Abstract:
A “smart” junction box for photovoltaic systems provides electrical measurements of strings of photovoltaic cells to detect premature photovoltaic cell degradation, bypass diode failure, and arcing, and report the same to a central location and/or to provide for automatic disconnection of a given string of photovoltaic cells. The smart junction box also provides general reporting of electrical characteristics to the central system and allows disconnection by command from the central system.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. provisional application 62/044,526 filed Sep. 2, 2014 and hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to photovoltaic systems for providing electricity from solar power and in particular to a junction box for electrically combining banks of photovoltaic cells. 
         [0003]    Sunlight, provides a promising source of energy that avoids many of the problems of burning fossil fuels. While sunlight can be readily concentrated and used for heating purposes, an increasingly attractive option is the direct generation of electrical power from sunlight using photovoltaic cells. Photovoltaic cells may be fabricated of semiconductor materials and take advantage of the photovoltaic effect to convert light into electrical voltage. 
         [0004]    The voltage provided by each photovoltaic cell is relatively small (approximately 0.5 volts for standard cell) and accordingly they are normally combined in series to provide a desired working voltage. This series combination of photovoltaic cells may be managed by a junction box which includes bypass diodes shunting individual photovoltaic cells or sets of series connected cells. The current produced by series connected cells is limited by the lowest current cell, for example, one that is in shade, and in this case, the diodes provide a bypass around the shaded cells and prevent current from being driven through the shaded cell such as can cause heating of the cell. The diodes may also bypass cells that have failed. 
         [0005]    Power from the junction boxes may be combined and provided to an inverter which converts the direct current of the photovoltaic cells into alternating current for connection to the grid or local electrical system. 
         [0006]    The effectiveness and safety of a photovoltaic system is strongly dependent on the proper operation of each of the photovoltaic cells; however, when the cells are combined and during normal variation in cell output their operating state is difficult to establish. This can lead to unexpected or catastrophic cell failure. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides a “smart” junction box for photovoltaic arrays that can monitor the electrical operating characteristics of individual or small groups of cells to detect early cell failure or bypass diode failure and reports that state to a central system. In one embodiment, the invention provides a method of detecting arcing in the cells or their interconnected wiring. 
         [0008]    Specifically, in one embodiment, the invention provides a smart junction box for series connected photovoltaic arrays providing bypass diodes. The smart junction box may include a housing providing terminals receiving electrical connections to series connected photovoltaic units at multiple electrical junctions between the series connected photo voltaic units as joined by bypass diodes, a remote communication circuit for communicating with a central station, and I/O circuitry communicating with the terminals to digitize electrical measurements of electrical junctions. A processor receives the digitized electrical measurements and executes a stored program to evaluate a history of electrical measurements to detect at least one of electrical damage to a photovoltaic unit or a bypass diodes to provide a signal through the remote communication circuit to the central station. 
         [0009]    It is thus a feature of at least one embodiment of the invention to provide rapid and distributed detection of cell degradation and diode failure. 
         [0010]    The detection of electrical damage to a photovoltaic unit may monitor a voltage across each photovoltaic unit to indicate damage to a photovoltaic unit whose voltage deviates from an average voltage for more than a predetermined time and amount. 
         [0011]    It is thus a feature of at least one embodiment of the invention to determine possible photovoltaic cell degradation despite series connection of the photovoltaic cells and natural variation in photovoltaic cell output caused by solar fluctuation. 
         [0012]    The detection of damage to a bypass diode may detect a forward bias voltage across a bypass diode of greater than a predetermined amount. 
         [0013]    It is thus a feature of at least one embodiment of the invention to employ natural variation in photovoltaic cell illumination to test for diode failure. 
         [0014]    The smart junction box may include an electrically controllable switch in series with the photovoltaic units and communicating with the processor for breaking the series connection of the photovoltaic units. 
         [0015]    It is thus a feature of at least one embodiment of the invention to permit local focused disconnection of photovoltaic cells for repair or hazard mitigation. 
         [0016]    The processor may respond to detection of a damaged bypass diode to control the electrically controllable switch for breaking the series connection of the photovoltaic units. 
         [0017]    It is thus a feature of at least one embodiment of the invention to allow automatic mitigation of potential hazards in photovoltaic cell assemblies. 
         [0018]    The processor may receive power from the photovoltaic units. 
         [0019]    It is thus a feature of at least one embodiment of the invention to allow for a simplified installation of a distributed monitoring system by allowing the junction box system to obtain power from the same photovoltaic cells that it monitors. 
         [0020]    The smart junction box may include a power storage element for storing power from the photovoltaic units for times when power cannot be obtained from the photovoltaic units. 
         [0021]    It is thus a feature of at least one embodiment of the invention to permit operation of the monitoring system using photovoltaic cell-scavenged power even at night. 
         [0022]    The smart junction box may include a current transducer measuring series current in the series connected photovoltaic units. 
         [0023]    It is thus a feature of at least one embodiment of the invention to provide additional information about the operation of the photovoltaic cells beyond photovoltaic cell voltage. 
         [0024]    The processor may further evaluate the history of electrical measurements and execute a stored program to detect electrical arcing in the photovoltaic units. 
         [0025]    It is thus a feature of at least one embodiment of the invention to detect electrical arcing in photovoltaic cell units such as may signal a variety of problems. 
         [0026]    The detection of arcing may monitor a voltage across at least one photovoltaic unit to (a) identify a dominant frequency over time; (b) subtract the dominant frequency over time from the measured voltage to produce a corrected voltage; and (c) analyze a trend of the spectral amplitude of the corrected voltage as a function of frequency to indicate arcing when the trend meets a predetermined criteria. 
         [0027]    It is thus a feature of at least one embodiment of the invention to provide improved detection of arcing by eliminating dominant frequencies from associated inverters and the like and evaluating a spectral signature associated with arcing. 
         [0028]    The detection of arcing may further monitor the voltage across at least one photovoltaic unit to: (d) identify a secondary dominant frequency in the corrected voltage; and (e) analyze a variation in the secondary dominant frequency over time to indicate arcing when that variation exceeds a predetermined value. 
         [0029]    It is thus a feature of at least one embodiment of the invention to detect arcing through an analysis of randomness of dominant frequency. 
         [0030]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a simplified representation of a photovoltaic system combining power from multiple array units and showing (in expanded form) each array unit comprising sets of series connected strings of photovoltaic cells combined at a junction box and showing (in expanded form) a block diagram of the junction box having an electronic processor monitoring electrical characteristics of the series connected photovoltaic cells; 
           [0032]      FIG. 2  is a figure showing an example plot of monitored voltage received by the processor of  FIG. 1  used for program analysis for detecting premature cell failure; 
           [0033]      FIG. 3  is a figure showing an example plot of monitored voltage of an individual cell showing analysis for the detection of a failed bypass diode; and 
           [0034]      FIG. 4  is a logical flow diagram of a program executed by the processor of  FIG. 1  for detecting arcing in or between the photovoltaic cells. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    Referring now to  FIG. 1 , a photovoltaic system  10  may provide a panel  12  for receiving sunlight  14  to expose multiple array units  16  positioned on the surface of the panel to the sunlight  14 . 
         [0036]    Electrical power from each of the multiple array units  16  may be combined and provided to an electrical inverter  18  producing a source of alternating current power  20 . A typical panel may provide a substantial power, for example,  250  watts with a short circuit current of 8.5 amps and open circuit voltage of forty-two volts. Ten to fifteen (or more) panels  12  may be connected in series to form a string and up to thirty two strings may be combined to form a sub-circuit to feed the inverter  18 . The inverter  18  may employ multiple battery units for storing electrical power. 
         [0037]    A central station  22  may communicate with the multiple array units  16  to monitor the condition of the array unit  16  and their constituent components as will be described. 
         [0038]    Referring to the first expanded inset of  FIG. 1 , each array unit  16  may hold multiple photovoltaic cells  24 , for example, providing a nominal operating voltage of 0.5 volts and connected in series in a string  27  to terminals  26  of a housing  25  of a smart junction box  28 . The smart junction box  28  combines electrical power from each string  27  to provide combined power on a common voltage bus  30  to be joined to a second junction box or to the inverter  18 . 
         [0039]    Referring to the second expanded inset of  FIG. 1 , the smart junction box  28  may include an electronic controller  32 , for example, including a computer processor  34  communicating with an electronic memory  36  holding a stored program  38 . The controller may also include I/O circuitry  41  that may receive voltage signals or output voltage signals under the control of the processor  34  and as intermediated by an analog-to-digital or digital-to-analog converter as is generally understood in the art. 
         [0040]    Analog input lines of the I/O circuitry  41  connect to each of the terminals  26  to measure a voltage at the terminals and to thereby capture voltages at junctions between the series connected photovoltaic strings  27 . For example, in a simplified system with three photovoltaic strings  27   a - c  connected to junction box  28 , photovoltaic string  27   a  will connect the terminals S 1 + and S 1 − so that during normal operation of the string  27   a  terminal S 1 + will have a higher voltage than terminal S 1 −. This convention continues with photovoltaic string  27   b  connected to terminals S 2 + and S 2 −, and photovoltaic string  27   c  connected to terminals S 3 + and S 3 −. 
         [0041]    In this example, a first analog input of the controller  32  receives the voltage from terminal S 1 +, a second analog input from the controller  32  receives the voltage from electrically joined terminals S 1 + and S 1 −, a third analog input from the controller  32  receives the voltage from electrically joined terminals S 2 − and S 3 +, and a fourth analog input from controller  32  receives the voltage from electrical terminal S 3 −. It will be understood generally that the term “junction” refers to both junctions between two photovoltaic strings  27  or between a given photovoltaic string  27  and other components of the photovoltaic system  10 . 
         [0042]    In one embodiment, bypass diodes  40   a - 40   c  are placed across each of the strings  27   a - 27   e  respectively with the cathode of diode  40   a  attached to terminal S 1 + and its anode attached to electrically joined terminals Si+ and S 1 −. Similarly the cathode of diode  40   b  has its anode attached to electrically joined terminals S 1 + and S 1 − and its anode attached to electrically joined terminals S 2 − and S 3 +. Finally, the diode  40   c  has its cathode attached to electrically joined terminals S 2 − and S 3 + and its anode attached to terminal S 3 −. 
         [0043]    Terminal S 3 − and the anode of diode  40   c  also attach to one terminal of a current measuring element  42  (such as a small resistor, Hall effect device or the like). The second terminal of the current measuring element  42  is attached to one terminal of an electrically controllable switch  44  such as a MOS transistor or relay or the like whose other terminal is then attached to a ground lead of voltage bus  30  while the corresponding positive lead of voltage bus  30  is attached to terminal S 1 +. 
         [0044]    In this manner, the current measuring element  42  may measure the current through each of the strings  27  (as must be equal because of their series connection) and this current can be broken by the electrically controllable switch  44 . An output from the current measuring dement  42  is received at an analog input of the controller  32  and a controller provides a digital output to control the state of the electrically controllable switch  44  for turning either on or off. 
         [0045]    The controller  32  may receive power from terminal S 1 + and be grounded at terminal S 3 − (or any two terminals having a voltage difference appropriate for powering the controller  32 ). The received power may be further processed by a voltage regulator  46  of the type known in the art to accommodate differences in voltages that will be obtained from the strings  27  under different illumination conditions. In addition, an energy storage element  48  such as a battery or a capacitor may be used to receive and store this power to provide power smoothing offsetting fluctuations of the power from the strings  27 . 
         [0046]    The controller  32  may also provide an output coupled through coupling capacitor  49  into the strings  27 , for example, through terminal S 1 +, so that the controller  32  may inject a high-frequency signal into the strings for measurement purposes as will be described. 
         [0047]    Finally, the controller  32  may communicate with a remote communication circuit  50 , for example, being a wireless transmitter using Bluetooth, ZigBee, 802.11 Wi-Fi communications, or carrier current communications such as X-10 or the like, to allow communication between the smart junction box  28  and the central station  22 . 
         [0048]    During operation, the smart junction box  28  may monitor the voltages at each of the terminals  26  and the current through the strings  27  and provide for regular reporting back to the central station  22  which may display or otherwise use this information combined with information from other array unit  16 . Generally this information can be used to generate energy statistics with respect to the photovoltaic system  10  for determining its efficiency or power output or to adjust the operation of the inverter  18  for maximum efficiency. 
         [0049]    The program  38  of the junction box  28  may further include routines for determining whether an individual string  27  has begun to degrade in performance such as may indicate imminent failure or the need for replacement of a photovoltaic cell  24 . 
         [0050]    Referring now to  FIG. 2 , in one embodiment, each of the voltages measured across the terminals of each string  27  may be averaged together and averaged over a time window (for example, 15 minutes) to produce a running average curve  54  and an upper and lower range curve  56  representing, for example, a given percentage deviation from the value of the running average curve  54 . Each individual voltage measurement from each string  27  aggregated with the other strings  27 , for example, individual string measurement  58 , may then be compared to the upper and lower range curve  56  and if the value of the individual string measurement  58  passes out of the range defined by the upper and lower range curve  56 , for example, at time t 0 , a clock maybe started measuring a deviation limit time  60 . After expiration of the deviation limit time  60 , the particular string  27  associated with the individual string measurement  58  may be marked as degraded indicating that it is providing a substantially lower voltage output likely not due to selective shading. In this respect, upper and lower range curve  56  and deviation limit time  60  are selected to avoid falsely indicating that a cell is defective when it is momentarily shaded, for example, by a cloud. The use of averaging over time and different voltages accommodates the seasonal and daily change of light input to the cells in combination. 
         [0051]    Upon detection of a degraded cell, a report may be generated and sent to central station  22  which may be displayed or otherwise used The central station  22  may in some circumstances communicate via the circuit  50  with the junction box  28  to open the electrically open, electrically controllable switch  44  to disconnect the strings, for example, to allow for maintenance or to prevent possible hazards. Alternatively, the junction box  28  may automatically provide for this disconnection according to program conditions in the program  38 . 
         [0052]    Referring now to  FIG. 3 , the junction box  28  may also check for failed diodes  40  by monitoring the voltages across the diodes  40  over time as indicated by voltage curve  62 . A diode failure in a shod condition will be indicated if the voltage across the diode is equal to zero volts for more than a predetermined time period  64  when the running average curve  54  is above a predetermined threshold indicating that the photovoltaic system  10  is illuminated. In addition, a diode failure in an open condition will be indicated if the reverse bias across the diode  40  rises to a level greater than the forward diode voltage (approximately 0.7 volts). 
         [0053]    Diode shorting may be detected in the alternative by injection of a high-frequency signal through capacitor  49  and observing the attenuation as one moves down an effective resistor ladder of successive voltage measurements at the terminals  26 . Each of the strings  27  will generally represent a current source  66  in parallel with a diode  67  and a resistor  73 . The current source  66  and diode  68  are reverse-biased with the injection of a high-frequency signal at S 1 + and so only the resistors  70  will be measured providing a regular drop in voltage at each successive terminal  26  unless those terminals are shorted by a diode  40 . 
         [0054]    Again, upon detection of damage to a diode, a report may be generated and sent to central station  22  which may be displayed or otherwise used. The central station  22  may in some circumstances communicate via the circuit  50  with the junction box  28  to open the electrically open, electrically controllable switch  44  to prevent hazards or allow maintenance. Alternatively, the junction box  28  may automatically provide for this disconnection according to program conditions in the program  38 . 
         [0055]    Referring now to  FIG. 4 , the smart junction box  28  may also provide for the detection of arcing, for example, caused by broken or eroded cable insulation, intermittent wire connections, or a short through the structure of the photovoltaic system  10 . This technique may also be used at the central station  22  and need not be incorporated into the smart junction box  28 . 
         [0056]    The process of arc detection may be implemented in the controller  32  which may, for example, employ a field programmable gate array to provide for the necessary high-speed processing. At a first step, a high-frequency sampling of the voltages at voltage bus  30  and/or the current at current measuring element  42  is made through the I/O circuitry  41 . 
         [0057]    The processor  34  following the program  38  may analyze the sample values of voltage and/or current to identify one or more dominant frequencies in the signal from voltage bus  30  or current measurement of block  42 . The dominant frequencies may be obtained by taking the Fourier transform of the signal at process block  65  and evaluating the amplitude of that transform (being the square root of the addition of the square of real part plus the square of imaginary part of the Fourier transform) and comparing that to the average value of the Fourier transform (F_FFT) within a predefined window of 3.5 kHz to 24k Hz (for example). The dominant frequencies will be any peak values more than a predetermined factor (for example, three) above the average value of the amplitude of the Fourier transform spectrum determined at process block  71 . Frequencies at those dominant frequencies and on either side of those dominant frequencies to half the amplitude of the peak are removed from the Fourier transform and replaced with the average value of the spectrum in the predefined window. 
         [0058]    Removal of the dominant frequencies is indicated by process block  63  identifying the dominant frequencies and subtractor block  69  for subtracting those frequencies from the spectrum. Removal of the dominant frequencies serves to partially eliminate background noise from measurements such as caused by inverter, charge controller, and other loads switching and the like that can obscure signals from arcing, 
       Parameter 1 
       [0059]    Once the dominant frequencies have been removed from the spectrum, a randomness value is determined from this modified spectrum by identifying the remaining next most dominant single frequency and comparing this peak between successive Fourier transform cycles. Each Fourier transform cycle may occur at a fixed frequency, for example, 64 kHz. This randomness value measures whether the next most dominant single frequency occurs in successive Fourier transform cycles or whether another peak having a frequency difference of at least 500 Hz replaces the previously identified peak as one moves to the next Fourier transform cycle. A value of zero through ten maybe assigned to this randomness value (FreqRandom), for example, by looking at ten successive Fourier transform cycles and counting how many times successive cycles have the same peak. This randomness value becomes a first parameter as calculated at process block  70   a.    
       Parameter 2 
       [0060]    The spectrum having the dominant frequency removed is also averaged as indicated by process block  70   b  over the above described frequency range and a magnitude of a difference is obtained between average value (A_FFT) for two successive Fourier transform cycles (A_FFT(n)-A_FFT(n+1)) to produce an amplitude variation value (E_FFT). When this value changes by less than a predefined percentage (for example, 50%) over a given time for example, 1 second) that value becomes a steady-state value (E_FFTSS). Process block  70   b  outputs the difference between the amplitude variation value and the steady-state value (E_FFT-E_FFTSS). 
       Parameter 3 
       [0061]    At process block  70   c  the average value of the Fourier transform within the predefined window after removal of the dominant frequencies (A_FFT) is compared by subtraction to its steady-state value obtained in the same way as described above with respect to E_FFT to obtain a value A_FFTSS. Process block  70   c  outputs the magnitude of the difference between the average value and the steady-state value (A_FFT-A_FFTSS). 
       Parameter 4 
       [0062]    At process block  70   d,  the output values of process block  70   c  is divided by the output of process block  70   b  and the result of that division output from process block  70   d  as: (A_FFT-A_FFTSS)/(E_FFT-E_FFTSS) 
       Parameter 5 
       [0063]    At process Hock  70   e,  a normalized average value (A_FFT %) is calculated by using the output of process block  70   c  and dividing it by the value A_FFTSS computed as described above with respect to process block  70   c  to provide: (A_FFT-A_FFTSS)/A_FFTSS. 
       Parameter 6 
       [0064]    At process block  70   f,  a normalized variability value (E_FFT %) is calculated by using the output of process block  70   b  and dividing it by E_FFTSS as described above to produce: (E_FFT- E − FFTSS)/E_FFTSS. 
       Parameter 7 
       [0065]    At process block  70   g,  a variability-adjusted average value is calculated by dividing A FFT  as calculated above by E_FFT also described above providing: A_FFT/E_FFT. 
       Parameter 8 
       [0066]    At process block  70   h,  a slope of the Fourier transform with the dominant frequencies removed is calculated as FFT_Ratio, for example, by a linear regression or by dividing the frequency window discussed above into two equal segments and summing the values of the spectrum (integrating) in each of the two segments and dividing the sum for the low segment by the sum for the high segment. Other methods of determining the slope can also be used including, for example, extending this process over additional segments. 
         [0067]    Once each of these parameters 1-8 is determined, they may be averaged or subject to a low pass filter over a number of Fourier transform cycles, for example, using the function: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0068]    for A_FFT, FFT_Ratio, and FreqRand. 
         [0069]    The averaging process block  71  may also be subject to this low pass filtering, however, using the above equation (1) with equation (3) below: 
         [0000]        x   2 =abs( x[ 0]+ x[ 1]) for F_FFT  (3)
 
         [0070]    The parameters 1-8 are next compared against a predetermined threshold value that may be determined empirically, this process as indicated by comparison blocks  72  to provide output flag values. The “&gt;” sign indicates that a high state output of the comparison process is set if the parameter is above the threshold value and the “&lt;” sign indicates that a high state output of the comparison process is set if the parameter is below the threshold value 
         [0071]    The number of set (high state) outputs (flags) is then counted as indicated by process block  74  and this count value provides the variable A_Flags. 
         [0072]    This value of A_Flags is then compared against a ninth predetermined threshold that may be empirically determined to provide a first final parameter. 
         [0073]    Next the difference in value of A_—FFT for two successive Fourier transforms cycles is compared (as calculated per process block  70   i ) against a tenth threshold that may be empirically determined to provide a second final parameter. 
         [0074]    Finally, a difference in the DC value (zero frequency) of the current passing through current measuring element  42  (as calculated per process block  70   j ) may be compared against its steady-state DC value (computed per the steady-state values described above) and compared against the eleventh predetermined empirically determined threshold to provide a third final parameter. 
         [0075]    The logical AND of the first, second, and third final parameters as indicated by AND gate  80  provides an indication that arcing is occurring. The occurrence of arcing as detected by this process may be reported back to the central station  22  or may be used by the program to automatically open electrically controllable switch  44 . 
         [0076]    It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments, including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.