  through communication means or module  760 , and cause the ISDS to open its shunt switch  220  ( FIG. 2C ) and close its series switch  230 . In one embodiment, the reconnect command is sent by a grid sensor which senses the reconnection of the output of the inverter to the electrical grid. 
       FIG. 8  is a block diagram of an example PV installation with a grid sensor. Grid sensor  800  is connected to the output of inverter  120  and senses whether inverter  120  is connected to grid  150 . When the grid sensor  800  detects that the inverter  120  is no longer connected to the grid  150 , it sends a disconnect command to ISDSs  201 ,  202  and  203 . Similarly, when the grid sensor  800  detects that the inverter  120  is again connected to the grid  150 , it sends a reconnect command to the ISDSs  201 ,  202 ,  203 . 
     In one embodiment, the grid sensor  800  measures the AC voltage at the output of the inverter  120  and before disconnect switch  140 . In this embodiment, when the inverter  120  is disconnected from the grid  150  by the opening of disconnect switch  140  the inverter will automatically enter anti-islanding mode and its AC output voltage will drop to substantially zero. The drop in output voltage is detected by the grid sensor  800 . The grid sensor  800  then sends a disconnect command to ISDSs  201 ,  202  and  203  and they disconnect their PV panels  111 ,  112 ,  113  from the string. In one embodiment, a disconnect command is sent if the AC output voltage of the inverter  120  drops below a minimum grid voltage specified by the grid authority. 
     When the inverter  120  is reconnected to the grid  150  by closing of grid disconnect switch  140  the AC grid voltage will be sensed by the grid sensor  800 . When the grid sensor  800  senses a grid voltage greater than a minimum value, it will send a reconnect command to ISDSs  201 ,  202  and  203 . In one embodiment, all PV panels  111 ,  112 ,  113  in the string receive the reconnect command and reconnect to the string. In another embodiment, only the minimum number of panels to start the inverter  120  reconnect. In one embodiment, the minimum AC voltage is the minimum AC grid voltage specified by the grid authority. In one embodiment, the grid sensor  800  instructs the ISDSs to remain connected for a minimum waiting period. In one embodiment, this waiting period exceeds the grid mandated anti-islanding waiting time of the inverter  120 . 
     Grid Sensor 
       FIG. 9  is a block diagram of one embodiment of a grid sensor. Grid sensor  800 , in the example shown, comprises AC voltage sensor  910  such as a voltmeter for monitoring the inverter output voltage, communication means or module  920  for communicating with the ISDSs, CPU  940  and memory  930 . Firmware for the operation of the example grid sensor is stored in memory  930  and executed on CPU  940 . User Interface (UI)  950  communicates the grid state (present or absent). These components are connected by control and data bus  970 . In one embodiment, the UI  950  includes a Light Emitting Diode (LED) that indicates the grid state (present or absent). It may indicate by its state of illumination (on or off) or by its color (green or red), for example. In one embodiment, UI  950  contains a switch which allows the connection state of the ISDSs to be set manually. In one embodiment, UI  950  allows for the programming of parameters such as, for example, a reconnect delay time after a disconnect. 
     In one embodiment communication means or module  920  is a wireless communication means supporting communications including but not limited to, for example, Zigbee, Wi-Fi, Bluetooth and/or a proprietary radio communication protocol. In another embodiment communication means or module  920  supports wired communications such as powerline communications. Powerline communication through inverter  120  ( FIG. 8 ) might not be possible for some inverter designs. In one embodiment of powerline communication, communication means or module  920  connects to the input side of inverter  120  for the purposes of communication with the ISDSs  201 ,  202 ,  203 . In another embodiment, communication is through dedicated wiring such as optical fiber or twisted pair. 
     Other embodiments of the grid sensor  800  are possible. For example, a state machine might be used instead of a CPU  940 . 
     In another embodiment, the grid sensor  800  is incorporated into the inverter  120  ( FIG. 8 ). In this embodiment, the anti-islanding state of the inverter  120  is communicated directly to the grid sensor  800  by the inverter&#39;s logic. The grid sensor  800  sends a disconnect command to the ISDSs  201 ,  202 ,  203  when it senses the inverter  120  entering an anti islanding state. In this embodiment the ISDS sends a reconnect signal to the ISDSs when the anti-islanding circuitry of the inverter  120  reports a return of the grid. 
     In another embodiment, the grid sensor  800  is incorporated into the disconnect switch  140 . In this embodiment, the grid sensor  800  senses the open or closed state of the disconnect switch  140  using a mechanical sensor, for example. The grid sensor  800  sends a disconnect command to the ISDSs  201 ,  202 ,  203  when it senses the opening of the disconnect switch  140 . 
     Gradual Reconnect with Communication 
     In another embodiment of the reconnect operation communication means or module  760  ( FIG. 7 ) is used to facilitate communication between ISDSs in a string for reconnection to the string. This method is similar to the Gradual Reconnect with Pulses method but uses communication means or module  760  rather than current pulses for connection detections between ISDSs. It is referred to herein as Gradual Reconnect with Communication.  FIG. 10A  is a flow diagram of an example 1000 of the Gradual Reconnect with Communication method. In this embodiment, all ISDSs have differing waiting times (T CONNECT ) before they reconnect their PV panels to the string. 
     An ISDS broadcasts a “CONNECT” signal, through its communication means or module  760 , to the other ISDSs in the string when it has connected. The variable COUNT measures the number of connected ISDSs and is incremented every time a CONNECT signal is received. Once the number of ready ISDSs reaches a minimum number (S MIN ) in this example, no further ISDSs are permitted to connect. In this embodiment, ISDSs connect to the string one by one. The string voltage gradually increases as more ISDSs connect. 
     In this embodiment, after a disconnect or after power-up in the morning, for example, an ISDS first sets its connected ISDS counter “COUNT” and timer “T” to zero at  1002  and then waits for a time T START . In one embodiment the ISDS measures the PV panel voltage prior to beginning a reconnect operation and will not commence a reconnect operation until the PV panel voltage is above a threshold value. In one embodiment, T START  is always chosen to be long enough such that, if the ISDS is the first ISDS to power up in the morning, the remaining ISDSs in the string will have also powered up before T START  expires. 
     During the waiting period T START  the ISDS listens with communication means  760  for a “CONNECT” signal from another ISDS in the string reconnecting. 
     If no “CONNECT” signal has been detected at  1004  when the T START  waiting period expires (as determined at  1006 ), then the ISDS reconnects its PV panel to the string at  1018  by opening bypass switch  220  and closing series switch  230  and monitors for the current pulse generated from its own reconnection. If no pulse is detected at  1020 , it means that there is no physical continuity in the string, the string is not connected to the inverter, or the ISDS did not generate a pulse of with the correct characteristics to be detected. In all cases the ISDS disconnects at  1022  by closing bypass switch  220  and opening series switch  230 . If an ISDS cannot generate a pulse with the correct characteristics to be detected by its own controller, then it is unlikely that the pulse can be detected by other ISDSs on the string. The ISDS should therefore disconnect. 
     If a pulse is detected at  1020 , then the ISDS broadcasts a “CONNECT” signal at  1021 , remains connected and becomes the first connected ISDS. The first connected ISDS will then wait for a time T MAX  for the DC string current I STRING  to meet or exceed I MIN . If the DC string current meets or exceeds I MIN  as determined at  1026  the operation terminates at  1028 . If the time exceeds T MAX  as determined at  1024  without the DC string current meeting or exceeding I MIN  then the ISDS disconnects at  1030  by closing bypass switch  220  and opening series switch  230  and the operation restarts at  1002 . 
     If a CONNECT signal is detected at  1004  during the T START  waiting period, then the connected ISDS counter COUNT is set to a value of one, the timer is reset to zero and a random second wait time (T CONNECT ) is generated, at  1008 . The ISDS monitors for additional CONNECT signals during the second waiting period T CONNECT . In this embodiment T CONNECT  is a random value between zero and T MAX . T MAX  is chosen such that the probability of two switches connecting simultaneously and generating overlapping current pulses before the required number of switches has connected is low. 
     The connected ISDS counter “COUNT” is incremented at  1010  when an additional connect signal is detected at  1012  within the time period T CONNECT . COUNT is compared at  1014  to the minimum number of connected switches (S MIN ). If COUNT equals S MIN  as determined at  1014 , then the minimum number of ISDSs and PV panels are now connected to the string and the ISDS should not yet attempt to connect. The ISDS therefore will wait for a time T MAX  for the DC string current I STRING  to meet or exceed I MIN . If the DC string current meets or exceeds I MIN  as determined at  1026 , then the ISDS may connect if it has not already (since I STRING  is now at or above I MIN ) and the operation terminates at  1028 . If the time exceeds T MAX  as determined at  1024  then the ISDS disconnects at  1030  if it previously connected and the operation restarts at  1002 . 
     If COUNT is less than S MIN  at the end of the T CONNECT  waiting period, which is detected at  1016 , then the ISDS connects to the string at  1018 , and as described above monitors for a current pulse generated from its connection to the string, at  1020 . If no pulse is detected, then the ISDS disconnects at  1022 . If a pulse is detected then the ISDS remains connected and will then wait for a time T MAX  for the DC string current I STRING  to meet or exceed I MIN . If the DC string current meets or exceeds I MIN  as determined at  1026  the operation terminates at  1028 . If the time exceeds T MAX  as determined at  1024  then the ISDS disconnects at  1030  and the operation restarts at  1002 . 
     In an embodiment, during the entire cycle the ISDSs continue to monitor the string current for a minimum value of DC current indicative of the start up of the inverter, and an ISDS might always connect to the string if it detects a DC string current greater than the minimum value. 
     Simultaneous Reconnect with Communication 
     In another embodiment of a reconnect method, communication means  760  ( FIG. 7 ) facilitates substantially simultaneous reconnection of multiple ISDSs in the string. This is referred to herein as Simultaneous Reconnect with Communication and is similar to the Simultaneous Reconnect with Pulses method. In this embodiment, all ISDSs have differing waiting times (T WAIT ) after a disconnect before they can reconnect their panels to the string. In this embodiment an ISDS broadcasts a READY signal through communication means  760  to the other ISDSs in the string when its T WAIT  has expired and it is ready to connect. The variable COUNT measures the number of ISDSs ready to connect and is incremented every time a READY signal is received. Once the number of ready ISDSs reaches the number (S MIN ) all the “READY” ISDSs connect and a measurement of the DC string current is made. The connected ISDSs measure the DC string current. If the current is greater than the minimum value I MIN  the ISDSs remain connected, and otherwise they all disconnect and the reconnect operation terminates. In this embodiment, the duration of any high string voltage can be minimized to only the time for a DC current to be established. 
       FIG. 10B  is a flow diagram of another example method, involving simultaneous reconnect with communication. In the example method  1050 , after a disconnect or after power-up an ISDS first sets its ready counter COUNT and timer (T) to zero and sets its status to “Not READY” at  1052 . During the waiting period T WAIT  the ISDS monitors for a READY signal from any other ISDS on the string reconnecting. If a READY signal is detected at  1054 , then the ISDS increments the ready counter at  1058 . 
     Once the T WAIT  waiting period expires, as detected at  1056 , the ISDS momentarily connects to the string at  1060  by opening bypass switch  220  and closing series switch  230  and looks for a current pulse generated from its connection to the string, at  1062 . If a pulse is detected, then it means that the ISDS is connected to the inverter output. The ISDS broadcasts a READY signal and the ready counter is incremented at  1064 . The ISDS then disconnects at  1066  by closing bypass switch  220  and opening series switch  230 . 
     The ISDS next checks the state of the ready counter at  1068 . If the ready counter is equal to the S MIN  then the minimum number of ISDSs (S MIN ) to provide the inverter start-up voltage are ready to connect, and the ISDS connects to the string at  1070  by opening bypass switch  220  and closing series switch  230 . The DC component of the string current is then measured and evaluated at  1072 . If the current is below the minimum value (I MIN ), then the ISDS disconnects at  1074  by closing bypass switch  220  and opening series switch  230 , and otherwise it remains connected and exits the reconnect operation at  1076 . 
     If the ready counter is less than S MIN  as determined at  1068 , then this indicates there are not enough ISDSs ready to reconnect. The ISDS will continue to listen for READY signals at  1078 . If additional READY signals are detected, then the ready counter is incremented at  1080  and re-evaluated at  1068 . 
     In this embodiment, when T WAIT  expires at  1056 , the ISDS verifies the physical continuity of the string by briefly connecting to the string at  1060  and measuring for a current pulse at  1062 . A READY signal is only sent to the other ISDSs in the string if a current pulse is detected. 
     General Overview 
       FIGS. 4 to 6C, 10A, and 10B  present detailed example methods. In a more general sense, a method could involve determining whether a reconnect condition, for reconnecting a PV panel to a power system from which the PV panel is disconnected, is satisfied. The reconnect condition could include a time condition related to a time T START , T WAIT , and/or T PULSE , for example. The determining in the case of a time condition could involve determining whether a time period has elapsed after disconnection of the PV panel from the power system and/or after some other event such as a previous reconnection attempt. 
     Some of the examples above refer to a connected PV panel limit condition relating to S MIN . Determinations in the case of such a PV panel limit condition could involve determining whether a predetermined number of other PV panels are reconnected to the power system within a predetermined period of time, or determining whether a predetermined minimum number of PV panels including the PV panel are ready to reconnect to the power system. The number of connected or ready to connect PV panels could be determined by detecting current pulses generated in the power system on reconnection of other PV panels to the power system or signals sent by other PV panels when they reconnect or are ready to reconnect to the power system. 
     A reconnect condition could include multiple conditions in some embodiments. For example, the determinations at  604 ,  616 ,  620 ,  624  in  FIG. 6B  illustrate multiple conditions involved in determining whether a PV panel should be reconnected to a power system at  608 .  FIGS. 6C, 10A, and 10B  also illustrate multiple conditions and determinations that could be made prior to reconnection at  670 ,  1018 ,  1070 . 
     As shown at  660  to  668  in  FIG. 6C , for example, a PV panel could be temporarily reconnected and then disconnected in determining whether the PV panel should subsequently be reconnected to the power system. In the example method  650 , the reconnect condition includes a current pulse condition, and determining whether the reconnect condition is satisfied involves reconnecting the PV panel to the power system at  660 , determining whether a current pulse is detected at the PV panel on reconnection of the PV panel to the power system at  662 , disconnecting the PV panel from the power system at  666  on detection of a current pulse or at  674  on expiry of a pulse detection time period, and determining that the reconnect condition is satisfied where a current pulse is detected at the PV panel at  662  during the pulse detection time period and the predetermined minimum of PV panels including the PV panel are ready to reconnect to the power system, as determined at  668 . The PV panel is then reconnected to the power system at  670 . 
     Another temporary reconnect embodiment is shown in  FIG. 10B . The temporary reconnection is shown at  1060  after T WAIT  expires at  1056 , a determination is made at  1062  as to whether a current pulse is detected at the PV panel on reconnection of the PV panel to the power system, a signal is broadcast from the PV panel at  1064  on detection of a current pulse during a pulse detection time period, and the PV panel is disconnected from the power system at  1066  on detection of a current pulse or at  1074  on expiry of the pulse detection time period. The reconnect condition is satisfied in this example where a current pulse is detected at the PV panel at  1062  during the pulse detection time period and the predetermined minimum of PV panels including the PV panel are ready to reconnect to the power system, as determined at  1068 . 
     In some embodiments, the reconnect condition could include a numeric value condition, in which case a random number associated with the PV panel is compared to a predefined value to determine whether to reconnect the PV panel to the power system. 
     Where a grid sensor is coupled to the power system as shown in  FIG. 8 , for example, the reconnect condition could be receipt of a connect command from a grid sensor. The reconnect condition is satisfied when the connect command is received. 
     A PV panel is automatically reconnected to the power system by its ISDS responsive to determining that the reconnect condition is satisfied. This could entail automatically reconnecting multiple PV panels to the power system, in embodiments where the reconnect condition relates to a predetermined minimum number of PV panels being ready to reconnect to the power system. 
     In order for a PV panel to remain connected to the power system, a power system operating condition must also be satisfied. A determination as to whether the power system operating condition is satisfied is made on reconnection of the PV panel to the power system. The determinations regarding I MIN , shown at  408 ,  510 ,  644 ,  626 ,  672 ,  1026 ,  1072  in  FIGS. 4 to 6C, 10A, and 10B  are illustrative of a current flow condition as the power system operating condition and an associated determination as to whether there is at least a minimum magnitude of current flow in the power system. Such a determination could involve determining whether there is at least a minimum magnitude of current flow in the power system over a time during which the PV panel remains reconnected on reconnection to the power system. 
     The power system operating condition could also include a current pulse condition and an associated determination as to whether a current pulse is detected at the PV panel on reconnection of the PV panel to the power system. This type of determination, to decide whether the PV panel should remain connected to the power system after reconnection, is shown at  640 ,  610 ,  1020  in  FIGS. 6A, 6B, and 10A , for example. 
     Responsive to determining that the power system operating condition is not satisfied on reconnection of the PV panel, the PV panel is automatically disconnected from the power system. This is shown, for example, at  412 ,  512 ,  649 ,  612 ,  630 ,  674 ,  1022 ,  1030 ,  1074  in  FIGS. 4 to 6C, 10A, and 10B . 
     Automatic reconnection and disconnection of a PV panel by an ISDS do not involve intervention by an operator or other personnel. In the case of a previous disconnection on detection of an arc fault, however, automatic reconnection is disabled, and a PV panel would not be reconnected automatically, to avoid possibly re-igniting the arc. 
     Regarding disconnection of a PV panel, a PV panel would be disconnected prior to its commissioning, but there could be other reasons to disconnect a PV panel after it has been initially installed. For example, in an embodiment, a low current condition or an arc fault condition in a power system could be detected at a PV panel that is connected to the power system. The PV panel could then be automatically disconnected from the power system responsive to detection of the low current condition or the arc fault condition. The low current condition or the arc fault condition could be independently detected at each of multiple PV panels that are connected to the power system, and each PV panel could be independently automatically disconnected from the power system responsive to detection of the low current condition or the arc fault condition at each PV panel. 
     In terms of apparatus implementations, a PV panel disconnect switching arrangement, referenced herein primarily as an ISDS, could include switches  220 ,  230  ( FIG. 2C ) to control connection of the PV panel to a power system and bypass of the PV panel on disconnection of the PV panel from the power system, and a controller  210 ,  710  ( FIGS. 3A, 3B, 7 ) operatively coupled to the switches, to determine whether a reconnect condition for reconnecting the PV panel to the power system is satisfied, to automatically reconnect the PV panel to the power system responsive to determining that the reconnect condition is satisfied, to determine whether a power system operating condition is satisfied on reconnection of the PV panel, to automatically disconnect the PV panel from the power system responsive to determining that the power system operating condition is not satisfied on reconnection of the PV panel. Various examples of reconnect conditions and power system operating conditions, and associated example determinations, are noted above. 
     The controller in a PV panel disconnect switching arrangement could also or instead detect a low current condition or an arc fault condition in the power system while the PV panel is connected to the power system, and automatically disconnect the PV panel from the power system responsive to detection of the low current condition or the arc fault condition. 
     Conclusion 
     What has been described is merely illustrative of the application of principles of embodiments of the present disclosure. Other arrangements and methods can be implemented by those skilled in the art. 
     For example, although described primarily in the context of methods and systems, other implementations are also contemplated. At least control features, for instance, could be implemented as instructions stored on a non-transitory computer-readable medium. 
     It should also be appreciated that embodiments disclosed herein are not necessarily restricted to single PV panel string implementations. Multiple PV panel strings could be connected in parallel to the same inverter, as shown in  FIG. 2B . With multiple parallel PV panel strings, V START  for a simultaneous reconnect operation could be the operating voltage of a PV panel string. When the PV panels of one parallel connected PV panel string connect to the string in the morning, for example, any other PV panel string must generate a voltage sufficient to forward bias its diode  214 ,  215 ,  216 , which might involve all of the PV panels in a string reconnecting simultaneously.