Abstract:
A method and system is provided for achieving power shut down or disconnect at individual solar panel level for a DC solar power system. The system provides fail safe power disconnect for emergencies or electrical system maintenance. A high-frequency current source signal is transmitted over DC power lines to each junction box of a photovoltaic panel array. The high frequency signal is transmitted concurrently with the DC power, and is isolated from an inverter by an inductor. A communications receiver circuit controls the PV panel to short circuit the PV panel output terminals and disconnect the PV panel from the output terminals in response to the high-frequency signal.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a safety shutdown system, and more particularly to a system and method for safely powering down a photovoltaic solar panel array. 
       BACKGROUND OF THE INVENTION 
       [0002]    Photovoltaic (PV) arrays, e.g., roof top mounted solar panels, are becoming more widely used as the manufacturing costs are reduced. In a large system a plurality of solar panels are connected in series to increase the power output by the system. 
         [0003]    For example, some large PV systems may include 15 to 33 solar panels connected in series. The output voltage associated with an individual PV array may be in the range of 38 to 48 VDC, which results in the operating voltages of 600 to 1500 volts for large PV systems. Such high voltages are output by the PV system when illuminated. 
         [0004]    Current systems provide no method of controlling or shutting down the output voltage of a PV system. The inability to controllably power down a PV system presents a problem for emergency personnel when an emergency arises in a building with PV systems providing power in the building. In traditional power systems a main power disconnect may be used to remove power from the building power system, to insure the safety of emergency personnel during their operations in the building. The danger to personnel in an emergency is further increased because the voltage of the large PV system may exceed the line voltage of traditional power systems. 
         [0005]    The higher voltage of the PV system is normally distributed over much of the roof area of a building where the solar panels are interconnected. In the event of fire for instance, water may be sprayed on the roof of the building, creating an electrical shock hazard if the panels and wire conductors are exposed to water. To overcome these hazardous situations, many governmental authorities are enacting regulations requiring some means of panel shutdown. 
         [0006]    There is a need for providing a means of safely disconnecting power from a PV system to prevent hazardous electrical conditions. 
       SUMMARY OF THE INVENTION 
       [0007]    This disclosure provides a method and system for achieving power shut down or disconnect at the individual solar panel level for all solar panels controlled from a single central or multiple locations. The system is fail-safe and power is disconnected if in the event that the wires burn completely from the solar panels. The disclosed system is simple and inexpensive to implement. Central control of the PV power disconnect system is provided, which enables remote control from a distance, and reliable communications. 
         [0008]    The disclosed PV system employs a high frequency, low level signal which enables the control circuit to be implemented with a smaller inductor. A high frequency signal is generated from a current source rather than a voltage. Although not wishing to be bound by any theory, using a current source to generate the signal, as the number of series connected PV panels increases, the voltage developed across the inductor does not change even if the inductance added by the series cable is many times greater than the signal developing inductance. The power level of the received signal remains approximately the same provided the current regulation of the current source is very good. The current signaling may also eliminate errors that may be caused by long runs of series cable resistance when using voltage signaling. Long power cables may be characterized by series inductance that may attenuate the high-frequency signal if voltage signaling is used. Also if the cable routing is uncontrolled large loops may be formed which can further increased cable inductance. 
         [0009]    Another advantage is that no additional wires need to be connected to the PV system to implement the shut-down control. 
         [0010]    The PV system may be expandable to full duplex. 
         [0011]    The PV system may be configured so that the PV system shuts down when the main power is shut off 
         [0012]    Yet another advantage is that the PV system is not subject to interference such as an RF link. 
         [0013]    A further advantage is that the PV system continuously monitors the shut down system for faults and shuts down when a fault is detected. 
         [0014]    An embodiment is directed to a method for achieving power shut down or disconnect at the individual photovoltaic panel level for multiple photovoltaic panels. The method includes: generating a low voltage high-frequency current source signal; transmitting the low voltage high-frequency current source signal concurrently with DC power to junction boxes of the multiple photovoltaic panels of a photovoltaic panel array; isolating the low voltage high-frequency current source signal from the DC power; and short circuiting output terminals of respective photovoltaic panels and disconnecting the respective photovoltaic panels from the output terminals in response to the low voltage high-frequency current source signal. 
         [0015]    An embodiment is directed to a method for achieving power shut down or disconnect of respective photovoltaic panels of a photovoltaic panel array. The method includes: generating a high-frequency current source signal; transmitting the high-frequency current source signal concurrently with DC power to junction boxes of the respective photovoltaic panels of the photovoltaic panel array; isolating the high-frequency current source signal from the DC power; comparing the value of the amplitude of the high-frequency current source signal to a predetermined threshold value; removing output power from the respective photovoltaic panels associated with a respective junction box if the value of the amplitude of the high-frequency current source signal is less than the threshold value; and short-circuiting output terminals of the respective photovoltaic panels if the value of the amplitude of the high-frequency current source signal is less than the threshold value, thereby providing zero output voltage on the photovoltaic panels. 
         [0016]    An embodiment is directed to a system for achieving power shut down or disconnect of respective photovoltaic panels of a photovoltaic panel array. The system includes a control unit which receives input power from a switch. The control unit is provided in electronic communication with respective junction boxes of the respective photovoltaic panels. The switch includes a mains input which receives AC power input from an electrical AC source. An AC to DC converter receives the AC input power from the switch when the switch is actuated. A current source signal generator receives DC power from the converter. The current source signal generator generates a low voltage high-frequency AC signal of a predetermined frequency when the switch is closed. 
         [0017]    Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  shows an exemplary block diagram of a PV system shut down control system. 
           [0019]      FIG. 2  shows a schematic diagram of the exemplary embodiment the PV system of  FIG. 1 . 
           [0020]      FIG. 3  shows an exemplary PV panel junction box. 
           [0021]      FIG. 4  shows an exemplary PV panel switch control configuration. 
           [0022]      FIG. 5  shows an exemplary communications receiver circuit for a PV panel switch control. 
           [0023]      FIG. 6  shows an exemplary a high voltage detection circuit for indication of output voltage level of the PV system. 
           [0024]      FIG. 7  shows an exemplary current source signal generator circuit for generating the high-frequency control signal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Referring to  FIG. 1 , a block diagram of the PV system power shut down function is shown. A main control unit  10  receives input power from a shut-down switch  12 . Control unit  10  is in electronic communication with each of a plurality of solar or PV panel junction boxes  14 . Control unit  10  communicates with PV panel junction boxes  14  over power transmission wires  18  carrying DC input power to an inverter  16  via control unit  10 . Shut-down switch  12  includes a mains input  20  sourced, e.g., from the electrical AC main service of a building, a building emergency AC power panel, or other similar AC power source. Mains input  20  is connected to a switch control unit  10  thru a switch  22 , e.g., an emergency pushbutton switch, a toggle switch or similar circuit disconnecting device. Control unit  10  includes an AC to DC converter  24  which receives the AC input power from switch  22  when switch  22  is actuated. The output of converter  24  provides DC power to a current source signal generator  26 . 
         [0026]    When switch  22  is closed, current source generator  26  generates a low voltage high-frequency AC signal of a predetermined frequency, which signal is transmitted to junction boxes  14  through an inductor  28 . When switch  22  is open, there is no high-frequency AC signal generated by current source signal generator  26 . Each junction box  14  includes an inductor  30  at the input of a PV panel control circuit  32 . High-frequency signals, when present, passes through inductor  30  to band pass filter  34 . Band pass filter  34  is configured to pass a band of frequencies that includes the predetermined frequency of the high frequency AC signal from inductor  30 , and to block frequencies outside of the frequency band of the filter. The output of band pass filter  34  is transmitted to a precision amplitude detector  36 . An amplitude detector  36  detects the amplitude of the signal received from band pass filter  34 , and a comparator  38  determines if the value of the signal amplitude detected by amplitude detector  36  exceeds a predetermined threshold value. In one embodiment the threshold voltage may be 0.6 volts. In this embodiment if the value of the signal amplitude detected by amplitude detector  36  is less than the 0.6 volt threshold, comparator  38  actuates a switch control  40  to remove output power from a PV panel  15  associated with a respective PV panel junction box  14 . Switch control  40  also short-circuits the output terminals of PV panel  15  in response to amplitude detector  36  sensing a low signal level, to ensure zero output voltage on PV panel  15 . The presence of the high frequency signal enables PV panel  15  to power up, and the absence of the high frequency signal causes PV panel  15  to shut down. 
         [0027]    In an alternate embodiment, if the predetermined threshold value is exceeded, a shut-down signal is transmitted to junction box  14 , and comparator  38  actuates a switch control  40  to remove output power from PV panel  15  associated PV panel junction box  14 . Switch control  40  also short-circuits the output terminals of PV panel  15  to ensure zero output voltage on PV panel  15 . 
         [0028]    Referring next to  FIG. 2 , an exemplary PV system  100  is shown. PV panels  15  are wired in series between a positive DC power line or bus  42  and a negative DC power line or bus  44 . The main control unit  10  is connected between PV panels  15  and inverter  16 . Shut-down switch  22  may be located remotely or locally relative to main control unit  10 . Shut-down switch  22  provides power to AC signal source generator  26 . The AC signal output of signal source generator  26  is coupled to positive DC power line  42  via DC capacitor  27  on the line side of inductor  28  relative to inverter  16 , which is connected on the load side of inductor  28 . In an alternate embodiment, inductor  28  may be connected to PV system  100  via negative DC power line  44  between AC signal source  26  and inverter  16 . 
         [0029]    PV panels  15  include a junction box  14  ( FIG. 3 ) to connect multiple modules within a PV panel. Modules may be interconnected with diodes to optimize the performance of the system if a PV module is shaded or otherwise fails to produce the rated output power. A shut down circuit  50  may be incorporated in junction box  14 . 
         [0030]    Electronic switches, or switch controls  40  are provided in junction box  14 . Electronic switches are configured to disconnect the voltage generated by PV panel  15  from the output terminals  54 ,  56  ( FIG. 4 ). Electronic switches  40  are also configured to short circuit the PV panel output terminals to ensure that no voltage is present on the output terminals. In one embodiment the electronic switches are configured so the default state is “OFF”. 
         [0031]    A communications signal is transmitted to PV panels  15  on the same cables  44 ,  42  that carry DC power from PV panels  15 . Main control unit  10  transmits a communications signal or high frequency signal from AC signal source  26  to communications receiver circuit  52  in PV panels  15 , which switches on PV panels  15 , i.e., when the high frequency signal is detected by communications receiver circuit  52 , electronic switches  40  route DC power from the internal panel cells of PV panel  15  to the output terminals of PV panel  15 . Switch  22  located in an accessible area is wired to main control unit  10 . When switch  22  is opened the high frequency signal from control unit  10  is transmitted on power wires  44 ,  42  to communications receiver circuit  52  for each PV panel  15  is muted. This then shuts down all of the PV panels  15  connected to the PV system  100 . 
         [0032]    Referring to  FIG. 4 , an exemplary circuit is shown for implementing shut-down control of a PV panel  15 . Output terminals  54 ,  56  in junction box  14  receive high-frequency AC current source signal  58  from control unit  10 . Signal  58  passes through inductor  30  and capacitor  60  connected in series, the inductor  30  and capacitor  60  forming a pass filter. In one embodiment, inductor  30  may have an inductance of approximately 10 micro-Henry When PV system is in operation signal  58  is present, and communications receiver circuit  52  controls electronic switches  40   a  and  40   b  (shown schematically in  FIG. 4  as single pole switches for simplicity) in response to the presence of signal  58 . Normally open switch  40   a  is closed when signal  58  is detected by communications receiver circuit  52 , and conducts power to output terminals  54 ,  56  from PV panel  15 , while normally closed switch  40   b  is in an open or non-conducting state, creating an open circuit between inductor  30  and negative output terminal  56 . It should be noted that in an alternate embodiment, switch  40  may be connected between the negative output terminal  56  and communications receiver circuit  52  to accomplish the same result. 
         [0033]    Once signal  58  is removed as a result of opening switch  22 , signal  58  is removed from communications receiver circuit  52 , and communications receiver circuit  52  causes electronic switches  40   a  and  40   b  to return to their default, or normal, states. Thus switch  40   a  opens to disconnect PV panel output from inductor  30 , and switch  40   b  closes, resulting in a short circuit between output terminals  54 ,  56 . In one embodiment switching circuits for switches  40   a  and  40   b  may include timing elements configured to cause normally closed switch  40   b  to close before normally open switch  40   a  opens in response to a shut-down command In the reverse situation, when applying power again timing elements are configured to cause normally open switch  40   a  to close before normally closed switch  40   b  opens. 
         [0034]    Referring next to  FIG. 5 , an embodiment of communications receiver circuit  52  is shown. Dashed lines indicate DC power flow from PV panels  15  to output terminals  54 ,  56 . Inductor  30  provides a high impedance to block high-frequency signal  58  from propagating to PV panels  15 . Receiver circuits  70  are configured to respond to signal  58  at the predetermined frequency. As discussed above, in an alternate embodiment inductor  30  may be connected in the negative output line rather than in the positive output line, by connecting inductor  30  between receiver circuits  70  and output terminal  56 . 
         [0035]    Referring to  FIG. 6 , an exemplary precision amplitude detector  46  comprises a high voltage detection circuit  80  as shown.  FIG. 7  shows an exemplary current source signal generator  26 . Current source signal generator  26  is implemented via an oscillator circuit  82  driving a current to voltage circuit  84 . 
         [0036]    While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.